The final projection lens element in a 193-nm immersion-based lithographic tool will be in direct contact with water during irradiation. Thus, any lifetime considerations for the lens must include durability data of lens materials and thin films in a water ambient. We have previously shown that uncoated CaF2 is attacked by water in a matter of hours, as manifested by a substantial increase in AFM-measured surface roughness.1 Thus, CaF2 lenses must be protected, possibly by a thin film, and the coatings tested for laser durability in water. To address the above lifetime concerns, we have constructed a marathon laser-irradiation system for testing thin film exposure to water under long-term laser irradiation. Coated substrates are loaded into a custom water cell, made of stainless steel and Teflon parts. Ultrapure water is delivered from a water treatment testbed that includes particle filtration, deionization and degassing stages. In-situ metrology includes 193-nm laser ratiometry, UV spectrophotometry and spectroscopic ellipsometry, all with spatial profiling capabilities. In-situ results are coupled with off-line microscopy, AFM measurements and spatial surface mapping with spectroscopic ellipsometry at multiple incidence angles. A variety of laser-induced changes have been observed, from complete adhesion loss of protective coatings to more subtle changes, such as laser-induced index changes of the thin films or surface roughening. Implications of the study on expected lifetimes of the protective coatings in the system will be discussed.
Pellicle materials for use at 157 nm must display sufficient transparency at this wavelength and adequate lifetimes to be useful. We blended a leading candidate fluoropolymer with silica nanoparticles to examine the effect on both the transparency and lifetime of the pellicle. It is anticipated that these composite materials may increase the lifetime by perhaps quenching reactive species and/or by dilution, without severely decreasing the 157-nm transmission. Particles surface-modified with fluorinated moieties are also investigated. The additives are introduced as stable nanoparticle dispersions to casting solutions of the fluoropolymers. The properties of these solutions, films, and the radiation-induced darkening rates are reported. The latter are reduced in proportion to the dilution of the polymer, but there is no evidence that the nanoparticles act as radical scavengers.
The photo-induced degradation of 157-nm anti-reflective (AR) coatings, and the role of water vapor in the ambient, have been studied with in-situ spectroscopic ellipsometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy. Using ellipsometric techniques, we find that MgF2 thin films develop a surface roughness layer under laser irradiation at an incident dose of ~0.1 MJ/cm2. These thin film changes occur well before any changes in 157-nm transmission are observed. The findings are confirmed by ex-situ post-irradiation AFM measurements. LaF3 does not exhibit this effect. Addition of ppm-levels of moisture suppresses surface roughness formation, suggesting that the surface roughness growth may be a precursor to the transmission degradation of full AR stacks that had been observed earlier.
Successful insertion of 157-nm lithography into production requires that materials comprising the optical train meet the lifetime requirements of the industry. At present, no degradation of bulk fluoride materials has been observed for at least up to 109 pulses. However, last year we reported on the surface damage to fluoride materials that appeared after 3-4x109 pulses at moderate fluences of 3-4 mJ/cm2/pulse2. This damage manifested itself as a precipitous transmission drop of up to 50% at 157 nm and was accompanied by the formation of a porous rough surface layer about 0.20 μm thick. Understanding this surface damage is important for the durability of laser windows and beam delivery optics, and it may also help elucidate fundamental 157-nm photophysics of fluoride surfaces. To understand the underlying phenomena, we have designed and constructed a new accelerated damage test chamber. The chamber utilizes 157-nm light from a lithography-grade laser operating at 1000 Hz. Inside the chamber, light is focused onto the sample to a submillimeter spot size. The chamber allows us to test in-situ transmission of multiple spots on a given sample over a range of fluences up to 140 mJ/cm2/pulse without breaking purge. We have used this chamber to understand the scaling of the damage mechanism for both uncoated and antireflectance (AR) -coated CaF2 samples as a function of laser repetition rate and fluence. Substrate damage appears to be governed by a complex set of mechanisms, both thermal and non-thermal in origin. Preliminary damage studies of AR-coated substrates show that AR-coating related degradation occurs well before the onset of the substrate surface damage.
