Our work presents and investigates the effectiveness of a model-based proximity effect correction method for helium ion beam lithography (HIBL). This method iteratively modulates the shape of a pattern by a feedback compensation mechanism until the simulated patterning fidelity satisfies specific constraints. A point spread function (PSF) is utilized to account for all phenomena involved during the scattering events of incident ion beam particles in the resist. Patterning prediction for subsequent correction process is derived from the energy intensity distribution due to convolution between the PSF and the pattern, with an adequate cut-off threshold. The performance of this method for HIBL is examined through several designed layouts from 15- to 5-nm in half pitches, under specific process parameters, including acceleration voltage, resist thickness, and resist sensitivity. Preliminary results show its effectiveness in improving the patterning fidelity of HIBL.
As semiconductor features shrink in dimension and pitch, the excessive control of critical-dimension uniformity (CDU) and pattern fidelity is essential for mask manufacturing using electron-beam lithography. Requirements of the electronbeam shot quality affected by shot unsteadiness become more important than before for the advanced mask patterning. Imperfect electron optical system, an inaccurate beam deflector, and imprecise mask stage control are mainly related to the shot unsteadiness including positioning and dose perturbations. This work extensively investigates impacts of variable shaped beam dose and positioning perturbations on local CDU using Monte Carlo simulation for various mask contrast enhancement approaches. In addition, the relationship between the mask lithographic performance and the shot count number correlated with mask writing time is intensively studied.
The availability of metrology solutions, one of the key factors to drive leading edge semiconductor devices and processes, can be confronted with difficulties in the advanced node. For developing new metrology solutions, high quality test structures fabricated at specific sizes are needed. Conventional resist-based lithography have been utilized to manufacture such samples. However, it can encounter significant resolution difficulties or requiring complicated optimization process for advanced technology node. In this work, potential of helium ion beam direct milling (HIBDM) for fabricating metrology test structures with programmed imperfection is investigated. Features down to 5 nm are resolvable without implementing any optimization method. Preliminary results have demonstrated that HIBDM can be a promising alternative to fabricate metrology test structures for advanced metrology solutions in sub 10 nm node.
Accurate and fast kernel-based proximity effect correction (PEC) models are indispensable to full-chip proximity effect simulation and correction. The attempt to utilize optical scatterometers for PEC model calibration instead of scanning electron microscopes is primarily motivated by the fact that scatterometry can be faster, more stable, and more informative if carefully implemented. Conventional scatterometry measures periodic patterns and retrieves their dimensional parameters by solving inverse problems of optical scattering with predefined libraries of the periodic patterns. PEC model parameters can be subsequently calibrated with the retrieved dimensional parameters. However, measuring only periodic patterns limits the usage of scatterometry, and the dimensional reconstruction is prone to generate estimation errors for patterns with complex three-dimensional geometry. Previously, we have proposed directly utilizing scattering light for PEC model calibration without the need for the intermediate step of retrieving the dimensional parameters. By iteratively comparing scattered light from predefined calibration patterns measured by a scatterometer to that predicted by the corresponding scattering and lithography models, PEC model parameters can be effectively calibrated with standard numerical optimization algorithms and one-dimensional periodic patterns. In this work, two-dimensional periodic circuit layouts are designed and utilized to study the applicability and potential limitations of the proposed method on the lithography of practical circuit designs.
Line edge roughness (LER) influencing the electrical performance of circuit components is a key challenge for electronbeam
lithography (EBL) due to the continuous scaling of technology feature sizes. Controlling LER within an acceptable
tolerance that satisfies International Technology Roadmap for Semiconductors requirements while achieving high
throughput become a challenging issue. Although lower dosage and more-sensitive resist can be used to improve
throughput, they would result in serious LER-related problems because of increasing relative fluctuation in the incident
positions of electrons. Directed self-assembly (DSA) is a promising technique to relax LER-related pattern fidelity (PF)
requirements because of its self-healing ability, which may benefit throughput. To quantify the potential of throughput
improvement in EBL by introducing DSA for post healing, rigorous numerical methods are proposed to simultaneously
maximize throughput by adjusting writing parameters of EBL systems subject to relaxed LER-related PF requirements.
A fast, continuous model for parameter sweeping and a hybrid model for more accurate patterning prediction are
employed for the patterning simulation. The tradeoff between throughput and DSA self-healing ability is investigated.
Preliminary results indicate that significant throughput improvements are achievable at certain process conditions.
KEYWORDS: Calibration, Lithography, Optical proximity correction, Data modeling, Signal processing, Scatterometry, Process control, Photomasks, Process modeling, Systems modeling
Fast and robust metrologies for retrieving large amount of accurate wafer data is the key to meet the ever stricter semiconductor manufacturing process control such as critical dimension (CD) and overlay as the industry moving towards 22 nm or smaller designs. Scatterometry emerges due to its non-destructivity and rapid availability for accurate wafer data. In this paper we simulate the ability of a new scatterometry method to show its accurate control over lithography model and OPC model calibrations. The new method directly utilizes scattering signals of scatterometry to control the process instead of using numerically analyzed dimensional parameters such as CD and side wall angle (SWA). The control can be achieved by optimizing the scattering signal of one process by tuning numerical aperture (NA), sigma, or lens aberration to match the signal of the target process. In this work only sigma is used for optimization. We found that when the signals of both processes are matched with minimized optimization error, CD of the grating profiles on the wafers are also minimized. This result enables valid lithography process control and model calibration with the new method.
