Naser Alijabbari

Design and Characterization of Integrated Submillimeter-Wave Quasi-Vertical Schottky Diodes

This work reports on a new approach to realizing vertically oriented Schottky diodes, with ohmic contact formed directly below the anode, that can be readily integrated into planar millimeter and submillimeter-wave circuits. The diode structure is based on backside processing and bonding of the diode epitaxy to a host, high-resistivity silicon substrate that supports both the vertical diode and its associated circuitry. A set of prototype diodes of different anode diameters are fabricated for characterization at both dc and (for the first time) submillimeter-wave frequencies (325-750 GHz) using micromachined on-wafer probes. Device equivalent circuit parameters extracted from these measurements are in good agreement with those expected from fundamental Schottky barrier diode theory and indicate the vertically oriented diodes yield series resistance values that are comparable to or lower than planar-oriented diodes of similar dimensions.

A 1.1 THz micromachined on-wafer probe

This paper presents a micromachined probe for on-wafer measurements of circuits in the WR-1.0 waveguide band (0.75 - 1.1 THz). The probe shows a measured insertion loss of less than 7 dB and return loss of greater than 15 dB over most of the band. These are the first reported on-wafer measurements above 1 THz.

160 GHz Balanced Frequency Quadruplers Based on Quasi-Vertical Schottky Varactors Integrated on Micromachined Silicon

This work reports on an integrated frequency quadrupler operating at 160 GHz with maximum efficiency of 30% and corresponding output power of 70 mW. 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 a 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. The chip is tailored to fit the multiplier waveguide housing, resulting in high reproducibility and consistency in manufacture and performance. Estimates of the varactor temperature for the multiplier were made using the diodes as integrated thermometers. These measurements estimate the operating temperature of the varactors in the quadrupler input stage to be 35 °C.

Characterization of Micromachined On-Wafer Probes for the 600–900 GHz Waveguide Band

A micromachined on-wafer probe has been designed to facilitate the development of integrated circuits in the 600-900 GHz frequency range. The probe tip is fabricated on a 5-micrometer thick high-resistivity silicon substrate using a silicon-on- insulator fabrication process. This letter updates previous work on WR-1.2 wafer probes and presents for the first time the full RF characterization of the probe. These are the first reported on-wafer measurements above 750 GHz.

Steady-state thermal analysis of an integrated 160 GHz balanced quadrupler based on quasi-vertical Schottky diodes

This work reports on a steady-state thermal analysis of a 160 GHz balanced quadrupler, based on a quasi-vertical varactor Schottky diode process, for high power applications. The chip is analyzed by solving the heat equation via the 3D finite element method. Time-Domain Thermoreflectance (TDTR) was used to measure the thermal conductivity of the different materials used in the model. A maximum anode temperature of 64.9°C was found from the simulation. The addition of an extra beam lead connected to the block, for heat sinking, was found to reduce this maximum temperature to 41.0°C.

An Epitaxy Transfer Process for Heterogeneous Integration of Submillimeter-Wave GaAs Schottky Diodes on Silicon Using SU-8

This letter describes a new approach for fabricating quasi-vertical submillimeter-wave GaAs Schottky diodes heterogeneously integrated to high-resistivity silicon substrates. The new method is robust and eliminates previous processing steps that were prone to result in wafer fracture and delamination. Diodes fabricated with the new process and measured in the 325–500 GHz range using on-wafer RF probes exhibits low parasitic capacitance and series resistance, achieving device characteristics comparable to the prior state-of-the-art submillimeter-wave diodes.