Preparation and characterization of unimorph actuators based on piezoelectric Pb(Zr0.52Ti0.48)O3 materials

This paper describes the preparation and characterization of unimorph actuators for deformable mirrors, based on Pb(Zr_0.52Ti_0.48)O₃ (PZT52) thin film. As comparison, two different designs, where the PZT layer in the unimorph actuators was driven by either interdigitated electrodes (IDT-mode) or parallel plate electrodes (d₃₁-mode), were investigated. The actuators utilize a unimorph membrane (diaphragm) structure consisting of an active PZT piezoelectric layer and a passive SiO₂/Si composite layer. To fabricate the diaphragm structures, n-type (1 0 0) silicon-on-insulator (SOI) wafers with 1 μm thermal SiO₂ were used as substrates (for d₃₁-mode actuators, the upper Si part of SOI need to be heavily doped and used as bottom electrodes simultaneously). Sol-gel derived PZT piezoelectric layers with PbTiO₃ (PT) bufferlayer in total of 0.86 μm were then fabricated on them, and 0.15 μm Al reflective layers were deposited and patterned into top electrode geometries, subsequently. The diaphragms were released using orientation-dependent wet etching (ODE) with 5-10 μm residual silicon layers. The complete unimorph actuators comprise 4 × 4 discrete units (4 mm² in size) with patterned PZT films for parallel plate configuration or 3 × 3 individual pixels (2 mm in IDT diameter) with continuous PZT films in graphic region for IDT configuration. The measurement results indicated that both of the two configurations can generate considerable deflections at low voltage. The measured maximum central deflections at 15 V were approximately 2.5 μm and 2.8 μm, respectively. The intrinsic strain conditions shaping the deflection profiles for the diaphragm actuators were also analyzed. In this paper, the behaviors of clamped parallel plate configuration without a diaphragm were also evaluated.     

Vertical comb array microactuators

A vertical actuator fabricated using a trench-refilled-with-polysilicon (TRiPs) process technology and employing an array of vertical oriented comb electrodes is presented. This actuator structure provides a linear drive to deflection characteristic and a large throw capability which are key features in many sensors, actuators and micromechanisms. The actuation principle and relevant theory is developed, including FastCap simulations for theoretical verification. Design simplifications have been suggested that enable one to use parallel plate analytical expressions which match simulation results with /spl sim/5.6% error. Several actuators were designed and fabricated using the 7-mask TRiPs technology with calculated drive voltages as low as 45 V producing 10 /spl mu/m of deflection. The actuators employed a mechanical structure that was 18 /spl mu/m tall using a polysilicon layer 1.5 /spl mu/m thick and occupying a total area of 750 /spl mu/m by 750 /spl mu/m. The actuators were successfully tested electrostatically and several microns of deflection were observed.     

Piezoelectric unimorph microactuator arrays for single-crystal silicon continuous-membrane deformable mirror

Micromachined deformable mirror technology can boost the imaging performance of an otherwise nonrigid, lower-quality telescope structure. This paper describes the optimization of lead zirconium titanate (PZT) unimorph membrane microactuators for deformable mirrors. PZT unimorph actuators consisting of a variety of electrode designs, silicon-membrane thickness, and membrane sizes were fabricated and characterized. A mathematical model was developed to accurately simulate the membrane microactuator performance and to aid in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. Using the above approach, we have successfully demonstrated a 2.5-mm-diameter PZT unimorph actuator. A measured deflection of 5 /spl mu/m was obtained for 50 V applied voltage. Complete deformable mirror structures consisting of 10-/spl mu/m-thick single-crystal silicon mirror membranes mounted over the aforementioned 4/spl times/4 4 PZT unimorph membrane microactuator arrays were designed, fabricated, assembled, and optically characterized. The fully assembled deformable mirror showed an individual pixel stroke of 2.5 /spl mu/m at 50 V actuation voltage. The deformable mirror has a resonance frequency of 42 kHz and an influence function of approximately 25%.     

