A discharge cell has been used to simulate conditions in fast flow, radio-frequency excited CO2 lasers using dielectric electrodes. The frequency dependence of the discharge structure and the Alpha to Gamma transition has been observed in the range 1 3 -40MHz. The radio frequency, the thickness of the dielectric layer, and the parallel capacitance of the matching circuit are all shown to control the discharge uniformity and excitation conditions at the centre of the discharge.
In the past the spatial intensity profile of a laser beam has been measured using such techniques as scanning pinholes, detector arrays and photography. This new system converts the Excimer beam into visible light using a glass fluorescer with a very small penetration depth giving a good 2D image of the beam. The fluorescence has been found to be linear over a wide range of input energy and is imaged by a CCD camera linked to the computer controlled video frame store. The digitised information is then computer processed to yield such results as the intensity profile along an arbitary line, peak fluence, the difference between two images and various statistical values. This new technique has been applied to beams with widely varying diameters including analysis in the focal region and in the early detection of damage in materials.
LIMA (laser induced mass analysis) has been used as a damage testing facility incorporating damage detection and analysis of all ionic species generated by breakdown and plasma formation on a surface. The laser radiation is frequency quadrupled Nd-YAG at 266 nm, enabling studies in the UV near to the most important KrF emission. Preliminary qualitative results give an insight into damage mechanisms and coating weaknesses. Two prototype coatings, an A.R. and an H.R., have been studied above and below the 50% damage fluence. The state of coating cleanliness is immediately obvious and it is possible to conclude the sources of sub-threshold damage and the extent of coating disruption. The ready availability of this information should led to forthcoming improvements from coating manufacturers.
The paper discusses the role of impurities in bulk laser breakdown. Crystals of CaF2 have been grown with different impurity content. The introduced impurities are mainly rare earths, these being the most common contaminent species found in CaF2. Crystals selectively doped with Ce as well as very pure, strain annealed specimens were also grown. The generation of defect centres as a function of incident 248 nm laser fluence was monitored by transient absorption measurements. Bulk damage measurements indicated that the Ce doped crystals had the lowest thresholds followed by the pure strain annealed crystals, despite a complete absence of excited state absorbing centres produced during laser irradiation. Those crystals which showed a significant production of transient defects and fluorescent levels had very high damage thresholds.
Electron-beam mastering of templates for patterned media presents a challenge to the toolmaker to simultaneously meet throughput, resolution and placement requirements. Fundamental to tool development is the ability to measure the placement to true grid of shapes as small as 7 nm over the whole substrate. In this article we describe a technique, consisting of acquiring and analyzing scanning electron (SE) micrographs, for measuring the placement errors in lithography similar to that required for patterned media, albeit over a few square microns and without scale and orthogonality components. The method enabled the measurement of placement errors of dots in an array with accuracy down to about 2 nm. The technique was used to benchmark current X-Y tool performance and the smallest 3× standard deviation of placement error was found to be 4.5 nm. A clearer understanding of the necessary tool improvements was obtained. The use of the technique as basis for measuring errors to true grid over the entire substrate is discussed.
This paper considers the effects of a 249nm excimer pulse on CaF2 substrates and is concerned with both surface damage and bulk colouration processes. The effect of different polishing procedures on the surface damage threshold of CaF2 has been investigated. CaF2 has been shown to develop colour centres under prolonged U.V. irradiation. A technique has been developed to monitor the transient excited states created in the bulk material when irradiated with a 25ns excimer pulse. This involves a simultaneously pumped dye laser pulse probe operating at the absorption wavelengths of the excited species. From the results information about the photon absorption cross sections can be derived, the concentration of defect states at any time during the excimer pulse may be estimated and a knowledge of the maximum safe fluence levels established.
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