Double Exposure Technique Research Articles

Evaluation of EBT radiochromic film using a multiple exposure technique

Radiochromic film is a self developing two-dimensional dosimeter system that is widely used in radiotherapy. Since its development some 40 years ago many improvements have been made; however the sensitivity can vary across the film and a non-linear dose response remains. The former can be addressed using a double exposure technique; however this technique assumes that the response is linear so that the dose distribution is incorrectly measured. We are proposing the use of two homogenous exposures; one performed prior, the other after the irradiation to be measured. In this 'multiple exposure technique' (MET), the first homogenous exposure can be used to correct for variations in response in different parts of the film, while the second homogenous exposure allows correction for non-linearity of response with dose. The MET was tested with a 60° wedged field and an Intensity Modulated Radiation Therapy fluence map produced by a computerised treatment planning system. In the wedge field measurements, the MET profile showed agreement within 0.6 cGy for 80% of the field compared to ionisation chamber dose values. A comparison of a TPS generated fluence map dose distribution with one measured using conventionally calibrated EBT, and another measured with MET calibrated EBT, showed similar agreement. The MET would be particularly useful for the assessment of highly inhomogenous dose distributions with high maximum dose such as encountered in hypofractionated radiation therapy.

Pattern definition employing prism-based deep ultraviolet lithography

Interferometric lithography provides a simple, cost-effective technique for the fabrication of large-area submicrometre periodic features. Biprism-based interferometric lithography employing a deep ultraviolet (DUV) light source is investigated for defining periodic patterns in large area with sub-500 nm pitch and sub-200 nm resolution. Collimated DUV light along with biprism with an angle of 30.4° was employed to fabricate grating lines using single exposure and square lattice features using double-exposure technique. Pyramidal prism was also employed to fabricate holes in square lattice using single exposure. Pitch size of around 410 nm and minimum feature size of 190 nm were realised employing this technique.

On inhomogeneous straining in compressed sylvinite

Spatiotemporal distributions of local components of the distortion tensor of quasi-plastic materials—saliferous rocks (sylvinite)—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity are considered for inelastic behavior of the material. Results obtained for the slow wave processes in deformed saliferous rocks are compared to analogous data available for ionic crystals.

Double Patterning Analysis Method by Emulation Using a Double-Exposure Technique

Double patterning lithography was utilized to break the resolution limit of lithography and is the most promising technology that enables shrinking of this limit. There are some barriers concerning evaluation of the double patterning process because it includes the hard mask fabrication and dry etching process. We proposed a new evaluation method for double patterning to analyze photomask performance with an easy and simple procedure using a double-exposure technique. Results showed this emulation method was useful in determining the impact of mask overlay and critical dimension (CD) error on double patterning lithography. We also evaluated the impact of mask overlay and CD. Both the overlay and CD difference were very good between the two masks. Correlations between mask and wafer cannot be detected because of the measured overlay error and the CD performance of masks was too small.

Efficient simulation and optimization of wafer topographies in double patterning

As the technology marches toward the 32-nm node and beyond in semiconductor manufacturing, double-patterning and double-exposure techniques are currently regarded as the potential candidates to produce lines and spaces and contact holes, respectively. We employ the Waveguide method, a rigorous electromagnetic field (EMF) solver, to investigate the impact of wafer topographies on two specific double-patterning techniques. At first, the topography effects induced by the first patterning on the second lithography process in a lithography-etch-lithography-etch process are demonstrated. A new methodology of the bottom antireflective coating optimization is proposed to reduce the impact of wafer topography on resist profiles. Additionally, an optical proximity correction (OPC) of the second lithography mask is demonstrated to compensate the wafer-topography-induced asymmetric deformations of line ends. Rigorous EMF simulations of lithographic exposures are also applied to investigate wafer-topography effects in a freezing process. The difference between the optical properties of the frozen (first) resist and the second resist potentially causes linewidth variations. Quantitative criteria for tolerable refractive index and extinction differences between the two resist materials are given. The described studies can be used for the optimizations of topographic waferstacks, the OPC of the second litho mask, and for the development of resist materials with appropriate optical properties.

