zno-on-nanocrystalline diamond lateral bulk acoustic resonators
ZNO-ON-NANOCRYSTALLINE DIAMOND LATERAL BULK ACOUSTIC RESONATORS Reza Abdolvand, Gavin K. Ho, James Butler* and Farrokh Ayazi Georgia Institute of Technology, Atlanta, GA, USA * Naval Research Laboratory, Washington DC Phone: 404-894-9496 E-mail:{rezaa,ayazi}@ece.gatech.edu
ABSTRACT
In this work, silicon is replaced with NCD to further increase the resonance frequency of the device and relax the requirements on lithographically-defined feature sizes of the resonator at high resonance frequencies. The temperature coefficient of frequency of diamond is relatively low (12ppm/ºC) [2], adding more value to its application as a substrate. A thin layer of oxide with a large positive TCF (~85ppm/ºC) [5] can be used as a passive temperature compensation technique. Although, lower acoustic velocity of the oxide film decreases the resonance frequency, it is offset by very large acoustic velocity of diamond. Therefore, both resonance frequency and temperature stability of thinfilm piezoelectric-on-diamond (TPoD) resonators are superior to devices made from other structural materials such as silicon or pure piezoelectric [6].
This paper reports, for the first time, on thin-film piezoelectric-on-diamond composite bulk acoustic resonators. These resonators benefit from the large elastic modulus of the nano-crystalline diamond to increase the resonance frequency, and the high electromechanical coupling of the piezoelectric transduction to reduce the motional impedance. More than 1.8× increase in the resonance frequency is measured for devices fabricated on 2µm thick diamond compared to the same size devices made on 6µm thick silicon on insulator substrate. A device with 5µm feature size exhibits a high resonance frequency of 1.2GHz. A small motional impedance of 225Ω is reported for a device operating at 173MHz. The temperature coefficient of frequency (TCF) is measured to be as small as -2ppm/ºC as a result of including an oxide layer in the device structure.
PROCESS DEVELOPMENT Although the fabrication process flow for TPoD resonators (Fig. 1) is very similar to that of TPoS resonators, processing on a NCD substrate requires development of new processing steps. Two to three micrometers of nanocrystalline diamond is deposited on silicon wafers in a microwave assisted chemical vapor deposition chamber at 800ºC to prepare the initial substrate. The surface roughness of the diamond film is directly related to the thickness and the grain size of the nano-crystalline diamond. This roughness imposes a major challenge in our technology by degrading the adhesion of subsequent deposited layers to diamond, and also reducing the ZnO piezoelectric coefficient by scattering the crystallographic orientation of the grains. To address this problem, a thin (
ABSTRACT
In this work, silicon is replaced with NCD to further increase the resonance frequency of the device and relax the requirements on lithographically-defined feature sizes of the resonator at high resonance frequencies. The temperature coefficient of frequency of diamond is relatively low (12ppm/ºC) [2], adding more value to its application as a substrate. A thin layer of oxide with a large positive TCF (~85ppm/ºC) [5] can be used as a passive temperature compensation technique. Although, lower acoustic velocity of the oxide film decreases the resonance frequency, it is offset by very large acoustic velocity of diamond. Therefore, both resonance frequency and temperature stability of thinfilm piezoelectric-on-diamond (TPoD) resonators are superior to devices made from other structural materials such as silicon or pure piezoelectric [6].
This paper reports, for the first time, on thin-film piezoelectric-on-diamond composite bulk acoustic resonators. These resonators benefit from the large elastic modulus of the nano-crystalline diamond to increase the resonance frequency, and the high electromechanical coupling of the piezoelectric transduction to reduce the motional impedance. More than 1.8× increase in the resonance frequency is measured for devices fabricated on 2µm thick diamond compared to the same size devices made on 6µm thick silicon on insulator substrate. A device with 5µm feature size exhibits a high resonance frequency of 1.2GHz. A small motional impedance of 225Ω is reported for a device operating at 173MHz. The temperature coefficient of frequency (TCF) is measured to be as small as -2ppm/ºC as a result of including an oxide layer in the device structure.
PROCESS DEVELOPMENT Although the fabrication process flow for TPoD resonators (Fig. 1) is very similar to that of TPoS resonators, processing on a NCD substrate requires development of new processing steps. Two to three micrometers of nanocrystalline diamond is deposited on silicon wafers in a microwave assisted chemical vapor deposition chamber at 800ºC to prepare the initial substrate. The surface roughness of the diamond film is directly related to the thickness and the grain size of the nano-crystalline diamond. This roughness imposes a major challenge in our technology by degrading the adhesion of subsequent deposited layers to diamond, and also reducing the ZnO piezoelectric coefficient by scattering the crystallographic orientation of the grains. To address this problem, a thin (
