HK1096768B - A process for filling vias or slots with thick film paste using contact printing - Google Patents

Description

Method of filling vias or trenches with thick film paste using contact printing

Technical Field

The present invention relates to a method of filling holes in an electronic device structure using thick film paste. The invention is particularly useful in the fabrication of electron field emission triode arrays where the holes are of fine size (< 100 μm in diameter) and electron emitter thick film pastes, which may contain carbon nanotubes, are of high value.

Background

Bouchard et al (WO 01/99146) describe a field emitter thick film paste mixture, a method of applying the thick film paste using conventional screen printing and photoimaging (photoimaging), and also a method for improving the field emitter.

However, the use of conventional screen printing has several disadvantages. First, because the thick film paste must be squeezed through the screen mesh, shadow marks of the screen mesh are always present on the printed paste film. These mesh marks can result in incomplete and uneven packing of fine-sized holes or slots. Second, screen printed films have a limited thickness and inevitably overfill the holes or trenches in the substrate. Third, since the area of the holes or grooves on the substrate is typically only 2 to 10% of the total printed area, too much paste is used for conventional screen printing. This excess paste not only results in significantly higher material costs, but also causes longer drying times and greater difficulty in removing all of the excess paste from undesired areas.

The present invention provides an improved method for plugging holes or trenches in a substrate. The contact printing method breaks away from the conventional screen printing method by eliminating the use of an outer screen and having the coated edge directly contact the substrate. The coating edge is the straight edge used to spread the thick film paste deposition. The coating edge may be a sheet or rod made of a rigid or elastic material. The coating edge may be provided with an opening at its edge through which thick film paste may be metered onto the substrate, or the thick film may be metered from a separate thick film container located in front of the coating edge.

Disclosure of Invention

The invention is a method comprising:

a) applying a deposit of thick film paste on a multilayer electronic device having holes;

b) spreading a deposit of thick film paste across the surface of the multilayer electronic device using a coating edge in direct contact with the upper surface of the multilayer electronic device to plug the holes with thick film paste.

The invention also includes a method comprising:

a) coating a photoresist on a substrate;

b) patterning holes in the photoresist;

c) applying a deposit of thick film paste on the photoresist using a coating edge;

d) a deposit of thick film paste is spread across the surface of the photoresist using a coating edge in direct contact with the upper surface of the photoresist to cause the thick film paste to fill the holes.

The invention also includes a method comprising:

a) coating a photoresist on a substrate;

b) patterning holes in the photoresist;

c) depositing a buffer layer on an upper surface of the photoresist such that the hole is coated by the buffer layer but not plugged by the buffer layer;

d) applying a deposit of thick film paste on the buffer layer using the coating edge;

e) a deposit of thick film paste is spread across the surface of the photoresist using a coating edge in direct contact with the upper surface of the photoresist to plug the holes with thick film paste.

In the above method, the thick film paste may be quantitatively provided through an opening provided at the coating edge or from a container located in front of the coating edge.

The invention also encompasses the above method wherein the thick film paste comprises an acicular emitting substance.

The present invention also encompasses the above method wherein the acicular emitting substance is a carbon nanotube.

In one embodiment, the coating edge includes one or more wings at either end that contact the upper surface. The wings are perpendicular to the coating edge and serve to prevent the paste from running off. The wings may be formed from any suitable material including materials such as metal, plastic, and the like.

In another embodiment, the thick film paste may be printed parallel to the coating edge rather than perpendicular to the sides of the maximum length of the aperture (edge). This results in more material being deposited in the pores.

The invention also describes an electronic device comprising a substrate coated with a thick film paste using the method set out above.

Drawings

Fig. 1 shows a device structure with patterned dielectric and conductive materials.

Figure 2 shows the device structure after the photoresist has been removed from within the holes.

Fig. 3 shows a device structure coated with a buffer layer.

Fig. 4 shows the deposition of the bead emitter paste and the positioning of the coating edge ready for contact printing.

Fig. 5 shows a structure of a thick film paste printed in holes using contact printing.

Figure 6 shows an alternative repeat print with the coating edge moving in the opposite direction.

Fig. 7 shows the back UV (ultraviolet) irradiation of the thick film paste in the vias.

