Photolithography - Semiconductor Device Fabrication

Photolithography is the process of transferring the window pattern (geometric shapes) on a mask to the wafer surface.
This shapes make up parts of the circuit in (bases, emitters, collectors) bipolar and (gates, sources, drains) MOS (metallic oxide semiconductor) transistors.

The final IC is made by essentially transferring the features from each mask level to the surface of the silicon.

Steps/Key of Photolithographic Processes

1.   Obtain Clean Water
2.   Coat water with photoresist
3.   Soft Bake
4.   Align Mask
5.   Expose Patterns
6.   Develop Photoresist
7.   Hard Bake
8.   Etch Windows
9.   Strip Photoresist

It is good to use clean materials and work in a clean environment because any dust particle on the wafer constitutes a defect which will eventually result in the failure of the circuit.

Soft baking (150C) of the wafer is done in a safe yellow light.

Selective Oxide Etching

In this process, the photo resist coated wafer is placed into a hydrofluoric acid bath to remove the exposed oxide.
The hydrofluoric acid will react chemically with the oxide to form wafer soluble products that dissolve in the water used to dilute the acid.

In negative photoresist emulsion, Hydroflouric acid is used since it does not react with silicon. Subsequently, the remaining hardened photoresist is finally 'stripped' with a mixture of sulphuric acid and hydrogen peroxide. A final wash and dry stage completes the photolithographic process.
Positive photoresists emulsion behaves in the same manner except that windows are opened where the mask is transparent. They are useful when large areas of oxide is transparent.

Photolithography has limitations as regards the minimum line widths and feature sizes of microelectronic devices that can be fabricated.

An alternative to photolithography is the patterning of the wafer directly without masks. In this technique, the wafers are coated with electron resists then direct writing with electron beam machine is used.

Electron beam machining of masks can produce very fine patterns with high resolution thus the mask can be produced by directly eliminating the photo reduction stages.

Advantages of Direct Writing with Electron Beam are:

1.   Fine Patterns
2.   Higher Reliability and Resolutions

Disadvantage of Direct Writing with Electron Beam is:

1.   It is more expensive.