Next-Generation Lithography | Clean, Bake and Develop
Each introduction of a new technology node necessitates the development of innovations in photomask processing. SUSS MicroTec works in close cooperation with customers, technology partners and OEMs SUSS to completely understand the requirements of its customers in mask shops and wafer fabs and to deliver solutions that guarantee the highest level of mask integrity.
SUSS MicroTec’s established suite of photomask processing equipment covers the full range of technology nodes, including next-generation lithography of 1x half-pitch ranges from stripping and cleaning to post-exposure bake and developing processes.
Contamination of the mask affects the correct imaging process in the lithography tool during device manufacturing. Sub-micron particles as well as organic and inorganic contamination threaten to endanger yield. Thus, preparation, cleaning and handling of photomasks are playing a vital part in lithographical production processes. Moreover, next-generation lithography with its drive to go to technology nodes down to 22 nm hp and beyond requires very efficient cleaning technologies by avoiding pattern damage and changes in optical properties as much as possible. A “zero particles” philosophy is therefore indispensable.
Substrates range from conventional binary photomasks of all kind of materials to phase-shift masks (PSM). Each requires matching techniques regarding cleaning in the photomask production process as well as during application by the end users (device manufacturers).
A comprehensive cleaning process involves:
Before applying wet cleaning the photomask surface needs to be activated by changing the surface properties from hydrophobic to hydrophilic. A possible method is the treatment of the surface with 172nm UV light which is a so-called dry technique. Wet-based approaches are the SPM (sulfuric-acid, hydrogen-peroxide mixture) process and the in situ UV-induced generation in liquid. They all aim at removing hydrophobic surface layers by an increase of hydroxyl groups on the photomask surface.
After final wet cleaning, it has to be ensured that the surface is kept in this clean state to extend the mask life time as much as possible and by that to decrease the customer’s cost of ownership. This becomes even more important with EUV lithography where scanner compatibility plays a critical role and any downtime of the exposure tool due to cross contamination by the photomask has to be avoided.
Therefore, different surface preservation and purification processes following the final wet cleaning are required. Residual organic and inorganic ions which are still embedded in the substrate surface and cause haze creation have to be removed as well as remaining molecular moisture.
SUSS MicroTec’s automated photomask cleaning equipment offers a variety of post-treatments. Depending on substrate type and challenges, either high temperature baking, soft RTP (Rapid Thermal Processing) or exposure with 172nm UV light can be applied to guarantee photomask pattern integrity.
At photomask cleaning, typically both physical and chemical cleaning techniques are used to remove particles as well as organic and inorganic contamination.
For advanced 193i optical and EUV lithography reticles, a combination of high precision megasonic and nano binary spray processes with different type of media results in extremely good particle removal efficiency rates.
In addition, the innovative “green” in situ UV technology offers a wide range of chemical free process capabilities like resist strip, organic contamination removal and final clean preserving the pattern integrity by avoiding pattern damage or changes of optical properties of the photomask.