Method of forming a semiconductor wafer containing a gallium-nitride layer and two diamond layers

Abstract

Wafers including a diamond layer and a semiconductor layer having III-Nitride compounds and methods for fabricating the wafers are provided. A nucleation layer, at least one semiconductor layer having III-Nitride compound and a protection layer are formed on a silicon substrate. Then, a silicon carrier wafer is glass bonded to the protection layer. Subsequently the silicon substrate, nucleation layer and a portion of the semiconductor layer are removed. Then, an intermediate layer, a seed layer and a first diamond layer are sequentially deposited on the III-Nitride layer. Next, the silicon carrier wafer and the protection layer are removed. Then, a silicon substrate wafer that includes a protection layer, silicon substrate and a diamond layer is prepared and glass bonded to the first diamond layer.

Claims

1. A method for fabricating a semiconductor wafer, comprising: disposing a nucleation layer on a silicon substrate; disposing at least one semiconductor layer on the nucleation layer, the at least one semiconductor layer including a III-Nitride compound; disposing a protection layer on the at least one semiconductor layer; bonding a carrier wafer to the protection layer; removing the silicon substrate, the nucleation layer and a portion of the at least one semiconductor layer; disposing an intermediate layer on a remaining portion of the at least one semiconductor layer, wherein the intermediate layer includes a first layer that is formed of a material including one or more of poly-Si, SiO.sub.2, Al.sub.2O.sub.3, and SiN and a second layer including diamond particles; disposing a first diamond layer on the intermediate layer; after disposing the first diamond layer on the intermediate layer, removing the carrier wafer and the protection layer; and after removing the carrier wafer and the protection layer, bonding a support wafer to the first diamond layer, the support wafer including a silicon substrate and a second diamond layer.

2. The method of claim 1, wherein the at least one semiconductor layer includes a GaN layer that is directly disposed on the nucleation layer and wherein the removed portion of the at least one semiconductor layer is a portion of the GaN layer near an interface between the nucleation layer and the GaN layer.

3. The method of claim 2, wherein the step of disposing at least one semiconductor layer includes: disposing the GaN layer on the nucleation layer; and disposing at least one of AlGaN layer and an InAlN layer on the GaN layer.

4. The method of claim 1, further comprising, after the step of bonding the support wafer to the first diamond layer: disposing an additional semiconductor layer on the remaining portion of the at least one semiconductor layer, the additional semiconductor layer including a III-Nitride compound.

5. The method of claim 1, wherein the protection layer is formed of a material that includes one or more of SiN, poly-Si, Al.sub.2O.sub.3, AlN and GaN.

6. The method of claim 1, further comprising, after the step of disposing the first diamond layer: lapping a surface of the first diamond layer to enhance a flatness of the surface of the first diamond layer.

7. The method of claim 1, wherein the support wafer further includes a second protection layer that is formed of a material including one or more of SiN, poly-Si, Al.sub.2O.sub.3, AlN and GaN.

8. The method of claim 7, wherein the step of bonding the support wafer to the first diamond layer includes: forming a first glass coating on the protection layer of the support wafer; forming a second glass coating on the first diamond layer; and melting the first and second glass coatings to form a glass bonding layer that bonds the support wafer to the first diamond layer.

9. The method of claim 1, further comprising: forming one or more semiconductor devices in the remaining portion of the at least one semiconductor layer.

10. The method of claim 9, wherein the one or more semiconductor devices includes one or more high electron mobile transistors (HEMTs).

11. The method of claim 1, wherein the carrier wafer includes a silicon substrate and wherein the step of bonding the carrier wafer to the protection layer includes glass bonding the silicon substrate of the carrier wafer to the protection layer.

12. The method of claim 1, wherein the nucleation layer is an AlN layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) References will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.

(2) FIGS. 1-12 show an exemplary process for fabricating a wafer that includes two diamond layers and a III-Nitride layer according to embodiments of the present disclosure.

(3) FIGS. 13-17 show an exemplary process for device processing the wafer in FIG. 12 to fabricate HEMTs according to embodiments of the present disclosure.

