A photoresist is a light-sensitive material used in photolithography to create a patterned coating on a surface. Some applications of photoresist are: microelectronics, patterning and etching of substrates, and fabrication of printed circuit boards. The electrical engineering industry relies heavily on resists for multiple applications, and expects them to be well formulated and consistent. Chemicals used in photoresist processes are PMMA, or Polymethyl methacrylate, PMGI, or Polymethyl glutarimide, DNQ/Novolac, or Phenol formaldehyde resin, and SU-8.
Deep Ultraviolet Photoresist
Strong deep ultraviolet absorption causes less photoresist sensitivity. More, and longer, UV exposure is required because the benzene-chromophore and DNQ-novolac absorption mechanisms together lead to stronger DNQ-novlac photoresists. Deep ultraviolet photoresists are usually polyhydroxystyrene-based polymers with a photoacid which is what creates the solubility change. There are deep UV photoresists that are widely used in semiconductor and TFH reader applications that require dry etching. The highly complex etch resistance properties of some UV photoresists are made for the dry etching environment.
Negative and Positive UV Resists
There are two types of deep UV resists: negative and positive. A negative resist is a photoresistthat becomes insoluble where the photoresist is exposed to light, while the unexposed section is dissolved by the photoresist developer. Therefore masks for negative UV photoresists contain the inverse of the pattern to be developed. Current negative photoresists on the market can exhibit better adhesion, though positive resists are now the dominant type of resist used in VLSI fabrication.
Positive deep UV photoresists act in just the inverse manner. For positive resists, the resist surface is exposed to the UV light wherever there is underlying material to remove. The UV light works by changing the chemical composition of the resist so that it is weakened and therefore soluble in a developer. The mask contains an exact copy of the pattern that is to remain.
E-beam, or electron-beam lithography, is at its most basic level, simply scanning a focused beam of electrons to inscribe shapes on an electron sensitive film surface that is also called a resist. E-beam resists work because the electron beam changes the solubility of the resist so that the desired material can be removed, leaving exposed regions. The resist surface is then “developed” in a solvent. While UV lithography has been the norm, and in many ways exceeded expectations, e-beam resists offer a promising alternative.