Double-Exposure Grayscale Photolithography

A double-exposure grayscale photolithography technique is developed and demonstrated to produce three-dimensional (3-D) structures with a high vertical resolution. Pixelated grayscale masks often suffer from limited vertical resolution due to restrictions on the mask fabrication. The double-exposure technique uses two pixelated grayscale mask exposures before development and dramatically increases the vertical resolution without altering the mask fabrication process. An empirical calibration technique was employed for mask design and was also applied to study the effects of exposure time and mask misalignment on the photoresist profile. This technology has been demonstrated to improve the average step between photoresist levels from 0.19 to 0.02 mum and the maximum step from 0.43 to 0.2 mum compared to a single pixelated exposure using the same mask design.

Resist charging effect in photomask: Its impact on pattern placement error and critical dimension

By the development of the double exposure technique or the double patterning technique, the pattern placement error of a photomask is interesting because of its impact on the size and position of wafer pattern. Among various sources to induce the pattern placement error, we have focused on the charging effect of the FEP-171 resist and have shown that the resist charging effect generates the pattern position error of a clear pattern and the critical dimension variation of a dark pattern. Based on experiment and simulation, we present quantitatively the dependence of position error on pattern density, pattern shape, and writing order. Furthermore, we have discussed the model to describe the charging effect and its agreement with experiment and the correction method to remove the resist charging effect.

Features of plastic strain localization at the yield plateau in Hadfield steel single crystals

Spatiotemporal distributions of local components of the plastic distortion tensor in Hadfield steel single crystals oriented for single twinning have been studied under active tensile straining conditions using the double-exposure speckle photography technique. Features of the macroscopically inhomogeneous strain localization at the yield plateau are considered. Relations between local components of the plastic distortion tensor in the zone of strain localization are analyzed.

Evaluation of the minimum iodine concentration for contrast enhanced subtraction mammography

The aim of this work is to investigate the use of iodine based contrast media in mammography with two different double exposure techniques: K-Edge Subtraction Mammography and Temporal Subtraction Mammography. Both techniques have been investigated by using monochromatic beams produced at a synchrotron radiation facility and conventional beams obtained with a clinical digital mammography system. A dedicated three-component phantom containing cavities filled with different iodine concentrations has been developed and used for measurements. For each technique, information about the minimum iodine concentration which provides a significant enhancement of the detectability of the pathology by minimizing the risk due to high dose and high concentration of contrast medium has been obtained. In particular for cavities of 5 and 8 mm of diameter filled with iodine solutions (0, 2, 4, 8, 16, 32 mg / ml ), the minimum concentrations needed to obtain a contrast-to-noise ratio of 5 with a mean glandular dose of 2 mGy have been obtained.

Strain localization in compressed ZrO2(Y2O3) ceramics

Spatiotemporal distributions of local components of the distortion tensor of a nonplastic material—yttria partially stabilized tetragonal zirconia (YTZ) ceramics—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity in the elastic state of YTZ ceramics are considered.

Pattern composed of two-dimensional diffusion dots for showing dynamic images

A new type of diffusion pattern is proposed. The proposed patterns are composed of 2D diffusion dots. The diffusion dots are created on a photoresist plate by recording the image of a local area of a piece of ground glass dot by dot. An imaging lens covered by a mask with a slit aperture is used to form the image. By changing the orientation of the slit aperture on the mask plane, the diffusion dots can have different microintensity distributions for the same incident light beam. Therefore the diffusion dots created by the same slit aperture orientation show the same brightness, and the diffusion dots created by different slit orientations show different brightness for the same illuminating and viewing conditions. Thus a proposed diffusion pattern can show dynamic images by changing its illuminating or viewing directions. By applying the double-exposure technique to the diffusion dots of a pattern, the pattern not only can show dynamic effects but also can possess several hidden features for identifying the pattern. Therefore the proposed patterns are dynamic and anticounterfeiting.