(4) FIG. 18 shows a flowchart of an exemplary process for fabricating a semiconductor wafer according to embodiments of the present disclosure.

(5) FIG. 19 shows a flowchart of an exemplary process for processing a semiconductor wafer according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) In the following description, for purposes of explanation, specific details are set forth in order to provide an understanding of the disclosure. It will be apparent, however, to one skilled in the art that the disclosure can be practiced without these details. Furthermore, one skilled in the art will recognize that embodiments of the present disclosure, described below, may be implemented in a variety of ways, such as a process, an apparatus, a system, a device, or a method on a tangible computer-readable medium.

(7) One skilled in the art shall recognize: (1) that certain steps may optionally be performed; (2) that steps may not be limited to the specific order set forth herein; and (3) that certain steps may be performed in different orders, including being done contemporaneously.

(8) Elements/components shown in diagrams are illustrative of exemplary embodiments of the disclosure and are meant to avoid obscuring the disclosure. Reference in the specification to “one embodiment,” “preferred embodiment,” “an embodiment,” or “embodiments” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the disclosure and may be in more than one embodiment. The appearances of the phrases “in one embodiment,” “in an embodiment,” or “in embodiments” in various places in the specification are not necessarily all referring to the same embodiment or embodiments. The terms “include,” “including,” “comprise,” and “comprising” shall be understood to be open terms and any lists that follow are examples and not meant to be limited to the listed items. Any headings used herein are for organizational purposes only and shall not be used to limit the scope of the description or the claims. Furthermore, the use of certain terms in various places in the specification is for illustration and should not be construed as limiting.

(9) FIGS. 1-12 show an exemplary process for forming a wafer that includes a diamond layer and a III-Nitride layer (or, equivalently semiconductor layer that includes III-Nitride compound) according to embodiments of the present disclosure. As depicted in FIG. 1, the wafer 100 may include a silicon substrate 102, an AlN layer 104 and a III-Nitride layer 106, and a protection layer 108.

(10) In embodiments, the III-Nitride layer 106 may include one or more layers that each include a GaN compound, such as hexagonal AlGaN/GaN, InAlN/GaN or cubic AlGaN/GaN. In the following sections, a III-Nitride layer may collectively refer to one or more layers that each include a III-Nitride compound. FIG. 2A shows an exemplary III-Nitride layer 106-1 that includes: a GaN buffer layer 106-14; and at least one of AlGaN layer and InAlN layer 106-12. FIG. 2B shows another exemplary III-Nitride layer 106-2 that includes only one GaN buffer layer 106-22.

(11) In embodiments, the protection layer 108 may protect the III-Nitride layer 106 from thermal and mechanical damages that may occur during the subsequent processes. For instance, if the glass coating 112 (in FIG. 3) is directly attached to the III-Nitride layer 106, the mismatch of coefficients of thermal expansion (CTE) between the glass coating 112 and the III-Nitride layer 106 may generate stress on the III-Nitride layer 106, generating negative impact on the performance of semiconductor devices to be formed in the III-Nitride layer 106. In embodiments, the material and thickness of the protection layer 108 may be selected to mitigate the stress due to the mismatch of CTEs. In embodiments, the protection layer 108, which may be formed of a material that includes one or more of SiN, poly silicon, Al.sub.2O.sub.3, AlN and GaN, may be deposited by a suitable deposition technique.

(12) In general, the large lattice mismatch between silicon in the silicon substrate 102 and GaN in the III-Nitride layer 106 may lead to cracks in the GaN buffer when cooling the heterostructure from the growth temperature to the room temperature. In embodiments, the AlN layer (or shortly, nucleation layer) 104 may be formed on the silicon substrate 102 to prevent cracks in the GaN buffer layer 106-14 (or 106-22) and/or growth of the cracks to the AlGaN/InAlN layer 106-12. In embodiments, the AN layer 104 and the III-Nitride layer 106 may be formed on the silicon substrate 102 by conventional wafer processing techniques, such as metal-organic chemical vapor deposition (MOCVD) technique.

