Evaporation system with improved collimation

Abstract

A system for depositing an evaporation material on a substrate is provided. The substrate has a central axis. The system includes an evaporation vacuum chamber, at least one nozzle assembly, and a shadow mask. The nozzle assembly has a plurality of point evaporation sources disposed adjacent to and spaced from the central axis of the substrate, whereby the nozzle assembly provides molecules of the evaporation material that reach the substrate at an incident angle of 5 degrees or less.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims priority to U.S. Provisional Patent Application No. 63 / 395,085, entitled "Nearly Collimated Evaporation System", filed on August 4, 2022, and U.S. Provisional Patent Application No. 63 / 418,393, entitled "Evaporation Chamber with Central Multiple Sources with Improved Collimation", filed on October 21, 2022.

Background Art

[0002] The present invention relates to substrate processing. More particularly, the present invention relates to high - resolution, high - precision substrate processing using a shadow mask to manufacture patterned organic light - emitting diode (OLED) microdisplays.

[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 referred to as "direct patterning".

[0004] In a typical shadow mask deposition process, the desired materials are vaporized at a source located at a predetermined distance from the substrate using a shadow mask positioned between them. As the vaporized atoms of the materials move towards the substrate, they pass through a set of through-holes within the shadow mask located just in front of the substrate surface. The through-holes (i.e., apertures) are arranged in the desired pattern for the material on the substrate. As a result, the shadow mask blocks the passage of all vaporized atoms except those that pass through the through-holes to deposit on the substrate surface in the desired pattern. Shadow mask-based deposition is similar to silk screening techniques used to form patterns (e.g., uniform numbers, etc.) on clothing or stencil printing used to develop artwork.

[0005] Shadow mask-based deposition has been used in the integrated circuit (IC) industry for many years to deposit a pattern of material on a substrate, in part because there is no need to pattern the material layer after deposition. As a result, using this method eliminates the need to expose the deposited material to strong chemicals (e.g., acidic etching agents, caustic photolithography development chemicals, etc.) to pattern it. In addition, shadow mask-based deposition requires less handling and processing of the substrate, thereby reducing the risk of substrate breakage and improving manufacturing yield. Further, many materials, such as organic materials, cannot be exposed to photolithography chemicals without damaging them, and require such materials to be deposited by a shadow mask.

[0006] Unfortunately, the feature resolution achievable by conventional shadow mask deposition is reduced due to the fact that the deposited material tends to spread laterally after passing through the shadow mask (referred to as "feathering"). Feathering increases with the magnitude of the separation between the substrate and the shadow mask. To mitigate feathering, this separation is maintained as small as possible without compromising the integrity of the chucks holding the substrate and the shadow mask. Furthermore, the non-uniformity of this separation across the deposition area causes variations in the amount of feathering. Such non-uniformities can arise, for example, from a lack of parallelism between the substrate and the shadow mask, bowing or sagging of one or both of the substrate and the shadow mask, etc.

[0007] Organic light-emitting diode (OLED) displays, as described above, can be manufactured by a number of methods including inkjet printing and vacuum evaporation through a shadow mask. The former method is widely used in the manufacture of large displays suitable for television screens. The second method using a shadow mask is well-suited for small-format high-resolution microdisplays. A typical OLED stack is a multilayer structure disposed between an anode and a cathode and is composed of at least one of each functional layer. Examples of functional layers can include, but are not limited to, a hole injection layer, a hole transport layer, an emitter layer, an electron transport layer, and an electron injection layer. In the case of tandem devices, the number of functional layers increases in proportion to the number of tandem units. The emitter layer may consist of a mixture of two or three materials including a host material and a dopant material. Therefore, in the manufacture of OLED microdisplays, it is necessary to deposit multiple materials on a silicon wafer, and for the emitter layer, it is necessary to simultaneously deposit the host material and the dopant material in a single layer to achieve complete mixing at the molecular level.

