Editing Photoresist (section)

==Simple resist polarity==
'''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.

=== Positive photoresist ===
[[File:Acid catalyze photoresist.tif|thumb|A positive photoresist example, whose solubility would change by the photogenerated acid. The acid deprotects the ''tert''-butoxycarbonyl (t-BOC), inducing the resist from alkali-insoluble to alkali-soluble. This was the first chemically amplified resist used in the semiconductor industry, which was invented by Ito, Willson, and Frechet in 1982.<ref>{{cite journal |last1=Ito |first1=H. |last2=Willson |first2=C. G. |last3=Frechet |first3=J. H. J. |title=New UV Resists with Negative or Positive Tone |journal=1982 Symposium on VLSI Technology. Digest of Technical Papers |pages=86–87 |date=1982-09-01 |url=https://ieeexplore.ieee.org/document/4480589}}</ref>|455x455px]]
[[File:Positive photoresist SO2.tif|thumb|282x282px|An example of single-component positive photoresist]]

A ''positive photoresist'' is a type of photoresist in which a portion is exposed to light and becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble in the photoresist developer.

Some examples of positive photoresists are:

'''PMMA''' (polymethylmethacrylate) single-component
* Resist for deep-UV, e-beam, x-ray
* Resin itself is DUV sensitive (slow)
* Chain scission mechanism

Two-component DQN resists:
* Common resists for mercury lamps
* Diazoquinone ester (DQ) 20-50% weight
** photosensitive
** hydrophobic, not water soluble
* Phenolic Novolak Resin (N)
** Frequently used for near-UV exposures
** Water soluble
** UV exposure destroys the inhibitory effect of DQ
* Issues: Adhesion, Etch Resistance

===Negative photoresist===
[[File:Polyisoprene negative photoresist.tif|thumb|346x346px|A crosslinking of a polyisoprene rubber by a photoreactive biazide as negative photoresist]]
[[File:Acrylate negative photoresist.tif|thumb|333x333px|A radical induced polymerization and crosslinking of an acrylate monomer as negative photoresist]]

A ''negative photoresist'' is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble in the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.

* Based on cyclized polyisoprene (rubber)
** variety of sensitizers (only a few % by weight)
** free radical initiated photo cross-linking of polymers
* Issues:
** potential oxygen inhibition
** swelling during development
*** long narrow lines can become wavy
*** swelling is an issue for high-resolution patterning
* Example: [[SU-8 photoresist|SU-8]] (epoxy-based polymer), good adhesion), [[KPR|Kodak Photoresist (KPR)]]

'''Modulation transfer function'''

MTF (modulation transfer function is the ratio of image intensity modulation and object intensity modulation and it is a parameter that indicates the capability of an optical system.

===Differences between positive and negative resist===

The following table<ref>{{cite book |title=Fundamentals of Microfabrication |last=Madou |first=Marc |publisher=CRC Press |isbn=978-0-8493-0826-0 |date=2002-03-13}}
</ref> is based on generalizations which are generally accepted in the [[microelectromechanical systems]] (MEMS) fabrication industry.
{| class="wikitable"
! Characteristic
! Positive
! Negative
|-
|Adhesion to silicon
|Fair
|Excellent
|-
|Relative cost
|More expensive
|Less expensive
|-
|Developer base
|Aqueous
|Organic
|-
|Solubility in the developer
|Exposed region is soluble
|Exposed region is insoluble
|-
|Minimum feature
|0.5&nbsp;μm
|7&nbsp;nm
|-
|Step coverage
|Better
|Lower
|-
|Wet chemical resistance
|Fair
|Excellent
|}