Material Processing Using Femtosecond Lasers: Repairing Patterned Photomasks
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Material Processing Using Femtosecond Lasers: Repairing Patterned Photomasks Richard Haight, Peter Longo, and Alfred Wagner Abstract The use of ultrafast laser pulses is having an impact on materials processing in profound ways. “Machining” with femtosecond pulses affords considerable advantages over nanosecond pulses, such as subdiffraction-limited material ablation, where ablated spot dimensions are below that achievable when longer pulses are focused to the minimum spot size dictated by optical physics. These properties have been exploited to address what had become a critical problem in the semiconductor industry, the repair of patterned photomasks. We will describe how the fundamentals of femtosecond laser ablation have been implemented in a machine designed to repair photomasks. We will also describe experiments designed to deposit Cr metal onto fused-silica substrates using 100-fs, 400-nm light pulses at atmospheric pressure. Multiphoton dissociation of Cr(CO)6 adsorbed on fused-silica substrates initiates Cr deposition. The mechanisms for deposition on both transparent (fused silica) and absorbing (Cr metal) substrates are discussed. Finally, we describe experiments that were carried out to extend the photomask repair process to shorter wavelengths (below 200 nm) using light generated by frequency-mixing of ultrashort, 30-fs pulses in an Ar-filled capillary. Keywords: Cr, laser ablation, machining, nanoscale.
Introduction Ablation of materials with femtosecond laser pulses has seen an increasing variety of applications in the area of precision micro- and even nanomachining. During the last 10 years, hundreds of papers have been published on various aspects and advantages of femtosecond laser ablation. Exceptional spatial precision is achieved because the fundamental mechanism of material removal with femtosecond laser pulses is dramatically different from that obtained with nanosecond pulses.1–3 In a nutshell, nanosecond laser pulses heat, melt, and then evaporate the metal intended for removal; this can result in
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metal splatter, rough edges, and, in the case we will discuss here, diffusion of metal into the underlying fused-silica photomask substrate, which stains the glass. The article by Reis et al. in this issue of MRS Bulletin describes the details of ultrafast ablation. The most important aspect is that the laser pulse is significantly shorter than the time it takes for electrons to transfer their energy to the lattice via electron– phonon coupling (τ e–ph ⬃ 1 ps in metals).4,5 Ablations exhibit sharp edges, consistent with a conversion of the material directly into a plasma with minimal thermal diffusion adjacent to the irradiated area.
Photomask Repair It has become widely recognized in the photomask industry that the soaring complexity of photomasks and the associated difficulty and cost in producing them place stringent requirements on methods for repairing defects that frequently occur during their manufacture. Photomasks are plates of high-quality fused silica coated with an absorber tha
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