[0008] To deposit two or several materials, such as a host / (multiple) hosts and a dopant / (multiple) dopants, simultaneously, there are two methods: evaporating from two or more point sources and evaporating from two or more linear sources. In a prior art evaporation chamber, point sources are typically located on the circumference of the chamber away from the central axis of the wafer where deposition is performed (see Figure 1A). In the example of Figure 1A, this can result in a limited overlap of the two material plumes. As shown in the example of Figure 1B, by evaporation from two linear sources horizontally traversing beneath the wafer, the materials are deposited sequentially, and thus, the mixing of the two materials at the molecular scale may be insufficient.

[0009] In addition, deposition from a point source positioned away from the wafer central axis can cause another problem related to the use of a high-resolution shadow mask. The resolution of microdisplays is rapidly improving, and more advanced methods for patterning sub-pixels with a size of less than 3.5 microns in the shortest dimension are becoming available. In the near future, sub-pixel dimensions of less than 2 microns are expected. Recent success in the direct patterning of OLED microdisplays formed by arranging red, green, and blue sub-pixels is due to the development of advanced high-resolution shadow masks with apertures on the scale of several microns.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

[0011] The implementation of such a high-resolution shadow mask used for direct patterning of OLEDs faces many problems. Among them is the existence of a small gap between the mask and the wafer, which can cause the "feathering" (described above) when the evaporated material reaching the wafer at an angle spreads beyond the area outlined by the sub-pixels (see FIG. 2). In the case of a microdisplay with a high fill-factor, the open space between two adjacent sub-pixels can be on the order of 1 micrometer or less. Therefore, in such a configuration, it is necessary to minimize the spread of the deposited material. From a practical point of view, feathering is not allowed if it exceeds half the distance between two adjacent sub-pixels. For example, if the distance between two adjacent sub-pixels is on the order of 1 micrometer, the feathering distance must be less than half of 1 micrometer. FIG. 3 shows a graph of the feathering distance versus the mask-wafer gap varying between 1 and 10 micrometers for two incident angles of 5 degrees and 10 degrees respectively. In this example, the threshold for feathering is set at 0.5 micrometer. When the incident angle is 5 degrees, the gap between the mask and the wafer must be 6 micrometers or less. Correspondingly, when the incident angle is 10 degrees, the gap cannot be greater than 3 micrometers.

[0012] As shown in FIG. 1A, in deposition from a point source, the incident angle is higher than the angular threshold of 5 degrees, resulting in an unacceptable level of feathering. MEANS FOR SOLVING THE PROBLEM

[0013] The present invention relates to a system for depositing an evaporation material on a substrate. The substrate has a central axis. The system includes an evaporation vacuum chamber, at least one nozzle assembly, and a shadow mask. The nozzle assembly has a plurality of point evaporation sources disposed away from the substrate adjacent to the central axis of the substrate, whereby the nozzle assembly provides molecules of the evaporation material that reach the substrate at an incident angle of 5 degrees or less.

[0014] The substrate has a diameter of 200 mm, and the irradiation distance between each of the plurality of evaporation sources and the substrate is 1200 mm or more. The nozzle assembly can provide an overlap of a plurality of evaporation material plumes originating from a point located near the central axis of the substrate. The point evaporation sources may be separated from each other by a plurality of similar actively water-cooled partitions in order to reduce thermal crosstalk between the sources. The water-cooled partitions may have built-in channels for propagating a flow of water supplied from an external chiller unit. The water-cooled partitions may be configured to maintain the temperature at about 20°C to 30°C. Each of the plurality of point evaporation sources can include a plurality of nozzles in a bundle that are close to each other and equidistant from the substrate in order to provide good mixing of the evaporation material at the molecular level by combining individual material plumes into a single plume. For example, there may be 3 or 4 bundles of point evaporation sources.

[0015] Each bundle may be disposed on one of a plurality of swing arms. Each swing arm may be driven by a step motor for positioning the bundle adjacent to the central axis of the substrate. The step motor is disposed outside the vacuum chamber and is operated using a belt drive via a seal.

Brief Description of the Drawings

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Figure 1A

Figure 1B

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DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to high-resolution, high-precision substrate processing using a shadow mask, for example, to manufacture a patterned organic light-emitting diode (OLED) microdisplay. Shadow mask-based (i.e., direct pattern) deposition is a process in which a layer of material is deposited on the surface of a substrate such that the desired pattern of the layer is defined during the deposition process itself.

