Takayuki Shibata

Ductile-regime turning mechanism of single-crystal silicon

Abstract Diamond turning of single-crystal silicon was carried out along all the crystallographic directions on the (001) and (111) planes at depths of cut of 100 nm and 1 μm, and then the mechanism involved in ductile-regime turning was studied. Pitting damage occurred along certain specific crystallographic orientations. The crystallographic orientation dependence of the surface features also changed with the depth of cut because of the difference in material removal mechanism between plastic deformation and brittle fracture. Transmission electron microscopy characterizations of the (111)(110) slip systems activated by turning revealed that the orientation dependence of the surface features was closely related to the ease with which slip deformation occurred. To predict the surface features turned along each crystallographic orientation, we proposed the slip orientation factor, which was determined from the Schmid factor, and demonstrated that it was useful for turning in the critical regime between ductile and brittle. Our slip model, based on the slip orientation factor, describes the ductile-regime turning mechanism well.

Cross‐section transmission electron microscope observations of diamond‐turned single‐crystal Si surfaces

Diamond turning was carried out along the [110] direction on the (001) plane of single‐crystal silicons at extremely small depths of cut of 100 and 500 nm. Cross‐section transmission electron microscope observations revealed that turning had converted the layers of the surface into an amorphous structure directly, as well as in continuous chips. The thickness of the layers was about 150 nm in spite of the depth of cut. Under the amorphous layer, another damaged layer with many dislocations was formed, the thickness of which was about 2 and 3 μm at 100‐ and 500‐nm cutting depth, respectively. At 500‐nm cutting depth, microcracks were formed through the accumulation of excessive dislocations. At both depths of cut, however, the dislocations were mostly oriented along the 〈110〉 directions within the {111} planes. It was found that ductile‐regime turning could be achieved by amorphization and deformation based on the {111}〈110〉 slip systems.

Micromachining of TiNi shape memory thin film for fabrication of micropump

Abstract In order to develop a micropump driven by shape memory actuation, we require a TiNi diaphragm structure with a cap to act as a chamber for applying bias pressure to the diaphragm. With the purpose of realizing such a structure, we studied the photoetching of TiNi thin film on a Si substrate and two bonding processes — diffusion bonding and anodic bonding — for patterning and assembling. TiNi thin film deposited on Si substrates by flash evaporation was etched in HF/HNO 3 /H 2 O solutions using negative photoresist masks. HF:HNO 3 :H 2 O=1:1:4 solution proved capable of etching it at a rate of about 30 nm/s without etching of the Si substrate. Patterned TiNi thin film of 6 μm in thickness on a Si substrate was diffusion-bonded to another Si substrate coated with the same TiNi thin film at a thickness of 300 nm. Bonding was conducted in a vacuum at a bonding pressure of 210 MPa. TiNi–TiNi diffusion bonding was obtained at temperatures of more than 300°C. A four-point bending test revealed that the bond strength of specimens bonded at 400°C was 15–20 MPa. Anodic bonding was conducted between TiNi thin film on a Si substrate and a Pyrex 7740 glass substrate at an applied voltage of 600 V. Two substrates were bonded in anti-oxidizing ambient at temperatures of more than 350°C, giving a bond strength of about 15–25 MPa.

Flash evaporation of TiNi shape memory thin film for microactuators

Abstract The flash evaporation, with which material for deposition is repeatedly evaporated in very small volumes, was investigated for formation of TiNi shape memory alloy thin film. Along with the flash method itself, the timing for opening the shutter proved a crucial factor in controlling thin film composition. Using our evaporation system, a thin film with a composition of around 50 at.% Ti-50 at.% Ni was obtained with an interval of about 5 s between the beginning of material evaporation and opening of the shutter. The deposited thin film had a lamella structure, corresponding to repeated deposition cycles, and showed small fluctuations in alloy composition in each deposition cycle. However, these effects on its shape memory properties could be considered tolerable. The deposited thin film showed martensitic (monoclinic structure) and reverse martensitic (B2 parent structure) transformation during cooling and heating cycles, respectively. In particular, thin film with a composition of 45–50 at.% Ni showed martensitic transformation at near room temperature and reverse martensitic transformation at about 70°C. This means that such thin film could be used as material for microactuators driven at room temperature without requiring any special cooling device.

Micromachining of diamond film for MEMS applications

We realized two diamond microdevices: a movable diamond microgripper and a diamond probe for an atomic force microscope (AFM), consisting of a V-shaped diamond cantilever and a pyramidal diamond tip, using a microfabrication technique employing semiconductive chemical-vapor-deposited diamond thin film. The microgripper was fabricated by patterning the diamond thin film onto a sacrificial SiO/sub 2/ layer by selective deposition and releasing the movable parts by sacrificial layer etching. The diamond AFM probe was fabricated by combining selective deposition for patterning a diamond cantilever with a mold technique on an Si substrate for producing a pyramidal diamond tip. The cantilever was then released by removing the substrate. We report the initial results obtained in AFM measurements taken using the fabricated diamond probe. These results indicate that this diamond probe is capable of measuring AFM images. In addition, we have developed the anodic bonding of diamond thin film to glass using Al or Ti film as an intermediate layer for assembly. This bonding technique will allow diamond microstructures to be used in many novel applications for microelectromechanical systems.