Self-aligned vertical electrostatic combdrives for micromirror actuation

In this paper, we analyze the effect of misalignment in electrostatic combdrives, and describe a fabrication technology that minimizes misalignment in vertical electrostatic combdrives by creating self-aligned, vertically staggered electrodes. Self-alignment of the interdigitated electrodes simplifies fabrication and minimizes failures due to electrostatic instability, thus enabling fabrication of narrow-gap, high-force actuators with high yield. The process is based on deep-reactive ion etching (DRIE) of buried-patterned silicon-on-insulator (SOI) wafers. Measurements on fabricated combdrives show relative misalignment of less than 0.05 /spl mu/m. This corresponds to less than 0.1% misalignment, which, according to our analysis, results in a travel range of 98% of that for perfectly aligned drives. The validity of the process is demonstrated by fabrication of scanning micromirrors measuring 300 /spl mu/m by 100 /spl mu/m. Optical angular deflections from 4/spl deg/ at low frequency to 40/spl deg/ at resonance were measured for an applied voltage of 75 Vpp. Resonant frequencies ranged from 5 kHz to 15 kHz for these devices, making them suitable for high-speed, high-resolution optical scanning and switching.     

A micromachining process for die-scale pattern transfer in ceramics and its application to bulk piezoelectric actuators

This paper reports on a batch mode planar pattern transfer process for bulk ceramics, glass, and other hard, brittle, nonconductive materials suitable for micromachined transducers and packages. The process is named LEEDUS, as it combines lithography, electroplating, batch mode micro electro-discharge machining (/spl mu/EDM) and batch mode micro ultrasonic machining (/spl mu/USM). An electroplating mold is first created on a silicon or metal wafer using standard lithography, then using the electroplated pattern as an electrode to /spl mu/EDM a hard metal (stainless steel or WC/Co) tool, which is finally used in the /spl mu/USM of the ceramic substrate. A related process (SEDUS) uses serial /spl mu/EDM and omits lithography for rapid prototyping of simple patterns. Feature sizes of 25 /spl mu/m within a 4.5/spl times/4.5 mm/sup 2/ die have been micromachined on glass-mica (Macor) ceramic plates with 34 /spl mu/m depth. The ultrasonic step achieves 18 /spl mu/m/min. machining rate, with a tool wear ratio of less than 6% for the stainless steel microtool. Other process characteristics are also described. As a demonstration, octagonal and circular spiral shaped in-plane actuators were fabricated from bulk lead zirconate titanate (PZT) plate using the LEEDUS/SEDUS process. A device of 20 /spl mu/m thickness and 450 /spl mu/m/spl times/420 /spl mu/m footprint produces a displacement of /spl ap/2/spl mu/m at 40 V.     

Kinematically Stabilized Microbubble Actuator Arrays

In this paper, the concept, fabrication, and characterization of a kinematically stabilized polymeric microbubble actuator (endoskeletal microbubble actuator) for a pneumatic tactile display application are presented. The kinematic stabilization is achieved by the combination of two polymeric layers with complementary functions: a microcorrugated parylene diaphragm layer as a ldquoskeletonrdquo to provide a directional deflection in a desired axial direction while suppressing undesired lateral deflections and an overcoated elastomer diaphragm layer as a ldquoskinrdquo to help the extended membrane recoil to its original shape, ensuring diaphragm stability. Arrays of microcorrugated diaphragms are implemented in a mass producible fashion using inclined rotational UV lithography, micromolding, and pattern transfer techniques. Both the number of corrugations and the corrugation profile of the endoskeletal actuator are determined through numerical analysis, taking into account the constraints of the microfabrication processes utilized. A prototype of a single endoskeletal bubble actuator with a diameter of 2.6 mm has been fabricated and characterized. For comparison purposes, elastomer microcorrugated diaphragm (skin only) actuators and parylene microcorrugated diaphragm (skeleton only) actuators of the same materials and dimensions have also been fabricated and tested. While the skin-only diaphragm actuators demonstrated undesired omnidirectional inflation and the skeleton-only diaphragm actuators have shown unstable and irreversible deformation during extension, the proposed endoskeletal microbubble actuators have shown stable reversible axial extensions with a deflection of approximately 0.9 mm. A 6 6 array of endoskeletal polymer microbubble actuators integrated with a microfluidic manifold has been successfully fabricated, demonstrating its mass manufacturability.     

