Wafer Level Thin Film Encapsulation for BAW RF MEMS

The BAW (Bulk acoustic Wave) RF filters that are used in mobile phone application are composed of pair of piezo electrical resonators which need an air gap above them to operate. This air gap consists of a cavity defined by a dielectric layer which is above each resonator. The size of this cavity is smaller than 200μm square and less than 5μm high. The process that has been developed to produce this uses conventional I.C. manufacturing technologies at wafer level with only three photolithography steps. The main advantages of this process are that it provides an area efficient process for high volume, and low cost production which results in high reliability. To summarise the results from this work, BAW filters have been exposed to such steps as: Lead free bumping, back side wafer grinding (to thickness of 150μm), die sawing, pick and place, flip chip, and plastic encapsulation (over moulding) after under filling and have not been damaged in any part of the process. . The resonance frequency of the BAW filter was not affected by either thin film or back end packaging.