Low-Temperature Direct Bonding of Diamond—Approach for Fabricating Low-Thermal-Resistance Widegap Devices

Characteristics of semiconductor electron devices such as output powers are limited by their temperature rise due to Joule heating in operation (self-heating effects). Device structures with the lowest thermal resistance are, consequently, one of the most important topics in R&D for advanced devices, especially for devices with widegap semiconductors that handle high output powers with high frequencies. Diamond is assumed to be the ideal material as heat spreader because of its high thermal conductivity and excellent electrical properties. In this presentation, we describe results that we obtained by directly bonding of diamond to Si and GaN and discuss the future prospective of direct bonding of diamond. We used surface activated bonding (SAB) technologies, in which two wafers are bonded after removing native oxides on their surfaces by irradiating them with fast atom beams of Ar (surface activation process). In SAB, bonding process is completed without heating bonded wafers in most cases, i.e., junctions of semiconductors with different coefficients of thermal expansion (CTE) can be fabricated.<br/>We bonded a high-pressure high-temperature (HPHT) diamond (001) wafer and a Si (100) wafer using SAB. We found that the diamond//Si junction withstood a 1000-°C annealing in spite of a large difference in CTEs between diamond and Si. A high-resolution TEM observation revealed that a poly crystalline SiC intermediate layer was formed at the bonding interface after the annealing, which was assumed to play a role of reducing impacts of the difference in CTEs.<br/>We prepared a GaN//diamond junction by bonding a GaN (0001) surface grown on Si (111) wafer to a diamond (001) wafer and removing the Si (111) wafer for the growth of nitride. We annealed the GaN//diamond junctions at temperatures up to 1000 °C and found that the junctions were also tolerant against such a high-temperature annealing.<br/>Recently we transferred an AlGaN/GaN epi layer to diamond using SAB and subsequently fabricated HEMTs. Ohmic contacts on nitride were formed by an annealing at 800-°C after bonding to diamond. The temperature rise of the GaN-on-diamond HEMTs was found to be lower than that of on-Si HEMTs with the same nitride structure. The results imply that direct bonding of diamond is promising for fabricating low-thermal-resistance semiconductor devices, especially after transferring active device layers to diamond wafers.<br/>Acknowledgement—This work was supported by the Feasibility Study Program of New Energy and Industrial Technology Development Organization (NEDO) and JSPS KAKENHI. TEM samples were fabricated at The Oarai Center and at the Laboratory of Alpha-Ray Emitters in IMR under the Inter-University Cooperative Research in IMR of Tohoku University. The observation of the TEM samples was supported by Kyoto University Nano Technology Hub. AlGaN/GaN epi layers were grown and supplied by Air Water Inc.