Cartridge structures for batch processing in epitaxial deposition operations and related methods

Abstract

In one embodiment, a cassette support system is disclosed and includes: a base assembly having a shaft; a plurality of arms coupled to and extending radially from the shaft, where at least two radially adjacent arms included in the plurality of arms are separated at an angle of about 130 degrees or greater; a plurality of cartridge support arms, each cartridge support arm extending from one end of an arm included in the plurality of arms; and one or more substrate support rings, where each substrate support ring includes a ridge defined along an inner circumference of the substrate support ring, where the ridge is configured to receive a substrate.

Description

Technical Field

[0001] The present disclosure relates to cassette structures and related methods for batch processing in epitaxial deposition operations. Background Art

[0002] Semiconductor substrates are processed for a variety of applications, including the manufacture of integrated devices and micro-devices. However, operations such as epitaxial deposition operations can be lengthy, expensive, and inefficient, and can have limited capacity and throughput. Operations can also be limited by film growth rates. In addition, the hardware can involve relatively large dimensions, occupying a large footprint in a manufacturing facility. In addition, operations can involve obstacles to temperature control, gas control, and / or substrate center-to-edge control and adjustability. This obstacle is exacerbated in relatively complex processing operations, and / or in operations requiring single-sided deposition.

[0003] Therefore, there is a need for improved apparatus and methods in semiconductor processing. Summary of the invention

[0004] The present disclosure relates to cassette structures and related methods for batch processing in epitaxial deposition operations.

[0005] In one embodiment, a cassette support system is disclosed and includes: a base assembly having an axis; a plurality of arms coupled to the axis and extending radially from the axis, wherein at least two radially adjacent arms included in the plurality of arms are separated by an angle of about 130 degrees or greater; a plurality of cassette support arms, each cassette support arm extending from an end of one of the plurality of arms; and one or more substrate support rings, wherein each substrate support ring includes a ridge defined along an inner circumference of the substrate support ring, wherein the ridge is configured to receive a substrate.

[0006] In another embodiment, a cassette support system is provided. The cassette support system includes a base assembly having an axis; a plurality of arms coupled to and extending radially from the axis, wherein at least two radially adjacent arms in the plurality of arms are separated by an angle of about 130 degrees or more; a plurality of cassette support arms, each cassette support arm extending from a different arm in the plurality of arms; and at least one substrate support ring including a ridge defined along an inner circumference of each of the at least one substrate support ring.

[0007] In another embodiment, a cassette support system is provided. The cassette support system includes a base assembly having an axis; three or more arms coupled to and extending radially from the axis, including two radially adjacent arms of the three or more arms separated by an angle of about 130 degrees or more; three or more cassette support arms, each cassette support arm extending from a different arm of the three or more arms; and three or more substrate support rings including a ridge defined along an inner circumference of each of the substrate support rings; and a first set of spacers disposed between a first substrate support ring and a second substrate support ring, wherein each spacer is disposed around a corresponding vertical cassette support arm, and a second set of spacers disposed between the second substrate support ring and a third substrate support ring, wherein each spacer is disposed around a corresponding vertical cassette support arm. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In order to be able to understand the above-mentioned features of the present disclosure in detail, the present disclosure, which has been briefly summarized above, may be described in more detail with reference to the embodiments, some of which are illustrated in the accompanying drawings. However, it should be noted that the drawings only show exemplary embodiments and are therefore not to be considered limiting of the scope, as the present disclosure may allow other equally effective embodiments.

[0009] Figure 1 is a schematic cross-sectional side view of a processing apparatus having a plurality of substrates in processing positions within a cassette in accordance with one or more embodiments.

[0010] Figure 2 According to one or more embodiments, Figure 1 A schematic cross-sectional side view of the device is shown in a removed position.

[0011] Figure 3 According to one or more embodiments, Figure 2 A cross-sectional side view of the apparatus is shown where a substrate is lifted by lift pins for placement on two fingers of a robotic arm and removal from the chamber.

[0012] Figure 4 According to one or more embodiments, Figure 2 and Figure 3 A cross-sectional side view of the apparatus shown with a substrate deposited on two fingers of the robotic arm with the lift pins retracted.

[0013] Figure 5 According to one or more embodiments, for supporting Figures 1 to 4 A perspective view of the upper portion of the base assembly of the cassette is shown.

