Doug Krajnovich

Experimental and theoretical studies of bump formation during laser texturing of Ni-P disk substrates

We provide here extended experimental data on the bump shape formed on Ni-P disk substrate under various laser and substrate surface conditions, and describe a model for the Sombrero-shape bump formation based on competition between thermocapillary and chemicapillary flows. This model rationalizes a large body of experimental data and provides insight into how one might tailor bump shapes to specific requirements. For comparison, laser bump formation on non-Ni-P substrates has also been studied.

Effect of intense and prolonged 248 nm pulsed‐laser irradiation on the properties of ultraviolet‐grade fused silica

We have studied changes in the absorption, temperature, ultraviolet (UV) spectrum, birefringence, and surface profile of various high‐purity ‘‘UV‐grade’’ fused silica samples during KrF excimer laser irradiation at 400 mJ/cm2 and 300 Hz and up to a total of 60 million pulses. All samples exhibit permanent and irreversible changes in the spectroscopic and physical properties. Some samples show a partial ‘‘self‐annealing’’ behavior during the irradiation in which the absorption first increases to a peak value of ∼10%/cm, and then decreases and levels off at a lower value. This may indicate the formation of a new stable state. The radiation‐induced effects are significantly affected by the laser repetition rate, annealing, and ambient temperature of the sample. For comparison, crystalline quartz is tested under similar conditions, and is found to be essentially unchanged by the radiation.

Testing of the durability of single-crystal calcium fluoride with and without antireflection coatings for use with high-power KrF excimer lasers

The transmission of uncoated and antireflection (AR)-coated CaF(2) windows has been measured during long KrF laser exposures. Samples were tested for up to 75 million pulses in front of an industrial-grade KrF excimer laser at high repetition rates (200-300 Hz) and moderately high energy fluence (220-550 mJ/cm(2)). In this fluence range bare CaF(2) performs much better than the best fused silicas currently available. Various AR coatings (whose structures were initially unknown to us) were deposited on CaF(2) substrates polished to two distinct surface finishes. We determined the coating structures by usingx-ray fluorescence spectroscopy to identify positively all elements that were heavier than fluorine and Rutherford backscattering spectroscopy to determine the layer structure and approximate layer thicknesses. We confirmed the structures by comparing measured UV transmission spectra with computer simulations. This study reveals excellent performance for SiO(2)/Al(2)O(3)/SiO(2)/Al(2)O(3) coatings on highly polished substrates, which provided two-sided transmission in excess of 99.8% at 248 nm and no measurable deterioration after 75 million pulses. AMgF(2)/Al(2)O(3) coating also performed quite well, while a hafnia-containing coating (MgF(2)/HfO(2)/LaF(3)) exhibited anomalous behavior. All-fluoride coatings consisting of MgF(2)/LaF(3)/MgF(2) and MgF(2)/LaF(3)/AlF(3) had good durability but lower starting transmission as a result of scatter losses in the LaF(3) layer. These results demonstrate the availability of AR-coated CaF(2) optics for high-power KrF laser applications with high transmission and no sign of degradation up to ~7 x 10(7) pulses.

Sudden onset of strong absorption followed by forced recovery in KrF laser-irradiated fused silica

A high-purity synthetic fused silica sample (Suprasil 2) was irradiated by a KrF laser at 248 nm, 300 Hz, and 500 mJ/cm(2). Transmission at 248 nm, transmission at 210 nm, and fluorescence at 650 nm were monitored in real time. The sample starts out in a weakly absorbing state. Then, after several million pulses, it experiences a sudden increase in 248-nm absorption with accompanying dramatic changes in its relaxation and fluorescence behavior. Further irradiation leads to (partial) bleaching of the UV absorption.

Excimer laser ablation of ferrites

Laser etching of ferrites was previously done by scanning a focused continuous‐wave laser beam on a ferrite sample in a chemical environment. We study the phenomenon of photo‐ablation of Ni‐Zn or Mn‐Zn ferrites by pulsed 248‐nm KrF excimer laser irradiation. A transfer lens system is used to project a grating pattern of a mask irradiated by the pulsed KrF laser onto the ferrite sample. The threshold fluence for ablation at the ferrite surface is about 0.3 J/cm2. A typical fluence of 1 J/cm2 is used. The etched grooves produced are typically 20–50 μm wide, with depths achieved as deep as 70 μm . Groove straightness is good as long as a sharp image is projected onto the sample surface. The wall angle is steeper than 60 degrees. Scanning electron microscopy of the etched area shows a ‘‘glassy’’ skin with extensive microcracks and solidified droplets being ejected that is frozen in action. We found that this skin can be entirely removed by ultrasonic cleaning. A fairly efficient etching rate of about 10 nm/pu...