Microwave micromachined cavity filters.

Abstract

Recent advances in RF technology, dominated by defense, national security and scientific research systems such as radar, communications, electronic warfare and radiometry, have occurred in the 1--100 GHz frequency band. With the advent of affordable systems, improved performance is in demand. Reduced size and weight for mobile and airborne platforms, and reliability for long-term satellite platforms, require innovation in RF system architecture. Traditional waveguides and coaxial lines are large and difficult to integrate with monolithic integrated circuits (MIC) and passive devices. This thesis addresses the issues related to the development of novel, high frequency, three-dimensional micromachined cavity filters, specifically the reduction of weight and volume and how loss and quality factor Q are consequently affected. The cavity filters are based upon slot-coupled, microstrip-fed, reduced height waveguide resonators in silicon. The design synthesis and fabrication for three unique filter architectures are presented at 10, 28 and 32 GHz. Both wet anisotropic etching and deep reactive ion etching techniques are used to fabricate the resonant cavities. Two direct-coupled Chebyshev filters and one cross-coupled linear phase filter are built and measured, employing both frequency and time domain design and different inter-cavity coupling schemes. The measured filters have reasonable losses, very good unloaded Q's and are low volume, lightweight and integrable into MIC circuits. For example, the 28 GHz linear phase filter overall circuit dimensions were 19.5 mm long x 15.4 mm wide x 1.9 mm high. The measured filter exhibited a 1.9% bandwidth at 27.604 GHz with 1.6 dB of de-embedded insertion loss and very good phase linearity in the ripple passband. An excellent unloaded Q of 1465 was measured, compared to a calculated theoretical value of 1614. Although this unloaded Q is a reduction from that seen for traditional machined waveguides, it is an improvement over micromachined membrane microstrip resonators, which have unloaded Q's of approximately 500 for this frequency range.