Shaoqun Shen

A piezoresistive accelerometer with a novel vertical beam structure

Abstract A micromachined piezoresistive accelerometer sensitive to an acceleration component in the chip plane and vertical to the beam direction has been developed. To sense a lateral acceleration, a beam with a cross section vertical to the (001) wafer surface has been developed by anisotropic etching in aqueous KOH. The vertical beam is along the 〈100〉 direction and has a high aspect ratio. Two n-type piezoresistors instead of p-type ones are positioned on the top edge (wafer top surface) of the beam with one resistor on each side of the neutral line of the beam. The top edge of the beam is widened slightly to make enough room to accommodate the piezoresistors and their interconnections so that, in fact, the beam has a T-shaped cross section. The sensitivity of the device fabricated is about 0.5 mV g−1 5 V1 and the resonance frequency is about 1.2 kHz. As the sensitivity is comparable to and the fabrication process is compatible with the conventional piezoresistive accelerometers for normal acceleration, this design leads the way to developing a monolithic piezoresistive multi-axis accelerometer.

Effects of electrostatic forces generated by the driving signal on capacitive sensing devices

Abstract In measuring the capacitance of a variable mechanical capacitor used in a capacitive mechanical sensor, an electrical driving signal is usually needed. The electrostatic forces caused by the driving signal on the mechanical capacitor may interfere with the measurement and the normal operation of the devices significantly. In this paper, quantitative analyses on the effects of driving signal are made for single-sided driving, double-sided driving and double-sided driving with voltage feedback (i.e., force-balanced measurement schemes). The effects caused by the driving signal are found to be: (1) the zero offset of the sensors for single-sided driving signal, (2) the change of the measurement sensitivity, and (3) the reduction of the critical measurand signal level causing the pull-in effect that hampers the normal operation of the device. The levels of critical measurand signal for specific driving signal levels are found quantitatively. Based on the analyses, the conclusions are: (1) the level of driving signal can be selected by the compromise among the requirements on the sensitivity, the accuracy and the reliability of the sensors devices for a specific configuration, (2) the side effects of the driving signal can be minimized by using the testing scheme of double driving with voltage feedback.

Maskless etching of three-dimensional silicon structures in KOH

Abstract A novel micromechanical technique using maskless etching of three-dimensional anisotropically etched silicon structures is investigated. The structures investigated are convex prismatic edges included by {100} and {111} planes and the convex corners of a rectangular m both formed by a previous masked anisotropic etching in KOH. Experimental results verify that the cutting planes developed at the mesa edges are {311} planes that make the contour change obviously. Analytical relations have been found to predict the evolution of the contour and the undercutting rate at the convex edges and corners based on the fact that the cutting planes at the convex edge are {311} planes while the cutting planes at the convex corner are {411}. The relations have been confirmed by experiments and the etching rate of {311} p found for various KOH concentrations. As an example of application, a symmetric cantilever beam-mass structure with beams horizontally located at the central plane of the wafer has been fabricated for a differential capacitive accelerometer with minimized cross-axis sensitivity.

Over-range capacity of a piezoresistive microaccelerometer

Over-range capacity is one of the most important parameters for an accelerometer. Over-range protection has been provided for commercially available micromechanical piezoresistive accelerometers to make them practical for applications. Described in this paper is an extensive analysis on the over-range protection capability for a cantilever beam accelerometer. Also analysed in the paper is the acceleration of a laboratory testing system using a drop hammer scheme for shock accelerations. Based on the theoretical analyses and experimental data, the system has been calibrated and used to provide shock accelerations for experimental testing in the laboratory.

A novel micromachining technology for multilevel structures of silicon

Abstract A novel micromachining technology for multilevel structures of silicon is described in this paper. Complicated multilevel structures can be formed by using a maskless etching following a conventional masked etching in aqueous KOH. The process consists of two successive steps. First, structures with {111} sidewalls are formed for a desired etching depth on the surface of a (100) silicon wafer by a conventional masked anisotropic etching process with a specially designed etching mask. Then, the etching mask is removed except for some areas (such as the frame of the chip) and a maskless etching follows. In addition to the downward etching of the upper and lower ( 100) planes during maskless etching, fast-etching {311} planes emerge at the upper edges of the {111} sidewalls and cut the {111} sidewalls, while the {111} sidewalls extend downward at the lower ends. As the {311} planes replace the {111} sidewalls with a rate faster than the extension rate of the {111} planes, the {111} sidewalls will finally be replaced by {311} planes. Moreover, by an appropriate mask design and an appropriate etching depth of the previous masked etching, the bottom areas of some etching cavities diminish temporarily so that the downward etching in these areas is stopped for time intervals after the intersection of {111} planes on the bottom and before the {111} planes are replaced completely by the {311} planes. Once the {111} sidewalls are replaced completely by the {311} planes, the bottom areas reappear and the etching downward resumes. Analyses show that levels with different depths can be created in this way for different window apertures and the number, positions as well as the depths of the new levels can be individually decided by the design of the single mask for masked etching and the etch depths of both masked and maskless etching processes. More than 10 interesting multilevel structures with up to five structure levels have been fabricated. The results show that the technology is very versatile and repeatable. Therefore, this technology is a very useful new tool for making silicon micromechanical structures to meet the ever-increasing demands for solid-state sensors and actuators.

