Deposited Energy Profile Research Articles

Effect of impact angle and projectile size on sputtering efficiency of solid benzene investigated by molecular dynamics simulations

Molecular dynamics computer simulations have been used to investigate the effect of the cluster size on the sputtering yield dependence on the impact angle. Ar366 and Ar2953 cluster projectiles with 14.75keV of incident energy are directed at the surface of a solid benzene crystal described by a coarse-grained representation at angles between 0° and 70°. It is observed that the shape of the angular dependence of sputtering efficiency is strongly affected by the cluster size. For the Ar366 cluster, the sputtering yield only slightly increases with the impact angle, has a broad maximum around 40°, and decreases at larger angles. For the Ar2953 cluster, the yield strongly increases with the impact angle, has a maximum around 45° followed by a steep decrease at larger angles. For both investigated cluster projectiles the primary energy is deposited so close to the surface so that the sputtering efficiency only weekly benefits from the shift of the deposited energy profile toward the surface which occurs at larger impact angles. In this study, molecular dynamics computer simulations are used to probe the effect of the impact angle on the efficiency of ejection molecules emitted from solid benzene by 14.75keV Ar366 and Ar2953 clusters.

Simulation of MeV electron energy deposition in CdS quantum dots absorbed in silicate glass for radiation dosimetry

We are currently developing 2D dosimeters with optical readout based on CdS or CdS/CdSe core-shell quantum-dots using commercially available materials. In order to understand the limitations on the measurement of a 2D radiation profile the 3D deposited energy profile of MeV energy electrons in CdS quantum-dot-doped silica glass have been studied by Monte Carlo simulation using the CASINO and PENELOPE codes. Profiles for silica glass and CdS quantum-dot-doped silica glass were then compared.

Significance of time-of-flight ion energy spectrum on energy deposition into matter by high-intensity pulsed ion beam

AbstractEnergy deposition by high-intensity pulsed ion beam into a metal target has been studied with time-of-flight (TOF) of ions which can be related to the original ion kinetic energy E0 and the ion mass with $t_{\rm TOF} \propto 1/\sqrt{2E_{0}/m_{i}}$. It is found that the TOF effect has a profound influence on the kinetic energy distribution of implanted ions and subsequent energy deposition process into the target. The HIPIB of mixed H+ and C+ was extracted from a magnetically insulated ion diode at a peak accelerating voltage of 350 kV, leading to an ion current density of 300 A/cm2 at the target. The widespread ion energy spectrum remarkably varied in shape as arriving at the target surface, from the original Gaussian-like of 80-ns duration to a pulse form of a sharp front and a long tail extending to about 140-ns duration. Energy loss of the mixed ions into a Ti target was simulated utilizing a Monte Carlo method. The energy deposition generally showed a shallowing trend and could be divided into two phases proceeded with sequent arrivals of H+ and C+. Note that, the peak value of deposited energy profile appeared at the beginning of mixed ion irradiation phase, other than the phase of firstly arrived H+ with peak kinetic energy and peak ion current. This study indicated that TOF effect of ions greatly affects the HIPIB-matter interaction with a kinetic energy spectrum of impinging ions at the target, noticeably differing from that of original output of the ion source; consequently, the specific energy deposition phenomena of the widespread ion energy can be studied with the TOF correlation of ion energy and ion current, otherwise not obtainable in common cases assuming fixed ion energy distribution in accordance with the original source output.

Modelling of radiation transfer in metallic powders at laser treatment

Absorptances and deposited energy profiles are calculated for thin layers of metallic powder placed on a reflective substrate at normal incidence of collimated radiation. Radiation transfer equation is analytically solved by the two-flux method. The effective optical parameters of the powder are estimated in geometrical optics approximation taking into account dependent scattering. The cases of specularly and diffusely reflecting particles are studied. Due to multiple reflections in an open pore system, laser radiation can penetrate in powder to considerable depths, much greater than the characteristic particle diameter. Total laser energy absorbed in a thin powder layer on a reflective substrate increases with its thickness but the deposited energy density decreases. Generally, the absorptance and the energy density for specular reflection are slightly greater than these values for diffuse reflection. The results obtained in the limit of deep powder bed essentially agree with known ray tracing simulations. The present model is in good agreement with experimentally observed correlation between effective absorptance of metallic powders and absorptance of corresponding dense metals.

Edge-Enhancement Writing for Electron Beam Nanolithography

A novel method of electron-beam (EB) writing to reduce line-edge roughness (LER) called edge-enhancement writing (EEW) is described. EEW involves writing a fine line along a pattern edge at a high dose. This provides a steep profile of the energy deposited at the edge. The high exposure dose breaks up aggregates of resist polymer at the edge, which cause LER. Moreover, in EEW the background energy in unexposed regions due to proximity effects is also reduced by control of the dose. This increases the energy contrast at the edge, thereby preventing the appearance of aggregates on the sidewall due to undesired dissolution of the edge region. The resulting energy profile is quite different from that obtained with conventional proximity effect corrections. Experimental results and simulations of the deposited energy profile have demonstrated the effectiveness of EEW.

