Modelling laser cleaning of low-absorbing substrates: the effect of near-field focussing

A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with the geometrical optics approximation. This permits the estimation of the beam width at the substrate surface and the focal distance of the radiation coming from the spherical particle. These parameters are used to find the distribution of intensity within the low-absorbing substrate from the formula for a focussed Gaussian beam. This is in contrast with most other models of laser cleaning, which assume that all absorption occurs at the surface of the substrate. The energy criterion was used to calculate the threshold fluence. The model predicts threshold fluences of the order of 10³ J/cm² for silica spheres having a diameter of the order of a micron on silica substrates, assuming adhesion by van der Waals force. As this is well above the damage threshold for silica, it effectively predicts that laser cleaning of silica spheres from silica will be impossible. For glass slides the threshold fluence is predicted to be a factor of 10^-4 times smaller than that for silica slides (about 0.1 J/cm²). This is due to the much higher absorption of glass compared to that of silica at 248 nm. PACS 42.62.Cf; 81.65.Cf     

Formation and growth of embedded indium nanoclusters by In<Superscript>2<Emphasis Type="Bold">+</Emphasis></Superscript> implantation in silica

Indium nanoclusters are synthesized in an amorphous silica matrix using an ion-implantation technique. Indium ions (In²⁺) with energy of 890 keV are implanted on silica to fluences in the range of 3×10¹⁶–3×10¹⁷ cm^-2. The formation of indium nanoclusters is confirmed by optical absorption spectrometry and glancing incidence X-ray diffraction studies. A low frequency Raman scattering technique is used to study the growth of embedded indium nanoclusters in the silica matrix as a function of fluence and post-implantation annealing duration. Rutherford backscattering spectrometry studies show the surface segregation of implanted indium. Photoluminescence studies indicate the formation of a small quantity of indium oxide phase in the ion-implanted samples. PACS 85.40.Ry; 78.67.Bf; 73.20.Mf; 82.75.Fq     

IR spectra of fused silica irradiated by fast neutrons

IR spectroscopy has been used to study the process of structural damage and changes in some characteristics of fused silica irradiated by fast neutrons over a very broad fluence range (10¹⁷–10²¹ cm⁻²). Features of the change in spectral characteristics of the bending and stretching vibrations of the bridge bonds have been identified, and also a comparative analysis has been carried out with radiation-induced changes in a series of optical spectra in the UV and visible regions, and structural parameters and other characteristics of wafers irradiated by different fluences. A correspondence has been established between the features of the radiation-induced changes in the optical, luminescence, and structural properties, and extremal points have been observed on the dose dependences. Based on the results obtained, a mechanism of radiation-induced rearrangement of the silica structure is suggested. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 494–497, July–August, 2006.     

中子辐照对AlGaN/GaN高电子迁移率晶体管器件电特性的影响

本文采用能量为1 MeV的中子对SiN钝化的AlGaN/GaN HEMT(高电子迁移率晶体管)器件进行了最高注量为1015cm-2的辐照.实验发现:当注量小于1014cm-2时,器件特性退化很小,其中栅电流有轻微变化(正向栅电流IF增加,反向栅电流IR减小),随着中子注量上升,IR迅速降低.而当注量达到1015cm-2时,在膝点电压附近,器件跨导有所下降.此外,中子辐照后,器件欧姆接触的方块电阻退化很小,而肖特基特性退化却相对明显.通过分析发现辐照在SiN钝化层中引入的感生缺陷引起了膝点电压附近漏电流和反向栅泄漏电流的减小.以上结果也表明,SiN钝化可以有效地抑制中子辐照感生表面态电荷,从而屏蔽了绝大部分的中子辐照影响.这也证明SiN钝化的AlGaN/GaN HEMT器件很适合在太空等需要抗位移损伤的环境中应用.     

