Leaching of resist components into the water has been reported in several studies. Potential effects of photo-acid generator (PAG) dissolved in water include photocontamination of the last optical surface and the formation of particulate defects on the wafer surface. In order to determine the impact of these phenomena on lithographic performance, such as optics lifetime and yield, we have initiated a set of controlled studies, where predetermined amounts of PAG were introduced into pure water and the results monitored quantitatively. One set of studies identified the complex, nonlinear paths leading to photocontamination of the optics. At concentrations typical of leached PAG, below 500 ppb, the in-situ self-cleaning processes prevent contamination of the optics. On the other hand, initial experiments with a nano-dropper show that micron-scale particles from the dissolved PAG are formed on the wafer surface when water evaporates. This phenomenon requires further systematic studies both at the fundamental science and the engineering levels.
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.
We have designed and constructed a microstepper for 157 nm immersion lithography. The lens, designed and fabricated at Newport, provides a numerical aperture of 1.3 and a field size of 60 μm with immersion liquids of index n=1.38. Because of a lack of system interferometer, final alignment has been ongoing in the field using actuators incorporated into the lens design. Lithography down to 250 nm has been demonstrated but lens alignment has proved difficult. We are currently implementing an image monitoring system to provide real-time feedback on lens performance and to allow expedited alignment.
Immersion lithography is proposed as a method for improving optical microlithography resolution to 45 nm and below via the insertion of a high-refractive-index liquid between the final lens surface and the wafer. Because the liquid acts as a lens component during the imaging process, it must maintain a high, uniform optical quality. One potential source of optical degradation involves changes in the liquid's index of refraction caused by changing temperatures during the exposure process. Two-dimensional computational fluid dynamics models from previous studies investigated the thermal and fluid effects of the exposure process on the liquid temperature associated with a single die exposure. We include the global heating of the wafer from multiple die exposures to better represent the "worst-case" liquid heating that occurs as an entire wafer is processed. The temperature distributions predicted by these simulations are used as the basis for rigorous optical models to predict effects on imaging. We present the results for the fluid flow, thermal distribution, and imaging simulations. Both aligned and opposing flow directions are investigated for a range of inlet pressures that are consistent with either passive systems or active systems using filling jets.
We have measured the intrinsic scattering of water with an eye toward its potential impact on immersion lithography. Quantitative measurements of the elastic Rayleigh scatter agree well with theory and show a loss of 0.001 cm-1. Qualitative measurements of the inelastic Raman scattering show a strong peak at 206 nm, consistent with the O-H stretch present in water. Both are expected to contribute flare of < 10-6 of the incident intensity. We have also examined the possibility for bubbles in the immersion liquid, and in particular those which form near the resist surface. We have measured scattering from single bubbles and estimate that bubbles as small as 5 μm should be detectable in this fashion. In addition, we have measured the potential for bubbles due to laser induced resist outgassing by direct imaging. In 2500 resist images (~235 mm2 of surface), we have seen only one bubble candidate which, due to its persistence in the water, we do not believe represents a true outgassing-induced bubble. Finally, using a technique borrowed from biology, rapid cryofixation/freeze fracture, we have examined nanobubbles which form spontaneously on hydrophobic surfaces and found that degassing the water prevents their formation.
A simulation package has been developed for predicting the influence of immersion, i.e., the presence of a uniform liquid layer between the last objective lens and the photoresist, on optical projection lithography. This technology has engendered considerable interest in the microlithography community during the past year, as it enables the real part of the index of refraction in the image space, and thus the numerical aperture of the projection system, to be greater than unity. The simulation program described here involves a Maxwell vector solution approach, including polarization effects and arbitrary thin film multilayers. We examine here the improvement in process window afforded by immersion under a variety of conditions, including λ = 193 nm and 157 nm, annular illumination, and the use of alternating phase shift mask technology. Immersion allows printing of dense lines and spaces as small as 45 nm with acceptable process window. We also examine the effect of variations in liquid index on the process window and conclude that the index of the liquid must be known to and maintained within a few parts per million. This has important implications for the temperature control required in future liquid immersion projection systems.
