SOI-Based HF and VHF Single-Crystal Silicon ... - Semantic Scholar
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SOI-BASED HF AND VHF SINGLE-CRYSTAL SILICON RESONATORS WITH SUB-100 NANOMETER VERTICAL CAPACITIVE GAPS Siavash Pourkamali, and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, GA 30332-0250 Email: [email protected]; Tel: (404) 385-4306; Fax: (404) 894-5028 resonators are made of polysilicon and the resonators are supported from the side.
ABSTRACT This paper reports on the fabrication and characterization of single-crystal silicon (SCS) capacitive resonators with operating frequencies in the HF (3-30MHz) and VHF (30-300MHz) range. In-plane ultra-stiff SCS resonators with polysilicon electrodes and self-aligned 90nm vertical capacitive gaps have been fabricated on SOI substrates using a HARPSS-like fabrication process. High frequency side-supported flexural disk resonators and clamped-clamped beam resonators have been implemented and tested. A 3µm thick, 30µm in diameter SCS disk resonator exhibited a quality factor of 40,000 in vacuum at 148MHz. When operated in atmosphere, the same device demonstrated a Q of 8,000. 1. INTRODUCTION Development of high quality factor silicon-based micromechanical resonators can have a great impact on the future of wireless communication systems by providing unprecedented levels of system integration and power reduction. A significant amount of research is currently underway to extend the operating frequency of MEMS resonators into the VHF and UHF range. Several types of HF and VHF MEMS resonators with polysilicon as the structural material have been implemented using surface micromachining techniques [1-3]. Single crystal silicon (SCS) is a superior structural material for microresonators compared to polysilicon due to its inherent high mechanical quality factor [4], lower internal stress and independence from various process parameters. However, since SCS cannot be deposited using conventional CVD techniques, fabrication of electrically isolated SCS structures with ultra-thin (10,000) high frequency (f>100MHz) single crystal silicon capacitive resonators. A fabrication technology based on the HARPSS process [8,9] has been used to implement in-plane SCS disk and beam resonators with sub-100 nanometer capacitive gaps on SOI substrates. The drive and sense electrodes for the fabricated
2. MOTIVATION The equivalent electrical output resistance of a capacitive micromechanical resonator is expressed by: Rio
KM d 4 QH o 2 L2 h 2V p 2
v
d4 Qh
where K and M are the effective stiffness and mass of the resonator, d is the capacitive gap size, Q is the resonator’s quality factor, Vp is the DC polarization voltage and L and h are the electrodes’ length and height, respectively. From this equation, it is evident that ultra-thin capacitive gaps, high Q, and large electrode area are needed to reduce the equivalent output resistance of the RF MEMS capacitive resonators to reasonable values. Achieving smaller output resistance will facilitate the insertion of MEMS capacitive resonators in various high frequency systems [10]. The high-Q SCS capacitive resonators reported in this paper provide all the necessary features to obtain reduced output resistance. Firstly, the capacitive gaps of these resonators are determined in a self-aligned manner by the thickness of the deposited sacrificial oxide layer and can be reduced to their smallest physical limits (10’s of nanometers and less) independent from lithography. Secondly, the thickness of the SCS HARPSS resonators can be increased to a few 10’s of microns while keeping the capacitive gaps in the nanometer scale, resulting in a lower equivalent motional resistance compared to their polysilicon counterparts which have limited thickness [1-3]. Finally, the ability of fabricating all-silicon resonators with single crystal silicon as the resonating element makes HARPSS a superior candidate for implementation of high-Q, high frequency integrated MEMS resonators. In this work, silicon-on-insulator (SOI) substrates have been utilized. The advantages of SOI substrate compared to regular silicon substrate previously used to implement low frequency SCS resonators [9], are: 1) Providing “electrical isolation” between the body of individual SCS resonators in an array implementation (e.g., for filter synthesis). 2) Enabling nano-precision fabrication of “ultra-stiff” SCS resonators with height-to-width-ratio
SOI-BASED HF AND VHF SINGLE-CRYSTAL SILICON RESONATORS WITH SUB-100 NANOMETER VERTICAL CAPACITIVE GAPS Siavash Pourkamali, and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, GA 30332-0250 Email: [email protected]; Tel: (404) 385-4306; Fax: (404) 894-5028 resonators are made of polysilicon and the resonators are supported from the side.
