Recent studies by this author and others has demonstrated that cavitation-erosion of NiTi coatings or bulk NiTi is exceptional. Studies were undertaken to ascertain whether this property is a consequence of either the general intermetallic properties of NiTi or by an adaptive stress-dissipation mechanism of the cavitation-generated shock wave by A microstructural mechanism related to the shape memory effect.

In cavitation, an oscillating pressure field causes the formation and implosion of air bubbles. As a surface easily nucleates bubbles, the subsequent implosion of the bubbles generates stresses approaching several MPa, which are large enough to ablate material, and are also high enough to generate stress-induced Martensite or Austenite, depending on whether the applied stress is tensile or compressive. The implication is that the stress wave may be partially accommodated by the stress-induced transformation, which can dissipate the energy as heat on retransformation to the materials unstressed phase.

Calculations concerning the variation of the cavitation-induced stresses and temperature distribution with depth into the TiNi coupled with the associated problems of heat transfer will be presented. It will be shown that an adaptive mechanism is plausible.

The B2 ⌊ R ⌊ B19′ phase transformations in a Ni-50.7 at.% Ti alloy were investigated under a series of uniaxial tensile stresses. A custom-build X-ray sample holder with hot, cold and stressing stages was used for structure analysis at temperatures up to 140 °C under a variety of tensile stresses (up to 250 MPa). Reorientation and growth of favorably oriented variant domains of R phase and martensite under stress were observed. In the stress-assisted phase transformations, the favorable (020)M and (300)R planes are expected to align preferably parallel to (011)B2 planes, and the unfavorable (111)M and (112)R planes perpendicular to the (011)B2 planes. In addition, the results of semi-quantitative analyses of the relative amounts of parent phase and martensite formed under a specific stress and temperature condition, x=x(σ,T), are presented.

It is shown that quite generally there is a correlation between insulator-metal transition pressures and indentation hardness numbers for covalent crystals. They are numerically equal for the Group IV elements. Three elements plus five III–V compounds acquire the β-Sn structure upon metallizing. Bondbending, rather than bond-shortening, appears to lead to their transformations. A simple version of the theory of covalent bonds gives the critical transformation bond-angle correctly.

Micromechanical structures designed for material characterization through analysis of their nonlinear dynamic response are presented. The structures consist of a rigid movable mass supported by beams which are attached to the wafer substrate. The structures are designed so that they are geometrically constrained, which is the source of their nonlinearity. The nonlinearity is shown to be well modeled by Duffing's equation for a stiffening spring and it is this model which is used to fit the test data to the desired mechanical properties, namely Young's modulus, intrinsic stress and damping.

We extend the method of Najafi and Suzuki [1] for the electronic determination of the modulus of elasticity and intrinsic stress of thin films using capacitive bridge structures. New theoretical concepts are introduced. The extended method does not require that the test structures be exercised to the point of snap-action.

In this work a blister test is applied to study the adhesion of thin films to substrates. In the blister test one injects a fluid at constant rate at the interface between the substrate and an overlayer to create a “blister”. The fluid pressure is measured as function of time. An analysis gives a reliable way of calculating the adhesion energy Ga. from the time-dependent pressure data. The method was applied to a variety of systems including polymer/polymer, polymer/silicon and polymer/metal interfaces. The results show that the test is very sensitive and is able to determine small adhesion energies inaccessible in conventional peel tests. This work demonstrates that the blister test provides a means of relating the mechanical strength of an interface to its microscopic dynamic and structural features.

Curvature measurements are generally employed in conjunction with elementary structural analysis to estimate deposition stresses in miniaturized electro-mechanical systems. In this paper the validity of this procedure is discussed by presenting a closed form solution for a bilayer subject to nonuniform intrinsic straining, and comparing the exact stress-curvature relations with the oft-used formulae of Stoney and Brenner-Senderoff.

A potentiometric method for the measurement of ultimate strain of thin films is presented. In this method, an electric potential is applied between two electrodes located one on each side of the thin film under investigation. The electrodes are immersed in an electrolytic solution. The thin film acts as an electrical current barrier. To determine the ultimate strain, a controlled load is applied to the film. Cracking of the film causes a sharp rise in the current from an initial small leakage value. The applied load at the onset of cracking is used to calculate the ultimate strain.

we have previously demonstrated the feasibility of the method for thin silicon nitride films deposited on aluminum strips. The method is very sensitive and can detect cracks too small to be observed with a microscope. We discuss extension of the method to the measurement of the ultimate strain of micromachined membranes. The load is applied by pressurizing one side of the membrane. Micromachined structures are used to determine residual stresses.

