WO2015061656A1 - Flexible permation barrier system deposited in a single process and having ultralow permeability and low mechanical stress over its service life - Google Patents

Abstract

Permeation barrier films and methods of manufacturing the same are provided, in which a barrier film layer having multiple sub-layers is deposited in a single deposition process. The barrier film layer may include multiple sub-layers that are fabricated by altering a process parameter of a single deposition technique.

Description

FLEXIBLE PERMATION BARRIER SYSTEM DEPOSITED IN A SINGLE PROCESS AND HAVING ULTRALOW PERMEABILITY AND LOW MECHANICAL STRESS OVER ITS SERVICE LIFE

CROSS-REFERENCE TO RELATED APPLICATIONS

[1] This application claims the benefit of U.S. Patent Application Serial No. 61/895,006, filed October 24, 2013, the entire contents of which is incorporated herein by reference.

PARTIES TO A JOINT RESEARCH AGREEMENT

[2] The claimed invention was made by, on behalf of, and/or in connection with one or more of the fol lowing parties to a joint university corporation research agreement: Regents of the University of Michigan, Princeton University, University of Southern California, and the Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

[3] The present invention relates to fabrication of permeation barrier layers and devices such as organic light emitting diodes and other devices, including the same.

BACKGROUND

[4] Opto-electronic devices that make use of organic materials are becoming increasingly desirabl e for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.

[5] OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several QLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.

[6] One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as "saturated" colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.

[7] One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppyj3, which has the following structure:

[8] in this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.

[9] As used herein, the term "organic" includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. "Small molecule" refers to any organic material that is not a polymer, and "small molecules" may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the "small molecule" class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter, A dendrimer may be a "small molecule," and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.

[10] As used herein, "top" means furthest away from the substrate, while "bottom" means closest to the substrate. Where a first layer is described as "disposed over" a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is "in contact with" the second layer. For example, a cathode may be described as "disposed over" an anode, even though there are various organic layers in between.

[11] As used herein, "solution processible" means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form,

[12] A ligand may be referred to as "photoactive" when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as "ancillary" when it is believed that the ligand does not contribute to the photoactive properties of an emissive material , although an ancillary ligand may alter the properties of a photoacti ve ligand,

[13] As used herein, and as would be generally understood by one skilled in the art, a first "Highest Occupied Molecular Orbital" (HOMO) or "Lowest Unoccupied Molecular Orbital" (LUMO) energy level is "greater than" or "higher than" a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level . Since ionization potential s ( IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A "higher" HOMO or LUMO energy level appears closer to the top of such a diagram than a "lower" HOMO or LUMO energy level,

[14] As used herein, and as would be generally understood by one skilled in the art, a first work function is "greater than" or "higher than" a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a "higher" work flmctioii is more negative. On a conventional energy level diagram, with the vacuum level at the top, a "higher" work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels fol low a different convention than work functions.

[15] More details on OLEDs, and the definitions described above, can be found in US Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.

SUMMARY OF THE INVENTION

[16] According to an embodiment, a method of fabricating a permeation barrier is provided, which includes providing a first silicon-containing precursor adjacent to a substrate; applying a single plasma-enhanced deposi tion process to form the first precursor into a sub- layer of a barrier film; and altering a process parameter controlling a plasma used during the plasma-enhanced deposition process at least twice during the plasma-enhanced deposition process. The barrier film may include at least three sub-layers, each sub-layer corresponding to a value of the process parameter. The process parameter may be a power supplied to the single plasma-enhanced deposition process, and altering the parameter may include decreasing the power from a first level to a second level, lower than the first level, during the single plasma-enhanced deposition process. The power also may be increased from the second level to a third level during the plasma-enhanced deposition process, which may be higher than the first level. In an

embodiment, a desired physical property of each of the plurality of sub-layers may be selected, and levels among which to alter the power may be selected based upon the desired physical property of each of the plurality of sub-layers. The physical property may be, for example, a diffusion coefficient, a mechanical stress, or the like. The permeation barrier may have a constant composition, with each of the sub-layers having a different morphology, or a morphology different from that of an adjacent sub-layer.

[17] In an embodiment, a barrier film is provided that is fabricated by providing a first silicon-containing precursor adjacent to a substrate; applying a single plasma-enhanced deposition process to form the first precursor into a sub-layer of a barrier film; and altering a process parameter controlling a plasma used during the plasma-enhanced deposition process at least twice during the plasma-enhanced deposition process. The barrier film may include at least three sub-layers, each sub-layer corresponding to a value of the process parameter. Each sublayer may have a different morphology than at least one other sub-layer. The barrier film may- have a uniform composition among the sub-layers.

[18] In an embodiment, a device is provided that includes a barrier film as disclosed herein. The device may be, or may include an OLED, which may be partially covered by the barrier film.

[19] in an embodiment, a device is provided that includes an OLED and a permeation barrier film at least partially disposed over the OLED, which includes a plurality of sub-layers, each of which has the same composition but different morphology.

[20] In an embodiment, a method of fabricating a permeation barrier layer is provided, which includes applying a single plasma-enhanced deposition process to form a first precursor into a first sub-layer of a permeation barrier layer; altering a first process parameter of the single plasma-enhanced deposition process to form the first precursor into a second sub-layer of the permeation barrier layer over the first sub-layer; and altering a second process parameter of the single plasma-enhanced deposition process to form the first precursor into a third sub-layer of the permeation barrier layer over the second sub-layer. The process parameters may be a power density of a plasma used during the single plasma-enhanced deposition process, a ratio of precursors within the first precursor, a separation distance of electrodes used during the single plasma-enhanced deposition process, and a composition of an inert gas used during the single plasma-enhanced deposition process. The specific values for the process parameter used during the deposition process may be selected based upon desired physical properties of the sub-layers.

BRIEF DESCRIPTION OF THE DRAWINGS [21] FIG. 1 shows an organic light emitting device. [22] FIG. 2 shows an inverted organic light emitting device that does not have a separate electron transport layer.

[23] FIG. 3 shows a schematic of an example bilayer barrier stmcture protecting an OLED according to an embodiment.

[24] FIG. 4A shows a hybrid barrier film that conformably coats a rough substrate according to an embodiment.

[25] FIG. 4B shows a hybrid barrier film that conformably coats an OLED fabricated on a substrate according to an embodiment.

[26] FIG. 4C shows a hybrid barrier film conformably coating an OLED with a contaminant particle according to an embodiment.

[27] FIG . 4D shows a hybrid barrier film coating an OLED, and distinguishes the bezel around the OLED according to an embodiment.

[28] FIG. 5 is a plot of the diffusion coefficient of water at 100 C into a hybrid barrier film, as a function of the radio frequency plasma power used in layer deposition according to an embodiment.

[29] FIG. 6 is a schematic partial cross section of an OLED display according to an embodiment.

[30] FIG . 7 shows a schematic profile of water concentration in the barrier film of FIG. 5 (shown in the inset), after exposure to water or water vapor according to an embodiment.

[31] FIG. 8 shows the built-in mechanical stress of single-layer hybrid barrier films in function of the radio frequency power used for the deposition of the film according to an embodiment.

[32] FIG. 9 shows schematically the evolution of the surface stress as a function of film deposition time in a three layer film of the type shown in FIG. 6 according to an embodiment. [33] FIG. 10 shows the change in mechanical stress in single-layer hybrid barrier films when exposed to boiling water, as a function of the radio frequency power used in film deposition according to an embodiment

[34] FIG. 1 1 shows schematically the change of surface stress in the film deposited with a compressive stress while it is exposed to humidity during use of the device that the film protects according to an embodiment.

