Investigation of N 2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films
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Investigation of N2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films H. J. Lee1, E. K. Lin1, W. L. Wu1, B. M. Fanconi1, J. K. Lan2,3, Y. L. Cheng2,3, H. C. Liou4, Y. L. Wang2, M. S. Feng3, and C. G. Chao3 1
National Institute of Standards and Technology, Polymers Division, Gaithersburg, MD, USA Taiwan Semiconductor Manufacturing Co., Hsin-Chu, Taiwan 3 National Chiao-Tung University, Hsin-Chu, Taiwan 4 Semiconductor Fabrication Materials KCI, Dow Corning, USA
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ABSTRACT The density depth profile and chemical bond structure of hydrogen silsesquioxane (HSQ) thin films treated with an N2 plasma with varying power and exposure time were measured using specular x-ray reflectivity (SXR) and Fourier transform infrared (FTIR) spectroscopy. The SXR data indicate that the density profile of an untreated HSQ film is not uniform and four layers with different electron densities were required to fit the SXR data. For HSQ films treated with either increasing plasma power or plasma exposure time, the film roughness increased and a densified layer was observed at the film/air interface. The thickness of the densified layer increased with both plasma power and plasma exposure time. As a result, up to seven distinct layers were used to model the experimental SXR data from plasma treated films. The FTIR data show that the plasma transforms the Si−H bonds in the HSQ film into Si−O bonds leaving more oxygen atoms around a Si atom. These data are also consistent with the densification observed in the SXR measurements. In general, the HSQ film is more sensitive to increasing plasma power than to increasing plasma exposure time. INTRODUCTION Materials with low dielectric constants (low-k) have attracted much interest because they can reduce RC delay, which becomes a limiting factor in improving device performance. Among low-k materials, siloxane-based hydrogen silsesquioxane (HSQ) resins have been received much attention because of its potential compatibility with conventional Si technology. In addition, new processing steps for HSQ are under development to address several important technical problems such as copper diffusion, the leakage current, water uptake, and damage during patterning in IC applications [1-3]. One of the new processes is designed to increase the surface density of an HSQ film. Although this solves some problems, a densified surface layer also increases the effective dielectric constant of film. To balance these two effects, the semiconductor industry would like to precisely control the thickness of the densified dielectric layer. To reach this goal, it is also essential to develop an accurate analytic technique to measure the thickness of the densified dielectric layer. However, few experimental techniques are able to perform high-resolution measurements of the density profile of dielectric thin films that are several thousand angstroms thick. In this work, we demonstrate the use of specular x-ray reflectivity (SXR) as a powerful technique to measure the depth profile of N2 plasma treated
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