CAPACITIVELY TRANSDUCED MICROMECHANICAL RESONATORS WITH SIMULTANEOUS LOW MOTIONAL RESISTANCE AND Q > 70,000

Abstract

Capacitively transduced micromechanical disk resonators that exhibit simultaneous low motional resistance (< 130 Ω) and high Q (>70,000) at 61 MHz are demonstrated using an improved ALDpartial electrode-to-resonator gap filling technique that reduces the Q-limiting surface losses of previous renditions by adding an alumina pre-coating before ALD of the gap-filling high-k dielectric. This work increases the Q over the ~10,000 of previous renditions by more than 6× towards demonstration of the first VHF micromechanical resonators in any material, piezoelectric or not, to meet the simultaneous high Q (>50,000) and low motional resistance R x (< 200Ω) specs highly desired for front-end frequency channelizer requirements in cognitive and software-defined radio architectures. This work finally overcomes the high impedance bottleneck that has plagued capacitively transduced micromechanical resonators over the past decade.

Capacitive-Piezoelectric Transducers for High-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> Micromechanical AlN Resonators

“…The 635-MHz quartz-MEMS device of [48] comes close, but does not quite satisfy the needed (C x /C o ) value. In fact, of the twelve resonators plotted only three simultaneously satisfy the stated Q and (C x /C o ) criteria: Two capacitive-gap transduced devices in the below 100 MHz regime [9], [49]; and one very recent capacitive-piezo disk device at 300 MHz [50]. This new capacitive-piezo device uses a different fabrication flow to achieve improved anchors, confirming that anchor losses constrain Q in the present devices.…”

“…The electrode geometry and configuration affects η Drive via the last term in (9). Given the same electrode area and static capacitance, the coupling coefficient η Drive is maximized by placing electrodes over locations that experience maximum strain.…”

“…2(c). From (9) and (16), the drive and sense coupling coefficients for a capacitive-piezo resonator are smaller by a factor α than those of an equivalently-sized conventional piezoelectric device using contacting electrodes. However, this does not necessarily mean that its port-to-port motional resistance is smaller.…”

“…The former approach is presently being explored by shrinking electrode-to-resonator gap spacings, such as done in [8] and [9]. So far, there has been some success at 60-MHz, at which a wine-glass disk resonator using partialfilled ALD gaps with spacings equivalent to 37.6 nm achieved a simultaneous Q of 73,000 and motional resistance of 130 , with a (C x /C o ) of 0.39% [9]. However, similar Q and (C x /C o ) marks are yet to be demonstrated at GHz frequencies, where shrinking resonator sizes reduce the total coupling afforded by capacitive-gap transducers, making it more difficult to achieve sufficiently large (C x /C o ).…”

2.97-GHz CVD diamond ring resonator with Q &#x003E;40,000

“…From (6), reducing the electrode-to-resonator gap d 0 of the device would yield a considerably smaller motional resistance. For example, if the gap were reduced from 75nm to 30nm via the partial ALD gap filling method of [19], the close-to-carrier phase noise reduction over the LC VCO would rise to 52dB, and the far-from-carrier phase noise shortfall would shrink to 6dB, as shown in Fig. 11.…”