Silica glass exhibits a permanent anisotropic response, referred to as polarization induced birefringence (PIB), when
exposed to short wavelength, polarized light. The magnitude of the PIB has been empirically correlated with the OH
content of the glass. Our recent studies pertaining to PIB have focused on careful characterization of PIB, with particular
emphasis on understanding all of the contributions to the measured birefringence signal and finally extracting only that
signal associated with birefringence arising from exposure to a polarized light beam. We will demonstrate that a critical
contributor to the total birefringence signal is birefringence that comes from exposure beam inhomogeneities. After
subtracting beam profile effects we are able to show that PIB is proportional to the OH content of the glass. Polarized
infrared (IR) measurements were performed on glasses that developed PIB as a consequence of exposure to polarized
157-nm light. These studies reveal that there is preferential bleaching of a specific hydroxyl (OH) species in the glass
with OH aligned parallel to the incident polarization undergoing more bleaching than those perpendicular. Further, we
observe a very strong correlation between the measured PIB of these samples and the anisotropic bleaching. From these
studies we propose a mechanism that can explain the role of hydroxyl in PIB.
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.
Calcium fluoride exhibits a fluence dependent transmission behavior at the 193 nm excimer laser wavelength of interest for photolithography applications. The transmission as a function of fluence is determined using a 193 nm excimer laser based transmission measurement system. Different transmission decay rate and saturation level is observed and correlated with structural defects and impurities. Thermally stimulated current measurements are applied to identify defects and correlate the relative concentration of mobile anion vacancies with fluence dependent transmission.
Silica glass exposed to pulsed UV excimer laser irradiation undergoes optical changes that can include either an optical path increase or a decrease. During a given exposure the sign of the induced optical path change can reverse as a function of pulse count. The reduced optical path and sign reversal are only observed in H2-containing glasses, and at high exposure fluence only optical path increase is observed. In past work we proposed an induced density change model invoking a dynamic equilibrium density to explain the high fluence experiments. Here we present a model that extends the density model to the low fluence regime by allowing the equilibrium density to be a function of the time-dependent break-up of the silica network during exposure. The network break-up is tracked by calculation of the induced SiH concentration in the glass. The agreement of optical path change obtained from experimental data with that deduced from the kinetic approach covers a wide range of exposure fluence and molecular hydrogen concentration. Using the model one can predict the change in optical path that arises from the excimer laser exposure.
We briefly review recent progress in the fabrication and characterization of air-core photonic band-gap fibers. These are silica fibers with an hexagonal array of air holes in the cladding, and a larger air hole creating the core. Improved structural uniformity transverse to the fiber axis and down the fiber axis has yielded fibers with better transmission characteristics. We have measured a minimum loss of 13 dB/km at 1500 nm for a 100 m length of our fiber. This is a marked improvement over previous loss measurements for air-core fibers of any kind. A comparison of observed spectra and calculated gap modes suggests that coupling between surface modes and core modes may be an important contributor to the remaining loss. We present a detailed analysis of the expected losses associated with mode crossings between the fundamental core mode and surface modes, showing that Lorentzian-shaped loss peaks are predicted.
Incorporation of fluorine into dry silica is known to extend the transmittance of the glass to shorter wavelengths relative to fluorine-free silica. Fluorine-doped silica has been shown to have greater than 80%/cm internal transmittance at 157-nm. Its high transmittance, low thermal expansion, ease of polishing and relative availability make it a perfect candidate for use at 157-nm for photomasks and potentially for other short wavelength optical elements. In the course of investigating composition-property relationships of F-doped, dry silica glasses, the effect of fluorine concentration on color center formation as a function of excimer laser exposure was considered. A series of glasses with fluorine concentrations from 0 to 1.8 wt% was studied. Using the 157-nm exposure wavelength, all glasses developed the silicon E' center, characterized by its absorption at 215-nm. Also noted was the non-bridging oxygen hole center (NBOHC) which appears in the UV at 260-nm. A band at 165-nm, identified as the oxygen deficient center (ODC) was also observed. This band was most pronounced in higher F concentration glasses, with the lowest F and non-F containing glasses showing no 165-nm band evolved. The assignment of the ODC is based on the fluorescence of the center and its reaction with oxygen, which causes diminishment of the band. The evolution of the color center formation (absorption) can limit the lifetime of optics by driving down transmission with use. For the 157-nm wavelength the absorption of importance is the one that appears at the 165-nm, the ODC. The exact atomic structure of the ODC is not important for this discussion; in fact, the defect is described by two different structures, the metal- metal bond and the divalent silicon: (O)3Si-Si(O)3 and O-Si-O(2+) The ODC has significant absorption at 157- nm and so minimizing its formation during exposure would be advantageous for optics life.
