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Dopants such as phosphine and boron trichloride can be added to the process gas to adjust conductivity and stress. Doped polysilicon requires caged wafer boats for better uniformity. Phosphorus decreases the deposition rate while boron increases it. This in situ doping is more uniform through the film thickness than can be achieved by sequential processing steps, and it is also done at a lower temperature. Drawbacks include process complexity, worse thickness uniformity, and increased process tube cleaning difficulty. Fine-grain polysilicon cannot be doped to the degree that the course-grain can, so it is mainly used as a structural material.
Applications: Polysilicon is used for resistors, MOSFET gates, thin-film transistors based on amorphous hydrogenated silicon (a-Si:H), DRAM cell plates, trench fills, and bipolar transistor emitters. Doped polysilicon is conductive enough to be useful for interconnects, electrostatic devices, and piezoresistive strain gauges. Polysilicon (mainly doped polysilicon) is also a popular structural material used in MEMS.
- Typical film thickness: 2.0 µm
- Deposition rate: 6-20 nm/min
- Refractive index at 550 nm
- Gases: silane, phosphine or boron trichloride
- Uniformity: < 3%
