POLY-SIGE: A HIGH-Q STRUCTURAL MATERIAL FOR INTEGRATED RF MEMS Sunil A. Bhave 1 , Brian L. Bircumshaw 2 , Wing Zin Low 1 , Yong-Sang Kim 1,† , Albert P. Pisano 2,1 , Tsu-Jae King 1 , and Roger T. Howe 1,2 Department of Electrical Engineering & Computer Sciences 1 and Department of Mechanical Engineering 2 Berkeley Sensor & Actuator Center, 497 Cory Hall, University of California, Berkeley, CA 94720-1774 † Current address: Department of Electrical Engineering, Myongji University, Kyongki, Korea. Travel support has been generously provided by the Transducers Research Foundation and by the DARPA MEMS and DARPA BioFlips programs. ABSTRACT This paper presents new material data for single- and multi- layer CMOS compatible poly-SiGe films, including mechanical quality factor (Q), Young’s modulus, and strain gradient. Using audio frequency folded-flexure comb-drive resonators, the mechanical quality factor of poly-SiGe was determined at 2µTorr pressure. As-deposited poly-SiGe has Q = 20,000 – 31,000. CMOS compatible rapid thermal annealing (RTA) at 525°C for 60 seconds results in a quality factor between 40,000 and 44,000. We have measured the highest Q factor yet reported for poly-SiGe (Q 61,100), as a result of RTA at 600°C for one minute. The measured resonant frequencies of the resonators were used to back-calculate the Young’s modulus of poly-SiGe: E = 155 ± 5GPa. The calculated elastic modulus is significantly higher than the metallurgical Young’s modulus of 146GPa. The as-deposited strain gradient of the tri-layer poly-SiGe film was found to be 1.75·10 -4 µm -1 (curl-up). RTA at 600°C for one minute drops the tri-layer strain gradient to 2.67·10 -5 µm -1 (curl-up). The graded Ge content multi-layer film was observed to induce a larger normalized strain gradient. I. INTRODUCTION Polycrystalline silicon-germanium (poly-SiGe) is a promising material for surface micro-machined MEMS (Micro- Electromechanical Systems) applications. Conformal deposition is possible using chemical vapor deposition techniques at temperatures below 425°C [1,2]. Consequently, poly-SiGe can be micro-machined directly on top of modern foundry CMOS. Moreover, research by Sedky et al. has shown that thermal annealing of CMOS devices at 525°C for up to 90 minutes leaves the underlying electronics largely unaffected [3]. Micromachined resonators can exhibit very high mechanical quality factors. Indeed, poly-Si MEMS resonators have been reported to have Q’s in excess of 80,000 [4]. Due to their high Q’s, MEMS resonators have superior frequency-selectivity compared to electronic active filters. Also, micromachined resonators are promising as replacements for discrete filters and oscillators in wireless communications systems [5,6]. Integrating RF MEMS directly with CMOS promises to drop parasitic capacitances and inductances, as well as reduce fabrication and integration costs, and the form factor of telecommunications devices. Using low-temperature poly-SiGe, it is our aim to make the MEMS CMOS integration process straightforward and modular, particularly for RF applications. This paper lays the groundwork for this goal by characterizing single- and multi-layer poly-SiGe films with processes relevant for RF MEMS applications. Section II presents the fabrication method used to micromachine the poly-SiGe test structures. Section III summarizes Q data collected from audio frequency poly-SiGe resonators. For MEMS microresonator filters and oscillators, Q is of prime importance. In Section IV, the measured resonant frequencies and geometric dimensions are used to back-calculate the Young’s modulus of poly-SiGe. Finally, strain gradient and the effects of RTA are discussed in Section V. II. FABRICATION All devices were fabricated using a single-mask, timed- release process with silicon dioxide as the sacrificial layer. Single crystal silicon wafers were used as the starting substrate. Two microns of low temperature oxide (LTO) was then deposited at 400ºC. The LTO acts as both the sacrificial layer and the mechanical anchor. LTO was used as opposed to a polycrystalline germanium (poly-Ge) sacrificial layer [1,2] because of the need for electrical isolation between the mechanical anchors and the substrate. Four structural films were studied: 1µm thick poly-SiGe films containing 62%, 65%, or 68% Ge, and a 3µm tri-layer sandwich (Figure 1). All films were deposited by LPCVD at 425ºC and in-situ boron-doped (using B 2 H 6 ), yielding a deposition rate of roughly 80Å per minute. The as-deposited RMS surface roughness of the films is 50Å for the tri-layer sandwich and under 30Å for the single-layer films. The tri-layer film was used to investigate whether films with different average stresses can be combined to cancel out the residual strain gradient [7]. The resonators and strain gradient test structures used for this paper were defined using a single-mask photolithography step, followed by a standard anisotropic HBr plasma etch using a poly- Si etch recipe. A timed dip in concentrated HF was used to release the structures while leaving SiO 2 pedestals for mechanical anchoring of the test devices. Single Crystal Silcon Substrate Single Crystal Silcon Substrate Single Crystal Silcon Substrate Single Crystal Silcon Substrate Poly-Si 35 Ge 65 Poly-Si 38 Ge 62 1µm 1µm 1µm 2µm 62% Ge-content Poly-SiGe 65% Ge-content Poly-SiGe 68% Ge-content Poly-SiGe 68/65/62 Tri-Layer SiO 2 Sacrificial Poly-Si 38 Ge 62 SiO 2 Sacrificial Poly-Si 35 Ge 65 SiO 2 Sacrificial Poly-Si 32 Ge 68 SiO 2 Sacrificial Poly-Si 32 Ge 68 1µm 2µm Single Crystal Silcon Substrate Single Crystal Silcon Substrate Single Crystal Silcon Substrate Single Crystal Silcon Substrate Poly-Si 35 Ge 65 Poly-Si 38 Ge 62 1µm 1µm 1µm 2µm 62% Ge-content Poly-SiGe 65% Ge-content Poly-SiGe 68% Ge-content Poly-SiGe 68/65/62 Tri-Layer SiO 2 Sacrificial SiO 2 Sacrificial Poly-Si 38 Ge 62 SiO 2 Sacrificial SiO 2 Sacrificial Poly-Si 35 Ge 65 SiO 2 Sacrificial SiO 2 Sacrificial Poly-Si 32 Ge 68 SiO 2 Sacrificial SiO 2 Sacrificial Poly-Si 32 Ge 68 1µm 2µm Figure 1. Cross-sectional views of the four types of structural films studied in this paper (depicted after HF release). 0-9640024-4-2 34 Solid-State Sensor, Actuator and Microsystems Workshop Hilton Head Island, South Carolina, June 2-6, 2002