A thermomechanical model for adhesion reduction of MEMS cantilevers

Presents a thermomechanical model that describes adhesion reduction in MEMS structures using laser heating. A fracture mechanics model is developed where the interface between the stiction-failed microcantilever and the substrate is treated as a crack, and the energy release rate is calculated using elastic theory. In order to include the effect of a temperature difference between the microcantilever and the substrate, an associated thermal strain energy is included in the fracture model. If the free length is longer than the critical buckling length, the beam buckles decreasing the strain energy of the system. For surface-micromachined polycrystalline silicon cantilevers with an initial crack length of 400 /spl mu/m, the model predicts that a temperature difference of 100 K repairs microcantilevers as long as 1300 /spl mu/m. The peeling of adhered beams from the substrate after laser irradiation is experimentally shown with measured crack lengths within 15% of predicted values indicating that the proposed model establishes the mechanism of adhesion reduction by laser irradiation.     

Process yields for laser repair of aged, stiction-failed, MEMSdevices

Stiction, the adhesion of micromachined components to each other or the substrate, decreases production yields and operational lifetimes for MEMS devices. A recent study demonstrated the feasibility of using a Nd:YAG, 1063 nm laser to repair 2 μm thick polycrystalline silicon microcantilevers with lengths up to 1000 μm which had adhered to the substrate. This investigation examines the influence of sample age at the time of laser irradiation on the repair of stiction-failed MEMS structures. The operating conditions for the 1064 nm, Nd:YAG laser included a fluence of 70 mJ/cm², a repetition rate of 20 Hz, a pulse duration of 20 ns, and an exposure time of 60 s. For samples irradiated on the same day of release, repair yields were 100% for beams up to 500 μm in length. For 1000 μm long beams, the 10- and 30-μm-wide cantilevers had same-day repair yields of 56% and 71%, respectively. The experimental results show that increasing the amount of time the microstructures are adhered to the substrate before laser irradiation decreases the ability to repair stiction-failed devices with delays of longer than 1 month resulting in a decreased repair yield     

Wafer-scale microdevice transfer/interconnect: its application in an AFM-based data-storage system

We have developed a robust, CMOS back end of the line (BEOL) compatible, wafer-scale device transfer, and interconnect method for batch fabricating systems on chip that are especially suitable for MEMS or VLSI-MEMS applications. We have applied this method to transfer arrays of 4096 free-standing cantilevers with good cantilever flatness control and high-density vertical electrical interconnects to the receiver wafer (typically CMOS). Such an array is used in a highly parallel, scanning-probe-based data-storage system, which we internally call "millipede." A very high-integration density has been achieved, even for wafer-scale transfer, thanks to the interlocking nature of the interconnect structure, which provides easy alignment with an accuracy of 2 /spl mu/m. The typical integration density is 100 cantilevers/mm/sup 2/ and 300 electrical interconnects/mm/sup 2/. Note that only the cantilevers, not a chip with cantilevers, are transferred and, unlike flip-chip technology, our method preserves the device orientation, which is crucial for MEMS applications, where often the MEMS device should have access to its environment (in our case, the cantilever tips are in contact with the storage medium). After device transfer, the system is mechanically and electrically stable up to at least 500/spl deg/C, allowing post-transfer wafer processing.     

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.     

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.     

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.     

Release processing effects on laser repair of stiction-failed microcantilevers

Undesirable adhesion in microelectromechanical systems (MEMS) is referred to as stiction and is a principal failure mechanism in surface-micromachined MEMS devices. Previous investigations demonstrated repairing stiction-failed polycrystalline silicon MEMS structures released from isopropyl alcohol (IPA) using Nd:YAG laser irradiation and predicting the laser repair process using a thermomechanical model. The current paper reports the effectiveness of the laser repair process and corresponding thermomechanical model predictions for microcantilevers that have failed during four release treatments: water, IPA, octadecyltrichlorosilane (OTS), and supercritical CO/sub 2/ drying. Model predictions and experimental measurements of the laser repair process are also provided for MEMS devices that failed due to contact during electrostatic actuation. The results indicate that the laser repair process is very effective for both failure modes and that the thermomechanical model predicts the laser repair of microcantilevers that failed during release much better than for microcantilevers that failed due to subsequent contact.     