Effect of implant temperature on defects created using high fluence of helium in silicon

Extended defects formed by high-fluence helium implantation in silicon have been studied as a function of the implantation temperature, from room temperature up to 800 °C. Transmission electron microscopy results show that the formation of cavities created by a 50 keV He implantation with a fluence of 5×1016 cm−2 can be divided into three stages. For implantation temperature up to about 300 °C the bubble size is relatively constant but the bubble density decreases due to the increase in dynamic annealing. Above 300 °C, where divacancies are no longer stable and when the helium is mobile, both the density and size of the cavities stay relatively constant. In this stage, helium starts to diffuse out and the cavities become more and more faceted as the temperature increases. Concurrently interstitial-type defects appear: small elongated rod-like defects at relatively low temperatures and large ribbon-like defects at 600 °C. Finally, for implantation at 800 °C, no cavities are formed and only dislocation loops and {113}’s (ribbon-like defects and rod-like defects), are observed depending on the deposited energy profile. At this temperature the defect annealing during implantation becomes efficient to convert ribbon-like defects into dislocation loops.

Inferring ejection distances and a surface energy profile in keV particle bombardment experiments

The disappearance cross-sections and kinetic energy distributions of fragment ions sputtered from polystyrene thin films under 12 keV 69Ga + ion bombardment are measured using a time-of-flight spectrometer. Even though the disappearance cross-sections are often used as an indicator of radial ion beam damage, the derived radii range from 2 to 10 Å depending on the particular hydrocarbon fragment. Therefore, the disappearance cross-sections cannot be directly related to a single global quantity of damage per incident particle. Likewise, the widths of the measured kinetic energy depend on the particular hydrocarbon fragment. Both the disappearance cross-sections and the widths of the kinetic energy distribution superficially relate to fragment mass but the correlation is not perfect. We develop a hypothesis that the disappearance cross-section and the width of the kinetic energy distribution actually correlate with the ejection radius of the particular fragment. Thus, the kinetic energy distributions provide an estimate of the radial extent of the energy density in the ejection region. Our interpretations are supported by molecular dynamics simulation results. For comparison with previously reported data, our results indicate that the deposited energy profile is 4–5 times narrower than for 72.3 MeV, electronic sputtering of PVDF [Phys. Rev. Lett. 77 (1996) 667].

A general expression for the width of a charged-particle track

Theoretical estimates of the dimensions of charged-particle tracks are conventionally based on Monte Carlo simulation of the trajectories of the primary as well as all secondary particles. The present note expresses the lateral width of a track by an integral over the primary cross-section and thus eliminates the need for simulating the primary trajectory. Expressions are given for the second-order and the fourth-order lateral average over the deposited-energy profile. These relations are applicable to tracks generated by either ionization or collision cascades. As an illustration, an analytical estimate is presented for the width of a collisional track generated by Au ions in Au which is of interest in current experimental work on sputtering by gold clusters.

Modelling the assessment of port wine stain parameters from skin surface temperature following a diagnostic laser pulse.

Laser treatment of port wine stains (PWS) has become an established clinical modality over the past decade. However, in some cases full clearance of the PWS cannot be achieved. To improve the clinical results, it is necessary to match the laser treatment parameters to the PWS anatomy on an individual patient basis. Therefore, knowledge of the PWS structure is of great importance. The objective of this study is to describe a diagnostic method to assess the PWS blood vessels depth and diameter from the skin surface temperature-time course following a diagnostic laser pulse. The Monte Carlo (MC) method was used to calculate the deposited laser energy into a port wine stain skin model following irradiation by a diagnostic laser pulse at 577 nm. The heat equation was solved numerically, using the deposited energy profile as the source term, yielding the temperature-time course at the skin surface. Subtraction of "bloodless" skin signal from that of the skin containing blood vessels gives us the net contribution of a heated dermal blood vessel to the skin surface temperature-time behaviour. The net blood vessel signal shows heat-diffusion behaviour and was found to be sensitive to the dermal blood vessel depth and diameter. The time delay for the peak signal temperature to occur depends quadratically on the blood vessel depth. The peak temperature relates linearly to the blood vessel diameter. The degree of epidermal melanin content can also be determined from the immediate temperature rise of the signal. The proposed method easily enables assessment of the blood vessel depth and diameter as well as the epidermal melanin content in a skin model. The method can be applied to a real PWS when using the adjacent normal skin as a reference.

Light distributions in a port wine stain model containing multiple cylindrical and curved blood vessels.

Knowledge of the light distribution in skin tissue is important for the understanding, prediction, and improvement of the clinical results in laser treatment of port wine stains (PWS). The objective of this study is to improve modelling of PWS treated by laser using an improved and more realistic PWS model. Light distributions are calculated by the Monte Carlo method for various PWS blood vessel configurations, such as single and multiple vessels oriented horizontally, curved vessels, and "vertically" oriented vessels. Various vessel sizes and wavelengths are used. Our modelling confirms the concept of selective photothermolysis; 577nm laser light gives maximal deposited energy at the top side of the blood vessels closest to the skin surface and 585nm gives a more uniform energy distribution in the vessel. In the distribution of deposited energy multiple vessels mutually influence each other, because of "shadowing" of diffuse light. Modelling PWS laser treatments with multiple vessels confirms the need for successive treatments of vessels layer by layer. The use of different wavelengths affects the local deposited energy profiles in the blood vessels. It predicts that the significance of 585nm laser light lies in the uniform energy distribution in the vessels rather than in gain in energy deposition with depth. The calculated light distributions provide a more realistic input for modelling thermal damage effects in PWS laser treatment and modelling of the epidermal response in thermal imaging of the PWS blood vessel structure.

