SiC LAYER TRANSFER BY REMOTE EPITAXY
The SiC layer transfer via remote epitaxy addresses the cost and mechanical support challenges by using a van der Waals layer and polycrystalline SiC handle wafer, enabling efficient and cost-effective fabrication of thin SiC substrates for larger devices.
Patent Information
- Application Number
- JP2025086210
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-11-25
- Filing Date
- 2025-05-23
- Publication Date
- 2025-12-11
AI Technical Summary
The high cost of silicon carbide (SiC) substrates and the need for mechanical support in larger devices are challenges in thinning SiC substrates, as it compromises material costs and mechanical integrity.
A method for SiC layer transfer via remote epitaxy involving a van der Waals layer on a donor wafer, epitaxial SiC layer growth, and bonding to a polycrystalline SiC handle wafer, followed by separation to create a final structure with an epitaxial SiC layer on the handle wafer, utilizing a low-cost handle wafer for mechanical support.
This method allows for the use of expensive single-crystal prime 4H—SiC wafers with efficient electrical properties while reducing costs and improving mechanical support by using a low-cost polycrystalline SiC handle wafer, enhancing thermal expansion coefficient matching and reducing on-resistance.
Smart Images
Figure 2025181721000001_ABST
Abstract
Description
[Technical Field]
[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 654,459, filed May 31, 2024, which is incorporated herein by reference in its entirety for all purposes.
[0002] The present disclosure relates generally to the fabrication of thin semiconductor substrates, and more particularly to silicon carbide layer transfer via remote epitaxy. [Background technology]
[0003] Silicon carbide (SiC) substrates add significant cost to the final SiC device. Thinning SiC substrates reduces material costs at the expense of mechanical support in the demand for ever larger devices. Therefore, those skilled in the art continue their research and development efforts in the area of providing thin SiC substrates for larger devices. Summary of the Invention
[0004] A method for fabricating a structure by SiC layer transfer via remote epitaxy is provided herein. The method includes forming a van der Waals layer on the carbon face of a donor wafer, growing an epitaxial SiC layer on the van der Waals layer, and wafer-bonding the epitaxial SiC layer to a handle. The handle is made of polycrystalline SiC. The method includes separating the epitaxial SiC layer from the van der Waals layer to produce a final structure including the epitaxial SiC layer on the polycrystalline SiC of the handle wafer.
[0005] The above summary is not intended to represent every embodiment or aspect of the present disclosure. Rather, the foregoing summary illustrates certain novel aspects and features described herein. These and other features and advantages of the present disclosure will be readily apparent from the following detailed description of exemplary embodiments and aspects for carrying out the disclosure, when taken in conjunction with the accompanying drawings and the appended claims. [Brief explanation of the drawings]
[0006] The drawings described herein are for purposes of illustration only, are schematic in nature, and are intended to be exemplary rather than limiting the scope of the present disclosure.
[0007] [Figure 1] FIG. 1 is a flow diagram of a fabrication method according to one or more exemplary embodiments. [Figure 2] 1A-1C are schematic cross-sectional views of a first layer stack option for an epitaxial layer stack according to one or more exemplary embodiments. [Figure 3] 1 is a schematic cross-sectional view of a second layer stack option for an epitaxial layer stack according to one or more exemplary embodiments. [Figure 4] FIG. 10 is a schematic cross-sectional view of a third layer stack option for an epitaxial layer stack according to one or more exemplary embodiments. [Figure 5] 1 is a table of C-plane epitaxial growth measurements according to one or more exemplary embodiments. [Figure 6] 1 is a graph of a C-plane epitaxial growth thickness profile as a function of radius according to one or more exemplary embodiments. [Figure 7] 1 is a graph of a C-plane epitaxial growth radial doping profile according to one or more exemplary embodiments. [Figure 8] 1 is a graph of a calibration curve according to one or more exemplary embodiments.
