Basis for a n-channel field-effect transistor is a p-doped (boron) silicon substrate.
On top of the substrate, a thin layer of silicon dioxide SiO2 (the gate oxide) is created via thermal oxidation. It is used for insulation of the later deposited gate and the substrate.
In a LPCVD process nitride is deposited, it is used later as an masking during the field oxidation.
On top of the nitride a photoresist is spun on, exposed and developed. Thus a structured coating layer is fabricated which serves as an etching mask.
Only at resist free sites nitride is removed using reactive ion etching.
6. Resist removal
Subsequent the resist mask is removed in a wet-chemical developer solution.
During field oxidation, the nitride serves as a mask layer, the thermal wet oxidation takes place only on the bare gate oxide. The grown field oxide is used for lateral isolation to adjacent devices.
Subseuqent to the oxidation, the nitride is removed in a wet chemical etching process.
Via low pressure CVD, polycrystalline silicon is deposited which represents the gate electrode.
Again a resist layer on top of the polysilicon is patterned.
The photoresist in turn serves as a mask layer, via reactive ion etching the gate is patterned.
12. Resist removal
The resist is removed via wet-chemical etching.
A thin oxide (post oxide) is deposited as an insulating layer for the gate electrode as well as a spacer for the subsequent source and drain implantation.
14. Ion implantation
Via ion implantation with phosphorus, the source and drain regions are introduced (n-type). Since the gate electrode acts as an mask during implantation, the width of the n-channel between the source and drain is preset. This is called "self alignment".
As an isolation a nonmetal is deposited (e.g. oxide). This happens in a LPCVD process with TEOS, which provides a good step coverage.
16. Photolithography and etching
In a further step a resist layer is structured and the edges of the contact holes are rounded in an isotropic etch process.
Subsequently the contact holes are opened in a highly anisotropic etch process.
The contact holes are filled with aluminum via sputtering.
In a final lithography step a new resist mask is patterned.
The pattern is transferred into the underlying metallization in an anisotropic dry etch process.
21. Resist removal
Finally, the resist is removed and aluminum conductors remain to actuate the transistor.
Actual the construction of a transistor is much more complex, since additional planarization layers for photolithography are neccessary or secondary drain and source implantations have to be done to adjust the threshold voltage accurately. On the slopes of the gate elektrode additional (side wall) spacers can be formed to set the exact length of the channel or fine tune the doping profile respectively.