High-resolution capacitive subnanometric motion detectors using branched finger electrodes with high-amplitude sense voltage

Abstract

This thesis presents a high-resolution capacitive subnanometric motion detector using branched finger electrodes with high-amplitude sense voltage. We reduce the mechanical noise level by increasing the mechanical stiffness based on deep RIE process of an SOI wafer. We reduce the electrical noise level by increasing the amplitude of AC sense voltage. The nonlinearity and instability problems arising in the high-amplitude sense voltage have been solved by the modification of electrode structure from conventional straight finger type into the branched finger type. Combined use of the branched finger electrode and the high-amplitude sense voltage generates stiffness increase effect for the linearity and bandwidth enhancement. We design, fabricate and test the branched finger capacitive motion detector for subnanometric motion sensing. The present nanodetector shows the total noise level of 0.0057 $Å/\sqrt{Hz}$ at the sense voltage of 16.5V, while the conventional capacitive detectors have shown the noise level of 0.018~0.007 $Å/\sqrt{Hz}$. The total noise of the capacitive nanodetector includes the mechanical noise of 0.0055 $Å/\sqrt{Hz}$ and the electrical noise of 0.0016 $Å/\sqrt{Hz}$ . The stiffness increase effect results in a 21% increase of the bandwidth from 1,585Hz to 1,923Hz. We apply the capacitive nanodetectors to the development of the navigation grade capacitive microaccelerometers. The total acceleration noise is achieved as $5.5μg /\sqrt{Hz}$ at the sense voltage of 19V. The stiffness increase effect results in a 140% increase of the bandwidth from 726Hz to 1,734Hz. For a fixed sense voltage of 10V, the total noise is measured as $2.6μg/\sqrt{Hz}$ at the air pressure of 3.9torr, which is the 51% of the total noise of $5.1μg/\sqrt{Hz}$ at the atmospheric pressure. We find that the branched finger capacitive microaccelerometer shows the positive stiffness effect when the total length of branched finger electrodes is greater than 1cm. From the excita...