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Title:
WAFER BASE FOR HIGH SPEED EPITAXIAL LIFTOFF PROCESS
Document Type and Number:
WIPO Patent Application WO/2022/047130
Kind Code:
A1
Abstract:
Various aspects of this disclosure relate to a wafer base or fixture that holds a wafer laminated with a plastic handle during Epitaxial Liftoff (ELO). The shape or configuration of the wafer base enables the ELO process to progress quickly without damaging film or wafer. In an aspect, a fixture is described that includes a pocket defining a space in which the wafer is placed, a set of first pegs on pads, a set of second pegs on the perimeter portion, a set of third pegs on a perimeter portion of the fixture outside the pocket, where a photovoltaic device grown on the wafer is attached to a bottom portion of a handle, and the handle with the attached photovoltaic device is separated from the wafer while in the fixture by the ELO process.

Inventors:
SANFORD ERIC (US)
SORABJI KHURSHED (US)
HESSLER KIRSTEN (US)
ARCHER MELISSA (US)
HE GANG (US)
Application Number:
PCT/US2021/047905
Publication Date:
March 03, 2022
Filing Date:
August 27, 2021
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
UTICA LEASECO LLC (US)
International Classes:
B32B38/10; H01L21/78; H01L21/20
Foreign References:
US20180135166A12018-05-17
US20140251547A12014-09-11
US6596086B12003-07-22
US20120247685A12012-10-04
Attorney, Agent or Firm:
MAIER, Paul et al. (US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A fixture for holding a wafer during an epitaxial liftoff (ELO) process, comprising: a pocket defining a space in which the wafer is placed, the pocket having a bottom portion, a perimeter portion raised from the bottom portion, and sidewalls around the perimeter portion; a set of first pegs on pads, the pads extending from the perimeter portion, the wafer resting on the pads and sides of the wafer being abutted by the first pegs to hold the wafer in place in the pocket, a bottom portion of the wafer not touching the bottom portion of the pocket; a set of second pegs on the perimeter portion, a handle on the wafer that extends over the sides of the wafer resting on the second pegs; and a set of third pegs on a perimeter portion of the fixture outside the pocket, sides of the handle being abutted by the third pegs to hold the handle in place in the fixture, wherein a photovoltaic device grown on the wafer is attached to a bottom portion of the handle, and the handle with the attached photovoltaic device being separated from the wafer while in the fixture by the ELO process.

2. The fixture of claim 1 , wherein the second pegs are taller than the first pegs such that the difference in height causes an angle between the handle and the wafer to enable an etch front to move inward during the ELO process to separate the handle with the attached photovoltaic device from the wafer.

3. The fixture of claim 1 , further comprising one or more slots configured to allow a mechanical fork to load or unload the wafer from the fixture.

4. The fixture of claim 3, wherein the one or more slots extend from one side of the fixture into the pocket.

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5. The fixture of claim 1 , wherein the pads for the first pegs are placed on or near corners of the pocket.

6. The fixture of claim 1 , further comprising one or more drainage holes defined on the bottom portion of the pocket and configured allow chemicals used in the ELO process to drain from the fixture.

7. The fixture of claim 1 , further comprising one or more drainage holes defined on the perimeter portion of the fixture outside the pocket and configured allow chemicals used in the ELO process to drain from the fixture.

8. The fixture of claim 1 , further comprising a foot in each corner of a bottom portion of the fixture, the feet in the fixture configured to rest on a top portion of another fixture positioned below the fixture.

9. The fixture of claim 1 , further comprising one or more bushings configured to enable the fixture to be stacked along with other fixtures in a wafer base carrier.

10. The fixture of claim 9, wherein the one or more bushings enable respective rods from the wafer base carrier to be inserted through them.

11. The fixture of claim 9, wherein the one or more bushings include a positioning bushing and an ovular bushing.

12. A pusher for use in a fixture to push on a wafer on a different fixture during an epitaxial liftoff (ELO) process, comprising: an upper circular portion; a lower circular portion having a smaller diameter than the upper circular portion; and a metal slug disposed within the pusher.

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13. The pusher of claim 12, wherein the upper circular portion extends out from a smaller diameter near the lower circular portion to a greater diameter.

14. The pusher of claim 12, wherein the upper circular portion and the lower circular portion are integrated together.

15. The pusher of claim 12, further comprising a pusher cap placed on an opening in the upper circular portion and configured to hold the metal slug in place within the pusher.

