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TimeDomain CVD, Inc. |
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| Thermal silicon nitride is generally produced
using a tube reactor operating
at low pressures. Dichlorosilane SiCl2H2 is a common precursor, with
silane also being used. The oxidant is ammonia. Typical conditions are
between 700 and 800 C, at around 1 Torr; deposition rates are in the 10
nm/minute range, but many applications employ very thin films, so
throughput is reasonable in batch processing.
The resulting films are dense and hard, and also highly stressed: 1E10 dynes/cm2 is typical. The films are excellent barriers to just about everything, including hydrogen, although 4-8 atomic % H is generally incorporated. By varying the gas mixture, silicon-rich films can be produced. Conformality of the deposition is significantly better than e.g. silane oxides. Hydrogen-free silicon nitride is nearly impervious to wet hydrofluoric acid (HF); thermal CVD films etch very slowly. Nitride films etch readily in fluorine-containing plasmas, and can be selectively etched with respect to silicon using hot (>150 C) phosphoric acid. The deposition process tends to produce copious amounts of powder in the exhaust system, in part due to the formation of NH4Cl (essentially an adduct of hydrochloric acid HCl and ammonia NH3). Thermal CVD films have various applications in semiconductor fabrication: combined with various thin oxide and polysilicon layers, they are used to mask regions for selective oxidation during LOCOS isolation. Nitride layers can be used as an etch stop for self-aligned contact holes, to allow lithographic misalignment to expose the polysilicon gate stack without excessive erosion during contact etch. Nitride layers can be used as a CMP (chemical mechanical polishing) stop layer for oxide polishing, and as a barrier to sodium diffusion to protect gate oxides. Deposited nitrides are frequently part of the dielectric stack for capacitors in DRAM (dynamic random access memory) fabrication processes, and sometimes used in gate dielectric stacks. [A few references:
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