Etching of buried photoresist layers and its application to the formation of three-dimensional layered structures

The fabrication of three-dimensional layered structures with 180-nm-thick TaO_x top layers supported by 1.5-μm-thick Mo pillars formed on a glass substrate is presented. The photoresist used for planarization was successfully removed through the TaO_x layers using heat treatment at 270 °C with mixed vapors of ethyl alcohol and pure water at high pressure for 3 h. Vacancies underlying the TaO_x layers were consequently formed. The possibility of rapid and lateral crystallization of amorphous silicon films was demonstrated when the silicon films formed on the TaO_x overlaying the vacancy regions were irradiated using a frequency-doubled YAG laser at 250 mJ/cm². Energy sensors using Cr/Al metal wires, with a high sensitivity of 0.07 mW/cm², were also demonstrated using the present structure with vacancy regions for reduction of heat diffusion. Received: 22 January 2001 / Accepted: 24 January 2001 / Published online: 27 June 2001     

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     

Fragmentation of dodecanethiol molecules: application to self-assembled monolayer damage in atom lithography

Atom lithography commonly employs self-assem- bled monolayers (SAMs) of alkanethiols which act as resists to protect prepared surfaces. Metastable atomic species such as helium are used to damage the resist, enabling pattern transfer via mask lithography, followed by wet chemical etching. The damage mechanism is, however, not well understood. Here we report studies of fragmentation of dodecanethiol (DDT) molecules embedded in helium nano-droplets that have been irradiated by an electron beam. The results of the charge-transfer fragmentation process provide the first experimental data on the damage mechanisms that occur in the metastable helium/SAM interaction. Received: 20 September 1999 / Revised version: 6 December 1999 / Published online: 8 March 2000     

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     

Near field induced defects and influence of the liquid layer thickness in Steam Laser Cleaning of silicon wafers

The removal of particles from commercial silicon wafers by Steam Laser Cleaning was examined. Polystyrene colloids were used as model contaminants due to their well defined size and shape. In contrast to previous studies, where the experimental conditions on the surface were only roughly determined, special care was taken to control the amount of liquid applied to the surface. We report measurements of the cleaning threshold for different particle sizes. The comparability of the results was ensured by the reproducible conditions on the surface. Moreover, we studied the influence of different liquid film thicknesses on the cleaning process. Investigations of laser induced liquid evaporation showed that the cleaning threshold coincides with the fluence necessary for the onset of explosive vaporization. After particle removal, the surface was examined with an atomic force microscope. These investigations demonstrated that near field enhancement may cause defects on the nm-scale, but also showed that Steam Laser Cleaning possesses the capability of achieving damage-free removal for a large range of different particle sizes. Received: 14 January 2003 / Accepted: 16 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-7531/88-3127, E-mail: florian.lang@uni-konstanz.de     

Frequency measurements of saturated-fluorescence-stabilized CO2 laser lines: comparison with an OsO4-stabilized CO2 laser standard

4 frequency grid, to test the performance of the saturated fluorescence method for CO₂ laser frequency stabilization. The Allan variance for the saturated-fluorescence-stabilized CO₂ laser reached 60 Hz for an integration time of 300 s. We also performed three absolute measurements of CO₂ laser frequencies stabilized by the saturated fluorescence method, with a sub-kHz accuracy. We obtained a 1.7 kHz redshift of the CO₂ saturated fluorescence lines, which we attribute essentially to pressure. This is in contradiction with the previous observation of a blueshift [K.L. SooHoo, C. Freed, J.E. Thomas, H.A. Haus: Phys. Rev. Lett. 53, 1437 (1984)]. Furthermore, significant deviations of the measured frequencies from the data of Maki et al. [A.G. Maki, C.-C. Chou, K. Evenson, L.R. Zink, J.T. Shy: J. Mol. Spec. 167, 211 (1994)] demonstrate the need for a new calibration of the CO₂ frequency grid. Received: 1 December 1997     

A method to pattern Pd over-layers on GdMg films and its application to increase the transmittance of metal hydride optical switches

