Thin Glass Characterization in the Radio Frequency Range

Glass wafers are being widely considered as a transparent and low-permittivity alternative to silicon substrates for radio frequency devices. To provide reliable material data to the device designer, the complex permittivity of three important wafer glasses with the thermal expansion coefficients matching the one of silicon, namely AF32 (R) eco, Borofloat33 (R), and MEMpaxe (R) from SCHOTT, has been investigated by two different methods (split cylinder and open resonator) in the range from 1 to 100 GHz. The data sets thus obtained for these glasses show a remarkable consistency, with a relative deviation of 1-2% for the real part of the permittivity and 10-25% for the effective dielectric loss tangent. In addition to this, the results have been confirmed by an indirect determination of the permittivity from the simulation of special radio frequency devices (resonant microstrip structures on wafers from said glasses as substrates). The relative deviation is about 1-3% for the real part of the permittivity and ca. 50% for the loss tangent this time. Concerning the effective dielectric losses, the performance of the three glasses can only be matched by (costly) high resistivity silicon rather than by commonly used low resistivity silicon.