Immersion lithography for the 32nm node and beyond requires advanced methods to control 193 nm radiation
reflected at the resist/BARC interface, due to the high incident angles that are verified under high numerical aperture
(NA) imaging conditions. Swing curve effects are exacerbated in the high NA regime, especially when highly reflective
substrates are used, and lead to critical dimension (CD) control problems. BARC reflectivity control is also particularly
critical when underlying surface topography is present in buried layers due to potential reflective notching problems. In
this work, a graded spin-on organic BARC was developed to enable appropriate reflectivity control under those
conditions. The graded BARC consists of two optically distinct polymers that are completely miscible in the casting
solution. Upon film coating and post-apply baking, the two polymers vertically phase-separate to form an optically
graded layer. Different characterization techniques have been applied to the study of the distribution of graded BARC
components to reveal the internal and surface composition of the optically graded film, which includes Variable Angle
Spectroscopic Ellipsometry (VASE) and Secondary Ion Mass Spectroscopy (SIMS). Also, optical constant optimization,
substrate compatibility, patterning defectivity and etch feasibility for graded BARC layers are described. Superior 193
nm lithographic performance and reflectivity control of graded BARC beyond 1.20 NA compared to conventional
BARCs is also demonstrated.
We provide an expanded description of the global algorithm for mask optimization introduced in our earlier papers, and discuss auxiliary optimizations that can be carried out in the problem constraints and film stack. Mask optimization tends inherently to be a problem with non-convex quadratic constraints, but for small problems we can mitigate this difficulty by exploiting specialized knowledge that applies in the lithography context. If exposure latitude is approximated as maximization of edge slope between image regions whose intensities must print with opposite polarity, we show that the solution space can be approximately divided into regions that contain at most one local minimum. Though the survey of parameter space to identify these regions requires an exhaustive grid search, this search can be accelerated using heuristics, and is not the rate-limiting step at SRAM scale or below. We recover a degree of generality by using a less simplified objective function when we actually assess the local minima. The quasi-binary specialization of lithographic targets is further exploited by searching only in the subspace formed by the dominant joint eigenvectors for dark region intensity and bright region intensity, typically reducing problem dimensionality to less than half that of the full set of frequency-domain variables (i.e. collected diffraction orders). Contrast in this subspace across the bright/dark edge will approximately reflect exposure latitude when we apply the standard fixed edge-placement constraints of lithography. However, during an exploratory stage of optimization we can define preliminary tolerances which more explicitly reflect constraints on devices, e.g. as is done with compactor codes for design migration. Our algorithm can handle vector imaging in a general way, but for the special case of unpolarized illumination and a lens having radial symmetry (but arbitrary source shape) we show that the bilinear function which describes vector interference within the film stack can be expressed in terms of three generic radial functions, enabling rapid numerical evaluation of the Hopkins kernel. By inspection these functions show that one can in principle recover classical scalar-like imaging even at high NA by exposing a very thin layer spaced above a reflective substack. The reflected image largely restores destructive interference in TM polarized fringes, if proper phasing is achieved. With an ideal reflector, the first-order azimuthal contrast loss term vanishes in all TCC components, and complete equivalence to scalar imaging is obtained in classical two-beam imaging.
The ability to extend 193 nm lithography resolution depends on increasing the numerical aperture (NA) of the exposure system, resulting in smaller depth of focus, which subsequently requires use of thinner photoresists. Bottom antireflective coatings (BARCs) are a necessity, but the organic composition of current 193 nm BARCs offers poor etch selectivity to the photoresist. As a result, image transfer with thin resists is becoming increasingly difficult. It is also more challenging to control reflectivity at high numerical apertures with a thin, single layer BARC.
To address these issues, IBM has developed a new class of silicon containing BARCs. These materials exhibit high etch selectivity that will significantly improve the performance of high NA 193 nm lithography. The incorporation of silicon in the backbone of the polymers comprising these BARCS affords a high etch selectivity to conventional organic resists and therefore these polymers can be used as thick planarizing BARCs. The optical constants of these BARCs have been tuned to provide good reflectivity control at NA > 1.2 These materials can also be used as part of a dual layer BARC scheme composed of the thin organosilicon based BARC coated over a planarizing organic underlayer. This scheme has also been optically tuned to provide reflectivity suppression at high incident angles. By utilizing a thick BARC, a novel contact hole shrink process is enabled that allows tapering of the sidewall angle and controlling the post-etch critical dimension (CD) bias. Structures of the silicon containing polymer, formulation chemistry, optical tunability, lithography at high NA and RIE pattern transfer are reported.
