bluuebunny/crossref_metadata_2025 huggingface dataset:
Transition from sub-Poissonian to super-Poissonian shot noise in planar cold cathodes
Authors: Krishnan Rajesh , Cahay Marc
Published: August 2003

In recently investigated cold cathodes, the energy distribution of the injected electron beam into vacuum is quite different from the hemi-Maxwellian distribution typical of thermionic cathodes. We use an ensemble Monte Carlo technique to study shot noise in planar cold cathodes in which the emission into vacuum is characterized by an average injection energy far in excess of the thermal energy typical of thermionic cathodes. For a sufficiently large voltage across the vacuum gap, the low frequency shot noise power spectrum of the anode current is found to agree with the Schottky classical result at low value of the cathode emission current. At the onset of current self-quenching due to space-charge effects, there is first a shot-noise reduction followed by a shot-noise enhancement in the anode current fluctuations for emission current densities far into the saturation regime. It is shown that this shot noise enhancement is due to large fluctuations in both energy and space of the maximum of the energy potential hump in front of the cathode.
Selective growth of carbon nanotubes on Si microfabricated tips and application for electron field emitters
Authors: Minh Phan Ngoc , Tuyen Le T. T. , Ono Takahito , Miyashita H. , Suzuki Y. , Mimura Hidenori , Esashi Masayoshi
Published: August 2003

In this article, we present the results on the synthesis and electron emission characteristics of an individual and a carbon nanotube (CNT) bundle on a Si tip. The Si tip with diameter of about 20 nm at the apex and the tip height of 5 μm was fabricated using Si microfabrication process. Individual and carbon nanotubes bundle were selectively grown at the apex of the Si tip using a hot-filament chemical vapor deposition with a mixture of C2H2 and H2 gases under a negative substrate biasing. Electron field emission characteristics of the Si tips with and without the individual CNT on the same substrate were measured in a vacuum of 1.7×10−4 Pa. Threshold or turn on voltages defined at the emission current of 10 pA of approximately 40 V (4 V/μm) and 200 V (20 V/μm) were found for the Si tips with and without the individual CNT, respectively. Emission current and emission light pattern of the emitted electrons from the carbon nanotubes bundle were also studied.
Flat-panel luminescent lamp using carbon nanotube cathodes
Authors: Chen Jun , Liang X. H. , Deng S. Z. , Xu N. S.
Published: August 2003

A diode-structure flat-panel luminescent lamp has been fabricated by using carbon nanotube (CNT) film as cathode. The CNT emitters were grown on silicon substrate by catalytic decomposition of acetylene. Studies of scanning electron microscopy and transmission electron microscopy show that as-grown CNTs are randomly orientated curved multiwall nanotubes. The I–V characteristics, uniformity, and brightness of the lamp are investigated. The result shows that CNT cathode flat-panel luminescent lamp is promising for lighting application.
Rectifying characteristics of sputter-deposited SiGe diodes
Authors: Ru Guo-Ping , Wang Guang-Wei , Jiang Yu-Long , Huang Wei , Qu Xin-Ping , Zhu Shi-Yang , Li Bing-Zong
Published: August 2003

Schottky and pn junction diodes with good rectifying characteristics have been prepared based on the polycrystalline SiGe (poly-SiGe) thin film deposited by the ion-beam-sputtering (IBS) technique. Boron and phosphorus diffusion techniques have been used to dope and crystallize as-deposited amorphous SiGe film. Rectification ratios as high as 4000 and 1800 have been achieved in Pt/n-poly-SiGe and Ti/p-poly-SiGe Schottky diodes, respectively, while rectification ratio higher than 1500 and breakdown voltage higher than 200 V have been achieved in poly-SiGe pn junction diodes. Schottky barrier height has been determined to be 0.62 and 0.59 eV for Pt/n-poly-Si0.81Ge0.19 and Ti/p-poly-Si0.81Ge0.19 contacts, respectively, which indicates that the band alignment of poly-SiGe may be substantially different from that of epitaxial SiGe.
Electron emission from porous silicon planar emitters
Authors: Mimura Hidenori , Miyajima Kentaro , Yokoo Kuniyoshi
Published: August 2003

Porous silicon planar emitters were fabricated by depositing a thin Au film on a conventional porous Si and their emission characteristics were examined. The emission currents and energy distributions were measured for the emitters with various Au thicknesses and for cesiated ones. The experimental results suggest that the emission mechanism of the porous silicon emitter studied in this work is conventional field emission, in which electrons are emitted from nanocrystals in the porous silicon directly into a vacuum.
In situ analyzer of electron emission properties: Fowler–Nordheim plotter and Seppen–Katamuki plotter
Authors: Gotoh Yasuhito , Ishizu Katsuyuki , Tsuji Hiroshi , Ishikawa Junzo
Published: August 2003

