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Grayscale lithography is a widely known but underutilized microfabrication technique for creating three-dimensional (3-D) microstructures in photoresist. One of the hurdles for its widespread use is that developing the grayscale photolithography masks can be time-consuming and costly since it often requires an iterative process, especially for complex geometries. We discuss the use of PROLITH, a lithography simulation tool, to predict 3-D photoresist profiles from grayscale mask designs. Several examples of optical microsystems and microelectromechanical systems where PROLITH was used to validate the mask design prior to implementation in the microfabrication process are presented. In all examples, PROLITH was able to accurately and quantitatively predict resist profiles, which reduced both design time and the number of trial photomasks, effectively reducing the cost of component fabrication.

Leaching of resist components into water has been reported in several studies. Even low dissolution levels of photoacid generator (PAG) may lead to photocontamination of the last optical surface of the projection lens. To determine the impact of this phenomenon on optics lifetime, we initiate a set of controlled studies, where predetermined amounts of PAG are introduced into pure water and the results monitored quantitatively. The study identifies the complex, nonlinear paths leading to photocontamination of the optics. We also discover that spatial contamination patterns of the optics are strongly dependent on the flow geometry. Both bare SiO2 surfaces as well as coated CaF2 optics are studied. We find that for all surfaces, at concentrations typical of leached PAG, below 500 ppb, the in situ self-cleaning processes prevent contamination of the optics.

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.

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 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.

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.

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.