Direct patterned deposition mask for rigid sapphire substrates

Abstract

The present disclosure provides a direct patterned deposition mask for OLED deposition, wherein the mask comprises: a sapphire substrate; and a silicon nitride (SiN) film. The thickness of the sapphire substrate can be between 0.7 mm and 2 mm. The diameter of the sapphire substrate may be in the range of 200 mm diameter to 300 mm diameter. The warpage of the substrate is preferably less than < 10 [mu] m.

Description

[0001] CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the pending U.S. Provisional Patent Application No. 63 / 403,964, filed on September 6, 2022, entitled “Rigid Sapphire Based Direct Patterning Deposition Mask.” Background Art

[0003] The present application is directed to direct patterned deposition (dPd). More particularly, the present invention is directed to dPd technology in displays.

[0004] Deposition of a shadow mask base is a process by 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 referred to as "direct patterning".

[0005] In a typical shadow mask deposition process, the desired material is vaporized at a source located at a distance from the substrate, with a shadow mask located 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 shadow mask located just in front of the substrate surface. The perforations (i.e., orifices) are configured in the desired pattern of the material on the substrate. Thus, the shadow mask blocks the passage of all vaporized atoms except those that pass through the perforations, which are deposited on the substrate surface in the form of the desired pattern. The deposition of the shadow mask base is similar to the silk screen technique used to form patterns (e.g., uniform numbers, etc.) on clothing or stencil printing used to develop artwork.

[0006] Deposition of shadow mask substrates has been used for many years in the integrated circuit (IC) industry to deposit patterns of materials on substrates, in part due to the fact that it avoids the need to pattern the material layer after it has been deposited. Thus, deposition using a shadow mask substrate eliminates the need to expose the deposited material to harsh chemicals (e.g., acid-based etchants, corrosive photolithography development chemicals, etc.) in order to pattern it. In addition, deposition of a shadow mask substrate requires less handling and processing of the substrate, thereby reducing the risk of substrate breakage and increasing manufacturing yields. Furthermore, many materials, such as organic materials, cannot survive exposure to photolithography chemicals without being damaged, which necessitates the deposition of such materials through a shadow mask.

[0007] High quality dPd masks are a key fixture for dPd manufacturing, especially for OLED microdisplays.

[0008] By using direct patterning of OLEDs using stencil lithography, efficient, high-resolution OLED microdisplays can be made. The deposition of color emitters for OLEDs uses a shadow mask that can have nanoscale features. The shadow mask has the precision and accuracy to match the underlying transistors of the microdisplay and forms the color emitters at higher resolution.

[0009] As can be seen in FIG. 1 , as is known, a flat substrate (e.g., a silicon wafer) is used to construct a shadow mask. A thin film (e.g., silicon nitride) is deposited on both sides of the substrate using chemical vapor deposition (CVD). This silicon nitride layer can act as an etching barrier on one side and a suspended diaphragm 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 subsequent through-substrate etching processes. For example, silicon nitride can be patterned using photolithography and dry etched to remove the silicon nitride. The other side of the thin film is patterned using photolithography and etched to form the desired shadow mask pattern. Again, this other side can use photolithography and dry etching. Of course, other patterning methods can be used. U.S. Pat. No. 9,385,323 (Chan et al.) describes this prior art process in detail.

[0010] Through-substrate etching leaves the thin film hanging freely, which enables the film to be used as a shadow mask.The substrate can be etched using, for example, potassium hydroxide.

[0011] Patterned evaporation can be performed through a shadow mask. The microdisplay substrate is placed close to or in contact with the shadow mask. The setup can be introduced into a deposition system to evaporate the material. After evaporation, there will be patterned material on the substrate. This is illustrated in Figure 2.

