1. Photoresist Application (Spinning)
A drop of light-sensitive liquid called photoresist is applied to the centre of the oxidized silicon wafer that is held down by a vacuum chuck. The wafer is then accelerated rapidly to a rotational velocity in the range 3000 to 7000 RPM for some 30 to 60 seconds. This action spreads the solution in a thin, nearly uniform coat and spins off the excess liquid. The thickness of the coat so obtained is in the range 5000 to 10000 A, as shown in the figure below. The thickness of the photoresist layer will be approximately inversely proportional to the square root of the rotational velocity.
Sometimes prior to the application of the photoresist the silicon wafers are given a “bake-out” at a temperature Of at least 100°C to drive off moisture from the wafer surfaces so as to obtain better adhesion of the photoresist. Typical photoresist used is Kodak Thin Film Resist (KTFR).
2. Prebake
The silicon wafers coated with photoresist are now put into an oven at about 80°C for about 30 to 60 minutes to drive off solvents in the photoresist and to harden it into a semisolid film.
3. Alignment and Exposure
The coated wafer, as above, is now placed in an apparatus called a mask aligner in very close proximity (about 25 to 125 micro meters) to a photomask. The relative positions of the wafer and the photomasks are adjusted such that the photomask is correctly lined up with reference marks or a pre-existing pattern on the wafer.
The photomask is a glass plate, typically about 125 mm square and about 2 mm thick. The photomask has a photographic emulsion or thin film metal (generally chromium) pattern on one side. The pattern has clear and opaque areas. The alignment of the photomask to the wafer is often required to be accurate to within less than 1 micro meter, and in some cases to within 0.5 micro meters. After proper alignment has been achieved, the wafer is brought into direct contact with the photomask. Photomask making will be described separately.
A highly collimated ultraviolet (UV) light is then turned on and the areas of the silicon wafer that are not covered by the opaque areas of the photomask are exposed to ultraviolet radiation, as shown in the figure. The exposure time is generally in the range 3 to 10 seconds and is carefully controlled such that the total UV radiation dosage in watt-seconds or joules is of the required amount.
4. Development
Two types of photoresist exist- negative photoresist and positive photoresist. In the present description negative photoresist is used in which the areas of the photoresist that are exposed the ultraviolet radiation become polymerized. The polymerization process increases the length of the organic chain molecules that make up the photoresist. This makes the resist tougher and makes it essentially insoluble in the developer solution. The resisting photoresist pattern after the development process will therefore be a replication of the photomask pattern, with the clear areas on the photomask corresponding to the areas where the photoresist remains on the wafers, as shown in the figure below.
An opposite type of process occurs with positive photoresist. Exposure to UV radiation results in depolymerization of the photoresist. This makes these exposed areas of the photoresist readily soluble in the developer solution, whereas the unexposed areas are essentially insoluble. The developer solution will thus remove the exposed or depolymerized regions of the photoresist, whereas the unexposed areas will remain on the wafer. Thus again there is a replication of the photomask pattern, but this time the clear areas of the photomask produce the areas on the wafer from which the photoresist has been removed.
5. Postbake
After development and rinsing the wafers are usually given a postbake in an oven at a temperature of about 150°C for about 30 to 60 minutes to toughen further the remaining resist on the wafer. This is to make it adhere better to the wafer and to make it more resistant to the hydrofluoric acid [HF] solution used for etching of the silicon dioxide.
6. Oxide Etching
The remaining resist is hardened and acts as a convenient mask through which the oxide layer can be etched away to expose areas of semiconductor underneath. These exposed areas are ready for impurity diffusion.
For etching of oxide, the wafers are immersed in or sprayed with a hydrofluoric [HF] acid solution. This solution is usually a diluted solution of typically 10: 1, H2O : HF, or more often a 10 : 1 NH4F [ammonium fluoride]: HF solution. The HF solutions will etch the SiO2 but will not attack the underlying silicon, nor will it attack the photoresist layer to any appreciable extent. The wafers are exposed to the etching solution ion enough to remove the SiO2 completely in the areas of the wafer that are not covered by the photoresist as shown in the figure.
The duration of oxide etching should be carefully controlled so that all of the oxide present only in the photoresist window is removed. If etching time is excessively prolonged, it will result in more undercutting underneath the photoresist and widening of the oxide opening beyond what is desired.
The above oxide etching process is termed wet etching process since the chemical reagents used are in liquid form. A newer process for oxide etching is a dry etching process called plasma etching. Another dry etching process is ion milling.
7. Photoresist Stripping
Following oxide etching, the remaining resist is finally removed or stripped off with a mixture of sulphuric acid and hydrogen peroxide and with the help of abrasion process. Finally a step of washing and drying completes the required window in the oxide layer. The figure below shows the silicon wafer ready for next diffusion.
Negative photoresists, as above, are more difficult to remove. Positive photoresists can usually be easily removed in organic solvents such as acetone.
The photolithography may employ contact, proximity, or projection printing. For IC production the line width limit of photolithography lies near 0.4 micro meters, although 0.2 micro meters features may be printed under carefully controlled conditions. At present, the photolithography occupies the primary position among various lithographic techniques.