Photo-induced contamination rates on 157-nm optical surfaces have been studied in controlled experiments with contaminants containing fluorocarbon, sulfur and iodine. The compounds investigated represent species generated in controlled outgassing studies of common construction materials and photoresists used in 157 nm steppers. No photocontamination was measured for highly fluorinated alkanes and ethers on an anti-reflective coating, at levels exceeding 10 ppm. Photocontamination with sulfur based compounds was similar to the behavior observed with hydrocarbon based derivatives. Sulfur containing residues, even from oxidized precursors, are fully cleanable in oxygen, with cleaning rates scaling proportionally with the level of oxygen. In contrast, at elevated levels of oxygen, non-volatile iodate complexes can form from iodine based contaminants. Sulfonium salts should therefore be considered over iodonium species in photoacid generators in 157 nm photoresists. In addition to studying these new classes of compounds, cleaning rates of hydrocarbon residues in trace levels of water were also studied.
We present results of the durability of antireflectance (AR) coatings under laser irradiation with emphasis on the interplay between coating materials and ambient. We find that introducing ppm-levels of water has a dramatic impact on the performance of certain coatings. In particular, no significant degradation of a coating was observed for up to 1MJ/cm2 dose in the presence of ~20 ppm H2O, whereas linear transmission drop of several percent was observed when irradiating a coating of similar design in <0.1 ppm H2O but under 1.5 ppm O2. Cycling water concentration on and off leads a corresponding cycling of transmission of the coatings. Adding water vapor to the ambient has a much greater benefit to coating durability than adding corresponding amounts of gas phase oxygen. In a series of experiments involving the same coating stack with different degrees of porosity of the outer layer, moisture was found to have the greatest impact on the most porous coating.
We present the methodology and recent results on the long-term evaluation of optical materials for 157-nm lithographic applications. We review the unique metrology capabilities that have been developed for accurately assessing optical properties of samples both online and offline, utilizing VUV spectrophotometry with in situ lamp-based cleaning. We describe ultraclean marathon testing chambers that have been designed to decouple effects of intrinsic material degradation from extrinsic ambient effects. We review our experience with lithography-grade 157-nm lasers and detector durability. We review the current status of bulk materials for lenses, such as CaF2 and BaF2, and durability results of antireflectance coatings. Finally, we discuss the current state of laser durability of organic pellicles.
Long-term durability tests of optical thin films and thin films designed for attenuating phase shifters have been performed in a chamber, which stresses clean protocols to eliminate extraneous effects of surface contamination. Most anti-reflective coatings tend to degrade several percent in transmission within 1 MJ/cm2 total dose. Attenuating phase
shifting materials usually show an increase in transmission during 6 kJ/cm2. In both types of films there are exceptions, indicating that there are no fundamental causes that would limit the performance of such films. A new phenomenon of laser-induced surface damage in calcium fluoride has been observed, and is being studied.
Photodeposition rates for ten hydrocarbon species have been measured on CaF2 substrates under 157-nm irradiation in the presence of ppm scale levels of oxygen. The species are representative of hydrocarbon based compounds observed in outgassing studies of common build materials used in 157-nm based lithographic systems. Photodeposition rates have also been measured for a subset of the hydrocarbon species on a MgF2 thin film, six anti-reflective dielectric stacks, and fluorine doped fused silica for comparison with the results on CaF2 substrates. Two contamination processes are observed. One is the formation of an equilibrium layer on the surfaces. The other is a quasi-permanent contamination which is most pronounced at elevated levels of contaminant.
Transmission loss during irradiation remains the critical limitation for polymer pellicle materials at 157 nm. In this work we establish a framework for calculating the necessary pellicle lifetime as well as a test methodology for evaluating the laser durability of candidate polymer films. We examine the role of key extrinsic environmental variables in determining film lifetime. Oxygen concentration affects pellicle lifetime, but there is not an oxygen level that effectively balances pellicle perforation and cleaning against the onset of photochemical darkening. Neither moisture level nor 172-nm UV lamp pre-cleaning were found to have a significant impact on pellicle lifetime.