Two-dimensional excitation operation mode and phase detection scheme for vibratory gyroscopes

We propose a novel operation mode of two-dimensional excitation and phase detection for vibratory gyroscopes. It has been proven that the angular rate output is both amplitude-modulated and phase-modulated when the structure is driven into vibration in both the x- and y-directions. The output can be converted into a narrow-band phase-modulated signal by an amplitude limiter. Therefore, the angular rate signal of a vibratory gyroscope can be measured by detecting the phase change of the output signal. This operation scheme features high accuracy, high immunity against interference and a low-temperature coefficient. A bulk micromachined gyroscope with piezoresistive sensing elements is tested in the two-dimensional excitation mode with the phase detection scheme and the experimental results are presented.

Study on linearization of silicon capacitive pressure sensors

Abstract The non-linearity of silicon capacitive pressure sensors with a single-sided island-diaphgragm structure and a diode-quad interface circuit is investigated. Based on the mathematical analysis of the sensor non-linearity, two types of linearization approaches have been developed. The first one uses feedback from the output to control the excitation voltage and, thereby, to adjust the non-linearity of a conventional structure sensor (consisting of a sensitive capacitor and a reference capacitor) conveniently to 0.1% or smaller. Another approach uses a novel push-pull capacitive structure, in which a pair of pressure-sensitive capacitors is operated in a push-pull configuration. The output is inherently linear if there is no parasitic capacitance. For a practical device with parasitic capacitance comparable with the working capacitance, the nonlinearity is usually of the order of 0.1%. The experimental results show that the linearity is improved by at least one order of magnitude by using either of the two approaches.

A novel technique for measuring etch rate distribution of Si

Three-dimensional etch rate distribution of Si is necessary in analyzing the anisotropic etching of Si. As three-dimensional etch rate distribution can be composed by a series of two-dimensional distributions, a novel method is suggested in this paper for measuring the two-dimensional distributions with simple tools and high efficiency. For example, vertical sidewalls of a series of U-shaped trenches in (0mn) silicon wafers are first created by deep reactive ion etching (DRIE) technique. By measuring the etch rates in normal directions of the vertical sidewalls, two-dimensional distributions of etch rate in (0mn) planes can be found. As the height of vertical sidewalls formed by DRIE can be very large, the planes of sidewall can withstand a relatively long etch time before they are replaced by emerging slow etching planes. No special measuring facilities but a conventional microscope is needed for the measurement. Presented in this paper are etch rate distributions in (001) and (011) crystal planes in 40% KOH and 25% TMAH.

A pressure transducer with a single-sided multilevel structure by maskless etching technology

A novel single-sided multilevel beam-diaphragm-mass structure has been designed and fabricated for high sensitivity silicon pressure transducers by using a novel anisotropic etching technology. The structure consists of three thin beams on a deep-etched thin diaphragm and two masses. Each mass consists of a mesa for stress concentration and a small island (post) on the mesa for overrange protection. As all the structures are on the back side of the wafer, the front side remains flat. The piezoresistors are diffused resistors on the front side in the locations with beams on the back. The multilevel structure features (1) flat front side facilitating metal interconnection; (2) small islands for overrange protection so that the electrostatic force caused by electrostatic bonding between the masses and the glass substrate is minimal; and (3) high burst pressure of the diaphragm. A prototype pressure transducer of 400 Pa operation range and 0.6% nonlinearity has been tested.

Maskless anisotropic etching-a novel micromachining technology for multilevel microstructures

The authors describe a novel bulk micromachining technology for silicon multilevel microstructures using a maskless etching following a masked etch in aqueous KOH. Deep multilevel structures with {311} sidewalls and <110> edges can be formed on a [001] silicon wafer and the number of levels is not limited in principle. The positions and the depths of individual levels are determined by the design for one mask and the masked-maskless etching process. The technology is likely to be very useful in micromechanical sensors and actuators.