Diffuser with corner HCG

Abstract

The present disclosure generally relates to a gas distribution plate for ensuring deposition uniformity. The gas distribution plate has a plurality of recessed portions on the downstream side to ensure uniform deposition in corner regions of the processing chamber.

Description

[0001] This application is a divisional application of an invention patent application with an application date of August 23, 2017, application number 201780059831.8, and invention name “Diffuser with corner HCG”. Technical Field

[0002]

[0013] Embodiments of the present invention generally relate to a gas distribution plate for a chemical vapor deposition (CVD) system designed to compensate for deposition non-uniformities. Background Art

[0003] Plasma enhanced chemical vapor deposition (PECVD) is a deposition method long used to deposit films onto semiconductor substrates. PECVD has recently been used to deposit films on large area substrates, such as solar panel substrates, flat panel display substrates, and large area thin film transistor substrates. Market forces continue to drive the cost of flat panel displays to decrease while increasing the size of the substrate. In flat panel display technology, it is not uncommon for substrate sizes to be greater than 1 square meter.

[0004] A gas distribution plate can be used to ensure that the deposition plasma is evenly distributed throughout the processing chamber. Even distribution of plasma can contribute to film uniformity across the substrate. However, as substrate size increases, obtaining even distribution of plasma within the processing chamber can be challenging.

[0005] Therefore, there is a need in the art for an improved gas distribution plate. Summary of the invention

[0006] The present disclosure generally relates to a gas distribution plate for ensuring deposition uniformity. In one embodiment, the plate includes a diffuser body having an upstream surface, a downstream surface, four sides, and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, each gas channel including a hollow cathode cavity: a central hollow cathode cavity disposed near the center of the diffuser body; a corner hollow cathode cavity disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; a first hollow cathode cavity disposed at a position between the central hollow cathode cavity and the corner hollow cathode cavity, the first hollow cathode cavity being larger in size than the central hollow cathode cavity and smaller in size than the corner hollow cathode cavity; and a second hollow cathode cavity disposed at a position between the corner hollow cathode cavity and the first hollow cathode cavity, the second hollow cathode cavity being smaller in size than the corner hollow cathode cavity and smaller in size than the first hollow cathode cavity.

[0007] In another embodiment, a gas distribution plate includes a diffuser body, the diffuser body having an upstream surface, a downstream surface, four sides and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, each gas channel including a hollow cathode cavity: a central hollow cathode cavity is arranged near the center of the diffuser body; a side hollow cathode cavity is arranged near the side of the diffuser body, and the side hollow cathode cavity is larger than the central hollow cathode cavity; a first hollow cathode cavity is arranged at a position between the central hollow cathode cavity and the side hollow cathode cavity, the size of the first hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the side hollow cathode cavity; and a second hollow cathode cavity is arranged at a position between the side hollow cathode cavity and the first hollow cathode cavity, the size of the second hollow cathode cavity is smaller than the side hollow cathode cavity and smaller than the first hollow cathode cavity.

[0008] In another embodiment, a plasma processing chamber includes: a chamber body; a substrate support disposed in the chamber body; and a gas distribution plate disposed in the chamber body and facing the substrate support, the gas distribution plate including: a diffuser body having an upstream surface, a downstream surface, four sides and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, each gas channel including a hollow cathode cavity: a central hollow cathode cavity disposed near the center of the diffuser body; a corner hollow cathode cavity disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; a first hollow cathode cavity disposed at a position between the central hollow cathode cavity and the corner hollow cathode cavity, the size of the first hollow cathode cavity being larger than the central hollow cathode cavity and smaller than the corner hollow cathode cavity; and a second hollow cathode cavity disposed at a position between the corner hollow cathode cavity and the first hollow cathode cavity, the size of the second hollow cathode cavity being smaller than the corner hollow cathode cavity and smaller than the first hollow cathode cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to be able to understand in detail the manner in which the above-mentioned features of the present disclosure are used, a more specific description of the present disclosure briefly summarized above may be made with reference to embodiments, some of which are shown in the drawings. However, it should be noted that the drawings only illustrate exemplary embodiments and are therefore not to be considered as limiting the scope of the present disclosure, as the present disclosure may allow for other equally effective embodiments.

