Improved resolution of submicron KrF laser ablation of polymers by a new filtered imaging irradiation
Résumé
Surface grating-like ablation with microscopic period (L~1-5 µm) is achieved at the surface of polymers with the aid of the KrF laser patterned with a mask projection technique using a precision imaging lens. The ablated patterns can be controlled with a submicron accuracy in most cases on smooth polymer surfaces. Prediction of these patterns can fur-thermore be made with the aid of theoretical models combining ablation curves, intensity profiles and hydrodynamic flow. We further demonstrate that by using harmonic filtering the grating period can be divided by a factor 2 thus leading to a convenient resolution enhancement. This improvement can be achieved by two experimental approaches. One is by changing the defocus and the other is the removal of low level orders (0 and ° 1) of the Fourier components of the image beams. Resulting grating patterns with doubled spatial frequency will be presented for a number of polymeric materials (AFM, SEM). It reveals to be a good method to overcome the theoretical limitations due to numerical aperture of the precision lens which prevent the transfer of details in the image by erasing the high spatial frequencies. This interesting optical set up and method offer also the advantage of a large working distance above the sample surface which is com-patible with irradiations in vacuum and liquid phase.