Cryogenic Etching of the Electroplating Mold for Improved Zone Plate Lenses

• Main Author: Larsson, Daniel
• Format: Others
• Language: English
• Published: KTH, Tillämpad fysik 2010
• Subjects: etching; plasma etching; zone plate; x-ray; nanofabrication; sidewall passivation; cryo; cryogenic; undercut; Engineering physics; Teknisk fysik; Biological physics; Biologisk fysik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-12109

The fabrication of zone plate lenses that are used for focusing X-rays relies on nanofabrication techniques such as e-beam lithography, reactive ion etching, and electroplating. The circular grating-like zone plate pattern can have a smallest half-period, a so-called zone width, of down to 20 nm while it also needs to have a height that is 5 to 10 times the zone width to have good diffraction efficiency. This high aspect ratio structuring is a very challenging field of nanofabrication. This diploma project has focused on improving the process step of fabricating the electroplating mold by cryo-cooling the polymer during the reactive ion etching with O2. The low temperature causes passivation of the sidewalls of the mold during etching which results in a more ideal rectangular profile of the high aspect ratio plating mold. By etching at -100 °C, structures with highly vertical sidewalls and no undercut were realized. The experiments showed that there is a tradeoff between the anisotropy of the zone profile and the formation rate of polymer residue, so-called RIE grass. Through a proper choice of process parameters the grass could be completely removed without introducing any undercut. === QC 20100414