In situ analyzer of electron emission properties that displays the Fowler–Nordheim (F–N) plot and its location on the Seppen–Katamuki chart directly from the current–voltage values of the field emission device, was developed. Field electron emission characteristics were acquired by applying the voltage that was modulated by an ac signal. An analog circuit, which consists of inexpensive operational amplifiers, was designed so that the circuit converts the current–voltage characteristics to the reciprocal voltage and the logarithm of the current over square voltage. The slope and intercept were also calculated by the analog circuit. The operation of the circuit was confirmed with a light-emitting diode. The electron emission property of an etched tungsten needle emitter was converted to F–N plot, and the property projected to the screen was a straight line.
Field emission characteristics of nodular carbon nanotubes
Authors: Li Qiong , Xu Jingfang , Yu Ke , Zhu Ziqiang , Feng Tao , Wang Xi , Liu Xianghuai , Kang W. P. , Davidson J. L.
Published: August 2003

A configuration of carbon nanotubes (CNTs) whose surfaces were fully covered with nanoscale nodes was developed in our laboratory. We named this material nodular CNTs or NCNTs. A hydrogen plasma process was used to turn common CNTs into NCNTs. Combined with this process, a technique has been developed to make planar NCNTs field emitters. Field emission experiments showed that planar NCNTs emitters had superior field emission characteristics. Specifically, the emission site density was increased by the order of 106/cm2. A suggestion was given to interpret the formation of NCNTs and a simple model was also proposed to explain the improvement of field emission properties. This technique was found to be an easy way to change the geometric character of CNTs in large quantities and will be useful for creating planar field emitters used in field emitter light sources or display devices.
Fundamental beam studies of deuterium and fluorine radical reaction kinetics on surfaces
Authors: Greer Frank , Fraser D. , Coburn J. W. , Graves David B.
Published: August 2003

The abstraction reaction kinetics of two radical surface reactions were investigated on silicon, photoresist (PR), alumina, and quartz using a high vacuum beam system with independent beams of D and F atoms. Modulated beam mass spectrometry was used to measure the following heteronuclear abstraction probabilities from Si, PR, Al2O3, and SiO2 at 286 K: F atoms abstracting adsorbed D atoms from PR (γFD,PR=0.25); Si (γFD,Si=0.22); quartz (γFD,SiO2=0.18); alumina (γFD,Al2O3=0.12); and D atoms abstracting adsorbed F atoms from PR (γDF,PR<0.02); Si (γDF,Si<0.02); alumnia (γDF,Al2O3<0.02); and quartz (γDF,SiO2<0.02). These probabilities were also measured at 344 K. The rate of D abstracting F increased from below the limit of detection at 286 K to γDF,Si=0.05 on Si at 344 K, suggesting that this process is thermally activated, but no significant effect of temperature was observed for F abstracting D on any surface tested. Reactive site densities were also estimated for these surfaces: PR (σPR=2.0×1015/cm2), Si (σSi=2.0×1015/cm2), quartz (σSiO2=8.0×1014/cm2), and alumina (σAl2O3=5.0×1014/cm2).
Growth of M-plane GaN films on γ-LiAlO2(100) with high phase purity
Authors: Sun Yue Jun , Brandt Oliver , Ploog Klaus H.
Published: August 2003

We present a comprehensive study of the growth of GaN(11̄00) on γ-LiAlO2(100) by plasma-assisted molecular beam epitaxy. Pure GaN(11̄00) layers are obtained preferentially on a particular face of the substrate. We provide a means by which to identify this face by simple chemical treatment. Nitridation of the substrate prior to growth as well as immediate N-rich nucleation conditions invariably induce phase mixture. In contrast, immediate Ga-rich nucleation is found to result in the growth of pure GaN(11̄00) films. Layers nucleated at low temperature exhibit the highest structural and morphological quality reported so far for non-[0001]-oriented structures.
Large blueshift in InGaAs/InGaAsP laser structure using inductively coupled argon plasma-enhanced quantum well intermixing
Authors: Djie H. S. , Arokiaraj J. , Mei T. , Tang X. H. , Ang L. K. , Leong D.
Published: August 2003

An inductively coupled plasma-enhanced quantum well intermixing technique has been developed to induce a shift in the band gap in quantum well structures using argon plasma. The emission of the InGaAs/InGaAsP laser structure was blueshifted as much as 104 nm with linewidth broadening of only 10.6 nm using 5 min plasma exposure and subsequent rapid thermal annealing. This large shift is attributed to inductively coupled plasma at high ion current density (with 100’s of eV ion impact energy) that promotes desirable point defects near the surface of the samples. The result has demonstrated an effective approach for large band gap tuning of InGaAs/InGaAsP laser structures.
Low temperature treatment of the (001) ZnTe substrate surface with the assist of atomic hydrogen
Authors: Tsutsumi K. , Terakado H. , Enami M. , Kobayashi M.
Published: August 2003

Recent success in the fabrication of the bulk ZnTe has enabled the realization of large area single crystal substrates. The growth and application of ZnTe homoepitaxial layers by molecular beam epitaxy should involve the pretreatment of the substrate surface prior to the nucleation. An atomic hydrogen treatment was performed to remove the surface oxide of ZnTe. The oxide could be removed at around 100 °C with the assist of the atomic hydrogen beam. The reconstructed surface could be achieved by annealing at around 230 °C. The two dimensional nucleation was also confirmed. The homoepitaxial layer has shown such a high quality that interface, or defects could not be confirmed by the transmission electron microscopy.
Effects of Si deposition on the properties of Ga-rich (4×6) GaAs (001) surfaces
Authors: Negoro Noboru , Anantathanasarn Sanguan , Hasegawa Hideki
Published: August 2003

