Integrated Schottky Structures for Applications Above 100 GHz Byron Alderman 1 , Hosh Sanghera 1 , Bertrand Thomas 1 , David Matheson 1 , Alain Maestrini 2 , Hui Wang 2 , Treuttel 2 , Jose V. Siles 3 , Steve Davies 4 , Tapani Narhi 5 1 Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK 2 Observatoire de Paris, LERMA, 75014 Paris, France 3 Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain 4 University of Bath, Bath, BA2 7AY, UK 5 ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk ZH, The Netherlands 1 B.Alderman@rl.ac.uk Abstract— Recent developments in the fabrication of GaAs integrated Schottky structures for applications above 100 GHz are presented. Two approaches are discussed; the fabrication of integrated circuits using a GaAs foundry service, coupled with the research based post-processing of these structures, and the fabrication of discrete and integrated Schottky structures using a bespoke research laboratory. I. INTRODUCTION Low capacitance GaAs Schottky diode technology is required for millimetre and sub-millimetre wave heterodyne receivers. Schottky diodes operate at both ambient and cryogenic temperatures and are uniquely able to cover the frequency range from DC to above 1 THz. Schottky diode technology has been evolving for many years and has traditionally been driven by the demands of radio astronomy and remote sensing of the atmosphere. Ground based applications, e.g. security imaging, are now increasing in importance. For these applications, Schottky based technology offers an attractive alternative to detectors and sources that require cryogenic cooling [1]. Despite the growing demand for Schottky devices operating above 100 GHz, there remains limited availability within Europe and there is currently no space-qualified process available. The European Space Agency (ESA) has initiated a programme to investigate the use of the GaAs foundry service from United Monolithic Semiconductors (UMS) to fill the current gap between demand and availability (ESA AO/1- 5084/06/NL/GLC). This programme aims to investigate the performance limitations of this GaAs foundry service and to explore ways of post-processing GaAs wafers to enhance device performance, for example, to reduce the dielectric loading around the anode to reduce the parasitic capacitance and the effect of dielectric loading. Using this approach, integrated Schottky structures have been designed for operation at frequencies upto 380 GHz. Schottky diode fabrication is a relatively simple process which can be established in a research environment using optical lithography with simple manual alignment, deposition and etching tools. Structures in which the Schottky contact is integrated with an embedding network can also be fabricated in such an environment. In fact, a small research laboratory, with its inherent flexibility, can be very effective in optimising Schottky structures. Whereas a GaAs foundry can be considered as a fixed process with a small number of wafers procured with a single reticule design repeated across a wafer, in a research environment it is often the case that relatively small samples are processed using contact lithography, rather than a stepper, and that each sample iterates to an optimum set of process conditions. A foundry therefore offers a stable and reliable fabrication process whereas a research laboratory offers the ability to develop novel structures. As operating wavelengths of Schottky structures move into the sub-millimetre wave range, novel integration and substrate transfer or removal techniques are required [2]. Here we report on the design, fabrication and test of devices fabricated at a GaAs foundry to which additional post-processing has been applied in order to improve their performance at higher frequencies. We also report on the fabrication and test of discrete and integrated Schottky structures designed and fabricated in a bespoke research laboratory which are being developed specifically to operate in the sub-millimetre wave range. In both cases, technology demonstrators are being targeted at the 200 to 400 GHz range. II. GAAS FOUNDRY DEVICES The use of a GaAs foundry service to fabricate high frequency Schottky structures has the potential to supply a large number of identical devices without the effort of developing the fabrication technology, which is often very expensive and time consuming. However, circuits operating above 100 GHz often require non-standard processing to reduce the parasitic capacitance and dielectric loading of waveguide cavities. These techniques are expensive to develop and there is little incentive for them to be available within a foundry service, given the current level of demand. For these reasons we have investigated the electromagnetic advantages and corresponding effort of post-processing foundry devices. Our results indicate that provided the quality of the Schottky contact can be retained for anodes that are smaller than is defined by typical design rules, the structure of the circuit metallisation can be designed to be suitable to at Jeanne 978-2-87487-007-1 © 2008 EuMA October 2008, Amsterdam, The Netherlands Proceedings of the 3rd European Microwave Integrated Circuits Conference 202