Dynamic actuation properties of TiNi shape memory diaphragm

Abstract In order to realize a micropump with a shape memory alloy (SMA) diaphragm actuator, TiNi thin film of about 7 μm in thickness was deposited by flash evaporation and its dynamic deformation–shape recovery properties were studied using a bulge test. The TiNi diaphragm was deformed by applying a gas pressure of 200 kPa, heated resistively to recover its initial flat shape, and then air-cooled to achieve deformation once again. During this thermal cycle, temperature and deflection of the diaphragm were monitored at its center. In order to monitor temperature, we fabricated a Cu–Ni micro thermocouple on the diaphragm by conventional evaporation. When the diaphragm was heated, shape recovery occurred at about 60°C. This continued after the temperature for termination of reverse martensitic transformation, Af (about 70°C), had been reached. Moreover, when the diaphragm was air-cooled, redeformation began even at temperatures higher than that for the commencement of martensitic transformation, Ms (about 60°C). From a FEM simulation and temperature measurements taken using thermography, these results could be explained by the temperature gradient formed in the diaphragm due to thermal conduction. When heating rate was increased, time required to complete shape recovery decreased and maximum displacement for shape recovery increased. This could also be explained in terms of the temperature gradient.

Dynamic thermo-mechanical properties of evaporated TiNi shape memory thin film

Abstract The shape recovery and re-deflection responses of shape memory alloy (SMA) thin film to thermal cycles were investigated using the bulge method. It was deposited by flash evaporation and had a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6 μm. After being released from silicon substrates and undergoing vacuum-annealing to obtain memorisation of an initial flat shape, it was deformed into a cap shape of 5 mm in diameter by pressurisation at 400 kPa. Then, by applying 100 ms voltage pulses, its initial shape was recovered by resistive heating at various energies. During these shape recovery and re-deflection cycles, change in displacement with time was measured continuously using a laser displacement meter. The thin film exhibited shape recovery at energies for heating of more than 1 J due to reverse martensitic transformation. Displacement due to shape recovery increased with increasing energy for heating, reaching saturation at around 100 μm at energies of more than 2 J. After heating was completed, the thin film deflected again due to martensitic transformation under pressure. The period for each shape recovery and re-deflection cycle was about 600 ms at an energy of 2.1 J. It exhibited stable shape recovery and re-deflection properties at up to 1000 cycles, which was the maximum number of thermal cycles tested. Finally, the pumping pressures and flow rates which might be expected with such an SMA micropump were also roughly estimated.

Thermo-mechanical properties of TiNi shape memory thin film formed by flash evaporation

Abstract TiNi thin film with a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6 μm was deposited onto silicon substrates by flash evaporation, and then released from those substrates to become free-standing film. After vacuum annealing at 500°C for 60 min so that an initially flat diaphragm could be formed, the thermo-mechanical properties of this thin film were evaluated using the bulge test, which causes deformation by pressurisation. Stress–strain curves revealed that it exhibited shape memory at temperatures of less than 60°C and super elasticity at temperatures of more than 80°C. It also showed shape recovery against pressures of up to at least 500 kPa. Under pressurised conditions, the temperature at termination of martensitic transformation, which causes deflection during cooling, increased with increasing pressure, although it was about 30°C without pressure. On the other hand, the temperature at termination of reverse martensitic transformation, which causes shape recovery during heating, remained almost constant at 75°C, independent of pressure. These two specific temperatures indicated that, under loading conditions, thin film could be driven easily, merely by heating and air cooling. At pressures of more than 100 kPa, slip deformation, which cannot be reversed, appeared and increased with increasing pressure. Moreover, displacement of deflection–recovery decreased during the early stages of the thermal cycle, because of work hardening due to slip deformation. After more than 50 cycles, however, displacement of deflection–recovery became constant, indicating that the thin film could give stable actuation, even under high load conditions.

Micromachining of fine ceramics by photolithography

Abstract The objective of this study is to establish a method of photolithography to enable fine patterns to be realised on ceramic materials. Phosphoric acid (PA) proved capable of etching ceramic materials such as alumina and silicon nitride at etch rates of more than 1 μm/min. With such methods, however, there is the problem of change in acid composition due to condensation. Therefore, we studied two methods aimed at avoiding this: refluxing the vaporised water and the addition of water. Refluxing and water addition proved superior in compensating for vaporised water and in maintaining a constant acid composition during heating. Two photoresist materials—polyimide precursor and cyclised polybutadiene rubber—were both tested for the etching of ceramics in boiling PA. Cyclised polybutadiene rubber showed superior chemical stability, thermal stability, and adhesion to the ceramic substrate.

Micromachining compatible metal patterning technique using localized decomposition of an organometallic compound by laser irradiation

Spin coated palladium acetate film was decomposed locally using argon ion laser irradiation and palladium thin film with a line pattern was formed. We studied the effects of the process parameters, such as laser power density and scan rate, on line pattern features and the basic characteristics of the formed palladium thin film. The film had a fine polycrystalline structure with a grain size of up to 100 nm, and high chemical purity, resulting in a low electrical resistivity of the order of cm. The formed line patterns had a smooth surface and flat features. Their thickness was 12% of that of the coated palladium acetate film. The width of the formed palladium line patterns was 100-200% of the spot size of the laser beam, depending on irradiation conditions, due to thermal conduction. This metal pattern deposition technique is not only very simple and flexible, but is also compatible with micromachining processes, since it is a maskless process and is easy to apply to three dimensional structures.