In order to achieve better surface passivation on technologically important (001) GaAs surfaces, microscopic and macroscopic properties of the clean (001) surface with Ga-rich (4×6) reconstruction, and effects of monolayer level Si deposition on this surface, were investigated. According to scanning tunneling spectroscopy and contactless capacitance–voltage (C–V) measurements, the Ga-rich genuine (G-)(4×6) surface had high-density acceptor-type states above EC−0.5 eV. However, the surface exhibited a much lower and wider surface state density (NSS) distribution as well as a much stronger band-edge photoluminescence (PL) intensity than the conventional As-rich surfaces. Irradiation of Si molecular beam on the (4×6) surface resulted in epitaxial growth of Si. This completely removed the acceptor-type surface states from the energy gap, and further enhanced the PL intensity. The result indicates that the Ga-rich G-(4×6) surface is more ordered and more stable against processing than conventional As-rich surfaces. Metal–insulator–semiconductor capacitors fabricated by further depositing a thick SiO2 layer indicated that the Fermi level was completely unpinned over the entire band gap.
Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors
Authors: Hashizume Tamotsu , Ootomo Shinya , Inagaki Takanori , Hasegawa Hideki
Published: August 2003

We have systematically investigated effects of plasma processing, formation of Si-based dielectrics, and formation of a thin Al2O3 film on the chemical and electronic properties of GaN and GaN/AlGaN heterostructure surfaces. The surface treatment in H2-plasma excited by electron-cyclotron-resonance (ECR) source, produced nitrogen-vacancy-related defect levels at GaN and AlGaN surfaces, while the ECR-N2-plasma treatment improved electronic properties of the surfaces. The deposition of a SiO2 film on GaN and AlGaN surfaces was found to induce high-density interface states, due to unexpected and uncontrollable oxidation reactions on the surfaces during the deposition process. In comparison, the SiNx/GaN passivation structure prepared by ECR-plasma assisted chemical vapor deposition showed good interface properties with the minimum Dit value of 1×1011 cm−2 eV−1. However, excess leakage currents governed by Fowler–Nordheim tunneling were observed in the SiNx/Al0.3Ga0.7N structure, due to a relatively small conduction band offset of 0.7 eV between SiNx and Al0.3Ga0.7N. A novel Al2O3-based passivation structure was proposed and fabricated by molecular beam deposition of Al and subsequent ECR O2-plasma oxidation. In situ x-ray photoelectron spectroscopy showed successful formation of the Al2O3 layer with a thickness of 3.5 nm and a large conduction band offset of 2.1 eV between Al2O3 and A0.3Ga0.7N. The GaN/AlGaN insulated-gate heterostructure field-effect transistors (IG HFETs) having the Al2O3-based passivation structure showed a good gate control of drain currents up to VGS=+3 V and achieved drain saturation current of 0.8 A/mm. The observed maximum gm value is 120 mS/mm. No current collapse was observed in the Al2O3 IG HFETs, indicating a remarkable advantage of the present Al2O3-based passivation structure.
Efficiency of cathodoluminescent phosphors for a field-emission light source application
Authors: Chubun Nikolai N. , Chakhovskoi Andrei G. , Hunt Charles E.
Published: August 2003

The efficiency of major types of cathodoluminescent phosphors has been investigated at high-brightness (up to 30 000 cd/m2) operational conditions using thermionic electron source. The accelerating voltage was varied from 4 to 14 kV and electron beam current density was independently varied from 2 to 50 μA/cm2. Under those conditions both thermal quenching and current saturation of the phosphors were observed. Due to combined influence of those factors, it was found that the best way to characterize the phosphor is plotting the efficiency (in lm/W) versus the specific power density (in mW/cm2) graph. The resulting plots show the integrated influence of current loading and elevated temperature on the phosphor efficiency. The best efficiencies at 200 mW/cm2 and 10 kV were measured for a green color phosphor −55 lm/W, for a blue one—10 lm/W, and for a red—13 lm/W.
Gated carbon-based cold cathode for high current applications
Authors: Jamison Keith D. , Zollars Byron G. , Patterson Donald E. , Schueller Randy , Windischmann Henry , Mulhollan Gregory A. , Kloba Anthony
Published: August 2003

We have developed a gated carbon-based cold cathode capable of operating at high current densities for use in applications such as cathode-ray tubes, microwave devices, and x-ray sources. The carbon-based electron-emitting material is a good choice for these applications as it does not readily form a native oxide when exposed to atmosphere. Cathodes can thus be pretested in a vacuum chamber before insertion into the final device. Given that carbon has a low sputtering yield and is relatively inert, this material is an ideal candidate for long-lived applications in moderate vacuum environments. Our present 500-μm-diam cold cathode array contains approximately 5000 individual microtips and emits 0.1–500 μA for a gate voltage of 70–118 V. The average current density over the array is about 0.25 A/cm2 at 500 μA emission current. Long-term testing of individual cold cathodes at current densities of 0.10 A/cm2 in cathode-ray tubes has shown lifetimes in excess of 5000 h. Peak currents exceeding 10 mA (current densities of 5 A/cm2) have been achieved.
Etching characteristics of porous silica (k=1.9) in neutral loop discharge plasma
Authors: Morikawa Yasuhiro , Mizutani Naoki , Ozawa Masanori , Hayashi Toshio , Chen Wei , Uchida Taijiro
Published: August 2003

