Showing papers in "Applied Physics A in 1992"

Nanosecond and femtosecond excimer laser ablation of fused silica

Abstract: Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 μm/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm2) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.

Gold and Platinum Diffusion: The Key to the Understanding of Intrinsic Point Defect Behavior in Silicon

Abstract: The study of gold and platinum diffusion is found to allow the separate observation of the intrinsic point defects, i.e., of silicon self-interstitials and of vacancies. The diffusion of gold in float zone (FZ) silicon is found to be dominated by the kick-out mechanism for temperatures of 800° C and higher. The diffusion of platinum in FZ silicon is described by the kick-out mechanism for temperatures above approximately 900° C, whereas for temperatures below approximately 850° C the dissociative mechanism governs platinum diffusion. As a result of numerical simulations, we suggest a complete and consistent set of parameters which describes the diffusion of platinum in silicon in the temperature range from 700° C to 950° C and the diffusion of gold in the temperature range from 800° C to 1100° C. The generation or recombination of self-interstitials and vacancies is found to be ineffective at least below 850° C. The concentration of substitutional platinum is determined by the initial concentration of vacancies at diffusion temperatures below 850° C. Platinum diffusion below 850°C can be used to measure vacancy distributions in silicon quantitatively.

Electrical and thermal properties of chalcogenide glass system Se75Ge25-xSbx

Abstract: The dc and thermal conductivities of five different compositions of the chalcogenide glass system Se75Ge25−xSbx have been studied in a temperature range below Tg. The dc conductivity results indicate that each composition has a single activation energy in the considered temperature range. The coefficient of thermal conductivity increased linearly with temperature below Tg for the compositions investigated. The increase of Sb content in the chalcogenide glass system leads to an increased coefficient of electrical conductivity σ, an increased coefficient of thermal conductivity ψ, and to a decreased activation energy Eσ and pre-exponential factor σ0. The observed compositional dependencies of σ and Eσ have been correlated with the increase of weak bond density and the decrease of covalent bond density in the structure of the compositions investigated with increasing Sb content at the expense of Ge content. The decrease in σ0 and the increase in ψ has been, respectively, correlated with the decrease in mobility and the increase in phonon velocity.

Verification of a generalized Planck law for luminescence radiation from silicon solar cells

Abstract: A generalization of Planck's radiation law based on the principle of detailed balance predicts the emission of luminescence radiation from absorption data for direct transitions in semiconductors. Its validity for indirect transitions is questionable due to the participation of phonons. We have tested the validity for the indirect transitions in Si by measuring absolute values of the emission intensity from Si solar cells under forward bias at room temperature and find good agreement with theoretical predictions based on existing absorption data. The generalized Planck law, thus verified for the indirect transitions in Si, allows to determine the performance of solar cell materials from measuring the absolute intensity of their emission of luminescence radiation when irradiated by the sun.

Excimer laser ablation of polyimide in a manufacturing facility

Abstract: High power excimer laser ablation projection tools have been used successfully in IBM manufacturing for about four years. One hundred fifty W XeCL, 300 mJ per pulse and 500 mJ per pulse lasers, running at 500 Hz and 300 Hz, respectively, have been run successfully for over 2 billion pulses. Excimer laser projection ablation tools are very similar to photo-expose tools, and their similarities and differences are explained. The laser and its gas handling, beam delivery, beam homogenization, optics, debris, part handling, and maintenance are discussed.

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

Abstract: H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p 1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p 1/3 power law is provided.

Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

Abstract: We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers.

Laser induced forward transfer: The effect of support-film interface and film-to-substrate distance on transfer

Abstract: A comparative study on metal pattern deposition of mm2-area by ablating chromium and titanium thin films from an optically transparent support and transferring the ablated material onto another substrate in close proximity with a single laser pulse (LIFT) is reported. The role of support-film interface and film-to-substrate distance in determining both ablation and transfer is discussed. The sequence of events as a function of processing fluence is interpreted by comparing experimental data with calculated temperature distributions. In the case of poorly adhering films the transfer yield is independent of film-to-substrate distance between 0 and 60 μm throughout the fluence range studied. The transmittance of the ablated areas of well adhering films decreases and that of the corresponding prints increases with increasing distance as evaporation becomes dominant.

