Evaporation system with improved collimation

Abstract

A system for depositing vapor deposition material on a substrate is provided. The substrate has a central axis. The system includes a vapor deposition vacuum chamber, at least one nozzle assembly, and a shadow mask. The nozzle assembly has a three-point multiple-point vapor deposition source disposed adjacent to a central axis of the substrate and at a distance from the substrate whereby the nozzle assembly provides molecules of vapor deposition material to the substrate at an angle of incidence of less than or equal to 5 degrees.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to pending U.S. Provisional Patent Application No. 63 / 395,085, filed on August 4, 2022, entitled Nearly Collimated Evaporation System, and pending U.S. Provisional Patent Application No. 63 / 418,393, filed on October 21, 2022, entitled Evaporation Chamber with Central Multiple Sources with Improved Collimation. Background Art

[0003] The present invention relates to substrate processing. More particularly, the present invention relates to high resolution, high precision substrate processing using shadow masks to fabricate patterned organic light emitting diode (OLED) microdisplays.

[0004] Shadow mask based deposition is a process by 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. This deposition technique is sometimes referred to as "direct patterning".

[0005] In a typical shadow mask deposition process, the desired material is evaporated at a source positioned at a distance from the substrate, with a shadow mask positioned between them. As the evaporated atoms of the material travel toward the substrate, they pass through a set of through holes in the shadow mask, which is located just in front of the substrate surface. The through holes (i.e., pores) are arranged in the desired pattern for the material on the substrate. Thus, the shadow mask blocks the passage of all evaporated atoms, except those that pass through the through holes, which are deposited on the substrate surface in the desired pattern. Shadow mask-based deposition is similar to screen printing techniques used to form patterns on clothing (e.g., uniform numbering, etc.) or stencil printing used to develop artwork.

[0006] Shadow mask based deposition has been used in the integrated circuit (IC) industry for many years to deposit patterns of materials on substrates, in part because it avoids the need to pattern the material layer after it has been deposited. Thus, the use of shadow mask based deposition eliminates the need to expose the deposited material to harsh chemicals (such as acid-based etchants, corrosive photolithography development chemicals, etc.) in order to pattern it. In addition, shadow mask based deposition requires less substrate handling and processing, thereby reducing the risk of substrate cracking and improving manufacturing yield. In addition, many materials, such as organic materials, cannot withstand photolithography chemicals without damaging them, which makes it necessary to deposit such materials through a shadow mask.

[0007] Unfortunately, the feature resolution achievable with conventional shadow mask deposition is reduced due to the tendency of the deposited material to diffuse laterally after passing through the shadow mask - this is 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 chuck holding the substrate and shadow mask. Still further, any non-uniformity in this separation across the deposition area will cause variations in the amount of feathering. This non-uniformity can be caused by, for example, 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.

[0008] Organic light emitting diode (OLED) displays can be manufactured by a variety of methods, including inkjet printing as discussed above and vacuum deposition through a shadow mask. The former method is widely used to manufacture large displays suitable for television screens. The second method using a shadow mask is very suitable for small high-resolution microdisplays. A typical OLED stack is a multilayer structure positioned between an anode and a cathode and consists of at least one of each functional layer. The functional layer may include, but is 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 a tandem device, the number of functional layers increases proportionally depending on 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, the manufacture of an OLED microdisplay requires the deposition of multiple materials on a silicon wafer, wherein the emitter layer requires the simultaneous deposition of the host material and the dopant material into a single layer to achieve thorough mixing at the molecular level.

[0009] There are two methods to deposit two or several materials simultaneously, such as a host / several hosts and a dopant / several dopants: one method can evaporate from two or several point sources, or can evaporate from two or several linear sources. In prior art evaporation chambers, the point sources are usually positioned on the periphery of the chamber, away from the central axis of the wafer on which deposition occurs (see FIG. 1A ). In the example of FIG. 1A , this can result in limited overlap of the two material plumes. As shown in the example of FIG. 1B , evaporation from two linear sources that pass horizontally under the wafer can result in sequential deposition of the materials and, therefore, less thorough mixing of the two materials at the molecular level.

[0010] In addition, deposition from a point source positioned away from the center axis of the wafer can create another problem associated with the use of high-resolution shadow masks. The resolution of microdisplays is increasing rapidly as more advanced methods can be used to pattern subpixels to sizes less than 3.5 microns in the shortest dimension. Subpixel sizes of 2 microns or even smaller are expected in the near future. Recent success in direct patterning of OLED microdisplays with red, green, and blue subpixels formed side by side can be attributed to the development of advanced high-resolution shadow masks with openings on the scale of several microns.