[35] FIG. 12A is a scanning electron micrograph showing a 3.4μχη glass fiber being encapsulated by a barrier film deposited at three plasma power levels according to an

embodiment.

[36] FIG. 12B is a schematic of features of the scanning electron micrograph in FIG. 12A according to an embodiment.

[37] FIG. 13 shows the calculated number of monolayers of water molecules that permeate single layer barriers of six different thicknesses deposited at 70W, in function of exposure time to 85% relative humidity in air at 85 C according to an embodiment.

[38] FIG. 14 shows the measured (circles) concentrations of water molecules reached at water vapor pressure of 1 atm, in single layer barrier films deposited at 70W F power, in function of reciprocal temperature according to an embodiment.

[39] FIG. 15 shows the diffusion coefficient of water in a barrier layer, in function of reciprocal temperature according to an embodiment.

[40] FIG. 16 shows the calculated number of monolayers of water molecules that permeate 70 W barriers of six different thicknesses in function of exposure time to 100% relative humidity in air at 30 C according to an embodiment.

[41] FIG. 17 shows the concentration profiles of Dueterium (D) and hydrogen (H), determined by secondary ion mass spectroscopy, in a 70W barrier film after exposure to 101 C boiling deuterium oxide (heavy water) for 12 hours according to an embodiment.

[42] FIG. 18 is a schematic cross section of the capacitor fabricated to measure water diffusion into barrier films by capacitance according to an embodiment. [43] FIG. 19 shows the square of the change in inverse capaci tance as a function of time in capacitor structures with a 70 W barrier, as shown in FIG. 18.

[44] FIG. 20 shows the square of the change in stress of a 70W barrier deposited on a silicon wafer as a function of time, when exposed to 100 C boiling water according to an embodiment.

[45] FIG. 21 illustrates the permeation time to a protected OLED along a bezel in relation to the permeation time from the top according to an embodiment.

DETAILED DESCRIPTION

[46] Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an "exciton," which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.

[47] The initial OLEDs used emissive molecules that emitted light from their singlet states ("fluorescence") as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds.

[48] More recently, OLEDs having emissive materials that emit light from triplet states ("phosphorescence") have been demonstrated. Baldo et aL "Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices," Nature, vol. 395, 151-154, 1998; ("Baido- I") and Baldo et al., "Very high-efficiency green organic light-emitting devices based on electrophosphorescen.ee," Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) ("Baldo-II"), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in US Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference. [49] FIG. 1 shows an organic light emitting device 100. The figures are not necessari y drawn to scale. Device 100 may include a substrate 110, an anode 115, a hole injection layer 120, a hole transport layer 125, an electron blocking layer 130, an emissive layer 135, a hole blocking layer 140, an electron transport layer 145, an electron injection layer 150, a protective layer 155, a cathode 160, and a barrier layer 170. Cathode 160 is a compound cathode having a first conductive layer 162 and a second conductive layer 164. Device 100 may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in US 7,279,704 at cols. 6-10, which are incorporated by reference.

[50] More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m- MTDATA doped with F₄-TCNQ at a molar ratio of 50: 1 , as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al, which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1 : 1 , as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/017 116, which is incorporated by reference in its entirety.

[51] FIG. 2 shows an inverted OLED 200. The device includes a substrate 210, a cathode 215, an emissive layer 220, a hole transport layer 225, and an anode 230. Device 200 may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device 200 has cathode 215 disposed under anode 230, device 200 may be referred to as an "inverted" OLED. Materials similar to those described with respect to device 100 may be used in the corresponding layers of device 200. FIG. 2 provides one example of how some layers may be omitted from the structure of device 100.

[52] The simple layered structure illustrated in FIGS. 1 and 2 is provided by way of non- limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device 200, hole transport layer 225 transports holes and injects holes into emissive layer 220, and may be described as a hole transport layer or a hole injection layer, in one embodiment, an OLED may be described as having an "organic layer" disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to FIGS. 1 and 2.

[53] Structures and materials not specifically described may also be used, such as O LEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et a!., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in FIGS. 1 and 2. For example, the substrate may include an angled reflective surface to improve out- coupling, such as a mesa structure as described in U.S. Pat. No. 6,091 ,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties.

[54] Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. Pat. No. 7,431,968, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat, Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alky! and ar l groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing, Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.

[55] Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The bamer layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023G98 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a "mixture", the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non- polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non- polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon.

[56] Devices fabricated in accordance with embodiments of the invention can be incorporated into a wide variety of electronic component modules (or units) that can be incorporated into a variety of electronic products or intermediate components. Examples of such electronic products or intermediate components include display screens, lighting devices such as discrete light source devices or lighting panels, etc. that can be utilized by the end-user product manufacturers. Such electronic component modules can optionally include the driving electronics and/or power source(s). Devices fabricated in accordance with embodiments of the invention can be incorporated into a wi de v ariety of consumer products that have one or more of the electronic component modules (or units) incorporated therein. Such consumer products would include any kind of products that include one or more light source(s) and/or one or more of some type of visual displays. Some examples of such consumer products include flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads-up displays, fully or partially transparent displays, flexible displays, laser printers, telephones, cell phones, tablets, phablets, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, 3-D displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18 C to 30 C, and more preferably at room temperature (20-25 C), but could be used outside this temperature range, for example, from -40 C to + 80 C,

[57] Organic light emitting diodes are sensitive to small quantities of moisture and oxygen, because chemical reactions with ¾0 and O2 may damage the device. To prevent such failures, OLEDs may be sandwiched between two glass plates. At their perimeter the plates are sealed to each other with polymer adhesive. The water and oxygen that permeate through the seal are captured by a desiccant that is packaged with the OLEDs. While many organic polymers are optically clear, they cannot serve as barrier materials because of their high permeability to water and oxygen. While metals are highly impermeable, they do not transmit light. To make the display flexible, glass-like permeation barriers typically must be made very thin, with thicknesses of the order of micrometers. Such thicknesses can be achieved with thin-film deposition techniques. However, glass-like thin-films of conventional optically clear barrier materials (silicate glass, silicon nitride, alumina) are permeable. The permeability is caused by microcracks that form in the films during their deposition, inadequate coverage of surface profiles in the substrate, and inadequate encapsul ation of particles that lie on the substrate.

[58] To overcome this disadvantage, multilayer barrier films have been proposed, in which layers of polymers alternate with inorganic, glass-like, layers. While not free of defects, such multilayer barrier films protect the underlying device by mechanically de-coupling the stiff inorganic layers f om each other and by forcing long permeation paths on water and oxygen, so that these molecules may take relatively long times to reach the OLED. In addition, polymer layers can be deposited under conditions that let the precursor molecule diffuse over the growth surface before it is anchored on it. This diffusion lets precursor molecules reach portions of the surface that are hidden below particles, thereby coating such hidden surfaces. In addition, particles often accumulate on OLED substrates during various processing steps. These particles may be several microns thick and can inhibit encapsulation of the OLED. To become impermeable the barrier must completely bury the particle. The polymer layers are used to conformably coat rough surfaces, surface profiles, and to encapsulate particles that contaminate the OLED surface. [59] Further, deposited barriers typically have built-in mechanical stress. Whether the stress is tensile or compressive and the magnitude of the stress depends on the deposition conditions. Because polymers are soft materials, they can be deposited without building up excessive mechanical stress that might cause the multilayer film to delaminate. Small stress becomes particularly important when the barrier does not adhere strongly to the substrate, because the barrier can delaminate when stress overcomes the strength of adhesion.

