Veena Rao

Comparison of the lithographic properties of positive resists upon exposure to deep- and extreme-ultraviolet radiation

Nineteen chemically amplified ultrathin resists were imaged using exposure to extreme-ultraviolet (EUV) (13.4 nm) and deep-ultraviolet (DUV) (248 nm) radiation. Direct comparisons were made of photospeed, resolution, and line edge roughness (LER). The photospeed of these resists at 248 nm shows a good correlation with photospeed at EUV for three polymer types, but appears independent of photoacid generator type. This result underscores the importance of the polymer in photoacid generation at EUV. Resolution showed poor correlation between DUV and EUV. Correlations were made between the line edge roughness of EUV-imaged features and unexposed film thickness loss, resist contrast, image log slope (ILS), and LER of resists exposed at DUV. Both contrast and image log slope play important roles in defining LER performance—where the best LER is achieved at high contrast and high ILS.

Ultrathin photoresists for EUV lithography

The strong attenuation of EUV radiation in organic materials has necessitated the use of a thin layer imaging (TLI) resist for lithographic patterning. We have studied several TLI processes for EUV and found the use of an ultra-thin single layer resist (UTR) over a hardmask is a plausible resist system. We have developed new EUV resist system based on DUV chemical approaches. These EUV resist pattern features as small as 70 nm L/S and 70 nm isolated features. The UTR process shows high sensitivity and low line edge roughness compared to other thin layer imaging resists processes such as top-surface imaging. The advantage of these UTR resists is the current familiarity in the industry with processing and materials development. We have also ben able to address one of the main concerns surrounding such thin resists, and we have found they are sufficient to pattern the hard mask with enough resist remaining.

Top surface imaging resists for EUV lithography

The strong attenuation of extreme UV (EUV) radiation by organic materials necessities the use of a thin layer imaging (TLI) process for EUV lithography. Several TLI processes have been identified for potential use for EUVL, and the common theme in these approaches is the transfer of the aerial image to a thin layer of refractory-containing material, which is then used as a dry O2 etch mask during a subsequent pattern transfer to the device layer. One TLI process that has been extensively examined for EUVL is the silylated top-surface imaging (TSI) technology, which is discussed in this paper. Using a new disilane silylation reagent, dimethylaminodimethyldisilane (DMDS) and 13.4 nm exposure, the TSI process has been sued to print 100 nm lines and spaces at equal pitch and 70 nm lines and spaces at a higher 1:2 pitch. The line edge roughness for the printed lines has been determined using a custom image analysis program and, as expected, varies with the particular EUV exposure system and numerical aperture. Exposures done with 193 nm lithography and the TSI process using DMDS are also shown for comparison to the EUV results.

Novel silicon-containing resists for EUV and 193-nm lithography

Two families of polymers have been prepared and evaluated as silicon-containing bilayer resist candidates at both 193 nm and 13.4 nm (EUV). Both families of polymers are based on a tertiary ester protecting group in which the ester group contains a silicon cluster. The PRB family of polymers are random methacrylate copolymers and the PRC family are alternating maleic anhydride/norbornene polymers. The PRB family shows good resolution and sensitivity at both 193 nm and EUV, but suffers from adhesion failure between the imaging layer and the underlayer. The PRC polymers show good adhesion to underlayers and can print features at

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.

Top surface imaging process and materials development for 193 nm and extreme ultraviolet lithography

The maturity and acceptance of top surface imaging (TSI) technology have been hampered by several factors including inadequate resist sensitivity and line edge roughness. We have found that the use of a chemically amplified resist can improve the sensitivity in these systems by 1.5– 2× without compromising the line edge roughness. In addition, we have shown improved line edge roughness by increasing the molecular weight of the polymeric resin in the resist. Using these materials approaches, we have been able to show excellent resolution images with the TSI process for both 193 nm and extreme ultraviolet (13.4 nm) patterning.