Characterization of selective polysilicon deposition for MEMS resonator tuning

Variations in micromachining processes cause submicron differences in the size of MEMS devices, which leads to frequency scatter in resonators. A new method of compensating for fabrication process variations is to add material to MEMS structures by the selective deposition of polysilicon. It is performed by electrically heating the MEMS in a 25/spl deg/C silane environment to activate the local decomposition of the gas. On a (1.0/spl times/1.5/spl times/100) /spl mu/m/sup 3/, clamped-clamped, polysilicon beam, at a power dissipation of 2.38 mW (peak temperature of 699/spl deg/C), a new layer of polysilicon (up to 1 /spl mu/m thick) was deposited in 10 min. The deposition rate was three times faster than conventional LPCVD rates for polysilicon. When selective polysilicon deposition (SPD) was applied to the frequency tuning of specially-designed, comb-drive resonators, a correlation was found between the change in resonant frequency and the length of the newly deposited material (the hotspot) on the resonator's suspension beams. A second correlation linked the length of the hotspot to the magnitude of the power fluctuation during the deposition trial. The mechanisms for changing resonant frequency by the SPD process include increasing mass and stiffness and altering residual stress. The effects of localized heating are presented. The experiments and simulations in this work yield guidelines for tuning resonators to a target frequency.     

Magnetic MEMS reconfigurable frequency-selective surfaces

A reconfigurable frequency-selective electromagnetic filter implemented by integrating hard magnetic materials with microelectromechanical systems (MEMS) provides a new variation of reconfigurable frequency-selective surfaces (FSS). By incorporating magnetically actuated dipole elements that are capable of being tilted away from the supporting surface, we can tune the FSSs operating frequency without having to physically alter the dimensions of the dipole elements. The 25/spl times/25 array of microactuators used in this work each consist of a 896/spl times/168/spl times/30 /spl mu/m/sup 3/ ferromagnetic plate made of 40Co-60Ni, layered with a 1-/spl mu/m-thick conductor (Au), attached to a pair of 400/spl times/10/spl times/1 /spl mu/m/sup 3/ polysilicon torsion beams, suspended just above the supporting substrate. The high remanent magnetization of the ferromagnetic material allows for relatively small magnetic fields (/spl sim/2.1 kA/m) to induce significant angular deflections (/spl sim/45/spl deg/). This innovative reconfigurable FSS design has successfully demonstrated electromagnetic-signal diplexing and tuning its resonant frequency over a bandwidth of 2.7 GHz at a frequency of 85 GHz.     

Mechanical characterization and design of flexible silicon microstructures

A variety of different silicon structures has been fabricated and characterized mechanically to optimize the design of silicon ribbon cables used in neural probes and multichip packaging structures. Boron-doped 3-/spl mu/m-thick silicon beams were tested in three modes: bending in plane, twisting (along beam axis), and pushing. Various cable configurations were investigated (straight beams, curved beams, meandered beams, etc.) as well the effects of length, width, cable termination, and the presence of reinforcing spans between multistranded cables. The results along with finite element modeling indicated that many simple modifications could be made to increase the strength and flexibility of silicon ribbon cables. One structure, a meandered beam 200-/spl mu/m wide and 2-mm long could be twisted up to 712/spl deg/. It also was seen that structures having multiple 20-/spl mu/m-wide beams were generally more robust than those with a single 500-/spl mu/m-wide beam. Finally, a method for easy determination of the bending fracture strain is analyzed and verified. It was seen that the silicon structures tested broke after a strain slightly above 2%.     

