VOLUMES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Provisional Application No. 63/071,829, filed August 28, 2020, the entire contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] This disclosure relates to a method and apparatus for supplying an etchant for epitaxial lift-off (ELO) processing, and more particularly, a method and apparatus for supplying for supplying small volumes of etchant instead of submerging an epitaxial stack into the etchant.

BACKGROUND

[0003] When a particular epitaxial structure is grown on a surface of a substrate, e.g, a semiconductor wafer, a lift-off process may be used in which a sacrificial material may also be grown between the epitaxial structure and the substrate such that removing the sacrificial material or layer allows for the epitaxial structure to be separated from the substrate. In many instances, the substrate may then be reused to grow a subsequent epitaxial structure. One example of such a process is known as an epitaxial lift-off (ELO) process which is used to separate an epitaxial structure from the substrate by using an etchant, such as hydrofluoric acid (HF). The etchant selectively etches the sacrificial material or layer that is grown between the epitaxial structure, also referred to as a device layer, and the substrate. One drawback of this method arises from the excessive amount of HF required to remove the sacrificial layer. For example, ELO is conventionally performed by submerging an epitaxial stack or epitaxial device stack, which typically includes the device layer (e.g., optoelectronic or photovoltaic layer), the sacrificial layer, the substrate, and a release handle (preferably a flexible release handle) into the etchant with the volume of etchant being 4 to 6 orders of magnitude of theoretically required amount of HF to completely remove the sacrificial layer and thereby release the device layer from the substrate. Often, the etchant is reused to save costs. However, reducing the amount of etchant needed per semiconductor wafer can also reduce costs and, moreover, can reduce safety risk as etchants tend to be hazardous materials, and can simplify or help with the permit requirements needed for manufacturing operations.

[0004] Therefore, techniques that substantially reduce the amount of etchants or other similar hazardous chemicals that are used during ELO processing are highly 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] In an aspect, the present disclosure is directed to a system for supplying an etchant during epitaxial lift-off (ELO) processing, where the system includes an etchant-resistant wafer base with a wafer pocket to accommodate a laminated wafer having a laminate and an epitaxial stack, and the wafer base having an etchant container along its perimeter for holding etchant that is to be in contact with an edge of the epitaxial stack to etch a sacrificial layer in the epitaxial stack. The system further includes a bottom plate and a top plate, where the wafer base is positioned between the bottom plate and the top plate, the laminated wafer and the wafer base provide a seal such that the etchant remains in the etchant container, and a substrate of the epitaxial stack is positioned to face down and the laminate is positioned to face up.

[0007] In another aspect, the present disclosure is directed to a method for supplying an etchant during ELO processing, where the method includes introducing an etchant into an etchant container in an etchant-resistant wafer base. The wafer base has a wafer pocket to accommodate a laminated wafer having a laminate over an epitaxial stack, the epitaxial stack having a device layer, a sacrificial layer, and a substrate facing down, the etchant container is positioned along a perimeter of the wafer base and is configured to hold the etchant in contact with an edge of the epitaxial stack to etch the sacrificial layer, and the wafer base is positioned between a bottom plate and a top plate, the laminated wafer and the wafer base providing a seal such that the etchant remains in the etchant container. The method further includes removing the sacrificial layer with the etchant in the etchant container to separate the laminate and the device layer from the substrate; and separating the bottom plate and the top plate from the wafer base to allow the laminate with the device layer to be removed for further processing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The novel features believed to be characteristic of aspects of the disclosure are set forth in the appended claims. In the description that follows, like parts are marked throughout the specification and drawings with the same numerals, respectively. The drawing figures are not necessarily drawn to scale and certain figures may be shown in exaggerated or generalized form in the interest of clarity and conciseness. The disclosure itself, however, as well as a preferred mode of use, further objects and advances thereof, will be best understood by reference to the following detailed description of illustrative aspects of the disclosure when read in conjunction with the accompanying drawings, wherein:

[0009] FIGS. 1A-1C illustrate an example process flow for a conventional ELO process.

[00010] FIG. 2 illustrates an example of a conventional environment for performing a ELO process.

[00011] FIG. 3 illustrates a diagram that shows different levels of etchant (hydrofluoric acid) volume and consumption per wafer.

[00012] FIG. 4 illustrates a top view of a system, assembly, or fixture for ELO with low etchant volume in accordance with aspects of the present disclosure.

[00013] FIG. 5 illustrates a bottom view of a system, assembly, or fixture for ELO with low etchant volume in accordance with aspects of the present disclosure.

