Role of Heating on Plasma-Activated Silicon Wafers Bonding

This paper reports on a comparative study of silicon wafer bonding using O2 reactive ion etching (RIE) vs sequential plasma-activated bonding (SPAB). The study shows the measurement of silicon surface roughness and the investigation of heating influences on the bonding strength and microstructures of silicon∕silicon bonded interfaces as a function of the plasma processing parameters such as plasma time and gas pressure. In SPAB, the surfaces were activated using nitrogen radicals after treatment with O2 RIE plasma for 60s . The surface roughness created via O2 RIE plasma is higher than that of the nitrogen radical. In both methods, although high strength bonding of silicon∕silicon interfaces was achieved before heating, bonding strength was reduced after heating except for the specimens activated for 10 and 60s heated at 600°C in the RIE method. This reduction may be attributed to the growing number of voids generated across the bonded interface. High resolution transmission electron microscope observations showed a silicon oxide interfacial layer in the SPAB-processed silicon∕silicon interface, which is thicker than that of the O2 RIE-processed interface without heating. After heating (at 600°C for 2h in air), the thicknesses of the interfacial oxide layers were increased for both processes. The increased oxide layer thicknesses after heating are a result of the addition of thermally activated oxygen from water absorbed by the silicon bulk wafers and oxygen intrinsic to bulk silicon.