[0010] Figure 1 is a schematic cross-sectional view of a processing chamber according to one embodiment.

[0011] Figure 2 is a schematic cross-sectional view of a gas channel.

[0012] Figure 3 is a top view of the gas distribution plate.

[0013] Figure 4 It is along Figure 3 Schematic cross-sectional view taken along line AA.

[0014] Figure 5 It is along Figure 3 A schematic cross-sectional view taken along line BB.

[0015] Figure 6 It is along Figure 3 Schematic cross-sectional view taken along line CC.

[0016] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0017] The present disclosure will be illustratively described below with reference to a PECVD system configured to process large area substrates, such as a PECVD system available from AKT, a subsidiary of Applied Materials, Inc., Santa Clara, CA. However, it will be understood that the present disclosure can be used with other system configurations, such as those used to process small or circular substrates. The present disclosure can also be used with processing systems manufactured by other manufacturers.

[0018] Figure 1 1 is a schematic cross-sectional view of a processing chamber 100 according to one embodiment. The processing chamber 100 includes a chamber body having a lid 102 and walls 108. In at least one wall 108, there may be one or more slit valve openings 122 to allow the substrate 106 to be inserted into and removed from the processing space 116. The processing space 116 may be bounded by the slit valve openings 122, the chamber walls 108, the substrate 106, and the gas distribution plate 110. In one embodiment, the gas distribution plate 110 may be biased by a power supply. The substrate 106 may be disposed on a substrate support 104, which may translate up and down as needed to raise and lower the substrate 106.

[0019] Gas may be introduced into the area between the gas distribution plate 110 and the cover 102, which is referred to as a plenum 114. Gas may be evenly distributed within the plenum 114 due to the presence of gas passages 112 extending through from the upstream side 118 to the downstream side 120 of the diffuser plate.

[0020] Figure 21 is a schematic cross-sectional view of a gas channel 112. The gas channel 112 includes an upper portion 202 extending from an upstream surface 204 of a gas distribution plate 110. The upstream surface 204 faces the cover 102, and the downstream surface 206 faces the substrate 106. The gas channel also has a choke 208 or pinch point and a hollow cathode cavity (HCC) 210. The choke 208 is the narrowest point in the gas channel 112 and is therefore the location of the gas that controls the flow through the gas channel. For each choke 208 in the gas distribution plate 110, the choke 208 generally has the same length and width, however, it should be understood that some mechanical tolerances may cause slight variations. In one embodiment, there is no upper portion 202, but the choke 208 extends to the upstream surface 204.

[0021] The HCC 210 may be conical or cylindrical or a combination of both. The dimensions of the HCC 210 are set to allow the ignition of a plasma within the HCC 210. In other words, the plasma may be ignited within the gas distribution plate 110 itself in addition to within the processing space 116. By igniting the plasma within the HCC 210, the shape of the plasma may be controlled because the shape and / or dimensions of the HCC 210 affect the shape and / or intensity of the plasma within the chamber 100.

[0022] Silicon nitride is a film that can be deposited in a PECVD chamber using silane gas. Silicon nitride can be used as a passivation layer, gate insulating layer, buffer layer, intermediate layer, and even as a barrier layer for amorphous silicon thin film transistors (TFTs), low temperature polycrystalline silicon TFTs, and active matrix organic light emitting diode (OLED) displays. In addition, silicon nitride can be used as a barrier layer in thin film encapsulation applications. The thickness and uniformity of the nitride layer have a significant impact on device performance, such as the uniformity of the drain current (i.e., mobility) and the threshold voltage in the TFT.

[0023] Since TFT device performance is sensitive to film thickness variations, thickness uniformity control is of concern to engineers. Uniformity expectations can range from up to 3% for amorphous silicon to 4% for silicon oxide to up to 5% for silicon nitride.

[0024] Uniformity within the substrate is not the only area of ​​concern, run to run uniformity is also closely monitored. The processing chamber is cleaned periodically and a run to run uniformity of 2% to 3% is expected in most scenarios up to eight processing times prior to cleaning.