Nitrogen plasma-based ion implantation of poly(tetrafluoroethylene): Effect of the main parameters on the surface properties

The surface of poly(tetrafluoroethylene) (PTFE or Teflon) was treated by nitrogen plasma-based ion implantation. Accelerating voltages between 15 and 30 kV, fluences between 1 × 10¹⁷ and 3 × 10¹⁷ cm⁻² and fluence rates between 3 × 10¹³ and 7 × 10¹³ cm⁻² s⁻¹ were applied. The effects of these main parameters were examined on the evolution of surface chemical composition, mean roughness, abrasive wear, wettability and surface electrical resistance. The aim was to obtain relationships, enabling to control the surface properties examined.The F/C atomic ratio determined by XPS strongly decreased, correlating inversely with voltage. The mean surface roughness characterized by topography measurements, increased, correlating directly with fluence and inversely with voltage. The wear volume obtained by multipass scratch tests did not show clear relationship with the main process parameters, however, it increased upon treatment with the increase of surface roughness and O/C atomic ratio. The water contact angle increased at low voltages and high fluences, due to preferential increase of roughness, and decreased at high voltages and low fluences, owing to intense defluorination and incorporation of N and O. The electrical resistance of the PBII-treated surfaces decreased by several orders of magnitude, showing a significant inverse correlation with fluence. It continued to decrease for samples exposed to air, primarily after treatments performed with low fluences, due to post-treatment type oxidation.     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Clusterization of vacancy defects in ZnO irradiated with 2 MeV O

Slow positrons have been used to study ZnO layers grown on a-axis sapphire and irradiated by 2 MeV O⁺ ions to fluences from 10¹² cm⁻² to 10¹⁷ cm⁻². At low fluences Zn vacancies are observed, and their introduction rate is estimated as 2000 cm⁻¹. At the highest fluences of 10¹⁶-10¹⁷ cm⁻² vacancy clusters are formed. The extent of the primary damage and its recovery is discussed.     

Investigations on the effect of argon ion bombardment on the structural and optical properties of crystalline gallium antimonide

The effects of bombardment of 250 keV argon ions in n-type GaSb at fluences 2×10¹⁵ and 5×10¹⁵ ions cm^?2 were investigated by high-resolution X-ray diffraction (HRXRD), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). HRXRD studies revealed the presence of radiation-damaged layer (strained) peak in addition to the substrate peak. The variation in the lattice constant indicates the strain in the bombarded region. The out-of-plane (?_⊥) and in-plane strains (?_|) determined from the profiles of several symmetric and asymmetric Bragg reflections, respectively, were found to change with the ion fluence. Simulations of XRD patterns using dynamical theory of X-ray scattering (single-layer model) for the damaged layer yielded good fits to the recorded profiles. FTIR transmission studies showed that the optical density (α·d) of GaSb bombarded with different fluences increases near the band edge with increase in ion fluence, indicating the increase in the defect concentration. The density of the defects in the samples bombarded with different fluences was in the range of 3.20×10²¹–3.80×10²¹ cm^?3. The tailing energy estimated from the transmission spectra was found to change from 12.0 to 58.0 meV with increasing ion fluences, indicating the decrease of crystallinity at higher fluences. SEM micrographs showed the swelling of the bombarded surface of about 0.33 μm for the fluence of 2×10¹⁵ ions cm^?2, which increased to 0.57 μm for the fluence of 5×10¹⁵ ions cm^?2.     

Preferential etching of ion-bombarded GaAs

Ion bombardment of GaAs above fluences of the order of 2 × 10¹³ 400-keV Xe-ions/cm² produces a highly strained surface which is elevated several hundred Angstroms above the adjacent masked surface. The irradiated area etches rapidly to yield circular etch pits ～ 0.3 μ in depth and 0.2 μ in diameter. At lower fluences, observations show much less expansion, slower crystalline etching, and some evidence of triangular etch pits and slip planes. These results are attributed to expansion in the implanted layer which results in high surface strain and the generation of dislocations to accommodate the mismatch of lattice parameters.     

Effect of ion irradiation on the thermoluminescence properties of K2Ca2(SO4)3 phosphor

This paper presents the thermoluminescence (TL) studies of ion-irradiated potassium–calcium mixed sulfate phosphor. The sample was prepared by the solid-state diffusion method. The X-ray diffraction study of the prepared sample suggests an orthorhombic structure with an average particle size of 0.16 μ m. The samples were irradiated with 1.2 MeV argon ions at fluences varying between 10¹¹ and 10¹⁵ ions/cm². The argon ions penetrate to a depth of 1.93 μ m and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects such as oxygen vacancies, radicals and color centers are formed in the sample. TL glow curves were recorded for each of the ion fluences. A linear increase in the intensity of TL glow peaks was found with an increase in the ion dose from 72 kGy to 720 MGy. The kinetic parameters associated with the prominent glow peaks were calculated using glow curve deconvolution, different glow curve shapes and sample heating rate methods.     

