A wide variety of MEMS micro-mirrors are being developed for various optical applications. One such application is Fourier Transform Spectrometry (FTS). The design, process optimization and fabrication of a micro-mirror for this application is presented. Large, non-tilting displacements of mirrors are required to achieve high FTS resolution. In order to obtain this without using Deep Reactive Ion Etching (DRIE), the micro-mirrors were fabricated on silicon using bulk micromachining techniques. This paper will present the process developed for fabrication of the mirror with the required specifications. In addition, results of the FTS experiments conducted with the micro-mirror will also be presented.
Due to the low Young's Modulus of porous silicon (PS), Si/PS composite membranes - where the silicon membrane is
converted into PS to a certain depth - deform more than silicon membranes and hence MEMS pressure sensors with
composite membranes have higher sensitivity. But the Si/PS composite membranes exhibit a smaller range of linear
response with applied pressure than silicon membranes with the linear range being less for Si/microPS as compared to
Si/macroPS composite membranes. In addition, while the composite membrane deformation saturates at high pressures
like silicon membranes, the deformation is irreversible unlike that seen with silicon membranes within reasonable limits.
With the possibility that the irreversible deformation could be due to stiction force between the collapsed pore walls at
high pressure, we investigate the effect of formation of self-assembled monolayer (SAM) antistiction coating on the
performance of Si/PS composite membranes.
Estimation of triglyceride concentration is important for the health and food industries. Use of solid state biosensors like
Electrolyte Insulator Semiconductor Capacitors (EISCAP) ensures ease in operation with good accuracy and sensitivity
when compared to conventional sensors. In this paper we report on packaging of miniaturized EISCAP sensors on
silicon. The packaging involves glass to silicon bonding using adhesive. Since this kind of packaging is done at room
temperature, it cannot damage the thin dielectric layers on the silicon wafer unlike the high temperature anodic bonding
technique and can be used for sensors with immobilized enzyme without denaturing the enzyme. The packaging also
involves a teflon capping arrangement which helps in easy handling of the bio-analyte solutions. The capping solves two
problems. Firstly, it helps in the immobilization process where it ensures the enzyme immobilization happens only on
one pit and secondly it helps with easy transport of the bio-analyte into the sensor pit for measurements.
The deposited thin films in surface micromachining have a lot of residual stress, and it is essential to measure this for both process development and monitoring. We estimate residual stress by electrical measurements on a series of fixed-fixed polysilicon beams designed to deflect laterally due to stress. To minimize errors in estimation during parameter extraction, the device dimensions also have to be measured accurately. Surface micromachining of an oxide-anchored polysilicon cantilever beam can result in beam undercut, reduction in beam thickness, and increase in the gap between the beam and the substrate. The undercut in the beam is estimated from the resonance frequency of the cantilever beam and also by using polysilicon resistors. Final device thickness is obtained by measuring the resistance in fixed-fixed beams.
Since porous silicon (PS) has a much lower Young's modulus than single crystalline silicon, Si/PS composite membranes deflect more and can be used to fabricate pressure sensors with improved sensitivity. However, PS has some drawbacks, like weaker structural stability and being more susceptible to humidity due to its large surface-to-volume ratio. We discuss the fabrication and testing of Si/PS composite membrane pressure sensors with MicroPS and MacroPS of varying porosity. For the same porosity, the composite membranes with Si/MicroPS show higher sensitivity than Si/MacroPS. The sensor output is linear and repeatable at pressures less than 1 bar. The deformation of composite membranes measured up to 10 bar showed that it saturates at high pressure and is irreversible. Composite membranes also exhibit higher offset voltage than single crystal silicon membranes, which could be attributed to the stress developed in the membrane during PS formation and subsequent processing. The composite membrane pressure sensors were packaged on TO 39 headers, and the effect of humidity and temperature variation were investigated.
