Rigid sapphire-based direct patterning deposition mask

Abstract

A direct patterning deposition mask for OLED deposition is provided, which includes a sapphire substrate and a silicon nitride (SiN) film. The thickness of the sapphire substrate can be between 0.7 and 2 mm. The sapphire substrate can have a diameter ranging from 200 mm to 300 mm. The substrate preferably has a warp of less than 10 μm.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to pending U.S. Provisional Patent Application No. 63 / 403,964, filed September 6, 2022, entitled Rigid Sapphire Based Direct Patterning Deposition Mask.

[0002] This application relates to direct patterned deposition (dPd). More specifically, the present invention relates to dPd technology in displays. [Background technology]

[0003] Shadow mask-based deposition is a process in which a layer of material is deposited onto the surface of a substrate such that the desired pattern of the layer is defined during the deposition process itself. This deposition technique is sometimes called "direct patterning."

[0004] In a typical shadow mask deposition process, the desired material is evaporated in a source positioned a distance from the substrate, with a shadow mask positioned between them. As the evaporated atoms of the material move toward the substrate, they pass through a set of through-holes in the shadow mask, which is positioned directly in front of the substrate surface. The through-holes (i.e., openings) are arranged in the desired pattern for the material on the substrate. As a result, the shadow mask blocks the passage of all evaporated atoms except those that pass through the through-holes, which are then deposited in the desired pattern on the substrate surface. Shadow mask-based deposition is similar to the silk-screen technique used to create patterns on clothing (e.g., uniform numbers) or stencils used to develop artwork.

[0005] Shadow mask-based deposition has been used for many years in the integrated circuit (IC) industry to deposit patterns of material onto substrates, in part due to the fact that it avoids the need to pattern material layers after deposition. As a result, its use eliminates the need to expose deposited materials to harsh chemicals (e.g., acid-based etchants, caustic photolithography developing chemicals, etc.) to pattern them. In addition, shadow mask-based deposition requires less handling and processing of the substrate, thereby reducing the risk of substrate damage and increasing manufacturing yields. Furthermore, many materials, such as organic materials, cannot be exposed to photolithography chemicals without being damaged, making it essential to deposit such materials with a shadow mask.

[0006] High-quality dPd masks are a key asset for dPd manufacturing, especially for OLED microdisplays.

[0007] High-efficiency, high-resolution OLED microdisplays can be produced by using direct patterning of OLEDs with stencil lithography. The deposition of color emitters for OLEDs uses shadow masks that can have nm-scale features. The shadow masks have the precision and accuracy to match the underlying transistors of the microdisplay and create color emitters at higher resolutions.

[0008] As seen in Figure 1, a flat substrate such as a silicon wafer is used to construct a shadow mask. Chemical vapor deposition (CVD) is used to deposit a thin film, such as silicon nitride, on both sides of the substrate. This silicon nitride layer can function as an etch barrier on one side and a free-standing 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 thin film is etched to expose the substrate for a subsequent through-substrate etching process. For example, the silicon nitride can be patterned using photolithography and then dry-etched to remove it. The other side of the thin film is patterned using lithography and then etched to create the desired shadow mask pattern. Again, this can use photolithography and dry etching. Of course, other patterning methods can also be used. U.S. Patent No. 9,385,323 (Chan et al.) describes this prior art process in detail.

[0009] Etching through the substrate leaves the membrane free-floating, allowing it to be used as a shadow mask. The substrate can be etched using, for example, potassium hydroxide.

[0010] Patterned evaporation can be performed through a shadow mask. The microdisplay substrate is placed near or in contact with the shadow mask. This setup can be brought into an evaporation system to evaporate the material. After evaporation, there will be patterned material on the substrate. This is shown in Figure 2.

[0011] dPd technology presents two main challenges. First, dPd masks must be fabricated as flat as possible. Traditionally, silicon (Si) wafers have been used as the frame material (see Figure 3). A SiN (silicon nitride) film is deposited, followed by high-resolution patterns. See Figure 4, which shows a typical 1 μm SiN mask for the dPd process. However, due to the limited rigidity of Si wafers (warpage of up to 35 μm for 0.7 mm of Si is typical in the integrated circuit (IC) industry), significant warpage and bending remain after dPd mask fabrication. As a result, the warpage of Si-based dPd masks can be as high as 30–40 μm (see Figure 5, which shows an example of the warpage of a dPd mask measured across an 8-inch wafer with a SiN film on a Si frame). Table 1 below shows an example of the warpage of a dPd mask across an 8-inch wafer with a silicon nitride film on a Si frame at points 1–4 in Figure 5.