Etching characteristics of a porous silica material (ISM-1.5™ produced in ULVAC, Inc.) were investigated and compared with those of thermal oxide. The etch rate of porous silica in magnetic neutral loop discharge plasma was approximately two times higher than that of thermal SiO2 film when linear saturated perfluorocarbon compounds were used. This may be due to the low film density of the porous silica. However, in the case of C4F8 (octafluorocyclobutane) plasma, the etch rate ratio to SiO2 was about 1.45. When C4F6 (CF2=CFCF=CF2: hexafluorobutadiene) was used, the etch rate ratio was also very low (0.6). So, the etch rate strongly depended on the gas structure, whereas the SiO2 etch rate did not depend on the gas species and was almost constant. Through mass spectrometry and x-ray photoelectron spectroscopy measurements, it was deduced that the fluorocarbon polymer formed in the pore suppressed the etch rate of porous silica in C4F8 or C4F6 plasmas.
Lithography-free fabrication of sub-100 nm structures by self-aligned plasma etching of silicon dioxide layers and silicon
Authors: Georgiev G. , Müller-Wiegand M. , Georgieva A. , Ludolph K. , Oesterschulze E.
Published: August 2003

A general scheme for the fabrication of silicon and silicon dioxide structures with lateral dimensions of less than 100 nm is introduced that avoids high-resolution lithography processes. For the self-aligned formation of extreme small openings in silicon dioxide layers at sharpened surface structures the angle dependent etch rate distribution of silicon dioxide against plasma etching with a fluorocarbon gas (CHF3) was exploited. Subsequent anisotropic plasma etching of the silicon substrate material through the perforated silicon dioxide masking layer results in high aspect ratio trenches of approximately the same lateral dimensions. The latter can be reduced and precisely adjusted between 0 and 200 nm by thermal oxidation of the silicon structures owing to the volume expansion of silicon during oxidation.
Impact of quantum well intermixing on polarization anisotropy of InGaAs/InGaAsP quantum well modulators
Authors: Ng S. L. , Djie H. S. , Lim H. S. , Lam Y. L. , Chan Y. C. , Dowd P. , Ooi B. S. , Aimez V. , Beauvais J. , Beerens J.
Published: August 2003

This article reports on the impact of the induced strain on the polarization anisotropy of a parallel set of electroabsorption intensity modulators on a single InGaAs/InGaAsP wafer chip. The strain build up due to the interdiffusion of atomic species across the quantum well region has been demonstrated experimentally using the gray mask-based quantum well intermixing process followed by an annealing step. A voltage swing of 5 V and an intensity modulation depth of more than −15 dB has been measured from these modulators. An interdiffusion process modeling has been developed to investigate the consequence of different interdiffusion ratios between the group III and the group V sublattices on the polarization behavior of these modulators, owing to the strain build up and the refractive index profiles for both transverse electric and transverse magnetic modes. The numerical modeling agrees with the experimental results, which indicates that the degree of intermixing on the group V sublattices is more significant compared to that of group III sublattices, hence causing a strained material system after an intermixing process and thus resulting in with polarization insensitive devices.
Integrating vertically aligned carbon nanotubes on micromechanical structures
Authors: Teh W. H. , Smith C. G. , Teo K. B. K. , Lacerda R. G. , Amaratunga G. A. J. , Milne W. I. , Castignolles M. , Loiseau A.
Published: August 2003

We present preliminary results on a microfabrication approach to enable the integration of high yield, uniform, and preferential growth of vertically aligned carbon nanotubes (VACNTs) on low-stress micromechanical structures using a combination of “electron-beam crosslinked” poly(methylmethacrylate) surface nanomachining and direct current plasma enhanced chemical vapor deposition of electric-field-aligned carbon nanotubes. In this article, selective placement of high yield and uniform VACNTs on a partially suspended Ni/SiO2/Ti microstructure has been demonstrated.
Current–voltage characteristics of nonplanar cold field emitters
Authors: Edgcombe Christopher John , Johansen Adam Michael
Published: August 2003

Conventional Fowler–Nordheim theory assumes that the emitter is planar, while most tips used in practice have curved emitting surfaces. Using a revised potential distribution, standard image theory and a dimensionless parameter x, we express the experimental current as a multiple σ of the current calculated using standard free-electron supply. A plot of σ(x) for one carbon emitter shows a maximum at a value of x corresponding to the known emitter radius. The calculated field strength at the emitter surface varies little with x. The values found for σ are sensitive to the accuracy of calculation and, to test the theory further, it is desirable both to improve the modelling of image effects and to obtain measurements of current–voltage characteristics and emitting radii together for more types of emitter.
Measurement of work function of transition metal nitride and carbide thin films
Authors: Gotoh Yasuhito , Tsuji Hiroshi , Ishikawa Junzo
Published: August 2003

