EUV Infrastructure: EUV photomask backside cleaning
Applied Materials as first author: Bruce J. Fender, Dusty Leonhard, Hugo Breuer, Jack Stoof
ASML: My Phung Van, Rudy Pellens, Reinout Dekkers, Jan Pieter Kuijten
Due to electrostatic chucking of the backside of EUV masks, backside cleanliness in EUV lithography is an important factor. Contamination on the backside can cause damage to reticle (e-chuck), cross-contaminate to the scanner or cause local distortions in the reticle. Cleaning of the masks offers a solution to reduce the defectivity level on reticles. However, repeated cleaning on masks is known to have an impact on absorber, CD and reflectivity. Ideally, cleaning should occur without any alterations to the critical features on the front side of the mask. With the introduction of pellicles for EUV, there could be an additional drive for backside-only cleaning.
In this work the GuardianTM Technology is introduced that enables backside cleaning without any cleaning impact on the reticle front side through a protective seal at the outer edge of the mask. The seal protects the front side during the backside clean. The cleaning process encompasses a single-sided pre-clean oxygen plasma treatment of the mask surface, followed by sonic cleaning, and ending with a rinse and dry step. Separating the mask backside from front side enables:
• Backside cleaning without any cleaning impact on features on the mask front side.
• The isolation allows an aggressive cleaning of the backside to ensure defect removal.
• Processing of reticle with studs on the front side. This prevents unnecessary actions of stud removal and removal of the remaining glue after stud removal and subsequent gluing of the studs after cleaning.
Just before chucking of a reticle, the defectivity level on the mask is initially inspected with an in-scanner reticle backside inspection tool. The GuardianTM cleaning process is able to remove the vast majority of the cleanable defects that could impact scanner performance. Post GuardianTM clean interferometric microscope defect review reveals the remaining defects > 25-μm-PSL are ~78% are indent/damage and 11% are defects with insignificant height to impact scanner performance or cleanliness.
Laser direct-writing is an important technique for the fabrication of complex patterns. There is a continuous need for structures with increasingly small features, i.e., enhanced resolution. Focused radially polarized light is known to exhibit a narrow longitudinal polarization component. Here, a proof-of-concept is shown of enhanced resolution through polarization-selectivity by the selective recording of the longitudinal polarization component in a polarization-selective homeotropic and smectic B photoresist. The full-width-at-half-maximum (FWHM) of the fabricated spots in the polarization-selective resist is up to 56% smaller compared to the FWHM of the same spot in a photoresist that is not polarization-selective, which supports simulations that predict a theoretical maximum reduction of 62%.
We developed a polarization-selective negative photoresist based on a smectic B liquid crystal monomer host functionalized with a dichroic photoinitiator. The smectic phase enables high-order parameter uniaxial alignment of the monomer host molecules. It is shown that the dichroic initiator aligns with the host which provides the polarization selectivity upon UV initiation of the polymerization of the monomer system. The polymerization contrast with respect to its sensitivity for polarized UV light can become infinitely high by the addition of an inhibitor. We tested the new lithographic material for its application in polarization holography. These experiments show that the polymerization contrast can be translated into the formation of well-defined structures but require further optimization.