Haruna Tada

Thermal expansion coefficient of polycrystalline silicon and silicon dioxide thin films at high temperatures

The rapid growth of microelectromechanical systems (MEMS) industry has introduced a need for the characterization of thin film properties at all temperatures encountered during fabrication and application of the devices. A technique was developed to use MEMS test structures for the determination of the difference in thermal expansion coefficients (α) between poly-Si and SiO2 thin films at high temperatures. The test structure consists of multilayered cantilever beams, fabricated using standard photolithography techniques. An apparatus was developed to measure the thermally induced curvature of beams at high temperatures using imaging techniques. The curvatures measured were compared to the numerical model for multilayered beam curvature. The model accounts for the variation in thermomechanical properties with temperature. The beams were designed so that the values of Young’s moduli had negligible effect on beam curvature; therefore, values from literature were used for ESi and ESiO2 without introducing si...

Effects of a butterfly scale microstructure on the iridescent color observed at different angles

Multilayer thin-film structures in butterfly wing scales produce a colorful iridescence from reflected sunlight. Because of optical phenomena, changes in the angle of incidence of light and the viewing angle of an observer result in shifts in the color of butterfly wings. Colors ranging from green to purple, which are due to nonplanar specular reflection, can be observed on Papilio blumei iridescent scales. This refers to a phenomenon in which the curved surface patterns in the thin-film structure cause the specular component of the reflected light to be directed at various angles while affecting the spectral reflectivity at the same time by changing the optical path length through the structure. We determined the spectral reflectivities of P. blumei iridescent scales numerically by using models of a butterfly scale microstructure and experimentally by using a microscale-reflectance spectrometer. The numerical models accurately predict the shifts in spectral reflectivity observed experimentally.

Evaluating the effects of thin film patterns on the temperature distribution of silicon wafers during radiant processing

A numerical model was developed to find the temperature distributions during radiant heating of a silicon wafer with SiO2 thin film patterns. The radiative properties of silicon and the film structure were found by considering the effects of partial transparency and thin film interference. The average total properties over simple patterns with feature sizes of the order of a few micrometers were found, using an average of the properties of each region within the pattern, weighted by their relative areas. In general, wafers with a single SiO2 film or pattern reach a higher steady state temperature than a plain Si wafer due to higher total absorptivity. This applies to thin films of any thickness below several micrometers, where coherent effects are dominant. The temperature of patterned wafers vary nonlinearly with film thickness, with the highest temperature discrepancy from Si wafer occurring at film thickness of ~0.2 ?m. For wafers with complex patterns, the temperature distributions can be estimated by the average of temperatures for simpler patterns, weighted by their respective areas. Due to limitations in the computational domain, the radiative processing of 3-in. wafers was modeled; however, results were confirmed for the 12-in. wafer for limited cases.

Novel imaging system for measuring microscale curvatures at high temperatures

An innovative system was designed to optically measure the curvature of microelectromechanical system at high temperatures. The system takes advantage of the limited numerical aperture of the imaging system to detect the curvature of cantilever beams. Images of the beam are used to determine beam curvature at high temperatures of up to 850 °C by analyzing the apparent change in beam length as seen by the camera during an experimental trial. The system is designed to operate at very high temperatures, which is difficult in conventional microscale curvature measurement techniques such as scanning electron microscopy or stylus profilometry due to excess heating of peripheral equipment. The system can measure curvatures as small as 300 m−1, which corresponds to tip deflections of 1.5 μm for a 100 μm beam. The resolution of the system is limited by the image resolution of the charge-coupled device camera, and increases at large curvatures. The maximum curvature that can be measured by the system is limited by ...

Determining the High-temperature Properties of Thin Films Using Bilayered Cantilevers

High-temperature applications of microelectromechanical systems (MEMS), especially in new temperature sensor designs, require an accurate knowledge of the temperature-dependent thermophysical properties of the materials. Although the measurement of the mechanical properties of materials at room temperature has been widely conducted, the same techniques often cannot be used for high-temperature property measurements. In this study, a new technique was developed to find the thermal expansion coefficient of thin films at high temperatures. Bilayered cantilever beams undergo thermally induced deflection at high temperatures, which can be measured and correlated to material properties. An imaging system was developed for the experimental measurement of the beam curvature for temperatures up to 1000°C. To find the high-temperature property of thin films, a bilayered beam, consisting of polycrystalline silicon and silicon dioxide, was designed such that the change in the property of SiO 2 had little effect on the curvature of the beam. Furthermore, numerical analysis showed that the Youngs modulus of Si also had negligible effect on the curvature. Therefore, the analytical model for beam curvature was simplified to be only a function of the thermal expansion coefficient of Si layer. Using this model, the thermal expansion coefficient of polycrystalline Si film was determined for temperature range between room temperature and 1000°C. The method can be easily modified to find the Youngs modulus of Si, as well as properties of SiO 2 .