[0008] The present disclosure may be modified or embodied in alternative forms, representative embodiments of which are shown in the drawings and described in detail below. The inventive aspects of the present disclosure are not limited to the disclosed embodiments. Rather, the present disclosure is intended to encompass alternative forms that fall within the scope of the present disclosure as defined by the appended claims. DETAILED DESCRIPTION OF THE INVENTION
[0009] Embodiments of the present disclosure generally define a method for fabricating the final structure using a release layer and direct transfer of an epitaxial layer from a donor wafer onto a handle wafer. The layer transfer technique allows for the utilization of expensive single-crystal prime 4H—SiC wafers with highly efficient electrical properties, while providing a low-cost handle wafer for mechanical support. The handle wafer is typically made from a sintered highly doped polycrystalline SiC wafer. Therefore, the thermal expansion coefficient is the same for both the transferred epitaxial layer and the handle wafer. Furthermore, the high doping of the handle wafer typically reduces the "on" resistance (R ON ) Improved. Referring to the drawings, like reference numerals refer to the same or similar components throughout the several views.
[0010] Referring to FIG. 1 , a flow diagram of an exemplary fabrication method according to one or more exemplary embodiments is shown. Method (or process) 100, as shown, generally includes steps 102-114. The order of the steps is shown as a representative example. Other order of steps may be implemented to meet the criteria of a particular application. Starting materials for method 100 include a donor wafer 120 and a handle wafer 140.
[0011] The donor wafer 120 implements a single-crystalline 4H—SiC wafer (where “4H” refers to the hexagonal polytype, the number 4 indicates the layer stacking order, and the letter H refers to the hcp Bravais lattice). The donor wafer 120 has a carbon-face side 122 and a silicon-face side 124. The silicon-face side 124 is opposite the carbon-face side 122. In step 102, the carbon-face side 122 of the donor wafer 120 may be cleaned and polished. The cleaning may include a hydrogen etch of several hundred nanometers to prepare the carbon-face side 122 for epitaxy growth and to reduce defect density. The hydrogen etch may be performed by exposing the carbon-face side 122 of the donor wafer 120 to hydrogen gas while the donor wafer 120 is heated to an elevated temperature. In various embodiments, the elevated temperature may be in the range of 1500°C to 1700°C.
[0012] The handle wafer 140 implements a polycrystalline SiC wafer. The handle wafer 140 has a first side 142 and a second side 144. The first side 142 is opposite the second side 144. In step 104, the second side 144 of the handle wafer 140 may be polished and cleaned.
[0013] In step 106, a van der Waals layer 126 is formed directly or indirectly on the carbon-face side 122 of the donor wafer 120. The van der Waals layer 126 is generally a two-dimensional layer that provides a weak force when bonded to the second side 144 of the handle wafer 140. The van der Waals layer 126 may be weakly bound to the surface (by active valence electrons that create the van der Waals force) to preserve a two-dimensional electronic band structure. In various embodiments, the van der Waals layer 126 implements a transfer layer or a release layer.
[0014] The van der Waals layer 126 may have a thickness of about 1.0 nanometer (e.g., in a range from about 0.3 nanometers to about 1.8 nanometers). Other thicknesses may be implemented to meet the design criteria of a particular application. The van der Waals layer 126 may include, but is not limited to, a graphene layer.
[0015] The van der Waals layer 126 may be formed as a graphene layer by SiC pyrolysis or chemical vapor deposition in an argon (Ar) atmosphere in a vacuum chamber. For example, the van der Waals layer 126 may be formed at a temperature of about 1550 degrees Celsius (°C) to 1650 degrees Celsius (°C) at 100 millibar (mbar) with an Ar atmosphere gas flow of about 20 liters / minute. In various embodiments, the van der Waals layer 126 may be a hexagonal boron nitrite layer or other two-dimensional van der Waals layer.
[0016] In step 108, an epitaxial SiC layer 130 is formed on the van der Waals layer 126. The epitaxial SiC layer 130 sandwiches the van der Waals layer 126 between the epitaxial SiC layer 130 and the original donor wafer 120. Formation of the epitaxial SiC layer 130 typically utilizes a remote epitaxial formation process in which the thin van der Waals layer 126 allows penetration of the potential field of the original donor wafer 120 and replication of the crystalline structure in the epitaxial SiC layer 130.