Work functions of transition metal nitride and carbide thin films were measured. The materials investigated were ZrN, NbN, HfN, TaN, HfC, and TaC. The films were prepared either by radio-frequency magnetron sputter deposition or by ion beam assisted deposition. The work function was measured by Kelvin probe in air. The work functions of ZrN and HfN ranged between 4.6 and 4.7 eV, and were similar to or slightly lower than that of NbN and TaN, 4.7 or 4.8 eV. The work function of TaC was approximately 5.0 eV. The higher work function of carbide may be attributed to lower electronegativity of carbon as compared to nitrogen.
Method on surface roughness modification to alleviate stiction of microstructures
Authors: Lee Cheng-Chang , Hsu Wensyang
Published: August 2003

Modification on surface roughness has been shown to effectively alleviate both release and in-use stiction in the previous literature. However, the modified materials in the previously reported methods were limited to polysilicon or single crystalline silicon with special properties. Here, the proposed modification method not only can apply to silicon without extra property requirements, but also has the potential to modify other materials, such as oxide, nitride, and some metals. The process here combines spin-on photoresist and reactive ion etching. The proposed low temperature process is simple, and no extra mask is needed. Consequently, there is more flexibility to add the roughness modification to the original fabrication process of microdevices. In this study, polysilicon and silicon nitride are demonstrated as the modified materials. Antistiction effects are characterized by calibrating the water contact angles on the modified surfaces and the detachment lengths of released cantilevers. The experimental results show that the detachment length is almost two times longer than the cantilevers without modified substrates, where the interfacial surface energy between solids is reduced about 15 times.
Synthesis of aligned bamboo-like carbon nanotubes using radio frequency magnetron sputtering
Authors: Lee Kuei-Yi , Ikuno Takashi , Tsuji Keita , Ohkura Shigeharu , Honda Shin-ichi , Katayama Mitsuhiro , Oura Kenjiro , Hirao Takashi
Published: August 2003

Vertically aligned bamboo-like carbon nanotubes (CNTs) are successfully synthesized on Ni-coated Si (Ni/Si) substrates by hot-filament-assisted radio frequency (rf) magnetron sputtering. Transmission electron microscopy analysis reveals that a bamboo-like structure is formed inside the nanotube that has a catalytic Ni nanoparticle on the tip. It is likely that moderate carbon particle energy plays an essential role in synthesizing CNTs by sputtering. To verify its importance, we conduct a bias-sputtering experiment to test the effect of negative voltages on the energy of the carbon particles. Furthermore, we find that the diameter and density of the aligned bamboo-like CNTs can be controlled by changing the number density of Ni nanoparticles on Si substrates.
Recombination dynamics of localized excitons in cubic InxGa1−xN/GaN multiple quantum wells grown by radio frequency molecular beam epitaxy on 3C–SiC substrate
Authors: Chichibu S. F. , Onuma T. , Aoyama T. , Nakajima K. , Ahmet P. , Chikyow T. , Sota T. , DenBaars S. P. , Nakamura S. , Kitamura T. , Ishida Y. , Okumura H.
Published: August 2003

Recombination dynamics of localized excitons in strained cubic (c-)InxGa1−xN/GaN multiple quantum wells (MQWs) grown on 3C–SiC (001) were summarized in terms of well thickness L, InN molar fraction x, and temperature T. Photoluminescence (PL) peak energy of c-In0.1Ga0.9N/GaN MQWs showed a moderate blueshift as L decreased, and the low-temperature PL lifetime did not change remarkably by changing L. These results proved that the quantum-confined Stark effect due either to spontaneous or piezoelectric polarization was inactive in cubic polytypes. Consequently, time-resolved PL (TRPL) data of c-InGaN MQWs reflect the intrinsic exciton dynamics. The TRPL signal showed stretched exponential decay and spectral redshift with time after excitation up to 300 K. The results are fingerprints that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures forming extended and localized states. Effective localization depth increased with the increase in x, which gave rise to fast exciton localization. However, nonradiative lifetime in the free or extended states decreased more rapidly with the increase in x and T, giving the emission efficiency maximum at particular x around 0.1.
Electron cyclotron resonance plasma etching of GaSb in Cl2/BCl3/CH4/Ar/H2 at room temperature
Authors: Langer J. P. , Dutta P. S.
Published: August 2003

Electron cyclotron resonance plasma etching of undoped and tellurium doped GaSb (100) samples using a combination of (Cl2/BCl3/CH4/Ar/H2) gases has been demonstrated. Etch rates up to about 5000 Å/min at room temperature with sharp sidewalls, low surface roughness, and no undercutting have been evinced. The etch rate obtained using the reported gas mixture is approximately two times the etch rate using existing methane based recipes and comparable to chlorine etches.
High-resolution pattern generation using the epoxy novolak SU-8 2000 resist by electron beam lithography
Authors: Aktary Mirwais , Jensen Martin O. , Westra Kenneth L. , Brett Michael J. , Freeman Mark R.
Published: August 2003

