Large Area Plasma Enhanced Chemical Vapor Deposition of Nonstoichiometric Silicon Nitride
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Large Area Plasma Enhanced Chemical Vapor Deposition of Nonstoichiometric Silicon Nitride Yue Kuo IBM Research Division, T. J. Watson Research Center, P. 0. Box 218, Yorktown Heights, N.Y. 10598 USA ABSTRACT This paper presents results on PECVD of low refractive index silicon nitride in a large area system. Film deposition characteristics, such as deposition rate and thickness uniformity, were investigated over a wide range of process parameters, such as gas composition, power, pressure, and N 2 pressure. Film properties, such as RI, absorbance, stress, and etch rate, have also been studied. A general model, which includes both the deposition and etching mechanisms, has been developed to explain these results. This model could explain the film uniformity issue in the process. I. Introduction Plasma enhanced, chemically vapor deposited (PECVD) silicon nitride (SiNs) thin films are widely used in microelectronics as intermetal dielectrics in Very Large Scale Integrated Circuits (VLSICs) or as gate dielectrics in thin film transistors (TFTs) (1). It is also commonly used as the passivation layer for packages (2). Most of the literature about PECVD SiNX is focused on VLSI applications, which require a refractive index (RI) between 2.0 and 2.1 and a Si/N ratio close to 1 (3). For a-Si:H TFT applications, the PECVD SiN, film is often used as the gate dielectric layer or part of the gate dielectric layer because of its good interfacial characteristics with a-Si:H (1). It requires different film characteristics, e.g., a Si/N ratio less than 1 and a high hydrogen content. Since a TFT is a multilayer structure, the film stress is also important and the deposition temperature should be low. For liquid crystal display (LCD) applications, the substrate size is usually very large, e.g., larger than 10 inch by 10 inch. The uniformity of the film is critical to the final product. A PECVD SiN1 process is usually carried out at a low power density, e.g., less than 0.1 W/cm 2 . Information on the higher power process is not available. In this paper, the above issues are investigated and discussed. 2. Experimental The PECVD system used in these experiments is Ulvac CPD-7433 with a 13.56 MHz rf power supply. The vertical anode is separated from the cathode by 30 mm. The power density is 0.048 W/cm 2 at 100 watts. For each experiment 1.5-inch size samples in five areas, i.e., one in the center and four on corners of a 13-inch square area, were analyzed. In this deposition system the anode is much larger than the substrate holder, the electric field can be taken as uniform along the path of the gas flow. Various combinations of SiH 4, NH 3, and N 2 under different pressure, power, and temperature were used. The effect of N 2, which is often taken as a diluent in the SiH 4/NH 3 stream, on the deposition process is studied. Although the partial pressure of each component in the stream is important in interpreting the process data, in this paper only the SiH 4 flow rate is varied in most cases to examine its effects on the deposition proces
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