[0011] The implementation of this high-resolution shadow mask for direct patterning of OLEDs faces many challenges. One of the challenges is that there is a small gap between the mask and the wafer, which causes the evaporated material, which arrives at the wafer at a certain angle, to spread outside the area outlined by the sub-pixel (see Figure 2 ), which can lead to "feathering" (as described above). For a high fill factor microdisplay, the open space between two adjacent sub-pixels can be on the order of one micron or less. In this arrangement, it is therefore necessary to reduce the spread of the deposited material to a minimum. From a practical point of view, feathering cannot be tolerated 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 one micron, then the feathering distance must be half a micron or less. Figure 3 A graph showing the feathering distance compared to the mask-to-wafer gap varying between 1 micron and 10 microns for 2 incident angles of 5 degrees and 10 degrees, respectively. In this example, the feathering threshold is set to 0.5 microns. For an incident angle of 5 degrees, the mask-to-wafer gap must be equal to or less than 6 microns. Correspondingly, for an incident angle of 10 degrees, the gap cannot be greater than 3 microns.

[0012] As shown in Figure 1A, deposition from a point source will result in incident angles above the angular threshold of 5°, causing unacceptable feathering levels. Summary of the invention

[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 adjacent to the central axis of the substrate and at a distance from the substrate, whereby the nozzle assembly provides molecules of the evaporation material to reach the substrate at an incident angle of less than or equal to 5 degrees.

[0014] The substrate may have a diameter of 200 mm, with a throw distance between each of the multiple evaporation sources and the substrate being 1,200 mm or greater. The nozzle assembly may provide an overlap of multiple evaporation material plumes originating from points positioned near the central axis of the substrate. The point evaporation sources may be separated from each other by a plurality of active water-cooled baffles to reduce thermal crosstalk between sources. The water-cooled baffles may have built-in channels to propagate a flow of water delivered from an external cooler unit. The water-cooled baffles may be adapted to maintain a temperature of approximately 20 to 30 degrees Celsius. Each of the multiple point evaporation sources may include a plurality of nozzles in a bundle, which are close to each other and equidistant from the substrate to provide good mixing of the evaporation materials at the molecular level by merging individual material plumes into a single plume. For example, there may be three or four bundles of point evaporation sources.

[0015] Each beam may be disposed on one of a plurality of swing arms. Each swing arm may be driven by a stepper motor for positioning the beam adjacent to the central axis of the substrate. The stepper motor is positioned outside the vacuum chamber with movement performed using a belt drive through a seal. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 1A is a simplified schematic diagram of a prior art combination of two point sources positioned away from the central axis of the wafer, resulting in limited overlap of the two material plumes and also resulting in large angles of incidence of the material molecules reaching the substrate (wafer).

[0017] 1B is a simplified schematic diagram depicting a prior art system for evaporation from two linear sources that pass horizontally beneath a substrate, which results in sequential deposition of the materials and, therefore, less thorough mixing of the two materials at the molecular level.

[0018] Figure 2 is a simplified schematic diagram depicting feathering, ie, undesired deposition of evaporated material in the areas between sub-pixels. Due to the presence of a gap between the shadow mask and the wafer, the evaporated material passes through the shadow mask openings to the wafer at a certain angle.

[0019] Figure 3 Graph depicting examples of feathering distances calculated for two angles when the wafer-to-mask gap is varied between 1 micron and 10 microns.

[0020] Figure 4A A simplified diagram depicting the throw distance, ie, the distance between the evaporation source nozzle and the wafer.

[0021] Figure 4B A graph depicting an example of incident angle at a 200 mm wafer edge versus throw distance H. When the throw distance is > 1,200 mm, angles equal to or less than a threshold angle of 5 degrees may be obtained in the described example.

[0022] Figure 4Cis a graphical illustration of the minimum throw distances for 75 mm, 100 mm, and 300 mm radius substrates for an evaporation system for depositing evaporated material on a substrate at an angle of incidence of less than 5 degrees at the substrate edge.

[0023] Figure 4D is a table showing the required throw distances for deposition angles of 5 degrees or less for an evaporation system for depositing evaporated material on a substrate without the nozzle being positioned at the center axis of the substrate.

[0024] Figure 5 is a simplified schematic diagram of a beam of three point sources with improved collimation, which results in almost complete overlap of the three evaporated material plumes, resulting in thorough mixing at the molecular level, in accordance with an exemplary embodiment of the present invention.

[0025] Figure 6 is an isometric view of a beam of three point sources with improved collimation.

[0026] Fig. 6A is a cross-sectional elevation view of an evaporation system for depositing evaporated material on a substrate having a collimated source nozzle configuration according to an exemplary embodiment of the present invention.

[0027] Figure 7 An isometric view of a beam of three collimated point sources on a swing arm driven by a stepper motor.