Fig. 8 shows the jet development and cleaning (ringing) of the thick film paste and buffer.

Figure 9 shows the device structure after the photoresist is removed.

Fig. 10 shows the device structure after the thick film paste bake.

Fig. 11 illustrates an alternative activation step for a thick film paste containing needle-like emitters.

Fig. 12 shows the structure and image of an actual device with a patterned photoresist containing a 2O μm array of holes and a buffer layer.

Fig. 13 shows a surface image of the actual device after contact printing and emitter paste drying.

Fig. 14 shows the surface image of the actual device after paste development.

Fig. 15 shows the emitter paste dots after clean removal of the photoresist.

Fig. 16 shows a surface image of the emitter dots after the adhesive stripe activation process.

Fig. 17 shows a surface image of an excitation tape showing the transfer of emitters to the tape.

Fig. 18 shows an illustration of electric field emission of an anode of an array of emitter dots deposited by using a contact printing method.

Detailed Description

The present invention is a method for manufacturing an electronic device comprising a substrate coated with a patterned thick film paste. The invention is also a device manufactured using the method of the invention. An example of such a device is an electron field emission triode array for use in a visual display in which a thick film paste includes acicular emitters such as carbon nanotubes to enhance the electron field emission properties of the thick film paste. Acicular emitters are materials with a high aspect ratio, a small radius of curvature (sharpradius) at the tip, and are electrically conductive to facilitate electron field emission under an applied potential.

In the method, the structure is fabricated on a transparent substrate, such as glass, which is coated with a conductive film, such as indium-tin oxide (ito). A dielectric layer is then deposited on the coated substrate and patterned by techniques such as photolithography or wet etching. In a specific design, holes or slots are made through the dielectric deposit extending to the coated substrate. A hole is a hole in a layer at the surface of a multilayer electronic device. The aperture may be of any shape. These holes or slots may be used to accommodate the electron emitter thick film material. A conductive gate electrode, which may be chromium, is then deposited over the entire dielectric layer, leaving the hole clean. The chromium gate electrode may be deposited by evaporation or sputtering techniques followed by lithographic and etching steps. At this point in the manufacturing process, an example of a device structure is shown in fig. 1, in which a Cr gate 1, a dielectric layer 2 and a hole 3 are shown. In other designs, no holes or trenches are created in the dielectric layer. Instead, a fine pattern of emitter thick film paste is deposited to the electrode structure with tight tolerances.

Using the contact printing method as disclosed in the present invention, a substrate with embedded holes or slots is prepared for holes plugged with emitter thick film paste. Unprotected substrates, however, are susceptible to damage or scratching from the coated edge. Therefore, it is recommended to apply a photoresist layer to protect the substrate surface. For substrates without embedded holes or slots, a photoresist layer may be used to define the location of the emitter thick film paste by precise patterning of the holes or slots in the photoresist. In addition to defining the location of the emitter paste, the photoresist layer also serves as a lift-off layer to ensure clear development of the thick film paste.

The photoresist may be of the positive or negative type and is typically deposited by spin coating or slot film coating. For positive photoresists, such as DNQ/novolac or chemically amplified resists, holes or trenches may be created in the photoresist layer by development after masked UV exposure in the hole or trench regions. The hole or trench illuminated areas of the photoresist are dissolved in a weak base developer such as 1% KOH or 2.6% tetramethylammonium aqueous solution and can be removed from the substrate. The device structure is shown in fig. 2 after the photoresist is removed from the holes or trenches.

Photoresists have been seen to be incompatible with the solvents used in certain formulations of thick film pastes. The solvent in the thick film can attack the photoresist, which can result in poor paste deposition. It has been found that the use of a buffer layer of material that is inert to the solvents used in the thick film paste formulation protects the photoresist and ensures accurate deposition. Polyvinyl alcohol (PVA) is an example of a suitable buffer layer. PVA may be applied to the device structure using spin coating or slot film coating. The use of buffer layer 5, though, depends on the chemical compatibility between the thick film paste and the photoresist. The device structure at this point in the process is shown in fig. 3.