[0025] Silicon nitride deposition can be challenging in large area substrate processing chambers. Within a single cycle before cleaning, the deposition rate of silicon nitride film can be higher at the corners of the substrate and at the edges of the substrate because the silicon nitride film can accumulate at the corners and edges of the gas distribution plate. The accumulation of silicon nitride can be referred to as a dielectric effect, and enhances the local plasma density by changing the surface electron emission conditions, thereby increasing the deposition rate of the dielectric film in the following deposition due to the locally enhanced plasma. The dielectric effect reduces the uniformity of the silicon nitride process, for example, from about 3% to about 6% of the high deposition rate peak mainly in the corners, and the average deposition rate varies by up to 6%. If the gas distribution plate is used for long-term production, the situation may become worse due to additional dielectric accumulation caused by the interaction of the cleaning gas with the gas distribution plate to produce aluminum fluoride.

[0026] Several attempts have been made to correct deposition uniformity, such as reducing plasma power (causing film quality to suffer), refurbishing the gas distribution plate more frequently, adjusting deposition time within a cleaning cycle, and inserting conductive seasoning (such as amorphous silicon), but no options to date can address uniformity variations within cleaning cycle variations and from the corner to the center of the gas distribution plate. Anodizing has been used in the past, but simply adding an anodized layer to a bare aluminum gas distribution plate causes silicon nitride non-uniformity at the corners because the silicon nitride coating is a dielectric coating at the corners.

[0027] To address the uniformity issue, a permanent dielectric layer (i.e., an anodized layer) of a material such as Al2O3, Y2O3, or other dielectric materials that can survive in a fluorine-based cleaning environment is formed on all exposed surfaces of the gas distribution plate 110. The anodized layer 212 can avoid additional dielectric effects in subsequent depositions, and thus can prevent degradation of continuous operational uniformity due to dielectric effects. The anodized layer 212 can be formed to a surface roughness of about 1 μm to about 20 μm with a total thickness of about 1 μm to about 20 μm to reduce the absorption of fluorine atoms during cleaning and minimize the risk of cracking and peeling of the anodized layer 212. In addition, the hollow cathode gradient (HCG) located not only in the center of the gas distribution plate but also in the edge and corner areas reduces the high deposition rate peaks at the corners and edges.

[0028] Anodization and corner HCG improve the thickness uniformity of the initial deposition by reducing the high deposition rate peaks in the corners, and also reduce the degradation of the deposition rate uniformity in the continuous operation by providing a permanent high-quality dielectric film (i.e., anodized layer 212). Corner HCG and anodization do not compromise process conditions to obtain better uniformity, do not require frequent refurbishment (the basic aluminum gas distribution plate needs refurbishment to restore silicon nitride uniformity), do not require adjustment of deposition time from one deposition to the next, do not require conductive dopants that affect substrate yield, and do not add initial thick silicon nitride dopants that affect particle performance.

[0029] Surprisingly, it was found that the corner HCG together with anodization controlled the deposition rate variation in the silicon nitride process to less than 1% within 8 substrate cycles and the thickness uniformity was about 2.9% to about 3.5%, which is significantly better than the bare aluminum gas distribution plate. There was also a 6% deposition rate increase within 9 substrate cleaning cycles and about 3.8% to about 6.3% uniformity. It should be understood that anodization and corner HCG can be applied to other film deposition processes, such as silicon oxynitride.

[0030] Figure 3 is a top view of the gas distribution plate 110 as viewed from the upstream side 204. For clarity, the gas channels 112 are not shown. Figure 3 The gas distribution plate 110 is shown having a first corner 302, a second corner 304, a third corner 306, and a fourth corner 308. Additionally, the gas distribution plate 110 has a first side 310, a second side 312, a third side 314, and a fourth side 316.