In this work we present progress on the long-term evaluation of optical materials for 157-nm lithographic applications. We review the unique metrology capabilities that have been developed for accurately assessing optical properties of samples both online and offline, utilizing VUV spectrophotometry with in-situ lamp-based cleaning. We review the current status of bulk materials for lenses, such as CaF2 and BaF2, and durability results of antireflectance coatings. Finally, we describe progress on materials testing of organic pellicles, both with 172-nm lamps as well as under 157-nm laser irradiation.
Photolithography utilizing 157-nm excimer lasers is a leading candidate technology for the post-193-nm generation. A key element required for successful insertion of this technology is the near-term performance and long-term reliability of the components of the optical train, including transparent bulk materials for lenses, optical coatings, photomask substrates, and pellicles. For instance, after 100 billion pulses at an incident fluence of 0.5 mJ/cm2/pulse optical materials, of which the primary candidate is calcium fluoride, should have an absorption coefficient of less than 0.002 cm-1, and antireflective layers should enable transmission of 98.5 percent for a two-sided coated substrate. Modified fused silica has emerged as a viable option as a transparent photomask substrate, and several approaches are being explored for transmissive membranes to be used as pellicles.
We have completed a comprehensive evaluation of bulk materials designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 6 X 109. The outcome of the study shows that most calcium fluoride materials should meet the industry lifetime targets for use in lens applications. Some fused silica material also appears to meet lifetime expectations of the industry; however, large grade-to-grade variability in both absorption and laser-induced densification has been observed. We also report on the impact of transient absorption in fused silica on lithographic dose control.
We have undertaken a systematic evaluation of both bulk material sand optical coatings designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 109. Measurements of absorption is fused silica show a large variation in performance for different samples in both initial and laser-induced absorption. Calcium fluorides samples show less variation in laser-induced absorption and appear to be more stable under irradiation of 0.2-1 billion pulses. Laser-induced densification of fused silica appears to follow an empirical power law; however, an order of magnitude spread in densification is observed among grades. For optical antireflectance coatings, we have characterized the initial 'laser-cleaning' phenomenon for various coatings. We have observed that laser-cleaned coatings deposited on CaF2 substrates exhibit higher initial optical losses at 193 nm than their counterparts on SiO2 substrates. However, the losses for coatings on CaF2 substrates are reduced over irradiation times of 0.2-1 billion pulses to final values comparable to their SiO2 counterparts. Finally, we have characterized various catastrophic failures of coating material, such as induced losses, adhesion failure and laser-induced thinning.
We present an assessment of bulk fused silica and calcium fluoride, and of antireflective coatings for 193-nm lithographic applications. In the course of extensive marathon studies we have accumulated 1-5 billion laser pulses on over 100 bulk material samples at fluences from 0.2 to 4 mJ/cm2/pulse. The result show large variation in both initial and induced absorption of fused silica and in densification of fused silica. For antireflective coatings, there are samples that undergo no appreciable degradation when irradiated for > 1 billion pulses at 15 mJ/cm2/pulse. However, initial losses in some coatings may be unacceptably high for lithographic applications.
We present an assessment of antireflective coatings for 193-nm lithography. Coatings from nine suppliers were exposed in a nitrogen ambient for up to 1.5 billion pulses at 15 mJ/cm2/pulse at 400 Hz. Sensitive metrology, developed for this study, included reflectance/transmittance measurements, in-situ ratiometric transmission measurements, and interferometric calorimetry for absorption measurements. The coatings from at least two suppliers withstood greater than 1 billion pulses with no observable degradation. Catastrophic damage observed on some samples included blistering and a dramatic transmission drop. Such damage occurred rather early (less than 100 million pulses).
We investigated laser-induced damage of pellicles for 193-nm lithography. We surveyed 193-nm-optimized material from three pellicle suppliers. Pellicles were irradiated under realistic reticle plane conditions (0.04 mJ/cm2/pulse - 0.12 mJ/cm2/pulse for up to 100 million pulses). Pellicles from two suppliers were found to meet lifetime requirements of the industry. Pellicles from the third supplier do not appear to meet the lifetime requirements. We present fluence scaling of pellicle damage and discuss effects of the ambient on pellicle degradation rates. We present results of the outgassing studies of pellicle material under irradiation using a separate gas chromatograph-mass spectrometer-based detection apparatus. From the results of these studies, we suggest possible photochemical pathways for pellicle degradation as a function of ambient.
