Pulse-parameter dependence of the configuration characteristics of a micro-structure in fused SiO2 induced by femtosecond laser pulses

The fabrication of a micro-structures in fused SiO₂ by femtosecond laser pulses was studied in the range of pulse duration 150∼500 fs, pulse energy 60 nJ to ∼7 μJ and wavelengths 400 nm and 800 nm. The characteristics of the cross section of the micro-structure were explained with the multiphoton-ionization and avalanche-ionization processes in the femtosecond laser-induced breakdown. The characteristics of the longitudinal length of the micro-structure relating to the direct-focusing and self-action effects are also discussed. Received: 10 January 2001 / Accepted: 7 April 2001 / Published online: 23 May 2001     

Characterisation of structured thin films made from complex materials by photoabsorption spectromicroscopy

2 Al₃ and YBa₂Cu₃O₇/PrBa₂Cu₃O₇. To investigate devices built from these complex materials we applied element-sensitive photoemission electron microscopy (PEEM). Information about the chemical composition of the imaged sample can be obtained by PEEM via tuning the photon energy to X-ray absorption edges. To apply spectromicroscopy we acquired microscopic images using photon energies near and at the edges. Such images give the lateral distribution of a specific element. Microspectroscopy is performed by recording the intensity of the true secondary electrons in selected spots during a sweep of the photon energy. The main aim of our work was to observe oxygen-related defects and changes in the composition affecting the physical properties of the materials. Therefore, we applied both methods to micro-patterned devices using soft-X-ray synchrotron radiation and found that small local defects and chemical differences can be easily detected. Such defects and chemical differences are quite critical to the physical properties of the devices, since they simulate spurious effects thus influencing the reliability of the devices. Received: 13 March 1998/Accepted: 22 April 1998     

Computer simulation of pulsed laser processing of amorphous Si

The paper introduces a 3D computer simulation model of the melting and recrystallization process of amorphous Si induced by pulsed laser irradiation. The model takes into account the temperature dependence of thermal and optical properties of crystalline, amorphous and liquid Si. The melting process is described by introducing for each volume element of melt pool the characteristic times of beginning of melting, end of melting and nucleation of a stable nucleus. The solution of heat equations of liquid and solid phases also provides one with the nucleation rates and temperatures. These data enable one to discriminate whether amorphous or crystalline phases are really allowed to be formed. Two examples of computer simulation are carried out to show the outputs of the model. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

Pulsed laser deposition of ZnO as conductive buffer layer of (001)-LiNbO3 thin films

1 (LO) mode peak at 579 cm^-1, indicating oxygen deficiency in the films. Significant dependence of the electrical property for the films on substrate temperature was shown. The I–V relation of the films exhibited non-ohmic behavior. Whereas the films deposited above 500 °C showed a metal-like property, at a lower substrate temperature semiconducting thin films were achieved. The (001)-oriented LiNbO₃/ZnO heterostructure was successfully prepared on quartz fused and (001) sapphire plates. Received: 6 April 1998/Accepted: 26 May 1998     

Extremely high-aspect-ratio patterns in macroporous substrate by focused-ion-beam etching: the realization of three-dimensional lattices

We show the very particular behavior of focused-ion-beam etching in macroporous silicon. We demonstrate that, contrary to bulk samples, a porous substrate allows extremely high-aspect-ratio patterns to be etched at submicrometer scales. Thanks to the pre-introduced porosity, the secondary effects that limit the pattern depth in bulk-sample etching, namely the sputtered material redeposition as well as the beam ‘self-focusing’ effects, are strongly reduced in a porous sample. In this case the walls between the pores are sputtered in an almost independent way. The etching of deep and straight patterns is feasible. Combined with photoelectrochemical etching that generates the initial macropores, three-dimensional (3D) lattices can be obtained, as demonstrated by 3D photonic crystal fabrication. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/6915-6086, E-mail: wang@lps.u-psud.fr     

Photoluminescence and capacitance transients in highly Mg-doped GaN

Behaviors of the photoluminescence blue-band and near-bandgap peak and the relevant thermal ionization energies of the shallow and deep Mg-related acceptors have been studied, respectively. The 2.989 eV blue-band is attributed to the deep donor–acceptor-pair transitions involving a deep Mg-related acceptor at E_v+0.427 eV. The blueshift with increasing excitation power is explained by variation in the contribution of close and distant donor–acceptor-pairs to the luminescence. The redshift with increasing temperature results from thermal release of carriers from close donor–acceptor-pairs. The 3.26 eV near-bandgap peak is attributed to the shallow donor–acceptor-pair transitions involving a shallow Mg-related acceptor at E_v+0.223 eV. The relevant thermal ionization energies of the shallow and deep Mg-related acceptors, being about E_v+0.16 and E_v+0.50 eV, are determined from deep-level transient Fourier spectroscopy measurements. Received: 11 July 2001 / Accepted: 9 August 2001 / Published online: 2 May 2002     

