We describe progress in low-k1 factor double patterning using 172 nm ultraviolet (UV) curing as a resist stabilization
method. Factors that have contributed to enhanced patterning capability include a) resists design and optimization for
both patterning and UV curing; b) use of unique R&D tooling capabilities to rapidly identify and optimize key process
variables; c) development of simple process metrics for characterizing double patterning process quality, and d) use of
172 nm-resistant antireflective materials. A designed resist, XP-7600A, was selected for detailed evaluation based on
superior patterning and curing behavior (less than 10 percent volumetric shrinkage during cure.) Process optimization
on 172 nm damage-prone antireflective coatings produced 60 nm cross-grid contact holes at 0.93 NA (litho k1 = 0.28)
with good uniformity when an ancillary 150 °C post-UV bake was used. Additional optimization on improved
antireflective coatings yielded superior process latitude (>20 percent 172 nm dose latitude) and also demonstrated that a
UV-cure-only resist stabilization process flow may be attainable. Under optimized conditions, highly uniform 60 nm
half-pitch cross-grid contacts with cross-sectional area uniformity (1σ) of approximately 200 nm2 (5 percent) are
produced at 135 nm resist film thickness.
The introduction of double patterning and double exposure technologies, especially in combination with hyper NA,
increases the importance of wafer topography phenomena. Rigorous electromagnetic field (EMF) simulations of two
beam interference exposures over non-planar wafers are used to explore the impact of the hardmask material and pattern
on resulting linewidths and swing curves after the second lithography step. Moreover, the impact of the optical material
contrast between the frozen and unfrozen resist in a pattern freezing process and the effect of a reversible contrast enhancement
layer on the superposition of two subsequent lithographic exposures are simulated. The described simulation
approaches can be used for the optimization of wafer stack configurations for double patterning and to identify appropriate
optical material properties for alternative double patterning and double exposure techniques.
Base titration methods are used to determine C-parameters for three industrial EUV photoresist platforms (EUV-
2D, MET-2D, XP5496) and twenty academic EUV photoresist platforms. X-ray reflectometry is used to measure the
density of these resists, and leads to the determination of absorbance and film quantum yields (FQY). Ultrahigh levels
of PAG show divergent mechanisms for production of photoacids beyond PAG concentrations of 0.35 moles/liter. The
FQY of sulfonium PAGs level off, whereas resists prepared with iodonium PAG show FQYs that increase beyond PAG
concentrations of 0.35 moles/liter, reaching record highs of 8-13 acids generated/EUV photons absorbed.
Etch resistance and post etch roughness of ArF photoresists still remain some of the critical issues during process
integration for sub-100nm technology nodes. Compared to phenyl-containing KrF polymers, methacrylate
polymers commonly used for ArF lithography show weak bulk etch resistance in addition to a highly damaged
surface after standard etch processes. Counter to the photoresist, the etch rates of BARC are required to be very
fast to prevent degradation of the photoresist before the image has been transferred to the substrate.
There are a number of etch models in the literature which attempt to describe the correlation between polymer
structure and blanket etch rates. Ohnishi Parameter and Ring Parameter are the most common etch models
correlating atomic and structural trends in the resist polymer and etch rates. These etch models have been tested
in two ways: systematically changing the composition of a terpolymer and using polymers with different
functional groups. By comparing the etch rates of this large series of polymer structures it was found that these
etch models were not sufficient in describing the relationship between the atomic or structural trends in polymer
with etch rates. New etch models that describe the structure property relationship and etch rate trends have been
developed. These new models show a better correlation with the observed experimental results. Finally, new
polymers have been developed, for both ArF and BARC applications. These materials show a significant
improvement in term of etch properties.
This paper describes characterization and lithographic results for one class of low absorbance fluoropolymers that were developed for use in 157 nm lithography. We discuss basic resist properties such as absorbance, hydrophobicity, thickness, resolution and profile for dense 1:1 and semi- dense 1:1.5-10 L/S features, reflection control and plasma etching resistance as a function of composition. Lithographic results were obtained on two types of substrates, silicon and SiON hardmask anti-reflectant. The results on the anti-reflectant were compared to those obtained from simulations using PROLITH. Some of the conclusions of this investigation are: Lower absorbance resists have higher hydrophobicity and better resolution; Resists with high hydrophobicity have very poor adhesion on SiOn, but have very good adhesion on Si and organic anti-reflectants; Only inorganic anti-reflectants have sufficient absorption to provide very low reflectance in <30nm thick films; 100 nm 1:1 L/S resolution is attained in 205 nm thick resist on Si at a resist absorption of 2.2/micrometers . The profile is tapered due to absorption; Adhesion to SiON has been achieved by polymer modification.
