Laser Cleaning for Removal of Nano/Micro-Scale Particles and Film Contamination – Elena

(size) In the dry laser cleaning technique, to understand the modes of substrate damage, both thermal and mechanical (stress) damage thresholds must be well understand and accurately model. A key complication in such analysis is that the material properties vary substantially with strain rates and temperature changes. And it is often not well understood how certain materials yiel under high thermoelastic strain rate excitations. These thresholds could then be use to avoid excessive heat deposition and/or stress levels. The complexity of the thermoelastic process requires that a detail analysis be carry out for the determination of optimal removal efficiencies.

The onset of the material damage due to thermal and mechanical fields in this technique was model for the first time in [46], for silicon particles on copper and silicon substrates.

It was report that for the DLC method the require laser fluence for removing a particular size particle. Depends on the coefficient of reflectivity of the substrate and the particle. Base on the computational model, the critical limit of DLC was determine. In terms of the minimum diameter of the silicon particles that could be remove from silicon and copper substrates without any damage.

Our research group at the Photo-Acoustics Research Laboratory at Clarkson University has report that the type of damage initiation (thermal or mechanical fracture). Depends on the particle size that has to be remove and particle-substrate system material properties. And the damage risk for sub-100 nm particle removal with DLC is found to be very high [46]. The thermal damage threshold is the melting temperature, and mechanical damage occurs above the yield stress of the substrate material. Thus, the thermo-elastic simulations from [46] indicate that when silicon particles smaller than 600 nm are remove from a silicon substrate. And when silicon particles smaller than 630 nm are remove from copper substrate using DLC. It would result in thermal and mechanical damage, respectively.

It should be note that the only effects consider in the report study were linear thermal and mechanical fields. And no other types of damage such as optical and electromagnetic damage mechanisms were consider. Delicate structures on substrates and processing techniques used to machine them sometimes substantially lower the local material yield properties. And such structures can amplify the laser beam on the textured surface due to diffraction. Thus their cleaning requires substantially greater care than flat substrates.