Au-compensated high resistivity silicon for low loss microwave devices - suppression of parasitic surface conduction effect

Abstract

Deep-level doping compensation process using elemental gold is used to create high resistivity silicon substrate for microwave application. Gold atoms are introduced into low resistivity Czochralski silicon substrates through an ion-implantation process and activated via high-temperature annealings. The highest substrate resistivity recorded for optimised substrates is 60 k-cm. A constant attenuation value of 0.19 dB/mm is measured at 20 GHz for a bias voltage range of -6 V to +6 V, for coplanar waveguides fabricated on this type of substrate, indicating full suppression of parasitic surface conduction effect at microwave frequencies. The attenuation results are supported by the capacitance-voltage characteristics, where the substrate is seen to be insensitive towards bias voltage. Based on a finite element analysis, this effect is caused by the reduced number of free carriers in the substrate and the increased interface trap densities at the oxide-silicon interface. Optimisation of substrate-processing stages are presented in this work. It is shown that the combination of slow-cooling and quenching for activation annealing provide a higher resistivity enhancement and a lower attenuation compared to single annealing. The removal of the near surface gold to increase substrate's resistivity is found to be unnecessary as it does not provide reduced attenuation. It can therefore be avoided to reduce process complexity. The influence of oxide type is studied, and thermal oxidation is seen to be unsuitable for oxide passivation. Bias-dependent attenuation characteristics suggest gold out-diffusion during the high-temperature treatment. Reactively sputtered oxides do, however, give excellent performance. In addition to coplanar waveguides attenuation responses, quality factor performance of meander inductors fabricated on gold-compensated high resistivity silicon substrate is evaluated. A higher quality factor is recorded for all inductance values for Au-compensated high resistivity silicon compared to Float-zone silicon. The highest quality factor value of 14 is measured for 0.7-nH inductors.

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ORCID for Cornelis De Groot: orcid.org/0000-0002-3850-7101

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