[0031] Figure 4 It is along Figure 3 Schematic cross-sectional view taken along line AA. For clarity, the anodized layer is not shown, but it should be understood that the anodized layer 212 is present. Figure 4 , the downstream surface 206 has a plurality of recessed portions 402, 404, 406 corresponding to the first corner 302 region, the center region 400, and the third corner 306 region. Figure 4As shown, the HCC 210 has different sizes at different locations along the cross-section. For example, the gas channel 112 near the center of the central area 400 of the gas distribution plate 110 has a first size HCC 210A, while the gas channel 112 near the first corner 302 has a second size HCC 210B, which is larger than the first size. Between the first corner 302 and the central area 400, there is another gas channel 112, which has a third size HCC 210C, which is larger than the first size but also smaller than the second size. Between the gas channel 112 with the third size HCC 210C and the gas channel 112 with HCC 210B near the corner 302 is another gas channel with a fourth size HCC 210D, which is smaller than HCC 210B and HCC 210C.

[0032] Figure 4 The other half of the cross section is a mirror image of the first half. Specifically, the gas channel 112 near the third corner 306 has a fifth size HCC 210E, which is greater than the first size. Between the third corner 306 and the central area 400, there is another gas channel 112 having a sixth size HCC 210F, which is greater than the first size but also smaller than the fifth size. Between the gas channel 112 having the sixth size HCC 210F and the gas channel 112 near the corner 306 having the HCC 210E is another gas channel 112 having a seventh size HCC 210G, which is smaller than the HCC 210E and the HCC 210F.

[0033] Figure 5 It is along Figure 3 A schematic cross-sectional view taken along line BB of FIG. Figure 4 , there are three recessed portions 502, 404, 506 corresponding to the first side 310 area, the central area 400 and the third side 314 area. Figure 5 As shown, the HCC 210 has different sizes at different locations along the cross section. For example, the gas channel 112 near the first side portion 310 has an eighth size HCC 210H, which is larger than the first size of the HCC 210A. Between the first side portion 310 and the central area 400, there is another gas channel 112, which has a ninth size HCC 210I, which is larger than the first size but also smaller than the eighth size. Between the gas channel 112 with the ninth size HCC 210I and the gas channel 112 with the HCC 210H near the side portion 310 is another gas channel 112 with a tenth size HCC 210J, which is smaller than the HCC 210H and the HCC 210I.

[0034] Figure 5 The other half of the cross section is a mirror image of the first half. Specifically, the gas channel 112 near the third side portion 314 has an eleventh size HCC 210K, which is greater than the first size. Between the third side portion 314 and the central area 400, there is another gas channel 112 having a twelfth size HCC 210L, which is greater than the first size but also smaller than the eleventh size. Between the gas channel 112 having the twelfth size HCC 210L and the gas channel 112 near the side portion 314 having the HCC 210K is another gas channel having a thirteenth size HCC 210M, which is smaller than the HCC 210K and the HCC 210L.

[0035] Figure 6 It is along Figure 3 A schematic cross-sectional view taken along line CC. Figure 4 and Figure 5 , there are three recessed portions 402, 604, 606 corresponding to the first corner 302 area, the central area 608 of the fourth side 316, and the fourth corner 308 area. Figure 6 As shown, the HCC 210 has different sizes at different locations along the cross section. For example, the gas channel 112 near the central area 608 of the fourth side portion 316 has a fourteenth size HCC 210N, which is smaller than the second size of the HCC 210B. Between the first corner 302 and the central area 608, there is another gas channel 112, which has a fifteenth size HCC 210O, which is larger than the fourteenth size but also smaller than the second size. Between the gas channel 112 with the fifteenth size HCC 210O and the gas channel 112 with HCC 210B near the corner 302 is another gas channel 112 with a sixteenth size HCC 210P, which is smaller than the HCC 210B and the HCC 210O.

[0036] Figure 6 The other half of the cross section is a mirror image of the first half. Specifically, the gas channel 112 near the fourth corner 308 has a seventeenth size HCC 210Q, which is greater than the fourteenth size. Between the fourth corner 308 and the central area 608, there is another gas channel 112 having an eighteenth size HCC 210R, which is greater than the fourteenth size but also smaller than the seventeenth size. Between the gas channel 112 having the eighteenth size HCC 210R and the gas channel 112 near the corner 308 having the HCC 210Q is another gas channel 112 having a nineteenth size HCC 210S, which is smaller than the HCC 210R and the HCC 210Q.

