NotesCMOS
Recall how CMOS are laid out.
We need to layer our CMOS somehow from the bottom up. For this, we use masks.
Masks
There are six generally used masks:
- n-well
- Polysilicon
- n+ diffusion
- p+ diffusion
- Contact
- Metal
We take each of these and create it step by step.
We start with a blank wafer, build the inverters from the bottom up. The first step is to form the n-well:
- Cover the wafer with protective SiO2
- Remove SiO2 layer where n-well should exist (expose that area)
- Implant or diffuse n-dopant into the exposed area
- Strip off the SiO2.
The common and preferred way to create the n-well is through ion-implantation. It is also possible to use arsenic gas in a furnace, among other methods.
Now that the n-well is created, put a very thin layer of gate oxide on top.