[60] While such polymeric/inorganic multilayer barriers perform well, their cost is high because of the high equipment and process cost that arises from combining two very different kinds or material. The present invention combines ail properties required of a flexible permeation barrier in a single film.

[61 ] To make OLED displays and similar electronic devices flexible and lightweight, thin fl exible barrier films must be used instead of rigid glass plates. At least one surface of the display must be protected with a barrier film that is transparent, to transmit the light generated by the OLEDs. When coated over OLEDs, such a film typically must be deposited at or near room temperature, because high temperatures will damage the OLEDs. Many inorganic materials such as SiNx, Si0₂ and AI2O3 have low permeability for atmospheric gases. The production of transparent encapsulant barrier films has been problematic in the past, as such films often become permeable when deposited as thin films at near room temperature. For example, they may contain microscopic defects that form pathways for permeation of atmospheric gases including water vapor.

[62] Patent numbers US 6,548, 912 Bl , US 6,268,695, US 6,413,645 Bl , US 6,522,067 describe "multiple" barrier stacks dyads to encapsulate moisture sensitive devices (such as OLEDs) and substrates. Each barrier stack or a "dyad" consists of an inorganic material/ polymer layer pair. The inorganic layer is the barrier layer; typically it is a metal oxide - such as polycrystailine AI2O3, which has low permeability for atmospheric gases. The polycrystalline AI2O3 is usually deposited by reactive sputtering at room temperature. These films contain microscopic defects such as pinholes, cracks, and grain boundaries that eventually form pathways for permeation of atmospheric gases including water vapor. The polymer layer is usually a polyacrylate material, which is deposited by flash evaporation of a liquid acrylate monomer that is subsequently cured by UV radiation or an electron beam. This polymer layer mechanically decouples the "defects" in the inorganic layers as disclosed in Patent number US 6,570,325. By using multiple dyads (around 3 to 5 dyads which is 6 to 10 layers), these barrier films protect the underlying device by mechanically de-coupling the rigid inorganic layers from each other and by forcing long permeation paths on water and oxygen, so that these molecules take long times to reach the OLED. Although this method provides a long lag time for top-down diffusion of water vapor through the dyads, it fails to address the lateral/edge diffusion of water vapor. Since the polymer/decoupling layer has a high diffusion co-efficient for water vapor, a very wide edge seal is required for protection.

[63] One way to reduce the edge seal width is disclosed in Patent number US 7,198,832. In this method; in a given barrier stack, the area of the inorganic layer, i.e., the barrier layer is made larger than the area of the decoupling, i.e., the polymer layer. Subsequently, the area of the second barrier stack need s to be larger than the area of the first barrier stack and so on. By adopting this structure, the barrier layer can provide protection against lateral/edge diffusion of water vapor and oxygen. However, this structure fundamentally poses a limit on the minimum edge width obtainable. The 'edge width' or 'bezel width' is a non-usable portion of the display, it is almost impossible to obtain an almost zero-edge or an edgeless display. Moreover, the deposition rate of the barrier layer, i.e., the sputtered metal oxide layer increases the TACT. Additionally, the substrate needs to be translated from the sputter chamber (vacuum) to an inert atmosphere chamber (non-vacuum) to flash evaporate the monomer layer and cure it and vice versa. Multiple transfers between chambers, intervening masking steps for barrier overlap, etc., considerably increase the TACT for such techniques.

[64] Patent number US 7,015,640 B2 discloses the use of graded composition diffusion barrier to encapsulate OLEDs and substrates. In this method, multiple alternate layers of SiOxNy/SiOxCy are deposited at room temperature by PEC YD. The composition of the layers is dependent on the reactant gases and process parameters. The SiO_xC_y tends to give the polymer- like effect, while the SiO_xN_y is the inorganic barrier layer. Other layers such as SiC, 8iN_x, SiO_x, AlOxCyNz can be deposited by the same method. Obtaining ultralow permeability in this barrier system depends on the inorganic layer, i.e., SiO_xN_y. Inorganic thin films (barrier layers) such as SiO_x, SiNx or SiO_xN_y, when deposited at room temperature, develop self-relief micro-cracks once they reach a critical thickness. The flexibility of the graded compositional diffusion barrier depends on its adhesion and the built-in stress of the inorganic layer.

[65] Other techniques have been reported, such as in Chen et al, which disclosed multilayer barrier deposition of SiOx/SiNx stacks, in which six stacks were needed to meet OLED requirements. Graham et al. disclosed the use of SiOx/SiNx/Parlyene (3 pairs) + SiOx/SiNx (3 pairs) in "Approaches to Barrier Coatings for the Prevention of Water Vapor Ingress" (available at http://wwwl .eere.energy.gov/soiar/pdfs/pvrw2010__graham.pdf) Obtaining ultralow permeability in this barrier system depends on the inorganic layer, i.e., SiO_x and SiN¾ . Inorganic thin films (barrier layers) such as SiO_x, SiN_x or SiO_xN_y when deposited at room temperature develop self-relief micro-cracks once they reach a critical thickness. The flexibility of this barrier depends on the adhesion and the built-in stress of the inorganic layer,

[66] Single layer encapsulation by Atomic Layer Deposition (ALD) was demonstrated in P. F. Carcia, R. S. McLean, M. H. Reilly, M, D. Groner, S. M. George, Applied Physics Letters, 2006, 89, 031915, which disclosed encapsulating a polymer substrate (PET) by ALD of AI2O3. A high density, pin hole free, conformal barrier coating was obtained with decent WVTR of 1.7 X10^"3 g /day. The coating is dominated by nano-defects rather than micro-defects. The disadvantage of ALD process is that the process has a large TACT time and thus depositing a thick layer for particle coverage will be a challenge.

[67] In general, single- or multi-layer inorganic thin films (barrier layers) such as SiNx or SiOx or SiOxNy, when deposited at room temperature develop self-relief micro-cracks once they reach a critical thickness. The flexibility of the graded compositional diffusion barrier will depend on the adhesion and built-in stress of the inorganic layer.

[68] A hybrid barrier layer grown by plasma enhanced chemical vapor deposition (PECVD) of an organic precursor with a reactive gas such as oxygen, e.g., HMDSO/O2 is described in Patent number US 7,968,146, the disclosure of which is incorporated by reference in its entirety. The barrier film is highly impermeable yet flexible. This material is a hybrid of inorganic S1O2 and polymeric silicone that is deposited at room temperature. The barrier film has permeation and optical properties of glass, but with a partial polymer character that gives a thin barrier film a low permeability and flexibility. At room temperature, a layer of this hybrid material is free of microcracks when deposited approximately thicker than 100 nm. The advantages of this harrier include: ultralow permeability to moisture and oxygen, particle coverage (confomial coating by PECVD), good edge seal with minimal edge/bezel requirement, transparency and flexibility. The deposition process is a cost-effective one step process with somewhat average TAC time.

