{"id":1536,"date":"2013-07-25T18:29:06","date_gmt":"2013-07-25T18:29:06","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/?p=1536"},"modified":"2013-08-05T19:04:20","modified_gmt":"2013-08-05T19:04:20","slug":"deep-trench-capacitor-drive-of-unreleased-si-mems-resonator","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/deep-trench-capacitor-drive-of-unreleased-si-mems-resonator\/","title":{"rendered":"Deep Trench Capacitor Drive of Unreleased Si MEMS Resonator"},"content":{"rendered":"

With frequency-quality factor products (f\u2022Q<\/i>) often exceeding 1013<\/sup>, MEMS resonators offer a high-Q<\/i>, small footprint alternative to conventional LC tanks and off-chip crystals for clocking and wireless communication. Over the past three decades, much progress has been made in the key figures of merit of MEMS resonators including small footprint, high Q<\/i>, low motional impedance, and efficient energy coupling kT<\/sub>2<\/sup><\/i>. In parallel, efforts have focused on system-level metrics including high yield, low cost, robustness, easy packaging, and integration with circuits. A key challenge in MEMS resonator design is to achieve high performance yet manufacturable devices. The unreleased deep trench (DT) resonators in this work address this challenge.<\/p>\n

Beyond the performance goals of high Q<\/i> and low loss, these devices target two key features desired for monolithically integrated MEMS resonators. First, lithographic definition of resonance frequency enables a broad range of frequencies to be fabricated on a single chip. Second, unreleased bulk-acoustic resonators do not require any low-yield, complex steps to freely suspend the moving structure and are robust in harsh environments without packaging. Unreleased resonators such as the HBAR[1<\/a>]<\/sup> have been demonstrated but have thickness-defined frequency. Lateral bulk acoustic resonators with lithographically defined frequency such as the LoBAR[2<\/a>]<\/sup> have achieved high Q<\/i> but require low d31<\/sub> coupling to drive and sense resonance. Meanwhile, sidewall AlN resonators[3<\/a>]<\/sup> excite lateral resonance with d33<\/sub> coupling but still require a release step. This work provides the benefits of all of these devices with high Q<\/i>, efficient dielectric transduction, lateral resonance, and no release step. The DT resonator implements deep trench capacitors as both electrostatic transducers and Acoustic Bragg Reflectors (ABRs), defined in a single mask and self-aligned (Figure 1). While ABRs provide acoustic isolation in a solid medium, the DT capacitors function as internal dielectric transducers[4<\/a>]<\/sup>, which have achieved the highest frequencies in Si to date[5<\/a>]<\/sup>. A 3.3-GHz unreleased Si resonator is demonstrated with Q<\/i> of 2057 and motional impedance RX<\/sub> of 1.2 k\u03a9 (Figure 2). This realization of high-Q<\/i> unreleased resonators in a bulk Si process provides a high yield, low cost, no packaging solution for on-chip clocking, wireless communication, and electromechanical signal processing.<\/p>\n\n\t\t