Wafer-level packaging of aluminum nitride RF MEMS filters

Abstract

Aluminum nitride (AlN) radio frequency (RF) MEMS filters utilize piezoelectric coupling for high-performance electrical filters with frequency diversity in a small form factor. Furthermore, the compatibility of AlN with CMOS fabrication makes AlN extremely attractive from a commercial standpoint. A technological hurdle has been the ability to package these suspended resonator devices at a wafer level with high yield. In this work, we describe waferlevel packaging (WLP) of AlN MEMS RF filters in an all silicon package with solder balls on nickel vanadium / gold (NiV/Au) bond pads that are subsequently ready for flip chip bonding. For this integration scheme, we utilize a 150 mm device wafer, fabricated in a CMOS foundry, and bond at the wafer level to a cavity silicon wafer, which hermetically encapsulates each device. The cavity wafer is then uniformly plasma etched back using a deep reactive ion etcher resulting in a 100 µm thick hermetic silicon lid encapsulating each die, balled with 250 µm 90/10 Pb/Sn solder balls and finally diced into individually packaged dies. Each die can be frequencytrimmed to an exact frequency by rapid temperature annealing the stress of the metallization layers of each resonator. The resulting technology yields a completely packaged wafer of 900 encapsulated die (14 mm 2 by 800 µm thick) with multiple resonators and filters at various frequencies in each package.

Micromachined piezoelectric Lamb wave resonators: a review

“…As for micromachined piezoelectric Lamb wave resonators, devices with working frequencies ranging from megahertz to gigahertz could be integrated on a single chip. Ring-shaped S0 mode AlN Lamb wave resonators with working frequencies ranging from 22 to 500 MHz have been demonstrated on a single wafer [173]. In another work, resonant frequencies ranging from 200 MHz to 1.9 GHz are reported by S0 mode AlN Lamb wave resonators fabricated on one SOI substrate [174].…”

“…© IOP Publishing Ltd. All rights reserved. structural strength of Lamb wave resonators, the mechanical stress weakens the resilience to external disturbance, which increases the risk of damage during the transportation or packaging of micromachined piezoelectric Lamb wave devices and lowers production yields [173]. Besides, deformations of the suspended resonance cavities resulting from mechanical stress can cause degeneration of resonator performance [205].…”

“…For example, SiO 2 films have been used to effectively compensate for thermally induced stress of suspended AlN Lamb wave resonators [76,78] (figures 13(a) and (b)), while structure optimizations can also efficiently relax mechanical stress [205]. From another perspective, techniques such as wafer level packaging provide efficient protection from external disturbance during the fabrication process for suspended devices with significant inner stress [173,209] (figures 13(c) and (d)). With efforts from multiple research and development directions, the stress problem faced by micromachined piezoelectric Lamb wave resonators can be further alleviated in the future.…”

(Invited) AlN and ScAlN Contour Mode Resonators for RF Filters

“…This feature of CMRs is fundamentally different than BAWs where frequency is set by film thickness [4][5][6] . CMRs have been fabricated and packaged to demonstrate different frequency RF bandpass filters on the same chip 6,7 . Every integration step to achieve these suspended acoustic devices, including lithographic patterning and plasma etching, is CMOS compatible and has been demonstrated in back end of the line (BEOL) integrations 8 .…”