Yu-Tian Shen

In situ beam drift detection using a two-dimensional electron-beam position monitoring system for multiple-electron-beam–direct-write lithography

Electron-beam lithography is one of the promising candidates to replace optical projection lithography due to its high resolution and maskless direct-write capability. In order to achieve the throughput requirement for high-volume manufacturing, miniaturized electro-optics elements are utilized to drive massively parallel beams simultaneously. In high-throughput multiple-electron-beam systems, beam positioning drift problems can become quite serious due to several factors such as thermal distortion and fabrication errors of electron optics. In single-beam systems, periodic recalibration with reference markers on the wafer can be utilized to achieve beam placement accuracy. This technique is not easy for multiple-beam systems. In this article, an innovative in situ two-dimensional electron-beam position monitoring system for multiple-electron-beam lithography is studied. An array of miniaturized electron detectors to measure scattered electrons from the substrate is placed above the wafer. It is assumed that the detector array signals are correlated with the distribution of electron trajectories, and the change of trajectory distortion due to the beam drift can be predicted by Monte Carlo electron-scattering simulation. A standard quadrant detection (SQD) method and a linear least-squares (LLS) method are used to estimate the beam drift from the detector array signals. Simulation results indicate that while the estimation uncertainty of both methods can be reduced substantially when the number of detected electrons is large enough. The LLS method always outperforms the SQD one regardless the detected electron numbers.

Model-based proximity effect correction for electron-beam direct-write lithography

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.

Impacts of point spread function accuracy on patterning prediction and proximity effect correction in low-voltage electron-beam–direct-write lithography

Electron-beam–direct-write lithography at lower accelerating voltages has been considered as a candidate for next-generation lithography. Although long-range proximity effects are substantially reduced with the voltage, proximity effect correction (PEC) is still necessary since short-range proximity effects are relatively prominent. The effectiveness of model-based PEC can be limited severely if an inaccurate point spread function (PSF) characterizing electron scattering within resist is adopted. Recently, a new PSF form using a promising calibration method has been developed to more accurately characterize the electron scattering and thus significantly improve patterning fidelity at 5 keV. However, influences of adopting the conventional and new PSF forms for the usage of patterning practical circuit layouts have not been intensively studied. This work extensively investigates impacts of PSF accuracy on patterning prediction and PEC under different resist thickness conditions suitable for various lithogr...

Electron-beam proximity effect model calibration for fabricating scatterometry calibration samples

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.

Direct-scatterometry-enabled lithography model calibration

Optical scatterometry is crucial to advanced nodes due to its ability of non-destructively and rapidly retrieving accurate 3D profile information.1, 2, 3 In recent years, an angle-resolved polarized reflectometry-based scatterometry which can measure critical dimensions, overlay, and focus in single shot has been developed.4, 6, 20 In principle, a microscope objective collects diffracted light, and pupil images are collected by a detector. For its application of calibrating lithography models, the pupil images are fit to a database pre-characterized usually by rigorous electromagnetic simulation to estimate dimensional parameters of developed resist profiles.5 The estimated dimensional parameters can then be used for lithography model calibration. In this work, we propose a new method which directly utilizes the pupil images to calibrate lithography models without needing dimensional parameter estimation. To test its feasibility and effectiveness by numerical simulation, a reference lithography process model is first constructed with a set of parameter values complying with ITRS. A to-be-calibrated process model is initialized with a different set of parameter values from those of the reference model. Rigorous electromagnetic simulation is used to obtain the pupil images of the developed resist profiles predicted by both process models. An optimization algorithm iteratively reduces the difference between the pupil images by adjusting the set of parameter values of the to-be-calibrated process model until the pupil image difference satisfies a predefined converging criterion. This method can be used to calibrate both rigorous first-principle models for process and equipment development and monitoring, and fast kernel-based models for full-chip proximity effect simulation and correction. Preliminary studies with both 1D and 2D aperiodic and periodic layouts indicate that when the pupil image difference is minimized, the lithography model can be accurately calibrated.

New method of optimizing writing parameters in electron beam lithography systems for throughput improvement considering patterning fidelity constraints

Abstract. 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.

Direct-scatterometry-enabled optical-proximity-correction-model calibration

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.

Design of an electron-optical system with a ball-tip emission source through a numerical optimization method for high-throughput electron-beam–direct-write lithography

To improve the throughput of electron-beam–direct-write lithography (EBDWL), we propose a new electron-optical system (EOS) design method with the capability of systematically optimizing EOSs with various types of emission source. A ball-tip emission source initially proposed by another group is further discussed. A numerical optimization method is utilized in the design method to maximize EOS emission current while satisfying constraints including the size of the electron beam spot and the amount of induced electric field on electrodes and insulators inside EOSs. For performance comparison, two EOSs, one with a generic emission source and the other with a ball-tip emission source, are designed using the proposed method. Preliminary results indicate that a sixfold emission current is obtained from the EOS with the ball-tip emission source. Furthermore, patterning fidelity investigations are conducted using the obtained design results. It is shown that the higher emission current from the ball-tip emission source is achieved without compromising patterning quality.

Optical scatterometry system for detecting specific line edge roughness of resist gratings subjected to detector noises

The Fourier scatterometry model was used to measure the ZEP 520A electron beam resist lines with specific line edge roughness (LER). By obtaining the pupils via an objective lens, the angle-resolved diffraction spectrum was collected efficiently without additional mechanical scanning. The concavity of the pupil was considered as the weight function in specimen recognition. A series of white noises was examined in the model, and the tolerant white noise levels for different system numerical apertures (NAs) were reported. Our numerical results show that the scatterometry model of a higher NA can identify a target with a higher white noise level. Moreover, the fabricated ZEP 520A electron beam resist gratings with LER were measured by using our model, and the fitting results were matched with scanning electron microscope measurements.