Mehran Mehregany

Microfabricated structures for the in situ measurement of residual stress, Young’s modulus, and ultimate strain of thin films

Two microfabricated structures for the in situ measurement of mechanical properties of thin films, a suspended membrane, and an asymmetric ‘‘released structure,’’ are reported. For a polyimide film on silicon dioxide, the membrane measurements yield a residual tensile stress of 30 MPa and a Young’s modulus of 3 GPa. The released structures measure the ratio of residual stress to Young’s modulus, and yield 0.011 at strains comparable to the suspended membranes, and 0.015 at larger strains. The ultimate strain as measured by both structures is approximately 4%.

Microfabricated electrohydrodynamic pumps

Abstract Pumping is often cited as a general application which motivates the development of microfabricated motors and other actuators. In that spirit, this paper studies microfabricated electrohydrodynamic fluid pumps. In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields and charges in the fluid. In contrast to forces generated by mechanical pumping using an impeller or bellows, EHD pumping requires no moving parts and consequently offers the possibility of simplified fabrication and high reliability. This paper discusses electrohydrodynamic pumping and issues concerning its use in micronsize scale systems. The fundamental operating principles of the EHD pump are outlined and examples of configurations which meet the requirement for inducing free electric charge are shown. The possible performance achievable in micron size-scale regimes is indicated. Issues concerning fluid conductivity, instability and surface tension are addressed. A microfabricated structure which demonstrates the EHD pumping of a highly insulating silicone oil is described. The structure consists of an array of 10 μm by 235 μm highly doped, LPCVD polycrystalline silicon electrodes patterned over silicon nitride. The electrode array is excited with a traveling wave of electric potential. Pumping results are qualitatively described. This paper describes a study of electrohydrodynamic pumping, including issues concerning its application to micron size-scale systems. The fundamental operating principles of EHD pumps are outlined and the possible performance achievable in micron size-scale regimes is indicated. Surface and bulk instabilities are addressed. Finally, the preliminary results of an EHD pumping experiment with a microfabricated structure are described. A practical requirement for EHD pumping is the induction of free electric charge in the volume of the fluid to be pumped or on its interface with another material. Charge accumulation on a material interface is readily achievable; however, if one material is a fixed rigid wall, such as the wall of a conduit, no pumping can take place. Consequently, the practical application of microfabricated EHD pumps may require the induction of free charge in the volume of the fluid, possibly by temperature-induced conductivity gradients. A further constraint on the usefulness of EHD pumping is its reduced effectiveness with conducting fluids. Hence its usefulness in many situations, including biological environments, may be limited. Another possible difficulty may be fluid instabilities; however, these instabilities may be useful for mixing and cooling purposes. Yet, it remains to be seen if they can compete with molecular diffusion in micron-size scale systems. Finally, it seems clear that surface tension will be a dominant force in virtually any micron-scale system with a liquid surface. In spite of these difficulties, electrohydrodynamic interactions may prove to be a reasonable way to achieve pumping without moving parts.

Thin-film processing of TiNi shape memory alloy

Abstract The deposition and processing of thin-film Nitinol, a shape memory alloy (SMA), are discussed and demonstrated. By using sputter-deposition techniques, 0.5 to 8.0-μm-thick films of Nitinol (an alloy of nickel and titanium) were deposited, patterned and etched. Although the processing of the individual constituents of the alloy are well understood, the processing of the shape memory alloy is not a simple extension or combination of the constituent metals processing. Requisite deposition conditions for the film to retain the shape memory effect of the bulk material and avoid stress-induced cracking are discussed. An appropriate wet-etch for patterning the thin-film Nitinol is presented. Results from surface micromachined test structures are presented, which verify the shape recovery property of the thin-film Nitinol using resistive heating as the source of heat energy for the shape recovery. Finally, we discuss an isotropic, dry-etch technique developed for the release of the thin-film test structures that also has potential use in other thin-film applications.

In situ friction and wear measurements in integrated polysilicon mechanisms

We describe a series of experiments, in situ measurements and theoretical models designed to provide estimates of the coefficients of friction and the nature of wear in integrated polysilicon micromechanisms. A laser-based measurement system was used to monitor the motion of the rotating micromechanisms and steady-state speeds of up to 10 000 rps (600 000 rpm) were recorded. Estimates of the dynamic coefficient of friction were obtained by incorporating sampled motion profiles and derived speed and distance measures into a dynamic model which included both coulomb friction and viscous damping forces. From such an analysis, we estimated dynamic coefficients of friction for polysilicon on silicon ranging in value from 0.25 to 0.35. The results were reproducible on numerous components, across structures of identical and different geometries, produced on different wafers from the same fabrication lot. Life-cycles for some of these structures were determined under various operating conditions, with typical components surviving for almost one million cycles.

A study of three microfabricated variable-capacitance motors

Abstract This paper discusses the design, microfabrication, operating principles and experimental testing of three types of rotary variable-capacitance micromotors. The advantages and disadvantages of these motors are discussed. The three motor types are top-drive, side-drive and harmonic side-drive. In this work, the micromotors are surface micromachined using heavily-phosphorus-doped polysilicon for the structural material, deposited oxide for the sacrificial layers and LPCVD nitride for electrical isolation. Frictional forces associated with electric pull-down forces on the rotor are dominant in the side-drive and harmonic side-drive motors fabricated and tested to date. Air drive and electric excitation have been used in studying these effects. Side-drive micromotors have been successfully operated by a three-phase electrical signal with the rotors air-levitated. With air levitation, successful operation is achieved at bipolar excitations greater than 80 V across 4 μm air-gap motors having eight rotor and twelve stator poles, with only half of the stator poles excited. Motor operation is sustained indefinitely.

