KrF Excimer Laser Source Research Articles

Pulsed laser removal of tungsten nanoparticle aggregates: Surface analysis and visualization of particle ejection dynamics

In this study, we report on some of the aspects that describe the interaction of nanosecond UV laser pulses with solid surfaces covered by layers of tungsten nanoparticles aggregates, with an interest in applications for next-generation fusion reactors. The safe and efficient operation of such reactors is challenged by their unavoidable contamination with dust by-products due to interactions between plasma and the surface of plasma-facing components. We investigate the cleaning possibility of tungsten nanoparticle layers from solid surfaces using a KrF excimer laser source with tailored spot shape. The samples that emulate dust contaminated surfaces are synthesized by magnetron sputtering combined with gas aggregation technique. In order to shed light into the particle removal efficiency, the tungsten samples are investigated by scanning electron microscopy, Raman spectroscopy, and profilometry. Furthermore, in order to gain insight into the W nanoparticles and fragments mobilization dynamics, a plasma imaging setup is used to visualize the speed and directionality of the ejected material. We find that a very good surface cleaning can be achieved, even for the lowest considered laser fluence (0.5 J/cm2) following 8 pulses. Material ejection from the laser irradiated surfaces occurs in two regimes, over very diverse time scales: (i) first, a fast plasma plume travelling at 40,000 km/h reaches the substrate in a few microseconds, (ii) followed by the ejection of large particle conglomerates at speeds of the order of few hundred km/h. The particle ejection dynamics is shown to differ depending on the type of atmosphere (ambient pressure and vacuum, respectively), whereas the plasma plume evolution could only be evidenced under vacuum conditions.

Pulsed Laser Deposition of Epitaxial ZnSxSe1-x Thin Films for Waveguiding Applications in Mid-IR Active Multilayered Structures

ABSTRACTChromium doped II-VI semiconductors (such as ZnSe and ZnS) feature broad mid-IR emission in the 2-3 μm spectral range due to intershell transitions of the Cr2+ions. These materials show much promise for development of a tunable, electrically-pumped, mid-IR laser source. For integration into a mid-IR active multilayered structure, the ternary alloy ZnSxSe1-xis an attractive waveguiding material due to its lattice-matching ability and lower index of refraction with respect to the Cr2+:ZnSe active material. Epitaxial growth of each layer is desired to achieve the electronic and optical properties necessary for successful integration into a lasing device, so a study was conducted on the effects of sulfur content and growth temperature on the crystal quality of the resulting thin films. Several films of ZnSxSe1-xwere deposited by pulsed laser deposition (PLD) using a 248 nm KrF excimer laser source at varying growth temperatures and with various compositional parameters onto (100) GaAs substrates. The samples were analyzed via x-ray diffraction (XRD) and energy dispersive x-rays (EDX) to investigate the crystal quality and elemental content of the films for device integration. Film-substrate epitaxy was achieved and upper bounds to the defect density were calculated for several regimes of compositional parameter and growth temperature. From all samples produced, the lowest defect density of 2.2 x 1010cm-2was observed for the x=0.06 film grown at 450°C, while the lowest lattice mismatch between the substrate and epilayer of 0.059% was observed for the x=0.02 film grown at 450°C.

Estimation of Local Crystallization of a-Si:H Thin Films by Nanosecond Pulsed Laser Irradiation Through Local Temperature Simulation

We present a detailed study of the wavelength influence in pulsed laser annealing of amorphous silicon thin films, comparing the results for material modification at different fluence regimes both in the three fundamental harmonics of standard DPSS laser sources, UV (355 nm), visible (532 nm) and IR (1064 nm), and KrF (248 nm) excimer laser sources. Samples of hydrogenated amorphous silicon thin films were irradiated and characterized with MicroRaman techniques. A finite element model (FEM) was developed in COMSOL to simulate the process. The numerical and experimental results are presented, proving that the process can be predicted with a model based on heat transport.

Electrical Properties of MIS Capacitors with AlN Films Synthesized by Pulsed Laser Deposition

AbstractSummary: We manufactured for the first time MIS capacitors based on aluminium nitride (AlN) thin films synthesized by Pulsed Laser Deposition (PLD). AlN films were deposited on Si substrates by PLD from AlN targets in nitrogen ambient by multi‐pulse ablation using a UV KrF excimer laser source (λ = 248 nm, τ = 7 ns). The structures we prepared were electrically characterized by C‐V and I‐V complementary measurements. Our studies evidence the formation of good interfaces, and of defects into the film bulk which are electrically active. This justifies further developments in view of future applications of PLD synthesized AlN thin films as a dielectric alternatives to SiO2.The distribution of the interface trap densities (Dit) in Si bandgap for AlN/Si structures synthesized at different N2 pressures.imageThe distribution of the interface trap densities (Dit) in Si bandgap for AlN/Si structures synthesized at different N2 pressures.

Characterization of a highly photorefractive RF-sputtered SiO2-GeO2 waveguide

We present the characterization of highly photorefractive Er3+/Yb3+-doped 75SiO2-25GeO2 planar waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering (RFMS) technique. Details of the deposition process are reported. The material presents an intense absorption band (alpha approximately 10;3/10;4 cm;-1) in the UV region. Irradiations by a KrF excimer laser source at lambda = 248 nm have produced large positive (up to 310-3) refractive index changes, without the need of particular sensitization procedures. Direct measurements of UV photo-induced volume densification demonstrates that glass compaction accounts for large part of the refractive index change. Highly efficient photo-induced phase gratings have thus been fabricated in the waveguide.