The performance of argon fluoride excimer lasers is an important issue in determining the practical feasibility of 193-nm exposure systems. This paper presents a summary of the experience gained at MIT Lincoln Laboratory regarding the long-term performance of 193-nm lasers, used under conditions similar to those expected in production-type lithographic systems.
We investigate the effect of 193-nm radiation on commercially available pellicles for 248-nm lithography. Pellicles from two suppliers were irradiated at a realistic reticle plane fluence (0.1 mJ/cm2/pulse) for 50 million pulses. Analysis of transmission spectra revealed loss of pellicle material, decreased refractive index and increased absorption in various combinations depending on pellicle type and ambient. Although one of the two materials may be suitable for use at 193 nm, the other showed unacceptable degradation. We also quantified outgassing rates of organic species during irradiation, and observed greatly accelerated material loss in a pure nitrogen ambient compared with air. Yield rates of perfluorinated fragments and polymer product exhibited two-photon scaling behavior.
The trend in microelectronics toward printing features 0.25 micrometers and below has motivated the development of lithography at the 193-nm wavelength of argon fluoride excimer lasers. This technology is in its early stages, but a picture is emerging of its strengths and limitations. The change in wavelength from 248 to 193 nm will require parallel progress in projection systems, optical materials, and photoresist chemistries and processes. This paper reviews the current status of these various topics, as they have been engineered under a multi-year program at MIT Lincoln Laboratory.
The effect of 193-nm excimer laser radiation on pellicles designed for 248-nm use is studied. The pellicles are transparent at 193 nm as well. However, prolonged irradiation causes gradual thinning and eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the approximately 0.1 mJ cm2/pulse fluences expected to be used in 193-nm steppers indicates that these pellicles will change little for several years in a full-production environment.
he effect of 1 93-nm excimer laser radiation on pellicles designed
for 248-nm use is studied. The pellicles are transparent at 1 93 nm
as well. However, prolonged irradiation causes gradual thinning and
eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the approximately 0.1 mJ cm2lpulse fluences expected to be used in
1 93-nm steppers indicates that these pellicles will change little for 5everal
years in a full-production environment.
Excimer laser irradiation of fused silica is shown to induce gradual changes in the material, which affect its optical properties. These changes include visible fluorescence, formation of absorption bands at approximately 215 nm (E' centers), and increases in density and index of refraction. The magnitude of these effects varies initially as the square of the laser fluence and linearly with the number of pulses, indicating that they are the result of a two- photon absorption process. Pre-or post-irradiation treatments can be used to reduce the amount of laser induced degradation, especially the formation of color centers.
Synthetic UV-grade fused silica, crystalline fluorides, and dielectric coatings have been evaluated for transparency and durability at 193 nm. Most bulk materials eventually develop color centers, and fused silica also changes its density and index of refraction. However, the rate at which these changes occur and their magnitude vary strongly with material, grade, and other more subtle details. Careful selection and possibly pretesting are recommended, in order to ensure optimal matching between the intended application and the material properties.
The effect of 193-nm excimer laser radiation on pellicles designed for 248-nm use has been studied. The pellicles are transparent at 193 nm as well. However, prolonged irradiation causes gradual thinning and eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose, but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the -0.1 mJ cm”2/pulse fluences expected to be used in 193-nm steppers indicates that these pellicles will change little for several years in a full production environment
The local modification of an integrated circuit (IC) requires in general the availability of three generic processes. First, a method for cutting conductors must be provided. Second, a process for depositing new conductors must be available. Finally, a means of opening via holes through the chip passivation to the underlying conductors is needed; this operation enables newly deposited conductors to make connections to the existing circuit elements, and also provides probe access to facilitate testing of the circuit.