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Concept
Photoresist
Summary
A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry.
The process begins by coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface.
In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed.
In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.
A BARC coating (bottom anti-reflectant coating) may be applied before the photoresist is applied, to avoid reflections from occurring under the photoresist and to improve the photoresist's performance at smaller semiconductor nodes.
Conventional photoresists typically consist of 3 components: resin (a binder that provides physical properties such as adhesion, chemical resistance, etc), sensitizer (which has a photoactive compound), and solvent (which keeps the resist liquid).
Positive: light will weaken the resist, and create a hole
Negative: light will toughen the resist and create an etch resistant mask.
To explain this in graphical form you may have a graph on Log exposure energy versus fraction of resist thickness remaining. The positive resist will be completely removed at the final exposure energy and the negative resist will be completely hardened and insoluble by the end of exposure energy. The slope of this graph is the contrast ratio. Intensity (I) is related to energy by E = I*t.
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