Uncooled multimirror broad-band infrared microbolometers

A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earth's Radiant Energy System) instrument to measure the radiation flux at the Earth's surface and the radiant energy now within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 /spl mu/m, a wide-band channel of 0.3 to 100 /spl mu/m and a window channel from 8 to 12 /spl mu/m each housing a 1.5 mm x 1.5 mm microbolometers or alternatively 400 /spl mu/m x 400 mm microbolometers in a 1 /spl times/ 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 /spl mu/m wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si/sub 3/N/sub 4/ bridge layer. The temperature coefficient of resistance was measured to be -2.8%/K. The voltage responsivities were over 10/sup 3/ V/W, detectivities above 10/sup 8/ cm Hz/sup 1/2//W, noise equivalent power less than 4 /spl times/ 10/sup -10/W/Hz/sup 1/2/ and thermal time constant less than 15 ms.     

Elimination of stress-induced curvature in thin-film structures

Argon ion machining of released thin-film devices is shown to alter the contour shape of free-standing thin-film structures by affecting their through-thickness stress distributions. In experiments conducted on MEMS thin-film mirrors it is demonstrated that post-release out-of-plane deformation of such structures can be reduced using this ion beam machining method. In doing so optically flat surfaces (curvature <0.001 mm/sup -1/) are achieved on a number of 3 /spl mu/m-thick surface micromachined silicon structures, including mirrors with either initially positive curvature or initially negative curvature measuring up to 0.02 mm/sup -1/. An analytical model incorporating the relevant mechanics of the problem is formulated and used to provide an understanding of the mechanisms behind the planarization process based on ion machining. The principal mechanisms identified are 1) amorphization of a thin surface layer due to ion beam exposure and 2) gradual removal of stressed material by continued exposure to the ion beam. Curvature history predictions based on these mechanisms compare well with experimental observations.     

Design and fabrication aspects of an S-shaped film actuator based DC to RF MEMS switch

This paper reports on design and fabrication aspects of a new microelectromechanical series switch for switching dc and RF signals. The switch consists of a flexible S-shaped film with the switching contact, rolling between a top and a bottom electrode in electrostatic touch-mode actuation. This design allows a low actuation voltage independent of the contact distance in the off-state. With a large contact distance, large overlapping switching contact areas are possible by obtaining a high off-state isolation. The RF transmission line and the MEMS part of the switch are fabricated on separate wafers, allowing an implementation of the switch with different RF substrates. The final assembly is done on device level for the first prototypes, even though the design provides the possibility of an assembly by full wafer bonding, leading to a near-hermetic package integrated switch. The measured prototype actuation voltages are 12 V to open and 15.8 V to close the contacts, with a resistance of 275 m/spl Omega/ of each contact at an estimated contact force of 102 /spl mu/N. The measured RF isolation with a contact distance of 14.2 /spl mu/m is better than -45 dB up to 2 GHz and -30 dB at 15 GHz, at a large nominal switching contact area of 3500 /spl mu/m/sup 2/.     

Cross-linked PMMA as a low-dimensional dielectric sacrificial layer

A surface nanomachining fabrication process using electron beam cross-linked poly(methyl) methacrylate (PMMA) has been developed and characterized. PMMA with different molecular weights (MW 100 K, MW 495 K, MW 950 K) in anisole casting solvent has been crosslinked with different electron beam irradiation levels ranging from 20 C/m/sup 2/ to 240 C/m/sup 2/. This is to investigate the quantifiable relationship between electron dose and its submicrometer remaining thickness after dissolving in acetone. This technique which uses electron beam lithography, offers a high resolution semi-three-dimensional (3-D) nanomachining of the sacrificial layer in a single run. Because of its low Young's modulus, it has been successfully integrated with nanoelectromechanical systems processing and has the advantage of producing low-stress submicrometer thick structures with lateral dimensions as low as, but not limited to 1 /spl mu/m. A fast dry release time from 55 to 100 s using oxygen plasma ashing has been demonstrated for a sacrificial layer aspect ratio of 125. This corresponds to an etch rate of about 0.6 /spl mu/m/s at an average temperature of 40/spl deg/C. The success of using cross-linked PMMA as a gate dielectric is demonstrated by the fabrication of multilayered gated lateral quantum dot devices. Periodic and continuous conductance oscillations arising from Coulomb charging effects are clearly observed in the transport properties at 50 mK.     

