Low Temperature Bonding Polycrystalline Diamond to Si by Using Au Thin-layer for High-power Semiconductor Devices

As the semiconductor devices are getting higher  frequency, higher power and smaller size, the management of  thermal dissipation becomes a big challenge. The application of  diamond as heat dissipation substrate for high-power  semiconductor devices has been placed great expectation due to  its ultra-high thermal conductivity. In this study, Au thin layer  was used for the low temperature bonding of polycrystalline  diamond and Si. The Au-Au atomic diffusion bonding was  successfully achieved. Clean processes were optimized. Scanning acoustic microscope (SAM) was used to determine the  bonding porosity, which typically exceeded 10%. Atomic force  microscope (AFM) tests indicated the diamond surface  roughness (Ra>1nm). The poor surface flatness of diamond  contributed to the degradation of bond-ability. The  technological route of diamond heat dissipation substrate for  high-power semiconductor devices needs more optimization. 

INTRODUCTION 

As the semiconductor devices are getting higher frequency,  higher power and smaller size, the management of thermal  dissipation becomes a big challenge. Thermal accumulation  increases rapidly in the active region of electronic devices,  forming local hotspots and leading to the significant  degradation of performance [1]. Therefore, how to manage the  heat dissipation becomes one of the key technical bottlenecks  restricting the further development of high-power  semiconductor devices. Recently, a falling price of artificial  polycrystalline diamond was achieved through mass  production. The application of diamond as heat dissipation  substrate has been placed great expectation due to its ultrahigh thermal conductivity, which could reach up to ~2000W/m·K [2-5]. Fig.1 shows the schematic diagram of a  diamond based high-power semiconductor device. Although diamond can be directly grown on the  semiconductor wafers by chemical vapor deposition (CVD),  the growth of diamond needs an environment of more than  700℃ [3], which is unacceptable for semiconductor devices  to withstand such high temperature. Surface Activated  Bonding (SAB) technology can bond a single-crystal diamond  to device at room temperature by using Ar beams for  activation [4]. But it needs extremely smooth surface, which  is very challenging for polycrystalline diamond. Another  method is soldering diamond to semiconductor devices [5].  However, the thermal conductivity of solder layer is typically  two orders of magnitude smaller than that of diamond, and the  bonding requires thick filler of metal layers. That introduces  huge thermal resistance and thus significantly degrades the  heat dissipation performance of diamond. In this study, novel Au thin layer was used for the low  temperature bonding of polycrystalline diamond and Si. The  Au-Au atomic diffusion bonding was successfully achieved.  Scanning acoustic microscope (SAM) was used to determine  the bonding porosity. The effects of surface roughness and flatness on the porosity were discussed. The technological  route of diamond heat dissipation substrate for high-power  semiconductor devices needs more optimization. 

Fig. 1. Schematic diagram of a diamond based high-power semiconductor  device. 

EXPERIMENTAL PROCEDURES 

The CVD deposited polycrystalline diamond films were  0.7 mm in thickness and were cut into plates with size of  10mm×10mm. The diamond plates had being ground and  polished. Then they were carefully cleaned. Physical vapor  deposition (PVD) method was used to deposit 5nm Ti/200nm  Cu/5nm Ti/20nm Au on diamond plates. At the same time, Si  plates (10mm×10mm×0.6mm) were cleaned and deposited  with 5nm Ti/20nm Au. Diamond-Si pairs were alignment  together and pressed by hand. They could be pre-bonded at  room temperature. Then they were bonded in a vacuum  chamber with 200℃ and 6MPa. As control experiments, SiSi pairs were bonded with the same parameters. Scanning  acoustic microscope (SAM) was used to determine the  bonding porosity. Atomic force microscope (AFM) was used  to characterize the surface roughness.

CONCLUSIONS

In this study, Au thin layer was used for the low  temperature bonding of polycrystalline diamond and Si. The  Au-Au atomic diffusion bonding was successfully achieved.  Clean processes were optimized. SAM was used to determine  the bonding porosity, which typically exceeded 10%. AFM  tests show that the typical diamond surface roughness was  Ra>1nm. In addition, the poor surface flatness (TTV≥2μm) of  diamond contributed to the degradation of bond-ability. The  effect of surface roughness on solid-state diffusion bonding  was discussed. Future work needs to flatten the diamond  surface and study the influence of diamond thickness and  thermal conductivity on the heat accumulation effect of  semiconductor devices. Develop and optimize the  technological route of diamond heat sink for high-power  semiconductor devices.