Sub-micron Capacitive Gaps Research Articles

Resonant pitch and roll silicon gyroscopes with sub-micron-gap slanted electrodes: Breaking the barrier toward high-performance monolithic inertial measurement units

This paper presents the design, fabrication, and characterization of a novel high quality factor (Q) resonant pitch/roll gyroscope implemented in a 40 μm (100) silicon-on-insulator (SOI) substrate without using the deep reactive-ion etching (DRIE) process. The featured silicon gyroscope has a mode-matched operating frequency of 200 kHz and is the first out-of-plane pitch/roll gyroscope with electrostatic quadrature tuning capability to fully compensate for fabrication non-idealities and variation in SOI thickness. The quadrature tuning is enabled by slanted electrodes with sub-micron capacitive gaps along the (111) plane created by an anisotropic wet etching. The quadrature cancellation enables a 20-fold improvement in the scale factor for a typical fabricated device. Noise measurement of quadrature-cancelled mode-matched devices shows an angle random walk (ARW) of 0.63° √h−1 and a bias instability of 37.7° h−1, partially limited by the noise of the interface electronics. The elimination of silicon DRIE in the anisotropically wet-etched gyroscope improves the gyroscope robustness against the process variation and reduces the fabrication costs. The use of a slanted electrode for quadrature tuning demonstrates an effective path to reach high-performance in future pitch and roll gyroscope designs for the implementation of single-chip high-precision inertial measurement units (IMUs).

Fabrication of high aspect ratio self-aligned stepped polysilicon electrode for single-crystal silicon microstructure

This paper reports a self-aligned process for the fabrication of a high aspect ratio stepped polysilicon electrode with sub-micron capacitive gaps, which increases the coupling factor in capacitive driving and sensing. In contrast to the previous works, the polysilicon electrodes, which are formed by refilling the trenches and etching back, are anchored on the surfaces of steps in the trenches and etched by two-step deep reactive ion etching. The polysilicon electrodes are self-aligned and perfectly symmetric. Because of the `U'-shaped profile produced by the refilling process, the polysilicon electrode is less sensitive to a stress gradient in Low-pressure chemical vapor deposition (LPCVD) polysilicon layer. A thin layer of heavily doped polysilicon and a low-stress polysilicon layer, instead of uniform, heavily doped polysilicon, are used to form the polysilicon electrodes without sacrificing the electrical performance, which decreases the deposition time for LPCVD significantly due to the much faster deposition of normal low-stress polysilicon. To validate the study, a single-crystal silicon length extensional mode resonator with a 300-nm capacitive gap is fabricated based on this process. The frequency of the resonator is measured to be 25.5955 MHz, with a quality factor of 67,656 under 20 Pa.

Micromechanical resonators with sub-micron capacitive gaps in 2 µm process

An innovative gap reduction technique is reported to achieve sub-micron capacitive gaps for micromechanical resonators to boost the output signal using the standard low-cost 2 µm commercially available foundry process from MEMSCAP. Electrostatic actuation was used to reduce the gap size below the fabrication limitation. To demonstrate the proposed idea, a 6.35 MHz Lame-mode square resonator was designed, fabricated and tested. The resonator gap size was experimentally measured to be 0.64 µm, which boosted the resonance peak by 20 dB.