[0037] When referring to the “sizes” of the various HCCs 210 , it should be understood that the “size” may refer to the volume of the HCC 210 or the diameter of the HCC at the downstream surface 206 .

[0038] When the reference is along Figures 4 to 6 When looking at the various recessed portions shown by the cross-sectional lines of FIG. 1 , it should be understood that there will be a recessed area adjacent to each side 310, 312, 314, 316, and there will be a recessed area adjacent to each corner 302, 304, 306, 308, and there will be a recessed area in the center region 400. Therefore, in one embodiment, it is contemplated that there will be nine separate and distinct recessed portions on the downstream side of the gas distribution plate 110.

[0039] By utilizing multiple HCGs on the downstream side of the gas distribution plate, uniform deposition can be achieved in a single substrate deposition process. By adding an anodized coating on the gas distribution plate, uniform deposition can be extended to not only a single substrate, but also to all substrates within a cycle. In this way, the number of substrates that can be processed within a single cleaning cycle can be increased and substrate throughput can be increased. Multiple HCG gradients and anodized coatings not only compensate for deposition non-uniformity in a single substrate, but also compensate for deposition non-uniformity within the entire cycle of substrates.

[0040] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the disclosure is determined by the claims that follow.

Claims

1. A gas distribution plate, comprising: A diffuser body having an upstream surface, a downstream surface, four sides and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, wherein the downstream surface has a plurality of recessed portions and each gas channel includes a hollow cathode cavity: a central hollow cathode cavity disposed proximate the center of the diffuser body; A corner hollow cathode cavity is disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; A first hollow cathode cavity is disposed between the central hollow cathode cavity and the corner hollow cathode cavity, wherein the first hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the corner hollow cathode cavity; and A second hollow cathode cavity is disposed between the corner hollow cathode cavity and the first hollow cathode cavity, and the second hollow cathode cavity is smaller in size than the corner hollow cathode cavity and smaller in size than the first hollow cathode cavity.

2. The gas distribution plate of claim 1, wherein: The diffuser body is anodized.

3. The gas distribution plate of claim 1, wherein: A second corner hollow cathode cavity is disposed near another corner, wherein the second corner hollow cathode cavity is larger than the central hollow cathode cavity; The third hollow cathode cavity is disposed between the central hollow cathode cavity and the second corner hollow cathode cavity, and the third hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the second corner hollow cathode cavity; and The fourth hollow cathode cavity is disposed between the second corner hollow cathode cavity and the third hollow cathode cavity, and the fourth hollow cathode cavity is smaller in size than the second corner hollow cathode cavity and smaller in size than the third hollow cathode cavity.

4. A gas distribution plate, comprising: A diffuser body having an upstream surface, a downstream surface, four sides and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, wherein the downstream surface has a plurality of recessed portions and each gas channel includes a hollow cathode cavity: a central hollow cathode cavity disposed proximate the center of the diffuser body; A side hollow cathode cavity is disposed near the side of the diffuser body, the side hollow cathode cavity being larger than the central hollow cathode cavity; A first hollow cathode cavity is disposed between the central hollow cathode cavity and the side hollow cathode cavity, wherein the first hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the side hollow cathode cavity; and The second hollow cathode cavity is disposed at a position between the side hollow cathode cavity and the first hollow cathode cavity, and the second hollow cathode cavity is smaller in size than the side hollow cathode cavity and smaller in size than the first hollow cathode cavity.

5. The gas distribution plate of claim 4, wherein the diffuser body is anodized.

6. The gas distribution plate of claim 4, wherein: A second side hollow cathode cavity is disposed adjacent to another side portion, the second side hollow cathode cavity being larger than the central hollow cathode cavity; The third hollow cathode cavity is disposed between the central hollow cathode cavity and the second side hollow cathode cavity, and the third hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the second side hollow cathode cavity; and The fourth hollow cathode cavity is disposed between the second side hollow cathode cavity and the third hollow cathode cavity, and the fourth hollow cathode cavity is smaller in size than the second side hollow cathode cavity and smaller in size than the third hollow cathode cavity.