The premise behind immersion lithography is to improve resolution by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer by inserting a high-index liquid in place of the low-index air that currently fills the gap. We present a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer. The objectives of this feasibility study are to identify liquid candidates that meet the fluid mechanical requirements and to verify modeling tools for immersion lithography. The filling process was analyzed to simplify the problem and identify important fluid properties and system parameters. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer are developed and used to investigate a passive technique for filling this gap, in which a liquid is dispensed onto the wafer as a puddle, and then the wafer and liquid move under the lens. Numerical simulations include a parametric study of the key dimensionless groups influencing the filling process, and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements.
The requirements of liquids for use in immersion lithography are discussed. We present simple calculations of the transmission and index homogeneity requirements of the immersion liquid (T > 0.95 and δn < 5×10-7 respectively for sin θ = NA/n = 0.9 and a working distance of 1 mm) along with the temperature and pressure control requirements which follow from them. Water is the leading candidate immersion liquid for use at 193 nm, and we present data on its chemical compatibility with existing 193 nm resists through dissolution/swelling and surface energy studies. We find that it has a minimal impact on at least some current 193 nm resists. At 157 nm, suitably transparent immersion fluids remain to be identified. Perfluorinated polyethers (PFPE) are among the most transparent organics measured. The lowest PFPE absorbance at 157 nm can be further reduced by roughly a factor of two, from 6 to 3 cm-1 through removal of dis-solved oxygen. We also discuss our efforts to understand the origin of the remaining absorbance through supercritical CO2 fractionation.
The premise behind immersion lithography is to improve the resolution for optical lithography technology by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer. This is accomplished through the insertion of a high index liquid in place of the low index air that currently fills the gap. The fluid management system must reliably fill the lens-wafer gap with liquid, maintain the fill under the lens throughout the entire wafer exposure process, and ensure that no bubbles are entrained during filling or scanning. This paper presents a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer in immersion lithography. The objective of this feasibility study was to identify liquid candidates that meet both optical and specific fluid mechanical requirements. The mechanics of the filling process was analyzed to simplify the problem and identify those fluid properties and system parameters that affect the process. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer were developed for simulating the process. The CFD simulations were used to investigate two methods of liquid deposition. In the first, a liquid is dispensed onto the wafer as a “puddle” and then the wafer and liquid move under the lens. This is referred to as passive filling. The second method involves the use of liquid jets in close proximity to the edge of the lens and is referred to as active filling. Numerical simulations of passive filling included a parametric study of the key dimensionless group influencing the filling process and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements. For active filling, preliminary simulation results characterized the influence of the jets on fluid flow.
A simulation package has been developed for predicting the influence of immersion, i.e. the presence of a uniform liquid layer between the last objective lens and the photoresist, on optical projection lithography. This technology has engendered considerable interest in the microlithography community during the past year, as it enables the
real part of the index of refraction in the image space, and thus the numerical aperture of the projection system, to be greater than unity. The simulation program described here involves a Maxwell vector solution approach, including polarization effects and arbitrary thin film multilayers. We examine here the improvement in process window afforded by immersion under a variety of conditions, including λ = 193 nm and 157 nm, annular illumination, and the use of alternating phase shift mask technology. Immersion allows printing of dense lines and spaces as small as 45 nm with acceptable process window. We also examine the effect of variations in liquid
index on the process window and conclude that the index of the liquid must be known to and maintained within a few parts-per-million. This has important implications for the temperature control required in future liquid immersion projection systems.
We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance α157 ∼ 10 cm−1 base 10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at numerical aperture ∼1.25 and thus resolution of 50 nm for k1 = 0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, de-ionized water, has α193 = 0.036 cm−1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with k1 = 0.4.
We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler
has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance α157~10 cm-1 base10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at NA~1.25 and thus resolution of 50 nm for k1=0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, deionized
water, has α193 = 0.036 cm-1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with
k1=0.4.
A 157nm interference lithography system which is capable of patterning features at sub-100-nm pitch has been implemented. Initial results demonstrate approximately 50 nm line and space patterns exposed in a commercial deep-UV photoresists. Little line edge roughness is observed, indicating that the intrinsic properties of the resist may meet CD-control requirements to at least 50 nm. In addition, this system may be used to measure the spatial coherence of the 157-nm F2 laser source. Preliminary estimates show that the coherence length is approximately 40 micrometers .
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