Dielectric Permittivity of Porous Si for Use as Substrate Material in Si-Integrated RF Devices

Abstract

Dielectric permittivity of porous Si (PSi) layers formed on a low-resistivity p-type Si (0.001-0.005 .cm) is thoroughly investigated using analytical expressions within the frame of broadband transmission line characterization method in the frequency range 1-40 GHz. It is demonstrated that the value of Si resistivity is critical for the resulting PSi layer permittivity even within the above limited resistivity range. The real part of PSi dielectric permittivity changes monotonically between 1.8 and 4 by changing the Si resistivity between 0.001 and 0.005 .cm. The above study was made for porosities between 70% and 84%. The quality factor and attenuation loss of the investigated coplanar waveguide transmission lines were found to be Q = 26 and a = 0.19 dB/mm, respectively, at 40 GHz. These values are competitive to those obtained on quartz, which is one of the off-chip RF substrates with the lowest losses. This confirms the superiority of the PSi material, mentioned above, for use as a local substrate for the on-chip RF device integration.

Porous Si dielectric parameter extraction for use in RF passive device integration: Measurements and simulations

“…The porous Si layer is then capped with a 500 nm SiO 2 layer in order to protect it from environmental variations. The authors also demonstrated in [16] that the structure and morphology of the PSi layer, and consequently its dielectric parameters, depend on the Si wafer resistivity, even in the above limited resistivity range used. So, six different porous Si layers were created, all starting from Si wafers of different resistivity value within the range of 1-5 mΩ.cm.…”

“…The resulting material is uniform throughout its thickness, which exceeds 150 μm. As demonstrated by the authors in [16], this thickness is enough to provide full electromagnetic (EM) isolation [17] from the underlying Si substrate. The porous Si layer is then capped with a 500 nm SiO 2 layer in order to protect it from environmental variations.…”

“…In a former work by the authors [16], an analytic formulation of the PSi dielectric parameter extraction has been developed. In this work, we used the developed formalism in order to extract the dielectric permittivity of PSi.…”

“…In the case of porous Si used as an RF substrate, this is a challenging task and one that has not been adequately addressed in the literature. Porous Si formation is a multi-parameter procedure and even small changes in any of the formation conditions can induce large variations in the dielectric parameters of the produced layers [16]. This offers the advantage of tunable dielectric parameters, but also increases the difficulty to accurately and reliably predict these parameters.…”

“…In this paper we show that by following a method similar to [16] we can extract PSi dielectric parameters that then can be used in commercial simulation programs in order to accurately predict the performance of a given RF passive device. More specifically, in a first step we extract the dielectric parameters of several PSi layers, with different porosity, through measurements on coplanar waveguide transmission lines (CPW TLines), in order to test the reliability of the extraction method.…”