[69] Flexible multilayer barrier films have been previously demonstrated as encapsulates. In these films layers of polymeric and inorganic material alternate in a multilayer structure. Because the equipment and processes for the deposition of polymeric and inorganic layers are completely different, such multilayer films are expensive. Barriers made of a single material in one process and single apparatus may be desirable owing to their lower cost of fabrication than that of multilayer barrier films. One such material is a hybrid of inorganic Si0₂ and polymeric silicone that is deposited by plasma-enhanced chemical vapor deposition (PECVD) on room temperature substrates from the source gases hexamethyl disiloxane and oxygen. The material is a hybrid with permeation and optical properties of glass, but wi th a partial polymer character that gives a thin barrier film a iow^r permeability and flexibility. At room temperature, a layer of this hybrid material is free of microcracks when deposited approximately thicker than 1 OOnm, unlike purely inorganic materials deposited at room temperature. Depositing a single type of material reduces the cost of making the barrier.

[70] Embodiments of the present invention provide a barrier film layer made of three or more sub-layers. The sub-layers may be made of the same material, with each sub-layer being deposited in different regime of a single deposition process. For example, three sub-layers may be deposited during a single plasma-enhanced deposition process using a high-low-high sequence of plasma power. The initial high power layer may provide the necessary adhesion between the OLED and the barrier and compressive mechanical stress. Deposition at low plasma power may provide a layer that conformably coats roughness, profiles, and particles on the substrate, and builds tensile mechanical stress into the layer. The subsequent change to high plasma power deposits a layer that has ultra-low permeability for water and compressive mechanical stress. The compressive-tensile-compressive stress structure keeps the stress within bounds through the deposition such that the film does not fracture during deposition.

[71] A barrier film as disclosed herein may combine ultra-low permeability for water with a relatively low mechanical stress during deposition of the barrier film, as well as a low mechanical stress during the operation of the device that the film protects, in addition, the barrier film may conformaily coat and thereby seal underlying surface profiles and particles that may contaminate the underlying surface. Thus, a barrier film as disclosed herein may coat particles and surface relief conformably, have an ultralow permeability for water, remain under a low mechanical stress over its lifetime, such that the bamer film does not peel off its substrate, and may be flexible and suitable for use with flexible polymer substrates.

[72] In an embodiment, a precursor may be formed into a permeation barrier layer having multiple sub-layers, each of which is formed while a single deposition process is performed using different process parameters. For example, a plasma-enhanced deposition process may be used to deposit a layer. One or more process parameters may be altered during the deposition process, with each value of the parameter resulting in deposition of a sub-layer.

[73] In an embodiment, a deposition process such as a plasma-enhanced deposition process may be used to deposit a permeation barrier layer. The process parameter altered during deposition may be a power, such as RF, A/C, or DC power that is supplied to generate the plasma. The power level may be, for example, increased from a first level to a second level, and then decreased to the first level or a third level from the second level, which each level corresponding to a sub-layer of the deposited permeation barrier layer. As described in further detail herein, the specific levels of the process parameter at which to operate the deposition process may be selected based upon desired physical properties of the sub-layers of the permeation barrier layer. Example physical properties include a diffusion coefficient, a mechanical stress, and the like. Example process parameters that may be altered to achieve the multiple sub-layers include the power density of the plasma, a ratio of precursors within the precursor used to deposit the permeation barrier layer, when the precursor includes multiple components, the separation distance of electrodes in a plasma-enhanced deposition process, and the composition of an inert gas used in the deposition process.

[74] FIG. 3 provides a schematic representation of a three layer structure as disclosed herein. In the example device shown, three sub-layers 310, 320, 330 of a permeation barrier may be deposited over an OLED 305 or other device disposed over a substrate 300. Each sub-layer 310, 320, 330 may be deposited using a single deposition process, such as a plasma-enhanced deposition process, with a process parameter of the deposition process being adjusted between deposition of adjacent sub-layers.

[75] As a specific example, the structure shown in FIG. 3 may be deposited using a plasma- enhanced deposition process such as plasma enhanced chemical vapor deposition (PECVD). The driving electrical power, such as RF and/or AC power, may be adjusted between deposition of each sub-layer, ai though all sub-layers are deposited during the single PECVD process. For example, the bottom-most sub-layer 310 may be deposited at a higher deposition power, the middle sub-layer 320 at a lower deposition power, and the topmost layer 330 at a higher deposition power.

[76] in general, the three sub-layers may have a common composition but different atomic morphology, and may be considered a single "layer" as disclosed herein. For example, all three sub-layers may be fabricated from a single precursor or multiple precursors that are used throughout the PECVD process. In this case, the only differences between the three layers 310, 320, 330 may be due to the difference in process parameters used during different times of the single PECVD process.

[77] Embodiments disclosed herein may apply to a range of compositions and properties of a barrier film as disclosed herein. For example, a permeation barrier film that is a hybrid between silicon dioxide and silicon polymer may be made by radio-frequency plasma enhanced chemical vapor deposition from source gases hexamethyl disiloxane and oxygen. The driving power (RF power) used to perform the PECVD technique may be varied during performance of the technique, to fabricate a film that meets the above-described functional requirements during the barrier's operational lifetime.

[78] In some embodiments, the different sub-layers disclosed herein may provide different attributes or perform different functions within the permeation barrier layer. This may be the result of different process parameters applied during a plasma enhanced deposition technique, which result in each sub-layer formed while a particular process parameter value is used having different physical properties.

[79] For example, the radio frequency deposition power applied during a plasma enhanced deposition process may determine the conformal nature of a deposited layer. Typically a lower deposition power results in a longer surface diffusion length of the growth species. As a result, a low deposition power yields a conformal layer. Hence a middle lower power may be used to conformally coat a device even when the device surface is rough or when particles are present on the surface. The bottom layer may provide adhesion and maintain the stress within desired bounds. The top layer may provide protection from water permeation.

[80] FIG. 4A schematically shows an example of a low RF power hybrid sub-layer that coats a substrate with a rough surface. FIG. 4B schematically shows how such a sub-layer may coat the edge steps associated with a patterned OLED. FIG. 4C shows schematically a hybrid sub-layer enveloping a dust particle that settled on the OLED prior to or during deposition. FIG. 4D shows the same configuration as in FIG. 4C, for a device having a bezel from the permeation barrier layer,

[81 ] Another physical feature of a permeation barrier sub-layer as described herein that may be controlled by controlling a process parameter of the deposition technique is the water permeation. The flux of water molecules through a hybrid barrier layer and into an OLED is proportional to the diffusion coefficient D of water in the layer. FIG. 5 5 shows how D depends on the RF deposition power with which the layer was made. The D data of FIG. 5 applies to one specific composition of the gaseous precursors used to deposit the barrier. They were measured on samples that had been held in boiling water at 100 C.

[82] These, and all other data unless indicated otherwise, were measured were obtained under accelerated test conditions, at higher temperatures and water vapor pressures than would exist in practical OLED use. The values of the diffusion coefficient of water D under typical operating conditions, near room temperature, were obtained by extrapolation as explained elsewhere herein. While layers deposited at low RF power typically have very low diffusion coefficients, even lower values may be obtained when the layer is deposited at high RE power, as shown by FIG. 5. Therefore, to reduce the flux of water molecules to the OLED to its lowest achievable value, in the three layer stnicture, the top layer may be deposited at high RF power on the conformable layer. This third layer is shown schematically as the top layer in FIGS. 3 and 4A-4D. [83] FIG. 6 shows a schematic cross section of a hybrid barrier film on an QLED fabricated on a substrate. It shows typical plasma power settings and layer thicknesses of a barrier film that is deposited to combine conformability with lowest possible permeability.

