Robert Clayton Wheland

Amorphous tetrafluoroethylene-hexafluoropropylene copolymers

Disclosed herein are novel amorphous tetrafluoroethylene-hexafluoropropylene (TFE-HFP) copolymers, which are more random than previous amorphous TFE-HFP copolymers, as well as a novel high productivity continuous process for making these polymers. The polymers are particularly useful in the form of coatings, films and encapsulants.

Novel hydrofluorocarbon polymers for use as pellicles in 157 nm semiconductor photolithography: fundamentals of transparency

With the advent of 157 nm as the next photolithographic wavelength, there has been a need to find transparent and radiation durable polymers for use as soft pellicles. Pellicles are � 1 mm thick polymer membranes used in the photolithographic reproduction of semiconductor integrated circuits to prevent dust particles on the surface of the photomask from imaging into the photoresist coated wafer. Practical pellicle films must transmit at least 98% of incident light and have sufficient radiation durability to withstand kilojoules of optical irradiation at the lithographic wavelength. As exposure wavelengths have become shorter the electronics industry has been able to achieve adequate transparency only by moving from nitrocellulose polymers to perfluorinated polymers as, for example, Teflon 1 AF 1600 and Cytop TM for use in 193 nm photolithography. Unfortunately, the transparency advantages of perfluorinated polymers fail spectacularly at 157 nm; 1 mm thick films of Teflon 1 AF 1600 and Cytop TM have 157 nm transparency of no more than 38 and 2%, respectively, with 157 nm pellicle lifetimes measured in millijoules. Polymers such as ‐[(CH2CHF)xC(CF3)2CH2]y‐, or ‐(CH2CF2)x[2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole]y‐ with chains that alternate fluorocarbon segments with either oxygen or hydrocarbon segments frequently show >98% transparency at 157 nm, if amorphous. These polymers are made from monomers, such as vinylidene fluoride (VF2) and hexafluoroisobutylene, which themselves exhibit good alternation of CH2 and CF2 in their structures. In addition, we find that ether linkages also can serve to force alternation. In addition, we find that fluorocarbon segments shorter than six carbons, and hydrocarbon segments less than two carbons or less than three carbons if partially fluorinated also promote 157 nm transparency. We also find that even with these design principles, it is advantageous to avoid small rings, as arise in the cyclobutanes. These results suggest a steric component to transparency in addition to the importance of alternation. Upon irradiation these polymers undergo photochemical darkening and therefore none has demonstrated the kilojoule radiation durability lifetimes required to be commercially attractive. This is likely because these exposure lifetimes require every bond to absorb � 10 photons, each photon having an energy roughly twice common bond energies. We have studied intrinsic (composition, molecular weight) and extrinsic (trace metals, impurities, environmental contaminants, oxygen, water) contributions to optical absorption and photochemical darkening in these polymers. Studies of photochemical darkening in model molecules illustrate the dynamics of photochemical darkening and that appreciable lifetimes can be achieved in fluorocarbons. To a first approximation the polymers that have lower 157 nm optical absorbance also tend to show the longest lifetimes. These results imply that quantum yield, or the extent to which the polymer structure can harmlessly dissipate the energy, can be important as well. # 2003 Elsevier Science B.V. All rights reserved.

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.

Second generation fluids for 193nm immersion lithography

Water is the first generation immersion fluid for 193 nm immersion lithography. With a fluid refractive index of 1.436 and an optical absorbance of 0.01/cm at 193 nm, water immersion technology can enable optical lithography for the ITRS’ 65 nm half-pitch node. However, to achieve numerical apertures above 1.35 and to go beyond the 45 nm node, low absorbance fluids with indices higher than 1.6 are needed for the second generation of immersion lithography. We have developed a number of Gen. 2 high index fluids for immersion lithography at 193 nm. These highly transparent fluids have 193 nm indices up to 1.67. 32 nm 1:1 line space imaging has been demonstrated using two of our Gen 2 candidate fluids, representing major advance in optical lithography. To understand the behavior and performance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis, and Lorentz oscillator modeling.

Imaging of 32-nm 1:1 lines and spaces using 193-nm immersion interference lithography with second-generation immersion fluids to achieve a numerical aperture of 1.5 and a k 1 of 0.25

Water-based immersion lithography using ArF illumination is able to provide optical solutions as far as the 45-nm node, but is not able to achieve the 38- or 32-nm nodes as currently defined. Achieving these lithographic nodes will require new, higher refractive index fluids to re- place the water used in first-generation immersion systems. We have developed a number of such second-generation high-index fluids for im- mersion lithography at 193 nm. These highly transparent fluids have 193-nm indices up to 1.664. To understand the behavior and perfor- mance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis and Lorentz Oscillator modeling. Interference imaging printers have long been available, and they now have a new use: a rapid, cost- effective way to develop immersion lithography, particularly at extremely high resolutions. Although interference printers will never replace classi- cal lens-based lithography systems for semiconductor device production, they do offer a way to develop resist and fluid technology at a relatively low cost. Their simple image-forming format offers easy access to the basic physics of advanced imaging. Issues such as polarization of the image-forming light rays, fluid/resist interaction during exposure, topcoat film performance, and resist line edge roughness LER at extremely high resolutions, can all be readily studied. 32-nm 1:1 line/space L/S imaging is demonstrated using two of the second-generation fluids. These resolutions are well beyond current lens-based system capabili- ties. Results on the performance of various resists and topcoats are also reported for 32-nm L/S features.

Transparent fluids for 157-nm immersion lithography

More than 50 fluorocarbon liquids are measured for transpar- ency over the wavelength range 150 to 200 nm for the purpose of iden- tifying a suitably transparent fluid for use in 157-nm liquid immersion lithography. Purification methods such as degasification, distillation, silica gel drying, and supercritical fluid fractionation are investigated to determine the impact of residual contaminants on absorbance. The pu- rification processes are monitored by gas chromatography-mass spec- trometry and Fourier-tranform infrared spectroscopy (for organics), 19 F-nuclear magnetic resonance spectroscopy (for molecular structure), gel permeation chromatography (for molecular weight), Karl Fisher analysis (for water), and for residual dissolved oxygen. We find that in most cases, the absorbance is dominated by dissolved oxygen and water. Once the contaminant levels are reduced, the most transpar- ent perfluoroether (PFE) measured is perfluoro-1,2-bis(2-

High-index immersion lithography with second-generation immersion fluids to enable numerical aperatures of 1.55 for cost effective 32-nm half pitches

To identify the most practical and cost-effective technology after water immersion lithography (Gen1) for sub-45 nm half pitches, the semiconductor industry continues to debate the relative merits of water double patterning (feasible, but high cost of ownership), EUV (difficulties with timing and infrastructure issues) and high index immersion lithography (single-exposure optical lithography, needing a suitable high index last lens element [HILLE]). With good progress on the HILLE, high index immersion with numerical apertures of 1.55 or above now seems possible. We continue our work on delivering a commercially-viable high index immersion fluid (Gen2). We have optimized several fluids to meet the required refractive index and absorbance specifications at 193 nm. We are also continuing to examine other property/process requirements relevant to commercial use, such as fluid radiation durability, last lens element contamination and cleaning, resist interactions and profile effects, and particle contamination and prevention. These studies show that both fluid handling issues, as well as active fluid recycling, must be well understood and carefully managed to maintain optimum fluid properties. Low-absorbing third generation immersion fluids, with refractive indices above 1.7 (Gen3), would further expand the resolution of singleexposure 193 nm lithography to below 32 nm half pitch.

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.