If we take a look inside of the chamber, we can see that we have all of the basic parts. There is an ultra-violet light emitting source. In the path of the ultra-violet light, there is a photomask.In the shadow of the mask, there is a Si wafer with a microcoating of photoresist.
The mask is brought into contact with the wafer, and the Ultra-violet light is turned on.
Only the regions on the wafer that are not blocked by the photomask are exposed to the ultra-violet light.
When the exposure is complete, the ultra-violet light is turned off, and the mask is brought out of contact with the wafer.
The pattern is thus transferred, and hence the term lithography. This process is very similar to stenciling.