The UV-photosensitivity effect in germania-doped optical waveguides has become an important area of research because of the ease and utility of making Bragg gratings. In this study we report on the finding of a large UV-induced refractive index change in a conventionally melted an alkali-alumino-boro-germano- silicate composition that has been loaded with molecular hydrogen. The exposure was done with either CW 244-nm light, or a pulsed KrF excimer laser at 248-nm. A modulated refractive index of the order of 2-3 X 10-4 has been measured.
Modified fused silica is the primary candidate material for 157-nm photomask substrates. Standard UV excimer grade silica, such as Corning HPFSR, does not transmit below about 175- nm because of its high OH content. In contrast, we have prepared fused silicas with low OH contents and low levels of fluorine with measured transmissions up to 73.8%/6.4 mm and internal transmittances up to 87.9%/cm at 157-nm. Refractive index measurements at 157-nm are presented from which we calculate a theoretical limit for the measured transmission (reflection losses only) of about 88.5% at 157-nm. Modified fused silica is shown to have high resistance to laser-induced color center formation. The thermal and mechanical properties of modified fused silica are shown to be similar to those of standard excimer grade fused silica but not identical. For example, the thermal expansion and Young's Modulus of modified fused silica are slightly lower than that of Corning HPFSR, while thermal conductivity is the same. Modified fused silica substrates have been shown to behave similarly to standard fused silica substrates in mask-making processes such as polishing and Cr film deposition. In summary, our property and process results support the selection of modified fused silica for the 157-nm photomask application.
The use of silica glass for the photomask material in 157-nm lithography tools is proposed. While fused silica enjoys widespread application for 248 and 193-nm optics, its use for 157-nm applications has been largely discounted, in part because of low transmittance at this wavelength. It is demonstrated here that silica glass can be made to have high transmittance at 157-nm. This is accomplished by minimizing the OH content of the glass. It is also noted that the thermal and mechanical properties of so-called dry silica are very close to higher OH silicas that are commonly used for lithography applications.
Laser irradiation of fused silica produces compaction. Irradiation at photolithographic wavelengths of 248nm or 193nm produces density increase of order parts per millon. While these changes are small, the accompanying increase in index of refraction may be large enough to degrade the performance of photolithographic exposure optics. This paper reports experimental of compaction by interferometry and theoretical analysis of the elastic response of the glass samples to extract the sample-independent unconstrained compaction. The power law behavior of compaction vs. exposure dose is presented, along with a Monte Carlo analysis of error bars on this curve.
The use of silica lenses in a photolithographic system employing a 193 nm excimer laser has been proposed. It is desirable to determine if, at the low intensity to be used in the system (approximately equals 0.1 mJ/cm2), the glass will withstand about ten years of use without objectionable induced absorption. At a pulse frequency of 1 Khz, this length of time corresponds to about 1011 pulses. Because of the long time involved, an accelerated test is needed to determine the susceptibility of silica to induced absorption. The mechanism of darkening must be understood in order that the behavior of the glass under use conditions be predicted with confidence from the results of the accelerated test. The most important processes in the mechanism of induced absorption are: (1) Two photon absorption creating an exciton; (2) Trapping of the exciton by a localized state; (3) Dissociation of the trapped exciton to form an E' center and a NBOHC; (4) Reaction of these centers with hydrogen to form SiH and SiOH bonds; and (5) Photolysis of the SiH bonds to produce more E' centers. The mechanism will be discussed in detail and the agreement with experimental results over a range of intensities and hydrogen levels will be presented.
The compaction (densification) of fused silica under low fluence (< 1 mJ/cm2/pulse), long term (hundreds of millions of pulses) 193 nm irradiation has been studied. With the use of a finite element analysis, the unconstrained densification, (delta) (rho) /(rho) , is extracted from the experimentally determined wavefront distortion. We find that the densification of silica in the low fluence exposure regime corresponds to what is predicted from the behavior described by: (delta) (rho) /(rho) equals 0.000117 (NI2)0.53 where N equals number of pulses, I equals intensity (mJ/cm2/pulse). Results are presented of high and low intensity studies that establish the utility of the above form as a predictive tool for densification in fused silica.
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