Excimer laser surface treatment of bulk ceramics

Changes of surface morphology following excimer laser (KrF, ArF) irradiation were investigated for three technical ceramic materials (TZP, PSZ, Si₃N₄). Zirconia samples exhibit a smooth and glass-like surface at the low power density regime, while at higher power densities colour changes were observed due to changes in the chemical composition of the bulk material. In the silicon nitride samples new crystals of specific orientation are formed, which could be crystalline silicon.     

Photochemical welding of silica microspheres to silicone rubber by ArF excimer laser

Transparent fused silica (SiO₂) microspheres 2.5 μm in diameter were photochemically welded to transparent, flexible silicone rubber ([SiO(CH₃)₂]_n) substrate by 193 nm ArF excimer laser induced photochemical modification of silicone into silicon oxide. Single layer of silica microspheres was easily formed on an adhesive silicone rubber before laser irradiation after dropping of silica microspheres dispersed in ethanol and subsequent tape peeling. The welding rate, the percentage of welded microspheres tested by ultrasonic cleaning with ethanol, was examined by varying the single pulse fluence and irradiation time of ArF excimer laser. The welding layer underneath microsphere, silicon oxide, was also found to emit white light of strong intensity under UV light illumination.     

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.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Catalytic action of the products of photodissociation of some atmospheric impurities on water vapor condensation

A significant effect of the photodissociation products of atmospheric impurities, such CF₂Cl₂, CCl₄,H₂S, and NH₃, on the condensation of supersaturated water vapor is demonstrated. The experiments are performed in a cloud chamber, with irradiation being carried out by an ArF excimer laser (λ = 193 nm). The results obtained in the absence and presence of photodissociation are interpreted. The impact of the initial water vapor pressure in the chamber on the process of condensation is observed and explained.     

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     

Atomic transport during growth of ultrathin dielectrics on silicon

Atomic transport in thermal growth of thin and ultrathin silicon oxide, nitride, and oxynitride films on Si is reviewed. These films constitute the gate dielectrics, the “heart” of silicon metal-oxide-semiconductor field-effect transistor (MOSFET) and dynamic random-access memory (DRAM) devices, which are usually thermally grown on the active region of the semiconductor Si substrate. The drive of ultra-large scale integration towards the 0.18 μm channel length and below requires gate dielectrics with thicknesses of 3–4 nm and less, establishing new and very strict material requirements. Knowledge on an atomic scale of dielectric film growth promoted by thermally activated transport mechanisms is essential to the engineering of this fabrication step. In the case of thermal growth of silicon oxide films on Si in dry O₂, the mobile species is O₂ and growth is essentially a diffusion–reaction phenomenon. The thermal growth of silicon nitride and oxynitride films on Si in NH₃, NO and N₂O, on the other hand, involves catalytic dissociation of the original gas molecules at the surfaces and interfaces and diffusion–reaction of different resulting species, like NH₂, NH, H, N, NO, O, and O₂. Hydrogen transport and incorporation is a crucial, ubiquitous issue in thermally grown dielectric films on Si which is also addressed here. A recall is made of the physico-chemical constitution of the involved surfaces and interfaces for each different dielectric material, as well as complementary studies of the gas, gas-surface, and solid phase chemistry. An outline of the unique tools of isotopic substitution and high resolution depth profiling is included.     

Plasma treatment studies of MIS devices

Silicon nitride films have emerged as the possible future dielectrics for ultra large scale integration (ULSI). Because the interface state density of silicon nitride/silicon interface in metal insulator semiconductor (MIS) configuration is more than an order of magnitude larger than that of silicon dioxide/silicon interface, plasma treatment studies on silicon nitride films have been undertaken for the possible improvement. Accordingly, silicon nitride films of various composition have been prepared by plasma enhanced chemical vapor deposition (PECVD) system using silane(SiH₄) and ammonia(NH₃) with nitrogen(N₂) as the diluent and MIS devices have been fabricated with as well as without plasma treated silicon nitride as the insulator. A considerable improvement in the silicon nitride/silicon interface is observed on ammonia plasma treatment while nitrous oxide(N₂O) plasma treatment studies have resulted in the establishment of a novel plasma oxidation process.     

