Heterogeneous Integration and Micromachining Technologies for Terahertz Devices and Components

Terahertz electronics has been a topic of research and development for many years, motivated largely by the technological needs of the radio astronomy and remote sensing scientific communities. Over the past decade, however, this field has experienced dramatic growth and intense, renewed interest from academic researchers and federal agencies, as well as from industry. This interest has arisen, in part, from recent funding initiatives from the federal government (such as DARPA's Terahertz Electronics Program), but is also largely due to the establishment of a commercial infrastructure that has made test and measurement instrumentation available to the engineers and scientists working at these frequencies. Moreover, the emergence of CMOS as a potential submillimeter-wave device technology has greatly expanded access to this spectral region by providing circuit designers with a platform for realizing terahertz circuits without need for specialized fabrication facilities or processes. The recent and rapid progress in terahertz electronics has created a demand for improved approaches to packaging and integration, as well as a need for new measurement instrumentation for characterizing emerging terahertz devices. This paper focuses on two recent research developments aimed at addressing these needs and broadening the technology base for both terahertz system implementation and terahertz metrology. These developments include (1) the development of a direct-contact probe technology that permits on-wafer scattering-parameter characterization and measurement of planar integrated devices at frequencies to 1 THz and beyond, and (2) the establishment of processing technologies that permit fabrication of highly-integrated submillimeter-wave diode-based circuits, such as heterodyne receivers and frequency multipliers, that are based on heterogeneous integration of III-V semiconductor devices with thin silicon membranes as a support and integration substrate. The technical foundation for each of these efforts is micromachining of silicon that allow the formation of mechanically-robust and low-loss membrane carriers to support terahertz devices and circuitry. Two examples of heterogeneous integration with silicon as an approach to packaging terahertz components are detailed in this paper. These include development of micromachined probes for on-wafer measurements of devices and circuits in the WR-1.0 waveguide band (0.75 – 1.1 THz). The probe design concept will be presented and methods for characterizing the probe described. Measurements demonstrate that the probes exhibit an insertion loss of less than 7 dB and return loss of greater than 15 dB over 750—1100 GHz band, yielding the first demonstration of on-wafer probe operating above 1 THz. In addition, an example of heterogeneous integration/packaging of a submillimeter-wave frequency quadrupler operating at 160 GHz with efficiency of 30% and corresponding output power of 70 mW will be discussed. The quadrupler design includes two frequency doubler stages in cascade and is based on a balanced circuit architecture that addresses degradation issues often arising from impedance mismatches between multiplier stages. A unique quasi-vertical diode fabrication process consisting of transfer of GaAs epitaxy to the thin silicon support substrate is used to implement the quadrupler, resulting in an integrated drop-in chip module that incorporates 18 varactors, matching networks and beamleads for mounting.