A numerical and experimental analysis on the natural convective heat transfer of a small heating strip located on the floor of a square cavity

In this paper it is analyzed the influence of the position of a small heating source placed on the bottom wall of a square cavity filled by air ( Pr = 0.71). We studied a strip of a determined dimensionless length of ε = 1/5 that is located at two different dimensionless distances from the left side of the cavity: the first one is δ = 0.4 (asymmetrical position) and the second one is δ = 0.5 (symmetrical position). The experimental analysis was carried out through an holographic interferometry and through a 2D PIV system. The holographic interferometry was used by the real-time technique and by the double-exposure technique. In the first case we analyzed the presence of the plume oscillations and we determined the achievement of the steady state; in the second case we concentrated on the air temperature distributions, consequently we measured the Nusselt numbers on the heated strip at different Rayleigh numbers. The experimental study also included an analysis through a 2D Particle Image Velocimetry (PIV), that allowed us to measure the velocity fields and the dynamic structures generated by the heat transfer at the same Rayleigh numbers. Measured quantities were compared with the numerical results which were obtained through the finite volume code Fluent 6.2.16.

Optical Pattern Generation Using a Spatial Light Modulator for Maskless Lithography

In this paper, we present a piston-type spatial light modulator (SLM) as a programmable optical pattern generator for maskless lithography. The SLM is a dual-lever-type actuator with pixel-overlapping springs to achieve low voltage actuation with a small capacitor area. Large arrays of up to ~40000 mirrors with 20-mum pixels and ~17000 mirrors with 30-mum pixels have been fabricated. The arrays have various prewired configurations to alleviate the need to control all the pixels individually. The mirrors have high resonance frequencies for fast response and achieve deflections of lambda/2 (lambda=193 nm for 20-mum pixels and lambda=633 nm for 30-mum pixels) at voltages less than 100 V. The mirrors are fabricated using thin-film processing and chemical mechanical polishing of the reflector layer. Using the SLM in an aerial imaging system, we demonstrate the generation of dark lines, dark areas, linewidth modulation, and subgrid positioning. We also demonstrate compensation of image shifts due to through-focus wafer drift using a double exposure (or phase-compensated exposure) technique

Prospects for quantitative two‐dimensional radiochromic film dosimetry for low dose‐rate brachytherapy sources

Radiochromic film (RCF) has been shown to be a precise and accurate two-dimensional dosimeter for acute exposure radiation fields. However, "temporal history" mismatch between calibration and brachytherapy films due to RCF dose-rate effects could introduce potentially large uncertainties in low dose-rate (LDR) brachytherapy absolute dose measurement. This article presents a quantitative evaluation of the precision and accuracy of a laser scanner-based RCF-dosimetry system and the effect of the temporal history mismatch in LDR absolute dose measurement. MD-55-2 RCF was used to measure absolute dose for a low dose-rate 137Cs brachytherapy source using both single- and double-exposure techniques. Dose-measurement accuracy was evaluated by comparing RCF to Monte Carlo photon-transport simulation. The temporal history mismatch effect was investigated by examining dependence of RCF accuracy on irradiation-to-densitometry time interval. The predictions of the empirical cumulative dose superposition model (CDSM) were compared with measurements. For the double-exposure technique, the agreement between measurement and Monte Carlo simulation was better than 4% in the 3-60 Gy dose range with measurement precisions (coverage factor k = 1) of <2% and <6% for the doses greater or less than 3 Gy, respectively. The overall uncertainty (k = 1) of dose rate/air-kerma strength measurements achievable by this dosimetry system for a spatial resolution of 0.1 mm is less than 4% for doses greater than 5 Gy. The measured temporal history mismatch systematic error is about 1.8% for a 48 h postexposure time when using the double exposure technique and agrees with CDSM's prediction qualitatively. This work demonstrates that the model MD-55-2 RCF detector has the potential to support quantitative dose measurements about LDR brachytherapy sources with precision and accuracy better than that of previously described dosimeters. The impacts of this work on the future use of new type of RCF were also discussed.

