TWI910465B - Direct patterning deposition mask for oled deposition and sapphire substrate manufacturing process - Google Patents

Abstract

A direct patterning deposition mask for OLED deposition is provided where the mask includes a sapphire substrate; and a Silicon Nitride (SiN) membrane. The sapphire substrate thickness may be between 0.7 and 2 mm. The sapphire substrate may have a diameter in the range of 200mm diameter to 300mm diameter. Warpage of the substrate is preferably less than 10um.

Description

Direct patterning deposition mask and sapphire substrate fabrication process for OLED deposition

This application pertains to direct patterned deposition (dPd). More specifically, this invention relates to dPd technology in displays.

The deposition of a photomask substrate is a process in which a material layer is deposited onto the surface of a substrate, so that the desired pattern of that layer is defined during the deposition process itself. This deposition technique is sometimes referred to as "direct patterning."

In a typical photomask deposition process, the desired material is vaporized at a source located at a certain distance from the substrate, with a photomask positioned between the substrate and the source. As the vaporized atoms of the material travel toward the substrate, they pass through a set of perforations in the photomask positioned just in front of the substrate surface. The perforations (i.e., apertures) are configured to form the desired pattern of the material on the substrate. Therefore, the photomask blocks all vaporized atoms except those passing through the perforations, which deposit on the substrate surface in the form of the desired pattern. The deposition of a photomask substrate is similar to screen printing techniques used to form patterns (e.g., uniform numbers) on clothing or stencil printing used to develop artwork.

Photomask deposition has been used in the integrated circuit (IC) industry for many years to pattern materials deposited on substrates. This is partly due to the fact that it avoids the need to pattern a material layer after it has been deposited. Therefore, photomask deposition eliminates the need to expose the deposited material to harsh chemicals (e.g., acid-based etching agents, caustic photolithography chemicals, etc.) for patterning. Furthermore, photomask deposition requires less substrate handling and processing, thereby reducing the risk of substrate breakage and improving manufacturing yield. Furthermore, many materials, such as organic materials, are susceptible to damage when exposed to photolithography chemicals, making the deposition of these materials using a light-blocking mask essential.

A high-quality dPd photomask is a key mounting element used in dPd manufacturing, especially in OLED microdisplays.

By directly patterning OLEDs using lithography, high-efficiency, high-resolution OLED microdisplays can be fabricated. A photomask, a nanoscale feature, is used in the deposition of color emitters for OLEDs. This photomask offers the precision and accuracy to match the underlying transistors of the microdisplay and enables the color emitters to be established at higher resolution.

As shown in Figure 1, and as is known, a flat substrate (such as a silicon wafer) is used to construct a photomask. A thin film (such as silicon nitride) is deposited on both sides of the substrate using chemical vapor deposition (CVD). This silicon nitride layer can serve as an etch barrier on one side and a suspended film on the other side. Silicon oxide, aluminum oxide, or other thin film materials have also been used instead of silicon nitride. One side of the film is etched to expose the substrate for a subsequent through-substrate etching process. For example, silicon nitride can be patterned using photolithography and removed by dry etching. The other side of the film is patterned using photolithography and etched to create the desired photomask pattern. Furthermore, photolithography and dry etching can be used on this other side. Of course, other patterning methods can also be used. This prior art procedure is described in detail in U.S. Patent No. 9,385,323 (Chan et al.).

Through-substrate etching allows the thin film to hang freely, enabling it to be used as a photomask. Potassium hydroxide, for example, can be used to etch the substrate.

Patterned evaporation can be performed using a photomask. A microdisplay substrate is placed close to or in contact with the photomask. This setup can be incorporated into a deposition system to allow material evaporation. After evaporation, a patterned material will be present on the substrate. This is illustrated in Figure 2.