We report the fabrication of high-resolution sub 50 nm patterns by electron beam lithography using the epoxy novolak SU-8 2000 resist formulation. The minimum linewidth achieved is on the order of 30 nm and corresponds to a threefold reduction in minimum linewidth over previous reports describing similar resist chemistries. Our results also show that it is possible to fabricate dense linear grating elements without proximity correction. The dry etch resistance of native SU-8 2000 was found to be nearly twice that of poly(methylmethacrylate), making it ideal for applications that require pattern transfer. These studies are intended to explore the feasibility of SU-8 2000 as an electron beam resist for pattern generation on length scales below 50 nm.
Signal detectability in digital radiography: Spatial domain figures of merit
Authors: Gagne Robert M. , Boswell Jonathan S. , Myers Kyle J.
Published: August 2003

The usefulness of Fourier‐based measures of imaging performance has come into question for the evaluation of digital imaging systems. Figures of merit such as detective quantum efficiency (DQE) based on Fourier domain parameters are relevant for linear, shift‐invariant systems with stationary noise. However, no digital imaging system is shift invariant, and realistic images do not satisfy the stationarity condition. Our methods for the task‐based evaluation of imaging systems, based on signal detectability in the spatial domain, do not require such assumptions. We have computed the performance of ideal and quasi‐ideal observers for the task of signal detection in digital radiography. Signal detectability in terms of an observer signal‐to‐noise‐ratio (SNR) has been compared to results obtained from a Monte Carlo simulation of the digital image‐acquisition process. The simulation incorporates the effects of random amplification and secondary quantum blur, integration over pixel area, and electronic noise. The observer figures of merit that have been previously shown to bracket human performance directly specify the usefulness of the images for the stated diagnostic task. In addition, the observer figures of merit give a task‐dependent measure of imaging system efficiency in terms of the ratio of an output to an input Thus, the concept of “detective quantum efficiency” reappears in a natural way but based in the spatial domain and not dependent on shift invariance and stationarity assumptions. With respect to the optimum amount of system blur, our simulations indicate that under certain task‐dependent conditions, large signals are fairly insensitive to blur in the x‐ray transducer, while an optimum blur is found for small signals.
Accounting for high Z shields in brachytherapy using collapsed cone superposition for scatter dose calculation
Authors: Carlsson Tedgren Åsa K. , Ahnesjö Anders
Published: August 2003

Common clinical brachytherapy treatment planning algorithms perform at best one‐dimensional corrections for high Z heterogeneities that will be inaccurate for intermediate energies (60–100 keV). The development of fast methods for a three‐dimensional dose calculation to account for high Z materials in this energy range is important, e.g., to fully utilize the potential of patient individualized shields using isotopes such as and In this work we use the collapsed cone superposition algorithm to calculate the scatter dose contribution around partly lead‐shielded point sources at 60, 100, and 350 keV. Methods to scale point kernels for water into kernels for high Z materials are derived. The scaling accounts for scattered photon spectral differences between materials and thus goes beyond the common density scaling approach. Compared to Monte Carlo simulations, the results of our algorithm yield agreements on the unshielded side to within 3% at 350 and 60 keV and to within 7% at 100 keV out to distances of 8 cm from the source. The effect of the shield in the center of the unshielded region is small at 350 keV but significant and occurs at short distances at 100 and 60 keV. At 60 keV, the shield causes a dose reduction of around 10%, 1 cm from the source on the unshielded side. At 100 keV, the reverse effect is seen, the insertion of shields leading to the total dose being increased by about 10% at 1 cm. That one‐dimensional algorithms are incapable of predicting these changes shows the importance of accounting for the full three‐dimensional geometry in correctly determining the scatter dose contribution.
Measurements of Gamma‐Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector
Authors: Araki Fujio , Ikegami Tohru , Ishidoya Tatsuyo , Kubo Hideo D.
Published: August 2003

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume ‐type silicon diode detector are used for the measurement of the output factors from Gamma‐Knife fields. The GRD system consists of small rod‐shaped glass chip detectors and an automatic readout device. The output factors measured with the GRD from the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with the ‐type silicon diode detector are 0.980, 0.949 and 0.867, respectively. The output factors are corrected for the end effect for each helmet. The output factors obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the manufacturer. The directional dependence of these detectors is also measured. For the Gamma‐Knife angle ranging from 6 to 36 degrees in the plane of the stereotactic space, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x‐ray beam. The response of the silicon diode detector indicates approximately 3–4% directional dependence for the same angular range for a 6 MV x‐ray beam. The Gamma‐Knife helmet output factors measured with the silicon diode detector are corrected for angular dependence.
Dependence of radiochromic film optical density post‐exposure kinetics on dose and dose fractionation
Authors: Ali I. , Costescu C. , Vicic M. , Dempsey J. F. , Williamson J. F.
Published: August 2003