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

Abstract: An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.

Calculated surface energies of the 4d transition metals: A study of bond-cutting models

Abstract: The surface energies of the low-index surfaces of the 4d transition metals were calculated using density-functional theory and the full-potential linear muffin-tin orbital method. The results are discussed in the context of bound-cutting models. It is found that the usual approach, which ignores the decrease of the bond strength with increasing coordination number, is incompatible with the calculated results. The coordination number — bond strength relation was calculated explicitly for several metals, showing that the bond strength decreases roughly as the reciprocal of the square-root of the coordination number. A modified model which takes this into account correctly describes the magnitude and the surface depencence of the calculated surface energies. In addition, we discuss the previously ignored role of the free-atom orbital structure in bond-cutting models.

CN temperatures above laser ablated polyimide

Abstract: Polyimide is readily ablated by UV excimer lasers pulses. There exists an ongoing effort to determine the temperature (T) of the surface during the ablation process. The present experiment evaluates the temperature by means of high resolution laser-induced fluorescence measurements near the bandhead of the CN radical. The rotational state occupation indicates T=1710±140 K for ArF (193 nm) ablation with 100 to 500 mJ/cm2. This temperature is near the extrapolation of earlier results at lower fluences (≤80 mJ/cm2). Furthermore, modeling predicts T values essentially equal to the present experimental result when the following are included: thermal diffusion in the solid, above surface UV absorption and increasing specific heat for temperatures above ambient. The present T-value reinforces the concept that thermal energy probably promotes the intermediate steps in photoablation.

Direct laser ablation of sub-100 nm line structures into polyimide

Abstract: Periodic line structures with a period of 167 nm and linewidths varying from 30 to 100 nm have been produced in polyimide by direct ablation with a KrF laser using an interferometric technique. The characteristics of this interferometer as it applies to the ablation of these line structures, including linewidth and alignment sensitivity, are analyzed. The ability to control the linewidth by varying the average incident fluence is described theoretically and demonstrated experimentally. This externally generated period of 167 nm also prevents the spontaneous growth of laser induced periodic surface structures (LIPSS).

Formation of interface bubbles in bonded silicon wafers: A thermodynamic model

Abstract: A thermodynamic model of the formation of unbonded areas or bubbles generated at the interface of bonded silicon wafers in the temperature range of 200–800°C is presented. Within this model it is assumed that the desorption of hydrocarbon contamination at the silicon wafer surfaces leads to small hydrocarbon molecules which are mobile at the bonding interface. When the vapor pressure generated by these molecules overcomes the interface bonding strength, interface bubbles are nucleated. These bubbles grow by incorporating further hydrocarbon and also possible hydrogen molecules. The model semiquantitatively explains all the essential features of interface bubble formation observed experimentally.

Growth, spectroscopic and photorefractive investigation of vanadium-doped cadmium telluride

Abstract: We present new results on the growth of semi-insulating vanadium-doped cadmium telluride crystals and their characterization by different optical techniques such as photoinduced current transient spectroscopy, absorption, photoconductivity spectra, and photorefractive wave mixing. Our joint research program aims at developing optimized crystals for efficient optical processing in the near infrared through the photorefractive effect.

Time-resolved spectra of excited-state absorption in Er3+ doped YAlO3

Abstract: A pump- and probe-beam technique is used for measuring time-resolved excited-state absorption (ESA) and stimulated-emission (SE) spectra of Er3+ doped YAlO3. The Er3+ 4I15/2 -> 4F7/2 transition of the sample is excited at 488 nm by an excimer laser pumped dye laser. The ESA and SE of broadband xenon flashlamp light is monitored between 300 and 860 nm by an optical multichannel analyzer (OMA). The analysis of the experimental results provides information on the effective cross sections ESA and SE originating from several levels and on the populations of these levels. To our knowledge this represents the first detailed investigation of time-resolved ESA and SE over a broad spectral range in rare-earth doped materials.

Enhanced adhesion of metal films on PET after UV-laser treatment

Abstract: The adhesion-force of thin metal films on PET foils can be significantly improved by UV excimer-laser irradiation of the polymer surface prior to metal deposition. The laser fluences required are well below the ablation threshold.

