KEYWORDS: Calibration, Electron beam lithography, Photomasks, Electron beams, Data corrections, Data modeling, Physics, Systems modeling, Monte Carlo methods, Human-machine interfaces
The main source of placement error in maskmaking using electron beam is charging. DISPLACE software
provides a method to correct placement errors for any layout, based on a physical model. The charge of a
photomask and multiple discharge mechanisms are simulated to find the charge distribution over the mask.
The beam deflection is calculated for each location on the mask, creating data for the placement correction.
The software considers the mask layout, EBL system setup, resist, and writing order, as well as other factors
such as fogging and proximity effects correction. The output of the software is the data for placement correction.
Unknown physical parameters such as fogging can be found from calibration experiments. A test layout on a
single calibration mask was used to calibrate physical parameters used in the correction model. The extracted
model parameters were used to verify the correction. As an ultimate test for the correction, a sophisticated
layout was used for verification that was very different from the calibration mask. The placement correction
results were predicted by DISPLACE, and the mask was fabricated and measured. A good correlation of the
measured and predicted values of the correction all over the mask with the complex pattern confirmed the high
accuracy of the charging placement error correction.
KEYWORDS: Calibration, Electron beam lithography, Photomasks, Electrons, Data corrections, Electron beams, Physics, Monte Carlo methods, Systems modeling, Distortion
In maskmaking, the main source of error contributing to placement error is charging. DISPLACE software corrects the placement error for any layout, based on a physical model. The charge of a photomask and multiple discharge mechanisms are simulated to find the charge distribution over the mask. The beam deflection is calculated for each location on the mask, creating data for the placement correction. The software considers the mask layout, EBL system setup, resist, and writing order, as well as other factors such as fogging and proximity effects correction. The output of the software is the data for placement correction. One important step is the calibration of physical model. A test layout on a single calibration mask was used for calibration. The extracted model parameters were used to verify the correction. As an ultimate test for the correction, a sophisticated layout was used for the verification that was very different from the calibration mask. The placement correction results were predicted by DISPLACE. A good correlation of the measured and predicted values of the correction confirmed the high accuracy of the charging placement error correction.
Improvements in the variation of critical dimensions (CD) and placement accuracy in electron beam
lithography (EBL) are of high importance in the modern maskmaking industry where acceptable variations
are on the one nanometer range over the mask area. In EBL, electrons backscatter from the resist and
substrate, reach the bottom of objective lens and come back to the resist, causing undesirable exposure and
charging far away from the point of exposure. This fogging affects both CD variation and placement
accuracy. The Monte Carlo software CHARIOT was upgraded to be capable of simulating this fogging effect.
The results of simulations are presented for variety of conditions. The results were used for the correction of
charging placement error. Fogging is one of the contributing factors to the charging placement error; the
DISPLACE software tool predicts the displacement map for any layout, system setup and exposure strategy,
which allows for the correction of placement error in maskmaking.
KEYWORDS: Scanning electron microscopy, Monte Carlo methods, Electrodes, Silica, Electromagnetism, Sensors, Scattering, Carbon, Nanofibers, 3D modeling
We are reporting the development of a simulation tool with unique capabilities to comprehensively model an
SEM signal. This includes electron scattering, charging, and detector settings, as well as modeling of the local and
global electromagnetic fields and the electron trajectories in these fields. Experimental and simulated results were
compared for SEM imaging of carbon nanofibers embedded into bulk material in the presence of significant charging,
as well as for samples with applied potential on metal electrodes. The effect of the potentials applied to electrodes on
the secondary emission was studied; the resulting SEM images were simulated. The image contrast depends strongly
on the sign and the value of the potential. SEM imaging of nanofibers embedded into silicon dioxide resulted in the
considerable change of the appeared dimensions of the fibers and as well as tone reversal when the beam voltage was
varied. The results of the simulations are in agreement with experimental results.
Propagation of high-power femtosecond laser pulse through random media is accompanied by transverse spatial distortions of the laser beam. Occurrence of high-intensity small-scale perturbations due to atmospheric disturbance may result in beam breakup and filament generation. Coherent scattering on random ensemble of aerosol particles result in these disturbance. On the other hand, plasma formation, owing to focusing of light into aerosol microdroplet, may hinder filamentation. The purpose of this paper is to numerical study the propagation of a femtosecond laser pulse through water aerosol. In particular, we will find the transverse intensity distribution resulting from coherent scattering of a 800 nm 45 fs pulse with 10 - 60 GW peak power on the ensemble of water droplets with the atmospheric size distribution and the density 100 cm-3. The forward scattering on aerosol particles takes into account the phase of the scattered radiation. The transverse distribution of the laser field behind the aerosol layer is calculated as the result of the interference of light fields formed by each particular particle. In the numerical simulations the input radius of a Gaussian beam was a = 2.5 mm. The average size of an aerosol particle was 4 μ. The length of a propagation path was set to the half of the diffraction length: 0.5.ka2. As the result we have shown that in aerosol medium it is possible forming of several hot spots containing approximately 1 critical power for self-focusing.
The problem of optimization of the adaptive optical system using local curvature sensor is discussed. Computer simulations of the sensor including closed-loop operation are based on ray-tracing technique and numerical solution of Poisson equation by FEM method. Wavefront restoration for both axial symmetric and non-axial symmetric phase distortions is examined. It is shown, that choosing appropriate geometry of the actuator external electrodes one can improve wavefront restoration for the case of non- circularly symmetric Zernike aberrations.
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