[0014] Figure 6 According to one or more embodiments, for supporting Figures 1 to 4 A top view of the upper portion of the base assembly of the cassette is shown.

[0015] Figure 7 is a perspective view of a cassette according to one or more embodiments.

[0016] Figure 8 is a cross-sectional partial side view of a cassette with a plurality of substrates accommodated therein according to one or more embodiments.

[0017] Fig. 9 It is a partial top view in cross section showing the upper portion of the base, lift pins, base plate and robot arm and fingers.

[0018] To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements in the drawings. It is contemplated that elements and features of one or more embodiments may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0019] The present disclosure relates to cassette structures and related methods for batch processing in epitaxial deposition operations.

[0020] Figure 1 FIG. 1 is a cross-sectional side view of a processing apparatus having a plurality of substrates in processing positions in a cassette according to one embodiment. The processing apparatus 100 includes a processing chamber having a chamber body 130 defining a processing volume 124 and upper and lower heat sources 106 and 138 .

[0021] The processing apparatus 100 includes a plurality of gas injection channels 182 formed in the chamber body 130 and in fluid communication with the processing volume 124, and one or more exhaust channels 172 ( Figure 1 1 ). One or more exhaust channels 172 are in fluid communication with the processing volume 124. The plurality of gas injection channels 182 and each of the one or more exhaust channels 172 are formed through one or more sidewalls of the chamber body 130 and through one or more liners 120 lining the one or more sidewalls of the chamber body 130.

[0022] The processing apparatus 100 includes a flow guiding structure 150 located in the processing volume 124. The flow guiding structure 150 includes one or more first flow dividers 151 that divide the processing volume into a plurality of flow levels 153. During operation (e.g., during epitaxial deposition operation), one or more processing gases P1 are supplied to the processing volume 124 through the supply conduit system 121 and a plurality of gas injection channels 182. The one or more processing gases P1 are supplied from one or more gas sources 196 that are fluidly connected to the plurality of gas injection channels 182. Each gas injection channel 182 is configured to direct the one or more processing gases P1 toward the cassette 230 in a generally radially inward direction. Thus, in one or more embodiments, the gas injection channel 182 may be part of a cross-flow gas injector. The gas flow of the one or more processing gases P1 is divided into a plurality of flow levels 153. Dividing the processing gas into a plurality of flow levels 153 facilitates uniform processing (e.g., deposition), center-to-edge uniformity, and process adjustability on the substrate.

[0023] The processing apparatus 100 includes an exhaust conduit system 190. One or more process gases P1 may be exhausted to an optional common exhaust box and subsequently exhausted through the conduit using one or more pumping devices 197 (eg, one or more vacuum pumps).

[0024] The cassette 230 is located in the processing volume 124 and is at least partially supported by the base assembly 300. In various embodiments, the cassette 230 supports multiple substrates 255a, 255b, 255c for simultaneous processing (e.g., epitaxial deposition). Figure 1 In the illustrated embodiment, the cassette 230 supports three substrates. However, the cassette 230 may support other numbers of substrates, including but not limited to two substrates 255, three substrates 255, six substrates 255, twelve substrates 255, and the like.

[0025] The processing apparatus 100 includes a window 193, such as a dome, disposed above the cassette 230 and below the upper heat source 106. The heat sources 106, 138 are positioned to provide uniform heating to the substrate 255. One or more heat sources may be radiant heat sources, such as lamps. For example, halogen lamps and / or other heat sources may be used in addition to or in place of the various heat sources described herein. In other examples, resistive heaters, light emitting diodes (LEDs), and / or lasers may be used in the various heat sources described herein.

[0026] The processing apparatus 100 includes a base assembly 300 disposed in a processing volume 124. One or more liners 120 are disposed in the processing volume 124 and surround the base assembly 300. The one or more liners 120 help protect the chamber body 130 from the processing chemicals in the processing volume 124. The one or more liners 120 are disposed between the processing volume 124 and the chamber body 130. In addition, a sealing ring 350 disposed on the base assembly 300 and below the cassette 230 sealingly protects the chamber body 130 from the processing gases when the cassette is in a processing position in the chamber. In the illustrated embodiment, during processing, the ring 350 seals the shield ring 111 adjacent the gas inlet and exhaust structure.

