Michael A. Huff

Thin-film shape-memory alloy actuated micropumps

Micropumps capable of precise handling of low-fluid volumes have the potential to revolutionize applications in fields such as drug delivery, fuel injection, and micrototal chemical analysis systems (/spl mu/TAS). Traditional microactuators used in micropumps suffer from low strokes and, as a result, are unsuitable for achieving large fluid displacement. They also suffer low-actuation work densities, which translate to low forces. We investigate the use of the shape-memory effect (SMA) in sputter-deposited thin-film shape-memory alloy (SMA) titanium nickel (TiNi) as an actuator for microelectromechanical systems (MEMS)-based microfluidic devices, as it is capable of both high force and high strains. The resistivity of the SMA thin film is suitable for Joule heating, which allows direct electrical control of the actuator. Two micropump designs were fabricated-one with a novel complementary actuator and the other with a polyimide-biased actuator-which provided thermal isolation between the heated microactuator and the fluid being pumped. A maximum water flow rate of 50 /spl mu/l/min was achieved.

The TiNi shape-memory alloy and its applications for MEMS

The shape-memory effect is a solid state phenomenon which exploits a reversible phase transformation to repeatedly achieve an initial shape, even after some deformation of the material. Numerous metal alloys exhibit this effect. One of the most widely used shape-memory alloys is TiNi, due to its large range of recoverable deformations and its relative ease of processing. In bulk and wire form, TiNi has been applied to a number of applications, and as a thin film, TiNi is an excellent material for use as a microactuator in microelectromechanical systems (MEMS), due to its large recovery forces and high recoverable strains. Several TiNi-actuated MEMS devices have already been reported.

A novel integrable microvalve for refreshable Braille display system

We introduce a novel integrable and electrostatic microvalve for the purpose of enabling a pneumatic refreshable Braille display system (RBDS). Physical design parameters of the microvalve such as orifice size, beam length, number of beams and beam profile are experimentally explored and found promising for use with the RBDS. Particularly, one design with an orifice of 70 /spl mu/m/spl times/70 /spl mu/m, beam length of 665 /spl mu/m, and beam count of 20 is electrostatically closed against a differential pressure of 82.7 kPa with an applied voltage of 68 V-rms. Also introduced is a steady-state mechanical model of the microvalve established on a coupled solution of fluid and solid domains. The model and experimental test results have been used to calculate the unknown discharge coefficient, elastic deflection, and entrance pressure. The model reveals that some of the designs have remarkably low discharge coefficient and entrance pressure, implying that pressure loss occurs mostly through and around the inlet port even at fairly large supply pressures. Experimental observations concerning the practical use of the microvalve are discussed.

A novel bulk micromachined electrostatic microvalve with a curved-compliant structure applicable for a pneumatic tactile display

Recent success of microelectromechanical systems (MEMS) in projection displays have raised similar expectation for an efficient, low power, affordable, full-page and pneumatic tactile display. Such design has not been achieved by the conventional technology but could bring significant improvement to current refreshable Braille displays. This paper demonstrates a novel bulk-micromachined electrostatic microvalve suitable for a pneumatic tactile display. The microvalve, a silicon perforated diaphragm juxtaposed to a silicon inlet orifice, requires relatively low closing voltage against a large supply differential pressure and flow rate, i.e., 72.9 V-rms for 19.3 kPa and 85 mi/min. Such an attractive characteristic is due to its unique curved-compliant structure that has, unlike other electrostatic microvalves, no tolerance for any initial air gap between its electrodes. As a design tool, a mechanical model of the microvalve is introduced based on the lubrication theory and large plate deflection theory. The model is established on a steady-state coupled field problem of fluid-solid mechanics. Reynolds and von-Karman equations were simultaneously solved for the microvalve geometry by finite difference approximation and double Fourier series expansion. The results of the model and experiments are compared and found to be in good agreement with a relative error less than 10%.

A titanium-nickel shape-memory alloy actuated micropump

The shape memory alloy (SMA) titanium-nickel (TiNi) is used in thin film form as the basis of actuation for a reciprocating micropump. A novel configuration comprising two complementary TiNi actuators is used to perform cyclic motion by utilizing the shape memory effect (SME). Check valves are fabricated from polyimide to ensure unidirectional flow through the pump chamber. The pump is driven by an electrical drive signal, which is passed directly through the TiNi thin films, causing a Joule heating-induced phase transformation and initiating the SME. Cyclic motion is achieved by heating the actuators out of phase. The pump was tested with filtered DI water and attained a pump rate of 50 /spl mu/L/min.