Electric micromotor dynamics

The dynamometry technique uses a strobe flash which is triggered from a phase excitation signal after a known time delay. This acts essentially as a video shutter allowing the position of the rotor as a function of the time delay to be recorded and measured. A dynamic model is developed that includes an electrostatic drive term, a velocity-dependent viscous drag term, and a Coulomb friction term that is dependent on the square of the drive voltage and the sign of the velocity. From the position-versus-time data, coefficients for this model are estimated using nonlinear least square error estimation. It is shown that both viscous drag and Coulomb friction terms are required if the model is to closely fit all the experimental data. The motor dynamics are shown to have a weak, if any, dependence on the rotor-bushing apparent area of contact. >

Novel microstructures for the in situ measurement of mechanical properties of thin films

This paper discusses microfabricated structures designed for the in situ measurement of the mechanical properties of thin films under residual tensile stress. The film is deposited and patterned on a (100) silicon substrate in which 5‐μm‐thick diaphragms have been fabricated. When the silicon diaphragm is etched from the backside in an SF6 plasma, the microstructures are released and deform under the residual tension. Measurement of this deformation in conjunction with appropriate mechanical models determines the mechanical properties of interest. We have used these structures to study benzophenonetetracarboxylicdianhidride‐oxydianiline/metaphenylene‐diamine polyimide films. Typical value for the residual stress to modulus ratio in this case was determined to be 0.011±0.001 while the ultimate strain at break was found to be 4.5% for 5.5‐μm‐thick films. For thicker films (8.5 μm), the film did not fail until 8% strain was reached.

Principles in design and microfabrication of variable‐capacitance side‐drive motors

This paper presents a detailed discussion of the critical issues it the design and fabrication of polysilicon, rotary, variable‐capacitance, side‐drive, electric micromotors. Three different side‐drive motor architectures with stator pole number to rotor pole number ratios of 3:1; 3:2, and 2:1 are considered. For each architecture, output torque characteristics of typical microfabricated motors are simulated using two‐dimensional finite‐element solutions in the plane of the substrate. The 3:2 design is shown to provide superior torque coverage with higher minimum torque values as compared to the other two designs. An examination of the contribution of the axial fringing fields shows that, for typical micromotors, the rotor–stator capacitance is more directly a function of the rotor–stator thickness and not of the vertical rotor–stator pole‐face overlap. Furthermore, since the rotor–stator capacitance is not very sensitive to a vertical offset between the rotor and the stator, electric forces tending to vertically align the rotor to the stator are significantly smaller than would be predicted from a simple parallel‐plate capacitance calculation. A standard and a localized oxidation of silicon (LOCOS)‐based side‐drive micromotor fabrication process are described. The standard process is used as a case study to provide a detailed discussion of practical issues that need to be considered in the development of a polysilicon surface‐micromachined motor fabrication process. Specific motor design examples are described and a brief history of our experimental findings is presented. Typical 3:2 micromotors have been operated with bipolar excitations as low as 37 V across 1.5 μm gaps and at speeds as high as 15 000 rpm.

Friction and wear in microfabricated harmonic side-drive motors

Polysilicon variable-capacitance rotary harmonic side-drive micromotors and results from operational and frictional studies of these motors are described. Quantitative studies of wear under electric excitation are also reported. Voltages as low as 26 V across 1.5- mu m gaps are sufficient for operation. Frictional force estimates of 0.15 mu N at the bushings and 0.04 mu N in the bearings are obtained from measurements of stopping voltages. Extended operation of these motors to near 100 million wobble cycles at excitation frequencies of 10000 r.p.m. and 25000 r.p.m. for operational durations of 150 h and 71 h, respectively, are studied. The results indicate that bearing wear is significant and results in changes in the gear ratio of the motors by as much as 20%. Typical gear ratios are near 90 at the start of motor operation and decrease to about 70 as the bearings wear out.<<ETX>>

Anisotropic etching of silicon in hydrazine

Abstract This paper contains a detailed discussion of the practical issues related to the anisotropic etching of single crystal silicon using a 5050 hydrazinewater solution. Characteristics of the etchant, etching reactor design, etch procedures, safety precautions, etch rate data for typical samples and appropriate etch-masks are among the topic discussed. The etching process is carried out in a atmospheric reflux reactor, continuously purged with nitrogen. The etch rate of (100) silicon at 115°C in this hydrazine solution is nearly 3 μm/min, which is much higher than that of ethylenediaminepyrocatecholwater (EDP) solutions. Silicon dioxide, silicon nitride and most metallic thin films, except aluminium, can be used to mask the etching process. The etch rate is reduced significantly in highly-boron-doped silicon; a boron concentration of 1.5 × 10 20 cm −3 practically stops the etch. The use of the hydrazine solution for micromachining thin silicon diaphragms, cantilevers and fibers is demonstrated.