WO2022047247A1

WO2022047247A1 - Systems and methods for supplying an etchant in a gaseous state during epitaxial lift-off (flo) processing

Info

Publication number: WO2022047247A1
Application number: PCT/US2021/048082
Authority: WO
Inventors: Melissa Archer; Gang He; Borirak OPASANONT
Original assignee: Utica Leaseco, Llc
Priority date: 2020-08-28
Filing date: 2021-08-27
Publication date: 2022-03-03

Abstract

This disclosure describes various methods and systems to introduce just enough etchant volume to an epitaxial stack via vapor phase transport during an epitaxial lift-off (ELO) process to reduce the amount of etchant used in the process. A method is described that includes converting an etchant from a liquid phase into a vapor phase in a first chamber, transferring the etchant in the vapor phase from the first chamber to a second chamber, and placing an epitaxial stack having a substrate, a sacrificial layer, and a device layer in the second chamber, wherein the etchant in the vapor phase condenses when in contact with the epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate. A system is described that includes the first chamber and the second chamber of the method described above and is configured to perform each aspect of the method.

Description

SYSTEMS AND METHODS FOR SUPPLYING AN ETCHANT IN A GASEOUS STATE DURING EPITAXIAL LIFT-OFF (ELO) PROCESSING

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Provisional Application No. 63/071,747, filed August 28, 2020, the entire contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] This disclosure relates to systems and methods for supplying an etchant in a gaseous state, and more particularly, systems and methods for supplying an etchant in a gaseous state during epitaxial lift-off (ELO) processing 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, and the substrate, 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 method for supplying an etchant during epitaxial lift-off (ELO) processing, the method including converting an etchant from a liquid phase into a vapor phase in a first chamber; transferring the etchant in the vapor phase from the first chamber to a second chamber; and placing an epitaxial stack having a substrate, a sacrificial layer, and a device layer in the second chamber, wherein the etchant in the vapor phase condenses when in contact with the epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate.

[0007] In another aspect, the present disclosure is directed to a system for supplying an etchant during ELO processing, the system including a first chamber configured to hold an etchant in liquid phase and to convert the etchant from the liquid phase to a vapor phase; and a second chamber coupled to the first chamber via at least one connector, the second chamber configured to have one or more epitaxial stacks placed within the second chamber, wherein each epitaxial stack has a substrate, a sacrificial layer, and a device layer. The etchant in the vapor phase is transferred from the first chamber to the second chamber via the at least one connector, and the etchant in the vapor phase condenses when in contact with each epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate. 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.

[0010] FIG. 2 illustrates an example of a conventional environment for performing a ELO process.

[0011] FIG. 3 illustrates an example of an environment for performing a ELO process in accordance with aspects of the present disclosure.

[0012] FIG. 4 illustrates an example of a chart that shows hydrofluoric acid weight percentage when in liquid or vapor phase in accordance with aspects of the present disclosure.

[0013] FIG. 5 illustrates an example method for supplying an etchant in a gaseous state or vapor phase during epitaxial lift-off (ELO) in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

[0014] 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 a gaseous state 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.

[0015] 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.

[0016] 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.

[0017] 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 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 that may be used to physically handle the device layer 115 once it is separated from the substrate 105.

[0018] FIG. 3 illustrates a environment for performing a ELO process in accordance with aspects of the present disclosure. The environment described in FIG. 3 allows for supplying etchant in vapor phase (also referred to as a gaseous state) instead of in a liquid phase (e.g., putting wafer into solution) when lifting off a device layer during an ELO process. As illustrated in FIG. 3, the environment 300 includes a first chamber 305 filled with an etchant 310, such as HF, and a second chamber 315 coupled to the first chamber 305 via a connector 320 (more than one connector 320 may be used). Although the present disclosure describes the etchant 310 as being HF, it should be understood by those of ordinary skill in the arts that other etchants are further contemplated in accordance with aspects of the present disclosure. In some implementations, the etchant 310 may be in a liquid phase/state, which may be transformed into a gaseous etchant 325. For example, the liquid etchant 310 (or etchant in liquid phase/state) may either heated to form a vapor or atomized into a fine mist. A person ordinary skill in the art should understand that etchants, like hydrofluoric acid, may have different acid composition in a gaseous state as compared to their liquid state, as defined by their vapor-liquid equilibrium. When using HF as an etchant, it should be understood that a weight percent of HF in its liquid state is the same weight percent in its gaseous state at approximately 36%, as shown in the acid composition relationship curve in FIG. 4. However, it should be understood by those of ordinary skill in the arts that other weight percentages for the liquid state of HF are further contemplated in accordance with aspects of the present disclosure. For example, a weight percent of HF of up to 40-50 % may be used in accordance with aspects of the present disclosure.

