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

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

A MEMS microphone has a backplate and a movable diaphragm that together form a variable capacitance. The backplate has a backplate surface and, in a corresponding manner, the diaphragm has a diaphragm surface that faces the backplate surface. At least one of the backplate surface and the diaphragm surface has at least a portion with a Hurst exponent that is less than or equal to about 0.5.

MEMS Device with Surface Having a Low Roughness Exponent

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Photoelectrochemical etching of isolated, high aspect ratio micorstructures in n-type silicon (100)

This article describes the preparation of mesoporous silicon granules with a layered structure directly from silicalite-1 single crystals. The silicalite-1 single crystals were thermally reduced in vacuum at 630 °C, with the original shape retained. The samples are confirmed as crystalline silicon by X-ray diffraction and transmission electron microscope. The silicon granule is composed of a monocrystalline surface and polycrystalline layered interior. A surface area of around 66 m2 g−1 and the pore size centered at 3.7 nm were obtained from nitrogen porosimetry, BET and BJH analysis. The ZnO clusters have been loaded into the porous silicon granule by thermal diffusion method. The photoluminescence emission centered at 3.44 eV originates from the small particles of ZnO and the band at 2.81 eV may be due to both an oxidized surface and quantum confinement effects. The microstructure in this silicon granule is very different from those in etched samples. The synthetic design demonstrates an interesting way from the microporous zeolite to mesoporous silicon and enlarges the structural diversity of porous silicon crystal.

Synthesis of mesoporous silicon directly from silicalite-1 single crystals and its response to thermal diffusion of ZnO clusters

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.

Stiction force estimation from attachment length and electrostatic measurements on cantilever beams

Stiction, or adhesion of components to one another, is a major failure mechanism in surface micromachined MEMS. In this paper we discuss roughening of the surface of polysilicon, which forms the structural layer in surface micromachining, by the formation of porous silicon and its effect on stiction. The adhesivity of the surface is investigated by measurements of contact angle and the roughness is measured by a surface profiler. Measurements to estimate stiction on surface micromachined cantilevers and accelerometers are reported.

Effect of porous silicon formation on stiction in surface micromachined MEMS structures

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.

U. Venu Babu

Papers

Effect of porous silicon formation on stiction in surface micromachined MEMS structures

Contact angle measurements on polysilicon for surface micromachining applications