Fredrick Claus Zumsteg

Design of very transparent fluoropolymer resists for semiconductor manufacture at 157 nm

Photolithography at 157 nm requires development of new photoresists that are highly transparent at this wavelength. Transparent fluoropolymer platforms have been identified which also possess other materials properties required for chemically amplified imaging and aqueous development. Polymers of tetrafluoroethylene (TFE), a fluoroalcohol-substituted norbornene and an acid-labile acrylate ester show the best combination of properties. A solution, semibatch, free-radical polymerization process was developed allowing synthesis of the terpolymers on a multikilogram scale. Further property enhancements may arise from replacing the norbornene with functionalized tricyclononenes. Formulated resists have been imaged in a 157 nm microstepper.

Fluoropolymers for 157-nm lithography: optical properties from VUV absorbance and ellipsometry measurements

With the introduction of 157 nm as the next optical lithography wavelength, the need for new pellicle and photoresist materials optimized for this wavelength has produced much activity in optical characterization of thin film materials. Here we focus on ultra transparent fluoropolymers for 157 nm pellicle applications where absorbances below 0.01/micrometers are necessary to achieve transmissions above 98 percent. Transmission-based absorbance/micrometers measurements performed using VUV spectroscopy are characterized by rapid turn-around time, and are essential during the materials design and screening phase of a new materials development program. Once suitable candidate materials families have been identified for development into 157 nm pellicles, VUV ellipsometry becomes essential to model the film structure, characterize the complex index of refraction, and to tune the pellicles etalon design. Comparison of VUV absorbance measurements of fluoropolymer thin films on CaF2 substrates with VUV ellipsometry measurements of the same polymers on silicon substrates demonstrates some of the artifacts in, and helps define the accuracy of transmission based absorbance measurements. Fresnel interference fringes can produce transmission oscillations that can lead to underestimation, or even negative values, of the film absorbance. Film thickness nonuniformity can serve to reduce the Fresnel interference fringes, leading to reduce variation in the apparent 157 nm absorbance for micrometers thick films. VUV ellipsometry coupled with Fresnel analysis of the thin film/substrate system formally takes into consideration all of these optical artifacts, while at the same time determining the complex index of refraction of the materials. Using VUV ellipsometry and Fresnel analysis, the absorbance values do not show the large apparent oscillations, the film thickness is directly determined in the measurement, and film microstructure is also modeled. We have identified ultra transparent fluoropolymers which have 157 nm absorbances below 0.01/micrometers . These materials have the appropriate optical properties for use as 157 nm pellicles with greater than 98 percent transmission. This is an important for the development of 157 nm lithography, since the lack of a 157nm pellicle has been identified as a critical path issue.

Materials design and development of fluoropolymers for use as pellicles in 157-nm photolithography

The introduction of 157 nm as the next optical lithography wavelength has created a need for new soft (polymeric) or hard (quartz) pellicle materials optimized for this wavelength. Materials design and development of ultra transparent fluoropolymers suitable for 157 nm soft pellicle applications has produced a number of promising candidate materials with absorbances below 0.03/micrometer as is necessary to achieve pellicle transmissions above 95%. We have developed 12 families of experimental TeflonAFR (TAFx) materials which have sufficient transparency to produce transmissions above 95%. For the successful fabrication of 157 nm pellicles from these materials, the fluoropolymers must have appropriate physical properties to permit the spin coating of thin polymer films and their lifting and adhesive mounting to pellicle frames, the processes which produce free standing pellicle membranes of micron scale thickness. Relevant physical properties include molecular weight, glass transition temperature, and mechanical strength and toughness. We have successfully developed various of the ultra transparent TAFx polymer families with these physical properties. Upon irradiation these 157 nm pellicle polymers undergo photochemical darkening, which reduces the 157 nm transmission of the material. Measurements of the photochemical darkening rate allow the estimation of the pellicle lifetime corresponding to a 10% drop in 157 nm transmission. Increasing the 157 nm lifetime of fluoropolymers involves simultaneous optimization of the materials, the pellicle and the end use. Similar optimization was essential to achieve the desired radiation durability lifetimes for pellicles successfully developed for use with KrF (248 nm) and ArF (193 nm) lithography.

Single layer fluropolymer resists for 157-nm lithography

We have developed a family of 157 nm resists that utilize fluorinated terpolymer resins composed of 1) tetrafluoroethylene (TFE), 2) a norbornene fluoroalcohol (NBFOH), and 3) t-butyl acrylate (t-BA). TFE incorporation reduces optical absorbance at 157 nm, while the presence of a norbornene functionalized with hexafluoroisopropanol groups contributes to aqueous base (developer) solubility and etch resistance. The t-butyl acrylate is the acid-catalyzed deprotection switch that provides the necessary contrast for high resolution 157 nm imaging. 157 nm optical absorbances of these resists depend strongly upon the amount of t-BA in the polymers, with the TFE/NBFOH dipolymers (which do not contain t-BA) exhibiting an absorbance lower than 0.6 μm-1. The presence of greater amounts of t-BA increases the absorbance, but also enhances the dissolution rate of the polymer after deprotection, yielding higher resist contrast. Formulated resists based upon these fluorinated terpolymer resins have been imaged at International Sematech, using the 157 nm Exitech microstepper with either 0.6 NA or 0.85 NA optics. We have carefully explored the relationship between imaging performance, resist contrast, optical absorbance, and t-BA content of these terpolymer resist resins, and describe those results in this contribution.

157 nm imaging using thick single layer resists

During the past year the probability that 157 nm lithography will precede next generation lithographies such as EUV or EPL has increased, partly due to encouraging advances in the design of polymeric materials, which have sufficient transparency at 157 nm to serve as platforms for single layer photoresists. We have identified several fluorinated resins which can be developed in aqueous 0.26 N TMAH, have reasonable etch resistances (comparable to poly-parahydroxystyrene), and can be formulated to yield photoresists with optical absorbancies at 157 nm which are low enough to be used at thicknesses of 150-200 nm. We have imaged a number of these formulated resists at 157 nm with the Exitech microstepper at International Sematech, and the results for formulated resists with optical absorption coefficients (base 10) as low as 2.1 per micron are described.

New Materials for 157 nm Photoresists: Characterization and Properties

The design of an organic material satisfying all of the requirements for a single layer photolithography resist at 157 nm is a formidable challenge. All known resists used for optical lithography at 193 nm or longer wavelengths are too highly absorbing at 157 nm to be used at film thicknesses greater than approximately 90 nm. Our goal has been to identify potential, new photoresist platforms that have good transparency at 157 nm (thickness normalized absorbance of 2.5 micrometer-1 or less), acceptable plasma etch resistance, high Tg and compatibility with conventional 0.26 N tetramethylammonium hydroxide developers. We have been investigating partially fluorinated resins and copolymers containing transparent acidic groups as potential 157 nm photoresist binders; a variety of material with promising initial sets of properties (transparency, etch resistance, solubility in aqueous TMAH) have been identified. Balancing these properties with imaging performance, however, remains a significant challenge.