Effect of Processing Conditions on the Surface Morphology of thin Polysilicon Films Used for Dram Cell Capacitors
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EFFECT OF PROCESSING CONDITIONS ON THE SURFACE MORPHOLOGY OF THIN POLYSILICON FILMS USED FOR DRAM CELL CAPACITORS
VIJU K.
MATHEWS
Micron Technology,
Inc.,
Boise,
ID 83706.
ABSTRACT The surface texture of polysilicon films, used for capacitor storage nodes, have been investigated for various deposition conditions. An increase in capacitance of approx. 70% has been observed in this work. The reproducibility of the surface texture and the resulting capacitance can be improved by optimizing the deposition pressure. The gain in capacitance was also observed to be affected by the thickness of the film. In the mass flow controlled regime the increase in capacitance was 12%. The increase in surface roughness and the gain in capacitance for the hot wall process provides a simple alternative for satisfying the charge capacity requirements in high density dynamic random access memories. INTRODUCTION The reliable operation of a dynamic random access memory (DRAM) device requires a minimum level of stored charge based on design and functional parameters. The reduction in cell sizes associated with high density DRAM's has a direct effect on this charge capacity since it reduces the area available for the storage node of the cell capacitor. The implementation of cell designs utilizing advanced trench [1] or stacked[2] structures, to overcome this problem, usually tends to increase the complexity of the fabrication process. The improvement in capacitance that can be achieved by reducing the thickness of the dielectric film is constrained by tunneling mechanisms
that
can lead
to high leakage
current.
The
application of high dielectric constant materials, like Ta205 [3], has not yet being realized on a large scale because of several unresolved issues related to leakage currents and reliability. The other potential option involves the "roughening" or "texturization" of the polysilicon film used as the storage plate. These techniques increase the surface to planar area ratio of the films. One approach has been the deposition of standard (smooth) polysilicon films followed by "texturization" using reactive ion etching [4], low temperature oxidation [5] or excimer laser processing [6]. Another approach employs the fact that the surface texture of polysilicon films can be altered by the deposition process [7]. The commonly used deposition conditions generate a smooth surface texture with a a columnar microstructure to satisfy lithographic and film doping considerations. Modification of these process conditions can result in the development of a very rough surface texture with a corresponding enhancement in the cell capacitance [8-10]. The changes in the surface texture of polysilicon films under various deposition conditions have been investigated in this study. The surface roughness of the films were evaluated using reflectance measurements and scanning electron microscopy. Electrical measurements were made to determine the gain in cell capacitance for the rough films.
Mat. Res. Soc. Symp. Proc. Vol. 230. ýc1992 Materials
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