A novel anti-spatter and anti-crack laser drilling technique: application to ceramics

This work reports a novel anti-spatter and anti-crack ceramic laser drilling method based on the gelcasting technique. A Nd:YAG solid state laser was used to drill micro-holes directly on a gelcast green alumina body. The results reveal that the irregular spatter at the periphery of the hole inherent to traditional laser drilling methods can be effectively prevented by direct drilling of the gelcast green body. Much more regular hole shapes without microcracks were also successfully obtained. Received: 02 October 2002 / Accepted: 07 October 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-10-62771160, E-mail: caikai99@mails.tsinghua.edu.cn     

Micromachining of quartz with ultrashort laser pulses

Received: 6 January 1997/Accepted: 1 April 1997     

Influence of storage time on laser cleaning of SiO<Subscript>2</Subscript> on Si

The influence of the ‘storage time’ τ_s on the threshold fluence φ_cl and the efficiency in dry laser cleaning is investigated. τ_s denotes the time between the deposition of particles and the cleaning. As a model system we employed silica spheres with diameters of 500 nm and 1500 nm on commercial silicon wafers and single-pulse KrF excimer laser radiation (τ_FWHM=28 ns). For the 1500-nm silica spheres, φ_cl was found to increase from about 65 mJ/cm² to 125 mJ/cm² for storage times of 4 h and 362 h, respectively. For 500-nm silica spheres the increase in the threshold fluence was less than 20% for storage times up to 386 h. Received: 12 July 2002 / Accepted: 12 July 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail:dieter.baeurle@jku.at     

Purification and magnetic properties of carbon nanotubes

Received: 5 January 1998     

Efficient submicron processing of metals with femtosecond UV pulses

The generation of submicron-sized holes on metal surfaces by applying femtosecond UV laser pulses was investigated. Different optical schemes based on a Schwarzschild-type reflective objective were used to reach optimized ablation quality and efficiency in different applications (hole ablation, through-hole drilling, generation of surface patterns consisting of holes, etc.). Submicron-sized holes and hole patterns were ablated onto metal surfaces and drilled through ∼5-μm-thick steel foils with 600-nm diameter on the output side. Using a special optical interferometric method, large-area surface processing of high-conductivity materials in the submicron regime was performed. Combining these techniques with the application of high-repetition-rate ultra-short UV laser sources, large-area sub-μm processing of all kinds of materials in industrial environments is possible. Received: 28 February 2002 / Accepted: 12 March 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503599, E-mail: psimon@llg.gwdg.de     

Effects of electron-beam irradiation on surface oxidation of polymer composites

The aim of this article was to show the effects of an electron radiation dose and presence of a compatibilizer on the oxidation of composites made of blends of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET) as well as of blends of LDPE, HDPE, and PP. As the compatibilizers, the styrene-ethylene/butylene-styrene elastomer grafted with maleic anhydride (SEBS-g-MA) and trimethylol propane trimethylacrylate (TMPTA) were used; they were added in the amounts of 5, 10, and 15 wt% and 1, 2, and 3 wt%, respectively. The oxidation of the surface layer (SL) was investigated by the X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It was found that the extent of the composite oxidation increased with the increasing dose of the electron radiation. The addition of the compatibilizers enhanced the oxidation of the SL but hindered the oxidation of the bulk of the material.     

Beam delivery of femtosecond laser radiation by diffractive optical elements

Received: 4 February 1998/Accepted: 22 April 1998     

Relaxed GexSi1−x layer formation with reduced dislocation density grown by ultrahigh vacuum chemical vapor deposition

A mixture of as-grown single-wall carbon nanotubes (SWNTs) and a monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) was sonicated and homogenized. As a result, SWNTs were separated from carbonaceous impurities and metal particles, which enabled us to purify the SWNTs by filtration. We also found that the number of short (about 1-μm) SWNTs and thin bundles of SWNTs increased. The thin bundles contained one to three SWNTs. These short, thin SWNTs suspended in the MCB solution of PMMA were spin-coated onto a Si wafer, and could be dispersed on it. Received: 18 July 2000 / Accepted: 20 July 2000 / Published online: 6 September 2000     

