With immersion lithography approaching the insertion in production, watermarks remain as one of the main concerns for
immersion specific defects. They require special attention because of their size and associated high kill-ratio, and their
increasing occurrence at higher scan speeds. IMEC has been working to understand the underlying mechanism of why
remaining water droplets cause these defects.
This work focuses on water uptake measurements and how this parameter correlates to watermark defectivity.
Ellipsometric Porosimetry (EP) is used to measure the water uptake tendencies of resist and top coat materials and stacks
thereof, and investigate what parameters are affecting it. The influence of material and process parameters and the
presence of a top coat on water uptake by the resist are evaluated. In parallel, the quartz crystal microbalance (QCM)
technique has been used as an alternative option to measure the water uptake. Though a one-to-one comparison between
the results is not straightforward, the main trends are identical for both techniques.
No perfect correlation of watermark defectivity with water uptake has been found in this study. Nevertheless, the results
show a tendency towards higher watermark sensitivity with higher water uptake by the film. It is recognized that the total
watermark defectivity is most probably a complex interplay of different parameters with water uptake being only one of
them.
Since the moment immersion lithography appeared in the roadmaps of IC manufacturers, the question whether to use top coats has become one of the important topics for discussions. The top coats used in immersion lithography have proved to serve as good protectors from leaching of the resist components (PAGs, bases) into the water. However their application complicates the process and may lead to two side effects. First, top coats can affect the process window and resist profile depending on the material's refractive index, thickness, acidity, chemical interaction with the resist and the soaking time. Second, the top coat application may increase the total amount of defects on the wafer. Having an immersion resist which could work without the top coat would be a preferable solution. Still, it is quite challenging to make such a resist as direct water/resist interaction may also result in process window changes, CD variations, generation of additional defects. We have performed a systematic evaluation of a large number of immersion resist and top coat combinations, using the ASML XT:1250Di scanner at IMEC. The samples for the experiments were provided by all the leading resist and top coat suppliers. Particular attention was paid to how the resist and top coat materials from different vendors interacted with each other. Among the factors which could influence the total amount of defects or CD variations on the wafer were: the material's dynamic contact angle and its interaction with the scanner stage speed, top coat thickness and intermixing layer formation, water uptake and leaching. We have examined the importance of all mentioned factors, using such analytical techniques as Resist Development Analyser (RDA), Quartz Crystal Microbalance (QCM), Mass Spectroscopy (MS) and scatterometry. We have also evaluated the influence of the pre- and pos- exposure rinse processes on the defectivity. In this paper we will present the data on imaging and defectivity performance of the resists with and without the use of top coats. So far we can conclude that top coat/resist approach used in immersion lithography needs some more improvements (i.e. process, materials properties) in order to be implemented in high volume manufacturing.
Defectivity has been one of the largest unknowns in immersion lithography. It is critical to understand if there are any immersion specific defect modes, and if so, what their underlying mechanisms are. Through this understanding, any identified defect modes can be reduced or eliminated to help advance immersion lithography to high yield manufacturing. Since February 2005, an ASML XT:1250Di immersion scanner has been operational at IMEC. A joint program was established to understand immersion defectivity by bringing together expertise from IMEC, ASML, resist vendors, IC manufactures, TEL, and KLA-Tencor. This paper will cover the results from these efforts. The new immersion specific defect modes that will be discussed are air bubbles in the immersion fluid, water marks, wafer edge film peeling, and particle transport. As part of the effort to understand the parameters that drive these defects, IMEC has also developed novel techniques for characterizing resist leaching and water uptake. The findings of our investigations into each immersion specific defect mechanism and their influencing factors will be given in this paper, and an attempt will be made to provide recommendations for a process space to operate in to limit these defects.
We have investigated the impact of water and top-coats on the resist in water immersion lithography by analyzing the dissolution behavior and the film constitution. We used a resist development analyzer (RDA) and a quartz crystal microbalance (QCM) to study the dissolution behavior. The film constitution was studied through the gradient shaving preparation (GSP) method in combination with TOF-SIMS. The GSP/TOF-SIMS method reveals the constitution of a top-coat/resist film. We found that, in a resist, the photo acid generator (PAG) anion at a depth of about 30 nm from the surface leached into water and a surface insoluble layer formed during immersion. The estimated amount of leaching was about 5% of the original content. The formation of an intermixing layer with a low dissolution rate was observed for some top-coat and resist combinations. The thickness of the intermixing layer and the formation behavior were made clear. We believe the intermixing layer was caused by the top-coat solvent eluting resist components. In a top-coat, a PAG existed within the top-coat and the PAG anion leached into the water. Top-coats blocked gaseous decomposed products from the resist film during PEB. These results are useful for estimating patterning characteristics and the defectivity due to materials for actual immersion exposure.
