The integrated circuits maybe viewed as a set of patterned layers of doped silicon, ,poly-silicon, metal and insulation silicon dioxide. The process to transfer a pattern to a layer on chip is called as lithography. Since each layer has its own distinct patterning requirements, the lithographic sequence must be repeated for every layer using a different mask.

Basic Fabrication Steps

1. Oxidation

The process of growing a layer of silicon dioxide (SiO2)on the surface of a silicon wafer.

Uses:

· Provide isolation between two layers

· Protect underlying material from contamination

· Very thin oxides (100 to 1000 Å) are grown using dry-oxidation techniques. Thicker oxides (>1000 Å) are grown using wet oxidation techniques.

2. Diffusion

Movement of impurity atoms at the surface of the silicon into the bulk of the silicon - from higher concentration to lower concentration. Diffusion typically done at high temperatures: 800 to 1400 °C.

3. Ion Implantation

Ion implantation is the process by which impurity ions are accelerated to a high velocity and physically lodged into the target.

· It is required to activate the impurity atoms and repair physical damage to the crystal lattice. This step is done at 500 to 800 °C.

· Lower temperature process compared to diffusion.

· Can implant through surface layers, thus it is useful for field-threshold adjustment.

· Unique doping profile available with buried concentration peak.

4. Deposition

Deposition is the means by which various materials are deposited on the silicon wafer.

Examples:

· Silicon nitride (Si3N4)

· Silicon dioxide (SiO2)

· Aluminum

· Polysilicon

There are various ways to deposit a meterial on a substrate:

· Chemical-vapor deposition (CVD)

· Low-pressure chemical-vapor deposition (LPCVD)

· Plasma-enhanced chemical-vapor deposition (PECVD)

· Sputter deposition

Materials deposited using these techniques cover the entire wafer.

5. Etching

Etching is the process of selectively removing a layer of material.

When etching is performed, the etchant may remove portions or all of:

· the desired material

· the underlying layer

· the masking layer

Important considerations:

· Anisotropy of the etch

A = 1 - (lateral etch rate / vertical etch rate)

· Selectivity of the etch (film to mask and film to substrate)

Sfilm-mask = film etch rate / mask etch rate

Desire perfect anisotropy (A=1) and invinite selectivity.

There are basically two types of etches:

· Wet etch, uses chemicals

· Dry etch, uses chemically active ionized gasses.

6. Photolithography

Components:

· Photoresist material

· Photomask

· Material to be patterned (e.g., SiO2)

Positive photoresist- Areas exposed to UV light are soluble in the developer

Negative photoresist- Areas not exposed to UV light are soluble in the developer

Steps:

1. Apply photoresist

2. Soft bake

3. Expose the photoresist to UV light through photomask

4. Develop (remove unwanted photoresist)

5. Hard bake

6. Etch the exposed layer

7. Remove photoresist