[84] FIG., 7 schematically illustrates the permeation of water into the barrier given in FIG. 6, with the concentration profile of water molecules after a typical accelerated testing. The concentration of water in the barrier is plotted against depth from the surface of the barrier at x=0. The QLED lies at x = 4 micrometers. The steep profile between the surface and 1 micrometer depth reflects the very low D in the top layer deposited at high RF power. The extreme shallow profile between 1 μπι and 3 iim reflects the higher D of the layer deposited at low RF power. The high power layer between 3 and 4 micrometer has lower diffusion coefficient than the top layer resulting in a shallow profile as well. Note that all permeabi lities are very low, as they correspond to diffusion coefficients of water, at 100°C, in the range of 10^{" i6} cm²/sec to 10^"u enV'/sec as shown in FIG. 5. A more detailed evaluation, as described in detail herein, shows that D = 10^{~i 6} cnrVsec at 100 C translates to an OLED lifetime of 20 years when the QLED is protected by a single 1 -micrometer thick barrier.

[85] Another physical property of a sub-layer, and thus the permeation barrier layer, that may be controlled by controlling as deposition process parameter is the mechanical force exerted by the barrier film on the substrate. The mechanical stress exerted generally is the product of the mechanical stress in the barrier film and the barrier thickness; such stress typically is called surface stress. For example, changing the RF power from high to low to high as a permeation barrier is deposited via a plasma enhanced deposition process keeps the mechanical stress in the permeation barrier film at the low values that are required to prevent layer delamination.

[86] FIG. 8 shows an example of measured mechanical stress in measured in hybrid sublayers over the same range of RF power as in FIG. 5. As shown, layers deposited at low RF power are under tensile (positive by definition) built-in stress, while layers deposited at high power are under compressive (negative) stress. Therefore, depositing the permeation barrier in the high-power to low-power to high-power sequence as described above results in stress compensation, as shown in. FIG. 9. [87] The middle layer deposited at low RF power may be used for conformal coating over particles with tensile surface stress. The middle layer may deiaminate during the deposition due to poor adhesion and excessive tensile stress if deposited directly on the OLED substrate. To improve adhesion and prevent the build-up of tensile stress, the deposition may be started with a bottom high-power barrier sub-layer that has compressive stress. This reduces the tensile stress which otherwise may be relatively very high and cause de lamination during deposition.

[88] FIGS. 9 are schematic plots of the surface stress in the film as a function of deposition time. The bottom layer and the top sub-layer are compressive. The tensile stress that builds up in the low-power layer is reduced by the compressive stress of the high-power bottom and top layers. The film of FIG. 9 is deposited in the same sequence as suggested by FIGS. 3, 4A-4D and 6. The two dashed horizontal lines show the critical surface stresses, tensile and

compressive, at which the hybrid barrier film would deiaminate. The curve with two slopes shows the development of film stress during the film's deposition. The initial deposition at high RF^' power builds a sub-layer with compressive stress (see FIG. 8). The following deposition at low RF power builds a tensile stress. The subsequent top sub-layer deposition at high power, by- adding a sub-layer with compressive stress, reduces the total built-in stress of the film. During the entire period of deposition the total stress in the film may be held within a band bounded by the critical values at which the film would deiaminate.

[89] A similar consideration applies to the surface stress in film stress during the operating life of the OLED that the film protects. During operation the hybrid barrier film is exposed to water vapor, and water molecules diffuse into the hybrid layer. This uptake of water swells the hybrid layer. The ensuing expansion of the hybrid layer is constrained by the substrate to which the layer is attached. This constraint causes the stress in the hybrid layer to become more compressive during the operating life of the OLED device. This compressive stress adds to the surface stress of the as-deposited film. The ensuing total surface stress also must stay within critical bounds.

[90] FIG. 10 shows by how much the mechanical stress in the hybrid barrier film changes when the sub-films are exposed to boiling water until the stress saturates, as a function of the deposition power used in making the film. The measurements were conducted under accelerated test conditions by boiling in water at 100 C. To arrive at the total stress in a film deposited at a given RF power, the data point for that power in FIG. 10 is to be added to the data point at the same RF power in FIG, 8. The schematic result is illustrated in FIG, 11, which plots the surface stress exerted by the hybrid barrier on the substrate during the operating life of the OLED that the film protects. Again schematically, the time scale (x-axis) of FIG. 11 is an extension of the time scale of FIG, 9. FIG, 11 illustrates that the initially tensile stress in the as-deposited, hybrid barrier is reduced as the barrier is exposed to humidity. FIG. 1 1 also illustrates the design goal of keeping the barrier within the critical stress limit during its operating life. The data of FIGS, 8 and 10 show that this can be achieved.

[91] One example of a permeation barrier film as disclosed herein may be made by plasma- enhanced chemical vapor deposition (PECVD) from hexamethyl disiloxane and oxygen, during which the radio-frequency power that is fed into the plasma is varied at least twice during the deposition. As described above, the variation of theRG power(I) the barrier conformably coats substrate roughness, profiles, and particulate contaminants on the substrate; (2) the barrier has ultralow permeability for water, which ensures a long operating life of the OLED device that the barrier protects; (3) the mechanical stress built into the barrier by deposition does not exceed critical values that would cause delamination; (4)the barrier coating is flexible. These features, of barrier d eposition and of barrier properties, combine to a permeation barrier with unique properties.

[92] Example apparatus and procedures for depositing barrier films of a hybrid material as disclosed herein are described in U.S. Patent No, 7,968,146, which is incorporated by reference in its entirety. A large number of si licon-bearing source gases may be used as well as a variety of oxidizing gases. In the following example, hexamethyl disiloxane was the source gas for silicon, and pure oxygen gas was used as the oxidizer. Other PECVD process parameters that may be varied to obtain different properties of sub-layers as disclosed herein include the pressure, the temperature at which the substrates is held on which the film is deposited, and the electric power that is fed into the reactor to maintain the plasma. W hile the electric pow^rer may be fed in with a direct current or an alternating current in a range of frequencies, for the present examples the radio frequency of 13.56 MHz was used. Unless otherwise indicates, all variables except for the electric power and pressure were held constant. Films were deposited in part at low and in part at high RF power. [93] The substrate electrode area is 182.3 cm^z. The barrier is deposited from

environmentally friendly source gases HMDSO and Oxygen. All gases except for HMD SO are taken from compressed -gas cylinders. HMDSO vapor is produced from liquid HMDSO that is held at a controlled source temperature-- The deposition power density is calculated by dividing the deposition power by the electrode area.

[94] The techniques employed are il lustrated on a standard barrier that is deposited at room temperature under the typical deposition conditions as shown in Table 1. This layer is not the optimal barrier.

