Tribological properties of mems materials measured on a small-scale device

Micro-electro-mechanical systems, MEMS, is a rapidly growing interdisciplinary technology within the general field of Micro-Systems Technology which deals with the design and manufacture of miniaturised machines with major dimensions at the scale of tens, to perhaps hundreds, of microns. Because they depend on the cube of a representative dimension, component masses and inertias rapidly become small as size decreases whereas surface and tribological effects, which often depend on area, become increasingly important. Although MEMS components and their areas of contact are small, tribological conditions, measured by contact pressures or acceptable wear rates, are demanding and technical and commercial success will require careful measurement and precise control of surface topography and properties. Fabrication of small numbers of MEMS devices designed to test potential material combinations can be prohibitively expensive and thus there is a need for small scale test facilities which mimic the contact conditions within a micro-machine without themselves requiring processing within a full semiconductor foundry. The talk will illustrate some initial experimental results from a small-scale experimental device which meets these requirements, examining in particular the performance of Diamond-Like-Carbon coatings on a silicon substrate. Copyright © 2005 by ASME.

A packaging technique for silicon MEMS strain sensors on steel

A packaging technique suited to applying MEMS strain sensors realized on a silicon chip to a steel flat surface is described. The method is based on adhesive bonding of the silicon chip rear surface on steel using two types of glue normally used for standard piezoresistive strain sensors (Mbond200/ 600), using direct wire bonding of the chip to a Printed Circuit Board, also fixed on steel. In order to protect the sensor from the external environment, and to improve the MEMS performance, the silicon chip is encapsulated with a metal cap hermetically sealed-off under vacuum condition with a vacuum adhesive in which the bonding wires are also protected from possible damage. In order to evaluate the mechanical coupling of the silicon chip with the bar and thestress transfer extent to the silicon surface, commercial strain sensors have been applied on the chip glued on a steel bar in alaboratory setup able to generate strain by inflection, yielding a stress transfer around 70% from steel to silicon. © 2008 IEEE.

Laminar to turbulent flow transition measurements using an array of SOI-CMOS MEMS wall shear stress sensors

The successful utilization of an array of silicon on insulator complementary metal oxide semiconductor (SOICMOS) micro thermal shear stress sensors for flow measurements at macro-scale is demonstrated. The sensors use CMOS aluminum metallization as the sensing material and are embedded in low thermal conductivity silicon oxide membranes. They have been fabricated using a commercial 1 μm SOI-CMOS process and a post-CMOS DRIE back etch. The sensors with two different sizes were evaluated. The small sensors (18.5 ×18.5 μm2 sensing area on 266 × 266 μm2 oxide membrane) have an ultra low power (100 °C temperature rise at 6mW) and a small time constant of only 5.46 μs which corresponds to a cut-off frequency of 122 kHz. The large sensors (130 × 130 μm2 sensing area on 500 × 500 μm2 membrane) have a time constant of 9.82 μs (cut-off frequency of 67.9 kHz). The sensors' performance has proven to be robust under transonic and supersonic flow conditions. Also, they have successfully identified laminar, separated, transitional and turbulent boundary layers in a low speed flow. © 2008 IEEE.

Improved infrared thermal imaging of a CMOS MEMS device

We report a technique which can be used to improve the accuracy of infrared (IR) surface temperature measurements made on MEMS (Micro-Electro-Mechanical- Systems) devices. The technique was used to thermally characterize a SOI (Silicon-On-Insulator) CMOS (Complementary Metal Oxide Semiconductor) MEMS thermal flow sensor. Conventional IR temperature measurements made on the sensor were shown to give significant surface temperature errors, due to the optical transparency of the SiO 2 membrane layers and low emissivity/high reflectivity of the metal. By making IR measurements on radiative carbon micro-particles placed in isothermal contact with the device, the accuracy of the surface temperature measurement was significantly improved. © 2010 EDA Publishing/THERMINIC.

Fabrication and testing of a high resolution extensometer based on resonant MEMS strain sensors

A novel type of linear extensometer with exceptionally high resolution of 4 nm based on MEMS resonant strain sensors bonded on steel and operating in a vacuum package is presented. The tool is implemented by means of a steel thin bar that can be pre-stressed in tension within two fixing anchors. The extension of the bar is detected by using two vacuum-packaged resonant MEMS double- ended tuning fork (DETF) sensors bonded on the bar with epoxy glue, one of which is utilized for temperature compensation. Both sensors are driven by a closed loop self-oscillating transresistance amplifier feedback scheme implemented on a PCB (Printed Circuit Board). On the same board, a microcontroller-based frequency measurement circuit is also implemented, which is able to count the square wave fronts of the MEMS oscillator output with a resolution of 20 nsec. The system provides a frequency noise of 0.2 Hz corresponding to an extension resolution of 4 nm for the extensometer. Nearly perfect temperature compensation of the frequency output is achieved in the temperature range 20-35 C using the reference sensor. © 2011 IEEE.