Maximizing EUV Photomask Lifetime with Backside Cleaning
EUV lithography requires backside holding of the reticle using an electrostatic chuck (E-chuck). Due to this fact, backside reticle cleanliness in EUV lithography is an important factor. Contamination on the reticle backside can cause damage to reticle e-chuck, cross-contaminate the scanner, and larger defects can cause local distortions in the reticle resulting in pattern placement errors (Overlay) on the wafer. Backside cleaning of the reticles is a requirement. However, repeated cleaning on masks is known to have an impact on absorber, CD and reflectivity. In addition, due to the nature of these defects an aggressive cleaning process is required. Backside cleaning should occur without any alterations to the critical features on the front side of the reticle. With the introduction of pellicles for EUV, there could be an additional drive for backside-only cleaning.
In this work the GuardianTM chamber enables backside cleaning without any impact on the reticle front side. The GuardianTM uses a protective seal that seals on the outer edge of the reticle. The seal protects and isolates the front side from the backside during the backside clean. The cleaning process encompasses a 1) single-sided pre-clean plasma treatment of the reticle surface, 2) followed by sonic cleaning, and 3) ending with a rinse and dry step. Isolating the reticle backside from front side enables backside cleaning requirements:
• No impact on critical features on the reticle front side.
• Aggressive clean to remove larger pattern placement (Overlay) surface defects.
• Aggressive clean to remove smaller surface defects leading to scanner protection
• Cleaning with pellicle studs attached.
The GuardianTM cleaning process is able to remove the vast majority of the surface defects that could impact scanner performance from a cleanliness and overlay standpoint. The GuardianTM process is capable of recovering out-of-specification reticles from backside defects. The GuardianTM process is aggressive and can recover reticles that have been previously cleaned using conventional cleans and are still unusable, due to remaining large overlay defects. This maximizes EUV mask lifetime and lowers Cost of Ownership. The process, also, effectively removes smaller defects that improve scanner cleanliness, performance, and uptime. In addition, the process does not enlarge subsurface (pits) non-removable defects.