Table 1 : Single layer permeation barrier deposition condit

[95] Particles may become embedded on a substrate surface and impede the smooth and defect free growth of a barrier layer. Changing a deposition condition, such as the power applied to a plasma enhanced deposition technique, may alter the properties of the deposited hybrid barrier. At low deposition powers, the barrier grows conformal, smoothens rough surfaces and provides rapid encapsulation of particles. FIG. 12A shows a scanning electron micrograph of a piece of glass fiber that was placed on a silicon substrate to function as a test particle. FIG. 12B is a schematic explaining the scanning electron micrograph. This particle was then encapsulated with the barrier film. Prior to putting a piece of glass fiber on the substrate, a 60nm thick thermal silicon dioxide was grown on the silicon to cause the substrate surface to resemble glass. The test glass fiber of diameter 3.4jim was encapsulated with a barrier containing three layers. The bottom layer was deposited at 70W (high power), middle layer at 30W (low power) and the top layer at TOW (high power). As shown, the bottom high-pow^rer sub-layer does not encapsulate the fi ber; this is evident by the thin gap between the deposition on top of the glass fiber and the deposition on the substrate for the thickness of the bottom layer. Once the deposition of the 30W (low power) middle sub-layer is started, the film coats the fiber and the gap vanishes. The top sub-layer deposited at a high power of 70 W (high power) then keeps coating the fiber as it continues to grow on the low power layer beneath it. The top sub-layer in the bilayer film grows over the already conformal bottom sub-layer. As a result, the top sub-layer is without any defects over the entire device region. The top sub-layer prevents water and moisture diffusion into the device.

[96] Because OLEDs are highly sensitive to chemical attack by water, a realistic but very demanding requirement is that during the entire lifetime of the protected OLED may be that one monolayer of water molecules permeate through the barrier into the O LED. The lifetime TIML so defined can be predicted if the thickness of the barrier H, and the concentration of water molecules at the surface of the barrier C(x=0) =Co, and the coefficient of diffusion D of water molecules in the hybrid barrier material are known. These three parameters may be set by the deposition process parameters, such as PECVD deposition parameters as previously described. The measurement techniques to identify these three parameters at different temperature and humidity are described in further detail below. H, Co, D and exposure time t may be used to calculate TIML.

[97] The quantity of water molecules that has permeated after time t of exposure to humidity is given by

where Q(t) is the quantity of water that has permeated through a film of thickness H in time t, D is the diffusion coefficient, and Co is the surface concentration of water.

[98] FIG. 13 shows the results of a calculation of the number of monolayers of water permeated through a hybrid barrier sub-layer deposited at the RF power of 70W as a function of time t at standard accelerated test conditions of 85 C and 85% relative humidity. The horizontal line in FIG. 13 denotes the penetration of 1 monolayer of water molecules. The curves in FIG. 13 stand for six values of hybrid barrier thickness H. The intersections of the horizontal line with these curves are the values of TIML(85/85), i.e., the time taken for 1 monolayer to permeate through a barrier film at 85 C and 85% humidity. For example, a Ι ηι thick barrier provides TiML ~ 400 hours.

[99] Knowledge of the concentration of water Co and of its diffusion coefficient D as functions of temperature enables extrapolating from values of lifetime obtained under accelerated test conditions to lifetimes under conditions of practical use of the OLEDs. The concentration of dissol ved water molecules Co in function of reciprocal temperature 1000/T is shown in FIG. 14. The concentrations C(x=0) in FIG. 14 from 100 C upwards were measured at 1 atmosphere of water vapor pressure, and below 100 C at 100% relative humidity. Normalizing the values obtained below 100 C also to 1 atmosphere of water vapor pressure makes all points fail on a straight line. In FIG. 14 this line is extrapolated to room temperatures, where it provides the C(x^zzz:0) values needed for permeation calculations under practical operating conditions of the OLEDs.

[100] Similarly, the diffusion coefficient of water D into the hybrid barrier was determined in accelerated tests at elevated temperature. The data are plotted against reciprocal temperature 1000/T in FIG. 15. Their extrapolation to room temperature provides the D values needed for calculating the water permeation through the barrier.

[101] At a given temperature T the Co is scaled to the water vapor pressure at that temperature and then is used together with D at that temperature in a numerical calculation that results in the one-monolayer penetration time T/ML at the particular temperature and relative humidity. Such calcul ations can be carried out to obtain curves of conversion factors from tests conducted at high temperature to operation at room temperature. FIG, 16 shows the monolayers permeated at 30 C and 100% relative humidity. Table 1 below shows the time taken for 1 monolayer to diffuse at 3 different conditions for different barrier thicknesses.

[102] To achieve a target TIML, for a given operating temperature and humidity, the appropriate deposition power and top layer thickness may be chosen accordingly. Table 2: Calculated time taken for 1 monolayer to permeate through a barrier fi lm

Time TiML for permeation of 1 monolayer through a

Barrier thickness barrier film deposited at 70W

7==85°C, Ri! 85% T M)^"X Ri! 100% 7-25°C, R/f 50% lOOnm 18 hours 160 days 1.6 years

200nm 45 hours 346 days 4 years

500nm 6 days 2.6 years 9 years

750nm 1 1 days 4.3 years 13 years lOOOnm 17 days 6.1 years 19 years

1500nm 31 days 1 1 years 30 years

[103] As previously described, another property of a barrier layer that can be controlled by varying a process parameter of a deposition techni que is the water vapor permeation of the l ayer. Water typically permeates into a barrier-protected OLED on three pathways: (i) through the bulk of the barrier, (ii ) through defects in the barrier, and ( iii) along interfaces between the barrier and the substrate. In a permeation barrier layer as disclosed herein, these pathways may be blocked by the various sub-layers formed when a process parameter is altered . For example, in a PECVD deposition technique as disclosed, they may be blocked in reverse order, by (iii) starting barrier film deposition with a layer at high plasma power then (ii) depositing a layer at low power, and (i) depositing a layer at high power.

[104] in many instances during an experimental program, permeation via pathways (ii) and (iii) dominates, in which case permeation via pathway (i), through the bulk, cannot be measured. However, the rate of permeation through the bulk film may be the most important measure of quality of the device. The permeation of water into the bulk of the barrier can be measured by evaluating changes of its bulk properties. Three bulk properties were measured: chemical concentration, mechanical stress and electrical capacitance.

[105] The data shown in FIGS. 5, 8, 10, 14, and 15 were obtained by holding hybrid barrier films in boi ling water at 100 C, or by exposing them to I atmosphere of steam at temperatures from 100 C to 200 C, or by exposing them to air with 100% relative humidity at temperatures below 100 C down to 65 C. The barrier films were deposited on substrates appropriate for the relevant test, in most instances on silicon wafers. For evaluation by secondary ion mass spectrometry the barriers were exposed to heavy water (D₂0) instead of light water. [106] The resulting concentrat ons of water C(x=0) in the hybrid barrier films were determined by secondary ion mass spectrometry (SIMS) as the primary measurement, and by measurements of electrical capacitance and of film/substrate curvature as secondary

measurements. The coefficients of diffusion were determined by secondary ion mass spectrometry, capacitance measurements, and measurements of film/substrate curvature.

[107] From SIMS measurements of concentration profiles of deuterium and hydrogen in a barri er layer, the solubility' of water in the barri er layer, and the concentration profile of water in the film, may be obtained which in turn provides the diffusion coefficient. The electrical capacitance of the barrier film is highly sensitive to the concentration of water in the film. Therefore the capacitance can be evaluated for relative values of amounts of water that have diffused in the film, hence for diffusion coefficients. By calibration against SIMS data the capacitance data can be converted to concentration of water. The diffusion of water into the barrier films makes the fi lm swell. When the film is bonded to a strong substrate, the swelling builds up compressive stress in the film, which makes the film/substrate couple bend. From the radius of curvature relative amounts of water that have diffused into the film can be evaluated, which in turn permits extracting diffusion coefficients. After calibration against SIMS data the curvature can be converted to concentration of water. All three techniques are sensitive to permeation through the bulk of the film.