[0027] In certain embodiments, the base assembly 300 can be raised to a first position so that the cassette 230 is located within the processing volume 124. The base assembly 300 can be lowered to a second position so that the cassette 230 of the chamber body 130 can load and / or unload one or more substrates 255 onto and / or off the cassette 230.

[0028] The lift pin assembly 360 is separate from the base assembly 300. In the illustrated embodiment, the lift pin assembly 360 is disposed outside the base assembly 300, and the coaxial shaft 365 is axially movable independently of the base shaft 305. Extending from the shaft 365 are three arms 370a, 370b, 370c ( Fig. 9 ), each arm has vertical lifting pins 372a, 372b, 372c at its distal end. Figures 1 to 4 As depicted, the lift pins 372 are used to lift and lower the substrates 255 as the substrates 255 are loaded onto and / or unloaded from the cassette 230 .

[0029] Figures 2 to 4 The manipulation and removal of substrates 255 from a cassette after processing is shown. Figure 2 yes Figure 1 A cross-sectional side view of the processing apparatus 100 is shown with the cassette 230 in the unloading position. In various embodiments, because the lift pin assembly 360 and lift pins 370a, 370b, 370c are located below the cassette 230 in the processing position, the substrates 255 are loaded into the cassette 230 from the top down and unloaded from the bottom up.

[0030] exist Figure 2, the lower substrate 255c has been removed and the lift pin assembly 360 is ready to lift the central substrate 255b to a position adjacent the opening 136 where the robotic arm 375 (not shown) can access the central substrate 255b. The opening 136 is formed through one or more side walls of the chamber body 130. The opening 136 can be used to move substrates 255 to or from the cassette 230, for example, into and out of the processing volume 124. In one or more embodiments, the opening 136 includes a slit valve. In one or more embodiments, the opening 136 can be connected to any suitable valve that enables the substrate 255 to pass therethrough. In some embodiments, the cassette 230 is rotatable during processing. In such an embodiment, the cassette 230 has a loading / unloading position in which the cassette 230 is oriented in a Fig. 9 The position shown enables substrates 255 to be loaded and unloaded through opening 136 without interference from cassette support arms 315 .

[0031] Figure 3 for Figure 2 FIG. 1 is a cross-sectional side view of the processing apparatus 100, wherein the substrate 255b is lifted by the lifting pins 372a, 372b, 372c to a position in front of the opening 136 and directly above the robot arm 375. The two fingers 375a, 375b (such as Fig. 9 The robot arm 375 (shown in FIG. 1 ) is constructed and arranged to straddle the lifting pins 372a and position itself directly below the substrate 255b. In this position, as the lifting pins 372a, 372b, 372c are lowered, the separated fingers of the robot arm 375 are ready to receive the substrate 255b.

[0032] exist Figure 3 , the lift pin assembly 360 lifts the lower substrate 255c for removal by the robotic arm 375. Once the lower substrate 255c has been removed, the lift pin assembly 360 will be raised through the interior of the lower substrate support ring 320c to lift the center substrate 255b from the center substrate support ring 320b for removal by the robotic arm 375. The process is then repeated to lift the upper substrate 255a for removal.

[0033] Figure 4 for Figure 2 and Figure 31. A cross-sectional top view of the processing apparatus 100 is shown, wherein the second substrate 255b is deposited on the robot arm 375 and the lift pins 372a, 372b, 372c are retracted. The robot can now remove the substrate 255b from the processing volume 124 through the opening 136. This process will be repeated until the remaining substrate 255a is removed and the cassette 230 is empty. It should be understood that although the above process is described as being repeated three times to remove the substrates 255a, 255b, and 255c, in various embodiments, the process can be repeated any number of times, such as 2 times, 4 times, 6 times, or 12 times, depending on the amount of substrates that the cassette 230 can accommodate.

[0034] The processing device 100 may include one or more temperature sensors 191, 192, such as optical pyrometers, which measure one or more temperatures within the processing device 100 (such as on the surface of the window 193, on one or more surfaces of the substrate 255, and / or the temperature of the cassette 230). In some embodiments, the one or more temperature sensors 191, 192 are disposed on the cover 104.

