Computer Simulation of Processes at Electron and Ion Beam Lithography, Part 2: Simulation of Resist Developed Images at Electron and Ion Beam Lithography

Abstract

Polymer resists are recording and transfer media in electron, ion and photon lithography. The primary aim of lithography process is mastering of relief image in polymer resist layer. This developed and baked pattern is performed to be transferred to substrate trough either: a) plasma or ion (dry) processes or b) wet chemical etching. The transfer of resist relief image can be realised by ion implantation or by “lift off” of deposited on its top thin film (only deposited in open windows in the resist pattern remains after treatment on a strong solvent namely acetone or methylene chloride). The use of these various microstructure technologies is to create optical, X-ray, electron or ion masks as well as for direct writing of the desired microstructure on the semiconductor wafers for production of integrated circuits. In the same way the resists are used at mastering the micro-structured elements of micro-devices. The conventional polymer resist is dissolved in a liquid solvent. Usually the liquid resist is dropped onto the sample, which then spins at 1000 to 6000 rpm to form a smooth and uniform film. Immersion deposition of very thin resist film is also applicable. After that the deposited resist layer is pre-baked to 70-90oC to dry the prepared film. During exposure with appropriate radiation scission and/or cross-linking of the polymer molecules are observed in the resists. An alternative for more complicated resist as chemically amplified resists (CAR’s) is creation of an acid agent (from an acid generator component in the compound of the twoor threecomponent resists) that changes the dissolving protection of the base resin. In this case there are two bake steps: a) pre-bake performed after the spin-coating, aiming to dry the resist by removing the solvent from the deposited resist layer as well as b) post exposure bake (PEB) – the purpose of which is to cause a thermal catalysis of the chemical reactions amplifying the latent bulk image. The second baking step (PEB) is performed in an oven or on a hot plate. The PEB temperature and time are critical for the resist acid generator modification and for acid diffusion and reduction (degradation losses) as well as its evaporation. At the end of the transfer process the resist is washed away in a solvent such as acetone or a special resist stripper. Often the strip of the resist is done by reactive ion etching in O2.