Application of ultra-thin silicon technology to submillimeter detection and mixing

Superconducting based SIS and HEB detectors continue to yield improved noise temperatures at submillimeter wavelengths. These higher frequencies present new challenges, particularly for waveguide based designs where the tolerances for mounting small mixer chips become quite narrow. Also, conventional millimeter wavelength techniques for making the IF and ground connections are more prone to error. As the device technology for these SIS and HEB-based detectors matures, there is also an increased interest in integrated receiver arrays. These challenges call for simpler mounting designs and more repeatable assembly techniques. Our research group, at the University of Virginia, is meeting these challenges with a new ultra-thin mixer chip technology, with integrated gold beam leads, first reported in [1]. We have since further developed and improved on this technology. We have several ongoing SIS, HEB and OMT projects which utilize these capabilities. Most important to this technology is the transition from the conventional use of quartz as a circuit substrate material to that of ultra-thin (<10 microns) silicon. We accomplish this by creating the mixer circuitry on a silicon-on-insulator (SOI) wafer and using a sophisticated backside release process to produce individual mixer chips. These 3 micron ultra-thin chips present less dielectric material within a waveguide channel and are actually much more robust than quartz chips that are an order of magnitude, or more, thicker. We use integrated 1-2 micron thick gold beam leads to simplify the electrical connection and placement of the chip within the receiver waveguide. Beam leads are another component of the mounting process that makes our modular mixer implementation possible. Based on our SOI process, we are currently developing several HEB mixers - two single element metal waveguide designs at 600 GHz and 1.6 THz, and an integrated array approach using silicon laser micromachined blocks centered at 900 GHz and 1.8 THz. We are also pursuing several SIS mixers-one single element 350-500 GHz design with ultra wide IF bandwidth and one 350 GHz receiver array. In this paper we will discuss our ultra thin silicon beam lead technology and the ongoing progress of these new receivers.