16. The pusher of claim 15, wherein the pusher cap includes a notch in its perimeter that is configured to catch a negative feature in the opening to secure the pusher cap in place.

17. The pusher of claim 12, wherein the pusher is made of a chemical resistant plastic.

18. The pusher of claim 12, wherein the lower circular portion is a pusher cup.

19. The pusher of claim 12, wherein the lower circular portion is configured to fit in an opening of the fixture.

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Description:
WAFER BASE FOR HIGH SPEED EPITAXIAL LIFTOFF PROCESS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Provision Application No. 63/071 ,246, filed August 27, 2020, the entire contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] This disclosure relates to fixtures used in the fabrication of photovoltaic devices, and in particular, to certain fixture designs and pushers used for high speed epitaxial lift-off (ELO) systems.

BACKGROUND

[0003] The wafer base holds wafers laminated with a flexible film (“laminated wafers”). This film is attached to the epitaxial layer (e.g., layer with photovoltaic functionality), which is deposited on the wafer in an upstream process. The shape or configuration of the wafer base interfaces with the edges of the film to lift it at a precise angle necessary for fast epitaxial liftoff (ELO). The laminated wafer is inserted into the wafer base with laminate on top. The laminated wafer rests within a central pocket which is slightly deeper than the thickness of the wafer. The wafer bases are designed to stack on each other, with a “pusher” in the center which holds the film and wafer in place during the ELO process. The pusher hangs from the center of the wafer base when the wafer bases are unstacked, and rests only on the laminated wafer when the wafer bases are stacked. The pusher is of a precise weight and shape such that it holds the wafer and laminate in place, but does not disrupt the etch process. The wafer base stack is submersed in a chemical bath which etches away a sacrificial layer between the epitaxial layer and the wafer. After the sacrificial layer is removed, the wafer is mechanically separated from epitaxial layer and film, which are still adhered to each other.

[0004] Improvements in both the design and configuration of the wafer base and the pusher to enable even higher speed ELO systems are desirable. SUMMARY OF THE DISCLOSURE

[0005] The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

[0006] The present disclosure describes aspects of a wafer base that is configured to hold a wafer laminated with a plastic handle during ELO. The precise shape or configuration of the wafer base enables the ELO process to progress quickly without damaging film or wafer.

[0007] In an aspect, a fixture or wafer base for holding a wafer during an ELO process is described that includes a pocket defining a space in which the wafer is placed, the pocket having a bottom portion, a perimeter portion raised from the bottom portion, and sidewalls around the perimeter portion; a set of first pegs on pads, the pads extending from the perimeter portion, the wafer resting on the pads and sides of the wafer being abutted by the first pegs to hold the wafer in place in the pocket, a bottom portion of the wafer not touching the bottom portion of the pocket; a set of second pegs on the perimeter portion, a handle on the wafer that extends over the sides of the wafer resting on the second pegs; and a set of third pegs on a perimeter portion of the fixture outside the pocket, sides of the handle being abutted by the third pegs to hold the handle in place in the fixture, where a photovoltaic device grown on the wafer is attached to a bottom portion of the handle, and the handle with the attached photovoltaic device being separated from the wafer while in the fixture by the ELO process.

[0008] In another aspect, a pusher for use in a fixture to push on a wafer on a different fixture during an ELO process is described that includes an upper circular portion; a lower circular portion having a smaller diameter than the upper circular portion; and a metal slug disposed within the pusher. [0009] The wafer base and pusher described herein are novel in that they allow for high speed epitaxial liftoff by pulling the film away from the wafer as the sacrificial layer is etched away. This speeds up the etch rate. The wafer base also interfaces with high speed automation, making it possible to process hundreds of wafers per hour.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates an example of a wafer base for use in high speed ELO processes in accordance with aspects of this disclosure.

[0011] FIG. 2 illustrates additional details of the wafer base in FIG. 1 in accordance with aspects of this disclosure.

[0012] FIG. 3 illustrates even more details of the wafer base in FIG. 1 in accordance with aspects of this disclosure.

[0013] FIG. 4 illustrates an example of a pusher to be used in high speed ELO processes in accordance with aspects of this disclosure.

[0014] FIG. 5 illustrates additional details of the pusher in FIG. 4 in accordance with aspects of this disclosure.

[0015] FIG. 6 illustrates even more details of the pusher in FIG. 4 in accordance with aspects of this disclosure.