A process to pattern Pd over-layers on reactive metal films was developed using ion milling through a tungsten trioxide mask patterned by photolithography and wet etching. The WO₃ mask exhibited a low Ar⁺ sputter yield and, unlike conventional mask materials (SiO₂, Si₃N₄), was easily etched in a mild alkaline solution. This procedure was applied to convert a 20-nm Pd cap over a 160-nm GdMg film to a Pd grid with ≈9-μm-diameter openings covering ≈40% of the surface. The Pd grid proved sufficient to catalyze the (de)hydriding reactions required to reversibly switch the GdMg film from reflecting to transparent. The maximum transmittance of the patterned Pd/GdMg hydride bi-layer was twice that of an otherwise identical sample with a continuous Pd cap, with similar hydriding kinetics. Received: 2 May 2000 / Accepted: 9 May 2000 / Published online: 13 September 2000     

Defect capture under rapid solidification of the melt induced by the action of femtosecond laser pulses and formation of periodic surface structures on a semiconductor surface

A theory of defect-strain instability with formation of periodic surface relief in semiconductors irradiated by ultra-short (τ_p=10^-13 s) powerful laser pulses is developed. The period and time of formation of surface relief are calculated. Regimes of multi-pulse laser ablation leading to formation of either a smooth surface or arrays of surface-relief spikes are pointed out and corresponding experimental results are interpreted from the viewpoint of the developed theory. Received: 4 December 2000 / Accepted: 23 July 2001 / Published online: 11 February 2002     

Reflection high-energy electron diffraction real-time monitoring of an etch process implemented in molecular beam epitaxy technology: hydrogen chloride versus GaAs(001) epilayers

Received: 13 November 1997/Accepted: 19 January 1998     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

Photochemical effects in the UV laser ablation of polymers: Implications for laser restoration of painted artworks

0 ∝t^1/2. The best results are expected for a circular top-hat beam shape. Received: 15 January 1999 / Accepted: 18 January 1999     

Neutral molybdenum atom and dimer formations in laser-induced plasma: emission study of 355-nm pulse laser photolysis of Mo(CO)6 in the gas phase

6 in the gas phase at room temperature. The transient behavior of emissive atoms and dimers were measured by using a time-resolved UV-vis emission spectroscopy. Most atomic emissions were found to consist of two distinct transient components, an “early” component with a similar time profile to the photolysis pulse, and a “late” component with a peak at around 300 ns after the photolysis. The fact that the addition of an electron scavenger decreased the late emission sensibly suggests that the late emission was caused by Mo^* produced in a neutralization process of ionic species. A plausible mechanism involving electron–ion recombination was proposed for the late emission and examined by the numerical simulation of the transient behaviors as a function of some experimental parameters such as total pressure, initial ion concentration, and electron scavenger concentration. Received: 9 April 1997/Revised version: 12 June 1997     

Ion-beam mixing in Fe/Si bilayers by singly and highly charged ions: evolution of phases,spike mechanism and possible effects of the ion-charge state

Applied Physics A - Silicide formation and ion beam mixing of Fe/Si bilayers due to Ar-, Xe- and Au-ion irradiations at room and liquid-nitrogen temperatures were investigated. For the study of...     

Near shot noise detection of oxygen in the A-band with vertical-cavity surface-emitting lasers

A dual-beam laser absorption spectrometer with balanced detection for high sensitivity detection of oxygen via the A-band at 760 nm is described. The 2×2 vertical-cavity surface-emitting laser arrays used for this set-up are characterized by their wavelength tuning behavior with temperature and current, amplitude modulation, side-mode suppression ratio and polarization contrast. The spectrometer performance is determined over time periods of up to 10 h using the variation in the differential absorption between two beam paths. With the R11R11 line at STP, 670 μW laser power and 200 mm-long absorption cells, we realized an excellent linearity (R=0.9999) and over a 5-min interval a record sensitivity for VCSEL-based spectrometers of 35 ppmV, corresponding to an optical density (O.D.) of 7×10^-7. For this specific set-up, this sensitivity is only a factor of 2.7 above the shot noise limit, giving us a normalized detection limit of 7.6 ppmV·m·. Over a 10-h interval we achieved a standard deviation of 65 ppmV. Received: 26 July 2000 / Revised version: 1 November 2000 / Published online: 6 December 2000     

Application of electron paramagnetic resonance to the study of Cu2+ ions in Cu-Na ion-exchanged glasses

The copper ion-exchange process in two different silicate glasses is experimentally studied by means of electron paramagnetic resonance analysis. Differences in the environment of divalent copper ions are found depending on the substrate. The Cu²⁺ concentration in-depth profile is determined and compared to the total copper diffusion profile as determined from secondary ion mass spectrometry. Received: 12 February 1998 / Accepted: 5 January 1999 / Published online: 7 April 1999     

Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

2 . Dynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation threshold, the ablated mass, and the particle size were investigated. The ablation plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm² the average initial velocity of the ejected particles was deduced from the extension of the plume to range from 120–400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopically determined absorption coefficient of cornea α_cornea. Where absorption due to proteins is high (at λ=6.2 and 6.5 μm), ablated mass, velocity and recoil momentum behave according to α_cornea. For the first time, variations of the ablation plume from pulse to pulse were observed. Those, as well as the particle size, not only depend on the absorption coefficient, but also on the predominant absorber. Received: 4 November 1997/Revised version: 7 September 1998     

The determination of the lifetime of hot electrons in metals by time-resolved two-photon photoemission: the role of transport effects, virtual states, and transient excitons

Experience has shown that theoretically determined lifetimes of bulk states of hot electrons in real metals agree quantitatively with the experimental ones, if theory fully takes into account the crystal structure and many-body effects of the investigated metal, i.e., if the Dyson equation is solved at the ab initio level and the effective electron–electron interaction is determined beyond the plasmon-pole approximation. Therefore the hitherto invoked transport effect [Knoesel et al.: Phys. Rev. B 57, 12812 (1998)] does not seem to exist. In this paper we show that likewise neither virtual states [Hertel: et al. Phys. Rev. Lett. 76, 535 (1996)] nor damped band-gap states [Ogawa: et al.: Phys. Rev. B 55, 10869 (1997)] exist, but that the hitherto unexplained d-band catastrophe in Cu [Cu(111), Cu(110)] can be naturally resolved by the concept of the transient exciton. This is a new quasiparticle in metals, which owes its existence to the dynamical character of dielectric screening at the microscopic level. This means that excitons, though they do not exist under stationary conditions, can be observed under ultrafast experimental conditions. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Pulsed blue-light generation by the frequency doubling of the 4F3/2 to 4I9/2 transition in Nd:YAG and Nd:YAlO3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO₃ lasers emitting at 946 and 930 nm, respectively (⁴F_3/2 to ⁴I_9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively. Received: 23 June 1999 / Revised version: 8 August 1999 / Published online: 3 November 1999     

Infrared-laser-pulse control of bond- and state-selective excitation, dissociation and space quantization: application to a three-dimensional model of HONO2 in the ground electronic state

Selective control over the vibrational excitation and space quantization of the dissociation fragments by optimally designed linearly polarized and shaped infrared (IR) laser pulses of the picosecond (ps) and subpicosecond duration is demonstrated by means of quantum-dynamical simulations within the Schr?dinger wave-function formalism for a three-dimensional (3-D) model of HONO₂ in the ground electronic state, wherein the OH and the ON single-bond stretches are explicitly treated, together with the bending angle between them, on the basis of the ab initio defined 3-D potential-energy surface and dipole function. The high-lying zeroth-order vibrational states of the OH bond are prepared selectively both below and above the dissociation threshold of the ON single bond, and demonstrate a quasi-periodic oscillatory behaviour, manifesting intramolecular vibrational energy redistribution (IVR) on the picosecond timescale. Selective breakage of the ON single bond in HONO₂ with more than 97% probability is demonstrated, along with control of the space quantization of the dissociation fragments: the OH fragments rotating clockwise, OH(c), and anticlockwise, OH(a), are prepared selectively, with the OH(a)/OH(c) branching ratio being as high as 10.975. The results obtained show that optimally designed strong and short IR-laser pulses can compete against IVR and manipulate vibrational excitation and dissociation of polyatomic molecules. Received: 3 November 1999 / Published online: 13 July 2000     

Effect of the lateral dimension on the grating formation and stability in the double-phase-conjugate mirror: application to interconnects between single-mode fibres

We develop a phenomenological model of the double-phase-conjugate mirror grating formation based on geometrical considerations. We show that the grating develops from one lateral side of the interaction zone towards the other side. This model is confirmed by the behaviour of different configurations of a 2-zone double-phase-conjugate mirror. Finally, we obtained stable operation in InP:Fe, this made it possible to implement a 1×8 switch between single-mode fibres at telecom wavelength with 12% global efficiency and 200 ms reconfiguration time. Received: 6 November 1998 / Revised version: 18 February 1999 / Published online: 7 April 1999