Johannes Moll

Laser-induced birefringence in fused silica from polarized lasers

Fused silica, when exposed to excimer laser light, exhibits permanent anisotropic birefringence and wavefront changes. These laser-induced changes depend on the silica composition and processing conditions. The optical anisotropy is most clearly observed in samples that are exposed with linear polarization. This polarization-induced effect has been known for several years, but has become much more important with the advent of immersion lithography and its associated very high numerical apertures. High numerical aperture optics require controlled polarization, notably linear polarization, in order to maintain phase contrast at the image. When birefringence and wavefront changes are induced by laser exposure, the image contrast at the wafer deteriorates. We interpret the changes in optical properties in terms of permanent anisotropic strain induced by laser damage, and the associated strain-induced optical effects. This is accomplished using the mathematics of tensors to account for anisotropic strain and optical anisotropy, and using finite element analysis to calculate the strain fields taking the sample and exposure geometries into account. We report the relations between underlying density and strain anisotropy changes and the induced birefringence and wavefront for a given experimental sample geometry. We also report some examples of the different degree of laser damage from silica with different compositions and processing conditions.

Laser resistance of fused silica for microlithography: experiments and models

Laser resistance of fused silica, used as lens material in DUV microlithography, is one of the keys to long-term high-level optical performance of steppers and scanners. The exposure of fused silica to high energy excimer laser pulses over long periods of time modifies the material in several different ways: the optical absorption increases due to laser-induced formation of color centers; the density of the material changes due to structural relaxation and formation of (beta) -hydroxyl (SiOH); and finally the index of refraction changes due to a photorefractive effect. All of these effects affect the imaging quality of illuminator systems and projection lenses, hence the need for fundamental understanding and modeling.

ArF laser induced absorption in fused silica exposed to low fluence at 2000 Hz

Excimer laser radiation changes the physical and optical properties of fused silica. These changes include laser induced absorption and density changes in the glass. Such effects may have an impact on the length of time for which optical elements made of fused silica can be used in DUV lithography systems. Corning Incorporated has recently developed and built a system for marathon testing of fused silica. The system consists of a 2000 Hz ArF laser and a specialized automated test bench. It allows the simultaneous testing of up to 10 samples under exposure conditions similar to the conditions expected in ArF lithographic exposure tools. First results of laser induced damage in samples exposed in this new system are presented.

Excimer laser induced absorption in fused silica

Excimer laser radiation changes the physical and optical properties of fused silica. These changes include compaction of the glass and induced absorption, both of which have an impact on the expected lifetime of silica lenses used in optical microlithography. We report on our ongoing study of excimer laser induced changes in fused silica. We use a fully automated experimental setup designed for marathon exposure of the sample at low fluence. In each setup, using either an ArF or a KrF laser, up to five samples are exposed simultaneously and their induced absorption is measured in situ. The spatial and temporal profiles of the laser beam can also be measured in the same setup. We present and discuss results from marathon test of fused silica at fluences close to the conditions expected in optical microlithography systems.

Boron and Phosphorus Implantation Induced Electrically Active Defects in p-Type Silicon

Electrically active defects induced by ion implantation of boron and phosphorus into silicon and their recovery under isothermal annealing at 450 °C were investigated using Deep Level Transient Spectroscopy (DLTS) and Energy Resolved Tunneling Photoconductivity (ERTP) spectroscopy at cryogenic temperatures. DLTS results show electrically active deep traps located at Ev+0.35 eV and Ev+0.53 eV in boron implanted Si and at Ev+0.34 eV, Ev+0.43 eV, and Ev+0.38 eV in phosphorus implanted Si. These meta-stable defect sites were found to be either eliminated or significantly reduced in thermally annealed samples. We assigned these defect sites to hydrogen and carbon incorporated complexes formed during ion implantation. Corroborating the DLTS results, the photocurrent measurement also revealed a strong reduction of electrically active defects states, extended from EC – 0.3 eV up to the conduction band edge of Si, upon isothermal annealing.

Investigation of Ion implantation induced electrically active defects in p-type silicon

We investigated ion implantation induced electrically active defects in p-type silicon using Deep Level Transient Spectroscopy (DLTS) and photoconductivity spectroscopy at cryogenic temperatures. Implantation related deep traps in H2, B11, and P31 implanted p-type Si and their recovery under isothermal annealing are described. We observed distinct deep trap levels located in the energy range between 0.25 eV up to 0.53 eV away from the valence band edge, which was either suppressed or eliminated upon thermal annealing below 600 °C. Supporting the DLTS results, photoconductivity shows strong recovery of a broad absorption band present near the conduction band edge upon thermal annealing. In this paper, we discuss the origin of the broad photoconductivity absorptions and DLTS emission in Si and their relation to the ion implantation induced damage to the lattice structure.

Advances in the use of birefringence to measure laser-induced density changes in fused silica

Birefringence mapping of fused silica samples is used to measure density change in the material after exposure to excimer laser radiation. The proper techniques and methods that should be used to perform the exposure of the samples and the analysis of the birefringence results will be discussed. The quantitative analysis of birefringence measurements includes the correct subtraction of the initial birefringence of the sample and the comparison with a theoretical birefringence map calculated for a 1 ppm unconstrained density change under consideration of material and exposure parameters. Proper experimental conditions include the use of samples with low initial birefringence and a round circularly polarized laser beam with top-hat intensity profile.