Speaker
Description
Introduction
Schottky components are required for future space instruments and for emerging terrestrial applications in millimetre-wave imaging employed in the generation of the local oscillator signal and/or downconversion of the signal. This activity aimed at the development of a novel Transverse diode.
Objectives
The main objectives of the activity were:
(1) To develop discrete Schottky diodes with good electrical and mechanical
characteristics, both for mixers and frequency multipliers,
and
(2) To demonstrate their good performance by designing, fabricating and testing of mixer
and multiplier Demonstrators for a receiver operating around 300 GHz.
Experimental
Tyndall National Institute was contracted with Farran Technology to develop the Tyndall Transverse diode. Techniques developed at Tyndall enable the thinning of the semiconductor substrate to micron thicknesses to minimise losses. This same technology is compatible with integration of the transverse diode to fabricate millimetre wave membrane circuits.
Several models evolved in the design of the transverse Schottky mixer and varactor diodes. Data from these studies was then used for the design of a mixer with a centre operating frequency of 340 GHz and a doubler at 170 GHz. Development of the transverse diode fabrication process was undertaken by going through many process runs to produce diodes that when DC tested achieved the target requirements. However RF testing both at Tyndall and Millilab showed a drop-off of impedance below around 4 GHz due to inflated capacitances. This effect was also present on simple transmission lines deposited on the semiconductor substrate.
In parallel to the diode fabrication, process development was extended to fabricate the mixer and multiplier membrane circuits. Mixer and doubler circuits were assembled into RF blocks and tested to measure conversion loss, noise, and efficiency. While some process runs showed good RF results others had indifferent performance.
A study was undertaken to identify and remedy the source of the variability. It was found that a thin charge layer existed at the interface of one of the etchstop layers and the GaAs membrane. An alternative etchstop material was substituted and experiments then showed that no charge layer now existed. A new substrate structure was specified and wafers were grown in house. However this modified substrate unexpectedly exhibits hugely elevated localised etch rates (cheese hole formations on the membranes) leading to high series resistances in the doubler diodes and high capacitances of the mixer diodes.
Conclusion
The goal of this activity was to produce mixers and multipliers that employ a complete circuit on a semiconductor membrane as opposed to using discrete diodes mounted in a quartz substrate circuit. It was demonstrated that this is feasible. Any future development work should concentrate on the MOVPE overgrowth step where a thin layer of n-GaAs is epitaxially overgrown on the mesa sidewall. Effort should also be spent on the epi-structure and growth of the wafer substrate. The other process steps are at this stage well understood and mature.
| ESA Technical Officer | Dr. Vaclav Valenta |
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