[00014] FIG. 6 illustrates a cross sectional view of a system, assembly, or fixture for ELO with low etchant volume in accordance with aspects of the present disclosure.

[00015] FIG. 7 illustrates an example method for supplying low volumes of an etchant during ELO in accordance with aspects of the present disclosure. DETAILED DESCRIPTION

[00016] This disclosure describes various structures, devices, methods, and arrangements that address the problem with conventional ELO processes. For example, the present disclosure is directed to supplying an etchant in small volumes during epitaxial lift-off (ELO) processing. In this way, the amount of etchant, i.e., microliters per wafer, that is needed to remove a sacrificial layer can be can be significantly reduced when compared to traditional techniques in which the wafer is submerged in the etchant. As such, the present disclosure makes use of the release handle (e.g., a laminate) as an etchant container wall, allowing small amount of etchant to be use efficiently (2 to 3 orders of magnitude over theoretical but significantly less than the 4 to 6 orders of magnitude currently used). The approach also eliminates the need to submerge the whole epitaxial stack into the etchant, which allows unprecedented access to the rear side of the release handle without requiring etchant chemical resistance. The rear side of the release handle can therefore be used for controlling the epitaxial lift-off process.

[00017] FIGS. 1A-1C illustrate an example process flow for a conventional ELO process. For example, as illustrated in FIG. 1A, an epitaxial device stack 100 (also referred to as an epitaxial stack) may include a substrate 105, a sacrificial layer 110 formed on a top surface of the substrate 105, and a device layer 115 formed on a top surface of the sacrificial layer 110. The device layer 115 includes an epitaxial structure that is intended to be used in a device, such as an optoelectronic structure for an optoelectronic device or a photovoltaic structure for a photovoltaic device. In some implementations, the presence of the sacrificial layer 110 permits the device layer 115 to be removed or separated from the substrate 105 without damaging the device layer 115. The sacrificial layer 110 may contain a suitable material, such as aluminum arsenide or an aluminum arsenide alloy. The sacrificial layer 110 may also be doped, such as an n-doped material, for example n-doped aluminum arsenide. It is to be understood that the thicknesses shown in FIGS. 1A-1C for the various layers are by way of illustration and to understand their arrangement and are not intended to represent a proportional thickness of the various layers.

[00018] The epitaxial device stack 110 can have attached a laminate or handle on top (not shown) to handle and manipulate the device layer 115 after being removed or separated from the substrate 105. The epitaxial device stack 110 may be referred to as a wafer and, when having the laminate or handle attached, as a laminated wafer. [00019] As illustrated in FIG. IB, the epitaxial device stack 100 may be exposed to a wet etch solution 120 in order to etch the sacrificial layer 110 and, once the sacrificial layer 110 is completely etched away, separate the device layer 115 from the substrate 105. That is, the sacrificial layer 110 may be etched and removed while separating the device layer 115 from the substrate 105 during the ELO process. Prior to being etched, the sacrificial layer 110 may be utilized to form the lattice structure for the subsequently and epitaxially grown layers contained within the device layer 115. In some implementations, the wet etch solution may be a hydrofluoric acid (HF), and may also contain various additives, buffers, and/or surfactants. The HF solution may selectively etch the sacrificial layer 110 while preserving the device layer 115 and the substrate 105. Once the sacrificial layer 110 is completely etched, as illustrated in FIG. 1C, the device layer 115 may be fully separated from the substrate 105 and then processed to form a variety of devices, including photovoltaic cells and modules or some other optoelectronic devices, as should be understood by those of ordinary skill in the art. Moreover, the substrate 105 may be processed and cleaned so that it can be reused to subsequently grow epitaxial layers.

[00020] FIG. 2 illustrates a conventional environment for performing a ELO process. For example, as illustrated in FIG. 2, a device stack, e.g., the epitaxial or device stack 100 of FIGS. 1A-11C, may be submerged in a chamber 200 filled with an etchant, such as the etchant 120 of FIG. IB, to selectively etch the sacrificial layer 110 between the device layer 115 and the substrate 105. The amount of time the device stack is submerged may vary depending on the characteristics of the sacrificial layer 110 and/or the concentration of the etchant in its liquid phase or liquid state. However, as illustrated in FIG. 2, the chamber 200 is filled with a sufficient amount etchant 120 to completely submerge the device stack 100. In some instances, the amount of etchant used may be 4 to 6 orders of magnitude higher than the theoretically required amount of etchant to completely remove the sacrificial layer 110 in order to release the device layer 115. In the example in FIG. 2, the epitaxial or device stack is shown with a release handle 205 (e.g., a laminate) that may be used to physically handle the device layer 115 once it is separated from the substrate 105.

