A MEMS Fabrication Process with Thermal-Oxide Releasing Barriers and Polysilicon Sacrificial Layers for AlN Lamb-Wave Resonators to Achieve fs·Qm > 3.42 × 1012.

Jicong Zhao,Chenguang Song,Zheng Zhu,Shitao Lv,Xingyu Wang,Haiyan Sun

doi:10.3390/mi12080892

Abstract

This paper presents a micro-electro-mechanical systems (MEMS) processing technology for Aluminum Nitride (AlN) Lamb-wave resonators (LWRs). Two LWRs with different frequencies of 402.1 MHz and 2.097 GHz by varying the top interdigitated (IDT) periods were designed and fabricated. To avoid the shortcomings of the uncontrollable etching of inactive areas during the releasing process and to improve the fabrication yield, a thermal oxide layer was employed below the platted polysilicon sacrificial layer, which could define the miniaturized release cavities well. In addition, the bottom Mo electrode that was manufactured had a gentle inclination angle, which could contribute to the growth of the high-quality AlN piezoelectric layer above the Mo layer and effectively prevent the device from breaking. The measured results show that the IDT-floating resonators with 12 μm and 2 μm electrode periods exhibit a motional quality factor (Qm) as high as 4382 and 1633. The series resonant frequency (fs)·Qm values can reach as high as 1.76 × 1012 and 3.42 × 1012, respectively. Furthermore, Al is more suitable as the top IDT material of the AlN LWRs than Au, and can contribute to achieving an excellent electrical performances due to the smaller density, smaller thermo-elastic damping (TED), and larger acoustic impedance difference between Al and AlN.

Highlights

In radio frequency (RF) wireless communications, micro-electro-mechanical system (MEMS) resonators have important applications, such as oscillators, filters, and duplexers, due to their small size, low power consumption, high performance, etc. [1,2]
Aluminum nitride (AlN) lamb wave resonators (LWRs) have drawn widespread attention; they combine the major virtues of surface acoustic wave (SAW) resonators and film bulk acoustic resonators (FBARs), including a multi-band integration, high performance, complementary-metal-oxide-semiconductor (CMOS)-compatible fabrication process, etc. [7,8,9]
This paper presents a MEMS fabrication process for AlN LWRs to minimize the effect from the undercut region and to obtain a high fs ·Q value

Summary

Introduction

In radio frequency (RF) wireless communications, micro-electro-mechanical system (MEMS) resonators have important applications, such as oscillators, filters, and duplexers, due to their small size, low power consumption, high performance, etc. [1,2]. SAW resonators can hardly achieve a high quality-factor (Q) value and a large power capacity Their frequencies are limited to 3 GHz due to the low phase velocity and lithography limit [5]. The extended undercut region between the resonator and the substrate causes an energy loss reducing the device’s Q value, and the device’s fracture and an eventual performance failure Another process involves depositing a SiO2 or polysilicon sacrificial layer on the Silicon wafer and etching the sacrificial layer to form an annular groove as well as define the releasing region. The process can define the releasing cavity well, the LPCVD-deposited polysilicon or SiO2 film is not dense enough, which may cause releasing gas to pass through the barrier and induce the hollowness of the inactive region.

Device Design

Fabrication Process and Results

Measurement and Discussions

Conclusions