Self-assembly of PZT actuators for micropumps with high process repeatability

In this paper, we report a novel capillary-driven self-assembly technique which proceeds in an air environment and demonstrate it by assembling square piezoelectric transducer (PZT) actuators for 28 diffuser valve micropumps on a 4-inch pyrex/silicon substrate: on the substrate, binding sites are wells of 24 /spl mu/m in depth and the only hydrophilic areas; on the bonding face of the PZT actuator, the central hydrophilic area is a square identical in size to the binding site, and the rim is hydrophobic; acrylate-based adhesive liquid is dispensed across the substrate and wets only the binding sites; the hydrophilic areas on the introduced PZT actuators self-align with the binding sites to minimize interfacial energies by capillary forces from the adhesive droplets; the aligned PZT actuators are pressed to contact the gold coated substrate by their rims and the adhesive is polymerized by heating to 85 /spl deg/C for half an hour, so permanent mechanical and electrical connections are established, respectively, at the center and rim of each PZT actuator. These pumps perform with high uniformity, which is indicated by a small standard deviation of their resonant frequencies to pump ethanol: the average resonant frequency is 6.99 kHz and the standard deviation is 0.1 kHz. Compared with the conventional bonding process with highly viscous silver epoxy, this assembly method has several major advantages: highly accurate placement with self-alignment, controllable adhesive thickness, tilt free bonding, low process temperature and high process repeatability.     

A hydrogel-actuated environmentally sensitive microvalve for active flow control

This paper reports on the fabrication and test of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. The microvalve consists of a thin hydrogel, sandwiched between a stiff porous membrane and a flexible silicone rubber diaphragm. Swelling and deswelling of the hydrogel, which results from the diffusion of chemical species through the porous membrane is accompanied by the deflection of the diaphragm and hence closure and opening of the valve intake orifice. A phenylboronic-acid-based hydrogel was used to construct a smart microvalve that responds to the changes in the glucose and pH concentrations. The fastest response time (for a pH concentration cycle) achieved was 7 min using a 30-/spl mu/m-thick hydrogel and a 60-/spl mu/m-thick porous membrane with 0.1 /spl mu/m pore size and 40% porosity.     

A wafer-scale membrane transfer Process for the fabrication of optical quality, large continuous membranes

This paper describes a new fabrication technique developed for the construction of large area mirror membranes via the transfer of wafer-scale continuous membranes from one substrate to another. Using this technique, wafer-scale silicon mirror membranes have been successfully transferred without the use of sacrificial layers such as adhesives or polymers. This transfer technique has also been applied to the fabrication and transfer of 1 /spl mu/m thick corrugated membrane actuators. These membrane actuators consist of several concentric-ring-type corrugations constructed within a polysilicon membrane. A typical polysilicon actuator membrane with an electrode gap of 1.5 /spl mu/m, fabricated using the wafer-scale transfer technique, shows a vertical deflection of 0.4 /spl mu/m at 55 V. The mirror membranes are constructed from single-crystal silicon, 10 cm in diameter, and have been successfully transferred in their entirety. Using a white-light interferometer, the measured average peak-to-valley surface figure error for the transferred single-crystal silicon mirror membranes is approximately 9 nm as measured over a 1 mm/sup 2/ membrane area. The wafer-scale membrane transfer technique demonstrated in this paper has the following benefits over previously reported transfer techniques: 1) No postassembly release process to remove sacrificial polymers is required. 2) The bonded interface is completely isolated from any acid, etchant, or solvent during the transfer process, ensuring a clean and uniform membrane surface. 3) Our technique is capable of transferring large, continuous membranes onto substrates.     