We have investigated the interaction of resists with water and perfluoropolyether (PFPE) as immersion fluids. We found that some unique behaviors occurred in immersion lithography. An acetal protected poly(p-hydroxystyrene) type resist in water immersion showed decreased resist thickness after exposure. The deprotection reaction during exposure appeared to be accelerated by water. A COMA (cycloolefine-mareic anhydride alt-copolymer) type resist in water immersion showed an increased dissolution rate. FT-IR measurements indicated that the hydrolysis of maleic anhydride occurred during exposure and post-exposure baking. A reduction in the dissolution rate was observed in the immersion lithography of most resists. In water immersion, the formation of a surface insoluble layer and swelling was observed. We confirmed that a photochemical acid generator (PAG) or generated acid eluted into the water by TOF-SIMS. In PFPE immersion, we think that PFPE penetrating across the resist film blocks the penetration of the alkaline aqueous developer.
We designed platforms for a 157-nm resist polymer that have both high transparency and high etch resistance. We measured the etching rate of various fluoropolymers under the conditions of poly etch and oxide etch processes. Most of the fluoropolymers showed good correlation with an etching rate ratio and a value of (NC-NO-NF) / NT (NT, NC, NO, and NF was the number of total atoms, carbon, oxygen, and fluorine atom in a monomer unit, respectively). This index, (NC-NO-NF) / NT was named IERF (index for etch resistance of fluoropolymers). It became easy to design a fluoropolymer that has high etch resistance using this index. Moreover, the acrylic polymer that has a particular kind of fluorinated alicyclic compound in its side chain was not in the above-mentioned correlation, however, it was found to have higher etch resistance. Base upon these results, two platforms were built, one of the copolymer type with polyacrylate (PA) and polystyrene (PS), the other of the copolymer type with PA and polycycloolefin (PC). Resists belonging to the two platforms were made and a high resolution of 85 nm or less was obtained by using the Microstepper (NA0.6, (sigma) 0.3, alternating phase shift mask). These resists showed a slightly higher etch resistance as compared with a COMA (cycloolefin-maleic anhydride) type resist with comparatively high etch resistance even during the ArF resist.
Utilizing ETS-1 laboratory tool in Himeji Institute of Technology (HIT), as for the fine pattern replicated by using the Cr mask in static exposure, it is replicated in the exposure area of 10 mm by 2 mm in size that the line and space pattern width of 60 nm, the isolated line pattern width of 40 nm, and hole pattern width of 150 nm. According to the synchronous scanning of the mass and wafer with EUVL laboratory tool with reduction optical system which consisted of three-aspherical-mirror in the NewSUBARU facilities succeeded in the line of 60 nm and the space pattern formation in the exposure region of 10mm by 10mm. From the result of exposure characteristics for positive- tone resist for KrF and EB, KrF chemically amplified resist has better characteristics than EB chemically amplified resist.
We investigated characteristics of resists consisting of well- known resist polymers by the F2 excimer laser exposure. Negative-resist behavior due to crosslinking or decarboxylation was observed for poly(p-hydroxystyrene) based resists, polysilsesquioxane based resists and polynorbornenecarboxylate based resists. On the other hand, polymethacrylate based resists indicated a clear positive- resist behavior with high contrast without crosslinking. Therefore we studied further details of the characteristics of resists consisting of photodecomposable polymers such as methacrylate polymers and found methacrylate polymers bearing alicyclic groups to be resist with high dissolution contrast and little outgassing. The 200 nm L/S pattern with vertical profile at 1000 angstrom thickness was obtained by F2 excimer laser contact exposure in spite of poor transmittance of 30% per 1000 angstrom thickness. It was showed that the resolution by a 157 nm scanner with the lens of NA0.7/(sigma) 0.7 was 70 nm L/S and 40 nm iso-line at 1000 angstrom thickness by PROLITH/3D lithography simulator using experimental parameters.
