Yuki Nishimori

Low-damage silicon etching using a neutral beam

A low-damage silicon etching technology for fabricating microelectromechanical system (MEMS) devices using a neutral beam is reported. Neutral beams were produced from Cl2 plasma in an etching apparatus and were used to etch silicon trenches and MEMS devices. Si trench etch rate depended on the bias voltage applied to an aperture, used to produce the neutral beam. Etch rate decreased with increasing Si trench aspect ratio. This trend was minimized by enlarging the aspect ratio of through-holes in the aperture. The silicon trench profile was influenced by the aspect ratio of through-holes in the aperture. Etched Si surfaces were smooth, and no damage/defects were observed by transmission electron microscopy. Si etching of MEMS devices with smooth surfaces and scallop free sidewalls was achieved. The mechanical characteristics of an oscillator etched with the neutral beam were superior to those of that etched using a conventional Bosch process.

Method to Evaluate the Influence of Etching Damage on Microcantilever Surface on Its Mechanical Properties

We propose a method to evaluate the effect of process damage on microcantilever surfaces, introduced by processes such as plasma etching, on their mechanical properties. Using this method, we can compare the mechanical properties before and after etching even if the process changes the microcantilever thickness. Defects at the microcantilever surface affect the quality (Q) factor of the microcantilever, but the Q factor cannot be used as an indicator to evaluate process damage because it also depends on the microcantilever thickness. On the basis of theoretical considerations, we propose using Q/f (f: resonance frequency) as an indicator because both Q and f are proportional to the thickness for very thin microcantilevers. We verified our method experimentally by etching microcantilever surfaces using conventional plasma etching and neutral beam etching, which can etch silicon without damage. As a result, the Q/f value markedly decreased after plasma etching but stayed nearly the same after neutral beam etching.

Effect of neutral beam etching on mechanical property of microcantilevers

As an effective application of neutral beam etching (NBE) to microelectromechanical systems (MEMS), here we propose a combination of conventional plasma processes and NBE to remove plasma-induced damage. To evaluate the effect of the combined approach quantitatively, we measured the resonance properties of a microcantilever before and after NBE treatment and compared them with a characteristic quantity. The thickness of the damage layer times the imaginary part of the complex Youngs modulus (δEds), which is a parameter of surface damage. Although a plasma process makes the damaged surface of the microcantilevers during their fabrication, the removal of that damage by NBE is confirmed as a reduction in δEds. NBE can provide a damage-free surface for MEMS devices without a high-temperature annealing process.

Modeling of Vibrating-Body Field-Effect Transistors Based on the Electromechanical Interactions Between the Gate and the Channel

A coupled analysis method for the mechanical and electrical systems of vibrating-body field-effect transistors (VB-FETs) is described. To accommodate energy transfer between the gate and the FET channel, we represent the FET with a resistance–capacitance ladder circuit and use the Lagrange function to derive motion equations. By solving the equations, we derive the typical electrical characteristics of VB-FETs, namely, the transconductance and the current gain. The results show that the current gain is obtained even above the cutoff frequency of the FET at the antiresonance frequency of the mechanical vibrator. These characteristics strongly depend on the device dimensions and operating conditions. This means that a coupled analysis is helpful for determining an appropriate design of VB-FETs.

Recovery of plasma-induced mechanical damage in resonators using Neutral Beam Etching: Wafer-scale validation by arrayed cantilevers

As an effective application of Neutral Beam Etching (NBE) to MEMS, we here propose a combined approach with conventional plasma process and NBE: removal of plasma-induced damage by NBE. If it is possible, we can obtain a damage-free surface for MEMS devices without a high-temperature annealing process. In order to evaluate the effect of this combined approach quantitatively; we focused on the resonance of a micro cantilever and derived a parameter of surface damage (δEds) theoretically from Q-factor and resonance frequency. And then we examined the change in δEds of the cantilevers on an 8-inch wafer before and after NBE treatment. The initial surface of cantilevers had been damaged by plasma processes during their fabrication, and the removal of those damage by NBE was confirmed as the reduction in δEds.

3DOF equivalent circuit model of a comb-drive actuator

We have derived a 3DOF (degree of freedom) mechanical equivalent circuit of a comb-drive actuator from Lagrange equation and applied it to simulation of an asymmetrical comb-drive actuator, and we can obtain that experimental data is in good agreement with the simulation. We also mentioned the quadric model of a comb-drive actuator which should be used under no DC bias operation. Using these models complicated MEMS devices are represented fully by an electrical equivalent circuit including mechanical parts, and can be simulated only by electrical simulator such as SPICE.