7. The gas distribution plate of claim 6, wherein: A corner hollow cathode cavity is disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; The fifth hollow cathode cavity is disposed between the central hollow cathode cavity and the corner hollow cathode cavity, and the fifth hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the corner hollow cathode cavity; and The sixth hollow cathode cavity is disposed between the corner hollow cathode cavity and the fifth hollow cathode cavity, and the sixth hollow cathode cavity is smaller in size than the corner hollow cathode cavity and smaller in size than the fifth hollow cathode cavity.

8. The gas distribution plate of claim 7, wherein: A second corner hollow cathode cavity is disposed near another corner, wherein the second corner hollow cathode cavity is larger than the central hollow cathode cavity; The seventh hollow cathode cavity is disposed between the central hollow cathode cavity and the second corner hollow cathode cavity, and the seventh hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the second corner hollow cathode cavity; and The eighth hollow cathode cavity is disposed between the second corner hollow cathode cavity and the seventh hollow cathode cavity, and the eighth hollow cathode cavity is smaller in size than the second corner hollow cathode cavity and smaller in size than the seventh hollow cathode cavity.

9. The gas distribution plate of claim 4, wherein: A corner hollow cathode cavity is disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; The third hollow cathode cavity is disposed between the central hollow cathode cavity and the corner hollow cathode cavity, and the third hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the corner hollow cathode cavity; and The fourth hollow cathode cavity is disposed between the corner hollow cathode cavity and the third hollow cathode cavity, and the fourth hollow cathode cavity is smaller in size than the corner hollow cathode cavity and smaller in size than the third hollow cathode cavity.

10. A plasma processing chamber comprising: Chamber body; a substrate support disposed within the chamber body; and A gas distribution plate, the gas distribution plate being disposed in the chamber body and facing the substrate support, the gas distribution plate comprising: A diffuser body having an upstream surface, a downstream surface, four sides and four corners, the diffuser body having a plurality of gas channels extending from the upstream surface to the downstream surface, wherein the downstream surface has a plurality of recessed portions and each gas channel includes a hollow cathode cavity: a central hollow cathode cavity disposed proximate the center of the diffuser body; A corner hollow cathode cavity is disposed near the corner of the diffuser body, the corner hollow cathode cavity being larger than the central hollow cathode cavity; A first hollow cathode cavity is disposed at a position between the central hollow cathode cavity and the corner hollow cathode cavity, the first hollow cathode cavity being larger in size than the central hollow cathode cavity and smaller in size than the corner hollow cathode cavity; and A second hollow cathode cavity is disposed between the corner hollow cathode cavity and the first hollow cathode cavity, and the second hollow cathode cavity is smaller in size than the corner hollow cathode cavity and smaller in size than the first hollow cathode cavity.

11. The chamber of claim 10, wherein the diffuser body is anodized.

12. The chamber of claim 10, wherein: A second corner hollow cathode cavity is disposed near another corner, wherein the second corner hollow cathode cavity is larger than the central hollow cathode cavity; The third hollow cathode cavity is disposed between the central hollow cathode cavity and the second corner hollow cathode cavity, and the third hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the second corner hollow cathode cavity; and The fourth hollow cathode cavity is disposed between the second corner hollow cathode cavity and the third hollow cathode cavity, and the fourth hollow cathode cavity is smaller in size than the second corner hollow cathode cavity and smaller in size than the third hollow cathode cavity.

13. The chamber of claim 12, wherein: A side hollow cathode cavity is disposed near a side of the diffuser body, the side hollow cathode cavity being larger than the central hollow cathode cavity; The fifth hollow cathode cavity is disposed between the central hollow cathode cavity and the side hollow cathode cavity, and the fifth hollow cathode cavity is larger than the central hollow cathode cavity and smaller than the side hollow cathode cavity; and The sixth hollow cathode cavity is disposed between the side hollow cathode cavity and the fifth hollow cathode cavity, and the sixth hollow cathode cavity is smaller in size than the side hollow cathode cavity and smaller in size than the fifth hollow cathode cavity.

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