In ArF excimer laser lithography, the bottom antireflective coating (BARC) technique is essential in inhibiting the effect of interference and reflective notching. We investigated the antireflective effect of commercially available organic BARCs, that had originally been designed for KrF and i-line lithography, and also the patterning characteristics of ArF resists with BARCs. The refractive indices of various materials were measured with a spectroscopic ellipsometer. The real part (n) and the imaginary part (k) of the complex refractive index at 193 nm were 1.4 to 1.7 and 0.1 to 0.8 respectively. Almost all the materials had sufficient antireflectivity at 193 nm. We investigated the patterning characteristics of chemically amplified ArF positive resists with suitable BARC materials. The resolution, the depth-of- focus of patterns below 0.16-micrometer lines and spaces, and the exposure latitude were improved and good critical dimensional control over topography was achieved by using BARC. An acceptable profile after gate structure (BARC, W-Si, and Poly-Si) etching could be obtained under the typical etching conditions used for KrF resists.
We investigated the dissolution properties of methacrylate polymers to study the development mechanism of chemically amplified resists. We measured the dissolution rate of methacrylate copolymers as a function of the hydrophilic unit content, and simulated dissolution phenomena based on percolation theory. The dissolution rate curves we obtained had strong nonlinear dependence on hydrophilic unit content. In the simulation analysis, base molecule diffusion through the hydrophilic path was modeled as particle diffusion through the percolation field. The diffusion constant was calculated based on conventional and modified diffusion models. The width of the diffusion path is not considered in a conventional model. We suggested a new variable to describe diffusion path width in the modified model. The simulation results indicated that our model reproduced the experimental results better than the conventional model.
We examine the process margins of chemically amplified ArF resists designed for the single layer process. We measured the dissolution characteristics and investigated how the dissolution contrast affected the lithographic performance. We confirmed that high dissolution contrast can improve both the resolution and process margin. A 0.13 micrometers line and space pattern can be obtained. However, the depth of focus is not good enough for device fabrication. To achieve an acceptable process margin, we applied resolution enhancement techniques to the high-contrast resist. Applying off-axis illumination and an attenuated phase-shifting mask can greatly improve the process margin. 1.2 micrometers and 0.8 micrometers defocus margins were obtained at 0.15 and 0.13 micrometers line and space patterns, respectively.
The etching rates of resist base polymers with several molecular weights were measured against fluorocarbon or chlorine plasma. The rate showed a minimum value at the weight-average molecular weight of approximately 10,000, and increased to a saturated value for higher molecular weights. For poly(p-hydroxystyrene) (PHS) protected with tert- butoxycarbonyl (BOC) or acetal (ACT) group, the rate became larger with increasing the protection ratio and the rate of BOC-protected PHS was lower than that of ACT-protected PHS. It was also found that the rate was significantly influenced by the prebake and increased as the temperature became higher. The etching rate of the resin with gradual cooling after the prebake was slower than that with quick cooling. These results may indicate that the dry etching resistance is independent of the density of resin film and influenced by the stability of arrangements of polymer molecules. The reduction of etching rate by the deep UV cure method has been tried. The rate decreased with the deep UV cure under N2 gas flow, while increased in the presence of O2 such as dry air. The films after deep UV cure under N2 and a dry air was scarcely dissolved in any organic solvents and this implies that the polymers are crosslinking. The absorbance band of carbonyl appeared in FT-IR spectra for the case under dry air may suggest that oxygen atoms in the polymer structure influence the dry etching resistance. We have tried deep UV cure under N2 gas flow for KrF chemically amplified PHS based resists and obtained the same level of etching rates as novolak resin. For ArF resists, the combination of the incorporation of acryl unit into the base polymer and deep UV cure under N2 gas flow may be an effective method for improving the dray etching resistance.
Germanium-containing resist material has been investigated as a new type of removable
bilayer resist , since the oxide of germanium is soluble in conventional acids. The
polymers derived from trimethylgermyl- styrene ( GeSt) show good resistance to 02 RIE , and
their surface has been •determined to be converted into GeO, by XPS measurement before
and after 02 RIE. The homopolymer of GeSt has been found to crosslink upon exposure to
deep UV or electron beam radiation and to behave as a negative resist. The sensitivity
is enhanced several times as high as that of the PGeSt by copolymerizing with 1 0 mol%
chloromethyl-styrene ( CMSt) . The copolymer gives fine resist patterns with vertical
sidewalls in a bilayer process. The germanium- containing resist pattern after 02 RIE
is not completely dissolved in some acids such as H2 SO4 . This is due to the organic
components remaining in the film. However, it has been found that it is perfectly
dissolved in oxidizing acids such as fuming HNO and H2S04/H202(2/l) without a residue.
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