Absorption-edge calculations of inorganic nonlinear optical crystals

Abstract: A theoretical model suitable for calculating absorption edges of inorganic nonlinear optical (NLO) crystals is introduced. This model is proved to be useful to elucidate the relationship between electronic structures of NLO-active groups and macroscopic properties of absorption edges on the UV side of most of the inorganic nonlinear optical crystals. A systematic calculation of absorption edges on the UV side for several important inorganic NLO crystals is carried out by means of DV-SCM-Xα method and all calculated results are in good agreement with experimental data. These inorganic NLO crystals include LiB3O5(LBO), β-BaB2O4(BBO), KB5, KDP, Na2SbF5, Ba2TiSi2O8, iodate and NaNO2. The calculated energy level structures of LiB3O5 and β-BaB2O4 crystals are compared with the measured XPS spectra. The unusual transparent spectra of KB5 and KDP crystals are partly explained from the microstructure point of view. The effect of lone electron pair in iodate and NaNO2 crystals on their absorption edges are discussed. All these results show that Anionic Group Theory of Nonlinear Optical Crystals is useful to evaluate the absorption edges of the inorganic nonlinear optical crystal and is a powerful tool in a Molecular Engineering approach to search for new nonlinear optical materials.

A study of blast waveforms detected simultaneously by a microphone and a laser probe during laser ablation

Abstract: We examine blast waves generated in air during irradiation of absorbing samples with Nd: YAG laser pulses of fluences exceeding the ablation threshold. Blast waves were detected simultaneously by a wideband microphone and a laser beam deflection probe. By a comparative analysis of both signals in the time and frequency domain we investigate characteristic features of their nonlinear waveform evolution. To explain the observed phenomena we employ the weak shock solution of the point explosion model.

NQR device for detecting plastic explosives, mines, and drugs

Abstract: We describe a NQR device to detect plastic explosives, mines, and drugs and discuss design considerations

Characterization of various aluminium oxide layers by means of spectroscopic ellipsometry

Abstract: This paper reports the characterization of both barrier type and porous type anodic oxide films on aluminium by means of spectroscopic ellipsometry (SE). In order to show the capabilities of the technique for quantitative determination of the layer characteristics, results based on ellipsometric data are correlated with complementary information from the analytical techniques transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). It is concluded that ellipsometry yields an accurate characterization for the thicknesses and the interfacial properties of both the barrier layer and the porous layer. The porosity of the porous layer, determined with SE, is found to be in good agreement with the results obtained from TEM.

Charge transport processes in LiNbO3:Fe at high intensity laser pulses

Abstract: Light-induced refractive index changes in LiNbO3:Fe crystals are investigated at high light intensities (>109 Wm−2). Holographic gratings are recorded and erased with frequency-doubled pulses of a Q-switched Nd:YAG laser. We find new intensity dependent contributions to the holographic sensitivity, to the photoconductivity, and to the saturation value of refractive index change. Light-induced absorption changes are also detected. These results indicate that the Fe2+/Fe3+ charge transport model, well established for low intensities, has to be modified for high intensities by assuming additional centers which trap and supply electrons.

Diffusion of silicon in titanium nitride films. Efficiency of TiN barrier layers

Abstract: Two kinds of reactively evaporated titanium nitride films with columnar (B0 films) and fine-grained film structure (B+ films) have been examined as diffusion barriers, preventing the silicon diffusion in silicon devices. The silicon diffusion profiles have been investigated by 2 MeV 4He+ Rutherford backscattering spectrometry (RBS) after annealing at temperatures up to 900° C, in view of application of high-temperature processes. The diffusivity from 400 to 900° C: D (m2 s−1)=2.5×10−18 exp[−31 kJ/mol/(RT)] in B0 layers and D (m2 s−1)=3×10−19 exp[−26 kJ/mol/(RT) in B+ TiN layers. The diffusivities determined correspond to grain boundary diffusion, the difference being due to the different microstructure. The very low diffusivity of silicon in B+ TiN layer makes it an excellent high-temperature barrier preventing silicon diffusion.