Micro-Raman spectroscopy study of surface transformations induced by excimer laser irradiation of TiO 2

Pressed disks of TiO 2 powder particles (≈1 μm in size) have been irradiated with a pulsed KrF (248 nm) excimer laser source at fluences between 0.1 and 1 J cm −2. Surface films (1.5–2 μm thick) have been studied by Raman microprobe spectroscopy and atomic force microscopy (AFM). The Raman study reveals a three-layer structure for the irradiated anatase powders. A dark layer of reduced oxide is sandwiched between a top coating of molten/resolidified rutile and an underlying defective, slightly oxygen-deficient mixed-phase of rutile and anatase. AFM measurements indicate that a smooth surface layer coexisting with the initial rough grain morphology gradually appears with increasing fluence. At low fluence, anatase is reduced in a dark film and further transformed into rutile. At intermediate fluence, a shiny coating of resolidified stoichiometric rutile forms on the dark film. It gets thicker as the fluence increases while darkening of the sublayer intensifies up to a maximum of approximately 700 mJ cm −2. At high fluence, however, melting and re-oxidation (and eventually ablation) prevail over reduction; the whole layer turns into a greyish crust of mostly resolidified rutile in non-ablated regions. A physico-chemical mechanism is proposed to explain the in-depth distribution of the various components as a function of fluence.

Pulsed laser deposition of yttria stabilized zirconia for solid oxide fuel cell applications

Yttria stabilized zirconia (YSZ) films were grown by pulsed laser deposition (PLD) from a 8YSZ target using a KrF excimer laser source (248 nm). The films have been deposited under oxygen atmospheres on porous NiO/YSZ substrates heated from room temperature to 600 °C. YSZ films were obtained in the range of 1–2 μm thickness. The films have been investigated with respect to surface morphology, microstructure and film–substrate interface interaction. The film morphology varied from columnar to an irregular crystalline structure depending on the oxygen pressure and the substrate temperature. In all cases the films consisted of YSZ with the cubic fluorite structure. The formation of oxide layers under low oxygen pressures on the NiO/YSZ substrates is due to a film–substrate redox interaction. The NiO grains close to the coating interface are partially reduced and serve as an oxygen source for the oxidation of the film. The measured He leakage rates to analyze the gas tightness of the YSZ films have so far shown no improvements as compared with uncoated substrates.

Performance of 0.2 μm optical lithography using KrF and ArF excimer laser sources

KrF and ArF excimer laser lithography are considered to be candidates for a next lithography tool to achieve a miniaturization below quarter micron. To improve the focus latitude and overlay accuracy, an in-house KrF excimer laser stepper with off-axis illumination has been developed. This developed system can successfully resolve 0.2 μm features using off-axis illumination combined with an in-house developed positive chemically amplified resist and can also attain overlay accuracy of better than 60 nm using heterodyne holographic alignment. Reducing the optical absorption of the resist and the lens material at 193 nm was a challenge. The newly developed 193 nm resist contains no aromatic groups, and the designed projection lens has aspherical elements to establish ArF excimer laser lithography.

Photodissociation of S4N4. I. 248 nm

S4N4 was photolyzed by the KrF excimer laser source at 248 nm. The excited state photofragments produced are the NS(B 2Π)r and NS(H 2Π)1/2, NS(G 2Σ−) and NS(I 2Σ+) states. Single band progression fluorescence out of v′=0 for NS(H)1/2 and v′=2 for NS(G,I) are observed. Franck–Condon factors have been calculated for NS(H) up to v′=3. The electronic transition moment for the NS(H–X) transition was found to decrease slightly with increasing r̄. The radiative lifetime of the 0,5 band of NS(H)1/2 is determined to be 87±11 ns. The collisional quenching constants at 335 K for various species including N2 (kq=3.4±0.7×10−10 cm3 s−1 ), CF4 (kq=1.8±0.4×10−10 cm3 s−1 ), SF6 (kq=3.9±0.7×10−10 cm3 s−1 ) and He (kq=5.6±2.2×10−11 cm3 s−1 ) are reported. A semiempirical calculation using a PM3 Hamiltonian was used to calculate the heats of formation of various (SN)x species. A mechanism is proposed to account for the presence of these excited states based on laser fluence, excited state time histories, spectral composition and calculated heats of formation. NS(B) is postulated to arise directly from an intermediate photolysis product which is assumed to be acyclic S3N3. The NS(H 2Π)1/2 subband, NS(G 2Σ−) and NS(I 2Σ+) states are thought to be produced from a resonant interaction with the KrF line at 248 nm and vibrationally excited ground state NS. Using the I2(D′2g−A2u) emission as an actinometer, the overall efficiency on the conversion of absorbed photons by S4N4 into NS(B 2Π) is 2.6±0.7%.

Recent advances in an excimer laser source for microlithography

This paper describes the advances made in a KrF excimer laser source designed specifically for the deep‐UV reduction stepper. Data is presented on key performance aspects that are required for the laser to be used in the next generation of excimer steppers. Major topics discussed will be spectral bandwidth reduction, center wavelength control, pulse‐to‐pulse energy stability, higher power performance, and the production worthiness of the system. The first generation excimer laser (CX‐2LS) for lithography operated at 200 Hz and 3 W, with a spectral bandwidth of 3 pm and a wavelength stability of ±0.5 pm. The new laser (ELS‐4000) is designed and engineered to perform at 400 Hz and 4 W with a narrower spectral bandwidth (2 pm) and a wavelength stability of better than ±0.25 pm. In addition, the pulse‐to‐pulse stability has been improved in stepper‐mode operation. The ELS‐4000 also employs a new microprocessor controlled gas injection system that permits uninterrupted operation for 10–20 h (7–10 million pulses). This level of performance assures the compatibility of the laser with the requirements of production excimer steppers being developed with higher numerical aperature lenses, larger fields, and higher wafer throughputs.