Polymer-based cantilevers with integrated electrodes

An innovative release method of polymer cantilevers with embedded integrated metal electrodes is presented. The fabrication is based on the lithographic patterning of the electrode layout on a wafer surface, covered by two layers of SU-8 polymer: a 10-/spl mu/m-thick photo-structured layer for the cantilever, and a 200-/spl mu/m-thick layer for the chip body. The releasing method is based on dry etching of a 2-/spl mu/m-thick sacrificial polysilicon layer. Devices with complex electrode layout embedded in free-standing 500-/spl mu/m-long and 100-/spl mu/m-wide SU-8 cantilever were fabricated and tested. We have optimized major fabrication steps such as the optimization of the SU-8 chip geometry for reduced residual stress and for enhanced underetching, and by defining multiple metal layers [titanium (Ti), aluminum (Al), bismuth (Bi)] for improved adhesion between metallic electrodes and polymer. The process was validated for a miniature 2/spl times/2 /spl mu/m/sup 2/ Hall-sensor integrated at the apex of a polymer microcantilever for scanning magnetic field sensing. The cantilever has a spring constant of /spl cong/1 N/m and a resonance frequency of /spl cong/17 kHz. Galvanometric characterization of the Hall sensor showed an input/output resistance of 200/spl Omega/, a device sensitivity of 0.05 V/AT and a minimum detectable magnetic flux density of 9 /spl mu/T/Hz/sup 1/2/ at frequencies above 1 kHz at room temperature. Quantitative magnetic field measurements of a microcoil were performed. The generic method allows for a stable integration of electrodes into polymers MEMS and it can readily be used for other types of microsensors where conducting metal electrodes are integrated in cantilevers for advanced scanning probe sensing applications.     

Electrothermally activated paraffin microactuators

A new family of electrothermally activated microactuators that can provide both large displacements and forces, are simple to fabricate, and are easily integrated with a large variety of microelectronic and microfluidic components are presented. The actuators use the high volumetric expansion of a sealed, surface micromachined patch of paraffin heated near its melting point to deform a sealing diaphragm. Two types of actuators have been fabricated using a simple three mask fabrication process. The first device structure consists of a 9 μm thick circularly patterned paraffin layer ranging in diameter from 400 to 800 μm all covered with a 4-μm-thick metallized p-xylylene sealing diaphragm. All fabricated devices produced a 2.7-μm-peak center deflection, consistent with a simple first order theory. The second actuator structure uses a constrained volume reservoir that magnifies the diaphragm deflection producing consistently 3.2 μm center diaphragm deflection with a 3-μm-thick paraffin actuation layer. Microactuators were constructed on both glass and silicon substrates. The actuators fabricated on glass substrates used between 50-200 mW of electrical power with response times ranging between 30-50 ms. The response time for silicon devices was much faster (3-5 ms) at the expense of a larger electrical power (500-2000 mW)     

Single mask, large force, and large displacement electrostatic linear inchworm motors

We have demonstrated a family of large force and large displacement electrostatic linear inchworm motors that operate with moderate to high voltages. The inchworm motor design decouples actuator force from total travel and allows the use of electrostatic gap-closing actuators to achieve large force and large displacement while consuming low power. A typical inchworm motor measures 3 mm /spl times/ 1 mm /spl times/ 50 /spl mu/m and can lift over 130 times its own weight. One motor has achieved a travel of 80 /spl mu/m and a calculated force of 260 /spl mu/N at 33 V. The force density of that motor was 87 /spl mu/N/mm/sup 2/ at 33 V and the energy efficiency was estimated at 8%. Another motor displaced the shuttle at an average velocity of almost 4 mm/s and achieved an estimated power density of 190 W/m/sup 3/. Motors were cycled 23.6 million times for over 13.5 h without stiction. This family of motors is fabricated in silicon-on-insulator (SOI) wafers using a single mask.     

Analog piezoelectric-driven tunable gratings with nanometer resolution

This work presents the design, fabrication, and characterization of a piezoelectrically actuated MEMS diffractive optical grating, whose spatial periodicity can be tuned in analog fashion to within a fraction of a nanometer. The fine control of the diffracted beams permits applications in dense wavelength-division multiplexing (DWDM) optical telecommunications and high-resolution miniaturized spectrometers. The design concept consists of a diffractive grating defined on a deformable membrane, strained in the direction perpendicular to the gratings grooves via thin-film piezoelectric actuators. The tunable angular range for the first diffracted order is up to 400 /spl mu/rad with 0.2% strain (/spl sim/8 nm change in grating periodicity) at 10 V actuation, as predicted by device modeling. The actuators demonstrate a piezoelectric d/sub 31/ coefficient of -100 pC/N and dielectric constant /spl epsiv//sub r/ of 1200. Uniformity across the tunable grating and the out-of-plane deflections are also characterized and discussed.     

