Jongwook Kye

A lithographic and process assessment of photoresist stabilization for double-patterning using 172-nm photoresist curing

We have developed a unique resist stabilization process for double patterning that uses 172 nm UV curing to freeze a first photoresist pattern prior to application and patterning of a second photoresist film. 172 nm cure offers many potential advantages over other resist stabilization processes, including improved pattern fidelity vs. other cure processes and track-based implementation scenarios that are relatively simple, compact, and inexpensive. Assessment of 172 nm double imaging process requirements and limitations indicates that pattern distortions in the frozen first photoresist may arise during all 2nd patterning steps, including coating, exposure, and development. Careful optimization to maximize overall pattern fidelity is needed. Process optimization using a conventional 193 nm photoresist suggests that pattern freeze approaches based on resist cure are best suited to extremely regular structures due to line-end and other resist distortions. Nevertheless, the method allows cross-grid contact printing at lithographic k1 = 0.385.

Polarization aberration analysis in optical lithography systems

The use of immersion technology extends the lifetime of optical lithography by enabling ultra-high NA much greater than 1.0. Ultra-high NA application for low k1 imaging strongly demands an adoption of polarization illumination as a resolution enhancement technology. It is typically assumed that the transmitted wavefront has uniform amplitude and a constant polarization state across the pupil. This assumption is not valid any more for the level required for low k1 imaging. This paper considers methods of polarization analysis including polarization aberration theory. Definitions of basic polarization phenomena and review of matrix representation are included in this discussion. Finally we propose Pauli spin matrix representation as preferred method to describe polarization aberration.

Polarization aberration analysis using Pauli-Zernike representation

Polarized illumination is a viable technique for improving the image quality and process latitude of hyper-NA lithography. On investigation of polarization effects, it is often assumed that the lens system can maintain the polarization state through optical path, which may not be the case with actual lenses. These polarization changes may cause CD variations and pattern placement errors. In this paper, we investigated a method of polarization analysis across the pupil and showed some examples of polarization aberrations. Also, we analyzed CD sensitivity and pattern placement errors from polarization aberrations. Specific terms of the Pauli Zernike representation have effects on CD and pattern placement errors, like the Zernike representation of conventional aberrations. The Pauli-Zernike representation is intuitive and useful for understanding and specifying polarization aberrations.

Cure-induced photoresist distortions in double patterning

Many processes are under evaluation as simplifications to current double patterning methods. Reduction in process complexity and cost may be achieved by use of track-based photoresist stabilization methods that eliminate one etch step by allowing a second resist to be patterned over a first resist. Examples of stabilization methods using numerous curing processes have been reported. At least some resist shrinkage during stabilization appears to be generally observed for these methods. We evaluate the link between shrinkage and three-dimensional pattern distortions at line ends and elbow corners using experimental and simulation-based methods. A 172-nm UV resist curing process was used to produce controlled shrinkage ranging from 5% to 30%: shrinkage was correlated with resist distortions. At cure dose sufficient to stabilize the resist, shrinkage of approximately 23% results in measured line-end pullback and elbow displacement of approximately 16% and 13% of nominal linewidth respectively, when measured at resist half-height. Finite element analysis of resist beam structures produces shrinkage distortions that are in good qualitative and semiquantitative agreement with these measurements and thus appears to provide a provisionally general and useful method for predicting pattern distortions that arise during cure-based resist stabilization methods used in double imaging.

Analysis of focus errors in lithography using phase-shift monitors

We present here a procedure to characterize focus behavior on a first generation prototype 193-nm scanner using phase-shift focus monitors, which clearly identifies the influence of full field dynamic effects and that of the wafer topography and flatness. These results are used to correct the systematic errors due to incorrect tool set-up and show that proposed procedure has capability to identify focus errors and on this basis to construct a focus budget for all components: reticle, wafer, tool. We also present results using a new focus monitor based on phase gratings, which is more sensitive than the traditional phase-shift focus monitor.

Inverse vs. traditional OPC for the 22nm node

The 22nm node will be patterned with very challenging Resolution Enhancement Techniques (RETs) such as double exposure or double patterning. Even with those extreme RETs, the k1 factor is expected to be less than 0.3. There is some concern in the industry that traditional edge-based simulate-then-move Optical Proximity Correction (OPC) may not be up to the challenges expected at the 22nm node. Previous work presented the advantages of a so-called inverse OPC approach when coupled with extreme RETs or illumination schemes. The smooth mask contours resulting from inverse corrections were shown not to be limited by topological identity, feedback locality, or fragment conformity. In short, inverse OPC can produce practically unconstrained and often non-intuitive mask shapes. The authors will expand this comparison between traditional and inverse OPC to include likely 22nm RETs such as double dipole lithography and double patterning, comparing dimensional control through process window for each OPC method. The impact of mask simplification of the inverse OPC shapes into shapes which can be reliably manufactured will also be explored.

