Yogesh B. Gianchandani

Bent-beam electrothermal actuators-Part I: Single beam and cascaded devices

This paper describes electrothermal microactuators that generate rectilinear displacements and forces by leveraging deformations caused by localized thermal stresses. In one manifestation, an electric current is passed through a V-shaped beam anchored at both ends, and thermal expansion caused by joule heating pushes the apex outward. Analytical and finite element models of device performance are presented along with measured results of devices fabricated using electroplated Ni and p/sup ++/ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is described. Nickel devices with 410-/spl mu/m-long, 6-/spl mu/m-wide, and 3-/spl mu/m-thick beams demonstrate 10 /spl mu/m static displacements at 79 mW input power; silicon devices with 800-/spl mu/m-long, 13.9-/spl mu/m-wide, and 3.7-/spl mu/m-thick beams demonstrate 5 /spl mu/m displacement at 180 mW input power. Cascaded silicon devices using three beams of similar dimensions offer comparable displacement with 50-60% savings in power consumption. The peak output forces generated are estimated to be in the range from 1 to 10 mN for the single beam devices and from 0.1 to 1 mN for the cascaded devices. Measured bandwidths are /spl ap/700 Hz for both. The typical drive voltages used are /spl les/12 V, permitting the use of standard electronic interfaces that are generally inadequate for electrostatic actuators.

Bent-beam strain sensors

We examine a new class of sensitive and compact passive strain sensors that utilize a pair of narrow bent beams with an apex at their mid-points. The narrow beams amplify and transform deformations caused by residual stress into opposing displacements of the apices, where vernier scales are positioned to quantify the deformation. An analytical method to correlate vernier readings to residual stress is outlined, and its results are corroborated by finite-element modeling. It is shown that tensile and compressive residual stress levels below 10 MPa, corresponding to strains below 6/spl times/10/sup -5/ can be measured in a 1.5-/spl mu/m-thick layer of polysilicon using a pair of beams that are 2 /spl mu/m wide, 200 /spl mu/m long, and bent 0.05 radians (2.86/spl deg/) to the long axis of the device. Experimental data is presented from bent-beam strain sensors that were fabricated from boron-doped single crystal silicon using the dissolved wafer process and from polycrystalline silicon using surface micromachining. Measurements from these devices agree well with those obtained by other methods.

Batch mode micro-electro-discharge machining

This paper describes a micro-electro-discharge machining (micro-EDM) technique that uses electrode arrays to achieve high parallelism and throughput in the machining. It explores constraints in the fabrication and usage of high aspect ratio LIGA-fabricated electrode arrays, as well as the limits imposed by the pulse discharge circuits on machining rates. An array of 400 Cu electrodes with 20 /spl mu/m diameter was used to machine perforations in 50-/spl mu/m-thick stainless steel. To increase the spatial and temporal multiplicity of discharge pulses, arrays of electrodes with lithographically fabricated interconnect and block-wise independent pulse control resistance-capacitance (RC) circuits are used, resulting in >100/spl times/ improvement in throughput compared to single electrodes. However, it was found to compromise surface smoothness. A modified pulse generation scheme that exploits the parasitic capacitance of the interconnect offers similarly high machining rates and is more amenable to integration. Stainless steel workpieces of 100 /spl mu/m thickness were machined by 100 /spl mu/m/spl times/100 /spl mu/m square cross-section electrodes using in 85 s using an 80-V power supply. Surface smoothness was unaffected by electrode multiplicity. Using electrode arrays with four circuits, batch production of 36 WC-Co gears with 300 /spl mu/m outside diameter and 70 /spl mu/m thickness in 15 min is demonstrated.

A bulk silicon dissolved wafer process for microelectromechanical devices

A single-sided bulk silicon dissolved wafer process that has been used to fabricate several different micromechanical structures is described. It involves the simultaneous processing of a glass wafer and a silicon wafer, which are eventually bonded together electrostatically. The silicon wafer is then dissolved to leave heavily boron doped devices attached to the glass substrate. Overhanging features can be fabricated without additional masking steps. It is also possible to fabricate elements with thickness-to-width aspect ratios in excess of 10:1. Measurements of various kinds of laterally driven comb structures processed in this manner, some of which are intended for application in a scanning thermal profilometer, are described. They comprise shuttle masses supported by beams that are 160-360 mu m long, 1-3 mu m wide, and 3-10 mu m thick. Some of the shuttles are mounted with probes that overhang the edge of the die by 250 mu m. Resonant frequencies from 18 to 100 kHz and peak-to-peak displacements up to 18 mu m have been measured. >

Bent-beam electrothermal actuators-Part II: Linear and rotary microengines

For Part I see L. Que, J.S. Park and Y.B. Gianchandani, ibid., vol.10, pp.247-54 (2001). This paper reports on the use of bent-beam electrothermal actuators for the purpose of generating rotary and long-throw rectilinear displacements. The rotary displacements are achieved by orthogonally arranged pairs of cascaded actuators that are used to rotate a gear. Devices were fabricated using electroplated Ni, p/sup ++/ Si, and polysilicon as structural materials. Displacements of 20-30 /spl mu/m with loading forces >150 /spl mu/N at actuation voltages 200 /spl mu/N at displacements >100 /spl mu/m were measured.