[0019] In an example of the differences in acid composition for HF in liquid and vapor phase, a 10 weight percent (wt%) liquid hydrofluoric acid source will form approximately 4 weight percent hydrofluoric acid in vapor phase (with remainder being water). Because of faster condensation kinetics and then equilibrium, the condensed etchant is also likely to have approximately 4 weight percent HF acid, rather than forming the equilibrium 10 weight percent. Therefore, if what is desired is 10 weight percent condensed etchant concentration, a 25 weight percent liquid etchant source may be needed. As noted above, for hydrofluoric acid etchant system, operating at the azeotropic point of approximately 36% is preferred for simplicity, as the vapor and liquid phases will have the same concentration.

[0020] In further implementations, after the liquid etchant 310 is transformed into the gaseous etchant 325 (e.g., by heating from a heating source 350), the gaseous etchant 325 may be transferred from the first chamber 305 to the second chamber 315 via the connector 320. In some implementations, the second chamber 315 may have heated walls. In some implementations, an epitaxial or device stack, such as the device stack 100 of FIG. 1, may be placed on a temperature controlled stage 340 in the second chamber 315. In this way, the device stack 100 (including the substrate 105) may be actively controlled at a process temperature, such as between room temperature (i.e., about 20-25° C, with an average of 23° C) and 90° C. For example, the temperature of the epitaxial or device stack 100 may be actively controlled at 60° C using the temperature controlled stage 340. By controlling the temperature of the epitaxial or device stack 100, the temperature of the liquid etchant 310 and gaseous etchant 325 may be set such that the gaseous etchant 325 condenses when in contact with the epitaxial or device stack 100. To achieve this, the liquid etchant 310 and the gaseous etchant 325 may be stored at a temperature higher than the temperature of the substrate 105. For example, when the gaseous etchant is HF, the liquid etchant 310 may be stored at a temperature that is 20° C higher than the epitaxial or device stack 100, but it should be understood by those of ordinary skill in the arts that any temperature between the temperature of the epitaxial or device stack 110 and the boiling point of the liquid etchant 310 are also contemplated in accordance with aspects of the present disclosure.

[0021] After being condensed, the gaseous etchant 325 (e.g., etchant in vapor phase or gaseous state) may etch the sacrificial layer 110 in order to separate the device layer 115 from the substrate 105. That is, the gaseous etchant 325 may selectively etch the sacrificial layer 110 in order to separate the device layer 115 from the substrate 105 during the ELO process. Once separated, the device layer 115 may be fully separated from the substrate 105 and then processed to form a variety of photovoltaic devices, including photovoltaic cells and modules, as should be understood by those of ordinary skill in the art. In some implementations, after the sacrificial layer 110 has been removed, the gaseous etchant 325 may be a either be discarded (i.e., the gaseous etchant 325 may be a single time) or the gaseous etchant 325 may be recycled for future use. [0022] FIG. 5 illustrates an example method 500 for supplying an etchant in a gaseous state during epitaxial lift-off (ELO) processing.

[0023] For example, the method 500 includes converting an etchant from a liquid phase (or liquid state) into a vapor phase (or gaseous state) in a first chamber (e.g., the chamber 305) at 505.

[0024] In some implementations, converting the etchant into the vapor phase includes heating the etchant in the liquid phase to form a vapor (e.g., by using the heat source 350). Alternatively, converting the etchant into the vapor phase includes atomizing the etchant in the liquid phase into a fine mist. In some implementations, the liquid etchant may be hydrofluoric acid having a weight percent of approximately 36%.

[0025] The method 500 further includes transferring the etchant in the vapor phase from the first chamber to a second chamber (e.g., the chamber 315) at 510.