[0012] [Table 1]

[0013] This high mask warpage creates a large gap between the mask and wafer during organic deposition, potentially resulting in undesirable feathering during lateral deposition. Deposited material tends to spread laterally after passing through the shadow mask, a phenomenon known as "feathering." Feathering increases with the magnitude of the separation between the substrate and the shadow mask. To mitigate feathering, this separation is kept as small as possible without compromising the integrity of the chucks holding the substrate and shadow mask. Furthermore, non-uniformity in this separation across the deposition area results in variable amounts of feathering. Such non-uniformity can arise, for example, from a lack of parallelism between the substrate and the shadow mask, or from curvature or sagging of either or both the substrate and the shadow mask. Furthermore, the shadow mask must be supported only at its periphery to avoid blocking the passage of evaporated atoms through the through-hole pattern. As a result, the center of the shadow mask can sag due to gravity, further exacerbating the feathering problem. See Figure 6, which shows an example of the calculated feathering distance for two deposition angles as the wafer-mask gap varies from 1 to 10 micrometers.

[0014] The second challenge concerns the manufacturability of the substrate: to integrate both the SiN membrane and the rigid substrate to fabricate the dPd mask, a process must be designed that is suitable for substrate etching, chemical compatibility, etc. The substrate properties and process integration must be taken into account. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] U.S. Patent No. 9,385,323 Summary of the Invention [Means for solving the problem]

[0016] The present invention relates to a direct patterning deposition mask for OLED deposition, which includes a sapphire substrate and a silicon nitride (SiN) film. The thickness of the sapphire substrate can be, for example, between 0.7 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 substrate warpage can be limited to, for example, less than 10 μm. This mask improves feathering of OLED pixel deposition and OLED performance.

[0017] A process for etching a sapphire substrate is also provided, which includes at least two of the steps of mechanical drilling, wet etching, dry etching, and laser-induced etching, plus wet etching. [Brief explanation of the drawings]

[0018] [Figure 1] An example of the major manufacturing steps for a prior art silicon nitride film includes: (1) silicon wafer, (2) silicon nitride deposition, (3) backside lithography, (4) frontside lithography, and (5) through-wafer etching from the backside. [Figure 2] FIG. 1 is a simplified schematic showing deposition through a shadow mask. [Figure 3] FIG. 1 is a top view of a typical prior art 1μ SiN mask for a dPd process. [Figure 4] FIG. 1 is a simplified diagram illustrating a prior art example of a cross section of a SiN mask. [Figure 5] (Above) Simplified diagram of an example of dPd mask warpage measured across an 8-inch wafer with a SiN membrane on a Si frame, as shown in Table 1. [Figure 6] 1 is a graphical depiction of an example of calculated feathering distances for two deposition angles as the wafer-to-mask gap varies from 1 to 10 μm. [Figure 7]FIG. 1 is a simplified diagram of the major fabrication steps for silicon nitride films, including: (1) sapphire wafer, (2) silicon nitride deposition, (3) backside lithography, (4) frontside lithography, and (5) wet etching through the sapphire wafer from the backside. [Figure 8] 1 shows simplified steps for an example process for fabricating a sapphire-based SiN mask. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention relates to a direct patterning deposition mask for OLED deposition. The mask includes a sapphire substrate and a silicon nitride (SiN) film. To reduce mask warpage, the present invention uses sapphire as the base material for SiN deposition and patterning. See FIG. 7, which shows a method for fabricating sapphire wafers as a dPd mask base material. Sapphire wafers, with their very good rigidity, have been widely used in the LED industry. Sapphire wafers have a Young's modulus approximately twice as high as that of Si wafers (as shown in Table 2 below, which shows the properties of sapphire and silicon compared to silicon nitride, diamond, and Invar).

[0020] [Table 2]

[0021] Based on the studies shown in Table 3 (below) which shows an example of silicon wafer warpage compared to Table 4 (below) which shows sapphire wafer warpage, the warpage of 1.3 mm thick sapphire can be controlled to less than 8 μm.