[0028] Figure 8 is a simplified top view of four point source beams in a vacuum chamber, which allow twelve materials to be deposited from the chamber center axis by moving each beam toward the center axis using four swing arms driven by four stepper motors.

[0029] Fig. 9 is an isometric bottom view of a vacuum deposition chamber showing the flange with electrical feedthroughs and four stepper motors and belt drives to perform movement of the swing arms within the chamber.

[0030] Fig.10 is a partial isometric side view of a beam of three point sources positioned beneath three crystal sensor head assemblies, each containing twelve crystal sensors to set and monitor evaporation rates.

[0031] Fig.11 is a side elevation view of a vacuum chamber with a removable central duct that acts as a shield for the evaporation plume reaching the crystal head assembly. The crystal head assembly is equipped with a focusing tube aligned with the aperture of the point source. DETAILED DESCRIPTION

[0032] The present invention relates to high resolution, high precision substrate processing using shadow masks to manufacture, for example, patterned organic light emitting diode (OLED) microdisplays. Shadow mask based (i.e. direct pattern) deposition 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.

[0033] The desired material is evaporated at a source positioned at a distance from the substrate, with a shadow mask positioned between them. As the evaporated atoms of the material travel toward the substrate, they pass through a set of through holes in the shadow mask, which is positioned just in front of the substrate surface. The through holes (i.e., apertures) are arranged in a desired pattern for the material on the substrate. Thus, the shadow mask blocks the passage of all evaporated atoms except those that pass through the through holes, which are deposited on the substrate surface in the desired pattern.

[0034] Another factor that affects the incident angle is the throw distance, which is the distance H between the evaporation source nozzle and the wafer. Figure 4A , which is a graphical illustration of the angle of incidence at the edge of the substrate as a function of throw distance and substrate radius. Figure 4B The dependence of the incident angle α on the throw distance at the edge of a 200 mm wafer is plotted in Fig. 1. Since the molecules travel in a straight line in a low vacuum, to get an angle of less than 5 degrees at the edge of the substrate (all other locations on the substrate away from the edge will have a smaller deposition angle), the throw distance must be greater than 1,135 mm. Fortunately, the mean free path of the molecules at a pressure of 10E-6 or 10E-7 Torr is up to 60 meters. Figure 4C is a graphical illustration of the minimum throw distances for 75 mm, 100 mm, and 300 mm radius substrates at angles of incidence less than 5 degrees at the substrate edge. Figure 4D is a table showing the throw distances required for deposition angles of 5 degrees or less if the nozzle is not positioned at the center axis of the substrate.

[0035] Thus, to obtain an angle of less than or equal to 5° at the edge of the wafer (all locations on the wafer away from the edge will have even smaller angles), a throw distance of more than about 1,200 mm is required. Another benefit of placing the evaporation source quite far from the wafer is to reduce the exposure of the wafer to radiant heat from the source. When the heating source is close to the wafer, the radiant heat can inadvertently affect the alignment of the high resolution shadow mask to the wafer due to the thermal expansion process. This is eliminated when the source is positioned at a long throw distance.

[0036] Referring now to the drawings, wherein like reference numerals refer to like elements throughout the several views, Figure 51 shows a system 10 for depositing evaporation materials on a substrate (wafer) 12 according to an exemplary embodiment of the present invention. The 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.

[0037] As shown, the solution to the problem of obtaining a complete overlap of plumes originating from several point sources according to the present invention is to place the point evaporation sources 18a, 18b, 18c in the center of the evaporation vacuum chamber 14, i.e., on or in close proximity to 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. Furthermore, according to the present invention, in order to form a small angle of incidence of the deposited material on the wafer (which is suitable for a mask-to-wafer gap of a few microns), the point evaporation sources 18a, 18b, 18c are placed at a throw distance of >1,200mm for a substrate having a diameter of 200mm.

[0038] like Figure 6 As shown, this can be achieved by implementing collimated point evaporation sources 18a, 18b, 18c equipped with nozzles 21a, 21b, 21c with small orifices. The offset of the nozzles 21a, 21b, 21c from the central axis X of each crucible 22a, 22b, 22c allows the nozzle orifices to be brought very close to each other. Thus, the assembly of the three point evaporation sources 18a, 18b, 18c in a single beam 23 provides very good mixing of the evaporated materials at the molecular level by merging the individual material plumes into a single plume (see Figure 5 ). This is important, for example, for combining two host materials with a single dopant material (or vice versa, ie combining a single host material with two dopant materials).