[0012] There are two main challenges with dPd technology. First, the dPd mask must be made as flat as possible. Conventionally, silicon (Si) wafers have been used as frame materials. See Figure 3. A SiN (silicon nitride) film is deposited and then a high-resolution pattern is made. See Figure 4, which depicts a typical 1μm SiN mask for a dPd process. However, due to the limited rigidity of the Si wafer (up to 35μm warpage, because 0.7mm Si is typical in the integrated circuit (IC) industry), significant warpage and curvature remain after the dPd mask is made. Therefore, the dPd mask warpage of the Si substrate can be as high as 30μm to 40μm (see Figure 5, which depicts an example of the dPd mask warpage measured across an 8-inch wafer, where the SiN diaphragm is located on top of the Si frame). Table 1 below shows an example of the dPd mask warpage across an 8-inch wafer, where the silicon nitride diaphragm is located on top of the Si frame at points 1 to 4 in Figure 5:

[0013] Location Mask 1(RG)μm Mask 2 (B) μm 1 38.8 30.4 2 39.7 32 3 36.7 30.67 4 38.8 33

[0014] Table 1

[0015] This high mask warp can create a large gap between the mask and the wafer during organic deposition and cause undesirable feathering in lateral deposition. The deposited material tends to expand laterally after passing through the shadow mask (known as "feathering"). Feathering increases with the amount of 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 chuck holding the substrate and shadow mask. Furthermore, any non-uniformity in this separation across the deposition area will cause variations in the amount of feathering. Such non-uniformity may arise from, for example, a lack of parallelism between the substrate and the shadow mask, bending or sagging of one or both of the substrate and the shadow mask, and the like. In addition, the shadow mask must be supported only at its periphery to avoid blocking the passage of vaporized atoms to the perforation pattern. As a result, the center of the shadow mask may sag due to gravity, which further exacerbates the feathering problem. See. Figure 6 , which depicts examples of calculated feathering distances for two deposition angles as the wafer-to-mask gap varies from 1 micron to 10 microns.

[0016] The second challenge involves the manufacturability of the substrate. In order to integrate both the SiN membrane and the rigid substrate together to make a dPd mask, a suitable process should be designed for substrate etching, chemical compatibility, etc. Substrate properties and process integration should be considered. Summary of the invention

[0017] The present invention is directed to a direct patterned deposition mask for OLED deposition, wherein the mask comprises a sapphire substrate and a silicon nitride (SiN) membrane. The sapphire substrate thickness may be, for example, between 0.7 mm and 2 mm. The sapphire substrate (wafer) diameter may be, for example, 200 mm diameter or 300 mm diameter. The sapphire wafer patterning process is preferably compatible with the SiN membrane process. The warpage of the substrate may be limited to, for example, less than 10 um. The mask improves OLED pixel deposition feathering and OLED performance.

[0018] The present disclosure 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 plus wet etching. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 1 is an example of the major fabrication steps for a prior art silicon nitride diaphragm, which include (1) silicon wafer; (2) silicon nitride deposition; (3) backside photolithography; (4) frontside photolithography; and (5) etching through the wafer from the backside.

[0020] FIG. 2 is a simplified diagram illustrating deposition through a shadow mask.

[0021] 3 is a top plan view of a typical prior art 1μ SiN mask for a dPd process.

[0022] 4 is a simplified diagram illustrating a prior art example of a SiN mask cross section.

[0023] 5 is a simplified view of an example of dPd quality warpage measured across an 8 inch wafer with a SiN membrane on top of a Si frame, as shown in Table 1 (above).

[0024] Figure 6 is a graphical depiction of an example of calculated feathering distance for two deposition angles as the wafer-to-mask gap varies from 1 μm to 10 μm.

[0025] Figure 7 is a simplified view of the major fabrication steps for a silicon nitride diaphragm, which include (1) sapphire wafer; (2) silicon nitride deposition; (3) backside photolithography; (4) frontside photolithography; and (5) wet etching through the sapphire wafer from the backside.

[0026] Figure 8 Simplified steps depicting an example of a process for making a SiN mask for a sapphire substrate. DETAILED DESCRIPTION

[0027] The present invention is directed to a direct patterned deposition mask for OLED deposition. The mask comprises a sapphire substrate and a silicon nitride (SiN) membrane. In order to reduce the warpage of the mask, the present invention is directed to using sapphire as a base material for SiN deposition and patterning. See Figure 7 , which describes a method for making sapphire wafers into dPd mask substrate materials. Sapphire wafers, which have excellent rigidity, have been widely used in the LED industry. The Young's modulus of sapphire wafers is about twice as high as that of Si wafers (as shown in Table 2 below, which shows a comparison of sapphire properties and silicon properties with silicon nitride, diamond, and invar steel).