[0017] The epitaxial SiC layer 130 may have a thickness of about 10 micrometers (e.g., in a range of about 5 micrometers to about 15 micrometers). Other thicknesses may be implemented to meet the design criteria of a particular application.
[0018] In step 110, direct wafer bonding is performed between the donor wafer 120 and the handle wafer 140 to form a silicon carbide structure 146. Bonding is typically performed between the epitaxial SiC layer 130 of the donor wafer 120 and the second side 144 of the handle wafer 140. The epitaxial SiC layer 130 is typically strained due to a doping mismatch with the heavily doped handle wafer 140. The wafer bonding step 110 may include an annealing process at a high temperature of 400° C. or higher to help bond the epitaxial SiC layer 130 to the handle wafer 140. In various embodiments, the high temperature may be held for approximately 60 to 180 minutes.
[0019] In step 112, the donor wafer 120 is separated from the handle wafer 140 where the van der Waals layer 126 is bonded to the epitaxial SiC layer 130. The van der Waals layer 126 remains on the donor wafer 120. The epitaxial SiC layer 130 remains on the handle wafer 140. The weak forces between the van der Waals layer 126 and the epitaxial SiC layer 130 may be overcome by application of ultrasound, a stressor layer incorporated during fabrication, or the like. Upon completion of step 112, the remaining donor wafer 120 may be cleaned and recycled in step 113 for use in another round of the method 100 to produce another final structure 132.
[0020] In step 114, a final structure 132 is established. The final structure generally includes a handle wafer 140 and an epitaxial SiC layer 130. In step 114, touch-up chemical mechanical polishing (CMP) of the outer surface 134 of the epitaxial SiC layer 130 can be performed to produce an exposed silicon surface 136 (or final surface). Touch-up CMP provides a fast, low-removal polishing step optimized for scratch removal and low surface roughness.
[0021] 2, a schematic cross-sectional view of a first exemplary layer stack option for an epitaxial layer stack 150a according to one or more exemplary embodiments is shown. The epitaxial layer stack 150a may be formed on the donor wafer 120 before the van der Waals layers 126 are formed.
[0022] In the illustrated embodiment, the first epitaxial layer stack 150a includes a first layer 152, a second layer 154, and a third layer 156. The third layer 156 may be more lightly doped than the second layer 154. The second layer 154 may be more lightly doped than the first layer 152. The first layer 152 may be about 10 18 dopant atoms / cm 3(cubic centimeters). The second layer 154 may be doped to about 10 17 dopant atoms / cm 3 The third layer 156 may be doped up to about 10 16 dopant atoms / cm 3 The carbon-face side 122 may be doped up to 0.05 wt. Cleaning of the carbon-face side 122 may be performed prior to deposition of the epitaxial layer stack 150a. Cleaning typically involves a hydrogen etch of several hundred nanometers to prepare the carbon-face side 122 for epitaxy and reduce defect density. The hydrogen etch may be performed by exposing the carbon-face side 122 of the donor wafer 120 to hydrogen gas while the donor wafer 120 is heated to a high etching temperature. In various embodiments, the etching temperature may be in the range of 1500°C to 1700°C. The epitaxial layer stack 150a grown on the carbon-face side 122 typically reduces the overall defect density within the epitaxial layers. Several existing fabrication techniques can be utilized to form the epitaxial layer stack 150a, improving crystal quality and induced strain. The van der Waals layer 126 may be formed directly on the third layer 156.
[0023] 3, a schematic cross-sectional view of a second exemplary layer stack option for an epitaxial layer stack 150b according to one or more exemplary embodiments is shown. The epitaxial layer stack 150b can be formed on the donor wafer 120 before the van der Waals layers 126 are added.
