Analysis of the bending T-MEMS：t-mems的弯曲分析.doc

Analysis of the bending T-MEMSThe bending structures are heated using the “rapid radiant heater” setup, which consists of a tunsgsten-halogen heat source, sample holder, reflector plates, CCD camera, and fiber optic illuminator (see figure). The W-halogen lamp radiatively heats the sample to up to 1000C. The reflector plates minimize the radiative heat loss from the sample, as well as supress the convective current above the sample which may distort the image seen by the CCD camera. The sample is illuminated by a collimated light source from fiber optics, and is monitored by the CCD camera as they bend. During the heating, temperature is monitored by a type-K thermocouple bonded to a small piece of Si wafer.The consequence of using collimated light source to view curved structure is that only a portion of the beam appear bright as seen by the CCD camera. This is due to the numerical aperture of the CCD (the extent of the “cone” of light entering the camera). The curvature of the beams can be found by geometrically relating the angle of the beam with respect to the collimated light, and the numerical aperture of the camera.Microscale Radiative EffectsMicroscale radiative effects are most prominent when the wavelength of radiation in question is on the same order of magnitude as the thickness of thin-films involved in the process. The T-MEMS use sub-micron thick films, and are subject to microscale effects when heated radiatively. The effect of thin film interference and the resulting change in radiative properties on the temperature distribution on the wafer was analyzed for the bending T-MEMS 1. Thin-film interference in the bending structures result in a highly non-linear spectral radiative properties. The figure below shows the spectral reflectivity for two cases of bending beams: etch depth of 1mm (5) and 2mm (). The figure also shows the normalized blackbody emissive power for a lamp temperature of 700K. The large fluctuations in reflectivity is due to the thin film interference between the two layers of the beam, as well as the cavity underneath the beam. The total reflectivity is found by integrating the product of the blackbody emissive power and the spectral reflectivity. Therefore, one can see that if a large peak in the spectral reflectivity coincides with high blackbody emission, then the result is a high total reflectivity. Similarly, a low spectral reflectivity in the high blackbody emission region of the spectrum will result in a low total refletivity. Because the peaks and valleys of the spectral property fluctuates with minor changes in thin film structure, the total radiative properties changes dramatically as well. Note, for opaque materials, absorptivity = 1 reflectivity (evaluated using blackbody emissive power at lamp temperature), and emissivity = 1 reflectivity (evaluated using blackbody emissive power at wafer temperature). The total radiative properties found by the above method was used to find the steady-state temperature distibution of a wafer. The results are shown in detail in 1 and 2. This analysis for T-MEMS is important for two reasons. First, the T-MEMS are designed to be a mechanically non-intrusive sensor. However, they can be indirectly intrusive by affecting the local temperature on the wafer, thereby causing unnecessary thermal stress. The degree of this effect must be evaluated and analyzed. Second, in order to be used effectively as temperature sensors, T-MEMS must accurately measure the wafer temperature. If the presence of T-MEMS affect the local temperature distribution, then the temperature indicated by the T-MEMS may not accurately represent the temperature of the wafer. A general rule can be assessed from the results from 2: a) temperature distribution on the wafer approaches that of a uniform wafer when the dies are small and far-spaced, and b) temperature uniformity improves when the dies are large and closely packed; however, in this case, the temperature approaches that of a wafer having the property of the die. References1. H. Tada, I. N. Miaoulis, and P. Y. Wong, Numerical Simulation of Radiant Thermal Processing of Bilayer Microcantilevers, American Society of Mechanical Engineers Conference Proceedings, DSC-Vol.66, pp.37-44, 1998.2. H. Tada, A. R. Abramson, I. N. Miaoulis, and P.