Characterization of Parylene-N as Flexible Substrate and Passivation Layer for Microwave and Millimeter-Wave Integrated Circuits

Investigation of Parylene-N (Pa-N) as a flexible substrate, multilayer dielectric material, and passivation layer for microwave and millimeter-wave integrated circuits is presented. For the first time, the electrical properties of Parylene-N have been characterized up to 60 GHz using various microstrip ring resonators and transmission lines. As a flexible substrate, Parylene-N measures a nearly invariant relative dielectric constant (epsiv
_r
) of 2.35-2.4, and a loss tangent (tan delta) of lower than 0.0006 for frequencies up to 60 GHz. Because of the above properties, as a passivation layer, Parylene-N causes insignificant modifications to the properties of underlying passive and active structures. Measurement of coplanar waveguide transmission lines before and after passivation reveals that a 5-mum Parylene-N barely changes the insertion loss (below measurement accuracy) while a 10-mum-thick Parylene-N layer increases the insertion loss by only 0.007 dB/mm (below measurement error) at 40 GHz. Ring resonators before and after a 5 or 10 mum passivation show a frequency shift of less than 0.05% or 1.51%, respectively, up to 40 GHz. The influence of Parylene-N passivation on the RF performance of GaAs MESFETs is also found to be negligible. Finally, humidity studies with dew point sensors reveal that with a 10- mum-thick passivation at 25
^deg
C and 100% relative humidity, the MTTF is about 481.6 days. In summary, the results indicate that Parylene-N is an excellent and promising material for application at microwave and millimeter-wave frequencies.