# The Challenges of Advanced CMOS Process from 2D to 3D
## 3 Process Integration of New Transistor Architecture
### Stress Engineering
The precursor molecules are piloted by hydrogen over the wafer and moves forwards along the chemical vapor deposition (CVD) quartz. A gas boundary is established over the wafer where the molecules may diffuse downwards (according to [PERSON]'s diffusion equation). Later these  
Figure 4: Schematic illustration of a contact etch stop layers (CESL) cap layer for implementing for example parallel tensile stress in a planar n-MOS. The different strain components, as determined by technology computer aided design (TCAD) simulations are shown in the right diagram as a function of the transistor pitch \(L_{p\bar{p}}\). Figure 5: The evolution of nano-scaled transistors having SiGe as stressor material in source/drain (S/D) for each node (**a**) 90 nm, (**b**) 65 nm, (**c**) 45 nm, and (**d**) 32 nm. Sigma shape recess was introduced for the first time in 45 nm. Reproduced with permission from [44]. Copyright SPIE, 2016.  
molecules are attracted within a spherical volume towards the dangling bonds which are available sites in the exposed Si areas of the chips. The total growth rate (\(R_{\textit{Tot}}\)) consists of components from vertical (\(R_{\textit{V}}\)) and lateral (\(R_{\textit{LC}}\) gas flow, as well as components from oxide surface around the transistor arrays (\(R_{\textit{ss}}\)) and oxide surface between the transistor openings (\(R_{\textit{SC}}\)) for SiH\({}_{2}\)Cl\({}_{2}\), GeH\({}_{4}\) and HCl gases. Thus, \(R_{\textit{Tot}}\) for SiGe growth in planar transistors is written as [50]:  
\[R_{\textit{Tot}}=R_{\textit{Si}}^{V}\ +\ R_{\textit{Si}}^{LG}+R_{\textit{Si}}^{SS}+R_{ \textit{Si}}^{SC}+R_{\textit{Ge}}^{V}+R_{\textit{Ge}}^{LG}+R_{\textit{Ge}}^{SS }+R_{\textit{Ge}}^{SC}-R_{\textit{E}}^{LG}-R_{\textit{E}}^{SS}\ -R_{\textit{E}}^{SC} \tag{1}\]  
The above equation can be modified to estimate the growth rate of SiGe on Si fins in 3D FinFETs (elevated S/D) when an extra component (\(R^{CO}\)) is added. This component is rooted from the diffusion of molecules on the surface of Si fins. Then Equation (1) is written as [51]:  
\[ \begin{split} R_{\textit{Tot}}=R_{\textit{Si}}^{V}+R_{\textit{Si} }^{LG}+R_{\textit{Si}}^{SC}+R_{\textit{Si}}^{CO}+R_{\textit{Si}}^{SS}+R_{ \textit{Ge}}^{V}+R_{\textit{Ge}}^{LG}+R_{\textit{Ge}}^{SO}+R_{\textit{Ge}}^ {CO}+R_{\textit{Ge}}^{SS}-R_{HCl}^{V}-R_{HCl}^{LG}\\ -R_{HCl}^{SO}-R_{HCl}^{CO}\ -R_{HCl}^{SS}\end{split}  \tag{2}\]  
The \(R^{CO}\) component makes a unique situation for the growth of 3D transistors when the molecules can move from a fin to another one creating a more uniform growth. Figure 6: The evolution of Ge content in SiGe S/D and stress created in channel for different technology nodes. Figure 7: Schematic view of gas flow in different directions over a chip containing (**a**) planar transistors and (**b**) FinFETs. The dashed circles (sphere in 3D) representing a volume where the molecules around the transistor arrays (or chips) are attracted. It is worth mentioning here that the components \(R^{SS}\) and \(R^{SC}\) are changed if the chip layout is changed. These components which are layout-dependent are the cause for pattern dependency in selective epitaxy growth.