Since porous silicon (PS) has a lower Young's Modulus as compared to silicon, Silicon/Porous Silicon (Si/PS)
composite membranes are expected to show higher sensitivity as compared to membranes of silicon alone. In this paper
we discuss the fabrication and testing of Si/PS composite membranes where a part of the silicon membrane depth is
converted into PS. Composite membranes with Si/ microPS and Si/ macroPS were fabricated with varying porosity and
same thickness. The composite membranes with micro PS show higher sensitivity than composite membranes with
macro PS. Formation of microporous and macroporous silicon produces stress on the membrane varying with the
porosity. The variation in compressive stress on the membrane with porosity for both micro and macro PS has been
studied by measuring the deformation of the composite membrane with a surface profiler and the stress is found to be
larger for microPS. The compressive stress results in an increase in the offset voltage by more than an order of
magnitude for composite membranes with porosity above 50% as compared to one with a single crystalline silicon one.
Though the composite membranes exhibit saturation and hysteresis at higher pressures, the response is linear and
repeatable at pressures below 1 bar making this a viable option for sensing low pressures.
In a surface micromachined cantilever the beam thickness, width and the gap get modified during fabrication. Since
performance of sensors and actuators strongly depend on the final geometry, it is essential to measure these parameters.
Electrical measurements have several advantages over microscopy and we present here two different electrical
techniques to measure undercut due to etching. In the resistance based approach, two polysilicon resistors of different
length and width are used and the undercut is extracted from the ratio of the measured resistances. In the second
technique, resonance frequency is measured for two sets of oxide anchored polysilicon cantilever beams with 20 and 30
µm widths. For a given length, the ratio between the measured frequencies for the two widths gives the value of
undercut. Measurements are done on both wet and dry etched polysilicon.
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.
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.
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.
An estimate of stiction force, rather than the more commonly reported surface energy, helps design reliable structures.
Stiction is a major cause of failure in surface micromachined structures. We report on the modeling and estimation of the stiction force from simple I-V curves on cantilever beams which can be measured even on packaged devices. We have fabricated oxide anchored cantilever beams of polysilicon by surface micromachining. Current is measured for an applied bias between the beam and the substrate. Pull-in and pull-out voltages are determined as the points of maximum slope calculated by differentiating a cubic spline fit to the measured I-V data. The commercial package CoventorWare was used to develop an empirical model for estimating the pull-out voltage for the cases when there is no stiction and in the presence of stiction. A model is developed for finding the stiction force from the simulated and the experimental pull-out voltages. The method uses only measured values of pull-in and pull-out voltages and the beam length and does not require the value of Young's modulus. We also discuss an independent visual method to estimate the process stiction force from the cantilever beam array that is normally used to estimate the surface adhesive energy. An analytical model is developed to calculate the stiction force from the attachment length of long stuck cantilever beams that are released in the same process.
In this paper, the design and fabrication of polysilicon piezoresistive pressure sensor are presented. The design considerations such as the membrane thickness and the arrangement pattern of polysilicon piezo-resistors on the membrane are discussed with emphasis on the use of SOI approach. The results obtained on the pressure sensors and the temperature coefficient of resistivity of polysilicon resistors are presented. The results presented include the electrical trimming of polysilicon resistors for compensating zero offset voltage in the pressure sensors.
Polysilicon layers deposited by Low Pressure Chemical Vapour Deposition (LPCVD) on sacrificial oxides are used for surface micromachined structures. Stiction is a major problem in surface micromachining both during processing and in use. Contact angle measurements on surfaces can give indication on the adhesivitiy of the surface. In this paper, contact angle measurements on polysilicon surface after different treatments are reported with a view to understand their stiction behavior. We also report on surface roughness measurements on these samples.
Stiction is a major failure mechanism during the operation of accelerometers and hence it is important to know the stiction force that the structures encounter during use. We explore the possibility of devising an electrical technique for the direct measurement of in use stiction force. We have designed and fabricated three terminal test structures to measure both vertical and horizontal in use stiction. The measurement is not visual and is based on I-V data with the possibility of automation in the future. The structure consists of cantilever beams of different lengths each with an actuating pad and a detection pad. We measure the pull in voltage applied to the actuating pad, VPI , required to bring the cantilever beam in contact with the detection pad and the pull out voltage, VPO, at which the contact is broken. Using the Finite Element tool, ANSYS, a coupled electromechanical model is developed to determine the stiction force from the pull-in and pull-out voltages. We discuss the measurements in terms of the advantages and the shortcomings. We also discuss the sensitivity of the model to various material and geometric parameters and to the accuracy of the measurement.