Reliability of Gap Capacitors

Abstract

The requirements for communication devices continue to become more challenging each year leading to rapid developments in varying technologies to meet the demands. Tunable filters, utilizing ferroelectric varactors, are merely one solution to the ever-demanding need for microwave communication systems. Barium Strontium Titanate (BST) can be employed as the dielectric material necessary to make tunable devices. The broad range of BST’s remarkable properties allow for easy integration into many devices, such as MEMS, oscillators, phase shifters, and tunable filters. BST deposition is done via RF magnetron sputtering for our purposes. Previous work completed here at NCSU has led to a sophisticated process to fabricate tunable filters chips that work over the 6-18 GHz range. Using polycrystalline alumina substrates with varactors made up of a pair of 4 μm silver gap capacitors in series as well as utilizing the BST dielectric creates excellent tunable chips. There are however issues related to the reliability of the gap capacitors composing the varactors.

Existing theory (Paschen’s Law) describing capacitor failure reveals gaps with relatively close spacings (below 5 μm) should maintain robust breakdown strengths. However, our research, as well as numerous additional literature, proves this theory to be inaccurate. Moore’s Law has led to a return to this research area, after almost a four decade hiatus, relatively recently as well as additional modifications to electrical systems that are presently occurring. Townsend discharge and electric field emission can be used to describe the mechanisms of gap capacitor breakdown. The Fowler-Nordheim equation and Weibull statistics, via the Bernard median rank equation for determining cumulative probability, are employed to analyze the breakdown data. Various mechanisms lead to breakdown occurring among these capacitors and further investigation into silver electromigration was undertaken to explore the possibility of migration as a significant factor towards failure.

The details involved in fabricating tunable filters is detailed and shown to be relatively similar to the construction of samples with loads of gap capacitors. BST thin-film deposition is completed initially, followed by photolithography processes to define the outline for the filters. Silver metallization is accomplished using DC magnetron sputtering, and patterning is completed by lift-off in an acetone solution, due to the lack of harmful effects to the BST thin-film. Samples with roughly 250 gap capacitors on them were also fabricated on alumina substrates, with gaps ranging between 2 to 5 microns and widths between 100 to 750 microns. Large number of the gap capacitors were placed under ramping DC bias to observe if failure occurred using a suitable probe station. Proper analysis was undertaken using Weibull statistics, and several critical variables were examined, including roughness, patterning processes, adhesion of silver metal to the substrate, as well as varying the type of ground metal. SEM was utilized in examining the capacitor failure, including carrying out experiments looking into silver electromigration.

In this thesis, we will show the inadequacies of Paschen’s Law for describing capacitor failure with gap spacings of less than 5 μm. Also, an explanation into the process improvements for utilizing Ag gap capacitors on alumina substrates, for our tunable filters purposes, to eventually using Cr-Au gap capacitors on sapphire substrates is provided. For silver gap capacitors, the existence of the electromigration process is established and verified confirmed through SEM and EDS analysis.