Fabrication Process Flow: Basic Steps
Each processing step requires that certain areas are defined on chip by appropriate masks.
The integrated circuit may be viewed as a set of patterned layers of doped silicon, polysilicon, metal, and insulating silicon dioxide. In general, a layer must be patterned before the next layer of material is applied on the chip.
The process used to transfer a pattern to a layer on the chip is called lithography
The Sequence starts with the thermal oxidation of the silicon surface, by which an oxide layer is created on the substrate.
The entire oxide surface is then covered with a layer of photoresist, which is essentially a light-sensitive, acid-resistant organic polymer, initially insoluble in the developing solution
If the photoresist material is exposed to ultraviolet (UV) light, the exposed areas become soluble so that they are no longer resistant to etching solvents.
To selectively expose the photoresist, we have to cover some of the areas on the surface with a mask during exposure.
Thus, when the structure with the mask on top is exposed to UV light, areas which are covered by the opaque features on the mask are shielded. In the areas where the UV light can pass through, on the other hand, the photoresist is exposed and becomes soluble and can be removed by a solvent.
The silicon dioxide regions which are not covered by hardened photoresist can be etched away either by using a chemical solvent.
We obtain an oxide window that reaches down to the silicon surface
The remaining photoresist can now be stripped from the silicon dioxide surface by using another solvent, leaving the patterned silicon dioxide feature on the surface
The surface is covered with a thin, high-quality oxide layer, which will eventually form the gate oxide of the MOS transistor. On top of the thin oxide layer, a layer of polysilicon is deposited.
After deposition, the polysilicon layer is patterned and etched to form the interconnects and the MOS transistor gates.
The entire silicon surface is then doped with a high concentration of impurities, either through diffusion or ion implantation
Once the source and drain regions are completed, the entire surface is again covered with an insulating layer of silicon dioxide.
The insulating oxide layer is then patterned in order to provide contact windows for the drain and source junctions.
The surface is covered with evaporated aluminium which will form the interconnects.
Finally, the metal layer is patterned and etched, completing the interconnection of the MOS transistors on the surface