Surface Morphology of P-Doped LPCVD Silicon Films
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RODICA PLUGARU*, E. VASILE**, P. COSMIN*, SILVIA COSMIN*, C. COBIANU*, D. DASCALU* *Institute of Microtechnology, P.O. Box 32-160, Bucharest, R-72225, Romania ** METAV-SA, P.O.Box 18, Bucharest, R-71259, Romania
ABSTRACT The formation of silicon hillocks at the surface of amorphous and polycrystalline LPCVD silicon films, as a function of phosphorus doping was investigated by SEM, TEM replica and X-ray diffraction techniques. The phosphorus doping was performed in the concentration 20 3 range from 9X 1019cm- 3 tol.5X 10 cm- . It was observed that the hillocks size increases with the dopant concentration for the as-doped and for annealed films. The experimental data were analysed in a model involving diffusion of silicon during the doping and recrystallization processes. INTRODUCTION Heavily doped low-pressure chemical vapor deposition (LPCVD) silicon films are widely used in double polysilicon structures for integrated circuit technology [1,2]. The simultaneous fabrication of junctions and the doping of the first level of polysilicon require heavy impurity doping and determine the structural and morphological transformations of these films [3]. In addition, some potential applications of highly level doped LPCVD silicon films in microelectromechanical (MEMS) structure fabrication have been proposed [4]. Therefore, it is of interest to control the surface morphology of the films by varying the doping process parameters. Hendricks et al. [5] reported that silicon hillocks are formed at the polycrystalline silicon films surface after POCI3 doping or P-implantation processes, for a dopant concentration in 20 3 the range IX 1020 cm- 3 to 5x 10 cm- . This has also been observed in the case of As doping of 3 21 heavily doped (8x10 cm" ) polycrystalline layers, as reported by Shibata et al. [6]. However, a reasonable explanation of the mechanism of hilllock formation is still not available. In order to get a deeper insight on this matter, we performed a detailed study of the effects of POCI3 doping on the surface morphology of amorphous and polycrystalline LPCVD silicon films by electron microscopy and X-ray diffraction. Also, we propose a phenomenological model for the hillock formation. EXPERIMENT 3 The silicon layers were obtained by LPCVD at 550 0 C, 15 cm .min-' silane flow rate, 1 torr 3 pressure and 620'C, 50 cm min"I silane flow rate, 0.33 torr pressure, on Si (100) p-type substrates covered with a 150 nm thick oxide layer grown at 1000'C in a dry 02 atmosphere. The doping process was performed in POCI3+ H2 + 02 atmosphere at 9500C, for 5, 15, and 30 min, respectively. The phosphorus concentration was determined from spreading resistance profiles (SRP), the measurements being carried out on monocrystalline silicon samples. After the doping processes the samples were annealed in a nitrogen atmosphere at 950'C for 20 min. Surface morphology of silicon layers after the doping and annealing processes were studied using scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was performed on a replic
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