Integrated self-assembling and holding technique applied to a 3-D MEMS variable optical attenuator

The application of polysilicon/gold bimorph stress-induced curved beams for three-dimensional self-assembly of MEMS devices is reported. The mechanical principle behind this self-assembling procedure is presented and comparison with current assembling methods are made. With this self-assembling technique, no postprocessing is required. A free-space optical MEMS device in the form of a variable optical attenuator (VOA) has been fabricated and self-assembled using this technique. The angular elevation of the self-assembled structures and the attenuation characteristics of the optical MEMS device are reported. The VOA has a measured dynamic attenuation range of 44 dB at 1.55 /spl mu/m optical wavelength. The bending of the bimorph beams is also temperature controllable, and the thermal behavior of the beams is also reported.     

Self-aligning MEMS in-line separable electrical connector

A MEMS in-line separable connector containing features for precision self-alignment is demonstrated. The concept relies on sliding connection between female and male halves to induce vertical deflections of a set of flexible conductors and establish stable electrical contacts. Electrodeposited photoresist is used to fabricate thick, nonplanar conductors, shaped by a silicon substrate that has previously been terraced by anisotropic etching. Further etched features ensure transverse and vertical self-alignment between conductor elements during mating. Prototype 10-way connectors are demonstrated with 200 /spl mu/m wide conductors on a 250-/spl mu/m pitch. Mechanical reliability of contacts during repeated mating and demating is demonstrated, and initial measurement of contact resistance reveals an encouraging value of 30 m/spl Omega/.     

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.     

A new methodology to investigate fracture toughness of freestanding MEMS and advanced materials in thin film form

This work presents a novel membrane deflection fracture experiment (MDFE) to investigate the fracture toughness of microelectromechanical systems (MEMS) and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing preexisting cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.5/spl plusmn/0.25 MP m/sup 1/2/. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 6.9 MPa m/sup 1/2/. Comparison of these two values, using the model proposed by Drory et al., provides a correction factor of two-thirds, which corresponds to a mean value of /spl rho//2x=1/2.     

Cubic millimeter power inductor fabricated in batch-type wafer technology

A hybrid technology for the realization of three-dimensional (3-D) miniaturized power inductors is presented. Our devices consist of planar Cu coils on polyimide substrates, and mm-size ferrite magnetic cores, obtained by three-dimensional micro-patterning of ferrite wafers using powder blasting. The coils are realized using an in-house developed high-resolution polyimide spinning and Cu electroplating process. Winding widths down to 5 /spl mu/m have been obtained and total device volumes are ranging between 1.5 and 10 mm/sup 3/. Inductive and resistive properties are characterized as a function of frequency; inductance values in the 100 /spl mu/H range have been obtained.     

The critical role of environment in fatigue damage accumulation in deep-reactive ion-etched single-crystal silicon structural films

The importance of service environment to the fatigue resistance of n/sup +/-type, 10 /spl mu/m thick, deep-reactive ion-etched (DRIE) silicon structural films used in microelectromechanical systems (MEMS) was characterized by testing of electrostatically actuated resonators (natural frequency, f/sub 0/, /spl sim/40 kHz) in controlled atmospheres. Stress-life (S-N) fatigue tests conducted in 30/spl deg/C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of silicon films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) are strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microscopy of high-cycle fatigue fracture surfaces (cycles to failure, N/sub f/>1/spl times/10/sup 9/) revealed clear failure origins that were not observed in short-life (N/sub f/<1/spl times/10/sup 4/) specimens. Reaction-layer and microcracking mechanisms for fatigue of silicon films are discussed in light of this empirical evidence for the critical role of service environment during damage accumulation under cyclic loading conditions.