Silicon Nitride LPCVD

Silicon nitride is a hard, dense material used for diffusion barriers, passivation layers, oxidation masks, etch masks, ion implant masks, insulation, encapsulation, mechanical protection, MEMS structures, gate dielectrics, optical waveguides, and CMP and etch stop layers. The use of dichlorosilane rather than silane improves uniformity and allows closer wafer spacing. Enriching silicon nitride films with silicon reduces stress and the HF etch rate. Other important process relationships are:

  • Increasing temperature decreases stress.
  • Increasing pressure and/or temperature increases the deposition rate.
  • Increasing deposition rate decreases uniformity.

The temperature dependence of film stress means that a temperature gradient along the process tube cannot be relied on to compensate for gas depletion from inlet to outlet. Thus, a Roots blower is stacked on the vacuum pump to increase the pumping speed, which allows more gas to be flowed while maintaining the process pressure. Increasing the gas flow rates partially mitigates the gas depletion effect.

Avoiding the "hazing" of silicon nitride films requires some method of keeping the NH4Cl byproduct from back-streaming out of the vacuum manifold during loading and unloading. Tystar's innovative gate valve solves this problem by allowing a small continuous flow of nitrogen from the process tube to the pump when the gate valve is closed.

LPCVD Processes