Chantal Combi

Design, realization and testing of micro-mechanical resonators in thick-film silicon technology with postprocess electrode-to-resonator gap reduction

This paper presents the design, fabrication and testing of high-Q high-frequency lateral-mode clamped–clamped beam micro-resonators driven by parallel-plate electrostatic transducers fabricated in a thick epipoly technology. An innovative approach is employed to reduce an intrinsically high transducer gap value (>3.0 μm) required by the need of 15 μm thick structural layer etching down to 0.2–0.4 μm after the fabrication. This is achieved by employing an electrostatic motor that approaches the actuating and sensing electrodes close to the resonator. The electrode motor is driven with 30 V dc voltage, without any dc current consumption. Two resonators having a resonance frequency of 10 MHz have been fabricated with gap values of 0.2 and 0.4 μm respectively. A comparative analysis of performances of the two resonators is given in the paper.

SURFACE ACOUSTIC WAVE PRESSURE SENSOR

A sensitive sensor for measurement of pressure and strain is created by fabricating a surface acoustic wave (SAW) delay line on a quartz substrate. The SAW device which acts as a band pass filter has a central frequency of 98MHz. When an external force, strain, pressure is applied to the acoustic gap, then the gap length and IDT finger spacing will change and thus cause a shift of the central frequency and of the phase. Our approach was first to generate a 98MHz signal at the input transducer and to evaluate the relative phase shift due to the pressure or the strain. The second approach was to introduce the SAW device in an oscillator loop to measure the relative variations of the central frequency. A simulation based on finite elements was carried out to compare the length variations and an experimental study of the IDTs position, substrate thickness and orientation on the sensor response.

Package design of pressure sensors for high volume consumer applications

In this abstract we outline the critical design aspects for the design of high volume pressure sensor for MEMS applications. Pressure sensor designs by their nature require the active device to be in contact with the pressure to be measured, this requirement has until now restricted the possibility of applying large volume semiconductor package manufacturing techniques to pressure sensors and hence their large scale deployment in consumer applications. The current designs of pressure sensors rely on either a separate protective membrane, a Gel-Liquid to transmit the pressure to the active device or a Gel applied on top of the sensor. The active part of the sensor is normally made up of two separate chips , the active membrane which flexes in response to external pressure the movement of which is translated into electrical signals by resistive or capacitive elements and a support chip with trapped between them a reference pressure volume. In this paper the design of a novel LGA based MEMS Pressure sensor and barometer/altimeter design are demonstrated. A single chip sensor solution is outlined with no need for the trapped reference volume between the support chip and the sensor membrane. In this single chip sensor solution the reference pressure volume is trapped within the chip itself improving both the chip dimensions and overall reliability by eliminating the joint between the two chips. The Packaging for a this single chip sensor is based on the now well established LGA sensor platform, the problem of interfacing the pressure sensor to external pressure is solved by using an exposed silicon in the over moulded package which allows the pressure sensor access to the outside pressure, reducing the overall package size for the absolute pressure sensor to 3 x 3 x 1 mm. MEMS sensor devices in general depend on their mechanical performance to translate mechanical movement into an electrical signal, the interface to the outside world, the package, is a mechanical structure that has a large influence on the device performance and must be thought of a part of the device not just as protective housing. In the case of the pressure sensor the flexible silicon membrane is influenced by the thermal stresses in the package. The design aspects that have influence on the performance of the MEMS devices are outlined, the Package and sensor devices were extensively modeled using FEA thermo-mechanical simulation with the results showing a good correlation to the final device performance. Problems encountered during the development related to device manufacturability are outlined and the solutions detailed. The final performance and reliability data is also presented demonstrating the robustness of the design solution.

Clamped-Clamped Beam Micro-Mechanical Resonators in Thick-Film Epitaxial Polysilicon Technology

This paper presents design and test results of clampedclamped beam resonators fabricated in thick-film epitaxial polysilicon technology. A new technique is used to achieve a 0.2 μm sensing electrod-to-resonator lateral gap in a structural polysilicon layer of 15 μm thickness. The frequencies of the designed resonators belong to the range of 2.3-16.4 MHz, the measured quality factors are of 450-15500 for different resonance frequencies.

Noise reduction in GaN-based radio frequencysurface acoustic wave filters

In this work we have fabricated and characterized GaN based surface acoustic wave (SAW) delay lines grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrate. The acoustic wave velocity of 0th Rayleigh and Sezawa modes, and the piezoelectric electromechanical coupling constant have been measured for different wave numbers in a 2μm-thick layer. The acoustic velocity resulted to be independent from the layer resistivity, which strongly affects the noise level. Through the introduction of a highly resistive GaN buffer layer, a noise level as low as −70dB has been measured. This result has been attributed to a reduced coupling between the input and output terminals.

Polysilicon Failure Stress and Young’s Modulus Evaluation in MEMS Devices

The main object of the paper is the implementation of two different approaches for the estimation of the modulus of elasticity and of the failure strain/stress in micro machined structures manufactured of epitaxial polycrystalline silicon (15 μm thick film). The micro-test-structures are produced using a standard THELMA process becoming an integral part of a MEMS actuator of comb-finger type. The micro-motor is designed to develop electrostatic in plane forces of desired intensity when fed by suitable electric potential difference.© 2003 ASME