Onset of Material Alterations Due to Laser-Induced Plasma Exposure in Nanofilms Deposited on Photomasks

Abstract

Damage-free removal of sub-100 nm particles from photomasks with deposited nanofilms is a challenge in lithography. Laser-induced plasma (LIP) is an emerging noncontact, chemical-free, dry, and selective nanoparticle removal technique. Investigation of the onset of material alterations on bonded nanofilms for optimizing LIP particle removal process is the objective of this paper. Shockwave thermomechanical excitation and radiation heating from the plasma core are major potential sources of damage. Computational analyses reported here indicate radiation heating as the chief damage source. Damage thresholds for the critical nanofilm surface temperature rise and radial stress component have been identified for a given nanosecond pulsed laser.Index Terms-Cleaning of thin films, damage threshold, material alteration, nanofilm, nanoparticle removal, photomasks, radiation intensity from LIP, shockwave.

Transient Thermoelastic Response of Nanofilms Under Radiation Heating From Pulsed Laser-Induced Plasma

“…The static yield stress of chrome film is 362 MPa. However, considering the experimental studies and the dynamic strain rate factor reported in [18], there would not be any damage at a threshold firing distance of 2.5 mm.…”

Laser-Induced Plasma Exposure on Extreme Ultraviolet Lithography Masks: Damage Analysis

“…Discoloration is the least damaging of the material alteration modes observed. LIP exposure on these EUVL mask blank samples, when compared to that on photomask blank samples [23], indicate more severe cracking on photomasks. In the experiments reported in is paper, no material alteration or damage was observed for LIP application at a clearance distance of d = 3.6 mm for n = 200 shots on the EUVL mask blank sample consisting of a 2.5-nm Ru on 105 nm of Mo/Si MLs on a LTEM substrate.…”

“…Previously, LIP shockwaves and the radiation intensity heating were characterized to determine whether material alterations would be initiated on EUVL masks. Utilizing transient pressure transducers, LIP shockwave pressure was acquired, shockwave temperature was estimated (from gas dynamics relationships and the measured pressure levels) [21], and the radiation intensity was approximated from measured radiation energy (utilizing a volume absorber-type radiant power meter) [22], [23]. A finite element (FE) analysis was conducted by using these loading conditions on thin films and obtaining the resultant stresses and surface temperature on the EUVL mask.…”

“…Thermomechanical analysis was conducted for LIP radiation intensity heating on an EUVL mask prototype to obtain sustainable film surface temperature and induced stresses as was done for the case of the photomask [21], [23]. A thermomechanical investigation of a Ru film on Mo/Si substrate (average properties of Mo and Si taken for a single layer instead of several MLs) is explored by a dynamic and fully coupled linear thermal-stress analysis (using explicit integration scheme) conducted using ABAQUS.…”

“…Assuming no direct plasma contact with the substrate, LIP shockwaves and transient radiation heating from the plasma core are also possible sources of material alteration of the EUVL mask. In previous reports [21], [23], material alteration studies and computational analyses due to LIP shockwave loading and radiation heating were conducted for photomasks with 100-nm chromium (Cr) films on quartz substrates. In current damage risk study, experimental and thermomechanical computational analyses of LIP-exposed EUVL mask blank samples with a 2.5-nm ruthenium (Ru) film on 105-nm-thick molybdenum (Mo) and silicon (Si) multilayer (ML) stack (i.e., 15 MLs of Mo and Si) with low thermal expansion material (LTEM) glass (with extremely low thermal expansion coefficient) as the substrate are reported.…”