• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.80-81
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• 2012

Recently, one of the critical issues in the etching processes of the nanoscale devices is to achieve ultra-high aspect ratio contact (UHARC) profile without anomalous behaviors such as sidewall bowing, and twisting profile. To achieve this goal, the fluorocarbon plasmas with major advantage of the sidewall passivation have been used commonly with numerous additives to obtain the ideal etch profiles. However, they still suffer from formidable challenges such as tight limits of sidewall bowing and controlling the randomly distorted features in nanoscale etching profile. Furthermore, the absence of the available plasma simulation tools has made it difficult to develop revolutionary technologies to overcome these process limitations, including novel plasma chemistries, and plasma sources. As an effort to address these issues, we performed a fluorocarbon surface kinetic modeling based on the experimental plasma diagnostic data for silicon dioxide etching process under inductively coupled C4F6/Ar/O2 plasmas. For this work, the SiO2 etch rates were investigated with bulk plasma diagnostics tools such as Langmuir probe, cutoff probe and Quadruple Mass Spectrometer (QMS). The surface chemistries of the etched samples were measured by X-ray Photoelectron Spectrometer. To measure plasma parameters, the self-cleaned RF Langmuir probe was used for polymer deposition environment on the probe tip and double-checked by the cutoff probe which was known to be a precise plasma diagnostic tool for the electron density measurement. In addition, neutral and ion fluxes from bulk plasma were monitored with appearance methods using QMS signal. Based on these experimental data, we proposed a phenomenological, and realistic two-layer surface reaction model of SiO2 etch process under the overlying polymer passivation layer, considering material balance of deposition and etching through steady-state fluorocarbon layer. The predicted surface reaction modeling results showed good agreement with the experimental data. With the above studies of plasma surface reaction, we have developed a 3D topography simulator using the multi-layer level set algorithm and new memory saving technique, which is suitable in 3D UHARC etch simulation. Ballistic transports of neutral and ion species inside feature profile was considered by deterministic and Monte Carlo methods, respectively. In case of ultra-high aspect ratio contact hole etching, it is already well-known that the huge computational burden is required for realistic consideration of these ballistic transports. To address this issue, the related computational codes were efficiently parallelized for GPU (Graphic Processing Unit) computing, so that the total computation time could be improved more than few hundred times compared to the serial version. Finally, the 3D topography simulator was integrated with ballistic transport module and etch reaction model. Realistic etch-profile simulations with consideration of the sidewall polymer passivation layer were demonstrated.

• Korean Chemical Engineering Research
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• v.56 no.4
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• pp.479-495
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• 2018

This study was carried out to research the multimedia fate modeling, concentration distribution and impact assessment of polycyclic aromatic hydrocarbons (PAHs) emitted from automobiles, which are known as carcinogenic and mutation chemicals. The amount of emissions of PAHs was determined based on the census data of automobiles at a target S-city and emission factors of PAHs, where multimedia fugacity modeling was conducted by the restriction of PAHs transfer between air-soil at the impervious area. PAHs' Concentrations and their distributions at several environmental media were predicted by multimedia fugacity model (level III). The residual amounts and the distributions of PAHs through mass transfer of PAHs between environment media were used to assess the health risk of PAHs at unsteady state (level IV), where the sensitivity analyses of the model parameter of each variable were conducted based on Monte Carlo simulation. The experimental result at S-city showed that Fluoranthene among PAHs substances are the highest residual concentrations (60%, 53%, 32% and 34%) at all mediums (atmospheric, water, soil, sediment), respectively, where most of the PAHs were highly accumulated in the sediment media (more than 80%). A result of PAHs concentration changes in S-city over the past 34 years identified that PAHs emissions from all environmental media increased from 1983 to 2005 and decreased until 2016, where the emission of heavy-duty vehicle including truck revealed the largest contribution to the automotive emissions of PAHs at all environment media. The PAHs concentrations in soil and water for the last 34 years showed the less value than the legal standards of PAHs, but the PAHs in air exceeded the air quality standards from 1996 to 2016. The result of this study is expected to contribute the effective management and monitoring of toxic chemicals of PAHs at various environment media of Metropolitan city.