Influence of SiNx:H film properties according to gas mixture ratios for crystalline silicon solar cells

The Hydrogenated silicon nitride (SiN_x:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the SiN_x:H film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. For optimizing surface layer in crystalline silicon solar cells, by varying gas mixture ratios (SiH₄ + NH₃ + N₂, SiH₄ + NH₃, SiH₄ + N₂), the hydrogenated silicon nitride films were analyzed for its antireflection and surface passivation (electrical and chemical) properties. The film deposited with the gas mixture of SiH₄ + NH₃ + N₂ showed the best properties in before and after firing process conditions.The single crystalline silicon solar cells fabricated according to optimized gas mixture condition (SiH₄ + NH₃ + N₂) on large area substrate of size 156 mm × 156 mm (Pseudo square) was found to have the conversion efficiency as high as 17.2%. The reason for the high efficiency using SiH₄ + NH₃ + N₂ is because of the good optical transmittance and passivation properties. Optimized hydrogenated silicon nitride surface layer and high efficiency crystalline silicon solar cells fabrication sequence has also been explained in this study.     

The effect of Si surface nitridation on the interfacial structure and electrical properties of (La2O3)0.5(SiO2)0.5 high-k gate dielectric films

Thermal stability, interfacial structures and electrical properties of amorphous (La₂O₃)_0.5(SiO₂)_0.5 (LSO) films deposited by using pulsed laser deposition (PLD) on Si (1 0 0) and NH₃ nitrided Si (1 0 0) substrates were comparatively investigated. The LSO films keep the amorphous state up to a high annealing temperature of 900 °C. HRTEM observations and XPS analyses showed that the surface nitridation of silicon wafer using NH₃ can result in the formation of the passivation layer, which effectively suppresses the excessive growth of the interfacial layer between LSO film and silicon wafer after high-temperature annealing process. The Pt/LSO/nitrided Si capacitors annealed at high temperature exhibit smaller CET and EOT, a less flatband voltage shift, a negligible hysteresis loop, a smaller equivalent dielectric charge density, and a much lower gate leakage current density as compared with that of the Pt/LSO/Si capacitors without Si surface nitridation.     

Field emission properties of a-CN<Subscript>x</Subscript> films prepared by pulsed laser deposition

Amorphous carbon nitride (a-CN_x) films were grown on silicon substrates by ArF excimer laser ablation of a graphite target in the presence of nitrogen at various gas pressures. By working at elevated pressures (up to 100 Pa), large amounts of nitrogen can be incorporated into the films (up to 40 at. %, which leads to a strong enhancement of their field emission properties. This behaviour was demonstrated to be mainly related to changes in the surface morphology of the samples, in connection with the development of graphite-like structures. Received: 22 February 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-(0)/388-106-230, E-mail: fogarassy@phase.c-strasbourg.fr     

Deep UV laser induced luminescence in oxide thin films

Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room temperature, Al₂O₃ coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al₂O₃ coatings, which suggests a strong single-photon interaction at 193 nm by F⁺ and F center absorption. Measurements on highly reflective thin-film stacks, consisting of quarter-wave Al₂O₃ and SiO₂ layers, indicate similar UV excitations, mainly from color centers of Al₂O₃. Received: 20 February 2002 / Accepted: 11 April 2002 / Published online: 5 July 2002     

Study of atomic transport mechanisms during thermal nitridation of silicon in ammonia using 15N and D labelled gas