A newly theoretical calculation for the Young's modulus Ey of poly-Si and a-Si thin films based on the combination of grain and grain boundary effects as well as the dependance of crystalline orientations is presented. The calculated results are in agreement with the experimental results in a wide range of grain size and hydrogen concentrations published in the literatures. The reason for aberration among experimental data of poly-Si and a-Si films caused by different hydrogen concentrations, texture and grain size has been discussed. The results offer a better understanding of. the effects of film structures on elastic properties of poly-Si and a-Si films.

In order to integrate micro actuators with III-V semiconductor devices, we have devised Micro-Electro-Mechanical devices (MEM's), constructed from materials and processes common to existing III-V device processing. These processes are substantially different from silicon based processes because of the requirements for low temperature processing and the use of gold-based metallizations.

Our material choices include, vacuum deposited and plated metal films, silicon oxide and nitride dielectric layers, and polyimide layers and structures. Sacrificial layers are implemented with photoresist rather than the more common silicon dioxide. The processes available are based on the ‘lift off’ of unwanted areas of the metal films, wet plating of metals through openings in photoresist masks, and wet and plasma etching of metals and dielectrics.

This paper will discuss why we are using these materials, the process constraints imposed by the materials, the measurement of some of the material properties, and will relate some progress in applications.

The new technique of scanned force microscopy which enables imaging surface features with sub nanometre resolution has been made possible by the development of highly sensitive, hysteresis free force sensing cantilevers and the availability extremely sharp probing tips. Such cantilevers with integral tips can be micromachined using IC compatible technology, and several processes have been described in literature for their fabrication. These are based on different etching schemes, and require two or more masking stages. A novel process using a single mask is described here for the fabrication of single crystal silicon cantilevers with integral sensing tips. The cantilever thickness can be tailored to provide a range of force constants and resonant frequencies, and the tip profile can be varied from pyramidal to highly cusped. As only a single mask is used in the fabrication, there are no mask alignment errors and precise location of the tip is thereby achieved. This eliminates any twist in the cantilever during scanning which could give rise to distorted imaging. The complete fabrication process and the mask design is described together with SEM photographs of the first batch of devices, which have been evaluated by retrofitting to a commercial atomic force microscope.

We are investigating organometallic materials which exhibit a novel fieldinduced reversible switching phenomenon. Silver and copper salts of tetracyanoquinodimethane (TCNQ) and its derivatives, prepared as polycrystalline thick films and vacuum-deposited thin films, have been imaged using a scanning tunneling microscope (STM). We have recently produced images with molecular resolution of these materials. We also have demonstrated a field-induced, charge-transfer reaction driven by the electric field at the STM tip when the field generated by the STM exceeds the switching threshold of the organic charge-transfer complex. The phase transition induced by the STM tip appears as nanometer to micrometer-sized domains on the metal-TCNQ and derivatives surface. This STM-induced chemical reaction is being explored for use in a molecular-based information storage system.

Microheater made of heavily Boron doped single crystal Si beam covered with SiO2 film, 1000×300×3 μm, is fabricated on the n type Si substrate by the anisotropic etching technique. As this microheater has an air bridge structure of low resistivity semiconductor material with positive but small temperature coefficient of resistance, a broad heating area up to 800 °C is easily obtained and it has quick response with the thermal time constant t of about 4 ms and has small power consumption. Since this heating area is made of p type layer in the n type substrate,this area can be electrically isolated from the substrate because of the formation of p-n junction.

A microheater was fabricated on a thermally isolated silicon (Si) membrane and its electrical and thermomechanical properties were assessed. The sensor design was adjusted for good thermal isolation and good resistance to thermal shock. The heater was a platinum (Pt) thin-film meandering filament deposited with electron beam evaporation and patterned with lift-off methods. In order to operate the heater at high temperatures with minimum loss of Pt by evaporation, the heater was encapsulated with a ceramic film. During the heater operation, most of the heat was dissipated via conduction through the Si membrane which supported the multi-layered heater. When the Si membrane was completely etched away to suspend the heater, the power needed to heat the filament to strong incandescence (about 700–800°C) was 110 mW. Structural failure was not observed when the device was subjected to thermocycling up to approximately 500°C. The importance of the thermomechanical match of layers in a multi-layered heater structure, especially for use at high temperatures, is addressed.

