The microstructure and residual stress of sputter-deposited yttria-stabilized zirconia (YSZ) films are presented as a function of thickness (5–1000 nm), deposition pressure (5–100 mTorr), and post-deposition temperature. The as-deposited residual stress of YSZ ranges from −1.4 GPa to 100 MPa with variations in sputtering conditions. Transitions from compressive to tensile stress are identified with variations in working pressure and film thickness. The origins and variations in as-deposited stress are determined to be from tensile stress due to grain coalescence/growth, and compressive stresses are due to forward sputtering/“atomic peening” of target atoms. The evolution of residual stress with post-deposition annealing shows a tensile stress hysteresis of up to 1 GPa for films deposited at low working pressures. This hysteresis is believed to be due to crystallization and the diffusive relief of compressive stresses initially generated by atomic peening during deposition. Discussion and evaluation of other common residual stress mechanisms are presented throughout.

The selection of actuators at the micro-scale requires an understanding of the performance limits of different actuation mechanisms governed by the optimal selection of materials. This paper presents the results of analyses for elastic bi-material actuators based on simple beam theory and lumped parameter thermal models. Comparisons are made among commonly employed actuation schemes (electro-thermal, piezoelectric and shape memory) at micro scales and promising candidate materials are identified. Polymeric films on Si subjected to electro-thermal heating are optimal candidates for high displacement, low frequency devices while ferroelectric thin films of Pb-based ceramics on Si/ DLC are optimal for high force, high frequency devices. The ability to achieve ∼10 kHz at scales < 100μm make electro-thermal actuators competitive with piezoelectric actuators considering the low work/volume obtained in piezoelectric actuation (∼ 10−8J.m−3.mV−2). Although shape memory alloy (SMA) actuators such as Ni-Ti on Si deliver larger work (∼ 1 J.m−3K−2) than electro-thermal actuators at relatively low frequencies (∼ 1 kHz), the critical scale associated with the cessation of the shape memory effect forms the bounding limit for the actuator design. The built-in compressive stress levels (∼ 1GPa) in thin films of Si and DLC could be exploited for realizing a high performance actuator by electro-thermal buckling.

Micromachined fuel cells are among a class of microscale devices being explored for portable power generation. In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells. Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition. Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 μm for YSZ and 525 μm for GDC, with high processing yields for YSZ. Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively. Tensile evaporated films fail via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio. Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films. Sputtered 1025-μm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K).

The stability of multilayered membrane structures is a major challenge in the development of microfabricated solid oxide fuel cells (SOFC). The work presented here explores residual stress in sputter-deposited yttria stabilized zirconia (YSZ) thin films (5nm – 1000nm thickness) as a function of deposition pressure and substrate temperature. The results indicate variations in intrinsic stress from ∼0.5GPa compressive to mildly tensile (∼50 MPa). Microstructure is characterized by x-ray diffraction (XRD). The evolution of intrinsic stress with temperature is investigated by thermally cycling YSZ films deposited on silicon wafers. Observed changes of 100s of MPa in the intrinsic stress component of the film serve as indicators of possible changes in microstructure. Such changes in microstructure are subsequently characterized using x-ray diffraction of as-deposited and annealed films. Correlations with relevant mechanisms and models of residual stress evolution are discussed. Finally, use of such residual stress data in the fabrication and design of mechanically stable multilayered membranes for micro SOFC devices is discussed.

The quality of wafer-level, gold thermocompression bonds is critically dependent on the interaction between the wafer topography, the thin film properties, the process parameters and tooling used to achieve the bonds. This study presents mechanics modeling of the effect of wafer topography. An analytical expression for the strain energy release rate associated with the elastic deformation required to overcome wafer bow is developed. Furthermore, a simple contact yielding criterion is used to examine the pressure and temperature conditions required to flatten nano-scale asperities in order to achieve bonding over the full apparent area. The analytical results combined with experimental data for the interface bond toughness obtained from four-point bend testing indicate that the overall wafer shape is a negligible contributor to bond quality. A micro-scale bond characterization based on microscopic observations and AFM measurements show that the bond yield is increased with increasing bond pressure.

The characteristics and mechanisms of damage and failure in microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass seals have been investigated. These joints are representative of seals for the MIT microrocket which is a silicon-based MEMS device. A key concern in such joints is the occurrence of cracks in silicon and glass due to residual stresses caused by a large thermal excursion during processing and the dissimilar coefficients of thermal expansion of the constituent materials. Joints with two types of glass compositions and joint configurations were fabricated, tested, and inspected. Axial tension tests were performed to investigate load carrying capability and the effect of thermally-induced cracks. Finite element models were used to obtain residual stresses due to the fabrication, and the location of the cracks from the experiments were found to coincide with the locations of the maximum principal stresses. The current work shows that the certain types of thermally-induced cracks are more detrimental to joint strength than others and a good bond between the Kovar tube and the silicon sidewall can help increase joint strength via shear load transfer.