[0039] like Figure 5 As shown, the assembly of the three-point evaporation source 18a, 18b, 18c of the present invention includes three nozzles 21a, 21b, 21c, three crucibles (i.e., material containers) 22a, 22b, 22c (which have heaters 24 and thermocouples 26 (only shown with respect to one point evaporation source) to control the temperature of the evaporated material in each crucible), three top nozzle heaters 25, three bottom nozzle heaters 27 - each equipped with a thermocouple to monitor and control the nozzle temperature up to 400°C, and three active water cooling baffles 28a, 28b, 28c to prevent thermal crosstalk. See Fig. 6A .

[0040] Figure 7A schematic diagram of a bundle 23 of three point evaporation sources 18a, 18b, 18c on a swing arm 30 driven by a stepper motor 32 is shown. The swing arm 30 includes a wire guide 34 that allows electrical and thermocouple connections to the point evaporation sources 18a, 18b, 18c. In addition, the swing arm 30 is equipped with a water line 35 connected to a water-cooled bulkhead of the source bundle. The stepper motor 32 is positioned outside the vacuum chamber 14 ( Figure 7 ), and by passing through a seal 36 (e.g. The movement of the swing arm 30 is activated by a feedthrough on the vacuum chamber side of the assembly (a seal).

[0041] Figure 8 A schematic diagram of four beams 23 of point evaporation sources 18a, 18b, 18c, 18d on four swing arms 30 is shown, which allows each beam to move from a parked position near a wall (as shown relative to point evaporation sources 18a, 18b and 18c) to the central axis X of the substrate 12, where evaporation of three materials is performed (point evaporation source 18d is shown moving towards the central axis in direction Y). In use, each of the four beams moves into a position at the central axis X.

[0042] Fig. 9 A bottom view of the system 10 is shown having a flange 37 consisting of electrical feedthroughs and four stepper motors 32 and belt drives 38 to perform movement of the swing arm 30 within the vacuum chamber 14 .

[0043] Fig.10 Three point evaporation sources 18a, 18b, 18c are shown positioned below three crystal sensor head assemblies 40, each containing twelve crystal sensors to set and monitor the evaporation rate.

[0044] Fig.11 The vacuum chamber 14 is shown with a removable central duct 42 that acts as a shield for the evaporation plume reaching the crystal head assembly 44. The crystal head assembly 44 is equipped with a focusing tube 46 that aligns with the apertures of the point evaporation sources 18a, 18b, 18.

[0045] It should be understood that the present disclosure teaches only one example of the illustrative embodiments and that those skilled in the art may readily devise numerous variations of the present invention after reading the present disclosure and that the scope of the present invention will be determined by the appended claims.

Claims

1. A system for depositing an evaporation material on a substrate, the substrate having a central axis, the system comprising: (a) Evaporation vacuum chamber; (b) at least one nozzle assembly having a plurality of point evaporation sources disposed adjacent to the central axis of the substrate and at a distance from the substrate, whereby the nozzle assembly provides molecules of evaporation material to arrive at the substrate at an incident angle of less than or equal to 5 degrees; and (c) A shadow mask disposed adjacent to the substrate.

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

3. The system for depositing evaporated material on a substrate of claim 1, wherein the nozzle assembly provides an overlap of multiple evaporated material plumes originating from points positioned near the central axis of the substrate.

4. The system for depositing evaporated materials on a substrate according to claim 1, wherein the point evaporation sources are separated from each other by a plurality of active water cooling baffles to reduce thermal crosstalk between sources.

5. The system for depositing evaporated materials on a substrate according to claim 4, wherein the water-cooled baffle has built-in channels to propagate a flow of water delivered from an external cooler unit.

6. The system for depositing evaporated materials on a substrate of claim 5, wherein the water-cooled baffle is adapted to maintain a temperature of approximately 20 degrees Celsius to 30 degrees Celsius.

7. The system for depositing evaporated materials on a substrate according to claim 1, wherein each of the plurality of point evaporation sources comprises a plurality of nozzles in a bundle, the plurality of nozzles being close to each other and equidistant from the substrate to provide good mixing of evaporated materials at a molecular level by merging individual material plumes into a single plume. 8 . The system for depositing an evaporation material on a substrate according to claim 7 , comprising three beams of the point evaporation sources. 9 . The system for depositing an evaporation material on a substrate according to claim 7 , comprising four beams of the point evaporation sources.

10. The system for depositing evaporated material on a substrate of claim 7, wherein each beam is disposed on one of a plurality of swing arms.

11. The system for depositing evaporated materials on a substrate according to claim 10, wherein each swing arm is driven by a stepper motor for positioning the beam adjacent to the central axis of the substrate.

12. The system for depositing evaporated materials on a substrate according to claim 11, wherein the stepper motor is positioned outside the vacuum chamber, wherein the movement is performed using a belt drive through a seal.

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