[0035] The processing device 100 includes a controller 1070 configured to control the processing device 100 or its components. For example, the controller 1070 can control the operation of the components of the processing device 100 using direct control of the components or by controlling controllers associated with the components. In operation, the controller 1070 can collect data and feedback from various chambers to coordinate and control the performance of the processing device 100.

[0036] The controller 1070 generally includes a central processing unit (CPU) 1071, a memory 1072, and support circuits 1073. The CPU 1071 may be one of any form of general purpose processor that may be used in an industrial environment. The memory 1072 or non-transitory computer readable medium may be accessed by the CPU 1071 and may be one or more types of memory, such as random access memory (RAM), read only memory (ROM), a floppy disk, a hard disk, or any other form of local or remote digital storage. The support circuits 1073 are coupled to the CPU 1071 and may include cache, clock circuits, input / output subsystems, power supplies, etc.

[0037] The various methods and operations disclosed herein may generally be implemented under the control of the CPU 1071 by the CPU 1071 executing computer instruction codes stored, for example, as software routines in the memory 1072 (or the memory of a particular processing chamber). When the computer instruction codes are executed by the CPU 1071, the CPU 1071 controls the components of the processing chamber 100 to operate according to the various methods and operations described herein. In one embodiment that may be combined with other embodiments, the memory 1072 (a non-transitory computer readable medium) includes instructions stored therein that, when executed, enable the methods and operations described herein to be implemented. The controller 1070 may communicate with, for example, a heat source, a gas source, and / or a vacuum pump of the processing device 100 to enable a plurality of operations to be performed.

[0038] Figure 5 For support Figures 1 to 4 A perspective view of the upper portion of the base assembly 300 of the cassette 230 is shown. Figure 6 3 is a top view of the base assembly 300. The base assembly 300 includes a vertical shaft 305 extending from a motor 164 (not shown), which is configured to independently raise, lower and / or rotate the cassette 230. At the upper end of the shaft 305 are a plurality of outwardly extending arms 310a, 310b and 310c, each arm having an upright cassette support arm 315a, 315b, 315c at its distal end. The base assembly can be a single piece or one-piece design made of a material such as quartz to allow a compact surface area to fit within the processing chamber. In the embodiment shown, the support arms are integrally formed on the arms.

[0039] like Figure 6As shown, adjacent pairs of arms 310a, 310b, 310c form different angles A, B, and C. In various embodiments, the angle A between the arms 310a, 310b is greater than the angles B and C formed between the arms 310b, 310c and 310c, 310a, respectively. Increasing the angle A relative to the angles B and C facilitates the substrate 255 to laterally pass through the two cassette support arms 315a, 315b and enter the cassette 230. Therefore, the length of the arms 310a, 310b and the angle A between the arms 310a, 310b result in a minimum distance width between the support arms sufficient to exceed the outer diameter of the substrate 255 and allow the substrate to pass between the arms 310a, 310b and enter the cassette 230. Angle A may have an angle greater than 120 degrees, such as an angle greater than 130 degrees, such as an angle greater than 150 degrees, such as an angle greater than 170 degrees, to ensure that the base assembly remains compact, but is still large enough to allow the substrate to be transferred into the cassette 230. For example, in the case where the substrate has an outer diameter of 300 mm, the inner distance between the arms 310a, 310b will exceed 300 mm. Although this example is a 300 mm substrate, it should be understood that the arms can be designed to exceed the outer diameter of substrates having a diameter greater than 100 mm, such as 200 mm, 400 mm, or any other diameter.

[0040] The purpose of the vertical cassette support arms 315a, 315b, 315c distal to each horizontal arm 310a, 310b, 310c of the base assembly 300 is to support various levels of cassettes 230, each level accommodating substrates 255 for processing. For example, Figure 7 The cassette of 230 includes three levels. Each level is composed of substrate support rings 320a, 320b, 320c, each ring including ridges 325a, 325b, 325c facing inward on the inner circumference of the substrate support ring. The inner diameter of the ridge 325 of each ring 320 is slightly smaller than the outer diameter of the substrate 255. Therefore, although the inner diameter of the ring 320 is slightly larger than the outer diameter of the substrate, the smaller inner diameter of the ridge 325 supports the substrate. In this way, each substrate can be maintained in a predetermined position in the cassette 230 during processing.