The next step in the process of the invention is the deposition of the thick film paste. Thick film pastes generally include solvent, organic and inorganic components. The solvent may typically be a high boiling point liquid such as butyl carbitol, butyl diglycol acetate, dibutyl diglycol, dibutyl phthalate, texanol ester alcohol (2, 2, 4-trimethyl-1, 3-pentanediol monoisobutyrate) and beta-terpineol. The organic components include binder polymers, photosensitive monomers, initiators, dispersants, and/or other rheology modifiers. The inorganic component includes glass frit, inorganic powder and/or metal powder. A specific example of a thick film paste is one that includes an acicular emitting material such as carbon nanotubes to enhance electron field emission from the paste.

For applying the paste to the substrate, conventional screen printing is often used. In conventional screen printing, a steel or polymer screen consisting of fine mesh stretched under high tension is disposed on top of a substrate. The through-holes of the screen mesh are first patterned with a polymer emulsion to define a print area. For photoimageable thick film pastes, an unpatterned flood print (floodprint) of the thick film paste is typically used to cover almost all of the substrate surface. In preparation for screen printing, excess thick film paste is first spread over the screen. The coating edge, typically a pellet made of elastomer of variable durometer, is in contact with the mesh of the screen. The sheet is then pushed across the screen over the thick film paste so that the thick film paste can be pressed through the open mesh areas of the screen and deposited on the substrate. In conventional screen printing, the coating edge never comes into contact with the substrate itself.

However, the use of conventional screen printing has several disadvantages. First, because the thick film paste must be squeezed through the fine mesh of the screen, shadow traces of the screen mesh are always present on the printed paste film. These mesh marks can lead to incomplete and uneven packing of the pores or channels. Second, screen printed films have a limited thickness and inevitably overfill the holes or trenches of the substrate. Third, conventional screen printing uses a large excess of paste, since the hole or slot area on the substrate is typically only 2 to 10% of the total printed area. This excess paste not only results in significantly higher material costs, but also causes longer drying times and greater difficulty in removing all of the excess paste from undesired areas.

The present invention provides an improved method, known as contact printing, to fill holes or trenches in a substrate. Contact printing methods differ from conventional screen printing by eliminating the use of an external screen and bringing the coating edge, similar to that used in conventional screen printing, into direct contact with the substrate surface. The coating edge is the straight edge used to spread the thick film paste deposition. The coating edge may be a sheet or rod made of a rigid or elastic material. The coating edge may be provided with an opening through which the thick film paste can be dosed onto the substrate. Thick film may also be metered onto the substrate from a separate thick film container at a position in front of the coating edge.

In one embodiment of the invention, a small bead of paste is first applied to the foremost edge of the substrate. The paste is conveniently applied using a metering pump connected to a syringe filled with thick film paste. The bead paste can be deposited accurately as the syringe traverses the foremost edge of the substrate. For large substrates, it is necessary to dose a plurality of bead pastes at distributed locations along the substrate, or to provide a plurality of openings at the coating edge through which thick film paste can be continuously dosed onto the substrate. The coated edge, a sheet or rod-like device similar to the elastomeric sheet used in conventional screen printing, is then brought into contact with the substrate at a location just in front of the paste bead. A constant pressure is used to maintain contact between the device and the substrate surface while it is advanced across the substrate surface. As the device is pulled through the bead of paste, it picks up the paste and pushes it into the holes or slots that are open on the substrate. Unlike conventional screen printing, only a very thin trace of paste in the form of a stripe is left on top of the substrate, photoresist or buffer layer surface. Except for paste hole plugging, almost all of the paste is pushed in front of the coating edge and away from the contact surface. The rim geometry and hardness, applied pressure and printing speed, all have an effect on the depth of the pore filling and the amount of residual paste deposition remaining on the contact surface. Complete packing of 20 μm diameter and 10 μm depth holes was achieved using a sheet with 80 shore a hardness, 30psi pressure, and 0.5 in/s print speed. However, for pore sizes greater than 100 microns orthogonal to the direction of motion of the coating edge, the contact printing process can result in incomplete filling of the pores. Alternatively, the coating edge may be traversed in the opposite direction for a second pass to further fill the vias with thick film paste. These steps in the fabrication of the device are illustrated in 4, 5 and 6.