Structure formation in UV-laser-ablated polyimide foils

Abstract: The formation of nap-type and wall-type structures on laser-irradiated polyimide (PI) films is reported for the first time. These investigations demonstrate that such structures can develop on non-melting polymer surfaces.

Protons and other defects in Fe-doped KTaO3

Abstract: The properties of defects in Fe-doped KTaO3, both in the form of single crystals and polycrystalline ceramics, have been investigated for a wide range of Fe concentrations. The techniques employed included infrared (IR) absorption, electron paramagnetic resonance (EPR), and ac electrical conductivity together with complex-impedance analysis. Samples were pretreated (at 900° C) in water vapor to introduce protons which take the form of OH· O defects, and were also treated at various oxygen partial pressures. A calibration of the OH− IR absorption band was carried out with the aid of a deuterium nuclear-probe method. EPR showed cubic and axial Fe3+ spectra, but only the axial spectrum appeared for crystals with high Fe concentrations. Pre-dominantly proton conductivity was observed for samples treated in reducing atmospheres, and a proton-hopping activation energy of 0.73 eV was deduced. For samples treated in high oxygen pressures, however, hole conduction dominated. Evidence for proton interaction effects was also found, but the nature of the traps is not clear.

Spatially confined laser-induced damage of Si under a liquid layer

Abstract: Experimental results are presented on the Si damage induced by a copper-vapor laser beam under a layer of chemically neutral liquid (H2O, DMFA, DMSO). The lateral dimensions of the damaged area coincide with those of laser beam, while the depth of the damaged area increases with time. The rate of hole formation may be as high as 75 μm/s. The qualitative interpretation of the results is given on the basis of laser generation of vacancies, and their combination and redistribution in the field of the inhomogeneous mechanical strains due to multiple heating/cooling cycles of the surface of the semiconductor.

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

Abstract: The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.

Efficient ultraviolet photorefraction in LiNbO3

Abstract: A nominally undoped LiNbO3 crystal with a slightly broadened absorption edge is used to study beam coupling effects in the UV at 351 nm. At this wavelength the crystal exhibits a diffusion-dominated charge transport mechanism, which allows steady state beam amplification of up to 700 times, comparable to BaTiO3 in the visible. The used crystal material was characterized by an absorption coefficient α=2.68 cm−1 at 351 nm and a maximal gain coefficient Г=13.94 cm−1. This high gain value in the UV can be attributed to a hole diffusion-dominated charge transport mechanism together with a low bulk photovoltaic effect. We measured photovoltaic fields of the order of 550 V/cm.

Thin Zirconium Nitride Films Prepared by Plasma-Enhanced CVD

Abstract: Thin films of zirconium nitride have been deposited at temperatures as low as 573 K by PECVD using tetrakis(diethylamido)zirconium, Zr[N(C2H5)2]4 as precursor. The influence of the various experimental parameters on film properties and deposition rates has been studied. Under most experimental conditions hard coatings of good adherence and low carbon contamination resulted.

An atomic view of crystal growth

Abstract: The ability to routinely observe individual metal atoms adsorbed on metal surfaces using field ion microscopy has made it possible to directly examine atomic events important in the growth of crystals from the vapor. On the atomically smooth (111) plane of iridium, there are now available observations of the binding sites for atoms and of condensation from the vapor, together with measurements of the diffusion of atoms over the surface, as well as studies of their subsequent incorporation at both ascending and descending steps. These results are briefly reviewed, revealing a picture generally more varied and interesting than envisioned in classical theories of crystal growth.

Treatment of roughness and concentration gradients in total Reflection X-ray fluorescence analysis of surfaces

Abstract: Total Reflection X-ray fluorescence (TXRF) spectrometry, a new technique for surface and layer analysis, was originally confined to ideal smooth interfaces. In practice, however, one has to cope with more or less rough surfaces. Therefore, modelling calculations have been conducted to consider the consequences of residual roughness on the fluorescence signal at grazing incidence. The model used was verified experimentally on surfaces which exhibit peak-to-valley roughnesses ranging from 5 to 4000 nm. In addition, concentration changes occurring in the zone of roughness in the surface layer of a high grade steel after exposure to nitric acid were determined.