DETAILED DESCRIPTION

[0016] This disclosure describes various aspects related to fixtures used in the fabrication of photovoltaic devices, and in particular, to certain fixture designs and related pushers for epitaxial lift-off (ELO) systems. In this disclosure, a fixture may refer to a wafer base or to some structure that includes a wafer base. As used herein the terms “photovoltaics,” “photovoltaic structures,” TVs,” TV cells,” and “solar cells” may be used interchangeably to refer to one or more portions of an optoelectronic component that produce voltage and/or electric current when exposed to light. A PV layer may refer to a layer (e.g., epitaxial layer, semiconductor layer) that includes one or more photovoltaics, photovoltaic structures, PVs, PV cells, and/or solar cells.

[0017] In this disclosure, a wafer base, which may also be referred to as a fixture, is described that can be made from chemical-resistant plastic and designed to hold a square wafer (e.g., a 4 inch square wafer), laminated with a plastic handle. In one example, the plastic handle can be 115mm to 130mm in edge length. The wafer base contains a square pocket which is slightly oversized to the wafer so that the wafer fits in the pocket. At the edges of the pocket there are mechanical features, or pegs. The inner set of pegs are shorter than the thickness of the wafer, and act to keep the wafer in place during the ELO process. The outer set of pegs are slightly taller than the thickness of the wafer, and act to lift the edges of the laminate during ELO. Outside of the pocket, there is a set of larger pegs which act to retain the laminate after it detaches from the wafer.

[0018] Around the perimeter of the wafer base there several holes to allow for drainage and reduce weight. Within the central pocket there are also drainage holes to prevent fluid from pooling. The wafer rests within the pocket on several pad (e.g., 8 pads, 2 at the each corner of the wafer). The center of the wafer need not make contact with the bottom of the pocket due to these raised pads.

[0019] In one implementation, at each corner of the wafer base there is a foot which inserts into a threaded hole (e.g., 4 feet in total). This foot is of a precise height which sets the pitch of a set of stacked wafer bases. When stacked, each foot rests on the top of the foot beneath it such that the total height of n wafer bases is n times the height of one foot. This allows for the wafer base stack height to be independent of the thickness of each wafer base.

[0020] Each wafer base may feature two bushings which mate with the rods of a wafer base carrier, on which the stack of wafer bases slides. One of these bushings may have an oblong hole to allow for mismatch between the rod spacing and wafer base hole spacing. The other bushing may have a precisely sized circular hole to ensure precise positioning of the wafer base relative to that carrier rod.

[0021] At the front edge of the wafer base there may be two slots to allow for robotic manipulation of the wafer, with or without laminate attached. To insert or remove the wafer/laminate composite from the wafer pocket, an end effector with two “forks” is inserted into these slots and grips the wafer from the underside. At the same time, a vacuum “paddle” end effector or similar design grips the laminate from the topside. These two end effectors grasp together and lift or drop the wafer/l am inate composite for insertion or removal from the wafer base.

[0022] In addition to the improved wafer base described above, a pusher is also described in this disclosure that can be made from chemical resistant plastic, with a metal slug inserted into the center to provide weight. This metal slug also does not interfere with the etch process. The metal slug is machined to a precise size so that it has a clearance fit with the outer pusher cup, with minimal open volume for fluid buildup. The pusher cup may be machined to a precise size to interface with the central wafer base hole, with shoulders which rest on the inside of the wafer base hole. The underside of the pusher cup is machined to a large radius which guarantees a single point of contact with the laminated wafer. The pusher cup has a negative feature which interfaces with a notch on the cap, securing it in place. The cap is pressed into place and is slightly raised above the rest of the pusher, so that no liquid will pool on the top of the pusher.

[0023] When in use, wafer bases are stacked in columns of arbitrary height. One pusher hangs from the center of each wafer base, interacting only with the wafer in the row below. The pusher is sized such that its weight does not interfere with the etch process, but still holds the laminated wafer below it in place. The wafer base stack is loaded one row at a time through an automated system which lifts all the wafer bases above the one to be loaded. This lifting process may be referred to as “singulation.” As each row of wafer bases is lifted, the pusher lifts with it and releases from the wafer below. After each row is singulated, an end effector loads a laminated wafer into each wafer base. Once loaded with laminated wafers, the stack is submersed in a chemical etch bath for a predetermined period of time. After the etch process is complete, the wafer base stack is removed and the stack is again singulated such that one row of etched wafers can be removed at a time, again by end effector.