[00021] FIG. 3 illustrates a diagram 300 that shows different levels of etchant (hydrofluoric acid) volume and consumption per wafer (see e.g., A.T.J. van Nifrik et al., “A Diffusion and Reaction Related Model of the Epitaxial Lift-Off Process,” Journal of the Electrochemical Society, volume 154, issue 11, 2007, D629-D635). For a traditional ELO submersion etchant technique, as illustrated by the picture in the top right comer (resembling the environment in FIG. 2), the amount of etchant, in this case hydrofluoric acid (HF), that is needed can be significant. For example, the HF volume per wafer (e.g., expitaxial device stack 100) can range from about 250 milliliters (mL) to as much as 1.4 liters (L). In the type of low volume techniques described in this disclosure, the HF volume per wafer can be as low as 1 mL or 2 mL. In another example, the HF consumption per wafer for submerge ELO processes can be greater than 54 mL, while for the type of low volume techniques described in this disclosure, the consumption can be in the range between 0.1 mL to 10 mL.

[00022] FIG. 4 illustrates a diagram 400 that shows a top view of a system, assembly, or fixture for ELO with low etchant volume in accordance with aspects of the present disclosure. The fixture includes a top plate 410, an etchant-resistant wafer base 420, and a bottom plate 430. Also shown are mechanical compression or fastening means 450, which in this example are four screws that can go from the top plate 410, through the wafer base 420, and into the bottom plate 430. The wafer base 420 also includes an etchant container 425 that runs along a perimeter of the top surface of the wafer base 420, and a wafer pocket 427 that provides a space or region in which to place a laminated wafer 440 (e.g., with a laminate or handle over a wafer or epitaxial device stack, see e.g., FIG. 5). The wafer base 420 also includes at least one hole 437, typically having one hole 437 as an inlet or input etchant hole to provide or introduce an etchant to the etchant container 425, and another hole 437 as an outlet or output etchant hole to remove etchant from the etchant container 425. Also shown are conduits, tubes, or ducts 435 that are used to provide and remove the etchant. These conduits 435 are connected to the holes 437.

[00023] FIG. 5 illustrates a diagram 500 that shows a bottom view of the system, assembly, or fixture described above in connection with FIG. 4. In the diagram 500 are shown the mechanical compression means 450, the top plate 410, the wafer base 420, the bottom plate 430, the conduits 435, and the hole(s) 437. Also shown is the laminated wafer 440 having a laminate or flexible handle 441 and a wafer or epitaxial device stack 442. The laminate 441 extends beyond the edges of the wafer 442 and this extended portion can be used to cover the etchant container 425, thus providing a top wall for the etchant container 425 as shown below in connection with FIG. 6.

[00024] FIG. 6 illustrates a diagram 600 that shows a cross sectional view of the system, assembly, or fixture described above in connection with FIGS.4 and 5. In the diagram 600 are shown the top plate 410, the wafer base 420 with the etchant container 425, and the laminate 441 and the wafer 442 of the laminated wafer 440. Also shown is an O-ring or gasket 610 in a groove 615 (e.g., annulus groove) on the top surface of the wafer base 420 and outside of the etchant container 420. The use of the O-ring or gasket 610 is to provide a better seal of the etchant in the etchant container 425 when the fixture is assembled. As shown in the diagram 600, the laminate 441 provides a top wall for the etchant container 425 to hold the etchant.

[00025] In connection with the description of FIGS. 4-6, the present disclosure proposes an example of a system, assembly, or fixture for supplying an etchant during ELO processing that includes an etchant-resistant wafer base (e.g., the wafer base 420) with a wafer pocket (e.g., the wafer pocket 427) to accommodate a laminated wafer (having a laminate and an epitaxial stack e.g., the laminated wafer 440 with the laminate 441 and the wafer 442), where the wafer base has an etchant container (e.g., the etchant container 425) along its perimeter for holding etchant that is to be in contact with an edge of the epitaxial stack to etch a sacrificial layer in the epitaxial stack (e.g., epitaxial device stack 100 and sacrificial layer 110). The system also includes a bottom plate (e.g., the bottom plate 430) and a top plate (e.g., the top plate 410), where the wafer base is positioned between the bottom plate and the top plate, the laminated wafer and the wafer base provide a seal such that the etchant remains in the etchant container, and a substrate (e.g., the substrate 105) of the epitaxial stack is positioned to face down and the laminate is positioned to face up.