(13) FIG. 3 shows a process of bonding a carrier wafer 110 to the wafer 109 according to embodiments of the present disclosure. As depicted, the carrier wafer 110 may include a silicon carrier wafer 116 and a glass coating 114 formed on its surface facing the wafer 109. In embodiments, the wafer 109 may be prepared by forming a glass coating on the protection layer 108. Then, the silicon carrier wafer 110 is mounted on the wafer 109 and heated at the temperature of 900-1000° C. to melt the glass coatings 112 and 114 and to form a glass bonding layer. FIG. 4 shows a wafer 118 that include the silicon carrier wafer 110 bonded to the wafer 109 by the glass bonding process, where the bonding layer 115 may be formed by melting the glass coatings 112 and 114.

(14) In embodiments, as depicted in FIG. 5, the silicon substrate 102, AlN layer 104 and a portion of the III-Nitride layer 106 in the wafer 118 may be removed to form the wafer 120. In embodiments, the silicon substrate 102 may be removed by one or more of grinding, lapping, polishing and dry etching processes, even though other suitable process may be used to remove the silicon substrate 102. In embodiments, the AlN nucleation layer 104 may be removed by a wet etching process, even though other suitable process may be used to remove the AlN layer 104.

(15) In embodiments, as discussed above in conjunction with FIG. 2A, the III-Nitride layer 106 may include the AlGaN/InAlN layer 106-12 and GaN layer 106-14. In alternative embodiment, as discussed above in conjunction with FIG. 2B, the III-Nitride layer 106 may include only GaN layer 106-22. In both cases, each of the GaN layers may include a portion near the interface between the GaN layer 106-22 and AlN layer 104, where the portion includes cracks developed during deposition of the GaN layer 106-22 and/or AlGaN/InAlN layer 106-12. As such, the portion of the GaN layer having the cracks may be removed so that the remaining GaN layer may not include any crack. In the wafer 120, the III-Nitride layer 107 refers to a III-Nitride layer that is formed by removing the portion of the GaN layer from the III-nitride layer 106.

(16) As depicted in FIG. 6, an intermediate layer 124 may be formed on the III-Nitride layer 107, where the intermediate layer 124 may include a first intermediate layer and a second intermediate layer (or equivalently seed layer). If a diamond layer is directly deposited on the III-Nitride layer 107, the mismatch of coefficients of thermal expansion (C 1E) between the diamond layer and the III-Nitride layer 107 may generate thermal stress on the III-Nitride layer 107 during formation of the diamond layer, and as such, the first intermediate layer may be deposited to mitigate the thermal stress. In embodiments, the material and thickness of the first intermediate layer may be selected to mitigate the stress due to the mismatch of CTEs. In embodiments, the first intermediate layer may be formed of a dielectric material, such as poly-Si, SiO.sub.2, Al.sub.2O.sub.3 or SiN.

(17) In embodiments, the second intermediate layer (i.e., a seed layer) may be formed on the top surface of the first intermediate layer. To form the seed layer, the wafer 122 may be submerged in an aqueous suspension of diamond nano particle (diamond seed particles) so that the top surface of the first intermediate layer may be in direct contact with the aqueous suspension. The diamond particles may be adsorbed onto the surface of the first intermediate layer, to thereby form the seed layer. Depending on the exposure time in the suspension and the concentration of the diamond particles, the density of the particles in the seed layer may be determined. In embodiments, the diamond particles may adhere to the seed layer better than to the first intermediate layer.

(18) In embodiments, after forming the intermediate layer 124, a diamond layer 128 may be deposited on the intermediate layer 124, to thereby form the wafer 126 in FIG. 7. In embodiments, the diamond layer 128 may be formed by chemical vapor deposition (CVD) technique, even though other suitable techniques may be used. Then, a lapping process may be carried out to reduce the surface roughness of the diamond layer 128. In FIG. 8, the wafer 130 may include a diamond layer 132 that has a top surface with enhanced flatness by the lapping process.