Mechanical properties of thick, surface micromachined polysilicon films

Polycrystalline silicon is the most widely used structural material for surface micromachined microelectromechanical systems (MEMS). There are many advantages to using thick polysilicon films; however, due to process equipment limitations, these devices are typically fabricated from polysilicon films less than 3 /spl mu/m thick. In this work, microelectromechanical test structures were designed and processed from thick (up to 10 /spl mu/m) undoped and in situ boron-doped polysilicon films. The elastic moduli of the doped films were 150/spl plusmn/30 GPa, and appeared to be independent of film thickness. The thermal oxidation of the polysilicon induced a compressive stress into the top surface of the films, which was detected as a residual stress in the polysilicon after the device fabrication was complete. The average nominal fracture toughness of the polysilicon was 2.3/spl plusmn/0.1 MPa /spl radic/m.

Applications of TiNi thin film shape memory alloys in micro-opto-electro-mechanical systems

Abstract The application of shape memory alloy (SMA) thin films in optical devices is introduced and explored for the first time. Physical and optical properties of titanium–nickel (TiNi) SMA thin films change as these films undergo phase transformation upon heating. An optical beam can be modulated either mechanically using a TiNi actuator or by the changes that occur in TiNis optical properties upon heating and phase transformation. Reflection coefficient of TiNi films were measured in their martensitic (at room temperature) and austenitic (elevated temperature) phases. The reflection coefficient of the austenitic phase were higher than those of the martensitic phase by more than 45% in the wavelength range between 550 and 850 nm. A microfabricated TiNi diaphragm with a 0.26-mm-diameter hole was used as a prototype light-valve. The intensity of the transmitted light through the hole was reduced by 10–17% when the diaphragm was heated.

Applications of shape memory alloys in optics.

The application of shape memory alloy (SMA) thin films in optical devices is introduced and explored for the first time. Physical and optical properties of titanium-nickel (TiNi) SMA thin films change as these films undergo phase transformation on heating. An optical beam can be modulated either mechanically with a TiNi actuator or by the changes that occur in TiNis optical properties upon heating and phase transformation. Reflection coefficients of TiNi films were measured in their so-called martensitic (room-temperature) and austenitic (elevated-temperature) phases. The reflection coefficients of the austenitic phase were higher than those of the martensitic phase by more than 45% in the wavelength range between 550 and 850 nm. Also, a microfabricated TiNi diaphragm with a 0.26-mm-diameter hole was used as a prototype light valve. The intensity of the transmitted light through the hole was reduced by 10%-17% when the diaphragm was heated. A novel TiNi light valve fabricated by using silicon micromachining techniques is also proposed and discussed. We present both optical data and structural data obtained by using transmission electron microscopies.

TiNi shape-memory alloy actuated micropump with fluid isolation

The shape memory alloy (SMA) titanium-nickel (TiNi) is used in thin film form as the basis of actuation for a reciprocating micropump. A novel configuration comprising a TiNi actuator and a polyimide diaphragm is used to perform cyclic motion while utilizing the one way shape memory effect (OWSME). The polyimide diaphragm acts as a spring to provide the bias force required for cyclic motion using the OWSME of TiNi. It also isolates the TiNi from the liquid pumped. An electrical drive signal is passed directly through the TiNi thin film. The current results in joule heating and hence a phase transformation in the SMA causing the shape memory effect. The cyclic motion of this actuator configuration cycles the volume of the pump chamber. Doubly fixed polyimide check valves ensure unidirectional flow through the pump chamber, resulting in pumping. The pump was tested with filtered DI water.

Modeling a geographically distributed MEMS fabrication network

Manufacturing is typically limited to fabrication of parts at a single location, with some sites assembling components from parts made elsewhere. The age of ubiquitous information transfer has made it conceivable to distribute manufacturing geographically, in order to provide access to unique manufacturing capabilities in a flexible manner. If the overhead of a distributed manufacturing network can be adequately reduced, it has the potential to make previously cost ineffective low volume and custom applications economically feasible. The MEMS-Exchange is an infrastructural service available to the domestic microelectromechanical systems community that provides an interface between MEMS designers and microfabrication facilities (academic, commercial, and government labs) which allows designers to develop and exercise custom process sequences in order to realize their devices.