• Progress in Medical Physics
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• v.27 no.1
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• pp.31-36
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• 2016

The paper discusses radiation dose of dual energy CT on which copper modulation layer, is mounted in order to improve diagnostic performance of the dual energy CT. The radiation dose is estimated using MCNPX and its results are compared with that of the conventional dual energy CT system. CT X-ray spectra of 80 and 120 kVp, which are usually used for thorax, abdominal, head, and neck CT scans, were generated by the SPEC78 code and were used for the source specification 'SDEF' card for MCNPX dose modeling. The copper modulation layer was located 20 cm away from a source covering half of the X-ray window. The radiation dose was measured as changing its thickness from 0.5 to 2.0 mm at intervals of 0.5 mm. Since the MCNPX tally provides only normalized values to a single particle, the dose conversion coefficients of F6 tally for the modulation layer-based dual energy CBCT should be calculated for matching the modeling results into the actual dose. The dose conversion coefficient is $7.2*10^4cGy/output$ that is obtained from dose calibration curve between F6 tally and experimental results in which GAFCHORMIC EBT3 films were exposed by an already known source. Consequently, the dose of the modulation layer-based dual energy cone beam CT is 33~40% less than that of the single energy CT system. On the basis of the results, it is considered that scattered dose produced by the copper modulation layer is very small. It shows that the modulation layer-based dual energy CBCT system can effectively reduce radiation dose, which is the major disadvantage of established dual energy CT.

• Progress in Medical Physics
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• v.21 no.3
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• pp.291-297
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• 2010

This study examined the dosimetric influence of implanted gold markers in proton therapy and the effects of their positions in the spread-out Bragg peak (SOBP) proton beam. The implanted cylindrical gold markers were 3 mm long and 1.2 mm in diameter. The dosimetric influence of the gold markers was determined with markers at various locations in a proton-beam field. Spatial dose distributions were measured using a three-dimensional moving water phantom and a stereotactic diode detector with an effective diameter of 0.5 mm. Also, a film dosimetry was performed using Gafchromic External Beam Treatment (EBT) film. The GEANT4 simulation toolkit was used for Monte-Carlo simulations to confirm the measurements and to construct the dose-volume histogram with implanting markers. Motion data were obtained from the portal images of 10 patients to investigate the effect of organ motions on the dosimetric influence of markers in the presence of a rectal balloon. The underdosed volume due to a single gold marker, in which the dose was less than 95% of a prescribed amount, was 0.15 cc. The underdosed volume due to the presence of a gold marker is much smaller than the target volume. However, the underdosed volume is inside the gross tumor volume and is not smeared out due to translational prostate motions. The positions of gold markers and the conditions of the proton-beam field give different impacts on the dose distribution of a target with implanted gold markers, and should be considered in all clinical proton-based therapies.

• Progress in Medical Physics
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• v.20 no.2
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• pp.72-79
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• 2009

Scatter correction for I-131 plays a very important role to improve image quality and quantitation. I-131 has multiple and higher energy gamma-ray emissions. Image quality and quantitative accuracy in I-131 imaging are degraded by object scatter as well as scatter and septal penetration in the collimator. The purpose of this study was to estimate scatter and septal penetration and investigate two scatter correction methods using Monte Carlo simulation. The gamma camera system simulated in this study was a FORTE system (Phillips, Nederland) with high energy, general-purpose, parallel hole collimator. We simulated for two types of high energy collimators. One is composed of lead, and the other is composed of artificially high Z number and high density. We simulated energy spectrum using a point source in air. We estimated both full width at half maximum (FWHM) and full width at tenth maximum (FWTM) using line spread function (LSF) in cylindrical water phantom. We applied two scatter correction methods, triple energy window scatter correction (TEW) and extended triple energy window scatter correction (ETEW). The TEW method is a pixel-by pixel based correction which is easy to implement clinically. The ETEW is a modification of the TEW which corrects for scatter by using abutted scatter rejection window, which can overestimate or the underestimate scatter. The both FWHM and FWTM were estimated as 41.2 mm and 206.5 mm for lead collimator, respectively. The FWHM and FWTM were estimated as 27.3 mm and 45.6 mm for artificially high Z and high density collimator, respectively. ETEW showed that the estimation of scatter components was close to the true scatter components. In conclusion, correction for septal penetration and scatter is important to improve image quality and quantitative accuracy in I-131 imaging. The ETEW method in scatter correction appeared to be useful in I-131 imaging.

