Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.     

Low-temperature growth of SnO2 film prepared by XeCl excimer laser MOD process

A SnO₂ film has been prepared by an excimer laser metal organic deposition (ELMOD) process using an XeCl laser. The effects of the laser fluence, shot number, and the pretreatment temperature of the Sn acetylacetonate (Sn-acac) on the crystallization of the SnO₂ film were investigated by X-ray diffraction and infrared spectroscopy. When the MO spin-coated film preheated at room temperature on a Si substrate was irradiated by the laser at a fluence of 100 mJ/cm² and at a repetition rate of 10 Hz for 5 min, a crystallized SnO₂ film was successfully obtained without heat treatment. At a fluence of 260 mJ/cm², the highest crystalline film was formed. On the other hand, when the amorphous SnO₂ film was irradiated by the laser at 260 mJ/cm², the crystallinity of the SnO₂ film was improved. SnO₂ films were also prepared by conventional thermal MOD in a temperature range from 300 to 900 °C. The crystallinity of the SnO₂ films prepared by the ELMOD process at room temperature was higher than that of the films prepared by heating at 900 °C for 60 min. PACS 81.15.Fg; 81.15.-z; 81.16.Mk; 82.50.Hp; 73.61.Le     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Direct observation of laser-induced crystallization of a-C:H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm². At fluences below 100 mJ/cm² an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm². The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.     

Formation of two-dimensional periodic nanostructures on fused quartz,polyimide, and polycrystalline diamond by pulsed four-wave interference laser modification

Atomic force microscopy is used to examine the topography of submicron periodic structures formed on the surfaces of synthetic polycrystalline diamond and polyimide films. The films are deposited on fused quartz substrates by four-wave interference modification using a pulsed 308-nm UV XeCl excimer laser. It is demonstrated that a two-dimensional periodic relief with a submicron period can be formed on the diamond surface directly by laser evaporation in the absence of a photoresist. Depending on the exposure, two mechanisms of polyimide film modification are observed. At exposures less than 100 mJ/cm², the relief is formed due to swelling at the positions of interference maxima. At exposures greater than 100 mJ/cm², holes are formed in the films. A periodic relief on the fused quartz surface is formed by using a UV photoresist exposed to pulsed interference laser radiation and subsequent Ar ion etching.     

High-resolution electron microscopy study of SiGeC thin films grown on Si(1 0 0) by laser-assisted techniques

PLIE was used for rapid crystallisation of a-SiGeC films deposited by LCVD on Si(1 0 0) substrates. HRTEM study of thin films grown with several laser energies shows that the combination of the two laser techniques gives an almost completely crystallised alloy, even for the lowest laser fluence. Island formation is observed below a certain threshold of fluence (about 450 mJ/cm²). In the case of the lowest energy (100 mJ/cm²) the material was partially crystallised (with the crystalline material being the predominant state), to a nanocrystalline alloy with a considerable amount of epitaxialy grown grains and with grain sizes of several tens of nanometers. Above the threshold of 450 mJ/cm² a rather smooth thin film is grown. The crystallisation is almost complete and the alloy is grown in an almost perfect epitaxial way.     

Effects of laser irradiation energy density on the properties of pulsed laser deposited ITO thin films

The properties of indium tin oxide (ITO) thin films, deposited at room temperature by simultaneous pulsed laser deposition (PLD), and laser irradiation of the substrate are reported. The films were fabricated from different Sn-doped In₂O₃ pellets at an oxygen pressure of 10 mTorr. During growth, a laser beam with an energy density of 0, 40 or 70 mJ/cm² was directed at the middle part of the substrate, covering an area of ∼1 cm². The non-irradiated (0 mJ/cm²) films were amorphous; films irradiated with 40 mJ/cm² exhibited microcrystalline phases; and polycrystalline ITO films with a strong 〈111〉> preferred orientation was obtained for a laser irradiation density of 70 mJ/cm². The resistivity, carrier density, and Hall mobility of the ITO films were strongly dependent on the Sn doping concentration and the laser irradiation energy density. The smallest resistivity of ∼1×10^-4 Ω cm was achieved for a 5 wt % Sn doped ITO films grown with a substrate irradiation energy density of 70 mJ/cm². The carrier mobility diminished with increasing Sn doping concentration. Theoretical models show that the decrease in mobility with increasing Sn concentration is due to the scattering of electrons in the films by ionized centers. PACS 81.15.Fg; 73.61.-r; 73.50.-h     

Direct observation of phase transitions by time-resolved pyro/reflectometry of KrF laser-irradiated metal oxides and metals

New experimental results are obtained by coupling both time-resolved reflectivity and rapid infrared pyrometry under a hemispherical reactor. The heating source KrF laser beam (28 ns, 248 nm) is homogenized and as for probing, a CW He-Ne laser beam (10 mW, 633 nm) is used.Using both methods infrared pyrometry with an IR detector cooled with liquid nitrogen and sensitive in the spectral range 1-12 μm, and time-resolved reflectivity with a rapid photodiode, we were able to study complex thermodynamic transitions with nanosecond time resolution. Three different materials are studied by varying the KrF fluence (energy/surface) from 100 to 2000 mJ/cm²: thin films melting (Au/Ni), the threshold of plasma formation (Ti), and complex liquid phase segregation under semi-conductor state (ZnO). The formation of a liquid Zn film induced by temperature gradient is well evidenced by our signals. Also melting of thin films irradiated by low laser fluences (less than 500 mJ/cm²) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400-2000 mJ/cm²) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000 mJ/cm² corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm² to the dynamics of plasma expansion.     