[0018] The desired material is vaporized at a source located at a predetermined distance from the substrate using a shadow mask positioned therebetween. As the vaporized atoms of the material move toward the substrate, they pass through a set of through-holes within the shadow mask positioned immediately in front of the substrate surface. The through-holes (i.e., apertures) are arranged in a desired pattern with respect to the material on the substrate. As a result, the shadow mask blocks the passage of all vaporized atoms except those passing through the through-holes that deposit on the substrate surface in the desired pattern.

[0019] Another factor that affects the angle of incidence is the irradiation distance, i.e., the distance H between the evaporation source nozzle and the wafer. Refer to FIG. 4A. This is a graph of the angle of incidence at the wafer edge as a function of the irradiation distance and the substrate radius. The dependence of the angle of incidence α at the edge of a 200 mm wafer on the irradiation distance is plotted in FIG. 4B. Since the molecules move linearly in a low vacuum, the irradiation distance must be greater than 1135 mm in order to achieve an angle of less than 5 degrees at the wafer edge (all other positions on the substrate away from the edge have a smaller deposition angle). Fortunately, -6 the mean free path of the molecules at a pressure of 10 -7 Torr or 10

[0020] Therefore, to achieve an angle of 5 degrees or less at the wafer edge (all positions on the wafer away from the edge have even smaller angles), an irradiation distance of more than about 1200 mm is required. Another advantage of placing the evaporation source quite far from the wafer is to reduce the exposure of the wafer to the radiant heat generated from the evaporation source. When the heat source is close to the wafer, due to the thermal expansion process, the radiant heat may inadvertently affect the alignment of the high-resolution shadow mask with respect to the wafer. This is eliminated by positioning the source at a long irradiation distance.

[0021] Next, referring to the drawings, FIG. 5 shows a system 10 for depositing an evaporation material on a substrate (wafer) 12 according to an exemplary embodiment of the present invention. Throughout several figures, like reference numerals refer to like elements. System 10 includes an evaporation vacuum chamber 14, a nozzle assembly 16 having a plurality of collimated point evaporation sources 18a, 18b, 18c, and a shadow mask 20.

[0022] As shown, a solution to the problem of achieving complete overlap of the plumes generated from several point sources according to the present invention is to place the point evaporation sources 18a, 18b, 18c at or near the center of the evaporation vacuum chamber 14, i.e., on the central axis X of the substrate 12. For the purposes of the present invention, the term "central axis" generally means on or near the central axis of the substrate. In addition, according to the present invention, to form a small angle of incidence of the deposited material on the wafer suitable for a gap of several micrometers between the mask and the wafer, the point evaporation sources 18a, 18b, 18c are arranged at an irradiation distance of more than 1200 mm with respect to a substrate having a diameter of 200 mm.

[0023] This can be achieved by implementing collimated point evaporation sources 18a, 18b, 18c with small orifice nozzles 21a, 21b, 21c as shown in FIG. 6. By offsetting nozzles 21a, 21b, 21c from the central axis X of each crucible 22a, 22b, 22c, the nozzle orifices can be brought very close to each other. In this way, by assembling the three point evaporation sources 18a, 18b, 18c into a single bundle 23 and combining the individual material plumes into a single plume, the evaporation materials can be very well mixed at the molecular level (see FIG. 5). This is important, for example, when combining two host materials and one dopant material, or vice versa, i.e., when combining one host material and two dopant materials.

[0024] The assembly of the invention of the three point evaporation sources 18a, 18b, 18c as shown in FIG. 5 includes three nozzles 21a, 21b, 21c, three crucibles (i.e., material containers) 22a, 22b, 22c having heaters 24 and thermocouples 26 (shown only for one point evaporation source) for controlling the temperature of the evaporation material in each crucible, three upper nozzle heaters 25, and three lower nozzle heaters 27 each provided with a thermocouple for monitoring and controlling the temperature of the nozzles up to 400° C., and three active water-cooled partitions 28a, 28b, 28c for preventing thermal cross-talk. See FIG. 6A.