The displacement loaded double cantilever beam (DCB), often referred to as the blade-insertion test or crack-opening method by the wafer bonding community, has become a common method for evaluating the work of adhesion of bonded wafer pairs. The test, while easy to perform, often yields results with large scatter and questionable accuracy. The mechanics of the specimen are investigated in detail in the current work. Expressions that demonstrate how wafer bow may lead to residual stresses that result in large errors in the calculated work of adhesion are developed. A three-dimensional finite element model is used to show that due to the circular wafer geometry and silicon anisotropy there is a large variation of the strain energy release rate across a straight crack front. The model is used to predict the actual crack front shape and shows good agreement with experiments. The results of the finite element simulations are compared to the traditional expression used for data reduction and implications of the model highlighted.

Embedded structural health monitoring systems are envisioned to be an important component of future transportation systems. One of the key challenges in designing an SHM system is the choice of sensors, and a sensor layout, which can detect unambiguously relevant structural damage. This paper focuses on the relationship between sensors, the materials of which they are made, and their ability to detect structural damage. Sensor selection maps have been produced which plot the capabilities of the full range of available sensor types vs. the key performance metrics (power consumption, resolution, range, sensor size, coverage). This exercise resulted in the identification of piezoceramic Lamb wave transducers as the sensor of choice. Experimental results are presented for the detailed selection of piezoceramic materials to be used as Lamb wave transducers.

A class of MEMS devices, which utilizes microfabrication technology and bulk piezoelectric material, is currently being developed to produce high power density transducers. A thin-film gold-tin eutectic solder bond has been developed to bond electrically and mechanically bulk piezoelectric elements to microfabricated silicon structures in these devices. A 4.3 [.proportional]m thick multilayer film structure, consisting of a titanium adhesion layer, a platinum diffusion barrier, a gold-tin (80 wt.% Au - 20 wt.% Sn) alloy layer, and a pure gold capping layer, was sputter deposited on the piezoelectric components to be bonded. Bonding was accomplished by mating the piezoelectric components with silicon components metallized with a titanium-platinum-gold multilayer film and heating to approximately 300°C in a reducing atmosphere. The bonding technology allows thin, electrically conductive bonds to be formed between dissimilar materials with minimal amounts of applied pressure during bonding. Successful bonding has been achieved between single crystal silicon and polycrystalline lead-zirconate-titanate (PZT-5H) as well as between silicon and single crystal lead zinc niobate-lead titanate (PZN-PT). The process was optimized to produce mechanically robust, void-free bonds. The absence of voids was verified through scanning electron microscope examinations of bond cross-sections. Tensile tests conducted on representative structures indicated that the strength of the bond was limited by the strength of the titanium – PZT-5H interface.

High power density micro-systems offer the potential to revolutionize technologies for portable electrical power generation, propulsion and flow control. Devices are being designed and fabricated which include micro-gas turbine engines, micro-rocket engines, micro-motor- compressors, micro-pumps and micro-hydraulic transducers. Common to all of this family of devices is the need to create packages that service the devices, and interface them with the macro-scale environment. Fluid interconnections are a particularly demanding packaging element for this class of devices. In order to achieve high power densities, these devices are required to operate at high pressures and, in some cases, high temperatures. This paper describes the design, analysis, fabrication and testing of high-pressure, high temperature fluid connections for the micro-engine and micro-rocket applications. A glass bonding technology has been developed to allow the creation of multiple fluidic connections consisting of Ni/Fe alloy tubes to silicon devices. Key strategies to achieve high strength connections are: to minimize the mismatch in coefficients of thermal expansion between the components, to eliminate voids from the glass and to promote adequate wetting of the glass to both the tubes and the silicon. Mechanical test results are presented which correlate the strength and statistical reliability of such bonds to the processing conditions, choice of glass and surface preparation prior to bonding. Successful examples of packaged micro-engine and micro-rocket devices are presented.

This paper presents residual stress characterization and fracture analysis of thick silane based PECVD oxide films. The motivation for this work is to elucidate the factors contributing to residual stress, deformation and fracture of oxide films so as to refine the fabrication process for power MEMS. It is shown that residual stress in oxide films strongly depended on thermal processing history. Dissolved gases were found to play an important role in governing intrinsic stress. The tendency to form cracks is a strong function of film thickness and annealing temperature. Mixed mode fracture mechanics was applied to predict critical cracking temperature, and there is a fairly good match between theoretical predictions and experimental observations.

