Influence of polysilicon thickness on the microwave attenuation losses of the CPWs fabricated on polysilicon-passivated high-resistivity silicon substrates

In this paper, the microwave performance of coplanar waveguides (CPWs) fabricated on high-resistivity silicon (HRS) substrates with various polysilicon film thicknesses was investigated. CPWs were fabricated on p-type silicon substrates with resistivities of 6000-8000 ¿-cm. Polysilicon films with thicknesses ranging from 100 to 900 nm were deposited by LPCVD, in which a 300 nm-deposited polysilicon was generally used as surface passication layer on the HRS substrates. Subsequently, the oxide layer was deposited on the polysilicon layer by PECVD, and a pure aluminum film was evaporated and patterned to form the 50-¿-CPW test structures. The CPWs were characterized through S-parameter measurements for up to 40 GHz by using an HP-8510 network analyzer. Experimental results show that the microwave attenuation loss can be influenced by a small variation of polysilicon layer thickness, especially when the thickness is smaller. Thicker polysilicon layers lead to higher grain boundary density, hence, free carriers can be rapidly trapped within grain boundary interfaces of the polysilicon, and the surface recombination velocity can be enhanced, thus, reducing the conductivity and bias-dependence of the surface layers. Furthermore, the bias effect on the HRS with various polysilicon thicknesses was taken into account as well.