Showing papers by "Enakshi Bhattacharya published in 2007"

Estimation of Stiction Force From Electrical and Optical Measurements on Cantilever Beams

Abstract: In this paper, we estimate the stiction force from electrical (current-voltage) measurements on surface micromachined polysilicon cantilever beams A bias voltage was applied between the beam and the substrate At the pull-in voltage, the beam collapsed to the substrate and the current rose rapidly from zero Similarly, at the pull-out voltage during bias sweep back, the current dropped rapidly to zero when the contact between the beam and the substrate was broken An analytic model for the stiction force was developed in terms of the pull-in and pull-out voltages and was used to estimate a stiction force of about 70 nN from the measured electrical characteristics This method of characterization is suitable for use in packaged devices An analytic model was developed to estimate stiction force from optical surface-profile measurements of the curvature of long collapsed cantilever beams in a cantilever-beam array, in the absence of any electrostatic actuation The force per unit length of about 14 nN/m thus obtained was used to compare the effects of surface roughness on stiction

Enhancement of the sensitivity of pressure sensors with a composite Si/porous silicon membrane

Abstract: Porous silicon (PS) has many interesting and unique properties that make it a viable material in the field of MEMS. In this paper, we investigate the application of PS in improving the sensitivity of bulk micromachined piezoresistive pressure sensors. The property of a low Young's modulus of PS and its dependence on porosity have been exploited to obtain a higher sensitivity compared to pressure sensors with single crystalline silicon membranes. Simulation was carried out to represent the Si/PS composite membrane by two layers with Young's modulus corresponding to silicon and PS. The behavior of this membrane was studied for various values of porosity and thickness of the PS layer. Composite Si/PS membranes were fabricated by converting a part of the silicon membrane thickness into PS by electrochemical etching in an HF-based electrolyte. Polysilicon piezoresistors were formed on the membrane in the form of a Wheatstone bridge for the measurement of sensitivity. When compared to membranes of silicon, the sensitivity of the composite Si/PS membrane is found to be higher showing improvement with an increase in the porosity and thickness of the PS layer.

Parameter extraction from simple electrical measurements on surface micromachined cantilevers

Abstract: In surface micromachined structures, many parameters like geometry and Young's modulus depend on the process steps and need to be measured for accurate prediction of their functionality. This work discusses simple electrical measurement techniques on surface micromachined cantilever beams to determine Young's modulus, the gap between the beam and the substrate, and the thickness of a deposited aluminum layer on the beam. Cantilevers are ubiquitous in most microelectromechanical system (MEMS) sensors and actuators, and hence are ideal test structures. Pull-in, and a novel resonance frequency measurement based on the pull-in technique, are done on oxide anchored doped polysilicon beams at the wafer level, and some of the device and material properties are extracted from these measurements. The extracted values are compared with those determined from established methods like vibrometry and surface profiler measurements, and show good agreement. Since the measurements are all electrical, they can be part of standardized testing and are also suitable for packaged devices.

Wafer bonding — A powerful tool for MEMS

Abstract: Wafer bonding techniques play a key role in the present day silicon bulk micromachining for MEMS based sensors and actuators. Various silicon wafer bonding techniques and their role on MEMS devices such as pressure sensors, accelerometers and micropump have been discussed. The results on the piezoresistive pressure sensors monolithically integrated with a MOSFET differential amplifier circuit have been presented to demonstrate the important role played by the Silicon Fusion Bonding technique for integration of sensors with electronics on a single chip.

Studies on cantilever based triglyceride biosensor

Abstract: We report detection of micromolar levels of triglycerides using surface micromachined polysilicon cantilever beams. Enzymatic hydrolysis of triglycerides produces glycerol which alters the viscosity and density of the solution. This affects the dynamic properties of cantilever beams immersed in the solution. The change in the resonance frequency of the cantilever beams in the solution is measured using Doppler Vibrometry and the concentration of triglyceride is determined by comparing with a predetermined calibration plot.

Simple measurement technique for resonance frequency of micromachined cantilevers

Abstract: This paper discusses a simple electrical measurement technique to determine resonance frequency of surface micromachined cantilever beams that is also suitable for packaged devices. Measurements are done on oxide anchored doped polysilicon beams. If the beam is driven by an AC signal riding on the DC bias, the beam starts vibrating. When the drive frequency matches the natural frequency of the beam, the oscillation amplitude is maximum. In this measurement, the DC bias is fixed at a value lower than the pull-in voltage. A small AC bias is then applied such that the sum of the DC and the maximum amplitude of the AC is less than the pull-in voltage. The frequency of the AC is then swept and at resonance, because of large displacement, the beam is pulled in and this is detected by a current flowing between the beam and the substrate. By iteratively adjusting the DC bias it is possible to make sure that pull-in occurs only due to resonance and the frequency setting at this point gives the natural frequency of the beam. Measured values for different beam lengths were compared with Doppler Vibrometry results and gave an excellent match.

Electrical measurement of undercut in surface micromachined structures

Abstract: In a surface micromachined cantilever, the gap (g), beam thickness (t) and width get modified during fabrication. Since performance of sensors and actuators strongly depend on the final geometry, it becomes essential that these parameters are measured. Two sets of oxide anchored cantilever beams of different lengths with 20 and 30 mum widths are realised. On the released beams resonance frequency (fi) measurements are performed. For a given length, by finding the ratio between measured frequencies for two widths, we get the value of undercut. Using this value in the slope of a fi 2 versus L-4 plot we get the Young's modulus (E) of beam material.

Sensitivity and stress in pressure sensors with composite silicon/macroporous silicon membranes

Abstract: Silicon/porous silicon (Si/PS) composite membranes can be fabricated by converting a part of the silicon membrane thickness into porous silicon (PS) with electrochemical etching in HF based electrolyte. In this paper we discuss the performance of composite membranes with MacroPS for pressure sensors. The sensitivity of the composite membrane is found to be higher compared to silicon and improves with the increase in the porosity due to the drastic reduction in Young's Modulus of PS with porosity. The sensor response is found to be linear for pressures below 1 bar. PS formation produces stress in the membrane raising the offset voltage in the pressure sensors.

A novel polysilicon based process for three terminal surface micromachined cantilever

Abstract: A three terminal cantilever beam can be used as a switch and also for material and process characterization. This paper deals with process optimization for its fabrication with surface micromachining steps compatible with standard integrated circuit manufacturing technology. The problems encountered in each step of fabrication and possible solutions are discussed in detail. The process makes extensive use of polysilicon deposited by Low Pressure Chemical Vapour Deposition for contacts, sacrificial oxide as well as the structural layer.

Silicon/Porous Silicon composite membrane for high sensitivity pressure sensor

Abstract: Porous Silicon (PS) has many interesting and unique properties that make it a viable material in the field of MEMS. In this paper we investigate the application of PS in improving the sensitivity of bulk micromachined piezoresistive pressure sensors. A part of the silicon membrane thickness has been converted into PS by electrochemical etching in HF based electrolyte. The property of low Young’s modulus of PS and its dependence on porosity have been exploited in obtaining higher sensitivity compared to pressure sensors with single crystalline silicon membranes. The sensitivity is found to increase with the porosity and thickness of PS layer and these can be easily controlled by varying the PS formation parameters.