Experimental analysis of natural convection in square cavities heated from below with 2D-PIV and holographic interferometry techniques

A numerical and experimental analysis was performed to study the natural convection heat transfer in a square cavity heated from below and cooled by the sidewalls. The enclosure was filled with air ( Pr = 0.71) and a discrete heater was mounted on its lower surface; the effect of three different positions was evaluated. The air temperature distribution and the Nusselt numbers at different Rayleigh numbers on the heated strip were measured by an holographic interferometry thanks to the real-time and the double-exposure technique. The double-exposure technique was performed at steady-state and it was used to obtain the isothermal lines in the cavity at different Rayleigh numbers; while the real-time technique was used to control the presence of the plume oscillations and to determinate the achievement of the steady-state. A 2D particle image velocimetry (PIV) was utilized to measure the velocity fields at the same Rayleigh numbers. In particular we analysed the distribution of the velocity vectors and their modulus inside the cavity. The convective phenomenon was studied and the Nusselt numbers were presented as well as the Rayleigh numbers analysed. Moreover experimental and numerical correlations were determined for each position analysed to connect the Rayleigh numbers with the Nusselt numbers. Measured quantities were compared with the numerical results which were obtained with the finite volume code Fluent 6.2.16.

Evaluation of the minimum iodine concentration for contrast-enhanced subtraction mammography

Early manifestation of breast cancer is often very subtle and is displayed in a complex and variable pattern of normal anatomy that may obscure the disease. The use of dual-energy techniques, that can remove the structural noise, and contrast media, that enhance the region surrounding the tumour, could help us to improve the detectability of the lesions. The aim of this work is to investigate the use of an iodine-based contrast medium in mammography with two different double exposure techniques: K-edge subtraction mammography and temporal subtraction mammography. Both techniques have been investigated by using an ideal source, like monochromatic beams produced at a synchrotron radiation facility and a clinical digital mammography system. A dedicated three-component phantom containing cavities filled with different iodine concentrations has been developed and used for measurements. For each technique, information about the minimum iodine concentration, which provides a significant enhancement of the detectability of the pathology by minimizing the risk due to high dose and high concentration of contrast medium, has been obtained. In particular, for cavities of 5 and 8 mm in diameter filled with iodine solutions, the minimum concentration needed to obtain a contrast-to-noise ratio of 5 with a mean glandular dose of 2 mGy has been calculated. The minimum concentrations estimated with monochromatic beams and K-edge subtraction mammography are 0.9 mg ml−1 and 1.34 mg ml−1 for the biggest and smallest details, respectively, while for temporal subtraction mammography they are 0.84 mg ml−1 and 1.31 mg ml−1. With the conventional clinical system the minimum concentrations for the K-edge subtraction mammography are 4.13 mg ml−1 (8 mm diameter) and 5.75 mg ml−1 (5 mm diameter), while for the temporal subtraction mammography they are 1.01 mg ml−1 (8 mm diameter) and 1.57 mg ml−1 (5 mm diameter).

Impact of registration error of reticle on total overlay error budget

As the overlay specification decreases drastically, it is necessary to consider how the total overlay is influenced by each contributing factor. In particular, it is expected that the contribution on overlay error budget can be quantitatively analyzed in terms of the correlation among registration errors of the reticle. The reticle contribution of about 25% is assessed by the breakdown of the sources of overlay metrology uncertainty through the double exposure technique (DET) process. A positive correlation of around 0.7 mitigates the reticle contribution by 180%, compared to the uncorrelated case. In both DET and double patterning technique (DPT) processes, it is needed to positively correlate the registration errors among many reticles in order to decrease the reticle contribution. To maintain the gain of 180% due to positive correlation, the correlation coefficient requisite has to be increased it is difficult to achieve highly positive correlation among more than three reticles. From an integration po...