Morphologies of GaN one-dimensional materials

GaN one-dimensional materials with different morphologies were formed on LaAlO₃ crystal, silicon crystal and quartz glass substrates through a simple sublimation method. They were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy. FE-SEM images showed that the morphologies of the one-dimensional materials included straight nanorods, curved nanowires, nanoribbons, zigzag nanorods and beaded or capture-tree nanorods. XRD and EDX studies indicated that all the one-dimensional materials were wurtzite GaN. Received: 14 July 2000 / Accepted: 17 July 2000 / Published online: 20 September 2000     

Structuring of aluminum films by laser-induced local oxidation in water

A simple method for patterning of thin (15–650 nm) aluminum films on glass substrates by direct, low-power, laser-thermal oxidation in water under common laboratory conditions is demonstrated. Local heating of the metal film enhances the formation of aluminum oxide (hydrargillite, Al₂O₃–3H₂O) and provokes breakdown of the passivation layer followed by local corrosion at temperatures close to the boiling point of water. Moving the focus of an Ar-ion laser (λ=488 nm) over the aluminum film with a speed of several μm/s yields grooves flanked by hydrargillite. Upon through oxidation of the metal these structures act as electrically insulating domains. Depending on the film thickness, the minimum width of the line structures measures between 266 nm and 600 nm. The required laser irradiation power ranges from 1.7 mW to 30 mW. It is found that the photo-thermal oxidation process allows for writing of two-dimensional electrode patterns. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

Topography and friction properties of macromolecular thin films using atomic-force-microscopy technology

Applied Physics A - The research work in this letter is on the microtribological properties of poly(ether ketone ketone) (PEKK) and sulfonated PEKK (S-PEKK) thin films. Polystyrene (PS) was used as...     

Hybrid pulsed laser deposition and Si-surface-micromachining process for integrated TiC coatings in moving MEMS

Titanium carbide (TiC) is one of the preferred coatings for improving the performance of macroscopic moving mechanical components due to its established wear-resistance. Pulsed laser deposition (PLD) is an excellent method for depositing TiC, because unlike any other deposition process for TiC, PLD offers the capability of producing high-quality films even at room temperature. Using a modified PLD technique, especially designed for the deposition of particulate-free films, TiC coatings have been deposited at room temperature on silicon (Si) and on several types of thin films typically employed for fabricating microelectromechanical systems (MEMS). Our results demonstrate that TiC coatings also offer a high wear-resistance to Si surfaces, which in turn has led to our application of TiC to “moving” Si MEMS devices. The performance of moving Si MEMS devices is limited by their poor operational lifetimes, which have been attributed to the excessive wear at sliding Si interfaces. The work presented here describes a hybrid process, whereby PLD is used in conjunction with a user-friendly Si surface micromachining scheme for inserting wear-resistant TiC coatings between critical sliding Si interfaces in MEMS devices. This paper describes the properties of PLD-TiC for MEMS and the hybrid PLD-surface micromachining process for the integration of TiC coatings into Si MEMS. Received: 23 January 2003 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-310/563-7614, E-mail: gouri.radhakrishnan@aero.org     

Dielectric constant of porous ultra low-k dielectrics by fractal-Monte Carlo simulations

The effective dielectric constant of porous ultra low-k dielectrics is simulated by applying the fractal geometry and Monte Carlo technique in this work. Based on the fractal character of pore size distribution in porous media, the probability models for pore diameter and for effective dielectric constant are derived. The proposed model for the effective dielectric constant is expressed as a function of the dielectric coefficient of base medium and the volume fractions of pores and base medium, fractal dimension for pores, the pore size, as well as random number. The Monte Carlo simulations combined with the fractal geometry are performed. The predictions by the present simulations are shown in good accord with the available experimental data. The proposed technique may have the potential in analyzing other properties such as electrical conductivity and thermal conductivity in porous ultra low-k dielectrics.     

Study of photoreaction in dichromated gelatin by laser photoacoustic technique

 An experimental method to study the photochemical reaction in solid-state dichromated gelatin during exposure is proposed, in which two laser sources with different wavelengths are used to induce photoreaction and to excite photoacoustic signal, respectively. This method is to obtain the real-time characteristics during the formation of the photoreaction product. The curve is fitted using the Rosencwaig-Gersho (R-G) theory and chemical kinetics. The reaction order and the rate of reaction may be obtained by fitting the parameters. The results show that photochemical reaction in dichromated gelatin conforms to the mechanism suggested by Watanable-Westheimer, and the photosensitivity of the samples, prepared under different conditions, is dependent on the initial concentration of photoactive ions. Received: 18 April 1995/Revised version: 3 May 1996