(19) In embodiments, the silicon carrier wafer 116, glass bonding layer 115, and protection layer 108 may be removed from the wafer 130, as depicted in FIG. 9. In embodiments, the thickness of the wafer 134 may be so thin that the wafer 134 may not have sufficient mechanical strength to withstand the thermal and mechanical stresses during the subsequent processes for forming semiconductor devices in the wafer 134. Thus, to bolster the mechanical strength of the wafer 134, a support wafer 140 may be bonded to the wafer 134, as shown in FIG. 10.

(20) FIG. 10 shows a process of bonding a support wafer 140 to a wafer 136 according to embodiments of the present disclosure. As depicted, the wafer 136 may be prepared by forming a glass coating 138 on the diamond layer 132 of the wafer 134. In embodiments, the support wafer 140 may be prepared by depositing a diamond layer 142 on a silicon substrate 144 on the bottom surface of the silicon substrate 144 and depositing a protection layer 146 and a glass coating 148 on the top surface of the silicon substrate 144. In embodiments, the diamond layer 142 may be deposited by a chemical vapor deposition (CVD) technique, even though other suitable techniques may be used. In embodiments, the protection layer 146, which may be formed of a material that includes one or more of SiN, poly silicon, Al.sub.2O.sub.3, AlN and GaN, may be deposited by a suitable deposition technique.

(21) In embodiments, the wafer 136 is mounted on the support wafer 140 and heated at the temperature of 900-1000° C. to melt the glass coatings 138 and 148 and to form a glass bonding layer 152. FIG. 11 shows a wafer 150 that includes the support wafer 140 bonded to the wafer 136 by the glass bonding process.

(22) As discussed above in conjunction with FIGS. 2B and 5, the III-Nitride layer may have a GaN buffer layer only i.e., the III-Nitride layer 107 may include only GaN layer. In such a case, another III-Nitride layer 162, such as AlGaN/InAlN layer, may be formed (regrown) on the GaN buffer layer, resulting in the III-Nitride layer stack 164. In the following sections, the III-Nitride layer stack 164 is referred to as a III-Nitride layer, even though more than one III-Nitride layer may be included in the III-Nitride layer 164.

(23) In embodiments, an edge trimming process may be optionally performed on the wafer 160 to make primary/flat zone in the wafer 160 before shipping to a foundry for further processing of the wafer 160.

(24) FIGS. 13-17 show an exemplary process for device processing the wafer 160 in FIG. 12 to fabricate HEMTs according to embodiments of the present disclosure. In embodiments, as shown in FIG. 13, the wafer 170 may include various semiconductor devices 171, such as semiconductor transistors, formed in the III-Nitride layer 164. In embodiments, the semiconductor devices 171 may be formed by suitable semiconductor processes.

(25) In embodiments, upon forming the semiconductor devices 171, a patterned metal layer 172 may be formed on the III-Nitride layer 164. In embodiments, the metal layer 172 may be formed of an ohmic alloy (preferably, but not limited to, Au, Ag, Ni, Ti, Al or any combination thereof) that alloys at 850° C. It is noted that various fabrication methods may be used to form the metal layer 172. In embodiments, the metal layer 172 may be annealed to reduce the contact resistance between the metal layer 172 and the III-Nitride layer 164. In embodiments, the metal layer 172 may be patterned by a suitable process, such as photolithography.

(26) In embodiments, as shown in FIG. 14, the wafer 174 may include one or more vias 176 formed by a laser drilling technique or any other suitable technique. In the conventional techniques, vias are drilled from a substrate side toward a III-Nitride side of the wafer. If the conventional techniques using laser beams are performed to drill vias from the diamond layer 142 toward the III-Nitride layer 164, the diamond layer 132, which may be formed of poly crystalline diamond, may scatter the laser beam, causing unintended subsidiary drilled spots or damages to the area of metal-semiconductor interface, especially gate contact area.