KEYWORDS: Electron beam lithography, Monte Carlo methods, Line edge roughness, Computer simulations, Critical dimension metrology, Transistors, Optical simulations, Diffusion, Optimization (mathematics), Point spread functions
Low-energy electron beam lithography is one of the promising next-generation lithography technology solutions for the 21-nm half-pitch node and beyond because of fewer proximity effects, higher resist sensitivity, and less substrate damage compared with high-energy electron beam lithography. To achieve high-throughput manufacturing, low-energy electron beam lithography systems with writing parameters of larger beam size, larger grid size, and lower dosage are preferred. However, electron shot noise can significantly increase critical dimension deviation and line edge roughness. Its influence on patterning prediction accuracy becomes nonnegligible. To effectively maximize throughput while meeting patterning fidelity requirements according to the International Technology Roadmap for Semiconductors, a new method is proposed in this work that utilizes a new patterning prediction algorithm to rigorously characterize the patterning variability caused by the shot noise and a mathematical optimization algorithm to determine optimal writing parameters. The new patterning prediction algorithm can achieve a proper trade-off between computational effort and patterning prediction accuracy. Effectiveness of the new method is demonstrated on a static random-access memory circuit. The corresponding electrical performance is analyzed by using a gate-slicing technique and publicly available transistor models. Numerical results show that a significant improvement in the static noise margin can be achieved.
KEYWORDS: Calibration, Line edge roughness, Scatterometry, Point spread functions, Process modeling, Scanning electron microscopy, Metrology, Scattering, Semiconducting wafers, Cadmium
Scatterometry has been proven to be effective in critical dimension (CD) and sidewall angle (SWA) measurements with
good precision and accuracy. In order to study the effectiveness of scatterometry measurement of line edge roughness
(LER), calibration samples with known LER have to be fabricated precisely. The relationship between ITRS LER
specifications and the feature dimension design of the LER calibration samples is discussed. Electron-beam-direct-write
lithography (EBDWL) has been widely used in nanoscale fabrication and is a natural selection for fabricating the
designed calibration samples. With the increasingly demanding requirement of lithography resolution in ITRS, the
corresponding LER feature of calibration samples becomes more and more challenging to fabricate, even for EBDWL.
Proximity effects in EBDWL due to electron scattering can cause significant distortion of fabricated patterns from
designed layouts. Model-based proximity effect correction (MBPEC) is an enhancement method for EBDWL to
precisely define fine resist features. The effectiveness of MBPEC depends on the availability of accurate electron-beam
proximity effect models, which are usually described by point spread functions (PSFs). In this work, a PSF in a double-
Gaussian function form at a 50 kV accelerating voltage, an effective beam size, and a development threshold energy
level of the resist are calibrated with EBDWL exposure tests. Preliminary MBPEC results indicate its effectiveness in
calibration sample fabrication.
Electron-beam-direct-write lithography has been considered a candidate next-generation technique for achieving high resolution. An accurate point spread function (PSF) is essential for reliable patterning prediction and proximity-effects correction. It can be derived via an effective parametric PSF calibration methodology, typically involving the fitting of the absorbed energy distribution (AED) from an electron-scattering simulation. However, the existing parametric PSF calibration methodology does not employ a systematic approach to obtain a new PSF form that is both compact and accurate when conventional PSF forms are not satisfactory. Only the AED fitting quality (rather than its patterning-prediction quality) is considered during the conventional calibration methodology. It also lacks a process to consider whether the predicted deviation (as simulated using the chosen PSF form) is satisfactory. This paper proposes a new parametric PSF calibration methodology to systematically obtain a PSF form consisting of the smallest number of terms, with a better combination of basis functions and that optimizes pattern accuracy. The effectiveness of using the new methodology is demonstrated in terms of fitting accuracy, patterning-prediction accuracy, and patterning sensitivity.
A model-based proximity effect correction methodology is proposed and tested for electron-beam-direct-write
lithography. It iteratively modulates layout geometry by feedback compensation until the correction error converges. The
energy intensity distribution is efficiently calculated by fast convolving the modulated layout with a point-spread
function which models electron beam shape and proximity effects primarily due to electron scattering in resist. The
effectiveness of this methodology is measured by iteration numbers required for meeting the patterning fidelity
specifications. It is examined versus process parameters including acceleration voltage and resist thickness with several
regular mask geometries and practical design layouts.
KEYWORDS: Monte Carlo methods, Scattering, Electron beam lithography, Silicon, Data modeling, Polymethylmethacrylate, Laser scattering, Direct write lithography, Computer simulations, Backscatter
Accelerating voltage as low as 5 kV for operation of the electron-beam micro-columns as well as solving the
throughput problem is being considered for high-throughput direct-write lithography for the 22-nm half-pitch node and
beyond. The development of efficient proximity effect correction (PEC) techniques at low-voltage is essential to the
overall technology. For realization of this approach, a thorough understanding of electron scattering in solids, as well as
precise data for fitting energy intensity distribution in the resist are needed. Although electron scattering has been
intensively studied, we found that the conventional gradient based curve-fitting algorithms, merit functions, and
performance index (PI) of the quality of the fit were not a well posed procedure from simulation results. Therefore, we
proposed a new fitting procedure adopting a direct search fitting algorithm with a novel merit function. This procedure
can effectively mitigate the difficulty of conventional gradient based curve-fitting algorithm. It is less sensitive to the
choice of the trial parameters. It also avoids numerical problems and reduces fitting errors. We also proposed a new PI to
better describe the quality of the fit than the conventional chi-square PI. An interesting result from applying the proposed
procedure showed that the expression of absorbed electron energy density in 5keV cannot be well represented by
conventional multi-Gaussian models. Preliminary simulation shows that a combination of a single Gaussian and double
exponential functions can better represent low-voltage electron scattering.
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