Design, fabrication, and measurement of high-sensitivity piezoelectric microelectromechanical systems accelerometers

Microelectromechanical systems (MEMS) accelerometers based on piezoelectric lead zirconate titanate (PZT) thick films with trampoline or annular diaphragm structures were designed, fabricated by bulk micromachining, and tested. The designs provide good sensitivity along one axis, with low transverse sensitivity and good temperature stability. The thick PZT films (1.5-7 /spl mu/m) were deposited from an acetylacetonate modified sol-gel solution, using multiple spin coating, pyrolysis, and crystallization steps. The resulting films show good dielectric and piezoelectric properties, with P/sub r/ values >20 /spl mu/C/cm/sup 2/, /spl epsiv//sub r/>800, tan/spl delta/<3%, and |e/sub 31,f/| values >6.5 C/m/sup 2/. The proof mass fabrication, as well as the accelerometer beam definition step, was accomplished via deep reactive ion etching (DRIE) of the Si substrate. Measured sensitivities range from 0.77 to 7.6 pC/g for resonant frequencies ranging from 35.3 to 3.7 kHz. These accelerometers are being incorporated into packages including application specific integration circuit (ASIC) electronics and an RF telemetry system to facilitate wireless monitoring of industrial equipment.     

Development of piezoelectric MEMS deformable mirror

This paper reports on piezoelectric micro-electro-mechanical systems deformable mirrors with high-density actuator array for low-voltage and high-resolution retinal imaging with adaptive optics. The deformable mirror was composed of unimorph structure of lead zirconate titanate (PZT) thin film deposited on Pt-coated silicon on insulator substrate and a diaphragm of 10?mm in diameter formed by backside-etching the Si handle wafer. 61 hexagonal electrodes were laid out on the PZT thin film for the high-density actuator array. In order to reduce the dead space for the lead lines between the electrodes and connecting pads, a polyimide layer with through holes on the electrodes was patterned as an electrical insulator. To confirm the application feasibility of the fabricated DMs, displacement profiles of the actuators were measured by a laser Doppler vibrometer. Independent applications of voltages on individual actuators were confirmed.     

Design of large deflection electrostatic actuators

Electrostatic, comb-drive actuators have been designed for applications requiring displacements of up to 150 /spl mu/m in less than 1 ms. A nonlinear model of the actuator relates the resonant frequency and the maximum stable deflection to the actuator dimensions. A suite of experiments that were carried out on deep reactive ion etched (DRIE), single-crystal silicon, comb-drive actuators confirm the validity of the model. Four actuator design improvements were implemented. First, a folded-flexure suspension consisting of two folded beams rather than four and a U-shaped shuttle allowed the actuator area to be cut in half without degrading its performance. Second, the comb teeth were designed with linearly increasing lengths to reduce side instability by a factor of two. Third, the folded-flexure suspensions were fabricated in an initially bent configuration, improving the suspension stiffness ratio and reducing side instability by an additional factor of 30. Finally, additional actuation range was achieved using a launch and capture actuation scheme in which the actuator was allowed to swing backward after full forward deflection; the shuttle was captured and held using the backs of the comb banks as high-force, parallel-plate actuators.     