[108] The in-diffusion of water was detected and measured by secondary ion mass spectrometry combined with sputter-etch depth profiling. The depth profile is obtained by ion bombardment of the surface, which control !abiy sputters (removes) materi al from the film. The sputtered-away particles are analyzed by mass spectrometry, which is the chemical

measurement. Therefore the mass spectrum reflects the film composition as a function of depth from its original surface. The mass spectrum as a function of depth from the surface provides a concentration profile. The absolute value of concentration of an element is determined by calibrating the ion intensity of the element on a compositional standard with known

concentration, in the present case against hydrogen and deuterium ion implanted into thennallv oxidized silicon dioxide layers. The diffusion coefficient can be extracted from the

concentration profile. [109] An example hybrid barrier material as disclosed herein may be nearly pure silicon dioxide, Si0₂, with small concentrations of hydrogen and carbon. Therefore, tracer techniques may be used to distinguish between the oxygen and hydrogen in the as-deposited film from the small concentrations of H introduced when water permeates into the film. The tracer atom is H-2 (deuterium, D).

[1 10] A 660nm thick barrier film was deposited at 70W power on a silicon wafer. The silicon wafer covered with the barri er layer was held for 12 hours in a bath of heavy water (D2O) boiling at 101°C. The SIMS concentration profiles of deuterium and hydrogen as a function of depth are shown in FIG. 17. The sum of the concentrations of deuterium and water at the surface (x = 0) corresponds to twice the surface concentration of water molecules, C(x=0). At T = 101 C this value is 1.6 x I (Y molecules/cc. This value provides the reference point for water concentration measurements by electrical capacitance and radius of curvature change. The results of water concentration in the barrier as a function of reciprocal temperature plotted in FIG, 14.

[I l l] The profiles for D (in-diffusion) and H (out-diffusion) both follow a complementary error function distribution with a diffusion coefficient of 4.2 x 10^""15 cm²fs. This value is close to 5.6 x 10^~lb cm^z/s and 4.4 x lG^~lb cm²/s, measured by electrical capacitance and mechanical stress induced bending radius of curvature.

[112] Water in liquid form has a very high dielectric constant of 80.2 (at 20°C, IMHz), while the example barrier layer has a dielectric constant of 3.9. As a result, even small a quantity of dissolved water causes a measurable change in dielectric constant of the barrier. To measure this dielectric constant, a capacitor structure as shown in FIG. 18 is fabricated. A l OOnm chrome bottom contact is deposited on a glass substrate in an evaporator. A 200nm blanket barrier is deposited at a radio frequency deposition power of 70W over the chrome following which a thin 15nm chrome contacts are deposited with a shadow mask. The top contact has an area of 7.9 x 10^"3cm². Such samples were held in boiling water at 100 C for up to 11 hours,

[113] For small dissolved quantities of water, the change in capacitance can be related to the water diffusion coefficient using the equation;

C(t) C(0)/ VC(∞) C(0)/ /-/ ^ where C(t) is the capacitance at time t, C(0) is the initial capacitance at f^:::0, C(∞) is the capacitance when the capacitor is saturated with water, and D is the diffusion coefficient.

[1 14] In F IG. 19 the measured capaci tances are plotted in form of the square of the left side of the equation above, against the exposure time t to boiling water. From the slope of the regression line the diffusion coefficient of water at 100 C in the 70 W barrier was calculated to be 5.6 x 10^~15 cm²/s.

[115] To determine the radius of curvature induced by film stress, Barrier film layers ranging in thickness from lOOnm to 2μη are deposited on 4-inch diameter, <100> oriented, 525um thick single-side polished silicon wafers were fabricated. If, after deposition, the barrier is under stress, the barrier/wafer sample bends with a radius of curvature R. The stress is then calculated using the Stone)' equation:

where R is the bending radius, Ew is the wafer electrical constant, hs is the substrate thickness, and His the barrier film thickness.

[116] As described earlier, FIG. 5 shows the physi cal stress of barriers deposi ted at different deposition powers. Negative and positive stress numbers represent compressive and tensile stress, respectively. When the barrier film is exposed to water, FI₂Q diffuses into the film, causing it to expand. When the deposited on a rigid substrate such as a silicon wafer or a rigid glass substrate, the barrier film tends to expand resulting in an increase in compressive stress or a reduction in tensile stress. The change is stress is proportional to the concentration of dissolved water in the barrier:

[117] The water diffuses in from an infinite source. Therefore the concentration profile of t he water follows a complementary error function that is a solution of Fick's law of diffusion. The stress a(t) at time t is given by:

where Aa(i) is the change in stress of the barrier on the silicon wafer at time t, a_s is the stress in the barrier when saturated with water, D is the diffusion coefficient, and H is the barrier thickness.

[1 18] Therefore, the diffusion coefficient D can be extracted from measurements of stress in the barrier film. The saturated barrier stress a_s is measured by saturating a lOOnm thick barrier with water at the test condition temperature and humidity. The barrier under study was deposited at 70W on a 4 inch, <!()()> oriented, 525um thick silicon wafer. FIG. 20 shows the square of the change in stress (o(t))² for a 1.5μηι barrier film deposited at 70W on a 4 inch, <100> oriented, 525 urn thick silicon wafer, held in a boiling water bath at 100 C as a function of time. This corresponds to an operating temperature of 100 C and 100% relative humidity. From the slope in FIG. 17 the diffusion coefficient D is calculated. At 100 C, the diffusion coefficient of water in a barrier deposited at 70W is 4.4 x ΚΓ¹⁵ cm²/s.

[119] Three example embodiments of a permeation barrier layer as disclosed herein will now be described, in which the layer is deposited using PECVD, and the process parameter varied during deposition of three sub-layers is the power supplied to the plasma, as previously described. They relate to the protection of an OLED in the confi guration of FIG. 4D. In this configuration, the barrier film layer is exposed to the atmosphere at its top and at its edges. It may be preferred to make the top layer sufficiently thick to achieve the target lifetime TIML , and to make the bezel sufficiently wide to obtain at least the same lifetime against lateral i -diffusion of humidity through the conformable middle layer. Therefore, the bezel may preferably be wide enough such that the permeation time xbezei from the edge of the barrier to the OLED is equal to or longer than the permeation time from the top TIML of the barrier to the OLED. The hybrid barrier films are made from the three different layers of thicknesses dbono , d mu and du>p and with mechanical stresses σι, θ2 and a?. Their average stress i¾ ¾ is (cn -dbomm +m -dmiddie + m-dtop) I (dbottom + dmiddie + dtop). The requirement for TIML = bezei is illustrated by FIG. 21.

[120] Table 1 shows that 1 monolayer of H₂0 will have penetrated through a Ιμτη (70 W) barrier in 6 years. A 150W deposited barrier has 4 times lower water diffusion coefficient than a TOW barrier as shown in FIG. 5. To the first order, it would take 24 years for 1 monolayer to permeate a 1 μηι of a barrier deposited at 150W. The water permeation at this time would have reached steady state and hence the diffusion l ength would be ½ the thickness of the barrier or 500nm. The steady state can be observed in FIG. 16, in which, for the Ι μπι 70W barrier, the rate of permeated water is increasing linearly with time suggesting constant permeation rate. A constant permeation rate corresponds to a steady state condition.

[121] An analogous first-order calculation can be made for the lateral permeation of H₂0 in the (50 W) polymer-like layer, along the width of the bezel. The result is that the bezel may be on the order of 20 μηι wide to meet the 20-year lifetime requirement. A more accurate value for the bezel width will depend on the thickness of the polymer-like layer. The bezel width may depend primarily on the rate of permeation along the interface between the substrate and the barrier film. This rate may be determined experimentally for each specific case.