Refractory metal silicides, such as tungsten silicide (WSix), have been used for integrated circuit interconnect and self-aligned MESFET gates because of their low resistivity and thermal and chemical stability. These same characteristics make refractory metal silicides interesting materials for prospective use in micromechanical structures. However, little information on residual stresses, elastic moduli, or other micromechanical properties has been available for refractory metal silicide films.

This paper presents morphology and stress characteristics of sputter-deposited tungsten silicide films, including orientation-dependent variations, as functions of deposition parameters. The biaxial elastic modulus and thermal coefficient of expansion are found for our sputtered films. Stress-measurement methods and annealing are discussed. Released diaphragms of different sizes and shapes, having controlled residual stress, have been fabricated.

The piezoresistive effect of materials is used as the basis for many types of microsensors. Polyimide, a material widely used in microelectronic fabrication, may be made to exhibit this effect by addition of small graphite particles to form a composite material. Polyimide / graphite based piezoresistive films have the advantage of being spin-castable, plasma-processable, highly chemically resistant, and thermally stable up to 400 °C in nitrogen atmospheres. In this work, piezoresistive polyimide films are formed by addition of various amounts (loadings) of graphite particles one micron in diameter or less to DuPont PI-2555 polyimide. Thin films of these materials are spin-cast on silicon wafers, and an in-situ load-deflection measurement technique is used to evaluate the following film properties: piezoresistive coefficient as a function of both strain and graphite loading; Young's modulus as a function of graphite loading; and residual film stress as a function of graphite loading. The observed piezoresistive coefficient is a strong function of graphite loading, with good piezoresistive properties exhibited in the loading range of 15–25 wt% graphite.

The key step in producing a movable microstructure is the controllable sacrificial layer etching process. For the current study, a 1 μm silicon-rich silicon nitride structural layer was deposited on a 2 μm phosphosilicate glass (PSG) sacrificial layer that had been deposited on a single-crystal silicon wafer. Silicon nitride, unlike the more common polycrystalline silicon (poly-Si) structural material, is transparent and, therefore, allows the observation of an etch front in the underlying PSG sacrificial layer. The PSG is an amorphous silicon dioxide, with phosphorus added as a controlled impurity, that can be selectively etched in hydrofluoric acid. Several experiments have been performed with four HF concentrations using these test structures. Diffusion limited etching is observed at long etching times for each concentration. Additional experimental work has been done to determine appropriate kinetic expressions for the PSG/HF reactions.

The sacrificial layer etching process has been modeled as a chemical reaction/diffusion system. Constants for the kinetic expression have been found from independent experimental work. This Deal-Grove type model assumes isothermal conditions, constant density solution, onedimensional etching, and the equality of the diffusion and reaction fluxes. The model calculates the HF surface concentration and the length of PSG underetched as a function of time. It fits the data well but requires the use of an unphysical diffusion coefficient. An extension of the Deal-Grove model to nonfirst order kinetics allows for a reasonable estimate of the reactant/product mixture diffusivity in water to be used when fitting the model to experimental data.

We developed designs in which hollow porous fibers filled with chemicals release them into a matrix over time. A coating covering the fibers is degraded by chemicals, pH change, or broken by structural loading, thus, releasing the chemicals. These stimuli for release are the very agents of environmental attack.

The design to alleviate cracking consists of hollow porous fiberglass fibers containing crack-closing chemicals. The chemicals are released from the fibers when the outer sheath cracks or the fibers flex due to loading. This is the ideal situation in which the agent of environmental degradation, namely loading, is the stimulus to release the repair chemical.

The design to prevent corrosion consists of an anticorrosion chemical in hollow porous polypropylene fibers. Change in chloride ion concentration dissolves the polyol coating and allows the chemical to be released from the fiber wall. The cause of deterioration, change in chloride level which causes corrosion, is the sensor (coating deterioration) and also the activator for the remedial or preventive action (release of anticorrosion chemical).

These are distributed systems responding to environmental stimuli for sensing and repairing when and where they are needed. The control of the amount of chemical released is being researched mainly as a function of coating design and also fiber type.