2.4.2 Deposition of SiO$_2$

The use of ${\mathrm{Si}}$ in microelectronic devices is very convenient because ${\mathrm{Si}}$ builds a native oxide on top of the ${\mathrm{Si}}$ surface following the oxidation reactions

$${{\mathrm{Si}}} + \mathrm{O_2} {\quad\stackrel{\! }{\rightarrow}}\quad {{\mathrm{SiO_2}}},$$ (2.138)

$${{\mathrm{Si}}} + 2 \mathrm{H_2O} {\quad\stackrel{\! }{\rightarrow}}\quad {{\mathrm{SiO_2}}} + 2\mathrm{H_2},$$ (2.139)

where the $\mathrm{SiO_2}$ is built by ${\mathrm{Si}}$ consumption from the surface. After the first oxide layer is built, oxygen and water have to diffuse to the ${\mathrm{Si}}$ - $\mathrm{SiO_2}$ interface to grow new oxide. With this type of oxidation, very thin layers of $\mathrm{SiO_2}$ can be deposited. However, thick oxide layers take a long time to build with this process technique and use a certain amount of ${\mathrm{Si}}$ as a source of the oxide layer.

Therefore, another technique has been introduced which transports all its sources via a carrier gas to the reactor where they react at the wafer surface, using the wafer surface as a catalytic material. A deposition process using silane ${\mathrm{SiH_4}}$ has been commonly established, which follows the reaction equation

$${\mathrm{SiH_4}}+ \mathrm{O_2} {\quad\stackrel{\! }{\rightarrow}}\quad {{\mathrm{SiO_2}}} + 2 \mathrm{H_2},$$ (2.140)

where the ${\mathrm{SiH_4}}$ reacts with oxygen at the hot wafer surface and $\mathrm{SiO_2}$ is built together with $\mathrm{H_2}$ . A optimal temperature for this reaction process is in the region of 1300 K [25]. The by-products from (2.138)-(2.140) are able to diffuse through the oxide to the plain ${\mathrm{Si}}$ and react with ${\mathrm{Si}}$ as oxygen or as water according to (2.138) and (2.139). This diffusion of oxygen and water can be controlled by regulating the temperature of the reaction process (2.140). However, despite of the regulative measures, some of the ${\mathrm{Si}}$ is alway consumed. In addition, a considerable concentration of $\mathrm{H_2}$ is built during the deposition of $\mathrm{SiO_2}$ and has to be taken into account for reliability issues during the further processing and the device operations [25,127].

The previously presented methods have shown how a $\mathrm{SiO_2}$ layer can be deposited by using ${\mathrm{Si}}$ from the target material (wafer). However, if ${\mathrm{Si}}$ consumption is not allowed at the surface, a more complex deposition method is required. A possible alternative, which provides that requirement is TEOS (Tetra-ethoxy-silane, ${\mathrm{Si(C_2H_5O)_4}}$ ). The deposition of $\mathrm{SiO_2}$ with TEOS uses a pyrolytic chemical reaction at a hot wafer surface in a LPCVD process (low pressure chemical vapor deposition) and follows the chemical reaction [25,127]

$${{\mathrm{Si(C_2H_5O)_4}}} {\quad\stackrel{\! 1000 \mathrm{K}}{\rightarrow}}\quad {{\mathrm{SiO_2}}} + 4\mathrm{C_{2}H_{4}} + 2\mathrm{H_{2}O},$$ (2.141)

where the semiconductor device structures on the wafer are heated at a temperature of approximately $1000$ K. The reactant TEOS is transported from a material reservoir to the reactor via a carrier gas typically consisting of $92\% \mathrm{N_{2}}$ and $8\% \mathrm{H_{2}}$ . At the hot surface of the wafer, the pyrolytic dissociation reaction (2.141) takes place. Typical $\mathrm{SiO_2}$ growth rates at these environment conditions are 100 Å/min [25] up to 1000 Å/min [127]. The growth rate can be controlled within a certain range by varying the temperature, pressure, and the TEOS concentration in the carrier gas. Typically, the deposition reactions follow an ARRHENIUS^2.38law [176]

$$R {=}A \exp{\left(-\frac{{\mathcal{E}_{\mathrm{A}}}}{{k_{\mathrm{B}}}T}\right)},$$ (2.142)

where the reaction rate $R$ depends exponentially on the activation energy ${\mathcal{E}_{\mathrm{A}}}$ . The proportionality constant $A$ depends on the surface shape and the chemical reaction which takes place and must therefore be determined separately for each different deposition type. The proportionality constant $A$ often depends on the temperature through a square-root law [127]

$$A {=}A_0 \sqrt{T}.$$ (2.143)