Radiochromic film (RCF) has been shown to be a precise and accurate secondary planar dosimeter for acute exposure radiation fields. However, its application to low dose‐rate brachytherapy has been questioned because of possible dose‐rate effects. To address this concern, we have measured the optical density (OD) of Model 55‐2 RCF as a function of time (interval between the completion of irradiation and densitometry using a 633 nm laser scanner) following exposure (from less than 1 hour to 90 days) for single and split doses from 1 Gy to 100 Gy. Our work demonstrates that film darkening as a function of post‐irradiation time depends significantly on total dose, with films exposed to lower doses developing faster than films given higher doses. At 1 Gy, the OD 90 days after exposure is 200% larger than that measured 1 h after exposure compared to a 20% increase over 90 days for doses larger than 20 Gy. An empirical model with time‐independent, fast and slow growth terms was used to fit single exposure data. The dependence of the resulting best‐fit parameters on dose was investigated. Splitting the dose into two fractions (20 Gy followed by doses of 1–80 Gy 24 h later) results in modest post‐irradiation time‐dependent changes in the total optical density (at most 15% at small doses), which dissipates within 20 hours following the second exposure. This experimental finding is consistent with the predictions of a simple cumulative dose superposition model. Overall, both experimental and empirical modeling suggest that dose‐rate effects may be relatively small despite the strong dependence of film darkening kinetics on total dose. However, more experimental evaluation of radiochromic film response dependence on dose rate and dose‐time‐fractionation patterns is needed.
Reducing uncertainties in volumetric image based deformable organ registration
Authors: Liang J. , Yan D.
Published: August 2003

Applying volumetric image feedback in radiotherapy requires image based deformable organ registration. The foundation of this registration is the ability of tracking subvolume displacement in organs of interest. Subvolume displacement can be calculated by applying biomechanics model and the finite element method to human organs manifested on the multiple volumetric images. The calculation accuracy, however, is highly dependent on the determination of the corresponding organ boundary points. Lacking sufficient information for such determination, uncertainties are inevitable—thus diminishing the registration accuracy. In this paper, a method of consuming energy minimization was developed to reduce these uncertainties. Starting from an initial selection of organ boundary point correspondence on volumetric image sets, the subvolume displacement and stress distribution of the whole organ are calculated and the consumed energy due to the subvolume displacements is computed accordingly. The corresponding positions of the initially selected boundary points are then iteratively optimized to minimize the consuming energy under geometry and stress constraints. In this study, a rectal wall delineated from patient CT image was artificially deformed using a computer simulation and utilized to test the optimization. Subvolume displacements calculated based on the optimized boundary point correspondence were compared to the true displacements, and the calculation accuracy was thereby evaluated. Results demonstrate that a significant improvement on the accuracy of the deformable organ registration can be achieved by applying the consuming energy minimization in the organ deformation calculation.
Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. I. The effect of shift invariance
Authors: Craig Tim , Battista Jerry , Dyk Jake Van
Published: August 2003

Convolution methods have been used to model the effect of geometric uncertainties on dose delivery in radiation therapy. Convolution assumes shift invariance of the dose distribution. Internal inhomogeneities and surface curvature lead to violations of this assumption. The magnitude of the error resulting from violation of shift invariance is not well documented. This issue is addressed by comparing dose distributions calculated using the Convolution method with dose distributions obtained by Direct Simulation. A comparison of conventional Static dose distributions was also made with Direct Simulation. This analysis was performed for phantom geometries and several clinical tumor sites. A modification to the Convolution method to correct for some of the inherent errors is proposed and tested using example phantoms and patients. We refer to this modified method as the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over different beam arrangements in the various phantom examples) was 21% with the Static dose calculation, 9% with Convolution, and reduced to 5% with the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over four clinical examples) was 9% for the Static method, 13% for Convolution, and 3% for Corrected Convolution. While Convolution can provide a superior estimate of the dose delivered when geometric uncertainties are present, the violation of shift invariance can result in substantial errors near the surface of the patient. The proposed Corrected Convolution modification reduces errors near the surface to 3% or less.
Microdosimetric intercomparison of BNCT beams at BNL and MIT
Authors: Burmeister Jay , Riley Kent , Coderre Jeffrey A. , Harling Otto K. , Ma Ruimei , Wielopolski Lucian , Kota Chandrasekhar , Maughan Richard L.
Published: August 2003

Microdosimetric measurements have been performed at the clinical beam intensities in two epithermal neutron beams, the Brookhaven Medical Research Reactor and the M67 beam at the Massachusetts Institute of Technology Research Reactor, which have been used to treat patients with Boron Neutron Capture Therapy (BNCT). These measurements offer an independent assessment of the dosimetry used at these two facilities, as well as provide information about the radiation quality not obtainable from conventional macrodosimetric techniques. Moreover, they provide a direct measurement of the absorbed dose resulting from the BNC reaction. BNC absorbed doses measured within this study are approximately 15% lower than those estimated using foil activation at both MIT and BNL. Finally, an intercomparison of the characteristics and radiation quality of these two clinical beams is presented. The techniques described here allow an accurate quantitative comparison of the physical absorbed dose as well as a measure of the biological effectiveness of the absorbed dose delivered by different epithermal beams. No statistically significant differences were observed in the predicted RBEs of these two beams. The methodology presented here can help to facilitate the effective sharing of clinical results in an effort to demonstrate the clinical utility of BNCT.
A real‐time biopsy needle segmentation technique using Hough Transform
Authors: Ding Mingyue , Fenster Aaron
Published: August 2003