[0024] In the illustrated embodiment, the second epitaxial layer stack 150b includes a first layer 162, a second layer 164, a third layer 166, and a fourth layer 168. The fourth layer 168 may be more heavily doped than the third layer 166. The third layer 166 may be more lightly doped than the second layer 164. The second layer 164 may be more lightly doped than the first layer 162. The first layer 162 may be about 10 18 atoms / cm 3 The second layer 164 may be doped up to about 10 17 atoms / cm 3The third layer 166 may be doped up to about 10 16 atoms / cm 3 The fourth layer 168 may be doped up to about 10 18 atoms / cm 3 ~about 10 19 atoms / cm 3 The carbon-face side 122 may be doped up to 0.05 wt. Cleaning of the carbon-face side 122 may be performed prior to deposition of the epitaxial layer stack 150b. Cleaning typically involves a hydrogen etch of several hundred nanometers to prepare the carbon-face side 122 for epitaxy and reduce defect density. The hydrogen etch may be performed by exposing the carbon-face side 122 of the donor wafer 120 to hydrogen gas while the donor wafer 120 is heated to a high etching temperature. Growing the epitaxial layer stack 150b on the carbon-face side 122 typically reduces the overall defect density within the epitaxial layers. Several existing fabrication techniques can be utilized to form the epitaxial layer stack 150b, improving crystalline quality and induced strain. The van der Waals layer 126 may be formed directly on the fourth layer 168.
[0025] 4, a schematic cross-sectional view of a third exemplary layer stack option for an epitaxial layer stack 150c according to one or more exemplary embodiments is shown. The epitaxial layer stack 150c can be formed on the donor wafer 120 before the van der Waals layers 126 are added.
[0026] In the illustrated embodiment, the doping level of the deformable layer 170 in the third epitaxial layer stack 150c is graded from a first side 172 adjacent to the donor wafer 120 to a second side 174 adjacent to the van der Waals layer 126. At or near the first side 172, the doping level is about 10 18 atoms / cm 3 At or near the second side 174, the doping level can be about 10 16 atoms / cm 3 ~about 10 19 atoms / cm 3Cleaning of the carbon-face side 122 may be performed prior to deposition of the epitaxial layer stack 150c. Cleaning typically involves a few hundred nanometers of hydrogen etching to prepare the carbon-face side 122 for epitaxy and reduce defect density. Hydrogen etching can be performed by exposing the carbon-face side 122 of the donor wafer 120 to hydrogen gas while the donor wafer 120 is heated to a high etching temperature. Growing the epitaxial layer stack 150c on the carbon-face side 122 typically reduces the overall defect density within the epitaxial layers. Several existing fabrication techniques can be utilized to form the epitaxial layer stack 150c to improve crystalline quality and induced strain. The van der Waals layer 126 may be formed directly on the second side 174 of the deformable layer 170.
[0027] 5, a table 180 (e.g., Table I) of exemplary C-face epitaxial growth measurements according to one or more exemplary embodiments is shown. Multiple (e.g., four) wafers were measured with various nitrogen (N) flow rates, thickness averages, thickness variations, doping averages, and doping variations in corresponding normalized arbitrary units (AU) and percentages.
[0028] Referring to FIG. 6, a graph 200 of an exemplary C-face epitaxial growth thickness profile as a function of radius is shown, according to one or more exemplary embodiments. Graph 200 has an x-axis 202 in terms of radius in millimeters (mm). A y-axis 204 has units of thickness normalized to a single arbitrary unit. Multiple curves 206 show the variation in epitaxial growth thickness for different wafers shown in Table I (see FIG. 5). Curve 206 shows that the maximum growth rate in this example is generally uniform from the center outward, peaking at a radius of approximately 50 mm. Results were achieved during process testing; variability is likely to be lower in the final process.
[0029] Referring to FIG. 7, a graph 220 of an exemplary C-face epitaxial growth radial doping profile according to one or more exemplary embodiments is shown. Graph 220 has an x-axis 222 in terms of radius in millimeters (mm). A y-axis 224 has an average doping concentration normalized to a single arbitrary unit. Multiple curves 226 show the variation in doping profile for different wafers shown in Table I (see FIG. 5). Curve 226 shows that the average doping concentration in this example is generally uniform from the center outward, increasing at a radius of approximately 55 mm. Results were achieved during process testing; variability is likely to be lower in the final process.