• Environmental and Resource Economics Review
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• v.13 no.1
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• pp.49-82
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• 2004

The study aimed to evaluate cancer damage cost due to risk from benzene inhalation. We performed health risk assessment based on US EPA guideline to estimate annual population risk in Ulsan metropolitan city. Also, we estimated a willingness-to-pay amount for reducing a cancer mortality rate to evaluate a value of statistical life. We combined the annual population risk and the value of statistical life to calculate the cancer damage cost. In the health risk assessment, we applied the benzene unit risk ($2.2{\times}10^{-6}{\sim}7.8{\times}10^{-6}$) in the US EPA'S Integrated Risk Information System to assess the annual population risk. Average concentration of benzene in ambient air is $7.88{\mu}g/m^3$(min: 1.16~max: $23.32{\mu}g/m^3$). We targeted an exposure population of 516,641 persons who aged over 30 years old. Using a Monte-Carlo simulation for uncertainty analysis, we evaluated that the population risk of benzene during ten years in Ulsan city is 2.90 persons (5 percentile: 0.32~95 percentile: 9.11persons). And the monthly average WTP for 5/1,000 cancer mortality reduction during ten years is 14,852 Won(95% C.I: 13,135~16,794 Won) and the implied VSL is 36 million Won(95% C.I: 30~40 million Won). Cancer damage cost due to risk from benzene inhalation during 10 years in Ulsan city is about 104 million Won(5 percentile: 13~95 percentile: 328 million Won). Health benefit cost to reduce a cancer mortality risk of benzene is about 50 million Won is Ulsan metropolitann city. But, it is very important that this cost is not for all health damage cost of cancer mortality in some area. We just recommended a model for evaluating a cancer risk reduction, so we must re-evaluate an integrated application of total VOCs damage cost including benzene.

• The Korean Journal of Pesticide Science
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• v.17 no.2
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• pp.117-125
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• 2013

A probabilistic exposure assessment was performed on the monitoring data of pesticides were assessed in agricultural products in Gyeonggi-do from 2006 to 2010. Chlorothalonil, chlorpyrifos, dicofol, endosulfan, EPN, ethoprophos, fenitrothion, methidathion, phenthoate and tebupirimfos were assessed. For this assessment, we used Monte Carlo simulation software and the distribution of concentration and intake were assumed to lognormal distribution by inputting mean and standard deviation. The hazard index (HI, %ADI) of average value and the $95^{th}$ percentile based on a probabilistic method were usually lower than those by a deterministic one. For the whole population, when non-detects data were assigned 0 mg/kg, HI of the average value and the $95^{th}$ percentile showed 0.05~0.70% and 0.11~1.94%, respectively. When nondetects data were assigned 0.005 mg/kg, HI of the average value and the $95^{th}$ percentile were 0.41~4.42% and 0.98~13.81%. For only consumers, when non-detects data were assigned 0 mg/kg, HI of the average value and the $95^{th}$ percentile were 1.24~10.16% and 3.72~33.81%, respectively. When non-detects data were assigned 0.005 mg/kg, HI of the average value and the $95^{th}$ percentile were 3.43~18.26% and 9.45~54.99%, respectively. Methidathion had highest values when both of 0 and 0.005 were assigned to non-detecs data for consumers only. This study showed that agricultural products in Gyeonggi-do were safe because they had less than 100 of HI (%ADI) based on probabilistic exposure assessment.

• The Journal of Korean Society for Radiation Therapy
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• v.19 no.2
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• pp.107-112
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• 2007

Purpose: The pelvic phantom was fabricated in the following purposes: (1) Dose verification of IMRT plan using Eclipse planning computer, (2) to study the interface effect at the interface between rectal wall and air. The TLD can be inserted in the pelvic phantom to confirm the dose distribution as well as uncertainty at the interface. Materials and Methods: A pelvic phantom with the dimension of 30 cm diameter, 20 cm height and 20 cm thickness was fabricated to investigate the dose at the rectal wall. The phantom was filled with water and has many features like bladder, rectum, and prostate and seminal vesicle (SV). The rectum is made of 3 cm-dimater plastic pipe, and it cab be blocked by using a plug, and film can be inserted around the rectal wall. The phantom was scanned with Philips Brillance scanner and various organs such as prostate, SV, and rectal wall, and bladder wall were delineated. The treatment parameters used in this study are the same as those used in the protocols in the SNUH. TLD chips are inserted to the phantom to evaluate the dose distribution to the rectal wall (to simulate high dose gradient region), bladder wall and SV (to simulate the high dose region) and 2 spots in anterior surface (to simulate the low dose region). The TLD readings are compared with those of the planning computer (ECLIPSE, Varian, USA). Results: The target TLD doses represented as the prostate and SV show excellent agreements with the doses from the RTP within +/-3%. The rectal wall doses measured at the rectal wall are different from the those of the RTP by -11%. This is in literatures called as an interface effect. The underdosages at the rectal wall is independent of 3 heterogeneity correction algorithm in the Eclipse RTP. Also the low dose regions s represented as surface in this study were within +/-1%. Conclusion: The RTP estimate the dosage very accurately withihn +/-3% in the high dose (SV, or prostate) and low dose region (surface). However, the dosage at the rectal wall differed by as much as 11% (In literatures, the underdosage of 9$\sim$15% were reported). This range of errors occurs at the interface, for example, at the interface between lung and chest wall, or vocal cord. This interface effect is very important in clinical situations, for example, to estimate the NTCP (normal tissue complication probability) and to estimate the limitations of the current RTP system. Monte-carlo-based RTP will handle this issue correctly.