In another embodiment of the present invention, the thick film may be first diffused over the entire surface substrate. This can be achieved by conventional screen printing or slot film coating. The coated edge is then brought into contact and pushed across the surface to fill the holes or slots with paste, and any excess paste is removed from the contact surface. This optionally obviously includes an additional step and is therefore not preferred.

The deposited paste is then dried under conditions to remove the solvent. Drying is typically carried out in a convection oven or using a heating lamp at a temperature in the range of 35-100 c for 15 to 30 minutes. After drying, the paste in the hole-filling area is exposed to UV radiation through an in-situ or external photomask. In the specific case illustrated in FIG. 7, the paste packed in the holes can be exposed to an appropriate UV dose (50 to 100) through the transparent substratemJ/cm²) And (6) exposing. The residual paste deposited on the contact surface is not irradiated by this process due to UV absorption/reflection by the electrode coating, dielectric material and/or photoresist. The thick film paste is constructed using a photopolymer so that the paste becomes insoluble in many solvents under irradiation.

After irradiation, the structure is developed by spraying or rinsing using a suitable developing solution, which removes the thick film paste that is not irradiated. For pastes containing acicular binding polymers, a 0.5% aqueous solution of sodium carbonate may be used. In the case of many paste formulations, a spray time of 30 to 45 seconds is required. If a buffer layer is used to separate the thick film paste from the photoresist, it may also be removed during this spraying and rinsing step. Fig. 8 shows the spraying and cleaning steps of the process.

The next step in the process is the removal of the photoresist. The photoresist may be removed by dipping the substrate in a suitable solvent. For DNQ/novolac photoresists, this can be 30 seconds exposure in acetone or 2 to 3 minutes exposure in 3% hydrated potassium hydroxide. The structure of the device after the photoresist is removed is shown in fig. 9.

The next step in the process is the firing of the thick film paste. This step burns off the organic binder and sinters the inorganic phase of the thick film paste. The inorganic phase may comprise glass frit, metal powder and carbon material. For the carbon nanotube containing paste, a baking condition of 460 to 525 ℃ for 10 to 20 minutes in nitrogen gas may be employed. Fig. 10 illustrates this process step.

In one embodiment, the thick film paste is printed at the coating edge parallel to, rather than orthogonal to, the longest via side (edge). This results in more material deposition in the hole.

The coating edge may have one or more "wings" at either end that contact the upper surface. The wings are perpendicular to the coating edge and serve to prevent the paste from flowing out.

If the device is a carbon nanotube thick film paste electron field emitter, it must be deposited using a binder contact method to stimulate the emitter. This is achieved by superimposing a sheet of polymer over the device. Bonding the polymer to the thick film paste deposition surface. The polymer sheet is then peeled from the device. The delamination of the polymer sheet breaks and rearranges the surface of the thick film paste field emitter to improve field emission. This step in the process is shown in fig. 11.

Examples of the present invention

The following examples illustrate the disadvantages of conventional screen printed filling of holes and the efficiency of filling holes using contact printing provided by the present invention.

Example 1

This example demonstrates the disadvantage of filling fine-sized holes with thick film paste using conventional screen printing as part of the process of manufacturing electron field emission devices.

First, a Cr layer of approximately 2000A was sputtered onto a glass substrate to prepare a glass substrate coated with an in-situ chromium photomask. The Cr layer was patterned with an array of 20 μm circles where the Cr coating was etched away. A novolac type photoresist, AZ4620 obtained from caliant Cooperationof Sulzbach am Taunus, Germany, was spin-coated on the Cr coating side of the glass substrate. A spin speed of 1000rpm and a spin time of 45 seconds were used. The phenolic resin polymer film was dried on a 95 ℃ hot plate for 10 minutes. After drying, a 12 μm thick phenolic polymer film was obtained. The photoresist was exposed to UV (350-450nm) radiation from the backside of the substrate through an in-situ Cr photomask layer. Using 450mJ/cm²UV dose of (a). The photoresist was developed for 90 seconds in a developer solution (containing 1% potassium hydroxide) also available from Cariant as AZA 21K. After development, the substrate was baked on a hot plate at 120 ℃ for 3 minutes.