[0030] 8, a graph 240 of an exemplary calibration curve according to one or more exemplary embodiments is shown. The graph 240 has an x-axis 242 in units of nitrogen (N2) flow rate in arbitrary units. The y-axis 244 has units of doping concentration per cubic centimeter (Nsl) normalized to a single arbitrary unit. Curve 246 shows a calibration curve based on center-point measurements. Curve 246 shows that the doping concentration is approximately linearly proportional to the flow rate.
[0031] Embodiments of the process methods and products generally utilize direct growth of future device layers on a graphene diffusion layer. The method can include preparing the C-face of a donor substrate using hydrogen etching and growing an epitaxial stack for strain-engineered remote epitaxy. The C-face of a donor single-crystal 4H-SiC substrate can be prepared by hydrogen etching, and an epitaxial stack can be grown for strain-engineered remote epitaxy. The epitaxial growth of van der Waals layers and strain-engineered remote epitaxy of SiC device layers can be performed in a single epitaxial process on the donor C-face 4H-SiC substrate without the intermediate step of graphene transfer from the donor substrate. The method / device further provides direct bonding of the donor substrate wafer to the graphene and SiC epitaxial layers on the handle wafer without the intermediate step of an additional layer transfer process. Delamination of the donor single-crystal 4H-SiC substrate is achieved by application of a stressor layer, ultrasound, or other methods. Delamination can be enhanced by strain-engineered remote epitaxy. A simple refresh process of the donor single-crystal 4H—SiC substrate after delamination has the potential for multiple (e.g., >20) reuse times.
[0032] Clause 1: The above-described method for fabricating a structure by silicon carbide (SiC) layer transfer via remote epitaxy, comprising: forming a van der Waals layer on a donor wafer; growing an epitaxial SiC layer on the van der Waals layer; wafer-bonding the epitaxial SiC layer to a handle wafer made from polycrystalline SiC; and separating the epitaxial SiC layer from the van der Waals layer to produce a final structure comprising the epitaxial SiC layer on the polycrystalline SiC of the handle wafer.
[0033] Clause 2: The method of clause 1, wherein the donor wafer is a SiC wafer having a carbon-face side and a silicon-face side, and the van der Waals layer is formed on the carbon-face side of the SiC wafer.
[0034] Clause 3: The method of clause 2, further comprising forming an epitaxial layer stack on the carbon-face side of the SiC wafer, wherein the van der Waals layer is formed directly on the epitaxial layer stack.
[0035] Clause 4: The method of clause 3, wherein forming the epitaxial layer stack includes forming a first layer on the carbon side of the SiC wafer, forming a second layer on the first layer that is more lightly doped than the first layer, and forming a third layer on the second layer that is more lightly doped than the second layer.
[0036] Clause 5: The method of clause 4, further comprising forming a fourth layer over the third layer, the fourth layer being more heavily doped than the third layer.
[0037] Clause 6: The method of clause 3 or 4, wherein forming the epitaxial layer stack includes forming a variable layer on the carbon face side of the SiC wafer, and wherein the doping level in the variable layer varies gradually from a first side adjacent to the carbon face side to a second side adjacent to the van der Waals layer.
[0038] Clause 7: The method of clause 3 or 4, further comprising hydrogen etching the epitaxial layer stack before forming the van der Waals layer.
[0039] Clause 8: The method of any one of clauses 1-5, wherein wafer bonding comprises annealing the epitaxial SiC layer and the handle wafer.
[0040] Clause 9: The method of any one of clauses 1 to 5, wherein the final structure has an exposed silicon surface of the epitaxial SiC layer.
[0041] Clause 10: The method of clause 9, further comprising chemical-mechanical polishing the exposed silicon surface of the epitaxial SiC layer.
[0042] Clause 11: The method of any one of clauses 1 to 5, wherein the van der Waals layer is one of graphene and hexagonal boron nitride.
