Photolithography is a technique used to pattern a film using light. The light is shaped using an opaque mask with clear patterns. The regions of the film that is exposed to light becomes more soluble and dissolves away in a developer. Photolithography can pattern a 4″ wafer in a few seconds with features as small as 1 μm.


Patterns are drawn in a CAD software such as KLayout and saved to the GDSII stream format, the de facto industry standard for micro and nano fabrication. A manufacturer will typically use the file to produce a mask with a dark background and clear shapes. The manufacturer can be told to invert the mask to produce an clear background with dark shapes. Masks cost $70 to $400 and can be made within a day.


The light sensitive film is called a photoresist (PR). A positive photoresist will dissolve when exposed to light and developed. A negative photoresist will dissolve when developed unless it is exposed to light. The easiest photoresist to work with is the positive type.

Photoresists are purchased from manufacturers. The manufacturers provide usage instructions, guides and technical support. Typically, dispense the photoresist (a liquid) on a wafer. Then spin the wafer at 3,000 rpm for 60 seconds to produce a uniform film. Place the wafer on a hot-plate at the specified temperature for 2 minutes to dry the film. Place a photomask on top of the film and expose it to UV light. Immerse the film in a developer for 2 minutes. Finally inspect the pattern with the microscope.


Photoresist is sensitive to ultraviolet (UV) light. An exposure system produces a uniform beam of UV light across a large area, typically 5″x5″. This enables all patterns on the mask to be exposed with the same amount of light. A specific amount of light is needed to replicate the mask patterns exactly in the photoresist. With too little light, the photoresist will not completely dissolve when developed. With too much light, the patterns in the photoresist will be larger than the patterns in the photomask.

Close Menu