The increased number of MEMS devices that are fabricated by bonding two or more bulk micromachined silicon wafers has highlighted the need to produce reliable silicon fusion bonds. The current study focuses on employing a four-point bend delamination specimen to measure silicon fusion bond strength as a function of processing conditions. The specimen, which is composed of two bonded layers and an initial notch, permits the measurement of a mixed-mode critical strain energy rate, GC, at the interface. This specimen geometry is advantageous because it does not require measurement of crack length to calculate the strain energy release rate and is insensitive to damage near the specimen edges. The fact that the interface is loaded under mixed-mode conditions presents difficulties in achieving stable crack propagation in well bonded specimens. Attempts were made to eliminate this problem by reducing the effective bond toughness by etching shallow grooves in the wafer surfaces to reduce the bonded area. Testing revealed that while this approach reduced the effective toughness of the interface, it did not prevent crack deflection in well bonded samples. Despite the limitations of the specimen, data was obtained for silicon fusion bonds fabricated under various annealing and contacting conditions. Test results indicate an increase in bond toughness with annealing temperature and time. The data also suggests that the contacting pressure and duration have little effect on bond quality. The specimen, while limited to characterizing bonds with lower toughness, proved straightforward to fabricate and test.

Low temperature, wafer-level bonding offers several advantages in MEMS packaging, such as device protection during aggressive processing/handling and the possibility of vacuum sealing. Although thermocompression bonding can be achieved with a variety of metals, gold is often preferred because of its acceptance in die bonding [1] and its resistance to oxidation. This study demonstrates that the simultaneous application of moderate pressure (0.5 MPa) and temperature (300°C) produces strong wafer-level bonds. A four-point benddelamination technique was utilized to quantify bond toughness. Test specimens exhibited constant load versus displacement behavior during steady state crack propagation. Two distinct fracture modes were observed: cohesive failure within the Au and adhesive failure at the Ti-Si interface. The strain energy release rate for Au-Au fracture was found to be higher than that associated with Ti-Si fracture, consistent with the greater plastic deformation that occurs in the metal during fracture.

A research and development program is underway to develop technology for a MEMS-based microgas turbine engine. The thermodynamic requirements of power-generating turbomachinery drive the design towards high rotational speeds and high temperatures. To achieve the specified performance requires materials with high specific strength and creep resistance at elevated temperatures. The thermal and mechanical properties of silicon carbide make it an attractive candidate for such an application. Silicon carbide as well as silicon-silicon carbide hybrid structures are being designed and fabricated utilizing chemical vapor deposition of relatively thick silicon carbide layers (10–100 μm) over time multiplexed deep etched silicon molds. The silicon can be selectively dissolved away to yield high aspect ratio silicon carbide structures with features that are hundreds of microns tall.

Research has been performed to characterize the capabilities of this process. Specimens obtained to date show very good conformality and step coverage with a fine (≈0.1 μm dia.) columnar microstructure. Surface roughness (Rq) of the films is on the order of 100 nm, becoming rougher with thicker deposition. Residual stress limits the achievable thickness, as the strain energy contained within the compressive film increases its susceptibility to cracking. Room temperature biaxial mechanical testing of CVD silicon carbide exhibits a reference strength of 724 MPa with a Weibull modulus, m =16.0.

The development of a high power-density micro-gas turbine engine is currently underway at MIT. The initial goal is to produce the components by deep reactive ion etching (DRIE) single crystal silicon. The capability of the silicon structures to withstand the very high stress levels within the engine limits the performance of the device. This capability is determined by the material strength and by the achievable fillet radii at the root of turbine blades and other etched features rotating at high speeds. These factors are strongly dependent on the DRIE parameters. Etching conditions that yield large fillet radii and good surface quality are desirable from a mechanical standpoint. In order to identify optimal DRIE conditions, a mechanical testing program has been implemented. The designed experiment involves a matrix of 55 silicon wafers with radiused hub flexure specimens etched under different DRIE conditions. The resulting fracture strengths were determined through mechanical testing, while SEM analysis was used to characterize the corresponding fillet radii. The test results will provide the basis for process optimization of micro-turbomachinery fabrication and play an important role in the overall engine redesign.

The development of power MEMS, such as the Microengine being developed at MIT, requires highly stressed structures to achieve high power densities. Material strength is, therefore, a critical issue for the design of such devices. Due to the stochastic nature of the strength of brittle materials, the length scales of the test specimens should be close to those of the structure in order to avoid excessive extrapolation of the test data. In this paper, strength characterization and supporting analysis of mesoscale biaxial flexure and radiused hub flexure single crystal silicon specimens are presented. The Weibull reference strength' of planar biaxial flexure specimens was found to lie in the range 1.2 to 4.6 GPa, depending on the surface quality. The local strength at stress concentrations was obtained by testing radiused hub flexure specimens. For the case of deep reactive ion etched (DRIE) specimens, the strength at fillet radii was found to be significantly lower than that measured from planar biaxial flexure specimens due to the inferior surface quality in such regions. It was found that strength could be significantly increased by the introduction of an additional isotropic etch after the DRIE step. The test results reported herein have important implications for the development of highly stressed microfabricated structures. The sensitivity of the mechanical strength to surface processing and etching techniques must be accounted for in the design cycle, particular with regard to the selection of the appropriate fabrication route. Furthermore, in the design of highly stressed MEMS devices, it is important to account for the stochastic nature of the material strength.