Editing Photoresist (section)

==Light sources==

===Absorption at UV and shorter wavelengths===
In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution.<ref>{{cite journal |last1=Bratton |first1=Daniel |last2=Yang |first2=Da |last3=Dai |first3=Junyan |last4=Ober |first4=Christopher K. |title=Recent progress in high resolution lithography |journal=Polymers for Advanced Technologies |language=en |volume=17 |issue=2 |pages=94–103 |s2cid=55877239 |issn=1099-1581 |date=2006-02-01 |doi=10.1002/pat.662 }}</ref> Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400&nbsp;nm). For example, [[diazonaphthoquinone]] (DNQ) absorbs strongly from approximately 300&nbsp;nm to 450&nbsp;nm. The absorption bands can be assigned to n-π* (S0–S1) and π-π* (S1–S2) transitions in the DNQ molecule.{{Citation needed|date=September 2018}} In the [[deep ultraviolet]] (DUV) spectrum, the π-π* electronic transition in benzene<ref>{{cite journal |last1=Ishii |first1=Hiroyuki |last2=Usui |first2=Shinji |last3=Douki |first3=Katsuji |last4=Kajita |first4=Toru |last5=Chawanya |first5=Hitoshi |last6=Shimokawa |first6=Tsutomu |title=Design and lithographic performances of 193-specific photoacid generators |journal=Advances in Resist Technology and Processing XVII |volume=3999 |pages=1120–1127 |bibcode=2000SPIE.3999.1120I |s2cid=98281255 |editor1-last=Houlihan |editor1-first=Francis M |date=2000-01-01 |doi=10.1117/12.388276}}</ref> or carbon double-bond chromophores appears at around 200&nbsp;nm.{{Citation needed|date=September 2018}} Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger [[photon energy]]. Photons with energies exceeding the ionization potential of the photoresist (can be as low as 5 eV in condensed solutions)<ref>{{cite journal |last=Belbruno |first=Joseph |year=1990 |title=Multiphoton-induced chemistry of phenol in hexane at 266 nm |journal=Chemical Physics Letters |volume=166 |issue=2 |pages=167–172 |bibcode=1990CPL...166..167B |doi-access=free |doi=10.1016/0009-2614(90)87271-r}}</ref> can also release electrons which are capable of additional exposure of the photoresist. From about 5 eV to about 20 eV, photoionization of outer "[[valence band]]" electrons is the main absorption mechanism.<ref>{{cite journal |year=2006 |title=Photoelectric Emission from Dust Grains Exposed to Extreme Ultraviolet and X-Ray Radiation |journal=The Astrophysical Journal |volume=645 |issue=2 |pages=1188–1197 |arxiv=astro-ph/0601296 |bibcode=2006ApJ...645.1188W |last1=Weingartner |first1=Joseph C |last2=Draine |first2=B. T |last3=Barr |first3=David K |s2cid=13859981 |doi=10.1086/504420}}</ref> Above 20 eV, inner electron ionization and Auger transitions become more important. Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the higher photon energy. The absorbed energy can drive further reactions and ultimately dissipates as heat. This is associated with the outgassing and contamination from the photoresist.

===Electron-beam exposure===
Photoresists can also be exposed by electron beams, producing the same results as exposure by light. The main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy gradually, and scatter within the photoresist during this process. As with high-energy wavelengths, many transitions are excited by electron beams, and heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; essentially the electron has to be at rest with respect to the molecule in order to react most strongly via dissociative electron attachment, where the electron comes to rest at the molecule, depositing all its kinetic energy.<ref>{{cite journal |year=2006 |title=IR photon enhanced dissociative electron attachment to SF6: Dependence on photon, vibrational, and electron energy |journal=Chemical Physics |volume=329 |issue=1–3 |pages=148 |bibcode=2006CP....329..148B |last1=Braun |first1=M |last2=Gruber |first2=F |last3=Ruf |first3=M. -W |last4=Kumar |first4=S. V. K |last5=Illenberger |first5=E |last6=Hotop |first6=H |doi=10.1016/j.chemphys.2006.07.005}}</ref> The resulting scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent, or else releases other chemical species (acids) which catalyze further scission reactions (see the discussion on chemically amplified resists below). It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.