Fabrication of terahertz frequency phonon cooled HEB mixers

We have successfully fabricated NbTiN phonon-cooled, hot-electron bolometers (HEBs) on silicon-on-insulator (SOI) substrates for use in terahertz-frequency waveguide mixers. At these frequencies our technology offers three significant advantages over the more common approach of using thin quartz substrates. Firstly, the SOI chips are more rugged than quartz, which become very fragile at the required thickness (<20 microns). Secondly, the outline of the chips is defined lithographically so that they can be given a nonrectangular shape that may be required for more complicated circuits. Thirdly, freestanding gold beam-leads are used to hold the chip in place in the waveguide block and to make good electrical connections at DC, IF (intermediate frequency) and signal frequencies. The SOI wafer is a 6 μm thick Si layer bonded to a 400 μm thick oxidized Silicon "handle" wafer. The HEBs and gold beam-leads are fabricated on front side of the SOI wafer. After processing the front side, the handle wafer is removed and the 6 μm Si layer is patterned and etched to complete the device. Mixer measurements have been made with such devices at a signal frequency of 19 GHz, giving an IF bandwidth of about 1.4 GHz (under conditions that maximize low-frequency conversion efficiency). This bandwidth is close to that measured with similar devices on thicker silicon and on quartz.