Mitsuo Konno

A high quality-factor silicon cantilever for a low detection-limit resonant mass sensor operated in air

The detection limit of a resonant cantilever mass sensor depends on its quality factor. The size and mode dependences of quality factors in silicon micro-cantilevers at atmospheric pressure are extensively investigated. Silicon micro-cantilevers of various lengths and widths are fabricated and their quality factors of flexural vibration modes are measured in air. The quality factor is found to vary largely depending on the cantilever length, width and vibration mode. The maximum quality factor obtained is 1550. These findings correlate well with a vibrating sphere model with an equivalent cross-section approximation.

Novel MEMS oscillator using in-plane disk resonator with sensing platform and its mass sensing characteristics

We developed a new disk resonator which has high quality factor, highly efficient mass detection ability, and area-effective layout. High quality factor was achieved by minimizing support loss using nodal-point position control on the structural design. Sensing platform in the center hole provides wide and high-sensitive mass detection area. We also developed an oscillation circuit using impedance transform circuit which is suitable to MEMS resonators with high motional impedance. Toluene gas detection was demonstrated using this oscillator.

Fabrication of 150-nm-Wide Transducer Gaps for Disk-Type Resonators by Single Dry Etching Process

We have newly designed and fabricated single crystal silicon (SCS) disk-type micromechanical resonators with 150-nm-wide vertical transducer gaps. The transducer gaps have been fabricated through a simple process including a single trench etching of SCS by deep reactive ion etching (D-RIE) with a resist mask patterned by electron beam lithography. The peak resonant frequencies measured for the fabricated resonators agreed well with those predicted by finite element simulations. Furthermore, the D-RIE process conditions for transducer gaps have been improved. Using the improved D-RIE process conditions, less tapered gaps of 2 µm deep were successfully fabricated for 100-, 150-, and 200-nm-wide gap patterns. The sidewall angle was from 88 to 89° depending on the gap width.

Frequency Stability of a Closed-Loop Oscillator Using Micro-Electro-Mechanical Cantilever Resonator Against Temperature Fluctuation

The frequency stability of a silicon cantilever resonator against temperature was investigated under open-loop and closed-loop conditions. The authors found that the adjustment of the circuit phase in the closed-loop condition induced a significant change in temperature coefficient. The observed temperature coefficient dependence on the circuit phase was analyzed with the temperature dependence of elasticity in silicon and the phase shift originated from the oscillation circuit. The existence of a phase shift of 1.37°/°C was evaluated from the measured results. The phase effect was comparable to the elasticity effect on the temperature coefficient.

Fabrication of two-point-supported annular-type microresonators with capacitive transducer gaps

Disk microresonators employing in-plane resonant modes are promising candidates as functional elements for high-sensitive mass sensing applications because they provide a high quality factor (Q) in air at atmospheric pressure. The authors have newly designed and fabricated single crystal silicon (SCS) annular micromechanical resonators with an inner to outer radius ratio of 0.17. The beam structures to support the resonator were connected to the two nodal points expected for a specified in-plane resonant mode. The resonator was electrostatically driven and detected with 150-nm-wide vertical transducer gaps that were fabricated by trench etching of the SCS using deep reactive ion etching (D-RIE) with a resist mask patterned by electron beam lithography. The fabricated resonators show a resonant peak at 50.10 MHz with a Q of 3000 in air. The measured peak resonant frequency agreed well with the frequency predicted by finite element simulations.

Area-Selective Polymer Deposition on Micro-Area Framed by Trenches with Falling Liquid Film

Additional formation of functional films on the defined micro-areas on the microstructure is often complicated and need advanced control technique. Any simple and faster deposition processes are favorable. In this paper, we propose a simple processing method to form thin films on the defined areas by falling dispersion liquid containing the coating materials on a tilted silicon substrate with trench frames. We attempted to hold the coating solution and to form a thin film within the framed micro-area. We fabricated 500 nm-wide trench frames on silicon substrates, and flew a diluted photoresist (PR) solution, as the coating solution, on the tilted surface with the frames. PR film was successfully formed on the framed area for the substrate modified with hexamethyldisilazane (HMDS). However, the area-selective PR deposition was difficult in case the substrate was not treated with HMDS. It is likely that lower contact angle for the substrate without HMDS treatment is unfavorable for the area-selective PR deposition for the excess adhesive properties of the coating solution to the surface. The effectiveness of the frames as the guide to hold a liquid droplet within the defined area was considered.