Three-dimensional Monte Carlo calculation of Ga+ ion penetration in an a-Se75Ge25 thin film

Three-dimensional (3D) Monte Carlo (MC) simulation for Ga+ ion penetration in an amorphous Se75Ge25 thin film which acts as a positive resist in focused ion beam lithography is presented. For 3D MC simulation, both the depth and the lateral distance in the resist are divided into discrete cubic pixels with 25×25×25 Å. Scattering and stop positions as well as deposited energy density by ∼104 ion trajectories are summed up in these pixels. Also, these MC data are expanded for a focused ion beam with a proper ion dose. Therefore, the 3D stop-ion distributions and the 3D deposited energy profiles by ions implanted into the resist are obtained. The peak concentration of MC simulation associated with a dose of 1015/cm2 and an incident energy of 80 keV amounts to 2×1020 ions/cm3 and is located at a distance of about 400 Å from the surface which is shifted about 40 Å to the surface in comparison with Lindhard, Scharff, and Schiott-based calculation.

Effect of beam condition in variable-shaped electron-beam direct writing for 0.25 μm and below

The effect of incident electron-beam conditions, which are acceleration voltage and beam blur of variable-shaped electron-beam direct writing, is investigated using the deposited energy distribution to realize a fine pattern of ≤0.25 μm in trilayer resist process. The deposited energy distribution is calculated using a three-dimensional Monte Carlo method. In a trilayer resist system, a thin bottom resist layer can be used, because the contrast value derived from the Monte Carlo calculation is independent of the bottom layer thickness. The beam blur of 0.05 μm does not degrade 0.25 μm line-and-space (L/S) patterns, but seriously degrades 0.1 μm L/S patterns. Higher acceleration voltage is effective for improving the contrast. At lower acceleration voltage, the slope of the deposited energy profile defined at the resist bottom is mainly influenced by electron scattering. On the other hand, at higher acceleration voltage, the slope of deposited energy profile mainly depends on the beam blur. The 0.1 μm L/S patterns are expected to be resolved at 30 kV when there is less than 0.02 μm beam blur with trilayer resist system. The possibility of using a single layer resist process for 0.1 μm L/S pattern will be barely realized at the conditions of 50 kV and 0.02 μm beam blur.

Direct determination of a radiation damage profile with atomic resolution in ion-irradiated platinum

The field-ion microscope technique has been employed to determine directly a radiation damage profile, with atomic resolution, in a platinum specimen which had been irradiated at 60 K with 20-keV Kr+ ions to a fluence of 5×1012 cm−2. It is shown that the microscopic spatial vacancy distribution (radiation damage profile) is directly related to the elastically deposited energy profile. The experimentally constructed radiation damage profile is compared with a theoretical damage profile—calculated employing the transport of ions in matter (TRIM) Monte Carlo code—and excellent agreement is obtained between the two. Thus, it is demonstrated that it is possible to go directly from a microscopic spatial distribution of vacancies to a continuous radiation damage profile.

In SituChanneling Study of Ni-P Amorphous Phase Formation

The nature and dose dependence of the disorder produced by 125-keV P implantation in Ni single crystals was studied via in situ channeling experiments using the 380-keV $^{4}\mathrm{He}$ beam of the implanter. At 90 K, disordering (and presumably amorphization) occurs around the depth of the maximum in the implanted P concentrations; at 300 K, it occurs mainly at the surface and progresses inwards until the P projected range and peak in deposited energy profile are reached. In both cases, the lattice is randomized at a local P concentration of \ensuremath{\sim}0.20.

Computer evaluation of primary deposited energy profiles in ion-implanted silicon under channeling conditions

Primary deposited energy profiles produced by 40-keV P ions channeled along the [110] direction in silicon are computed. The model takes into account impact-parameter-dependent electronic energy loss and dechanneling mechanisms, such as thermal vibrations, electronic scattering, and a surface oxide layer. The deposited energy is converted to damage by using Kinchin and Pease's model and distributed along the range of primary recoils or dechanneled ions in two ways: either uniformly or localized at the average damage depth, evaluated using Winterbon, Sigmund, and Sanders's theory. The model is tested for the random case with experiments available. When no dechanneling mechanism is present, radiation damage is reduced by a factor of 10 compared to the random case. Thermal vibrations increase damage almost uniformly along all the channeled path, while the oxide layer increases the damage mostly at the surface. The combined effect of these mechanisms gives a reduction of radiation damage of only a factor of [inverted lazy s]2 compared to the random case. The results show also that damage is mostly concentrated at the surface: about 85% in the first 2000 Å compared to a maximum penetration of 7600 Å.