The rate constants for acid generation (C parameter) in chemically amplified photoresist are determined for four photoacid generators (norbornene dicarboximidyl triflate, triphenyl sulfonium triflate, bis-4-t-butylphenyl iodonium perfluorooctane sulfonate, and bis-4-t-butylphenyl iodonium triflate) under exposure to 157nm radiation using a standard addition technique. The technique utilizes an in film neutralization of photogenerated acid by base quencher to determine the increase in exposure energy necessary to produce an equivalent fee acid concentration at each loading of base. We present a general model to interpret the data that also accounts for the strong absorption of radiation by the resist film. An average absorption coefficient of 13.2micrometers -1 (base e) has been measured at 157nm for these resist films. Results from 157nm irradiation are compared to deep ultraviolet and ionizing radiation, indicating that resist photochemistry at 157nm includes processes important to both energy regimes.
The thickness and complex refractive indices of the thin films on a silicon wafer during lithographic imaging are critical factors affecting the processing of integrated circuits. The inorganic materials involved, such as the silicon substrate and inorganic anti-reflection coatings, are usually well characterized or present few difficulties. The optical properties of organic materials, such as photoresists and anti-reflection coatings have been more difficult to determine with confidence. In general, for each material on the substrate, three values need to be determined; they include the thickness and the real and imaginary parts of the refractive index at the exposure wavelength. Single measurements of reflectance or ellipsometric parameters do not provide sufficient degrees of freedom for determining these three unknowns. Typically, this problem is resolved by collecting reflectance or ellipsometric data over a range of wavelengths. However, because n and k functions of wavelength, two additional unknowns are present for each additional wavelengths. This problem is typically resolved by fitting n and kn to various spectral functions in order to reduce the number of unknowns. Unfortunately, the functional forms used are frequently inappropriate for the organic materials of interest. The essence of the new method is the use of data from coatings having different thicknesses in order to provide the degrees of freedom necessary for a solution. This is achieved at a single wavelength and thereby avoids the spectral model fits that are frequently fraught with problems. The new method is demonstrated by determining thicknesses and n and k values at the exposure wavelength for a photoresist and an anti-reflection coating designed to be used with 193 nm exposures. The optical properties of two 248 nm anti-reflection coatings are also determined over a spectral range.
During the exposure of photoresist, standing waves form within the film, modulating the intensity of the illumination with depth in a sinusoidal fashion. Significant changes in the standing wave period are observed when the exposure tool numerical and illumination settings are altered. These changes are explained and demonstrated by means of both experimentation and lithography simulation for conventional and annular illumination. A function is derived which fully characterizes standing wave extrema separation in terms of numerical aperture and partial coherent for conventional illumination. It is shown that when extrema separation is normalized by a factor of (lambda) /4n that the resulting function is independent of (lambda) and exposure tool magnification ratio, but is sensitive to changes in n. Although not fully characterized, the effect of annular illumination on extrema separation is explored and some key aspects of the behavior are identified.
Traditional methods of measuring the Dill C Parameter involve monitoring the absorbance of a resist as a function of exposure. In chemically amplified resist, absorbance changes with exposure are small and frequently have little correlation to the amount of photoacid generated.
Organic anti-reflective coatings (ARCs) continue to play an important role in semiconductor manufacturing. These materials provide a convenient means of greatly reducing the resist photospeed swing and reflective notching. In this paper, we describe a novel class of ARC materials optimized for lithographic applications using 193 nm exposure tools. These ARCs are based upon polymers containing hydroxyl-alkyl methacrylate monomers for crosslinkable sites, styrene for a chromophore at 193 nm, and additional alkyl-methacrylate monomers as property modifiers. A glycouril crosslinker and a thermally-activated acidic catalyst provide a route to forming an impervious crosslinked film activate data high bake temperatures. ARC compositions can be adjusted to optimize the film's real and imaginary refractive indices. Selection of optimal target indices for 193 nm lithographic processing through simulations is described. Potential chromophores for 193 nm were explored using ZNDO modeling. We show how these theoretical studies were combined with material selection criteria to yield a versatile organic anti-reflectant film, Shipley 193 G0 ARC. Lithographic process data indicates the materials is capable of supporting high resolution patterning, with the line features displaying a sharp resist/ARC interface with low line edge roughness. The resist Eo swing is successfully reduced from 43 percent to 6 percent.