There are two main challenges with dPd technology. First, the dPd mask must be fabricated as flat as possible. Conventionally, a silicon (Si) wafer is used as the frame material. See Figure 3. A SiN (silicon nitride) film is deposited, and then a high-resolution pattern is created. See Figure 4, which illustrates a typical 1μm SiN mask used in a dPd process. However, due to the limited stiffness of the Si wafer (maximum warpage of 35μm, since 0.7mm Si is typical in the integrated circuit (IC) industry), a noticeable warpage and curvature remain after the dPd mask is fabricated. Therefore, the warpage of a dPd mask on a Si substrate can reach 30 μm to 40 μm (see Figure 5, which illustrates an example of dPd mask warpage measured across an 8-inch wafer, where a SiN film is located on top of a Si frame). Table 1 below shows an example of dPd mask warpage across an 8-inch wafer, where a silicon nitride film is located on top of a Si frame at points 1 to 4 in Figure 5:

This high mask warpage can create a large gap between the mask and the wafer during organic deposition and cause unwanted feathering in lateral deposition. The deposited material tends to extend laterally after passing through the mask (a phenomenon known as "feathering"). Feathering increases with the amount of separation between the substrate and the mask. To mitigate feathering, this separation should be kept as small as possible without compromising the integrity of the chuck holding the substrate and mask in place. Furthermore, any non-uniformity in this separation across the deposition area will cause variations in the amount of feathering. This non-uniformity can arise from, for example, a lack of parallelism between the substrate and the photomask, bending or sagging of one or both of the substrate and the photomask, and the like. Furthermore, a photomask must be supported only at its periphery to avoid obstructing the passage of vaporized atoms toward the via pattern. Therefore, the center of the photomask can sag due to gravity, further exacerbating the feathering problem. See Figure 6, which illustrates one example of the feathering distance calculated for two deposition angles varying from 1 micrometer to 10 micrometers in the wafer-to-mask gap.

The second challenge concerns the manufacturability of the substrate. To integrate the SiN film and the rigid substrate to create a dPd photomask, a suitable process must be designed, taking into account substrate etching, chemical compatibility, and other factors. Substrate properties and process integration must be considered.

This invention relates to a direct patterning deposition photomask for OLED deposition, wherein the photomask comprises a sapphire substrate and a silicon nitride (SiN) film. The thickness of the sapphire substrate can be, for example, between 0.7 mm and 2 mm. The diameter of the sapphire substrate (wafer) can be, for example, 200 mm or 300 mm. The sapphire wafer patterning process is preferably compatible with the SiN film process. The warpage of the substrate can be limited to, for example, less than 10 μm. This photomask improves OLED pixel deposition feathering and OLED performance.

This invention also provides a process for etching a sapphire substrate, the process comprising at least two of the following steps: mechanical drilling; wet etching; dry etching; and laser-induced etching combined with wet etching.

Figure 1 illustrates one of the main fabrication steps for a prior art silicon nitride film, which includes (1) silicon wafer; (2) silicon nitride deposition; (3) back-side lithography; (4) front-side lithography; and (5) back-side through-wafer etching.

Figure 2 is a simplified view illustrating the deposition through a light-blocking mask.

Figure 3 is a top view of a typical prior art 1μm SiN mask used in a dPd process.

Figure 4 is a simplified view illustrating a cross-section of a SiN photomask, representing a prior art example.

Figure 5 is a simplified view of one example of dPd quality warpage measured across an 8-inch wafer, where a SiN film is located on top of a Si framework, as shown in Table 1 (above).

Figure 6 illustrates one example of the feathering distance calculated for two deposition angles as the wafer-to-mask gap varies from 1 μm to 10 μm.

Figure 7 is a simplified view of one of the main manufacturing steps for silicon nitride films, which includes (1) sapphire wafer; (2) silicon nitride deposition; (3) back-side lithography; (4) front-side lithography; and (5) wet etching through the sapphire wafer from the back side.

Figure 8 illustrates a simplified step-by-step example of a procedure used to fabricate a SiN photomask on a sapphire substrate.

Cross-referencing of related applications

This application claims priority to U.S. Provisional Patent Application No. 63/403,964, filed on September 6, 2022, entitled "Rigid Sapphire Based Direct Patterning Deposition Mask," which is still pending.

This invention relates to a direct patterning deposition mask for OLED deposition. The mask comprises a sapphire substrate and a silicon nitride (SiN) film. To reduce mask warpage, this invention addresses the use of sapphire as the substrate material for SiN deposition and patterning. See Figure 7, which illustrates a method for fabricating a sapphire wafer into a dPd mask substrate material. Sapphire wafers, with their excellent rigidity, are widely used in the LED industry. Sapphire wafers have a Young's modulus approximately twice that of Si wafers (as shown in Table 2 below, which compares the properties of sapphire and silicon with those of silicon nitride, diamond, and rigid steel).