The influence of laser-induced surface modifications on the backside etching process

Spectroscopic measurements in the UV/VIS region show reduced transmission through laser-induced backside wet etching (LIBWE) of fused silica. Absorption coefficients of up to 10⁵ cm⁻¹ were calculated from the transmission measurements for a solid surface layer of about 50 nm. The temperatures near the interface caused by laser pulse absorption, which were analytically calculated using a new thermal model considering interface and liquid volume absorption, can reach 10⁴ K at typical laser fluences. The high absorption coefficients and the extreme temperatures give evidence for an ablation-like process that is involved in the LIBWE process causing the etching of the modified near-surface region. The confinement of the ablation/etching process to the modified near-surface material region can account for the low etch rates observed in comparison to front-side ablation.     

Non linear volume expansion in helium implanted erbium metal films

Large dimensional expansion has been observed at room temperature in erbium metal films implanted at room temperature with high fluences of helium. The interferometrically measured film thickness increases linearly with fluence up to a critical dose of 3 × 10¹⁷ He+/cm² (E = 160 keV) and is superlinear at higher fluences. Annealing at 400°C causes a reduction of the induced expansion for fluences below the critical dose without apparent release of helium. Annealing of samples implanted to fluences greater than 3.5 × 10¹⁷ He⁺/cm² causes accentuated expansion which is accompanied by formation and rupture of bubbles at the film surface.     

Dynamic behavior of implanted nitrogen in zirconium at various stages of implantations

Abstract

Depth profiles of nitrogen implanted into Zr with an energy of 50 keV were calculated by dynamic SASAMAL code with three different assumptions for the diffusion of excess atoms over stoichiometry, i.e., ‘no diffusion', ‘both-sides-diffusion’ and ‘upward-diffusion'. To distinguish nitrogens implanted certain stage of implantations, alternate implantations of ¹⁵N and ¹⁴N were used. The results were compared with the experimental results by the resonance nuclear reaction analysis, NRA. For ¹⁵N implantation with fluences from 1 × 10¹⁷ to 1 × 10¹⁸ ions/cm², the calculated results with ‘upward-diffusion’ agreed very well with the NRA results for all fluences. For the depth profile of pre-implanted ¹⁵N (1 × 10¹⁷ ions/cm²), which was changed by the subsequent ¹⁴N implantation with fluences of 1 ～ 10 × 10¹⁷ ions/cm², the agreement with the NRA results was satisfactory until the ¹⁴N fluence did not exceed 5 × 10¹⁷ ions/cm², but for higher fluences, the retained probabilities of ¹⁵N obtained by the ‘upward-diffusion’ code were too low compared with the experimental value obtained by NRA. For the depth profiles of ¹⁵N (1 × 10¹⁷ ions/cm²) implanted following after implantations of ¹⁴N with fluences of 1 ～ 10 × 10¹⁷ ions/cm², the agreement with the NRA results was quite good for all ¹⁴N fluences. It is concluded that the approximation of ‘upward-diffution’ is proper satisfactorily for the treatment of atoms implanted at the final stage of implantations, but a problem is left for the treatment of atoms implanted at the early stage of implantations.     

50 MeV Li3+ ion irradiation effect on structural, electrical, and dielectric properties of PVA-based nanocomposite polymer electrolytes

Nanocomposite polymer electrolyte thin films of polyvinyl alcohol (PVA)-orthophosphoric acid (H₃PO₄)-Al₂O₃ have been prepared by solution cast technique. Films are irradiated with 50 MeV Li³⁺ ions having four different fluences viz. 5?×?10¹⁰, 1?×?10¹¹, 5?×?10¹¹, and 1?×?10¹² ions/cm². The effect of irradiation on polymeric samples has been studied and characterized. X-ray diffraction spectra reveal that percent degree of crystallinity of samples decrease with ion fluences. Glass transition and melting temperatures have been also decreased as observed in differential scanning calorimetry. A possible complexation/interaction has been shown by Fourier transform infrared spectroscopy. Temperature-dependent ionic conductivity shows an Arrhenius behavior before and after glass transition temperature. It is observed that ionic conductivity increases with ion fluences and after a critical fluence, it starts to decrease. Maximum ionic conductivity of ～2.3?×?10^?5 S/cm owing to minimum activation energy of ～0.012 eV has been observed for irradiated electrolyte sample at fluence of 5?×?10¹¹ ions/cm². The dielectric constant and dielectric loss also increase with ion fluences while they decrease with frequency. Transference number of ions shows that the samples are of purely ionic in nature before and after ion irradiation.     

