Dirk Scheibner

Bonding and deep RIE: a powerful combination for high-aspect-ratio sensors and actuators

In this paper we present the very promising results for two methods of the so-called Bonding and Deep RIE (BDRIE) technology, characterised by bonding of two wafers with pre-patterned vertical gaps and subsequent RIE trench etching of the active layer. In case of the anodically bonded silicon-glass compound detection electrodes for vertical movement are integrated. The silicon layer contains the movable structure as well as drive and detection electrodes for lateral movement. It is advantageous that finally the mechanical active elements consist of single crystalline silicon without any additional layers. The BDRIE approach allows a great variation of parameters. The active layer thickness can be defined due to application issues. Our examples show active layers thickness ranging from 30 up to 200 μm, patterned by dry etching steps with maximum aspect ratio between 20:1 and 30:1. Structures with trench width variations of more than 50 (widest/smallest trench) have been fabricated successfully. Methods and results of preventing notching and backside etching of the active layer are presented as well. The size of the vertical gap can be as small as 1.5 μm for a very sensitive detection or several tens or hundreds of microns in order to reduce damping and parasitic capacitance. Holes for release in the movable structure are not necessary and will therefore not restrict the design. However, restrictions are given by the minimum size of bond area and the relation between layer thickness, free standing area above the groove and bond pressure, which are discussed within the paper. Applications of BDRIE are inertial sensors like gyroscopes, step-by-step switchgears as well as micro mirrors.

In-process gap reduction of capacitive transducers

This paper presents a MEMS fabrication technique for reducing the trench width of microstructures below the technological limitations of the deep reactive ion etching (DRIE) process, in order to increase the aspect ratio of the sensing electrode gap of capacitive transducers. The in-process trench width reduction is based on the displacement of a substructure actuated by a buckling beam mechanism. Compressive stress causes a longitudinal force in the acting beams which results in the buckling to a predefined direction. This way, the capacitive sensitivity and hence the signal to area ratio of a transverse comb structure could be increased by a factor of 5.

Wide range tuneable resonators for vibration measurements

In this paper an array of frequency selective capacitive vibration sensors with electrically tuneable band selectivity is presented. The band selectivity is based on the resonance characteristics of the structures, the centre frequency of which is tuned by electrostatic forces directly applied to the seismic mass. The eight-cell array fabricated in surface near silicon bulk micro-technology (SCREAM) covers the frequency range from 1 to 10 kHz. Measurement results illustrate the tuning characteristics of the array and the linearity of the capacitive comb structures. Furthermore the functionality of the micro-system consisting of the sensor array, analogue signal detection circuitry and an eight-bit micro-controller for array control and signal evaluation tasks is described.

A Frequency Selective Silicon Vibration Sensor with Direct Electrostatic Stiffness Modulation

In the field of industrial machinery an increasing demand for low cost vibration measurement equipment exists. The described frequency selective structures represent a low-cost alternative to the commonly used wide band accelerometers. To continuously cover a wide frequency range with a small number of resonators the sensor structures possess a resonance frequency tuning capability. Electrostatic forces applied to the seismic mass lead to a softening of the system and thus a lowering of the resonance frequency. Experimental results gained from fabricated test structures demonstrate the suitability of the tuning principle for wide range resonance tuning and the obtained linearity of the sensor behaviour. By grouping eight cells with stepped base frequencies and overlapping tuning ranges into an array one decade from 1 to 10 kHz is continuously covered with tuning voltages below 40 V.

Tunable resonators with electrostatic self test functionality for frequency selective vibration measurements

In this paper we present an array of tunable resonators for frequency selective vibration measurements. The resonance frequency tuning is carried out by electrostatic forces directly applied to the seismic mass affecting the total stiffness. Frequency selective sensor systems require the exact knowledge of the resonance frequency. Therefore a method of in-system calibration using electrostatic excitation and capacitive vibration detection was investigated. Coupling problems of the excitation signal into the detection path were solved by using two excitation signals 180/spl deg/ out-of-phase and by contacting the substrate with a backside metallization.

Frequency selective sensor arrays for vibration measurement

Describes a new microsystem for vibration measurement based on frequency selective sensor arrays with electrically tunable band selectivity for the frequency range from 1 to 10 kHz. The band selectivity is achieved by electrostatic forces.

Improved Coupled-Field FE Analysis of Micromachined Electromechanical Transducers

FE analysis of electrostatic-structural interaction in MEMS is enhanced by three-dimensionally sensitive so-called transducer elements. As demonstrated at a surface micromachined tunable vibration sensor, they enable detailed transfer function simulation at flexible parts, capturing resonance shift, levitation, cross-axis sensitivity and distributed resistive voltage drop. By drastic order reduction based on pre-calculated 3D capacitance tables a component level analysis with minor model simplification and many solution cycles becomes fast and convenient on a desktop computational environment.

Adjustable Force Coupled Sensor-Actuator System for Low Frequency Resonant Vibration Detection

The paper reports on a novel micro-electro-mechanical (MEM) sensor-actuator system for adjustable resonant vibration detection at frequencies below 1 kHz. The MEM system operates at standard pressure and utilizes the superheterodyne principle well known from information technologies. By mixing the low frequency vibration with a high frequency carrier, the information of the vibration is transformed into a higher frequency range. A resonant oscillator amplifies and filters the information.

A Tunable Resonant Vibration Measurement Unit Based on a Micromachined Force Coupled Sensor-Actuator System

In this paper we present a novel microsystem for frequency selective vibration measurements in a lower frequency range. It is based on a force coupled sensor-actuator system which transforms vibration signals from a low into a high frequency range and filters the desired spectral line by means of resonance amplification. Commonly, vibration signals are detected by wideband accelerometers and subsequent fast Fourier transformation. In contrast to electronic signal transformation from a time to a frequency domain, the presented approach performs most signal conditioning in the mechanical domain. Thanks to the high Q-factor of MEMS, the signal is amplified in a very narrow band with high signal-to-noise ratio. The sense frequency can be tuned by variation of the carrier frequency and motion signals are detected by capacitive pick-up. The sensor is fabricated by BDRIE (bonding & deep reactive ion etching) silicon technology.