Crystallisation of TiO2 thin films induced by excimer laser irradiation

Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass substrate and subsequently irradiated by UV radiation using a KrF excimer laser. In this work, we have study the influence of the laser fluence (F) ranging between 0.05 and 0.40 mJ/cm² on the constitution and microstructure of the deposited films. Irradiated thin films are characterized by profilometry, scanning electron microscopy and X-ray diffraction. As deposited films are amorphous, while irradiated films present an anatase structure. The crystallinity of the films strongly varies as a function of F with maximum for F = 0.125 J/cm². In addition to the modification of their constitution, the irradiated areas present a strongly modified microstructure with appearance of nanoscale features. The physico-chemical mechanisms of these structural modifications are discussed based on the theory of nucleation.     

Preparation of Y2O3:Eu thin films by excimer-laser-assisted metal organic deposition

Europium-doped yttrium oxide (Y₂O₃:Eu) thin films were successfully deposited on quartz and ITO/glass substrates by excimer-laser-assisted metal organic deposition (ELAMOD) at low temperatures. The effects of laser wavelength and thermal temperature on the films’ crystallinity and photoluminescence properties were investigated. Films irradiated by an ArF laser at 80 mJ/cm² and 400–500°C were highly crystallized compared with those prepared by thermal MOD. In contrast, when the film was irradiated by a KrF laser at 500°C, no crystalline Y₂O₃:Eu was formed. The Y₂O₃:Eu film irradiated by the ArF laser at 80 mJ/cm² and 500°C showed typical PL spectra of Eu³⁺ ions with cubic symmetry and a ⁵D₀→⁷F₂ transition at ∼612 nm. The PL intensity at 612 nm was much higher for the film prepared with ELAMOD than for that prepared by the thermal-assisted process, and the photoemission intensity of the film prepared with ELAMOD strongly depended on the substrate material.     

Solvent-related effects in MAPLE mechanism

To study the role of the solvent and of the laser fluence in the matrix-assisted pulsed laser evaporation (MAPLE) process, we used a soft polymer (polydimethylsiloxane—PDMS) as “sensing surface” and toluene as solvent. Thin films of the PDMS polymer were placed in the position of the growing film, while a frozen toluene target was irradiated with an ArF laser at the conventional fluences used in MAPLE depositions (60–250 mJ/cm²). Apart the absence of solute, the MAPLE typical experimental conditions for the deposition of thin organic layers were tested. The effects on the PDMS films of the toluene target ablation, at different fluences, were studied using atomic force microscopy and contact angles measurements. The results were compared with the effects produced on similar PDMS films by four different treatments (exposure to a drop of the solvent, to saturated toluene vapors and to plasma sources of two different powers). From this comparative study, it appears that depending on the MAPLE experimental conditions: (1) the MAPLE process may be “semidry” rather than purely dry (namely the solvent is likely to be present in the deposition environment near the growing film), (2) the solvent, if sufficiently volatile, is in form of vapor molecules (neutral, ionized and probably dissociated) rather than in liquid phase near the substrate and (3) at relatively high laser fluences (>150 mJ/cm²), the formation of an intense plasma plume results which can damage/affect a soft substrate as well as a growing polymer film.     

Ablation of silicon suboxide thin layers

We investigate the ablation of SiO _x thin films on fused silica substrates using single-pulse exposures at 193 nm and 248 nm. Two ablation modes are considered: front side (the surface of a film is irradiated from above) and rear side (a film is irradiated through its supporting substrate). Fluence is varied from below 200 mJ/cm² to above 3 J/cm². SiO _x films of thickness 200 nm, 400 nm, and 600 nm are ablated. In the case of rear-side illumination, at moderate fluences (around 0.5 mJ/cm²) the ablation depth corresponds roughly to the film thickness, above 1 J/cm² part of the substrate is ablated as well. In the case of front-side ablation the single-pulse ablation depth is limited for all film thicknesses to less than 200 nm even at fluences up to 4 J/cm². Experimental results are discussed in relation to film thickness, fluence, and ablation mode. Simple numerical calculations are performed to clarify the influence of heat transport on the ablation process.     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La_0.8Sr_0.2MnO₃ (LSMO) films were prepared on LaAlO₃ substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm² for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K. PACS 81.15.-z; 81.15.Fg; 81.15.Np; 73.61.-r; 71.30.+h     