[00026] In an aspect of this system, the etchant is hydrofluoric acid and the wafer base is resistant to hydrofluoric acid.

[00027] In an aspect of this system, the laminate is positioned over the epitaxial stack and provides a top wall for the etchant container to prevent evaporation of the etchant.

[00028] In an aspect of this system, the wafer base includes a sealable inlet hole (e.g., holes 437) to introduce the etchant to the etchant container, and a sealable outlet hole (e.g., holes 437) to remove the etchant from the etchant container.

[00029] In an aspect of this system, the system further includes a mechanism (e.g., mechanism 450) that compresses and holds the wafer base between the bottom plate and the top plate. In one example, the compression mechanism may include multiple screws or other fasteners. The etchant is introduced into the etchant container prior to the mechanism compressing and holding the wafer base between the bottom plate and the top plate. [00030] In an aspect of this system, the wafer base includes an annulus groove or channel (e.g., groove 615) with an O-ring or gasket (e.g., O-ring or gasket 610) positioned below an outer portion of the laminate such that the laminated wafer and the wafer base provide the seal.

[00031] In an aspect of this system, a size of the wafer pocket is configurable to control an amount of etchant volume used for the laminated wafer.

[00032] In an aspect of this system, the top plate is configured to contain a fluid that is pressurized or placed under partial vacuum to exert a differential pressure with the laminate or the etchant.

[00033] In an aspect of this system, the top plate is configured to exert pressure to the laminate or the etchant through mechanical means, wherein the mechanical means include springs, deflected beams, or both.

[00034] In an aspect of this system, the wafer base is a first wafer base, and the system further includes one or more additional wafer bases with corresponding laminated wafers, where the first wafer base and the one or more additional wafer bases are stacked on top of each other, and where an upper wafer base sealing a laminate in the laminated wafer of a lower wafer base.

[00035] In an aspect of this system, the top plate or a flexible layer different from the top plate and different from the laminate is configured to provide a top wall for the etchant container to prevent evaporation of the etchant.

[00036] In an aspect of this system, the epitaxial stack includes a device layer that is separated from the substrate during the ELO processing, and the device layer includes an optoelectronic structure or a photovoltaic structure that is further processed after being separated from the substrate.

[00037] In an aspect of this system, the laminated wafer is a rectangular or square component, and the wafer pocket has a matching shape.

[00038] FIG. 7 illustrates an example method 700 for supplying low volumes of etchant during ELO processing. Aspects of the method 700 may be performed using the system, assembly, or fixture described above in connection with FIGS. 4-6.

[00039] The method 700 at 705 includes introducing an etchant into an etchant container in an etchant-resistant wafer base, where the wafer base has a wafer pocket to accommodate a laminated wafer having a laminate over an epitaxial stack, the epitaxial stack having a device layer, a sacrificial layer, and a substrate facing down, the etchant container is positioned along a perimeter of the wafer base and is configured to hold the etchant in contact with an edge of the epitaxial stack to etch the sacrificial layer, and the wafer base is positioned between a bottom plate and a top plate, the laminated wafer and the wafer base providing a seal such that the etchant remains in the etchant container.

[00040] The method 700 at 710 includes removing the sacrificial layer with the etchant in the etchant container to separate the laminate and the device layer from the substrate.

[00041] The method 700 at 715 includes separating the bottom plate and the top plate from the wafer base to allow the laminate with the device layer to be removed from the wafer base for further processing.

[00042] Aspects of the method 700 may further include introducing the etchant into the etchant container through a sealable input hole in the wafer base.

[00043] Aspects of the method 700 may further include removing the etchant from the etchant container through a sealable output hole in the wafer base.

[00044] Aspects of the method 700 may further includes applying a mechanism to compress and hold the wafer base between the bottom plate and the top plate.

[00045] Aspects of the method 700 may further include having the wafer base be a first wafer base, the method 700 may further include stacking on top of each other multiple wafer bases with corresponding laminated wafers, where the multiple wafer bases include the first wafer base, and where an upper wafer base seals a laminate in the laminated wafer of a lower wafer base.

[00046] 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. Implementations 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 implementations, and the various modifications that are suited to the particular uses contemplated. It is therefore intended that the scope of the techniques described herein be limited not by this Detailed Description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of various implementations is intended to be illustrative, but not limiting, of the scope of the embodiments, which is set forth in the following claims.