(27) In contrast, in embodiments, the one or more vias 176 may be drilled from the metal layer 172 toward the protection layer 144. This drilling technique of the present disclosure may reduce the scattering of the laser beam by the diamond layer 132. Also, the heat energy, which may be accumulated in the gate contact area during the drilling process of the metal layer 172 and the III-Nitride layer 164, may be discharged to the diamond layer 132 by heat transfer, which further reduces the thermal damages to the III-Nitride layer 164. In embodiments, the one or more vias 176 may extend from the metal layer 172 into the protection layer 146 of the support wafer 14

(28) In embodiments, as shown in FIG. 15, a chemical or electrochemical gold plating technique may be used to deposit a thin layer of gold 180 on the metal layer 172 and in a top portion of the one or more vias 176. It is noted that other suitable metal may be used in place of the gold to form the metal layer 180 and other suitable techniques may be used to form the metal layer 180.

(29) It is noted that the processes in FIGS. 14 and 15 are exemplary processes performed on the wafer 160. As such, it should be apparent to those of ordinary skill in the art that other suitable processes may be performed on the wafer 160 to form various semiconductor devices in the wafer 160.

(30) As discussed above, in embodiments, the support wafer 140 is bonded to the wafer 134 so as to bolster the mechanical strength of the wafer 134. As such, upon completion of the final passivation process (or any other process that may cause unintended mechanical deformation (such as bending and warping) of the wafer 178, the support wafer 140 may be removed from the wafer 178. In embodiments, the diamond layer 142, silicon substrate 144, protection layer 146 and glass bonding layer 152 may be removed from the wafer 178 by a lapping process and/or any other suitable processes, as shown FIG. 16. In embodiments, the bottom surface of the diamond layer 132 may be further cleaned by a dry etching technique so as to remove remaining glass material on the bottom surface of the diamond layer 132.

(31) In FIG. 17, the wafer 200 may include a metal layer 202 that is deposited on the bottom surface of the diamond layer 132 and the side surfaces of the one or more vias 176 by the Au plating technique. It is noted that other suitable metal may be used in place of the gold for the metal layer 202, and other suitable techniques may be used to form the metal layer 202 on the diamond surface 132. In embodiments, the metal layer 202 may be in electrical contact with the metal layers 172 and 180 through the vias 176.

(32) In embodiments, other processes, such as street etching, may be performed on the metal layer 202. Upon completion of the processes to form semiconductor devices in the wafer 200, the wafer 200 may be diced for singulation.

(33) FIG. 18 shows a flowchart 1800 of an exemplary process for fabricating a semiconductor wafer according to embodiments of the present disclosure. At step 1802, the nucleation layer 104 and the III-Nitride layer (or equivalently, a semiconductor layer that includes a III-Nitride compound) 106 may be disposed on the silicon substrate 102. In embodiments, the nucleation layer 104 may be formed between the silicon substrate 102 and the III-Nitride layer to prevent cracks in the III-Nitride layer 106. In embodiments, the III-Nitride layer 106 may include only GaN layer 106-22. In alternative embodiments, the III-Nitride 106 layer may include the GaN layer 106-14 and the AlGaN/InAlN layer 106-12.

(34) At step 1804, the protection layer 108 may be disposed on the III-Nitride layer 106, where the protection layer may be formed of a material that includes one or more of SiN, poly silicon, Al.sub.2O.sub.3, AlN and GaN.

(35) At step 1806, the carrier wafer 110 having the silicon carrier wafer 116 may be bonded to the protection layer 108. In embodiments, the glass coating layers 112 and 114 may be formed on the protection layer 108 and silicon carrier wafer 116, respectively, and heated at the temperature of 900-1000° C. Then, at step 1808, the silicon substrate 102 and nucleation layer 104 may be removed.