Selective patterning and scribing of Ti thin film on glass substrate by 532 nm picosecond laser

In this paper, the feasibility of Ti film coated on glass substrate scribed via a 532 nm picosecond laser is investigated. Laser irradiations from the film side and from the transparent substrate side are performed for comparison. Optical microscopy, SEM, surface stylus and contact resistance measurement reveal that the Ti film can be completely removed with no damage to the glass substrate, using optimized process parameters. The complete removal threshold for the film for front-side scribing is found at 120 mJ/cm², while the minimum laser fluence for complete scribing is 70 mJ/cm² in the case of back-side scribing. The lines scribed from the front side exhibit obvious thermal effects such as heat affected zones, burr and micro cracks. Back-side scribing exhibits non-thermal behavior, which also can increase the process speed for the scribing of a Ti film on glass to 1000 mm/s. This makes the back-side laser scribing of Ti film a promising technique.     

Dynamics of excimer laser ablation of thin tungsten films monitored by ultrafast photography

The time course of laser light induced transport of tungsten films from a glass support is followed by ultrafast photography using delayed dye laser pulses. The photographs provide unambiguous evidence that the material transport in the 40–200 mJ/cm² intensity domain takes place via removal of solid pieces from the film material. These results are consistent with heat flow calculations which predict the overall melting of the metal layer above 380 mJ/cm². The series of photographs presented give detailed insight into the melting process and have revealed an unexpected in-flight phase separation of solid fracture pieces and molten droplets throughout the 200–900 mJ/cm² domain. The faster propagating molten droplets form a condensed halo in front of the solid pieces, thereby providing an efficient shield between the processing laser light and the solid phase.     

Exploring the influence of a XeCl laser treatment on biocompatibility of polyethersulfone film

The effect of XeCl laser irradiation on biocompatibility of polyethersulfone (PES) film surface was investigated. For this purpose, the surface of PES film was irradiated with different number of pulses at different fluences. The treated surfaces were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. The platelet adhesion and cell culture measurements were done on the treated surface for investigation of the biocompatibility. It was shown that, irradiation of surface with 500 pulses at a fluence of 25 mJ/cm² is the most optimal condition for improving the platelet adhesion on the PES surface with a XeCl laser.     

Patterned laser annealing of silicon oxide films

UV-absorbing silicon monoxide (SiO _x , x≈1) thin films on fused silica substrates are irradiated by an ArF excimer laser (wavelength 193 nm) in the sub-ablation threshold regime. Multi-pulse irradiation of films with ∼200-nm thickness at a fluence of about 100 mJ/cm² leads to a significant increase of the UV transmission, indicating the oxidation of SiO _x to SiO₂. The quality of the obtained films after this laser annealing process depends on the oxygen content of the environment. Irradiation in air at atmospheric pressure leads to the formation of sub-micron-sized oxide particles on top of the film. Structured illumination is applied either to form areas of the film with changed transmission and refractive index, or for the formation of regular particle patterns with sub-micron periods. These processes can be utilized for the fabrication of phase masks or for various types of surface functionalization.     

Deposition of aluminum nitride thin film on Si(1 1 1) by KrF excimer laser and its characterizations

We present the deposition of aluminum nitride (AlN) thin film by KrF excimer laser sputtering and the study of the effects of substrate temperature and laser fluences. Deposition rate of AlN thin film at 0.3 Å/pulse has been achieved with laser fluence of 1500 mJ/cm² and at substrate temperature of 250 K, and this shows the enhancement of the deposition rate at low substrate temperature. Surface morphology of the deposited films is characterized by atomic force microscopy (AFM). In addition, the electrical performance of the MIS devices with AlN thin films prepared in this experiment has been characterized.     

Evidence for self-sputtering during pulsed laser deposition of Zn

Thin films of Zn have been prepared by pulsed laser deposition with a KrF excimer laser (248 nm). The laser energy density (E.D.) on the target has been varied in the 1 to 5 J/cm² range. The results show that as the E.D. increases the material distribution changes. For low E.D.( 1.6 J/cm²) the maximum of the distribution is at the substrate center, for intermediate E.D. it is displaced to the side, and a clear minimum appears at the center of the substrate for the higher E.D. (4.5 J/cm²). The growth velocity at the center of the substrate reaches a maximum value for E.D. of 2.8 J/cm², and decreases for higher E.D. as a result of the competition between deposition and self-sputtering. Virtually a zero growth velocity is obtained for E.D. above 4.5 J/cm². The self-sputtering process is most likely responsible for the increase of the film surface roughness as a function of the laser E.D. The low cohesive energy for metal Zn, compared to other metals (Fe, Ag, Cu) is correlated with the high efficiency of the self-sputtering for this material. PACS 81.15.Fg, 68.55.Ac, 68.37.-d