An aqueous solution of polyvinyl alcohol (PVA) is prepared for use as a buffer layer. A fully hydrolyzed (99%) grade of PVA with a molecular weight of 130,000 was chosen because of its high resistance to decomposition by organic solutions as well as cold water. A fully hydrolyzed PVA solution was prepared by dissolving 4 grams of polymer in 200ml of hot water (> 90 ℃ C.). The solution was allowed to cool to room temperature. A200 ml mixture was prepared by mixing 100ml each of isopropyl alcohol (IPA) and water which can improve the wetting of the photoresist. The mixture was gradually stirred into the PVA solution to complete 400ml of buffer layer solution containing about 1 weight percent (wt%) PVA.

A single layer of PVA buffer was spin coated on the substrate on which the phenolic photoresist was pre-patterned. A spin speed of 1000rpm and a spin time of 45 minutes were used. The buffer layer was spin dried at room temperature to give a dry thickness of 0.5 μm. Figure 12 shows a patterned photoresist containing a 20 μm aperture array coated with a buffer layer.

A photoimageable thick film paste containing a binder polymer, a photoinitiator, monomers, silver particles, glass frit, and Carbon Nanotubes (CNTs) was prepared using β -terpineol as a paste solvent. This CNT paste is effective in the fabrication of diode and triode electron field emission devices. Using a conventional screen printing process, a blanket film of CNT paste is deposited over the substrate, over-coating the patterned photoresist. A C400 mesh screen was used for printing. The CNT paste film was dried in a forced air convection oven at 50 ℃ for 30 minutes. On the buffered photoresist, a thickness of up to 8 μm thick dry CNT paste film was established. Close inspection of the dried paste film showed the presence of mesh traces and rheology induced orange peel defects. These defects lead to incomplete filling of the holes in the photoresist. The total weight of the dried emitter film was determined to be 0.52g for coverage of 25 square inches.

Approximately 300mJ/cm is used from the back of the substrate through the embedded photomask²The thick film paste film is exposed to UV rays. 0.5% NaCO was used₃The aqueous solution sprayed the exposed CNT paste film for 1 minute, during which the CNT paste film was brushed away from the areas where the paste was not irradiated. At the paste exposed to UV radiation, an array of CNT paste dots was preserved. The weight of the emitter paste hole packing was determined to be 0.03 g. Therefore, over 94% of the screen printed emitter paste was brushed off during development, illustrating the very inefficient use of CNT emitter paste using conventional screen printing.

Example 2

This example describes the contact printing process and illustrates the benefit of using contact printing to fill the vias with a photoimageable thick film paste containing carbon nanotubes.

As in example 1, a glass substrate coated with an in-situ photomask patterned using a 20 μm array of open circles was prepared. A novolac type photoresist, AZ4620 obtained from jiantcooperation, germany, was spin-coated on the Cr-coated side of the glass substrate. A spin speed of 1000rpm and a spin time of 45 seconds were used. The phenolic resin polymer film was dried on a 95 ℃ hot plate for 10 minutes. After drying, a 12 μm thick phenolic polymer film was obtained. The photoresist was exposed to UV (350-450nm) radiation from behind the substrate through the internal Cr photomask layer. Using 400mJ/cm²The dosage of (a). The photoresist was developed for 45 seconds in a developer solution, AZA21K also available from Cariant. After development, the substrate was baked on a hot plate at 120 ℃ for 3 minutes. As in example 1, a single layer of PVA polymer was spin coated on the photoresist as a buffer layer. Fig. 12 again shows a patterned photoresist containing a 20 μm array of holes and coated with a buffer layer.

The amount of photoimageable CNT thick film paste, as used in example 1, was filled into a syringe optionally fitted with a 2mm open needle. A bead of paste of approximately 2mm thickness is metered along the front edge of the substrate. The substrate is mounted on a substrate holder of a conventional screen printer. A conventional screen printing sheet with a shore hardness of 80 is in direct contact with the substrate coated with photoresist at a position a few millimeters in front of the paste bead. A constant pressure of 30psi (pounds per square inch) was used to maintain contact between the sheet and the substrate surface while the sheet was advancing across the substrate at a rate of 0.5 inches per second. As the sheet is pushed over the bead paste, it picks up the paste and pushes it into the holes on the substrate. Unlike conventional techniques, only very thin traces of paste are left on the substrate surface, in the form of stripes. Except for hole plugging, most of the paste is pushed through before the chip and eventually off the substrate surface.