[0028]

[0029] Table 2

[0030] Based on investigations presented in Table 3 (below), which depicts an example of silicon wafer warpage, as compared to Table 4 (below), which depicts sapphire wafer warpage, 1.3 mm thick sapphire warpage can be controlled to < 8 μm.

[0031]

[0032] Table 3

[0033]

[0034]

[0035] Table 4

[0036] However, for sapphire, typical dry etching only gives an etching rate of nm(s) / min. Basically, this means that a cycle of 2 to 3 weeks is required to complete the etching of one wafer, which is impractical. Alternatively, the newly developed high-temperature wet etching can give an etching rate of um(s) / min, which reduces the etching time of a wafer to 1 day or less.

[0037] In the past, there has been a 190°C limit for etching baths. Sapphire etch rates increase geometrically with temperature. It would be desirable to achieve an etching bath temperature of 300 degrees.

[0038] During wet etching at relatively high temperatures (e.g., 300 degrees), the wafer masked with SiN is placed in a high-temperature processing tank with a mixture of etchant and buffer. Before immersion, a plasma-enhanced chemical vapor process adds a silicon dioxide mask to the sapphire substrate, and photolithography exposes the desired pattern. The mixture is at a temperature of, for example, 260°C to 300°C.

[0039] White Knight's Accubath TM Quartz tanks and specially designed automated stations make sapphire wet etching safe, reliable, and suitable for high-volume manufacturing. See https: / / wkfluidhandling.com / resources / sapphire-etching / .

[0040] High temperature wet etching processes have advantages over dry etching in terms of speed, cost, and scalability.

[0041] In the present invention, the thickness of the sapphire substrate is preferably between 0.7 mm and 2 mm. The diameter of the sapphire substrate is preferably in the range of 200 mm diameter to 300 mm diameter. The warpage of the substrate is preferably <10 um.

[0042] According to another exemplary embodiment of the invention, as is known, selective laser induced etching (SLE) can be used in a two-step process. In the first step, the sapphire is modified internally by laser irradiation to improve chemical etchability. To prevent crack formation in the brittle material, short pulse durations (fs-ps) and small focal volumes (several μm) are used. 3 During the laser modification, the crystallinity of the sapphire is degraded (eg, from crystalline to amorphous). In a second step, the modified sapphire is removed by wet etching, for example, using potassium hydroxide (KOH).

[0043] In the first step, ultrashort pulsed laser radiation is focused into a certain volume of the substrate. The pulse energy is absorbed only into the focal volume based on a multiphoton process. The process modifies the substrate without cracking it, thereby changing the chemical properties of the substrate. In this way, materials can be selectively chemically etched.

[0044] In addition, a combination of several etching methods can be used with respect to sapphire etching. For example, mechanical drilling, laser processing, KOH etching, high temperature wet etching (described above), Cl-based 2 Inductively coupled plasma (ICP) etching and Cl 2 , BCl 3 , ICP reactive ion etching (RIE), 20C etching. Table 5 below shows a comparison of several sapphire thinning and etching methods.

[0045]

[0046] Figure 8 An example of a process for making a SiN mask for a sapphire substrate is depicted. The process starts with a sapphire substrate with a SiN diaphragm. A pattern is placed on the SiN diaphragm by one or more of mechanical drilling, wet etching, dry etching, selective and laser induced etching wet etching. A photoresist is applied to the substrate, the sapphire is removed from the surface of the diaphragm opposite the pattern (mechanical thinning) (e.g., 0.8 mm from 1.3 mm sapphire), and then laser treatment wet etching is performed to remove the remaining 0.5 mm of sapphire.

[0047] It should be understood that the present disclosure teaches only one example of illustrative embodiments and that those skilled in the art may readily conceive of many variations of the invention after reading this disclosure and that the scope of the invention is determined by the following claims.

Claims

1. A direct patterning deposition mask for OLED deposition, the mask include: (a) Sapphire substrate; and (b) Silicon nitride (SiN) membrane.

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

3. The direct patterned deposition mask of claim 1, wherein the diameter of the sapphire substrate is in the range of 200 mm diameter to 300 mm diameter. The direct patterning deposition mask according to claim 1 , wherein the warpage of the substrate is less than 10 um.

5. A process for etching a sapphire substrate, comprising at least two of the following steps: (a) Mechanical drilling; (b) wet etching; (c) dry etching; and (d) Laser induced etching wet etching.

Images