Polysilsesquioxane-based 193 nm positive bilayer resists are described. In this design Si for etch resistance is placed in every repeat unit and acid-labile protected and acidic groups (and polar units) are in the side chain, allowing to incorporate each lithographically critical functionality in sufficient quantity. Fluoroalcohol is employed as an acid group instead of carboxylic acid because of its more attractive dissolution properties. Polymers were carefully analyzed by 19F, 13C, and 29Si NMR to determine composition and to quantify residual acetyl, silanol, and Q/T. Hydrogen-bonding between tertiary ester and fluoroalcohol in the polysilsesquioxanes was investigated by FT-IR and the effect of lactone incorporation on the thermal deprotection temperature elucidated. In order to better understand the dissolution behavior of exposed resist films, the silsesquioxane resist polymers were partially (ca. 30%) and fully deprotected in solution with acid and their dissolution kinetics investigated by using a quartz crystal microbalance (QCM). It has been found that the exposed areas of the silsesquioxane resists can have a very fast dissolution rate (Rmax) of >20,000 A/sec (or even >100,000 A/sec). Heating the fully deprotected model polymers to 150°C did not reduce the dissolution rate much, suggesting thermal condensation of silanol end groups is insignificant. Model deprotected polymers containing triphenylsulfonium nonaflate were exposed to 254 nm radiation, baked, and subjected to QCM measurements in order to determine whether or not acid-catalyzed silanol condensation would reduce the dissolution rate. A combination of high dose and high temperature bake resulted in significant reduction of the dissolution rate in the silsesquioxane polymer containing a small trifluoroalcohol. However, the dissolution behavior of the polymer bearing a bulky norbornene hexafluoroalcohol was unaffected by exposure and bake. Chemical and development contrast curves were generated by using a temperature gradient plate and bake temperature effects investigated. While a postapply bake temperature variation from 95 to 130°C did not affect the contrast (chemical and development) and sensitivity, a postexposure bake temperature effect was quite small. The process window of the silsesquioxane bilayer resists was comparable to that of a high performance commercial 193 nm single layer resist for both isolated and nested contact hole imaging, Superior performance of our silsesquioxane resist was demonstrated in patterned etch in comparison with a COMA-based bilayer resist.
Polyhedral Oligomeric Silsesquioxane (POSS) derivatives have been investigated as potential candidates for high resolution resists. POSS materials are cage compounds with defined mono-disperse molecular weights. These materials are attractive candidates for molecular resist development because of their commercial availability and the ease with which they can be derivatized. These resists are more suited for bilayer resist applications because of their high silicon content. We have developed positive bilayer resists suitable for 193-nm and other emerging lithographic applications.
While evaluating 193 nm, and early versions of 157 nm and EUV resists, the lithography community has focused on post-develop LER values derived from image analysis of top-down SEM micrographs. These numbers, however, do not capture the tendency of a resist to facet and roughen during plasma etching processes. They also do not convey any information about the role of the anti-reflective coatings/hard masks in the transfer of resist roughness into the underlying substrate. From a manufacturing perspective, it is the "LER" of the final etched substrate that is more important. This paper systematically studies the impact of resist polymer platform and thickness, etching conditions, and presence of organic and inorganic anti-reflective coatings/hard masks on substrate roughening. An AFM technique, previously developed by Reynolds and Taylor, is used to measure the feature sidewall roughness as a function of etch depth. This technique enables us to calculate the sidewall roughness of the resist, ARC/hard mak and substrate surfaces simultaneously, and determine correlations that may exist between these values. The paper identifies and demonstrates patterning methodologies that can be used to achieve "smooth" substrate surfaces even when the resist is "thin".
The importance of hardmask technology is becoming increasingly evident as the demand for high-resolution imaging dictates the use of ever-thinner resist films. An appropriately designed etch resistant hardmask used in conjunction with a thin resist can provide the combined lithographic and etch performance needed for sub-100 nm device fabrication. We have developed a silicon-based, plasma-enhanced chemical vapor deposition (PECVD) prepared material that performs both as an antireflective coating (ARC) and a hardmask and thus enables the use of thin resists for device fabrication. This ARC/hardmask material offers several advantages over organic bottom antireflective coatings (BARC). These benefits include excellent tunability of the material's optical properties, which allows superior substrate reflectivity control, and high etch selectivity to resist, exceeding 2:1. In addition, this material can serve as an effective hardmask etch barrier during the plasma etching of dielectric stacks, as the underlying silicon oxide etches eight times faster than this material in typical fluorocarbon plasma. These properties enable the pattering of features in 1-2 μm dielectric stacks using thin resists, imaging that would otherwise be impossible with conventional processing. Potential extendibility of this approach to feature sizes below 100nm has been also evaluated. High resolution images as small as 50nm, have been transferred into a 300nm thick SiO2 layer by using Si ARC/hardmask material as an etch mask. Lithographic performance and etch characteristics of a thin resist process over both single layer and index-graded ARC/hardmask materials will be shown.
Extending 193nm lithography to well below 100nm resolution will depend on high NA tooling coupled with thin resist processing. Semiconductor manufacturing uses BARC's (Bottom Antireflective Coating) based on organic spin coatable polymers, to improve the resolution by absorbing light that otherwise will be reflected back into the resist. However, the use of organic BARC's for patterning sub 100nm features will be limited due to poor etch selectivity to the photo resist. IBM has developed a new class of polymers that can function as planarizing BARC's. These materials show an etch selectivity to the photo resist in excess of 3:1 in fluorocarbon based ARC-open RIE chemistry. The hardmask properties of these materials for oxide open are equivalent to typical resists. Furthermore these materials can be implemented like organic ARC's and are stripped in resist strips available in manufacturing. Basic materials characterization data, optical tunability, lithographic performance with different resists, process window data, and complete integration schemes will be presented.