[0043] Clause 12: The method of any one of clauses 1 to 5, further comprising removing the van der Waals layer from the donor wafer to prepare the donor wafer for reuse in fabricating another final structure.
[0044] Clause 13: The method of any one of clauses 1-5, further comprising polishing the handle wafer before bonding to the epitaxial SiC layer.
[0045] Clause 14: A silicon carbide structure fabricated according to any one of clauses 1 to 5.
[0046] Clause 15: A silicon carbide (SiC) structure comprising: a donor wafer; an epitaxial layer stack formed on the donor wafer; a van der Waals layer formed on the epitaxial layer stack; an epitaxial SiC layer grown on the van der Waals layer; and a handle wafer made of polycrystalline SiC bonded to the epitaxial SiC layer.
[0047] Clause 16: The silicon carbide structure of clause 15, wherein the donor wafer is a SiC wafer having a carbon-face side and a silicon-face side, and the epitaxial layer stack is formed on the carbon-face side of the SiC wafer.
[0048] Clause 17: The silicon carbide structure of clause 16, wherein the epitaxial layer stack includes: a first layer formed on the carbon face side of the SiC wafer; a second layer formed on the first layer, the second layer being more lightly doped than the first layer; and a third layer formed on the second layer, the third layer being more lightly doped than the second layer.
[0049] Clause 18: The silicon carbide structure of clause 17, wherein the epitaxial layer stack further comprises a fourth layer formed on the third layer, the fourth layer being more heavily doped than the third layer.
[0050] Clause 19: The silicon carbide structure of clause 16, wherein the epitaxial layer stack includes a variable layer formed on the carbon face side of the SiC wafer, the doping level in the variable layer varying gradually from a first side adjacent the carbon face side to a second side adjacent the van der Waals layer.
[0051] Clause 20: The silicon carbide structure of any one of clauses 15 to 19, wherein the van der Waals layer has a thickness in the range of about 0.3 nanometers to about 1.8 nanometers.
[0052] While several modes for carrying out many aspects of the present teachings have been described in detail, those skilled in the art to which these teachings pertain will recognize various alternative ways of implementing the present teachings that fall within the scope of the appended claims. The above description and accompanying drawings are illustrative and exemplary of the entire range of alternative embodiments that those skilled in the art will recognize as being structurally and / or functionally equivalent based on the content contained therein, or as being implied by what is otherwise apparent, and are not limited to only the embodiments expressly shown and / or described.
[0053] Furthermore, the present concepts explicitly include combinations and subcombinations of the described elements and features. The Detailed Description and drawings supplement and explain the present teachings, the scope of which is defined solely by the claims. Approximate terms such as "about," "almost," "substantially," "generally," and "approximately" may each be used herein to indicate, for example, "in, near, or approximately there," or "within 0-5%," or "within acceptable manufacturing tolerances," or any logical combination thereof.
Claims
1. 1. A method for fabricating structures by silicon carbide (SiC) layer transfer via remote epitaxy, comprising: forming a van der Waals layer on a donor wafer; growing an epitaxial SiC layer on the van der Waals layer; wafer bonding the epitaxial SiC layer to a handle wafer made of polycrystalline SiC; separating the epitaxial SiC layer from the van der Waals layer to produce a final structure comprising the epitaxial SiC layer on the polycrystalline SiC of the handle wafer; and A method comprising:
2. the donor wafer is a SiC wafer having a carbon-face side and a silicon-face side; the van der Waals layer is formed on the carbon face side of the SiC wafer. The method of claim 1.
3. 3. The method of claim 2, further comprising forming an epitaxial layer stack on the carbon-face side of the SiC wafer, wherein the van der Waals layer is formed directly on the epitaxial layer stack.
4. forming the epitaxial layer stack; forming a first layer on the carbon side of the SiC wafer; forming a second layer on the first layer, the second layer being more lightly doped than the first layer; forming a third layer on the second layer, the third layer being more lightly doped than the second layer; The method of claim 3, comprising:
5. The method of claim 4 further comprising forming a fourth layer over the third layer, the fourth layer being more heavily doped than the third layer.