Bent-beam electro-thermal actuators for high force applications

This paper describes in-plane microactuators fabricated by standard microsensor materials and processes that can generate forces up to about a milli-newton. They operate by leveraging the deformations produced by localized thermal stresses. Analytical and finite element models of device performance are presented along with measured results of fabricated devices using electroplated Ni, LPCVD polysilicon, and p/sup ++/ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is outlined. Test results show that static displacements of /spl ap/10 /spl mu/m can be achieved with power dissipation of /spl ap/100 mW, and output forces >300 /spl mu/N can be achieved with input power <250 mW. It is also shown that cascaded devices offer a 4/spl times/ improvement in displacement. The displacements are rectilinear, and the output forces generated are 10/spl times/-100/spl times/ higher than those available from other comparable options. This performance is achieved at much lower drive voltages than necessary for electrostatic actuation, indicating that bent-beam thermal actuators are suitable for integration in a variety of microsystems.

A micromachined 2D positioner with electrothermal actuation and sub-nanometer capacitive sensing

This paper reports on a multi-purpose two-axis micropositioner with sub-nanometer position sensing for precise feedback control. Along each axis it has an electrothermal actuator, a capacitive position sensor and a displacement amplifier that provides a gain of 3.37 for the sensor. It is fabricated from custom SOI wafers using dry etching, and each component is electrically and thermally isolated by silicon nitride. For a fabricated device of 65 µ mt hickness, the measured displacement sensitivity is 0.333 fF nm −1 ,w hich corresponds to 0.3 nm resolution with available laboratory instrumentation. The range is ≈19 µ ma long each axis for the positioner, which corresponds to 66 µ mt ravel in the sense combs. Using an external parallel inductor, a positioning displacement of 9.6 µ mo ffers as hift of 240 kHz in L–C resonance, corresponding to a sensitivity of 25 Hz nm −1 . (Some figures in this article are in colour only in the electronic version)

On-chip vacuum generated by a micromachined Knudsen pump

This paper describes the design, fabrication, and testing of a single-chip micromachined implementation of a Knudsen pump, which uses the principle of thermal transpiration, and has no moving parts. A six-mask microfabrication process was used to fabricate the pump using a glass substrate and silicon wafer. The Knudsen pump and two integrated pressure sensors occupy an area of 1.5 mm /spl times/ 2 mm. Measurements show that while operating in standard laboratory conditions this device can evacuate a cavity to 0.46 atm using 80 mW input power. The pumpdown time of an on-chip chamber and pressure sensor cavity with a total volume of 80 000 cubic micrometers is only 2s, with a peak pump speed of 1/spl times/10/sup -6/ cc/min. High thermal isolation is obtained between the polysilicon heater and the rest of the device.

Spectral detection of metal contaminants in water using an on-chip microglow discharge

This paper reports on the detection of trace contaminants in water by spectroscopy of micro glow discharges that operate in air or at moderate vacuum using liquid electrodes. A liquid electrode spectral emission chip (LEd-SpEC) has been developed to perform this function. The device is fabricated by a four mask process, and provides a reservoir and channels in a glass substrate, along with electrodes that bias the water sample. Liquid from the cathode is sputtered into the discharge, for spectroscopic detection of impurities. Using a commercial spectrometer, Na concentrations <10 ppm, and Pb concentrations of 5 ppm, and Al and Cr concentrations of 10 ppm have been measured. The ratio of Na spectral intensity to that of ambient N/sub 2/ is shown to be a suitable measure of Na impurity concentration over several orders of magnitude. The addition of HNO/sub 3/ to lower the pH of the liquid solution increases this ratio by almost an order of magnitude. By selectively doping the solution, the device can also be used as a customizable optical source for UV and visible wavelengths.

Micromachined Antenna Stents and Cuffs for Monitoring Intraluminal Pressure and Flow

This paper describes two stainless steel microstructures that are microelectrodischarge machined from 50-mum-thick planar foil for intraluminal measurements of pressure and flow (with potential for applications ranging from blood vessels to bile ducts). The first structure is an inductive antenna stent (stentenna) with 20-mm length and 3.5-mm expanded diameter. It is coupled with capacitive elements to form resonant LC tanks that can be telemetrically queried. The resulting LC tanks are deployed inside silicone mock arteries using standard angioplasty balloons and used in a passive telemetry scheme to sense changes in pressure and flow. Using water as the test fluid, the resonant peaks shift from about 215 to 208 MHz as the flow is increased from 0 to 370 mL/min. The second structure is a ring-shaped intraluminal cuff with two 400times750-mum2 electrodes that are used to provide a direct transduction of flow velocity in the presence of a magnetic field. It is fabricated in a manner similar to the stentenna, but with an insulating segment. The voltage has a linear dependence on flow rate, changing by 3.1-4.3 muV per cm/s of flow (of saline) over a 180 cm/s dynamic range, with a magnetic field of about 0.25 T