[0025] FIG. 7 shows a schematic view of a bundle 23 of three point evaporation sources 18a, 18b, 18c on a swing arm 30 driven by a stepper motor 32. The swing arm 30 includes a wire conduit 34 that enables electrical and thermocouple connections to the point evaporation sources 18a, 18b, 18c. Also, the swing arm 30 is provided with a water pipe 35 connected to the water-cooled partition of the source bundle. The stepper motor 32 is disposed outside the vacuum chamber 14 (not shown in FIG. 7) and operates the movement of the swing arm 30 through a seal 36, e.g., a Ferrotec® seal, through the feed-through separation vacuum chamber side of the assembly.

[0026] FIG. 8 shows a schematic view of four bundles 23 of point evaporation sources 18a, 18b, 18c, 18d on four swing arms 30, each bundle being movable from a stop position near the wall (shown for point evaporation sources 18a, 18b, 18c) to the central axis X of the substrate 12 where evaporation of three materials is carried out (point evaporation source 18d is shown as moving in the Y direction towards the central axis). In use, one of the four bundles is moved to the position of the central axis X.

[0027] FIG. 9 shows a bottom view of a system 10 having a flange 37 consisting of an electrical feedthrough, four stepper motors 32, and a belt drive 38 for carrying out the movement of the swing arm 30 within the vacuum chamber 14.

[0028] FIG. 10 shows three point evaporation sources 18a, 18b, 18c located below three crystal sensor head assemblies 40, each including twelve crystal sensors for setting and monitoring the evaporation rate.

[0029] FIG. 11 shows a vacuum chamber 14 having a removable central chimney 42 that functions as a shield from the evaporation plume reaching the crystal sensor head assembly 44. The crystal sensor head assembly 44 includes a focusing tube 46 aligned with the orifices of the point evaporation sources 18a, 18b, 18.

[0030] It should be understood that this disclosure teaches only one example of an exemplary embodiment, 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 should be determined by the following claims.

Claims

1. A system for depositing an evaporation material on a substrate, wherein the substrate has a central axis, and the system comprises: (a) an evaporation vacuum chamber; and (b) at least one nozzle assembly having a plurality of point evaporation sources, the point evaporation sources being adjacent to and spaced apart from the central axis of the substrate, whereby the nozzle assembly provides molecules of the evaporation material reaching the substrate at an incident angle of 5 degrees or less; and (c) a shadow mask disposed adjacent to the substrate. A system for depositing an evaporation material on a substrate, comprising the above components.

2. The system for depositing an evaporation material on a substrate according to claim 1, wherein the substrate has a diameter of 200 mm and the irradiation distance between each of the plurality of evaporation sources and the substrate is 1200 mm or more.

3. The system for depositing an evaporation material on a substrate according to claim 1, wherein the nozzle assembly provides an overlap of a plurality of evaporation material plumes originating from a point located near the central axis of the substrate.

4. The system for depositing an evaporation material on a substrate according to claim 1, wherein the point evaporation sources are separated from each other by a plurality of similarly actively water-cooled partitions to reduce thermal crosstalk between the sources.

5. The system for depositing an evaporation material on a substrate according to claim 4, wherein the water-cooled partition has a built-in channel for propagating a flow of water supplied from an external cooling unit.

6. The system for depositing an evaporation material on a substrate according to claim 5, wherein the water-cooled partition is configured to maintain the temperature at about 20°C to 30°C.

7. The system for depositing an evaporation material on a substrate according to claim 1, wherein each of the plurality of point evaporation sources is provided with a plurality of nozzles in a bundle that are close to each other and equidistant from the substrate to provide good mixing of the evaporation material at the molecular level by combining individual material plumes into a single plume.

8. The system for depositing an evaporation material on a substrate according to claim 7, comprising three bundles of the point evaporation sources.

9. The system for depositing an evaporation material on a substrate according to claim 7, comprising four bundles of the point evaporation sources.

10. The system for depositing an evaporation material on a substrate according to claim 7, wherein each bundle is disposed on one of a plurality of swing arms. **Claim 11** The system for depositing an evaporation material on a substrate according to claim 10, wherein each swing arm is driven by a stepper motor to position the bundle adjacent to the central axis of the substrate. **Claim 12** The system for depositing an evaporation material on a substrate according to claim 11, wherein the stepper motor is disposed outside the vacuum chamber and is operated using a belt drive via a seal.

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