Photolithography is the process of mask fabrication. This includes the process of transferring a pattern from a mask to the silicon surface. This step is applied many times. The number of masks applied to make the IC is often considered to be a measure of how complex the circuit is.

The idea of a mask is that hole or window allow impurities to contact the desired portion of the wafer surface.  The animation below shows the overall process.  First a mask is fabricated with the desired “pattern” that is to be transferred to the surface of the silicon.   The actual transfer is through a process called photolithography.  The process can be likened to a photoengraving process.  First the patterns are transferred to a light sensitive material called a photoresist. Then either chemical or plasma etching is used to transfer the pattern from the photoresist to the silicon wafer surface.

The conditions under which this occurs must be extremely clean. Naturally called a cleanroom)  Any dust particles falling on the substrate  can result in defects on the final chip.  If even 10% of the time a defect occurs.. After 8 or more masks up to 80% of the chips could have defects!  Something called a laminar flow hood like the one show below from Rockwell’s plant, are used to prevent contamination.

A typical stepper used for lithography. (Click on the image for a better view!)

Summary

Masking is used to protect one area of the wafer while working on another. This process is referred to as photolithography or photo-masking. A photoresist or light-sensitive film is applied to the wafer, giving it characteristics similar to a piece of photographic paper. A photo aligner aligns the wafer to a mask and then projects an intense light through the mask and through a series of reducing lenses, exposing the photoresist with the mask pattern.  Precise alignment of the wafer to the mask prior to exposure is critical. Most alignment tools are fully automatic.
 

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