Thin-Film Piezoelectric Unimorph Actuator-Based Deformable Mirror With a Transferred Silicon Membrane

This paper describes a proof-of-concept deformable mirror (DM) technology, with a continuous single-crystal silicon membrane reflecting surface, based on$ PbZr _0.52 Ti_0.48 O _3$(PZT) unimorph membrane microactuators. A potential application for a terrestrial planet finder adaptive er is also discussed. The DM comprises a continuous, large-aperture, silicon membrane “transferred” onto a 20$,times,$20 piezoelectric unimorph actuator array. The actuator array was prepared on an electroded silicon substrate using chemical-solution-deposited 2-$mu m$-thick PZT films working in a$d _31$mode. The substrate was subsequently bulk-micromachined to create membrane structures with residual silicon acting as the passive layer in the actuator structure. A mathematical model simulated the membrane microactuator performance and aided in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. The resulting piezoelectric unimorph actuators with patterned PZT films produced large strokes at low voltages. A PZT unimorph actuator, 2.5 mm in diameter with optimized PZT/silicon thickness and design showed a deflection of 5.7$~mu m$at 20 V. A DM structure with a 20-$mu m$-thick silicon membrane mirror (50 mm$times,$50 mm area) supported by 400 PZT unimorph actuators was successfully fabricated and optically characterized. The measured maximum mirror deflection at 30 V was approximately 1$~mu m$. An assembled DM showed an operating frequency bandwidth of 30 kHz and an influence function of approximately 30%. 1738     

A monolithic three-axis micro-g micromachined silicon capacitive accelerometer

A monolithic three-axis micro-g resolution silicon capacitive accelerometer system utilizing a combined surface and bulk micromachining technology is demonstrated. The accelerometer system consists of three individual single-axis accelerometers fabricated in a single substrate using a common fabrication process. All three devices have 475-/spl mu/m-thick silicon proof-mass, large area polysilicon sense/drive electrodes, and small sensing gap (<1.5 /spl mu/m) formed by a2004 sacrificial oxide layer. The fabricated accelerometer is 7/spl times/9 mm/sup 2/ in size, has 100 Hz bandwidth, >/spl sim/5 pF/g measured sensitivity and calculated sub-/spl mu/g//spl radic/Hz mechanical noise floor for all three axes. The total measured noise floor of the hybrid accelerometer assembled with a CMOS interface circuit is 1.60 /spl mu/g//spl radic/Hz (>1.5 kHz) and 1.08 /spl mu/g//spl radic/Hz (>600 Hz) for in-plane and out-of-plane devices, respectively.     

Performance of nonplanar silicon diaphragms under large deflections

The successful realization of many high-performance microactuators, including many microvalves and micropumps, depends critically on the development of diaphragms which are capable of large displacements and free from fatigue. Thin nonplanar silicon diaphragms are promising candidates for such applications since they can be batch fabricated using techniques and materials that are compatible with the other portions of these devices. This paper reports the detailed simulation, fabrication, and characterization of such diaphragms, which are corrugated-bossed structures that unfold, accordion-like, to produce high boss deflections. Boron-doped diaphragms 1 mm on a side, 3 μm in thickness, and containing five 10-μm-deep corrugations produce boss deflections of more than 30 μm at 760 mmHg, in close agreement with simulations. The maximum deflection measured at diaphragm fracture is 38 μm under a 1050 mmHg differential pressure. The effects on load-deflection performance due to changes in diaphragm internal stress (residual stress), corrugation profile, and diaphragm thickness are also explored     

A micromechanical flow sensor for microfluidic applications

We fabricated a microfluidic flow meter and measured its response to fluid flow in a microfluidic channel. The flow meter consisted of a micromechanical plate, coupled to a laser deflection system to measure the deflection of the plate during fluid flow. The 100 /spl mu/m square plate was clamped on three sides and elevated 3 /spl mu/m above the bottom surface of the channel. The response of the flow meter was measured for flow rates, ranging from 2.1 to 41.7 /spl mu/L/min. Several fluids, with dynamic viscosities ranging from 0.8 to 4.5/spl times/10/sup -3/ N/m, were flowed through the channels. Flow was established in the microfluidic channel by means of a syringe pump, and the angular deflection of the plate monitored. The response of the plate to flow of a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m was linear for all flow rates, while the plate responded linearly to flow rates less than 4.2 /spl mu/L/min of solutions with lower dynamic viscosities. The sensitivity of the deflection of the plate to fluid flow was 12.5/spl plusmn/0.2 /spl mu/rad/(/spl mu/L/min), for a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m. The encapsulated plate provided local flow information along the length of a microfluidic channel.