Thermal silicon nitridation mechanisms in ammonia gas at 950°C and under 2 × 10⁻⁵ Torr were investigated by using ¹⁵N as a tracer. Nitride layers first grown in ¹⁴NH₃ were further grown in ¹⁵NH₃. ¹⁵N profiling was obtained by combining nuclear microanalysis with step-by-step chemical dissolution of the nitride layer. Thus one can show that the nitride film consists of a ¹⁴N layer located near the Si/nitride interface and a ¹⁵N layer located near the external surface of the film. It was also shown that there is no isotope exchange between the nitrogen of the nitride network and of the NH₃ gas. These results indicate that the growth does not occur by interstitial transport through the nitride film of the NH₃ molecules nor by any other nitrogenated species resulting from a decomposition of the ammonia gas. A possible incorporation of hydrogenated species was also examined using ND₃ nitridation. As the relative concentration of deuterium in such a film was less than 10⁻⁴, ammonia most probably dissociates at the outer surface and hydrogenated species do not react appreciably with the network.From these experiments, it can be concluded that the growth of the silicon nitride layer occurs by either:

• -transport of silicon atoms through the layer which then react with the nitridant gas;
• -or a step-by-step migration of a nitrogen network defect (vacancy or interstitialcy) in only one direction. In such a case, the defect should be electrically charged and the hoping direction should be imposed by a strong electrical field arising from the charging of nitrogenated species adsorbed at the external nitride surface by electrons coming from the silicon substrate because of the very small thickness of the nitride film (< 40 Å) (tunnel or thermionic effects)

.     

Excimer ArF laser with an output energy of 1.3 J at 2.0% efficiency on the He:Ar:F2 mixture

In this paper it is shown that to achieve a maximum efficiency and high output energy of an ArF (193 nm) excimer laser, one should use optimal pump intensity. It has been shown experimentally that the optimal pump intensity for an ArF excimer laser with the mixture of He:Ar:F₂ has a value of 4.5–5.0 MW/cm³. The results of an experimental study of the pump and active medium parameters effect on the efficiency and output energy of the ArF excimer laser on the mixture of He:Ar:F₂ are presented. To provide high pump intensity of an active medium, the excitation scheme of the LC-inverter type has been used where the current return conductor inductance had been increased from 30 to 80 nH. This allows the pump to achieve levels of intensity above 5.0 MW/cm³. By using the pump intensity of 5.0 MW/cm³ in an active medium of He:Ar:F₂–79.7:20:0.3 at total pressure of 2.4 atm, we are the first to obtain the output energy of 1.3 J at the total efficiency of 2.0%. The pulse duration (FWHM) was 15±1 ns and the peak pulse power was 85 MW. PACS 42.55.Lt; 42.60.Lh     

Deep UV laser induced fluorescence in fluoride thin films

Fluorescence experiments have been performed to study the interaction of 193-nm laser radiation with dielectric thin films of LaF₃, AlF₃, and MgF₂. Spectral- and time-resolved measurements reveal the presence of cerium in LaF₃ and the influence of hydrocarbons in MgF₂ and LaF₃. Virtually no fluorescence response is observable in the case of AlF₃. Supplementary measurements on multilayer stacks confirm the contribution of hydrocarbon and cerium emission in high-reflective UV mirrors upon ArF excimer laser irradiation. Energy density dependent measurements indicate a linear absorption process as the origin of UV laser induced fluorescence in LaF₃. Luminescence calculations are applied as a helpful tool in order to account for interference effects that are inherently to be found in the multilayer emission spectra. Received: 21 May 2002 / Accepted: 23 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/807-601, E-mail: heber@iof.fraunhofer.de     

Boron doping of silicon by excimer laser irradiation in a reactive atmosphere

UV excimer lasers have been used to dope semiconductors by a one-step process in which the laser serves both to melt a controlled thickness of a sample placed in dopant ambient and to photodissociate the dopant molecules themselves. Here we report the boron doping of silicon by means of an ArF (193 nm) excimer laser. Dopant atoms are obtained by photolysis of BCl₃ or pyrolysis of BF₃ molecules. The doping is performed both in gas ambient and using only an adsorbed layer. We have investigated the dependence of doping parameters such as laser pulse repetition and gas pressure on the subsequent boron impurity profiles and the dopant incorporation rate. These results indicate that the laser doping process is dopant-flux limited for BF₃ and externally rate limited for BCl₃.     

