A Novel Cleaning Approach to Remove the Carbon Contaminant from Ru Surface of EUV Mask

Monday, 2 October 2017: 15:40
Chesapeake I (Gaylord National Resort and Convention Center)
M. S. Kim (Hanyang University), H. J. Song (Hanyang university), H. T. Kim, and J. G. Park (Hanyang University)
The surfaces of extreme ultraviolet (EUV) mask and optics are very vulnerable to carbon contamination during EUV lithography process. The top surface of EUV mask is coated with Ru capping layer to protect the mask substrate. Carbon contamination occurs on Ru surface when the EUV light of 13.5 nm interacts with the surface. The carbon contamination on mask surface is known to lose its reflectivity and change the critical dimension (CD). Thus, the removal of carbon contaminant from mask surface is required to maintain the integrity of EUV lithography process. Conventional cleaning methods such as SPM and UV cleaning are widely used for the removal of organic contaminants but can cause surface damages on EUV mask such as surface oxidation and etching of absorber pattern.  
In this study, a novel cleaning mechanism is proposed to remove carbon contaminant using a quaternary ammonium hydroxide solution and an organic solvent mixture solution. According to the SN2 (nucleophilic substitution) reaction, hydroxyl ions are ready to react with carbon atoms as nucleophiles. Also, polar aprotic solvents such as DMSO solvate cations to actively assist hydroxyl ion reactions. Therefore, a study on the removal of carbon contaminants was carried out using a mixture of quaternary ammonium hydroxide solution and polar aprotic solvent. There have been limited studies reported on organic solvent cleaning in a semiconductor process for removal of photoresist. However, physical cleaning such as megasonic and UV treatment were applied to the organic solvent cleaning process to improve organic removal. Few studies have also been carried out to remove the photoresist by applying an additional cleaning process such as CO2 aerosol, megasonic, and UV pretreatment to the organic solvent. So far no studies have been reported on the removal of carbon contaminants using quaternary ammonium hydroxide solution with an organic solvent. This study investigated the effect of cleaning solution on hydrocarbon film removal by using four kinds of quaternary ammonium hydroxide (TMAH, TEAH, TPAH and TBAH) as alkaline chemicals and DMSO and THF organic solvents as polar aprotic solvents. Instead of EUV carbon contaminant, a hydrocarbon film with similar properties to EUV carbon contaminant was prepared and evaluated using alkaline chemicals and organic solvents. The hydrocarbon film was effectively removed from Ru surface in the mixture of TBAH and THF. This is because of low polarity nature of both TBAH and THF. When DMSO used with quaternary ammonium hydroxide solution, hydrocarbon was also removed with longer process time. The carbon removal efficiency is improved and reduced the cleaning process time for TBAH+DMSO cleaning process with megasonic due to the easy lift-off of hydrocarbon film from Ru surface. The polarity of carbon contaminant and cleaning solution have to be considered for removal carbon contaminant from Ru surface.