Based on a survey presented in Table 3 (below), the warpage of a 1.3 mm thick sapphire wafer can be controlled to <8 μm. Table 3 illustrates examples of silicon wafer warpage, compared to Table 4 (below), which illustrates sapphire wafer warpage.

Table 4

However, for sapphire, typical dry etching only yields an etching rate of one nm(s)/min. Essentially, this means a cycle of 2 to 3 weeks is required to complete the etching of a wafer, which is impractical. Instead, newly developed high-temperature wet etching can provide an etching rate of μm(s)/min, reducing wafer etching time to one day or less.

Previously, an etching bath with a temperature limit of 190°C was considered suitable. The etching rate of sapphire increases geometrically with temperature. An etching bath with a temperature of up to 300°C is now expected.

During wet etching at relatively high temperatures (e.g., 300°C), a SiN-masked wafer is placed in a high-temperature processing bath containing a mixture of an etchant and a buffer. Prior to immersion, a plasma-enhanced chemical vapor deposition (PECVD) process adds a silicon dioxide photomask to the sapphire substrate, and lithography exposes the desired pattern. The mixture is maintained at temperatures, for example, between 260°C and 300°C.

White Knight's Accubath ^™ quartz tanks and specially designed automated stations make wet sapphire etching safe, reliable, and suitable for high-volume manufacturing. See https://wkfluidhandling.com/resources/sapphire-etching/.

High-temperature wet etching processes offer advantages over dry etching in terms of speed, cost, and scalability.

In this invention, the thickness of the sapphire substrate is preferably between 0.7 mm and 2 mm. The sapphire substrate preferably has a diameter in the range of 200 mm to 300 mm. The warpage of the substrate is preferably <10 μm.

According to another exemplary embodiment of the present invention, as is known, selective laser-induced etching (SLE) can be used in a two-step procedure. In a first step, the sapphire is chemically etchable by internally modifying it with laser radiation. To prevent crack formation in the brittle material, a short pulse duration (fs-ps) and a small focal volume (a few ^μm³ ) are used. During the laser modification, the crystallinity of the sapphire is degraded (e.g., from crystalline to amorphous). In a second step, the modified sapphire is removed by wet etching, such as using potassium hydroxide (KOH) etching.

In this first step, ultrashort pulsed laser radiation is focused onto a substrate of a specific volume. The pulse energy, based on a multiphoton process, is absorbed only into the focused volume. This process modifies the substrate without causing it to crack, thereby altering its chemical properties. In this way, selective chemical etching of the material can be performed.

In addition, several etching methods can be combined for sapphire etching. Examples include mechanical drilling, laser treatment, KOH etching, high-temperature wet etching (described above), _Cl₂ -based inductively coupled plasma (ICP) etching, and _Cl₂ , _BCl₃ , ICP reactive ion etching (RIE), and 20°C etching. Table 5 below compares several sapphire thinning and etching methods.

Figure 8 illustrates an example of a process for fabricating a SiN photomask on a sapphire substrate. The process begins with a sapphire substrate having a SiN film. A pattern is placed on the SiN film using one or more of mechanical drilling, wet etching, dry etching, selective laser-induced etching with wet etching. Photoresist is applied to the substrate, and sapphire is removed from the surface of the film opposite the pattern (mechanical thinning) (e.g., removing 0.8 mm from a 1.3 mm sapphire), followed by laser-induced wet etching to remove the remaining 0.5 mm of sapphire.

It should be understood that this disclosure is merely one example of an illustrative embodiment, and those skilled in the art can readily conceive of many variations of the invention after reading this disclosure. The scope of the invention is determined by the following patent application.

Claims (2)

A direct patterning deposition mask for OLED deposition, the mask comprising: (a) a sapphire substrate having a thickness between 0.7 mm and 2 mm and having a diameter in the range of 200 mm to 300 mm, and wherein the warpage of the substrate is <10 μm; and (b) a silicon nitride (SiN) film, wherein the SiN film is used to form a pattern on an OLED microdisplay. A process for fabricating a sapphire substrate with the direct patterned deposition mask as described in claim 1, comprising at least two of the following steps: (a) mechanical drilling; (b) wet etching; (c) dry etching; and (d) laser-induced etching combined with wet etching; wherein the process is configured to control the warpage of the substrate to be less than 10 μm.