[0026] The method 500 also includes placing an epitaxial stack having a substrate, a sacrificial layer, and a device layer in the second chamber at 515, wherein the etchant in the vapor phase condenses when in contact with the epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate.

[0027] In some implementations, the gaseous etchant may condense when in contact with the epitaxial or device stack and may selectively etch the sacrificial layer to release the device layer. In some implementations, the method 500 may optionally include controlling a temperature of the substrate using a temperature controlled stage (e.g., temperature controlled stage 340) in the second chamber at 520.

[0028] In some instances, the temperature of the epitaxial or device stack (e.g., including the device layer, the substrate, and the sacrificial layer) is between 20° C and 90° C. For example, the temperature of the epitaxial or device stack may be 60° C.

[0029] In further implementations, the method 500 may optionally include storing the liquid etchant (e.g., the etchant in the liquid phase) at a temperature higher than the temperature of the substrate at 525.

[0030] 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.

Claims

CLAIMS What is claimed is:

1. A method for supplying an etchant during epitaxial lift-off (ELO) processing, the method comprising: converting an etchant from a liquid phase into a vapor phase in a first chamber; transferring the etchant in the vapor phase from the first chamber to a second chamber; and placing an epitaxial stack having a substrate, a sacrificial layer, and a device layer in the second chamber, wherein the etchant in the vapor phase condenses when in contact with the epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate.

2. The method of claim 1, wherein the converting comprises heating, in the first chamber, the etchant in the liquid phase to form a vapor for the vapor phase.

3. The method of claim 1, wherein the converting comprises atomizing, in the first chamber, the etchant in the liquid phase to form a fine mist for the vapor phase.

4. The method of claim 1, wherein the etchant comprises hydrofluoric acid.

5. The method of claim 4, wherein the hydrofluoric acid comprises a weight percent of approximately 36% such that an acid concentration of the etchant in the liquid phase is approximately the same as an acid concentration of the etchant in the vapor phase.

6. The method of claim 1, further comprising controlling a temperature of the epitaxial stack using a temperature controlled stage in the second chamber to condense a desired volume of etchant on the epitaxial stack.

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7. The method of claim 6, wherein the temperature of the epitaxial stack is between 20° C and 90° C.

8. The method of claim 6, wherein the temperature of the epitaxial stack is 60° C.

9. The method of claim 1, further comprising storing the etchant in the liquid phase in the first chamber at a temperature higher than a temperature of the epitaxial stack in the second chamber.

10. A system for supplying an etchant during epitaxial lift-off (ELO) processing, the system comprising: a first chamber configured to hold an etchant in liquid phase and to convert the etchant from the liquid phase to a vapor phase; and a second chamber coupled to the first chamber via at least one connector, the second chamber configured to have one or more epitaxial stacks placed within the second chamber, wherein each epitaxial stack has a substrate, a sacrificial layer, and a device layer, wherein the etchant in the vapor phase is transferred from the first chamber to the second chamber via the at least one connector, and wherein the etchant in the vapor phase condenses when in contact with each epitaxial stack and selectively etches the sacrificial layer to separate the device layer from the substrate.

11. The system of claim 10, wherein the first chamber is configured to convert the etchant from the liquid phase to the vapor phase by heating the etchant in the liquid phase to form a vapor for the vapor phase.

12. The system of claim 10, wherein the first chamber is configured to convert the etchant from the liquid phase to the vapor phase by atomizing the etchant in the liquid phase to form a fine mist for the vapor phase.

13. The system of claim 10, wherein the etchant comprises hydrofluoric acid.

14. The system of claim 13, wherein the hydrofluoric acid comprises a weight percent of approximately 36% such that an acid concentration of the etchant in the liquid phase is approximately the same as an acid concentration of the etchant in the vapor phase.

15. The system of claim 10, further comprising at least one temperature controlled stage in the second chamber configured to control a temperature of the one or more epitaxial stacks.

16. The system of claim 15, wherein the temperature of the one or more epitaxial stacks is between 20° C and 90° C.

17. The system of claim 16, wherein the temperature of the one or more epitaxial stacks is 60° C.

18. The system of claim 10, wherein the etchant in the liquid phase is stored in the first chamber at a temperature higher than a temperature of the one or more epitaxial stacks in the second chamber.

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