Roger F. Sinta

Immersion liquids for lithography in the deep ultraviolet

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.

Extending optics to 50 nm and beyond with immersion lithography

Numerical imaging simulations demonstrate the capability of immersion lithography to print features smaller than 45 nm (35 nm) with good depth of focus at a vacuum wavelength of 193 nm (157 nm). The optical impact of index variation of the immersion liquid is simulated and found to be a shift of focus of 1 nm for each 1 ppm change in the bulk index of the liquid. For an index which varies through the thickness of the liquid (e.g., due to nonuniform temperature), the focus shift is found to be proportional to the total change in optical path length (OPL), with a 1 nm change in OPL leading to a ∼1.5 nm focus shift at 1.3 numerical aperture. A focus offset of 1–3 nm can be expected due to heating during scanning exposure. The possible formation of nanobubbles at resist surfaces is also discussed. While simulations show that even 10 nm thick bubbles at the surface of the resist cause 30% modulation in the aerial image intensity, no evidence of bubbles is seen in open frame immersion exposures. Imaging of 100 ...

Polymer photochemistry at advanced optical wavelengths

As lithography is extended to 157 nm, the molecular absorptivity becomes high for most organic polymers. Polymer photochemistry depends on photon absorption, and the higher energy associated with 157 nm light should lead to higher quantum yields of photoproducts. Polymers representative of those commonly employed in 193 or 248 nm resists were selected for this study. A gel permeation chromatography based method was developed to determine quantum yields for chain scission and crosslinking on thin polymers films coated on silicon wafers. This method was applied to determine the ΦS and ΦX of a number of lithographically significant homopolymers and copolymers at both the 157 and 248 nm wavelengths. It was found that polymers containing hydroxystyrene only undergo crosslinking while acrylate and methacrylate polymer only undergo chain scission. The film loss of 157 nm exposed poly-t-butyl acrylate and polymethyl methacrylate was found to be very high and attributed primarily to side chain cleavage of the este...

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-resolution fluorocarbon-based resist for 157-nm lithography

Lithography with 157 nm fluorine lasers is rapidly emerging as the next evolutionary step in optical lithography and is clearly seen as the likely successor to 193 nm lithography. In fact, it may become the technology of choice for the sub-100-nm node features. The photoresists used for this technology will be required to be extendable to less than 70 nm. As has been demonstrated with the transition to shorter wavelengths in the past, the photoresist materials that were developed for the longer wavelength applications are too absorbent for practical use as high-resolution single layer resist with 157 nm radiation. This high absorbency will force the coated resist thicknesses to be well under 100 nm. Fluorine containing polymers have been demonstrated to be more transparent in this spectral region than pure hydrocarbon polymers. We have developed and evaluated a number of unique 4-hexafluoroisopropanol styrene based polymer systems which we previously termed FESCAP resists and have developed new acetal partially blocked 4-hexafluoroisopropanol styrene based copolymers. These resists can have absorbencies of under 3 micrometers -1 at 157 nm which could allow imaging to thicknesses of 150 nm. Our recent resist designs are shown to have imaging capability down to 70 nm with a 0.60 NA microstepper.

Contributions to innate material roughness in resist

A method has been developed to probe the Innate Material Roughness (IMR) of resist materials. We have applied this to EUV and 248 nm resists to deconvolute the material contributions to roughness: 1) the polymer alone, 2) interaction between the polymer, photoacid generator (PAG), base quencher, and photolysis byproducts, 3) the effects of exposure, and 4) development. We studied ESCAP based resists (with more limited data on APEX polymers), an iodonium nonaflate PAG, a tetabutyl ammonium hydroxide (TBAH) base quencher, and standard tetramethylammonium hydroxide (TMAH) development.

Fluorine—an enabler in advanced photolithography

Abstract Fluorine—its chemical and physical properties—plays a critical role in the development of advanced photolithography. It is widely expected that, when in a few years the critical dimensions in microelectronic devices will be less than ∼70xa0nm, the patterning technology of choice will be based on 157xa0nm light generated from molecular fluorine laser medium. Lithography at 157xa0nm also requires major advances in areas of fluorine science, from the growth of near-perfect calcium fluoride crystals for lenses to fluorine doping of fused silica for masks to the synthesis of fluoropolymers that would serve as pellicles, photoresists, and immersion liquids. With the expected resolution of these issues, the prospects are excellent that photolithography will continue as the main semiconductor patterning technology for at least another decade.

Dissolution inhibition mechanism of ANR photoresists: crosslinking vs. -OH site consumption

This paper reports our recent studies of crosslinking of phenolic resins with melamines through 1H and 13C NMR, GPC, and dissolution rate changes. For the NMR studies, we used model phenolic compounds such as 4-ethylphenol, and the hexafunctional crosslinker, hexamethoxymethylmelamine (HMMM). The NMR clearly reveals that the crosslinking reaction occurs quantitatively at the hydroxyl site of the phenol. This result raises the question of whether the dissolution inhibition observed in the ANR resists is due to -OH site consumption or to the rapid rise in molecular weight of the phenolic polymer. Comparison of tetrahydrofuran (THF) extraction vs. aqueous tetramethylammonium hydroxide (TMAH) development shows that the dose required to insolubilize the resist is much higher for THF. Gel permeation chromatography on the soluble fraction extracted into THF showed a fraction with molecular weights up to 400,000 Daltons. We believe that crosslinking and -OH site protection provide synergistic dissolution selectivity in TMAH, leading to high contrast and high resolution. Finally, we present results on the effect of (chi) , which is proportional to the ratio of phenolic hydroxyl groups to melamine methoxy groups, on the lithographic performance of ANR photoresists. At low (chi) , the DUV resists can be used as increased absorption resists over topography, and development times can be shortened significantly. We have also found that increasing the melamine loading can lessen the degree of bridging residue observed between lines.

Experimental VUV absorbance study of fluorine-functionalized polystyrenes

A number of fluoro-functionalized poly(4-hydroxystyrene) derivatives, consisting of both blocked and unblocked hexafluoroisopropanol-substituted stryrenes, were prepared and their vacuum-ultraviolet absorption spectra were measured. From our efforts, we find that a wide range of synthetic flexibility exists and allows for a variety of fluorinated analogs of APEX-like and ESCAP-like copolymers and terpolymers with 157nm absorption coefficients less than 4.0micrometers . From these findings, we conclude that facile routes to high-performance 157nm resins are possible with optimum imaging thicknesses of 100 to 130nm.

Deconstructing the resist to probe innate material roughness

We have developed an AFM-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, we have deconstructed the resist into component materials and established that the PAG is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nano-scale dissolution inhomogeneities. Small differences in PAG concentration as a result of standing waves in the resist can lead to large changes in surface roughness due to PAG or PAG-photoproduct segregation and the resultant non-linear change in nano-scale dissolution rates. The temperature dependence of the PAG segregation suggests that increased mobility of the PAG occurs due to a lowering of the film Tg during the deprotection process.