Processing of Dielectric Optical Coatings by Nanosecond and Femtosecond UV Laser Ablation

J Ihlemann,J Békési,P Simon,J.-H Klein-Wiele

doi:10.1155/2008/623872

Abstract

Microprocessing of dielectric optical coatings by UV laser ablation is demonstrated. Excimer laser ablation at deep UV wavelengths (248 nm, 193 nm) is used for the patterning of thin oxide films or layer stacks. The layer removal over extended areas as well as sub-μm-structuring is possible. The ablation of SiO2, Al2O3, HfO2, and Ta2O5 layers and layer systems has been investigated. Due to their optical, chemical, and thermal stability, these inorganic film materials are well suited for optical applications, even if UV-transparency is required. Transparent patterned films of SiO2 are produced by patterning a UV-absorbing precursor SiOx suboxide layer and oxidizing it afterwards to SiO2. In contrast to laser ablation of bulk material, in the case of thin films, the layer-layer or layer-substrate boundaries act as predetermined end points, so that precise depth control and a very smooth surface can be achieved. For large area ablation, nanosecond lasers are well suited; for patterning with submicron resolution, femtosecond excimer lasers are applied. Thus the fabrication of optical elements like dielectric masks, pixelated diffractive elements, and gratings can be accomplished.

Highlights

Optical coatings have a variety of applications
The thermal diffusion length L of around 50 to 500 nm which is related to the laser pulse length τ by L∼τ1/2 and characterizes the heat-affected zone (HAZ) is short enough to minimize lateral damage, but sufficiently long to provide heat flow within the typical layer thickness, which promotes the liftoff of a complete layer with a single pulse [6, 7]
At about 1 J/cm2 the fused silica substrate is ablated, too. These observations can be explained by treating the layer-substrate boundary as predetermined end points, so that precise depth control and a very smooth surface can be achieved, even if the laser beam is somewhat inhomogeneous within the limits given by the process window

Summary

Introduction

Optical coatings have a variety of applications. Design, fabrication, and applications of dielectric optical interference coatings are subject of numerous studies [1]. The production of dielectric coatings has been optimized to obtain such high quality coatings nearly without defects on large substrates. There are other applications, where the coating is needed in locally well-defined areas, for example, masks or waveguides. Coating technology is not well developed in this direction. Deposition through stencil masks is possible but not with high spatial resolution. In this case, the coatings have to be processed following the deposition process in order to generate spatially well-defined patterns. Lithographic processes are applied, but they have limited applicability, because the required etching processes are complicated and not sufficiently developed for all used materials

Methods

Results

Conclusion