Mask topography effect with polarization at hyper NA

As ArF immersion lithography is adopted beyond the 45 nm node, the minimum mask feature size will become equal to or smaller than the wavelength of the light. For such situations, polarization by the mask will play a very important role on imaging quality. In addition, TM and TE diffraction efficiencies for very narrow grating masks will depend significantly on the mask materials. Also, they are affected by variations of absorber thickness, sidewall angle and material optical properties. In this paper, we investigate how the aquatic images with unpolarized and properly polarized illumination on binary image masks, attenuated phase shift masks (attPSM) and alternating aperture phase shift masks (altPSM) are affected by those mask parameters, using rigorous electro-magnetic field simulator. In terms of mask topography effects, there are some difficulties with phase shift mask technology with unpolarized illumination beyond 45 nm node. We will need to control absorber thickness within 2.6% for attPSM with unpolarized illumination and sidewall angle of π-shifter trenches within 1 degree for altPSM with unpolarized illumination.

Overcoming the challenges of 22-nm node patterning through litho-design co-optimization

Historically, lithographic scaling was driven by both improvements in wavelength and numerical aperture. Recently, the semiconductor industry completed the transition to 1.35NA immersion lithography. The industry is now focusing on double patterning techniques (DPT) as a means to circumvent the limitations of Rayleigh diffraction. Here, the IBM Alliance demonstrates the extendibility of several double patterning solutions that enable scaling of logic constructs by decoupling the pattern spatially through mask design or temporally through innovative processes. This paper details a set of solutions that have enabled early 22 nm learning through careful lithography-design optimization.

Polarization aberrations in hyper-numerical-aperture projection printing: a comparison of various representations

Various representations of polarization aberrations are described and compared for optical lithography. Polarization aberrations, which are potentially important with hyper-numerical-aperture tools, are a complicated phenomena that refer to induced polarization-dependent wavefront distortions as light propagates through an imaging system. Pupil representations based on the following concepts are discussed: the physical polarization properties, the Mueller matrix, the Jones matrix, and the Jones matrix decomposed into a Pauli spin matrix basis. Although each has its own advantages and disadvantages, it is concluded that the Jones matrix representation decomposed into a Pauli spin matrix basis offers the most useful format for the lithographer due to its compact notation, physically intuitive interpretation, ability to be implemented into standard imaging equations, and its usefulness as an input into a lithographic simulator. Depending on the assumptions that can be made, the pupil specification consists of three to eight independent functions, where a normalization constant is calculated to ensure a physically realizable pupil. An example is shown to illustrate the usefulness of this strategy. Additionally, a simple metric for lens polarization quality based on this representation is proposed.

Image-blur tolerances for 65-nm and 45-nm node IC manufacturing

The deployment of 157nm lithography for manufacturing of integrated circuits is faced with many challenges. The 65 and 45nm ITRS nodes, in particular, require that the lithographic imaging technology be pursued to its theoretical limits with full use of the strongest resolution enhancement techniques. Stringent demands are therefore placed on the quality of the imaging optics to attain the optimal image fidelity for all critical IC device structures. Besides aberrations and light scatter in projection optics, image quality is also strongly influenced by the dynamics of the wafer and reticle stage. The tradeoffs involved in increasing scan speeds and exposure slit-widths, to achieve the ever-important productivity improvements as well as aberration, distortion, and pulse-energy averaging, must be carefully gauged against the image quality impacts of scan-induced errors. In this work, we present a simulation methodology, based on incoherent image superposition, for treatment of the general aerial image effects of transverse image-blur in two dimensions. Initial simulations and experimental results from state-of-the-art 193nm scanner exposures are discussed. The requirements for the transverse image stability during a step-and-scan exposure are defined in the context of 193nm and 157nm lithography, based on generalized image contrast and process window criteria. Furthermore, careful consideration of actual mask layout (post resolution enhancement and optical proximity correction) is necessary in order to understand the implications on CD control. Additionally, we discuss the contributors to transverse image blur in scan-and-repeat lithography, and show that the fading requirements for 65nm and 45nm node imaging notably differ from predicted exposure set-up and process contributions in manufacturing. The total fading budget, or tolerance, for the 65nm node is 15nm, and less than 10nm for the 45nm node given the present imaging strategy assumptions. This work concludes that image-blur contributors must be well controlled, and as such are enablers of 65nm and 45nm lithographic imaging.