[0022] [Table 3]

[0023] [Table 4]

[0024] However, for sapphire, typical dry etches only provide etch rates of nm(s) / min. Essentially, this means that it would take two to three weeks to complete the etching of a single wafer, which is impractical. Instead, newly developed high-temperature wet etches can provide etch rates of um(s) / min, thereby reducing wafer etching times to one day or less.

[0025] In the past, etching baths had a limit of 190°C. The etching rate of sapphire increases exponentially with temperature. Etching baths that achieve temperatures of 300°C are desirable.

[0026] During wet etching at relatively high temperatures, such as 300°C, a SiN masked wafer is placed in a high-temperature process tank with an etching and buffer mixture. Prior to immersion, a silicon dioxide mask is added onto the sapphire substrate by a plasma-enhanced chemical vapor process, and the desired pattern is lithographically exposed. The mixture is at a temperature of, for example, 260 to 300°C.

[0027] White Knight's Accubath™ quartz tank and specially designed automated station make sapphire wet etching safe, reliable, and suitable for high-volume production. See https: / / wkfluidhandling.com / resources / sapphire-etching / .

[0028] High temperature wet etching processes offer advantages over dry etching in terms of speed, cost, and scalability.

[0029] In the present invention, the thickness of the sapphire substrate is preferably between 0.7 and 2 mm. The sapphire substrate preferably has a diameter ranging from 200 mm to 300 mm. The warpage of the substrate is preferably less than 10 μm.

[0030] According to another exemplary embodiment of the present invention, the known selective laser-induced etching (SLE) can be used in a two-step process. In the first step, the laser irradiation internally modifies the sapphire to make it more chemically etchable. To prevent crack formation in this brittle material, short pulse durations (fs-ps) and small focal volumes (a few μm) are used. 3 During laser modification, the crystallinity of the sapphire is reduced, for example, from crystalline to amorphous. In a second step, the modified sapphire is removed by wet etching, for example, potassium hydroxide (KOH) etching.

[0031] In the first step, ultrashort pulsed laser radiation is focused onto a specific volume of the substrate. The pulse energy is absorbed exclusively in the focal volume through a multiphoton process. This process modifies the substrate without cracking it, thereby changing its chemical properties. In this way, materials can be selectively chemically etched.

[0032] Additionally, for etching sapphire, a combination of several etching methods can be used, such as mechanical drilling, laser processing, KOH etching, high temperature wet etching (as described above), Cl2-based inductively coupled plasma (ICP) etching, and Cl2, BCl3, ICP, reactive ion etching (RIE), 20C etching. Table 5 below shows a comparison of several sapphire thinning and etching methods.

[0033] [Table 5]

[0034] Figure 8 shows an example process for fabricating a sapphire-based SiN mask. The process begins with a sapphire substrate with a SiN membrane. A pattern is placed on the SiN membrane by one or more of mechanical drilling, wet etching, dry etching, selective and laser-induced etching plus wet etching. Photoresist is applied to the substrate, and the sapphire on the surface of the membrane opposite the pattern is removed (mechanically thinned)—for example, 0.8 to 1.3 mm of sapphire is removed—and then the remaining 0.5 mm of sapphire is removed by laser treatment plus wet etching.

[0035] It should be understood that this disclosure teaches only one example of an illustrative embodiment, and that many variations of the invention can be readily devised by those skilled in the art after reading this disclosure, and that the scope of the invention is to be determined by the claims which follow.

Claims

1. 1. A direct patterning deposition mask for OLED deposition, comprising: (a) a sapphire substrate; (b) a silicon nitride (SiN) film; A direct patterning deposition mask comprising:

2. 10. The direct patterning deposition mask of claim 1, wherein the thickness of the sapphire substrate is between 0.7 and 2 mm.

3. 10. The direct patterning deposition mask of claim 1, wherein the sapphire substrate has a diameter ranging from 200 mm to 300 mm.

4. The direct patterning deposition mask of claim 1 , wherein the substrate has a warpage of less than 10 um.

5. (a) mechanical drilling; (b) wet etching; (c) dry etching, and (d) laser-induced etching, plus wet etching; 1. A process for etching a sapphire substrate, comprising:

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