• Korean Journal of Materials Research
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• v.5 no.1
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• pp.42-54
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• 1995

$Ta_2O_5$ thin film IS a promising material for the high dielectrics of ULSI DRAM. In this study, $Ta_2O_5$ thin film was grown on p-type( 100) Si wafer by thermal metal organic chemical vapo deposition ( MCCVD) method and the effect of operating varialbles including substrate temperature( $T_s$), bubbler temperature( $T_ \sigma$), reactor pressure( P ) was investigated in detail. $Ta_2O_5$ thin film were analyzed by SEM, XRD, XPS, FT-IR, AES, TEM and AFM. In addition, the effect of various anneal methods was examined and compared. Anneal methods were furnace annealing( FA) and rapid thermal annealing( RTA) in $N_{2}$ or $O_{2}$ ambients. Growth rate was evidently classified into two different regimes. : (1) surface reaction rate-limited reglme in the range of $T_s$=300 ~ $400 ^{\circ}C$ and (2: mass transport-limited regime in the range of $T_s$=400 ~ $450^{\circ}C$.It was found that the effective activation energies were 18.46kcal/mol and 1.9kcal/mol, respectively. As the bubbler temperature increases, the growth rate became maximum at $T_ \sigma$=$140^{\circ}C$. With increasing pressure, the growth rate became maximum at P=3torr but the refractive index which is close to the bulk value of 2.1 was obtained in the range of 0.1 ~ 1 torr. Good step coverage of 85. 71% was obtained at $T_s$=$400 ^{\circ}C$ and sticking coefficient was 0.06 by comparison with Monte Carlo simulation result. From the results of AES, FT-IR and E M , the degree of SiO, formation at the interface between Si and TazO, was larger in the order of FA-$O_{2}$ > RTA-$O_{2}$, FA-$N_{2}$ > RTA-$N_{2}$. However, the $N_{2}$ ambient annealing resulted in more severe Weficiency in the $Ta_2O_5$ thin film than the TEX>$O_{2}$ ambient.

• Progress in Medical Physics
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• v.20 no.1
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• pp.21-29
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• 2009

Breast cancer is the most common form of cancer among korean woman. Therefore, the early detection activities of breast cancer such as breast self-examinations, clinical breast examinations, mammography are important. A yearly mammography examination has been recommended for women aged 40 and older for the early detection of breast cancer in asymptomatic periods. However, the glandular tissue of breast is the most radiation-sensitive tissue, and the determination of average glandular dose (AGD) forms an important part of the quality control of the mammographic systems. Because of the difficulty of estimating AGD directly, it is often estimated from the measurements of the incident air kerma and by applying the appropriate conversion factors. The primary objective of this study was to standardize the method of measuring AGD. The secondary objective was to evaluate the relationships between AGD per various composition and thickness of the breast using Monte Carlo simulations. As a result, we standardized the method of measuring AGD according to International Atomic Energy Agency (IAEA) guidelines (CoP: an international code of practice). Overall, AGD for mammographic practice in Korea was less than 3.0 mGy recommended by the Korea Food and Drug Adminstration (KFDA) protocol, and Korean Institute for Accreditation of Medical Image (KIAMI). The measured and simulated AGD for a given condition were calculated as 1.7 and 1.6 mGy, respectively. For the AGDs obtained, there was no significant difference between them. The simulated AGD was dependent on the fraction of glandular tissue of the breast. The AGD increases with increasing of the breast glandularity due to increasing absorption of low energy photons. The AGD also increases as a function of breast thickness. In conclusion, the results of this study could be used as a baseline to establish a reference level of radiation dose in mammography.