Dosimetric characteristics of the Leipzig surface applicators used in the high dose rate brachy radiotherapy

The nucletron Leipzig applicator is designed for (HDR) 192Ir brachy radiotherapy of surface lesions. The dosimetric characteristics of this applicator were investigated using simulation method based on Monte Carlo N-particle (MCNP) code and phantom measurements. The simulation method was validated by comparing calculated dose rate distributions of nucletron microSelectron HDR 192Ir source against published data. Radiochromic films and metal-oxide-semiconductor field-effect transistor (MOSFET) detectors were used for phantom measurements. The double exposure technique, correcting the nonuniform film sensitivity, was applied in the film dosimetry. The linear fit of multiple readings with different irradiation times performed for each MOSFET detector measurement was used to obtain the dose rate of each measurement and to correct the source transit-time error. The film and MOSFET measurements have uncertainties of 3%-7% and 3%-5%, respectively. The dose rate distributions of the Leipzig applicator with 30 mm opening calculated by the validated MC method were verified by measurements of film and MOSFET detectors. Calculated two-dimensional planar dose rate distributions show similar patterns as the film measurement. MC calculated dose rate at a reference point defined at depth 5 mm on the applicator's central axis is 7% lower than the film and 3% higher than the MOSFET measurements. The dose rate of a Leipzig applicator with 30 mm opening at reference point is 0.241+/-3% cGy h(-1) U(-1). The MC calculated depth dose rates and profiles were tabulated for clinic use.

An evaluation of radiation exposure from portal films taken during definitive course of pediatric radiotherapy

Purpose Recently, considerable attention has been directed toward computed tomography radiation doses (estimated 1 to 4 cGy) received by pediatric patients, because of the potential for increasing a pediatric patient's risk for developing a secondary malignancy. However, minimal attention has been given to the radiation exposure outside the treatment field resulting from the use of portal films to visualize surrounding anatomy. The objective of this study was to quantify the radiation dose from portal imaging delivered within and outside the radiation treatment field during a course of radiation therapy. Methods and materials A retrospective review was conducted of the port film dose for 56 consecutive pediatric patients who underwent definitive radiation therapy between January 2001 and January 2002. Treatment locations were classified as brain, 27 patients; abdomen, 11 patients; extremities, 9 patients; pelvis, 6 patients; and thorax, 3 patients. Using the dose per monitor unit and total number of port films taken, the total port film dose for each patient was calculated. In addition, port film dose was quantified for 5 pediatric patients undergoing intensity modulated radiation therapy. Results The mean total port dose varied from a maximum of 46 cGy for brain to a minimum of 17 cGy for thorax. The mean total port dose as a percentage of prescribed dose was less than 1.25% for all locations in this study; however, most of the port dose is a result of the open-field dose from the double-exposure technique. Conclusions Care should be exercised while exposing port films of pediatric patients to minimize both the number of films and corresponding radiation exposure without compromising the quality of treatment delivery. Specifically, the number of monitor units used to image regions outside the treatment field should be kept to a minimum, because such exposure could lead to an increased risk of development of secondary neoplasms.

Resonant holographic interferometry for species concentration measurements with saturated anomalous dispersion

A double-exposure resonant holographic interferometry measurement technique for full-field, instantaneous species is presented. This technique utilizes tuned pulsed dye lasers, separate reference beams, and a corrective holographic optical element. The resulting interferogram fringes represent the species number density of interest with variations in the bulk refractive index due to the temperature field automatically subtracted. Expressions for relating resonant absorption with fringe shift are disclosed. The relations suggest that RHI is well suited for optically dense media and long path lengths, where absorption measurements can be difficult. A saturation related modification of the dispersion near a broadened resonant transition is described. The depopulation of the ground state due to saturation by the probe beam causes an asymmetric modification of the resonant refractive index, which when convolved over the probing beam does not affect the measured fringe shift as measured by RHI. Resonant interferometry is demonstrated as an accurate and quantitative measure of the concentration of sodium vapor in a low pressure cell. Furthermore, as a combustion diagnostic, RHI is demonstrated on a minor combustion species, OH, in a nonpremixed slot burner.