Improved Conformality of CVD Titanium Nitride Films
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ABSTRACT We demonstrate a novel approach to improving the step coverage of thin films made by chemical vapor deposition (CVD). Titanium nitride (TiN) films were deposited by atmospheric pressure CVD using tetrakis(diethylamido)titanium vapor (TDEAT) and ammonia gas (NH3 ) carried in nitrogen gas. Trimethylamine (NMe 3 ) gas was added during some of the depositions. The substrates were patterned silicon wafers having holes with aspect ratio of 3.5 through a silicon dioxide layer. We discovered that the step coverage was significantly increased for TiN films made with NMe 3 . At 320 °C, the step coverage was increased from 70% to nearly 100%. Within the range of deposition temperatures used in our study, 320 oC to 370 oC, the amount of improvement increased as the deposition temperature decreased. The trimethylamine did not increase the resistivity or the impurity levels in the films, but it did reduce the growth rate slightly. We suggest that the trimethylamine adsorbs onto the surface, temporarily blocking some of the sites on which growth could take place. Thus the effective sticking coefficients for the precursors are decreased, and the step coverage is increased. INTRODUCTION
Chemical vapor deposition (CVD) is a very important technique in making thin films. One of the major advantages of CVD over physical vapor deposition (PVD) is that CVD can provide a much better conformality for films deposited on non-planar structure, which is required in many applications such as microelectronic device fabrication. For some applications even a step coverage as high as 80% may not be acceptable. For example, in tungsten plug fabrication, a step coverage of 80% could still leave a void in the plug, and cause device failure [1 ]. Therefore, techniques to make films of very high conformality are desirable. TiN film has been extensively studied world wide because of its unique mechanical, chemical, electrical and optical properties. It is the standard material for adhesion layers, anti-reflective coatings and diffusion barriers in microelectronics industry. Currently, TiN films are deposited by sputtering. As device feature sizes are being further miniaturized, thinner TiN films are needed and will soon reach the limit of PVD capabilities. CVD TiN has been recognized as the most promising successor to PVD TiN because CVD TiN films can achieve much better step coverage, and also have a denser microstructure and thus may be better diffusion barriers [2]. In this paper, we propose a novel approach to improve step coverage of CVD films by control of their growth rate. Our approach was based on the fact that one of the most critical 135
Mat. Res. Soc. Symp. Proc. Vol. 555 © 1999 Materials Research Society
determining the step coverage of CVD films are the sticking coefficients (Sc) of species involved in the deposition reaction [3]. Sc is defined to be the ratio of amount of material deposited on the substrate to the amount of material flowing onto the substrate. Lower Sc yields better (higher) step coverage. The most common way
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