Excimer Laser Optics Research Articles

Calcium Fluoride and Barium Fluoride Crystals in Optics

AbstractCalcium fluoride has a long tradition as an optical material in microlithography for semiconductor production. Based on the limited choice of optical materials transmitting well in the deep UV wavelength range, calcium fluoride could establish as an industry standard material for excimer laser optics in projection and illumination optical designs for the microlithography process. But beyond the UV, calcium fluoride and barium fluoride crystals also exhibit an excellent broadband transmittance and feature unique optical properties. These open up a variety of advanced applications covering the full visible range up to the mid‐infrared wavelength range.

Evaluation of coated and uncoated CaF2 optics by variable angle spectroscopic ellipsometry

CaF2 is one of the dominating optical materials used for ArF excimer laser optics. Surface quality of optically finished CaF2 plays an important role in the components' lifetime. A variable angle spectroscopic ellipsometry was employed to evaluate surface quality of optically finished CaF2 in terms of top surface and subsurface damage. The subsurface damage was revealed by removing the top surface. Combining plasma ion assisted deposition and ellipsometric measurement, a dense smooth F-SiO2 film was developed to prevent fluorine loss of CaF2 optics under ArF excimer laser irradiation, leading to an extended lifetime. In addition, an integrated protective coating approach was established on fluoride multilayer coated CaF2 optics, resulting in environmentally stable optical performance.

Effects of H2 impregnation on excimer-laser-induced oxygen-deficient center formation in synthetic SiO2 glass

We examined the effect of H2 impregnation on defect formation upon F2 laser (7.9 eV) and ArF excimer laser (6.4 eV) irradiation. It was revealed that H2 impregnation enhanced the formation of oxygen-deficient center (Si–Si bond) as well as suppressed the formation of E′ center and nonbridging oxygen hole center. A Si–Si bond gives an intense absorption band peaking at 7.6 eV, which contributes the absorption at the wavelength of F2 laser light. These results indicate that H2-free SiO2 glass, which is clearly inappropriate for KrF and ArF excimer laser optics, is more suitable for F2 laser optics than H2-impregnated glass.

Generation and healing behavior of radiation-induced optical absorption in fluoride phosphate glasses: The dependence on UV radiation sources and temperature

High purity fluoride phosphate (FP) glasses have a large transmission range from the vacuum ultraviolet to the infrared. They are attractive candidates for lens systems in microlithography equipment and excimer laser optics. Fluoride single crystals and vitreous silica are well-known traditional materials for ultraviolet optics. Crystal sizes are limited and glass is better for fabricating optics. For lens systems, a variety of glasses with different refractive indices and dispersion are required. The UV resonance wavelengths of FP glasses with a low content of phosphate using a two-term Sellmeier dispersion formula are comparable with those of silica and fluoride single crystals. It is known, that UV radiation induces the generation of several defect centers leading to additional absorption bands. The investigation of the kinetics of defect generation is very important for the prediction of transmission losses in case of long-time irradiation. To predict the kinetics of defect generation, first the separation of absorption bands is necessary. Experiments were carried out using UV-lamps, the KrF excimer laser (ns- and fs-pulses) and the ArF excimer laser (ns-pulses). The healing behavior of radiation-induced absorption bands was investigated using thermal treatment (temperature-dependence) and bleaching experiments (radiation-dependence). The results of these experiments enable the separation of absorption bands and the prediction of the defect generation depending on the used radiation source. The healing of strong defects at room temperature is postulated to be a diffusion-controlled process.

Interaction of F2 excimer laser with SiO2 glasses: Towards the third generation of synthetic SiO2 glasses

Changes in optical absorption spectra and defect formation of different types of synthetic SiO2 glasses were examined by irradiation with F2 excimer laser pulses (157 nm). Fluorine-doped, OH-free SiO2 glasses exhibit high optical transmittance (∼80%) at 157 nm in an as-delivered state and the intensity of F2-laser-induced absorption is much less than that in wet or dry F-free samples. The effect of F-doping on the blue shift of the absorption edge and suppression of color center formation was conspicuous up to 1 mol% but was slight upon further doping. It is suggested that elimination of strained Si–O–Si bonds upon F-doping is the primary reason of the improvement of resistance of SiO2 glasses to F2-laser light. Novel optical phenomena by F2-laser irradiation, bleaching of the vaccum UV (VUV) absorption edge and changes in the SiOH infrared absorption, were found in H2-impregnated, or wet SiO2 glasses. These results lead to the conclusion that F-doping to 1 mol% is an effective and practical method to obtain synthetic SiO2 glasses for F2 excimer laser optics as a photomask in optical lithography.