[122] Example 1. Barrier designed for covering 5-μιη particles and TIML = 20 years at 30 C and 100% R.H. The three layer barrier with 1 μτη bottom layer deposited at 70 W followed by a 2 μτη middle layer deposited at SOW and finally finished with a 1 μτη top layer deposited at 150W will have an as deposited stress, <¾¾? = 0 MPa (stress neutral). Due to the low diffusion coefficient of the 150W top layer, a conservative approximation would be that the permeation is limited only by the top layer. It is assumed that as soon as the water permeates the top layer, it- reaches the OLED. The stress calculations are made on the basis of this approximation. At the end of life, when the SiOa-like layer deposited at 150 W will have let 1 monolayer of H2O permeate, it will have picked up FI2O to saturate the equivalent of 1/2 of its thickness. This follows from the finding that water permeation would have achieved steady state. The concentration profile would be similar to that of FIG, 7. Water permeation will have changed the stress <7ave from 0 MPa to -12 MPa.

[123] Example 2. The thickness of the top layer of Example 1 is raised from 1 μιη to 2 μτη, which will make the initial o_ave = -20 MPa; over 64 years a maximum of 1 monolayer would have diffused; o_ave will increase to -40 MPa. [124] Example 3. The bottom layer thickness of Example I is reduced from 1 to 0.5 μιη and the middle layer from 2 to 1.5 um, which will make the initial O ve = 0 MPa; over 24 years 1 monolayer would have diffused; Gave will change to - 17 MPa.

[125] Examples disclosed herein are described in terms of various layers, which may include multiple sub-layers. Generally, as used herein, a single "layer" refers to a film of a single, heterogeneous composition, which may include multiple sub-layers having the same composition but different atomic morphology or other structural differences. Tests, calculations, simulations, and descriptions of individual layers may refer to complete permeation barrier layers having multiple sub-layers, or to individual single-structure layers that correspond or are equivalent to structures that would be expected for sub-layers within a layer having multiple sub-layers, as will be apparent from the description and context.

[126] Embodiments disclosed herein are described with respect to a "single" deposition process. As will be apparent to one of skill in the art, a "single" process refers to an

uninterrupted deposition process, during which a complete "layer" as described with respect to the deposition process is fabricated. As disclosed herein, the layer may include multiple sublayers that are deposited during the single deposition process due to variation in process parameters that control operation of the deposition process. Such variation of process parameters is not considered as interrupting the single deposition process, since the deposition process need not be stopped and re-started in order to change such parameters.

[127] it is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. it is understood that various theories as to why the invention works are not intended to be limiting.

Claims

CLAIMS We claim:

1. A method of fabricating a permeation barrier, the method comprising:

providing a first silicon-containing precursor adjacent to a substrate;

applying a single plasma-enhanced deposition process to form the first precursor into a sub-layer of a barrier film; and

altering a process parameter controlling a plasma used during the plasma-enhanced deposition process at least twice during the plasma-enhanced deposition process;

wherein the barrier film comprises at least three sub-layers, each sub-layer corresponding to a value of the process parameter,

2. The method of claim 1, wherein the process parameter is a power supplied to the single plasma-enhanced deposition process.

3. The method of claim 2, wherein the altering the process parameter comprises decreasing the power from a first level to a second level, lower than the first level, during the single plasma- enhanced deposition process.

4. The method of claim 3, wherein the altering the process parameter further comprises, subsequent to decreasing the power, increasing the power from the second level to a third level during the plasma-enhanced deposition process.

5. The method of claim 4, wherein the third level is higher than the first level.

6. The method of claim 2, further comprising:

sel ecting a desired physical property of each of the plurali ty of sub-layers; and selecting a plurality of levels among which to alter the power, based upon the desired physical property of each of the plurality of sub-layers, during the single plasma-enhanced deposition process,

7. The method of claim 6, wherein the physical property is a diffusion coefficient.

8. The method of claim 7, wherein each diffusion coefficient is independently selected from the group consisting of: not more than 10^"10 cm²/s, not more than 1ίΤ° crrrVs, and not more than 10 ¹⁵ c s.

9. The method of claim 8, wherein each of the plurality of levels among which to alter the power is independently selected from the group consisting of: not less than 30W, not less than SOW, and not less than 70W.

10. The method of claim 6, wherein the physical property is a mechanical stress.

11. The method of claim 10, wherem the mechanical stress is selected from the group consisting of: not more than 200 MPa, not more than 100 MPa, and not more than -100 MPa.

12. The method of claim 8, wherein each of the plurality of levels among which to alter the power is independently selected from the group consisting of: not less than 30W, not less than SOW, and not less than 70W.

13. The method of claim 1, wherein each of the plurality of sub-layers has a different atomic morphology.

14. A barrier film fabricated according to the method of claim 1.

15. The barrier film of claim 14, wherein each of the plurality of sub- layers has a different atomic morphology.

16. A device comprising a barrier film fabricated according to the method of claim 1.

17. The device of claim 16, further comprising an organic light emitting device (OLED) at least partially encapsulated by the barrier film.

18. A d evice compri sing :

an OLED; and

a permeation barrier film at least partially disposed over the O LED, the barrier film comprising a plurality of sub-layers, each of which has the same composition but different morphology.

19. A method of fabricating a permeation barrier layer, the method comprising:

applying a single plasma-enhanced deposition process to form a first precursor into a first sub-layer of a permeation barri er layer;

altering a first process parameter of the single plasma-enhanced deposition process to form the first precursor into a second sub-layer of the permeation barrier layer over the first sublayer; and

altering a second process parameter of the single plasma-enhanced deposition process to form the first precursor into a third sub-layer of the permeation barrier layer over the second sublayer.

20. The method of ciaim 19, wherein each of the first and second process parameters is independently selected from the group consisting of: a power density of a plasma used during the single plasma-enhanced deposition process, a ratio of precursors within the first precursor, a separation distance of electrodes used during the single plasma-enhanced deposition process, and a composition of an inert gas used during the single plasma-enhanced deposition process.

21. The method of claim 19, wherem the second process parameter is the same as the first process parameter.

22. The method of claim 21 , wherem the first process parameters is selected from the group consisting of: a power density of a plasma used during the single plasma-enhanced deposition process, a ratio of precursors within the first precursor, a separation distance of electrodes used during the single plasma-enhanced deposition process, and a composition of an inert gas used during the single plasma-enhanced deposition process.

23. The method of claim 19, wherein the first process parameter is an electrical power supplied to the single plasma-enhanced deposition process.

24. The method of claim 23, wherein the altering the first process parameter comprises decreasing the process parameter from a first level to a second level, lower than the first level, during the single plasma-enhanced deposition process.

25. The method of claim 24, wherein the altering the first process parameter further comprises, subsequent to decreasing the process parameter to the second level, increasing the first process parameter from the second level to a third level during the plasma-enhanced deposition process.

26. The method of claim 25, wherein the third level is higher than the first level .

27. The method of claim 19, further comprising:

selecting a desired physical property of each of the plurality of sub-layers; and selecting a plurality of levels among which to alter the process parameter, based upon the desired physical property of each of the plurality of sub-layers, during the single plasma- enhanced deposition process.

28. A barrier film fabricated according to the method of claim 19.