Deposition and Characteristics of Polysilicon Films for Integrated-Circuit Applications
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DEPOSITION AND CHARACTERISTICS OF POLYSILICON FILMS FOR INTEGRATED-CIRCUIT APPLICATIONS TED KAMINS Hewlett-Packard, Deer Creek Laboratories 3500 Deer Creek Road, P 0 Box 10350 Palo Alto CA 94303-0867 Abstract The deposition conditions used to form polysilicon layers are determined to a large extent by the geometry of the reactor used to deposit the films. The structure of the films and the electrical behavior of devices incorporating the polysilicon layers are, in turn, influenced by the deposition conditions. Operation in the reaction-rate limited regime is necessary for film uniformity in a high-volume reactor; this is achieved by operating at low temperatures and low pressures. The temperature gradient often used in this reactor changes the structure of the deposited films, and cannot be used for devices which depend sensitively on the structure. Diffusion in polysilicon is influenced by the grain boundaries and, consequently, depends sensitively on the deposition conditions. The electrical properties of devices with their active layers within the polysilicon layer itself depend strongly on the structure of the polysilicon. Even conventional MOS transistors in which the polysilicon serves only as a conducting electrode can be affected by the structure of the polysilicon. Therefore, the deposition conditions of the polysilicon films and the resulting structure must be considered to obtain the desired electrical properties in the devices being fabricated.
Introduction Polysilicon is employed in a wide variety of integrated-circuit applications. It is used for gate-electrodes and interconnections in MOS integrated circuits [11; for this application it must have low resistivity. When used for high-value load resistors [21, its resistivity is about eight orders of magnitude higher. In bipolar transistors polysilicon is widely used to make contact to the base and emitter regions and can improve the device properties both by decreasing the capacitance and by modifying the basic physics of the device [3]. In most applications, the device behavior can depend on the structure of the polysilicon, which, in turn, is controlled by the deposition and processing conditions. In this discussion, we will briefly examine the factors which determine the deposition conditions generally employed. Then we will consider how the deposition and processing conditions influence the structure and, consequently, the behavior of the devices in which polysilicon is used. Mot. Res. Soc. Symp. Proc. Vol. 106. 9 1988 Materials Research Society
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The Deposition Process Polysilicon has been used in integrated circuits for almost two decades. The development of polysilicon technology was initially motivated by the use of polysilicon as the gate electrode in MOS integrated circuits [1]. Although evaporated films were briefly considered, the step coverage of these films was not adequate, and alternate techniques were examined. Chemical vapor deposition in horizontal, atmospheric-pressure reactors was a well developed technique used to form epitaxi
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