AlN thin films prepared by ArF plasma assisted PLD. Role of process conditions on electronic and chemical–morphological properties

Aluminium nitride thin films were deposited on n-Si <100> substrates by RF plasma activated reactive pulsed laser deposition (PLD). An ArF excimer pulsed laser, 10 Hz and 2.5 J/cm² energy fluence, has been used to ablate a pure Al target in a reactive atmosphere of N₂ plasma (generated by a RF source), at varying processing parameters (substrate temperature, time, and N₂ plasma configuration). We studied the dependence and correlation of structural and electronic properties with the experimental conditions. The chemical composition of deposited material has been determined by both Raman and X-ray photoelectron spectroscopy (XPS). Electrical resistivity has been evaluated by the sheet resistance method. Both spectroscopic characterizations (Raman and XPS) show a strong dependence in the formation of AlN on the deposition temperature. At low temperatures, there is little formation of nitride, with a prevalence of aluminium oxide, while at higher temperatures the N uptake increases, with AlN formation. Raman analysis also highlights the formation of nano-structures, for temperatures ≥400^°C. These material characteristics have a fundamental influence on the electronic properties. Indeed, electrical resistivity properties have been found to be strongly dependent on the film structure, nitrogen incorporation, and presence of mixed oxide compounds, closely related to deposition temperature.     

Deep etching of epitaxial gallium nitride film by multiwavelength excitation process using F2 and KrF excimer lasers

The deep etching of GaN(0001) thin films epitaxially grown on Al₂O₃(0001) has been investigated by laser ablation using F₂ and KrF excimer lasers. The simultaneous irradiation with F₂ and KrF excimer lasers markedly improves etching quality compared with single-KrF excimer laser ablation and provides almost the same quality as that in the case of single-F₂ laser ablation with a high etching rate. Additionally, the present method achieves the deep etching of a GaN film of more than 5 μm in depth with steep side walls at an angle of 87° by changing the laser incidence angle. PACS 52.38.Mf; 61.82.Fk; 81.65.Cf     

AES analysis of nitridation of Si(100) by 2–10 keV N+2 ion beams

Ion beam nitridation of Si(100) as a function of N⁺₂ ion energy in the range of 2–10 keV has been investigated by in-situ Auger electron spectroscopy (AES) analysis and Ar⁺ depth profiling. The AES measurements show that the nitride films formed by 4–10 keV N⁺₂ ion bombardment are relatively uniform and have a composition of near stoichiometric silicon nitride (Si₃N₄), but that formed by 2 keV N⁺₂ ion bombardment is N-rich on the film surface. Formation of the surface N-rich film by 2 keV N⁺₂ ion bombardment can be attributed to radiation-enhanced diffusion of interstitial N atoms and a lower self-sputtering yield. AES depth profile measurements indicate that the thicknesses of nitride films appear to increase with ion energy in the range from 2 to 10 keV and the rate of increase of film thickness is most rapid in the 4–10 keV range. The nitridation reaction process which differs from that of low-energy (< 1 keV) N⁺₂ ion bombardment is explained in terms of ion implantation, physical sputtering, chemical reaction and radiation-enhanced diffusion of interstitial N atoms.     

Parametric characteristics for a broadband Gaussian beam in free space

In this work, the evidence of SF₆ gas decomposition at the vicinity of SiO₂ glass has been investigated using various laser wavelengths: at 193, 248, 532 and 1064 nm. It was shown that SiF₄ gas and S₂F₁₀ clusters were simultaneously created during ArF excimer laser irradiation, while no by-products were seen in the irradiation cell using Q-switched Nd:YAG laser. The gas content analysis was carried out using laser breakdown spectroscopy (LIBS) and Fourier transform IR spectroscopy (FTIR). Moreover, the fluorine penetration into the glass surface was studied by energy dispersive X-ray (EDX) microanalysis and wavelength dispersive X-ray (WDX) mapping to support the suggested mechanisms.