[0041] In some embodiments, the levels are separated vertically using hollow spacers 330 that are coaxially mounted around the cassette support arms 315a, 315b, 315c ( Figure 5 ). For example, in Figure 7, the cassette 230 has three levels for accommodating three substrates 255. The arrangement utilizes two sets of spacers 330 to facilitate separation of the upper two levels, such as rings 320a, 320b. The lower substrate support ring 320c may include three holes (not shown) corresponding to the three vertical cassette support arms 315a, 315b, 315c. The three holes will be coaxially aligned around the cassette support arms 315a, 315b, 315c so that the lower substrate support ring 320c can be positioned against the upper surface of each horizontal arm 310a, 310b, 310c. Subsequently, the first set of spacers 330 is positioned around each corresponding vertical cassette support arm 315a, 310b, 310c so that the lower surface of each spacer 330 is positioned against the upper surface of the lower substrate support ring 320c.

[0042] The middle substrate support ring 320b may include three through holes aligned with the three holes of the lower substrate support ring 320c. The three through holes will be coaxially aligned around the cassette support arms 315a, 315b, 315c so that the middle substrate support ring 320b can be positioned against the upper surface of each of the first set of spacers 330. The process is then repeated with the second set of spacers 330 and the upper support ring 320a. By selecting the height of the spacer 330, the vertical distance between the support rings can be predetermined. Although the current embodiment is shown as having three vertical cassette support arms 315a, 315b, 315c and three support rings 320a, 320b, 320c. It should be understood that the cassette 230 may have more than three vertical cassette support arms 315, such as four vertical cassette support arms 315, or five vertical cassette support arms 315, and more or less than three support rings 320, such as two support rings 320, six support rings 320, or twelve support rings 320. In general, any number of combinations of vertical cassette support arms 315 and substrate support rings 320 may be implemented, such as five vertical cassette support arms 315 and two substrate support rings 320, or three vertical cassette support arms 315 and twelve substrate support rings 320. It should also be understood that there may be any number of holes through each substrate support ring, and any number of spacers 330 between each pair of adjacent substrate support rings.

[0043] Figure 8 FIG. 2 is a partial side view of a sectional view of the cassette 230 . Figure 8The right side of the cassette is shown with a cassette support arm 315c having two spacers 330 stacked thereon with a substrate support ring 320b held between the pair of spacers 330. Two additional support rings 320a, 320c are shown, one below the lower spacer adjacent the sealing ring 350 and one at the upper end of the upper spacer 330, with the support ring 320a being held by a fastener 317. A total of three support rings 320 are shown, each having an inwardly facing flange 325a, 325b, 325c for supporting a substrate 255. Towards the rear of the cassette 230, a second cassette support arm 310b with its own spacer 300 can be seen (the third vertical member is at Figure 8 (not visible in the image).

[0044] Fig. 9 3 is a partial top view in cross section, depicting base arms 310a, 310b, 310c and their cassette support arms (view is blocked by spacers 330 and fasteners 317) and their connection to substrate support ring 320a. Lift pins 372a, 372b, 372c are also shown. A robot arm 375 with two bifurcated fingers 375a, 375b and substrate 255 are shown in phantom.

[0045] Fig. 9 1 and 2. In various embodiments, a substrate 255 is generally illustrated as being loaded into a cassette 230 having a base design as described herein (note arrow 905). As shown, the substrate 255 is insertable (or removable) by utilizing the expanded angle A formed between the cassette support arms 315 at the ends of the horizontal arms 310a, 310b. Figure 6 consider Fig. 9 , it can be appreciated that the length of the horizontal arms 310a, 310b and the angle A between the horizontal arms 310a and 310b produce dimension 901, which is the inner distance between the cassette support arms 315a, 315b. Comparing dimension 901 with dimension 902 (the outer diameter of the substrate 255), it can be appreciated that as long as the base assembly 300 is in Fig. 9 In the rotational position shown, the difference in dimension 903 provides sufficient space for the substrate to be inserted laterally into the cassette 230 .

[0046] In addition, lift pins 372a, 372b, 372c are also radially arranged in the loading / unloading position so as not to interfere with the forked robot fingers 375a, 375b extending into the cassette 230 to deposit the substrate 255 on the lift pins 372 as the lift pins 372 are raised, thereby lifting the substrate from the fingers. Thereafter, the lift pins 372 lower the substrate 255 to its predetermined position in the substrate support ring 320a of the cassette 230. The same process is repeated for each substrate to be batch processed in the chamber. Each time a new substrate is loaded, the cassette axially aligns the next empty support ring with the opening 136.