Fluorine-doped SiO_2 glasses for F_2 excimer laser optics:?fluorine content and color-center formation

Color-center formation in F-doped, OH-free synthetic SiO(2) glasses by irradiation with F(2) excimer lasers (157 nm) was examined as a function of the F content. The concentration of photoinduced E(') centers was reduced to approximately 1/20 by 1 mol.% F(2) doping and remained almost constant on further doping to 7.3 mol. %. The absorption edge was considerably shifted to a lower wavelength (157.4 nm -->153 nm for a 5-mm-thick sample) by 1-mol. % doping and decreased only slightly on further doping. The intensities of the Raman bands that are due to three- and four-membered ring structures were significantly reduced by 1-mol. % F doping. These results strongly suggest that elimination of strained Si-O-Si bonds by F doping plays a central role in the improvement of radiation resistance of SiO(2) glasses to F(2) laser light.

Preparation of CaF2 precursors for laser grade crystal growth

The expansion of commercial excimer lasers has increased the need of high-quality optics manufactured from CaF2. The key technical parameters, affecting the long-term quality of CaF2 excimer laser optics, are controlled by the purity of the raw material. The new purifying techniques provide 99.99% purity of the starting (material which rises up to 99.999% into the grown crystals. However, the techniques are rather expensive which leads to a significant rise in the price of the final crystal product. The present paper describes an alternative highly effective and relatively low-cost technique providing a precise control over a set of preparatory and purifying procedures and crystal growth. Using an original apparatus for electro-chemical grinding of the thermo-broken starting material, the grained product, uncontaminated by unintentional impurities, may be highly purified by two complementary methods to produce precursors used for UV-and laser-grade CaF2 crystal growth.

Xaar Greyscale Technology

Xaar technology can print at 360 dpi with 8 levels of grey; this capability is reaching the market in the form of the XaarJet XJ500S.Xaar achieves greyscale printing by rapid ejection of multiple droplets, which merge to produce a single dot on the paper. The basic piezoelectric operation of the technology is explained, whereby shear mode deformation of shared walls causes acoustic waves in channels, resulting in ejection of droplets. The waveforms used to operate the printhead are explained.The nozzle plate is made of polyimide; the nozzles are ablated, nine at a time, using an excimer laser and special optics. Great care is taken over cleanliness, in order to avoid nozzle blockages; integrated filters are provided to each of the four colour manifolds.The printheads incorporates an RS232 interface for printhead configuration and a 4-bit bus for data communication, a microcontroller, EEPROM for configuration storage, voltage control according to actuator temperature, and special purpose drive chips.The actuator and electronics are mounted on an aluminium chassis to provided good heat sinking, and special waveforms can be used on non-fired channels to maintain temperature uniformity.

Growth of ultra-violet grade CaF 2 crystals and their application for excimer laser optics

A crystal growth method for growing UV grade CaF 2 crystals from fluorspar has being developed using purified Bulgarian fluorspar mixed with 2% PbF 2. Excimer laser damage characteristics of the grown crystals were performed in a specially designed laboratory system into a high pressure camera. It was found that the fluorine mixture does not change, neither on the outer surface of the crystals nor in the bulk even after 3500 h of contact; no color centers were observed on the windows of the XeCl laser and that there are no remarkable differences between the surface and the bulk damage thresholds. Thus, the grown crystals of CaF 2 are suitable for manufacturing windows for excimer lasers and for other optical elements used in UV and vacuum UV spectra regions.

OH Content Dependence of the Intensity of ArF-Excimer-Laser Induced Absorption in Synthetic Fused Silica.

OH content dependence of the intensity of ArF-excimer-laser induced absorption in fused silica synthesized under reducing conditions was investigated. The absorption spectrum of each silica has a peak at 220 nm, and the intensity of which increases with decreasing OH content. Above the OH content about 1150 ppm, ArF-laser induced absorption was not observed. Annealing in the He ambient, suppressed the creation of a 220 nm band, and no absorption was observed in the samples containing OH of greater than 800 ppm. The annealing in the H2 ambient was not more effective as compared with He. These results indicate that, the fused silica synthesized under reducing conditions and containing OH more than about 1150 ppm was found desirable for excimer laser optics. The fused silica containing OH of more than 800 ppm annealed in the He ambient can also be used as excimer laser optics.

Laser projection patterned processing of semiconductors

This paper is concerned with laser projection patterned processing of material. We first describe the technique and discuss its limitations and its prospects for utilization in the fabrication of a complete IC. Then, experimental results on laser projection patterned etching of GaAs in a chlorine atmosphere performed with an KrF excimer laser and deep UV optics are reported. The dependence of etch rate, surface quality and spatial resolution on the various etching parameters are discussed. The overall etching process is thermally driven and not concerned with photochemical reactions.

Modification of Fused Silica Materials for Excimer Laser Optics—measurement of laser induced defects and its reformation—

ABSTRACTFused silica is widely used material in window and lens applications for excimer laser processing. The transmittance of the laser beam in fused silica is attenuated approximately 40% by high fluence ArF excimer laser irradiation. The attenuation of the transmittance corresponds to the growth of an absorption band at 215nm. This phenomenon is troublesome for laser lithography. To investigate this effect, we examined the laser induced luminescence and absorption under various conditions. The 215nm absorption band was diminished by annealing at 900 °C for 2 hours in He ambient. Ke could successfully obtain good optical material, whose transmittance remains constant with increasing ArF laser(193nm) shot exposure.