Gary Taylor, Peter Trefonas, Charles Szmanda, George Barclay, Robert Kavanagh, Robert Blacksmith, Lori Joesten, Michael Monaghan, Suzanne Coley, Zhibiao Mao, James Cameron, Ricky Hardy, Dana Gronbeck, S. Connolly
Methacrylates were the first class of resist to be examined for use in 193nm lithography. They are still useful today, but have a very different molecular structure because of the requirements for development in 0.262N tetramethyl ammonium hydroxide and high etching resistance. A major driving force for their continued use is the availability of a wide variety of methacrylate monomers and the use of free racial polymerization which imparts a wide range of properties to the polymers and makes them very cost effective.
This paper describes some of the basic physicochemical considerations necessary to design a resist for use in 193 nm lithography. Of fundamental importance are the photoreaction which generates the photoacid, the reactivity of the photoacid the dissolution of the resist in the developer, and the adhesion of the images to the substrate. These phenomena are discussed and we show results that demonstrate progress in these areas. In addition, we show preliminary etch resistance of our polymer system and selected lithographic results.
Information theory has been used to quantify the amount of information transferred during a lithographic process. By calculating the information, a quantity isomorphous with the entropy, we are able to determine the points in the process space where most of the lithographic information is lost and suggest modifications that will increase the overall information content. In this study we use the threshold image density model to evaluate the information content of the latent and printed images in a given set of resist features. The model includes contributions from exposure, acid diffusion, crosslinking or deblocking chemistry, dissolution selectivity and percolation effects. At each step we compute the information gain or loss relative to a given 'prior' distribution and display the results. Selected experimental examples will be shown to illustrate the point.
KEYWORDS: Polymers, Diffusion, Chemistry, Picture Archiving and Communication System, Semiconducting wafers, Systems modeling, Molecules, Data acquisition, Information operations, Spatial analysis
The universality of a single percolation field scaling law to the tetramethylammonium hydroxide dissolution of derivatized novolac and poly(4-vinylphenol) polymers was investigated. According to this hypothesis, the dissolution of phenolic polymers occurs through a percolative mechanism resulting from the diffusion of base along nascent channels formed by the spatial proximity of phenolic hydroxyl groups (presumably diffusion of the cation is rate limiting). Dissolution inhibition results from the removal of sites from the percolation field. In this study, the polymers were derivatized with increasing amounts of either 2,1,5- diazonaphthoquinone groups or methylsulphonyl ester groups and the dissolution rates of the films were measured. While our experimental data supported adherence to the percolation law (p equals the scaled amount of free hydroxyl sites remaining on the polymer), Rate equals Rate0 (p-pc)t, we did not find that a single exponent of t equals 2 universally described the dissolution behavior. Rather, our data indicated that t varied with different systems, with values of t greater than 5 being observed. These results are explained in terms of multiple simultaneously operant mechanisms of dissolution creating an environment where multiple percolation can occur. The relative shielding effects of the blocking groups are also compared.
This paper describes a method to measure acid diffusion in different negative I-line resist systems. Diffusion of this acid plays a critical part in the image formation process. While some diffusion is necessary to achieve high resist sensitivity, excessive acid diffusion can cause an unacceptable loss of resolution. For this reason, diffusion must be controlled within certain limits during standard resist processing. The threshold crosslink theory of image formation, which states that a minimum concentration of acid is required to render the resist insoluble for a given development condition can be used in conjunction with a reaction- diffusion model to determine the magnitude of acid diffusion in the resist. This relatively straightforward method of measuring acid diffusion is then applied to determining an optimum resist process. Several different negative I-line acid catalyzed resists are investigated and the differences in the magnitude of acid diffusion are determined.
Numerical simulations of printing of a bar in photoresist over a MOSFET gate using positive and negative, i-line and deep-UV resists are presented. The masks were chosen to produce the same nominal structure. The resist process was simulated in three separate phases: exposure, post-exposure bake, and development. Three-dimensional relief images of the printed bar are given for these cases.
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