Once the masks are ready, the wafer will be coated with a chemical called photo resist, sometimes called “resist”. The resist is sensitive to light. The resist will be dissolved in a solvent and poured over a spinning wafer. Due to the rotation of the wafer, the solution will spread uniformly over the wafer (Figure 2.4- Stage 1).

Figure 2.4 Lithographic process

After some solution is poured, the wafer will be made to spin at high speed (Figure 2.4- Stage 2). The solution will spread very thin and if the wafer is heated at this time, the solvent will evaporate.  The resist will form a film on top of the wafer. This process is called soft baking. All these operations would be conducted in a dark room.
After this, the wafer and mask are brought into alignment.  The lens system will be adjusted so that the focus is correct. Then light of particular wavelength will be made to fall on the wafer, through the mask, for a short time. Typically, the exposure is for a second.  Wherever the light has fallen, the chemical reaction would have taken place (Animation 2.4 stage 3 and stage 4). Note that the image on the wafer is smaller than the image on the mask.

A mask will expose only a small area. The area may be about 5 cm x 5 cm. So, the whole wafer cannot be exposed in one shot. The wafer must be moved to the next location (e.g. 5 cm), and then the process of alignment, focus and exposure must be repeated (Figure 2.4 Stage 5). This procedure will continue until the entire wafer is exposed. Since it is done by moving the wafer in steps, the litho tool is also called “stepper”. After the whole wafer is exposed, it is sent for developing. The wafers are dipped in suitable solutions. Wherever the light has fallen, a chemical reaction would have occurred. Those areas will be dissolved in the developing solution. The areas where light did not fall will not dissolve (Figure 2.4 Stage 6)
If the wafer is heated now, the resist will harden a bit. This process is called hard baking (Figure 2.4 Stage 6). After baking, the resist will not move or show much flexibility. The litho process ends here.
Let us assume that the wafer is sent for etching after this. Here, the wafer may be dipped in a corrosive solution (wet etching) or corrosive gases may be passed on the wafer surface (dry etching). The areas protected by photo resist will not be corroded, while the areas left open will corrode or etch (Figure 2.4  Stage 7).
After etching for a certain time, the wafer will be cleaned. Then it will be dipped in a different solution, which is capable of removing the hardened photo resist (Figure 2.4 Stage 8)
Similarly, in each level, whenever a material has to be removed in certain areas, photolithography is used. Making a normal IC will need more than 30 masks.