Ablation-Induced Stresses in Fused Silica by 157-nm F 2 -Laser irradiation
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Ablation-induced stresses in fused silica by 157-nm F 2-1aser irradiation Igor A. Konovalov and Peter R. Herman Department of Electrical and Computer Engineering University of Toronto 10 King's College Road, Toronto, ON, Canada, M5S 3G4
ABSTRACT The F2 laser is a promising source for direct etching of microstructures and the precise shaping of optical-grade surfaces on wide bandgap materials such as fused silica. We report here on residual tensile stresses induced in fused silica (Coming 7940, UV grade) by 157-nm laser ablation. Plastic strain of 160-mm thick rectangular strips, monitored with an optical interferometric microscope, revealed the presence of residual tensile stresses in the near-ablated surface. HF chemical thinning of the sample showed the thickness of ablation-affected layer provoking strain was -275 nm, a value independent of laser fluence (1.9-4.7 J/cm 2) and scanning speed (94 - 220 pmis). A near-surface mean residual tensile stress of -80 MPa was inferred from a thin film-substrate approximation. INTRODUCTION Thin-film devices and systems consisting of dissimilar materials or layers frequently exhibit substantial residual stresses as noted in numerous studies of thin films [1]. Large residual stresses can also develop during surface treatment involving ion, electron, or laser beams. While such treatments offer improved surface properties, the residual stresses become increasingly important in microsystems where feature sizes are shrinking to the dimensions of the affected surface layer. Residual stresses can hinder the operation of microelectronic, photonic or biological components and can lead to bending or buckling of microelectromechanical devices (MEMs). In laser material processing, a field of widespread commercial significance, residual stresses arise in the shaping and machining of materials and in the texturing and modification of surfaces. Laser-induced residual stresses can grossly affect the macroscopic properties of materials, when using ultraviolet lasers such as excimers to define feature sizes on the scale of 100's of nanometers. This paper examines the stresses formed by laser ablation of fused silica, a high-quality material widely used in optical and photonic applications. Laser damage and laser ablation of fused silica has been extensively studied using conventional sources such as C0 2, YAG, visible, and excimer [2-7]. Several kinds of damage related to residual stresses have been noted for fluences below the ablation threshold. Material densification and concomitant refractive index changes lead to residual tensile stresses in irradiated glasses [13]. Another form of damage includes the generation of surface or internal cracks, and the manifestation of residual tensile-stress fields around the crack [14-15]. Microcrack formation is common for fluences above the ablation threshold. In these cases, residual stresses increase with increasing fluence or number of laser pulses. For a special class of short wavelength sources such as the vacuum-ultraviolet Raman laser [6-9] and t
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