[0047] Benefits of the present disclosure include a reduction in the number of individual parts (due to incorporating the cassette support arms into the base assembly), and a smaller overall footprint of the cassette assembly (eg, in terms of width), enabling loading, unloading, and processing of multiple substrates simultaneously.

[0048] 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 of the disclosure, the scope of which is determined by the claims that follow.

Claims

1. A cassette support system, the cassette support system comprising: A base assembly, the base assembly comprising: axis; a plurality of arms coupled to the shaft and extending radially from the shaft, wherein at least two radially adjacent arms in the plurality of arms are separated by an angle of about 130 degrees or more; a plurality of cassette support arms, each cassette support arm extending from an end of an arm included in the plurality of arms; and One or more substrate support rings, each substrate support ring comprising: A ridge is defined along an inner circumference of the substrate support ring, the ridge being configured to receive a substrate.

2. The cassette support system of claim 1, wherein the base assembly comprises quartz.

3. The cassette support system of claim 1, wherein the shaft and the plurality of arms are formed as one body.

4. The cassette support system of claim 1, wherein the base assembly comprises one or more spacers, wherein each spacer is disposed around a corresponding cassette support arm between a pair of adjacent substrate support rings included in the one or more substrate support rings.

5. The cassette support system of claim 1, wherein the angle is approximately 150 degrees or greater.

6. The cassette support system of claim 1, wherein the angle is approximately 170 degrees or greater.

7. The cassette support system of claim 4, wherein the one or more substrate support rings include at least three substrate support rings separated by at least two sets of spacers included in the one or more spacers.

8. The cassette support system of claim 4, wherein the one or more substrate support rings include at least four substrate support rings separated by at least three sets of spacers included in the one or more spacers.

9. The cassette support system of claim 1, wherein the plurality of arms includes at least two horizontal arms.

10. The cassette support system of claim 1, wherein the base assembly includes at least three arms and at least three cassette support arms.

11. The cassette support system of claim 4, wherein a distance between the substrate support rings corresponds to a height of the one or more spacers disposed on the arm.

12. A processing system, the processing system comprising: a processing chamber; and The cassette support system of claim 1.

13. The processing system of claim 12, further comprising a lift pin assembly having a plurality of lift pins, wherein the lift pin assembly is vertically adjustable.

14. The processing system of claim 13, wherein the shaft of the lift pin assembly is coaxially disposed outside of the shaft.

15. The cassette support system of claim 1, wherein at least one of the one or more substrate support rings comprises a plurality of through holes.

16. The cassette support system of claim 15, wherein the plurality of cassette support arms are disposed within the plurality of through holes.

17. The cassette support system of claim 1, wherein the angle forms a minimum distance between the ends of the radially adjacent arms, wherein the minimum distance is greater than a diameter of the inner circumference of the substrate support ring.

18. The cassette support system of claim 17, wherein the minimum distance is greater than 200 mm.

19. A cassette support system, the cassette support system comprising: A base assembly, the base assembly comprising: axis; a plurality of arms coupled to the shaft and extending radially from the shaft, wherein at least two radially adjacent arms in the plurality of arms are separated by an angle of about 130 degrees or more; a plurality of cassette support arms, each cassette support arm extending from a different arm included in the plurality of arms; and At least one substrate support ring, the substrate support ring comprising: A ridge is defined along an inner circumference of each of the at least one substrate support ring.

20. A cassette support system, the cassette support system comprising: A base assembly, the base assembly comprising: axis; three or more arms coupled to the shaft and extending radially from the shaft, wherein two radially adjacent arms of the three or more arms are separated by an angle of about 130 degrees or more; three or more cassette support arms, each cassette support arm extending from a different one of the three or more arms; and Three or more substrate support rings, the substrate support rings comprising: a ridge defined along an inner circumference of each of the substrate support rings; a first set of spacers disposed between the first substrate support ring and the second substrate support ring, wherein each spacer is disposed around a corresponding vertical cassette support arm; and A second set of spacers is disposed between the second substrate support ring and the third substrate support ring, wherein each spacer is disposed around a corresponding vertical cassette support arm.

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