[0024] Additional details related to this disclosure are provided in connection with the description of FIGS. 1-6 provided below.

[0025] FIG. 1 shows a diagram 100 that illustrates an example of a wafer base for use in high speed ELO processes in accordance with aspects of this disclosure. In this example, a top view of the wafer base shows a wafer pocket 110 outlined where a wafer is to be held by the wafer base. Although the wafer pocket 110 is a square pocket to handle a square wafer, it need not be so limited and different size and/or shape wafer and pockets may be used. Also shown in the diagram 100 are load/unload slots 120, inner drainage holes 130, outer drainage holes 135, and a central wafer base hole 125 in which a pusher can be placed.

[0026] FIG. 2 shows a diagram 200 that illustrates additional details of the wafer base in FIG. 1. This top view of the wafer base shows that at the edges of the wafer pocket 110 there are mechanical features, or pegs. The wafer retention pegs 210, the film lift pegs 230, and the film retention pegs 240. Also shown are wafer rest pads 220 on which the wafer rests. In an example, the wafer base may include 8 wafer retention pegs, 8 wafer rest pads, 16 film lift pegs, 4 film retention pegs, and 8 wafer rest pads, 2 in each corner. As described above, the inner set of pegs 210 are shorter than the thickness of the wafer, and act to keep the wafer in place during the ELO process. The outer set of pegs 230 are slightly taller than the thickness of the wafer, and act to lift the edges of the laminate during ELO. The set of larger pegs 240 outside the wafer pocket 110 act to retain the laminate after it detaches from the wafer.

[0027] FIG. 3 shows a diagram 300 that illustrates even more details of the wafer base in FIG. 1. This bottom view of the wafer base shows the central wafer base hole 125, a positioning bushing 320, an ovular bushing 310, and multiple feet 330 (e.g., the wafer base may have 4 feet, one in each corner). As noted above, multiple wafer bases may be stacked during the ELO process and the precise height of the feet sets the pitch of a set of stacked wafer bases. Moreover, the bushings mate with rods of a wafer base carrier (not shown), on which the stack of wafer bases slides. The ovular bushing 310 may have an oblong hole to allow for mismatch between the rod spacing and wafer base hole spacing, and the positioning bushing 320 may have a precisely sized circular hole to ensure precise positioning of the wafer base relative to that carrier rod.

[0028] FIG. 4 shows a diagram 400 that illustrates an example of a pusher to be used in high speed ELO processes in accordance with aspects of this disclosure. In this example, the pusher is shown to have an upper circular portion and a lower circular portion, where the upper circular portion is wider or has a larger diameter than the lower circular portion that allows the pusher to move up and down in the central wafer base hole 125 and also rest in central wafer base hole 125. More specifically to the diagram 400, there is shown a pusher cap 420 and a pusher cup 430 (e.g., lower circular portion). The pusher cap 420 is also shown to be raised over a top surface (e.g., raised cap 410).

[0029] FIG. 5 shows a diagram 500 that illustrates a cross sectional view of the pusher in FIG. 4 to show additional details. In this example, there is shown a notch 510 in the pusher cap 420 that matches with a negative feature in an opening formed in the upper circular portion of the pusher that allows for the pusher cap 420 to be secured in place. Also shown is a metal slug 520 inserted in the center of the pusher to provide weight. Accordingly, the pusher includes an opening or open area in which the metal slug 520 is placed and the pusher cap 420 is provided to close the opening and keep the metal slug 520 in place.

[0030] FIG. 6 shows a diagram 600 that illustrates a bottom view of the pusher in FIG. 4 to show even more details. In this example, the upper circular portion of the pusher is shown to have shoulders 610. That is, the shoulders 610 are used to enable the pusher to move up and down the central wafer base hole 125 and also rest on the central wafer base hole 125. Also shown in the diagram 600 is a radius 620 on an underside of the pusher cup 430 that is large enough to guarantee that there is a single point of contact with the laminated wafer below.

[0031] The above description of various embodiments of the claimed subject matter has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. Many modifications and variations will be apparent to one skilled in the art. Embodiments were chosen and described in order to best describe certain principles and practical applications, thereby enabling others skilled in the relevant art to understand the subject matter, the various embodiments and the various modifications that are suited to the particular uses contemplated.