Materials for High Frequency Filters

Abstract

Different types of frequency filters (bandpass, bandstop, lowpass, and highpass) are used to obtain the desired frequencies and enable wireless communications. As 5G operates in higher frequency bands, new resonating structures are needed for frequency filters, which has been a significant challenge for industry. High percent bandwidth, high selectivity, low insertion loss, small form factor, increased rejection, and reliability under a broad range of temperatures are some of the important criteria for 5G filters. The most popular structures are acoustic wave filters, including surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters, but SAW and BAW filters are only practical up to approximately 2.5 and 6 GHz, respectively. The piezoelectric layer used in BAW filters must have a precise crystal orientation to attain the finest electromechanical coupling. Aluminum nitride (AlN) is the most common material used for BAW filters, providing the greatest balance of performance, reliability, and manufacturability. For 5G, increasing the filter bandwidth requires enhanced piezoelectric coupling. Scandium-doped aluminum nitride (ScAlN) can solve this problem, but manufacturing becomes more challenging, motivating the transition to lithium niobate (LiNbO3) materials. The precise deposition of the piezoelectric material, electrode configuration, acoustic reflector, and parasitic effects are key factors for enhanced coupling. Innovative structures such as XBAR and other options, including dielectric waveguides, on-chip filtering structures, cavity waveguides, and microstrips, can lead to filters operating at even higher frequencies. In the 5G era, the filter needs to be more miniaturized and better integrated with other devices. Great opportunities for stable ceramic materials with low dielectric constants/loss tangents and high quality factors (Q-values) exist. This chapter will provide a brief review of various material selections for different types of high-frequency filters used in 5G communication systems.