(36) It is noted that a portion of the III-Nitride layer 106 is also removed at step 1808. In embodiments, when the III-Nitride layer 106 is formed of a GaN only, a portion of the GaN near the interface between the GaN layer 106-22 and nucleation layer 104 may include cracks developed during deposition of the GaN layer 106-22. Similarly, in alternative embodiments, when the III-Nitride layer 107 includes the GaN buffer layer 106-14 and the AlGaN/InAlN layer 106-12, a portion of the GaN layer near the interface between the GaN layer 106-14 and nucleation layer 104 may include cracks developed during deposition of the GaN layer 106-14 and the AlGaN/InAlN layer 106-12. In both cases, a portion of the GaN layer having the cracks may be removed so that the remaining III-Nitride layer 107 may not include any crack, at step 1808.

(37) At step 1810, the diamond layer 128 may be disposed on the III-Nitride layer 107. In embodiments, the intermediate layer 124 may include first and second layers, where the first layer may be disposed between the III-Nitride layer 107 and the diamond layer 128 and may mitigate the stress due to the mismatch of CTEs of the diamond layer 128 and the III-Nitride layer 107. In embodiments, the first layer may be formed of a dielectric material, such as poly-Si, SiO.sub.2, Al.sub.2O.sub.3 or SiN. In embodiments, the second layer (which is a seed layer) may be disposed between the first layer and the diamond layer 128, where the seed layer may include diamond nano particle (diamond seed particles). In embodiments, after the step 1810 is completed, a lapping process may be carried out to enhance the surface flatness of the diamond layer 128.

(38) At step 1812, the carrier wafer 110 and the protection layer 108 may be removed. At step 1814, the support wafer 140 may be bonded to the diamond layer 132, where the support wafer 140 may include the diamond layer 142, silicon substrate 144, and protection layer 146. In embodiments, the glass coatings 148 and 138 may be formed on the diamond layer 132 and the protection layer 146, respectively, and heated at the temperature of 900-1000° C. for glass bonding.

(39) At step 1816, another III-Nitride layer (or equivalently, another semiconductor layer having a III-Nitride compound) 162 may be optionally disposed on the III-Nitride layer 107. In embodiments, when the III-Nitride layer 107 includes only the GaN buffer layer 106-22, an additional AlGaN/InAlN layer 162 may be formed on the GaN buffer layer.

(40) FIG. 19 shows a flowchart of an exemplary process for processing a semiconductor wafer according to embodiments of the present disclosure. At step 1901, semiconductor devices 171, such as semiconductor transistors, may be formed in the III-Nitride layer (or a semiconductor layer) 164 of the semiconductor wafer 160. As discussed above, the semiconductor wafer 160 may include the support wafer 140, glass bonding layer 152, diamond layer 132, intermediate layer 124 and the III-Nitride layer 164. In embodiments, the support wafer 140 may include the protection layer 146, silicon substrate 144 and diamond layer 142. At step 1902, the metal layer 172 may be formed and patterned on the III-Nitride layer 164.

(41) At step 1904, a laser drilling technique or any other suitable technique may be performed to drill one or more holes from the metal layer 172 toward the silicon substrate 144 to thereby form the one or more vias 176, where the one or more vias 176 may extend from the metal layer 172 into the protection layer 146 of the support wafer 140. Then, at step 1906, the metal layer 180 may be formed on the metal layer 172 and in a top portion of the one or more vias 176. Next, at step 1908, the support wafer 140 and the glass bonding layer 152 may be removed to expose the bottom surface of the diamond layer 132. Optionally, in embodiments, the bottom surface of the diamond layer 132 may be further cleaned by a dry etching technique so as to remove the remaining glass material on the bottom surface of the diamond layer 132, at step 1910.

(42) At step 1912, the metal layer 202 may be disposed on the exposed surface of the diamond layer 132 and the side surfaces of the one or more vias 176 by the Au plating technique. It is noted that other suitable metal may be used in place of the gold for the metal layer 202, and other suitable techniques may be used to form the metal layer 202 on the diamond surface 132. In embodiments, the metal layer 202 may be in electrical contact with the metal layers 172 and 180 through the vias 176. Then, at step 1914, the wafer 200 may be diced for singulation.

(43) While the invention is susceptible to various modifications and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims.