Editing Electron-beam lithography (section)

==Electron-beam resist performance==
Due to the scission efficiency generally being an order of magnitude higher than the crosslinking efficiency, most polymers used for positive-tone electron-beam lithography will also crosslink (and therefore become negative tone) at doses an order of magnitude <!--higher or lower?--> higher than the doses used to cause scission in the polymer for positive tone exposure.<!--This is confusing. If scission efficiency is higher why would crosslinking happen easier?--> In the case of PMMA, exposure of electrons at up to more than 1000 μC/cm<sup>2</sup>, the gradation curve corresponds to the curve of a “normal” positive process. Above 2000 μC/cm<sup>2</sup>, the recombinant crosslinking process prevails, and at about 7000 μC/cm<sup>2</sup> the layer is completely crosslinked which makes the layer more insoluble than the unexposed initial layer. If negative PMMA structures should be used, a stronger developer than for the positive process is required.<ref>J. N. Helbert et al., ''Macromolecules'', vol. 11, 1104 (1978).
</ref> Such large dose increases may be required to avoid shot noise effects.<ref>{{cite conference |first1=M. |last1=Wieland |first2=G. |last2=de Boer |first3=G. |last3=ten Berge |first4=R. |last4=Jager |first5=T. |last5=van de Peut |first6=J. |last6=Peijster |first7=E. |last7=Slot |first8=S. |last8=Steenbrink |first9=T. |last9=Teepen |first10=A.H.V. |last10=van Veen |first11=B.J. |last11=Kampherbeek |title=MAPPER: high-throughput maskless lithography |book-title=Alternative Lithographic Technologies |volume=7271 |series=SPIE Proceedings |date=2009 |doi=10.1117/12.814025 |pages=72710O }}</ref><ref>{{cite conference |first1=Frederick |last1=Chen |first2=Wei-Su |last2=Chen |first3=Ming-Jinn |last3=Tsai |first4=Tzu-Kun |last4=Ku |title=Complementary polarity exposures for cost-effective line-cutting in multiple patterning lithography |book-title=Optical Microlithography XXV |volume=8326 |series=SPIE Proceedings |date=2012 |doi=10.1117/12.912800 |pages=83262L }}</ref><ref>{{cite journal |last1=Kruit |first1=P. |last2=Steenbrink |first2=S. |last3=Jager |first3=R. |last4=Wieland |first4=M. |title=Optimum dose for shot noise limited CD uniformity in electron-beam lithography |journal=Journal of Vacuum Science & Technology B  |volume=22 |issue=6 |pages=2948–55 |date=2004 |doi=10.1116/1.1821577 |bibcode=2004JVSTB..22.2948K }}</ref>

A study performed at the Naval Research Laboratory<ref>{{cite journal|doi=10.1116/1.591134|title=Low-energy electron-beam effects on poly(methyl methacrylate) resist films|year=1999|last1=Bermudez|first1=V. M.|journal=Journal of Vacuum Science and Technology B|volume=17|page=2512|bibcode = 1999JVSTB..17.2512B|issue=6 }}</ref> indicated that low-energy (10–50&nbsp;eV) electrons were able to damage ~30&nbsp;nm thick PMMA films. The damage was manifest as a loss of material.

*For the popular electron-beam resist ZEP-520, a pitch resolution limit of 60&nbsp;nm (30&nbsp;nm lines and spaces), independent of thickness and beam energy, was found.<ref>H. Yang ''et al.'', Proceedings of the 1st IEEE Intl. Conf. on Nano/Micro Engineered and Molecular Systems, pp. 391–394 (2006).</ref>
*A 20&nbsp;nm resolution had also been demonstrated using a 3&nbsp;nm 100&nbsp;keV electron beam and PMMA resist.<ref>{{cite journal|doi=10.1063/1.116073|title=Fabrication of 3&nbsp;nm wires using 100&nbsp;keV electron beam lithography and poly(methyl methacrylate) resist|year=1996|last1=Cumming|first1=D. R. S.|last2=Thoms|first2=S.|last3=Beaumont|first3=S. P.|last4=Weaver|first4=J. M. R.|journal=Applied Physics Letters|volume=68|page=322 |bibcode = 1996ApPhL..68..322C|issue=3}}</ref> 20&nbsp;nm unexposed gaps between exposed lines showed inadvertent exposure by secondary electrons.
*[[Hydrogen silsesquioxane]] (HSQ) is a negative tone resist that is capable of forming isolated 2-nm-wide lines and 10&nbsp;nm periodic dot arrays (10&nbsp;nm pitch) in very thin layers.<ref>{{cite journal | last1 = Manfrinato | first1 = Vitor R. | last2 = Zhang | first2 = Lihua | last3 = Su | first3 = Dong | last4 = Duan | first4 = Huigao | last5 = Hobbs | first5 = Richard G. |authorlink6=Eric Stach| last6 = Stach | first6 = Eric A. | last7 = Berggren | first7 = Karl K. | author7-link = Karl K. Berggren | year = 2013 | title = Resolution limits of electron-beam lithography toward the atomic scale | url =https://dspace.mit.edu/bitstream/1721.1/92829/1/STEM%20Lithography%20Manuscript-after_review-preprint.pdf | journal = Nano Lett. | volume = 13 | issue = 4| pages = 1555–1558 | doi = 10.1021/nl304715p | pmid = 23488936 | bibcode = 2013NanoL..13.1555M | hdl = 1721.1/92829 | s2cid = 1060983 | hdl-access = free }}</ref> HSQ itself is similar to porous, hydrogenated SiO<sub>2</sub>. It may be used to etch silicon but not silicon dioxide or other similar dielectrics.

In 2018, a thiol-ene resist was developed that features native reactive surface groups, which allows the direct functionalization of the resist surface with biomolecules.<ref>{{Cite journal|last1=Shafagh|first1=Reza|last2=Vastesson|first2=Alexander|last3=Guo|first3=Weijin|last4=van der Wijngaart|first4=Wouter|last5=Haraldsson|first5=Tommy|year=2018|title=E-Beam Nanostructuring and Direct Click Biofunctionalization of Thiol–Ene Resist|journal=ACS Nano|volume=12|issue=10|pages=9940–6 |doi=10.1021/acsnano.8b03709|pmid=30212184|s2cid=52271550 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-236089}}</ref>