Progress in 193-nm top-surface imaging process development

The maturity and acceptance of top surface imaging (TSI) technology has been hampered by several factors including inadequate resist sensitivity and silylation contrast, defects and line edge roughness and equipment performance/reliability issues. We found that the use of a chemically amplified resist can improve the sensitivity by a factor of 1.5 - 2X, without compromising line edge roughness. While the post-silylation contrast of this chemically amplified material is poor ((gamma) < 1), the post-etch contrast is excellent ((gamma) >> 10) and the use of advanced silylation chemistries (disilanes) can further reduce the dose-to-size and increase the contrast. We have also demonstrated that using sulfur dioxide in the plasma etch process can improve the sidewall passivation of the resist lines, thus reducing the overall line edge roughness. Finally, we have been able to successfully use the TSI process to pattern deep sub-micron polysilicon and metal patterns.

Defects and metrology of ultrathin resist films

Defectivity in spin-coated, but unpatterned ultrathin resist (UTR) films (<EQ 1000 Angstrom) was studied in order to determine whether defectivity will present an issue in EUV (13.4-nm) and 157-nm lithographic technologies. These are the lithographic regimes where absorption issues mandate the use of ultrathin resists. Four resist samples formulated from the same Shipley UV6 polymer batch and having the same polymer molecular weight properties but different viscosities, were spin-coated at spin speeds ranging from 1000 to 5000 RPM on a production-grade track in a Class 1 pilot line facility. Defect inspection was carried out with KLA SP1/TBI tool, while defect review was carried out with JEOL 7515 SEM tool and KLA Ultrapointe Confocal Review Station (CRS) Microscope. The results obtained are related to the physical properties of the resist polymers, as well as to spin coating parameters. Also, the results of the defect inspection, review, characterization, and pareto are compared to those obtained on baseline thick resists (>= 3500 Angstrom) processed under similar condition as the ultra-thin resists. The results show that for a well-optimized coating process and within the thickness range explored (800 - 4200 Angstrom), there is no discernible dependence of defectivity on film thickness of the particular resists studied and on spin speed. Also assessed is the capability of the current metrology toolset for inspecting, reviewing, and classifying the various types of defects in UTR films.

Lithographic and chemical contrast of single-component top-surface imaging (TSI) resists

A variety of different approaches were used in an effort to improve the photospeeds of single component TSI resists based on poly(4-hydroxystyrene). The variations included molecular weights, co-monomer partners, and selected substituents. The factors that were studied dramatically affected silylation rates, in one case by as much as an order of magnitude. However, when the silylation times were adjusted to compensate for the rate differences and silylation depths, only minimal differences in photospeed were observed. The apparent contrast measured by swelling upon silylation was very poor ((gamma) equals 1.5) while the contrast measured after etching was quite high, approximately ten times that of the silylation value.

Progress in 157-nm lithography development at Intel: resists and reticles

Intel is aggressively pursuing the use of 157 nm lithography for the 0.1 mm patterning node. Two areas of concentration have been in photoresist and reticle materials development. Over the six months, we have seen considerable progress in new materials development in both areas. In the photoresist area, the use of ultra-thin resists of currently used chemistries appear to be capable of providing short-term layer development and tool testing patterning capability. We have obtained imaging results using a 0.5 NA Schwartzchild optics system. Our best result to data show 70-80 nm lines printed on a pitch of 180 nm. While this small field system has considerably immature optics, it can be used effectively to do basic resist development. In the area of reticle materials development, we have seen considerable improvement in the reduction of OH in blank materials, resulting in higher transmission. We expect to see substrates with greater than 80 percent transmission within the next year at the current rate of accelerated progress. Furthermore, we are not seeing any major processing differences with these new blank materials. Overall, we have seen an accelerated pace of learning in materials development for both resist and new blank materials. Overall, we have seen an accelerated pace of learning in materials development for both resist and reticle materials for 157 nm lithography.