Nondestructive determination of Young’s modulus, coefficient of thermal expansion, Poisson ratio, and thickness of a thin film has long been a difficult but important issue as the film of micrometer order thick might behave differently from that in the bulk state. In this paper, we have successfully demonstrated the capability of determining all these four parameters at one time. This novel method includes use of the digital phase-shifting reflection moire´ (DPRM) technique to record the slope of wafer warpage under temperature drop condition. In the experiment, 1-um thick aluminum was sputtered on a 6-in silicon wafer. The convolution relationship between the measured data and the mechanical properties was constructed numerically using the conventional 3D finite element code. The genetic algorithm (GA) was adopted as the searching tool for search of the optimal mechanical properties of the film. It was found that the determined data for Young’s modulus (E), Coefficient of Thermal Expansion (CTE), Poisson ratio (ν), and thickness (h) of the 1.00 um thick aluminum film were 104.2Gpa, 38.0 ppm/°C, 0.38, and 0.98 um, respectively, whereas that in the bulk state were measured to be E=71.4 Gpa, CTE=23.0 ppm/°C, and ν=0.34. The significantly larger values on the Young’s modulus and the coefficient of thermal expansion determined by this method might be attributed to the smaller dislocation density due to the thin dimension and formation of the 5-nm layer of Al2O3 formed on top of the 1-um thick sputtered film. The Young’s Modulus and the Poisson ratio of this nano-scale Al2O3 film were then determined. Their values are consistent with the physical intuition of the microstructure.
A Michelson-type, whole-field residual stress measurement method has been set up. From this method, two-dimensional out-of-plane partial slope contours of optical thin films were obtained. The linear and the rotational mismatch techniques in the experiment help to improve the sensitivity of measurement and determine the signs of residual stress. Whether aluminum film, the most popular material used in reflective optical components, is coated with protective layers (SiO or MgF2) or not, exposure to the background humidity is unavoidable because moisture penetrates into aluminum film. In this paper, we point out that the residual stress of aluminum film changes drastically because of moisture absorption. This highly sensitive residual stress measurement method also shows that protective layers cannot prevent moisture penetration.
With the rapid trend for miniaturization of electronic products, new optical techniques are increasingly needed for measuring the surface profile or deflection/warpage in the micro-scale for electronic packages mounted on printed circuit boards. In this paper, we reduced the grating spacing of the digital projection moire´ (DPM) by directing the grating pattern into a stereo zoom microscope and performed surface profile measurement under this microscope. In this method, the reference grating is generated digitally having sinusoidal intensity. Another digitally generated grating is projected by the means of a digital light processing (DLP) projector and a set of carefully arranged optical lens into the microscope to form the object grating. The pitch of micro-scaled object gratings can be adjusted by the reduction ratio of the microscope. As a result, this micro-scaled digital projection moire´ method produces micro object gratings on the order of 10-um pitch and is suitable for surface profile measurement in a square dimension on the order of 100-um. The method of linear mismatch is utilized to obtain more fringes in each measurement and the guideline to achieve the optimal degree of linear mismatch between the reference and the object gratings is proposed. In addition, the phase shifting technique is employed to extract the data between the recorded fringes. Verification of the method is demonstrated by measuring an inclined plane of a micro prism. The deviation between the measured data and the given values was found to be less than 5%, which demonstrates the validity of the developed method.
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