Steven M. Marazita

Integrated GaAs Schottky mixers by spin-on-dielectric wafer bonding

A novel wafer bonding process has been used to integrate high quality GaAs devices on quartz substrates. The method of adhesion by spin-on-dielectric temperature enhanced reflow (MASTER) uses a spin-on-dielectric as a bonding agent to achieve a robust bond that in no way degrades either high frequency performance or reliability. A 585 GHz integrated mixer fabricated using this process has achieved record double-sideband mixer noise temperatures of 1,150 K at room temperature and 880 K at 77 K. Furthermore, the integrated mixers require no mechanical tuning, are easy to assemble, and repeatable. Precise control of the circuit geometry, coupled with the reduction of parasitic elements, allows greater accuracy of computer simulations and will therefore lead to better high frequency performance and bandwidth. This new technology is easily extended to other circuit designs and will allow the development of a new generation of submillimeter-wave integrated circuits.

Improved 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne applications

Low-noise broad intermediate frequency (IF) band 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne radiometers have been developed and characterized. The planar GaAs Schottky diodes are fully integrated with the RF/IF filter circuitry via the quartz-substrate upside-down integrated device (QUID) process resulting in a robust and easily handled package. A best double-sideband-mixer noise temperature of 490 K was achieved with 3 mW of local-oscillator power at 2-GHz IF. Over an IF band of 1.5-10 GHz, the noise temperature is below 1000 K. This state-of-the-art performance is attributed to lower parasitic capacitance devices and a low-loss waveguide circuit. Device fabrication technology and the resulting RF mixer performance obtained in the 200-250-GHz frequency range will be described.

Fabrication and test results for integrated submillimeter-wavelength planar Schottky mixers on quartz utilizing MASTER

Sensitive and robust heterodyne mixers are needed for future atmospheric remote sensing missions. This data from satellites such as NASAs Earth Observing System (EOS) lends great insight into molecular interactions in our environment. The Microwave Limb Sounder (MLS) on EOS will detect radiation emitted from 03, ClO, and OH molecules which are critical to our understanding of ozone depletion and greenhouse warming. The heterodyne mixers on MLS must exhibit sufficient spectral sensitivity, wide bandwidth, low noise, and minimal LO power requirements. Planar GaAs Schottky diodes currently are the most promising technology for space-borne radiometers where cryogenic cooling is not desirable. In this work we present progress on a novel wafer bonding technology, MASTER, used to integrate submillimeter wavelength planar GaAs Schottky mixer diodes with quartz microstrip circuitry. Problems associated with wafer expansion after bonding, open- circuited devices, and Ti/Pt/Au metallization removal have been solved and device yield is significantly improved. FTIR measurements of the bonding adhesives properties at submillimeter wavelengths are discussed. We have fabricated 640 GHz subharmonic mixers for EOS-MLS which nearly match state-of-the-art performance at this frequency with DSB Tmix equals 2396 K and Lmix equals 10.98 dB using 4.67 mW of LO power. RF testing of a new higher yield batch of MASTER mixers is in progress.

Integrated GaAs Diode Technology for Millimeter and Submillimeter-wave Components and Systems

GaAs Schottky barrier diodes remain a workhorse technology for submillimeter-wave applications including radio astronomy, chemical spectroscopy, atmospheric studies, plasma diagnostics and compact range radar. This is because of the inherent speed of these devices and their ability to operate at room temperature. Although planar (flip-chip and beam-lead) diodes are replacing whisker contacted diodes throughout this frequency range, the handling and placement of such small GaAs chips limits performance and greatly increases component costs. Through the use of a novel wafer bonding process we have fabricated and tested submillimeterwave components where the GaAs diode is integrated on a quartz substrate along with other circuit elements such as filters, probes and bias lines. This not only eliminates the cost of handling microscopically small chips, but also improves circuit performance. This is because the parasitic capacitance is reduced by the elimination of the GaAs substrate and the electrical embedding impedance seen by the diodes is more precisely controlled. Our wafer bonding process has been demonstrated through the fabrication and testing of a fundamental mixer at 585 GHz (T mix < 1200K) and a 380 GHz subharmonically pumped mixer (T mix < 1000K). This paper reviews the wafer bonding process and discusses how it can be used to greatly improve the performance and manufacturability of submillimeter-wave components.