Thermal Expansion of Low Dielectric Constant Thin Films by High-Resolution X-Ray Reflectivity
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Thermal Expansion of Low Dielectric Constant Thin Films by High-Resolution X-Ray Reflectivity Kazuhiko Omote and Yoshiyasu Ito X-Ray Research Laboratory, Rigaku Corporation, Akishima, Tokyo, 196-8666, JAPAN ABSTRACT By introducing high precision sample alignment technique, repeatability of incident angle to the sample surface for x-ray reflectivity (XRR) measurement is achieved to be within 0.3 arcsec. As a result, film thickness and density are possible to be measured repeatability within 0.03% and density within 0.26%. This accuracy realized to detect very small change of thermal expansion of thin films. The coefficient of thermal expansions (CTE) for porous low-k films deposited by CVD method were measured up to 400°C. The obtained values are in the range from 40 to 80 x10-6 K-1 and they are very large compare to that of copper (16-20 x10-6 K-1). INTRODUCTION In recent years, new low dielectric constant materials have been considerable interest for reducing dielectric constant of interlayer insulators [1]. For introducing such new materials into microelectronic devices, thermal properties of the film is very important, because of the film will undergo the thermal stresses in the fabricating processes of the devices. X-ray reflectivity (XRR) technique is well known to measure density and thickness of thin films very accurately [2, 3]. We are trying to measure density and thickness changing with rising to the film temperature by this technique. However, thermal expansion coefficients of materials are typically in the order of 10-5 K-1 and the changing may be less than one percent, even if the temperature is rising 100°C. Therefore, we need a very accurate XRR measurement for detecting thermal expansion of the film materials. For this purpose, we have developed a high-precision goniometer with high-resolution crystal beam conditioner and analyzer. The repeatability of the measurements has been achieved to be within ±0.3 arcsec. The samples are deposited by CVD method and including very small pores (about one nanometer in diameter). Experiment was carried out in nitrogen atmosphere up to 400°C after preheating for evaporating adsorbed water. The data were corrected at both heating and cooling processes for confirming the film materials was not deteriorated. The density and thickness of the film was determined by the least square fitting for minimizing the residual error between the observed experimental data and XRR calculation. The estimated error of the obtained density and thickness are less than 0.3% and 0.05%, respectively. From the thickness changing of the film, we could estimate Z-axis (normal direction) coefficient of linear thermal expansion (Z-axis CTE), beside from the density changing, we could estimate coefficient of volumetric thermal expansion, independently. We will discuss the relation between obtained Z-axis CTE of the film bound to the substrate and that of freestanding state. In addition, we have measured Z-axis and volumetric CTE of copper film on the silicon substrate for confirming rel
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