The CNT paste film was dried in a forced air convection oven at 50 ℃ for 10 minutes. Due to the lower paste volume, a very short drying time can be used, illustrating one of the advantages of the contact printing process. The thickness of the dried CNT paste film is only 1-2 μm in the stripe region on the photoresist and 5-6 μm in the pores. This illustrates the advantage of using contact printing rather than overfill holes. A close check of the dried paste film showed good paste filling of all the holes, illustrating another advantage. The total weight of the dried emitter film was determined to be 0.04g for 25 square inches of coverage. Thus, the use of contact printing proved to greatly reduce the use of emitter paste by more than an order of magnitude. Fig. 13 shows an image of the surface of the actual device at this point of processing.

Approximately 300mJ/cm is used from the back of the substrate through the embedded photomask²The thick film paste film is exposed to UV rays. 0.5% NaCO was used₃The aqueous solution sprayed the exposed CNT paste film for 1 minute, during which the CNT paste film was washed away from the areas of the paste that were not irradiated. At the paste exposed to UV radiation, an array of CNT paste dots was preserved. The weight of the emitter paste hole packing was determined to be 0.02 g. Fig. 14 shows the surface image of the actual device after paste development.

In preparation for stripping the photoresist from the substrate, the substrate is further rinsed in room temperature water for 1-2 minutes to remove the buffer layer from the areas not covered by the CNT paste. Subsequently, the photoresist was removed by dipping in a 3% KOH aqueous solution for 2 minutes. Figure 15 shows the CNT emitter paste after clean removal of the photoresist.

The substrates were baked in a nine zone belt furnace apparatus at a maximum temperature zone with a maximum temperature of 465 c for a 20 minute dwell time. The baked substrate is activated by an adhesive activation method using a tape coated with a pressure sensitive adhesive. Surface images of the actual device and the activated tape are shown in fig. 16 and 17.

A substrate deposited with an array of CNT paste dots is used as a cathode in an electron field emission diode device comprising a cathode and an anode. Cathode comprising ITO coatingFrom the substrate, P13 phosphorus particles were deposited on the ITO surface. The cathode and anode were separated by a glass space of 0.9mm thickness. The diode assembly has a cathode connected to a high voltage pulsed power supply and an anode connected to ground through an electricity meter, which is then placed in a vacuum chamber and evacuated to less than 1x 10^-6(1xE-6) Torr. High current electron field emission was observed when the cathode was energized with a high voltage pulse train comprising negative polarity voltage pulses at 100Hz and 3 microseconds duration. At an applied voltage of 4KV, 12. mu. Amp/cm were measured²Average anode field emission current density. Figure 18 shows an illustration of electron field emission from the anode of an array of CNT paste dots deposited using a contact printing process.

Claims (8)

1. A method of filling vias or slots with thick film paste using contact printing, comprising:

a) coating a photoresist on a first side of a substrate;

b) patterning holes or trenches in the photoresist;

c) depositing a buffer layer on the upper surface of the photoresist such that the holes or trenches are coated but not plugged;

d) providing a coating edge on said photoresist in direct contact with said buffer layer to deposit thick film paste in said holes or slots;

e) irradiating the thick film paste.

2. The method of claim 1, wherein said thick film paste is metered through an opening in the coating edge for deposition in said hole or slot.

3. A method according to claim 1, wherein said thick film paste is metered from a container located in front of the coating edge for deposition in said holes or slots.

4. The method of claim 1, wherein the coating edge includes one or more wings for inhibiting the flow of paste away.

5. The method of claim 1, wherein said thick film paste is deposited in said holes or slots in a direction parallel to the side of the hole of greatest length.

6. The method of claim 1, wherein the buffer layer comprises polyvinyl alcohol.

7. The method of claim 1, wherein the thick film paste comprises an acicular emitting substance.

8. The method of claim 7, wherein the acicular emitting substance is a carbon nanotube.