ABSTRACT This paper reports on the fabrication and characterization of single-crystal silicon (SCS) capacitive resonators with operating frequencies in the HF (3-30MHz) and VHF (30-300MHz) range. In-plane ultra-stiff SCS resonators with polysilicon electrodes and self-aligned 90nm vertical capacitive gaps have been fabricated on SOI substrates using a HARPSS-like fabrication process. High frequency side-supported flexural disk resonators and clamped-clamped beam resonators have been implemented and tested. A 3µm thick, 30µm in diameter SCS disk resonator exhibited a quality factor of 40,000 in vacuum at 148MHz. When operated in atmosphere, the same device demonstrated a Q of 8,000. 1. INTRODUCTION Development of high quality factor silicon-based micromechanical resonators can have a great impact on the future of wireless communication systems by providing unprecedented levels of system integration and power reduction. A significant amount of research is currently underway to extend the operating frequency of MEMS resonators into the VHF and UHF range. Several types of HF and VHF MEMS resonators with polysilicon as the structural material have been implemented using surface micromachining techniques [1-3]. Single crystal silicon (SCS) is a superior structural material for microresonators compared to polysilicon due to its inherent high mechanical quality factor [4], lower internal stress and independence from various process parameters. However, since SCS cannot be deposited using conventional CVD techniques, fabrication of electrically isolated SCS structures with ultra-thin (10,000) high frequency (f>100MHz) single crystal silicon capacitive resonators. A fabrication technology based on the HARPSS process [8,9] has been used to implement in-plane SCS disk and beam resonators with sub-100 nanometer capacitive gaps on SOI substrates. The drive and sense electrodes for the fabricated
2. MOTIVATION The equivalent electrical output resistance of a capacitive micromechanical resonator is expressed by: Rio
KM d 4 QH o 2 L2 h 2V p 2
v
d4 Qh
where K and M are the effective stiffness and mass of the resonator, d is the capacitive gap size, Q is the resonator’s quality factor, Vp is the DC polarization voltage and L and h are the electrodes’ length and height, respectively. From this equation, it is evident that ultra-thin capacitive gaps, high Q, and large electrode area are needed to reduce the equivalent output resistance of the RF MEMS capacitive resonators to reasonable values. Achieving smaller output resistance will facilitate the insertion of MEMS capacitive resonators in various high frequency systems [10]. The high-Q SCS capacitive resonators reported in this paper provide all the necessary features to obtain reduced output resistance. Firstly, the capacitive gaps of these resonators are determined in a self-aligned manner by the thickness of the deposited sacrificial oxide layer and can be reduced to their smallest physical limits (10’s of nanometers and less) independent from lithography. Secondly, the thickness of the SCS HARPSS resonators can be increased to a few 10’s of microns while keeping the capacitive gaps in the nanometer scale, resulting in a lower equivalent motional resistance compared to their polysilicon counterparts which have limited thickness [1-3]. Finally, the ability of fabricating all-silicon resonators with single crystal silicon as the resonating element makes HARPSS a superior candidate for implementation of high-Q, high frequency integrated MEMS resonators. In this work, silicon-on-insulator (SOI) substrates have been utilized. The advantages of SOI substrate compared to regular silicon substrate previously used to implement low frequency SCS resonators [9], are: 1) Providing “electrical isolation” between the body of individual SCS resonators in an array implementation (e.g., for filter synthesis). 2) Enabling nano-precision fabrication of “ultra-stiff” SCS resonators with height-to-width-ratio
