12.6 The Mechanism of Deposition of Silicon Nitride

When modeling topography processes it is in general possible to write down quite complicated reaction paths. This route was taken, e.g., in the investigations of surface reactions in [48,152,151,153], where dozens of reactions are listed. In the conclusion of [153] it is remarked that it is not straightforward to determine the vital reactions and their constants. Therefore it is mandatory to reduce the possible reaction paths to an essential minimum which can reproduce the observed phenomena (cf. Chapter 6).

Silicon nitride films can be deposited from dichlorsilane ( $\mathrm{SiCl_2H_2}$) and ammonia ( NH$_3$) in a LPCVD process. This process is, however, difficult to handle because of the generation of excess ammonium chloride ( $\mathrm{NH_4Cl}$) which accumulates at surfaces in the reactor with temperatures below its sublimation temperature of $338\,\text{\textcelsius}$. These accumulations may release particles when turbulent gas flows occur [111].

More important is the deposition of silicon nitride films by PECVD (Plasma Enhanced Chemical Vapor Deposition) from silane ( SiH$_4$), NH$_3$, and N$_2$. This process is carried out at temperatures below $400\,\text{\textcelsius}$. The reactions are

$$\textrm{SiH$_4$}\xspace + \textrm{NH$_3$}\xspace \overset{}{\rightarrow} \mathrm{SiNH} + 3 \textrm{H$_2$}\xspace$$

$$2\textrm{SiH$_4$}\xspace + \textrm{N$_2$}\xspace \overset{}{\rightarrow} 2\mathrm{SiNH} + 3 \textrm{H$_2$}\xspace$$

depending on the choice of reaction partners [111]. Here SiNH denotes nitride containing lots of Hydrogen. Thus silicon nitride deposited from CVD is an amorphous substance with varying concentration of hydrogen.

For the purposes of the process simulated in Section 12.8, we will use

$$\textrm{SiH$_4$}\xspace + \textrm{NH$_3$}\xspace \overset{}{\rightarrow} \mathrm{SiNH} + 3 \textrm{H$_2$}\xspace$$

as the essential reaction. Finally we note that a detailed model of triaminosilane condensation can be found in [125].