A CMOS compatible process for improved RF performance on highly doped substrates

In this paper we present a CMOS compatible process for CMOS-grade wafers in order to create specific areas where radio frequency (RF) devices can be implemented without the high losses associated to this substrate. The process is based on refilling of deep trenches, which allows the local replacement of the silicon substrate by silicon nitride, which has very good RF properties (tan δ = 5-9 10^-4). The trenches are in the order of 30 μm deep and 2 μm wide, leaving a space of 2 μm in between where the silicon still remains. In this way, half of the lossy substrate is replaced by silicon nitride. We present measurement results which indicate that the RF performance of CMOS-grade wafers can be significantly improved, as well as a careful study of the most relevant fabrication parameters and the consequences for the final RF performance of the substrate. An additional advantage of this new technique is the possibility of using it as a pre-CMOS process, thus allowing monolithic integration of CMOS electronics and RF and microwave components.