Laser induced ion emission from wide bandgap materials

At fluences well below the threshold for plasma formation, we have characterized the direct desorption of atomic ions from fused silica surfaces during 157 nm irradiation by time-resolved mass spectroscopy. The principal ions are Si⁺ and O⁺. The emission intensities are dramatically increased by treatments that increase the density of surface defects. Molecular dynamics simulations of the silica surface suggest that silicon ions bound at surface oxygen vacancies (analogous to E′ centers) provide suitable configurations for the emission of Si⁺. We propose that emission is best understood in terms of a hybrid mechanism involving both antibonding chemical forces (Menzel-Gomer-Redhead model) and repulsive electrostatic forces on the adsorbed ion after laser excitation of the underlying defect.     

Study of water uptake in poly(ether ether ketone) irradiated by MeV O+ and Au+ ions

Poly(ether ether ketone) (PEEK) was irradiated with 4?MeV O⁺ and 5 and 10?MeV Au⁺ ions to the fluences from 10¹² to 10^14?cm^?2 and then treated in 5 M/l water solution of LiCl for one month at room temperature. After drying and removal of LiCl surface contamination, the depth distribution of LiCl embeded in PEEK was measured by the neutron depth profilig method (NDP) sensitive to ⁶Li isotope. Embeded LiCl is believed to map distribution of water diffusing into PEEK interior. The results show that the PEEK irradiated to the fluences above 1.10¹³cm^?2 is prone to water penetration to the depths of few microns. On the pristine PEEK and that irradiated to lower ion fluences only a surface Li contamination is observed.     

Change of sheet resistance of high purity alumina ceramics implanted by Cu and Ti ions

High purity alumina ceramics (99% Al₂O₃) was implanted by copper ion and titanium ion in a metal vapour vacuum arc (MEVVA) implanter, respectively. The influence of implantation parameters was studied varying ion fluence. The samples were implanted by 68 keV Cu ion and 82 keV Ti ion with fluences from 1 × 10¹⁵ to 1 × 10¹⁸ ions/cm², respectively. The as-implanted samples were investigated by scanning electron microscopy (SEM), glancing X-ray diffraction (GXRD), scanning Auger microscopy (SAM), and four-probe method. Different morphologies were observed on the surfaces of the as-implanted samples and clearly related to implantation parameters. For both ion implantations, the sheet resistances of the alumina samples implanted with Cu and Ti ion fluences of 1 × 10¹⁸ ions/cm², respectively, reached the corresponding minimum values because of the surface metallization. The experimental results indicate that the high-fluence ion implantation resulted in conductive layer on the surface of the as-implanted high purity alumina ceramics.     

Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pump-probe microscope. It allows recording images of the laser-excited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within ≈1.5 ps, while the normalized transmission decreases within ≈400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma is ≈50 ps, and its maximum electron density amounts to 5.5×10²² cm⁻³.     

Supra-atomic structure of radiation-induced defects in synthetic quartz from neutron scattering data

The nanostructure of synthetic quartz samples irradiated with fast reactor neutrons with energies E n > 0.1 MeV has been studied by small-angle neutron scattering. The fluences are varied from 10¹⁷ to 2 × 10²⁰ neutrons/cm². In the quartz samples irradiated with fluences higher than 10¹⁷ neutrons/cm², point, extended (dislocation loops), and volume defects, namely, thermal peaks up to 50 nm in radius, are observed over the entire volume. At a fluence of 2 × 10²⁰ neutrons/cm², the total fraction of the formed defect regions, where the material is in a noncrystalline state, exceeds 10% of the sample volume. The data on the formation of a metamikt glassy phase in the quartz sample have been obtained.     

Structure of defects in Pt induced by neutron and ion irradiation

The effect of formation of a nanocrystalline structure in the near-surface layer of platimun (99.99%) as a result of 30-keV Ar ion bombardment up to fluences of 10¹⁶–10¹⁷ cm^?2 was discovered by the direct method of field ion microscopy. The spatial distribution and structure of radiation damage in Pt was established in the case where Pt is bombarded by fast neutrons (E > 0.1 MeV) up to fluences of 6.7 × 10¹⁷ and 3.5 × 10¹⁸ cm^?2 in the RWW-2M reactor at a temperature of ～310 K.