Temperature-Dependent Coefficient of Thermal Expansion of Silicon Nitride Films Used in Microelectromechanical Systems

Microelectromechanical systems (MEMS) have potential application in high temperature environments such as in thermal processing of microelectronics. The MEMS designs require an accurate knowledge of the temperature dependent thermomechanical properties of the materials. Techniques used at room temperature often cannot be used for high-temperature property measurements. MEMS test structures have been developed in conjunction with a novel imaging apparatus designed to measure either the modulus of elasticity or thermal expansion coefficient of thin films at high temperatures. The MEMS test structure is the common bi-layered cantilever beam which undergoes thermally induced deflection at high temperatures. An individual cantilever beam on the order of 100 νm long can be viewed up to approximately 800°C. With image analysis, the curvature of the beam can be determined; and then the difference in coefficient of thermal expansion between the two layers can be determined using numerical modeling. The results of studying silicon nitride films on silicon oxide are presented for a range of temperatures.

Mechanical and Thermophysical Properties of Silicon Nitride Thin Films at High Temperatures Using In-Situ Mems Temperature Sensors

The optimization of microelectronic devices and Microelectromechanical Systems (MEMS) technology depends on the knowledge of the mechanical and thermophysical properties of the thin film materials used to fabricate them. The thickness, stoichiometry, structure and thermal history can affect the properties of thin films causing their mechanical and thermophysical properties to diverge from bulk values. Moreover, it is known that the mechanical and thermophysical properties of thin films vary considerably at different temperatures. Bulk properties of semiconductors have been characterized over a wide range of temperatures; however there is limited information on thin film properties of silicon-based compounds such as silicon nitride, specially at high temperatures. In our work, MEMS devices designed to record the localized maximum temperature during high temperature thermal processes, which we call Breaking T-MEMS, will be presented as a way to determine some of the mechanical properties (Youngs modulus and fracture strength) and thermophysical properties (coefficient of thermal expansion) of silicon-rich nitride thin films at high temperatures. The Breaking T-MEMS device consists of a thin film bridge suspended over a substrate. During testing, the devices are thermally loaded in tension by heating the sample. The low coefficient of thermal expansion of the film relative to that of the substrate causes the thin film bridge to break at a specific temperature. Through a combination of indirect experimental measurements, analytical expressions, numerical and statistical analysis, and if the experiments are conducted using at least two different substrates of known temperaturedependent coefficients of thermal expansion, some of the material properties of the film can be calculated from the breaking temperatures of various devices. The two candidate materials for the substrate are silicon and aluminum oxide (sapphire).

MEMS-Based Microcalorimeter for Liquid Samples

Advances in MEMS technology has enabled the development of microcalorimeters that have significant improvement in resolution over conventional calorimeters and require smaller samples. However, microcalorimeters have yet to accomplish the variety of tasks that are done by conventional calorimeters, such as study of the thermal properties of polymers and proteins that are typically in liquious state. These applications are especially important in the biomedical field, where combinatorial approaches are used to test a large number of variations in biomaterials. We present a design for a MEMS microcalorimeter that can be applied for a variety of liquid samples. The basic design of the microcalorimeter consists of low stress silicon nitride thin film with nickel resistive heater and thermometer. The heater and thermometer are thermally isolated from each other so that the thermometer accurately measures the temperature of the sample. The silicon nitride film containing the device is suspended over silicon substrate to achieve thermal isolation. Modulation calorimetry technique is used to determine the specific heat of the sample based on the temperature response of the sample when subjected to an AC-modulated heat source. A numerical model was developed to model the thermal behavior of the device. Initial numerical studies found that the device operates optimally at low frequencies, where with appropriate corrections, the device can yield values of heat capacity that are within one percent of the actual value.Copyright