Larry L. Chu

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.

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)

Measurements of material properties using differential capacitive strain sensors

This paper describes a laterally deflecting micromachined device that offers high sensitivity and wide dynamic range to electronically monitor the thermal expansion coefficient, tensile and compressive residual strain and Youngs modulus of microstructural materials, as well as the temperature dependence of these properties. The device uses sidewall capacitance between interdigitated tines to sense displacement caused by the release of residual stress in a bent-beam suspension. Electrostatic force is used to obtain load-deflection profiles. The suspensions and tines are arranged such that output is a differential readout, immune to common mode parasitic capacitance. Analytical and numerical modeling results are presented and the device concept is verified by three different fabrication approaches using polysilicon and nickel as structural materials. Measured values of residual strain, thermal expansion and Youngs modulus are very consistent with measurements taken by other approaches and those reported previously. For example, the residual strain in certain electrodeposited Ni structures was tracked from 68.5 microstrain at 23/spl deg/C to -420 microstrain at 130/spl deg/C, providing an expansion coefficient of 8.2 ppm/K; the best fit Youngs modulus provided by the device was 115 GPa.

Long throw and rotary output electro-thermal actuators based on bent-beam suspensions

This paper reports on several aspects of the performance and application of bent-beam microactuators. Orthogonal pairs of p/sup +/ Si cascaded actuators with 500-1000 /spl mu/m long, 6.5 /spl mu/m thick beams designed for rotary drives are shown to produce 20-30 /spl mu/m non-resonant displacement with >150 /spl mu/N loading at less than 8 V, 300 mW. Inchworm type devices using 500-1500 /spl mu/m long, 8-14 /spl mu/m thick bent-beams are shown to produce up to 104 /spl mu/m displacement against 204 /spl mu/N loading farce with 250-750 mW DC and pulse actuation. Integrated passive locks reduce standby power to zero. Measured frequency response of bent-beam actuators and refinements in modeling that include non-linear thermal expansion coefficients and buckling are also reported.

A micromachined Kelvin probe with integrated actuator for microfluidic and solid-state applications

This paper reports on a micromachined Kelvin probe structure with an integrated scanning tip and an integrated electrothermal actuator that provides axial dithering motion. The device is fabricated from metal foil by a modified microelectrodischarge machining process that allows electrical isolation within the device. In particular, it permits the incorporation of a wide epoxy plug that creates an insulating gap with low parasitic capacitance between the probe and the actuator. The epoxy structures are found to withstand the thermal and mechanical conditions encountered during device operation. The device is used to measure changes in the external surface potential of a parylene microfluidic channel as a function of varying pH of liquid inside the channel. A contact potential difference of /spl ap/6 V is measured for a change in pH from 4 to 8 within the channel. The device is also used to map embedded charge in a thin SiO/sub 2/ layer on a Si substrate, showing it to be suitable for monitoring microelectronics manufacturing processes.

Lifetime studies of electrothermal bent-beam actuators in single-crystal silicon and polysilicon

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners, thus creating a need for information on the longevity of these devices. This paper reports on the dc and pulse mode lifetime testing of this class of actuators constructed using polysilicon and p/sup ++/ doped single crystal silicon. The relative temperature profile along the top surface of an actuator is experimentally verified by scanning probe microscopy. Displacement measurements are used to explore links between aging behavior and the design variables and operating conditions. At low power levels (which result in average operating temperatures of 300-400/spl deg/C) both polysilicon and p/sup ++/ Si devices provide continuous dc operation for >1400 min, in air without change in amplitude. While some types of p/sup ++/ Si devices show monotonic loss of amplitude in pulse tests, others have been operated up to 30 million cycles without degradation. The displacement for polysilicon actuators can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.

A micromachined strain sensor with differential capacitive readout

This paper describes a laterally deflecting micromachined device that can be used to electronically monitor residual strain and Youngs modulus of microstructural materials. Residual strain is indicated by a change in the differential capacitance change, whereas the Youngs modulus is provided by the slope of a simple CV (capacitance vs. voltage) test. The device is suitable for automated wafer level as well as post-packaging readout. Typical strain sensitivities are 0.1-1 fF/MPa, and the CV slope essentially doubles (e.g. from 0.23 fF/V to 0.47 fF/V at 70 V bias) as the Youngs modulus changes from 220 to 130 GPa. Nickel plated and polysilicon strain sensors were fabricated by surface micromachining techniques and coated with self-assembled monolayers using an ODS-based process. Both stress and Youngs modulus measurements from these structures were found to closely match theoretical models.

Performance enhancement of polysilicon electrothermal microactuators by localized self-annealing

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners for scanning microscopy, creating an important need for information on the longevity of these devices. This paper reports on the lifetime pulse testing results of polysilicon actuators. Devices have been operated up to 60 million cycles without failure, but over tens of millions of cycles the displacement for a given actuator design can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. In certain cases actuator displacement increased by more than 50% (up to 100%) of the initial displacement, while for other cases it decreases by more than 25%. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.

A micromachined Kelvin probe for surface potential measurements in microfluidic channels and solid-state applications

This paper reports on a micromachined Kelvin probe structure with integrated scanning tip and dither actuation mechanism. It is fabricated by a modified micro electro-discharge machining process which allows electrical isolation within the micromachined structure using epoxy plugs. The device is used to measure changes in the external surface potential of a parylene microfluidic channel as a function of varying pH of liquid inside the channel. A contact potential difference of /spl ap/6 V is measured for a change in pH from 4 to 8 within the channel. The device is also used to map embedded charge in a thin SiO/sub 2/ layer on a Si substrate, showing it to be suitable for monitoring microelectronics manufacturing processes.

Temperature coefficients of material properties for electrodeposited MEMS

This paper presents the use of micromachined differential capacitive strain sensors to investigate mechanical properties of electroplated Ni deposited under two different conditions on Si and glass substrates. The thermal expansion coefficient (/spl alpha/), Youngs modulus, and residual strain were studied as a function of temperature. The measured a was 8-16 ppm/K over 23-150/spl deg/C; the residual strain changed from neutral to -880 microstrain over 23-100/spl deg/C in one case and +68.5 microstrain to -420 microstrain over 23-130/spl deg/C in another case; and the Youngs modulus ranged from 115-135 GPa at room temperature. The sensitivity of the device to structural non-idealities was evaluated by numerical modeling.