Finding the right filter for frequency ranges above the 3 GHz range is a perennial challenge for RF system engineers. Designers are typically looking for repeatable performance at production volume and a small, surface-mount form factor robust enough to withstand reflow onto their existing printed wiring board (PWB). Lumped element filters utilizing discrete wire-wound inductors and chip capacitors meet these criteria handily for passbands below about 3 GHz, but frequency response becomes more sensitive to variations in the physical structure of the device and temperature at higher frequencies, rendering this approach impractical.

Several filter technologies support application bands higher into the microwave range, but each comes with compromises, some of which may be non-starters for real-world system requirements. Suspended substrate stripline and cavity filters, for example achieve passbands up to around 40 GHz with outstanding rejection and selectivity, but they’re big, heavy and expensive. Also, because these are not surface-mount technologies, using them with integrated microwave assemblies requires launching the signal off the board to the connectorized filter interface and back onto the board, which adds loss and mismatch at the transitions. This is rarely viable.

Integrating the filter directly onto the PWB in the form inductor-capacitor pairs (L-C) in the conductive traces is one economical choice, and this might serve the engineer well for filtering a discrete signal as in an LO chain, for instance. When it comes to filtering complex waveforms in modulated or broadband signals, however, the band of interest and the filter requirements are often defined by a specified filter mask. In these applications, an on-board filter comprised of etched copper traces will struggle with performance and repeatability at higher frequencies. Most filters constructed using chemical etching processes will be limited in their repeatability due to tolerances inherent to the etching process. This technique sacrifices precious board space as typical PWB dielectric constants are not particularly high, rendering on-board filters larger than a discrete filter component. 

Thin film filter technology is one solution for addressing these design concerns. Their high-accuracy fabrication process provides excellent  repeatability, and ceramic substrates make for rugged construction and easy reflow onto existing PWBs. Some ceramic substrates exhibit high D_k characteristics, enabling very compact physical filter realizations. Thin film filters built on ceramic substrates are naturally temperature-stable over a broad operating temperature range. These filters exhibit a variety of performance characteristics that make them unique among other surface mount filter technologies including wide passband width up to 40 GHz, outstanding Q-factor, high power handling and rejection floor on the order of -60 dB and deeper.

The Thin-Film Filter Fabrication Process

RF thin film filters are typically fabricated by depositing thin layers of metal or dielectric material onto a substrate using a technique known as Physical Vapor Deposition (PVD), or sputtering. Sputtering utilizes plasma, usually generated from a gas such as Argon to deposit material from a target a few atoms at a time and onto a host substrate in specific pattern and thickness (from fractions of a nanometer to several micrometers). The thickness of each layer, the materials and the patterns utilized are carefully controlled to create the desired filter characteristics such as passband, selectivity, insertion loss, and rejection.  These methods achieve extremely tight tolerances for material thickness and trace width. The benefit of processing filters using these sputtering techniques is shown in Figure 1, where the trace edges exhibit far superior uniformity for the filter processed using ion bean etching.¹