Real‐time needle segmentation and tracking is very important in image‐guided surgery, biopsy, and therapy. Due to its robustness to the addition of extraneous noise, the Hough Transform is one of the most powerful line‐detection techniques nowadays and has been widely used in different areas. Unfortunately, its high computation needs often prevent it from being applied in real‐time applications without the help of specially designed hardware. In order to solve this problem, a variety of fast implementation algorithms have been proposed. However, none of them can be performed in a real time on an affordable computer. In this paper, we describe a fast implementation of the Hough Transform based on coarse‐fine search and the determination of the optimal image resolution. Compared to conventional techniques, our approach decreases the time for needle segmentation by an order of magnitude. Experiments with agar phantom and patient breast biopsy ultrasound (US) image sequences showed that our approach can segment the biopsy needle in real time (i.e., less than 33 ms) on an affordable PC computer without the help of specially designed hardware with the angular rms error of about 1° and the position rms error of about 0.5 mm.
Guidance document on delivery, treatment planning, and clinical implementation of IMRT: Report of the IMRT subcommittee of the AAPM radiation therapy committee
Authors: Ezzell Gary A. , Galvin James M. , Low Daniel , Palta Jatinder R. , Rosen Isaac , Sharpe Michael B. , Xia Ping , Xiao Ying , Xing Lei , Yu Cedric X.
Published: August 2003

Intensity‐modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add‐on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three‐dimensional (3‐D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf‐collimator‐based IMRT delivery systems, goal‐based inverse treatment planning, and clinical implementation of IMRT with patient‐specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.
Determining optimal gel sensitivity in optical CT scanning of gel dosimeters
Authors: Xu Y. , Wuu Cheng‐Shie , Maryanski Marek J.
Published: August 2003

A method for determining the gel sensitivity that is necessary for obtaining optimal image contrast in optical CT scanning of gel dosimeters is presented. The effective dynamic range of the OCTOPUS‐ONE™ research scanner (MGS Research, Inc., Madison, CT) is analyzed. Optical density increments for selected straight‐line paths across a gel cylinder to be scanned are calculated based on the optical properties of the polymer gel and the dose distribution from a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA). Maximum optical density increment across the entire gel is obtained by searching the gel cylinder over a set of transverse planes at different rotational angles. The application of this quantity as a criterion for optimizing the quality of the optical CT scanning is demonstrated through dose verification of two representative treatment plans. When the MU dependence of the dose distribution for a treatment plan is linear, as is the case for static field irradiation, it is possible to scale the treatment plan such that the intensity variation of the signals received by the photodetector spans its entire dynamic range. For treatment plans that are possibly nonlinear, IMRT plans, for example, modification of the sensitivity of the gel material is necessary for the high‐dose signals to be collected at a certain signal‐to‐noise ratio. Results obtained using the optimized CT scanning approach are compared with those from the treatment planning system and the film measurement.
Aging Research Grows Up
Authors: Leslie Mitch
Published: August 2003

Long viewed as an insoluble enigma, aging is shedding its cloak of mystery as scientists start to understand why and how we age. Many studies support the theoretical argument that aging occurs because natural selection weakens with age, leaving us vulnerable to harmful, late-acting genes. As for what causes aging, scientists have narrowed the pack of candidates to a handful, including free radicals and reactions between glucose and proteins. In recent decades, many mechanisms for lengthening life in animals have come to light. By extending this research, scientists may be closing in on ways to lengthen the human life-span.
Using Yeast to Discover the Fountain of Youth
Authors: Kaeberlein Matt , McVey Mitch , Guarente Leonard
Published: August 2003

The budding yeast
Saccharomyces cerevisiae
has long served as a model organism for the study of basic cellular processes. Its short generation time, well-established molecular genetics, and fully sequenced genome have made this organism a favorite of researchers in diverse fields. Much of the information obtained has been shown to apply to higher eukaryotes, including humans. Recently, researchers have begun using yeast to tackle one of the outstanding questions in science: How and why do organisms age? The identification of individual genes in yeast that can affect the aging process itself has elevated this single-celled fungus to full contender status in the aging field. In this Perspective, we present two fundamentally different measures of aging in yeast: replicative life-span and stationary phase survival (chronological life-span). We describe the benefits and limitations of each and present models that attempt to explain these “aging” phenomena. Finally, we present compelling evidence that the use of yeast as a model system will ultimately prove beneficial to the study of human aging.
Biomarkers of Aging
Authors: Miller Richard A.
Published: August 2003

As a subgenre of biogerontology, biomarker research has developed a thoroughly tarnished reputation, and those who believe that biomarker studies should be a major focus in experimental aging research must now try to prove to their doubting colleagues that under the tarnish lies true metal, rather than more layers of tarnish. In this Perspective, the author argues that the discovery of biomarkers of aging is possible.
Deciphering the Gene Expression Profile of Long-Lived Snell Mice
Authors: Becker Kevin G.
Published: August 2003

In a recent publication, gene expression profiling experiments for the long-lived Snell dwarf mouse were reported. In this Perspective, the author argues that although the authors have developed a solid analytical approach, further refinements are needed to produce robust conclusions.