Optical reflectivity of si above the melting point

The variation of the optical reflectivity of a Si single during irradiation with two successive Nd:YAG laser pulse is investigated with ns resolution. The first pulse melts the surface and therefore the reflection coefficient increases up to the value of the metallic liquid at the melting temperature (1685 K). Upon further heating the surface with the second, time-delayed pulse, a decrease of the reflection coefficient is observed, resulting from the temperature-dependent dielectric function of the molten Si. The largest decrease in the reflectivity that could be reached before damaging the surface amounted to 9% for a probe wavelength of 633 nm. The application of a simple Drude model for the optical constants above the melting point is discussed

Study of Nucleation Processes during Laser Cleaning of Surfaces

Bubble nucleation and growth dynamics on a nanosecond lime scale induced by pulsed laser heating or ucoustlc cavitation are scudied experimentally. Sensitive lest methods such as optical reflection and light hcatterlng, piezoelectric transducer, and surface plasmons are used to monitor threshold fluences for bubble nucleation, bubble growth velocities, and pressure effects in various liquids. A long-term memory effect, i.e., enhanced acoustic cavitation following laser-induced bubble formation at a liquid-solid interface could be demonstrated and its decay mode determined. Recent experimental results show that surface plasmons are especially qensitive for the study of the early stages of bubble nucleation, indicating that bubble nucleation sets on at a lower liquid superheat than previously determined using optical reflectance or piezoelectric transducer methods

Absolute pressure measurements on a nanosecond time scale using surface plasmons

Transient acoustic waves generated by laser-induced bubble formation at a liquid solid interface are sensitively monitored using optically excited surface plasmons. This method enables the detection of both the compressive and tensile waves with high accuracy as demonstrated for the propagation and reflection of acoustic pulses at a quartz water interface. Unique advantages of this new technique are the high sensitivity of 0.1 0.2 MPa that could be achieved for absolute pressure measurements on a nanosecond time scale and its ability to probe exact pulse profiles due to the localized probe depth of surface plasmons

Laser cleaning of silicon surfaces

The continuing trend towards miniaturization of integrated circuits requires increasing effots and new concepts to clean wafer surfaces from dust particles. We report here about our studies of the "steam laser cleaning" process first described by Tam and coworkers: In order to remove sub-micron particles from a surface, first a thin liquid layer is condensed onto the substrate from the gas phase, and is subsequently evaporated momentarily by irradiating the surface with a short laser pulse. We have investigated the nucleation and growth of gas bubbles in the liquid, by which the whole process is started, with optical techniques like light scattering and surface plasmon resonance spectroscopy. The experiments indicate that the temperature where nucleation sets in is surprisingly low, which facilitates the application of this phenomenon for cleaning purposes. On the basis of these results and in order to study the cleaning effect for the particularly interesting surface of silicon in a quantitative way, we have deposited well-characterized spherical polymer and silica particles of different diameters from several ten to hundred nanometers on commercial Si wafers and have studied systematically the cleaning efficiency of the explosive evaporation process. The results show that steam laser cleaning is a promising and suitable method for removing sub-micron particles from semiconductor surfaces

Bubble nucleation and pressure generation during laser cleaning of surfaces

Bubble nucleation and growth dynamics, on a nanosecond time scale, induced by pulsed laser heating of a liquid solid interface are studied experimentally. A surface-plasmon probe is implemented as a novel, highly sensitive method for the study of submicroscopic bubbles, providing accurate information on the nucleation thresholds, growth velocities, and transient pressure generation by rapid bubble growth. Owing to the higher sensitivity of the surface plasmon probe to small bubbles, it is demonstrated that bubble nucleation sets in at a lower liquid superheating than previously determined with the use of optical reflectance or piezoelectric transducer measurements. A comparison of experimentally determined bubble growth velocities with computational results confirms that bubble growth is governed by the heat transfer from the solid surface into the liquid. Reconstructed surface plasmon resonance curves from transient signals are used to estimate the fractional volume and number density of bubbles in the superheated liquid layer. Further, a surface plasmon probe is utilized for the absolute measurement of bubble-growthinduced pressure amplitudes on a nanosecond time scale. The measurements yield peak pressure amplitudes in the range of ~1 5MPa with a pressure pulse width of ~40 ns. Additionally, the phase of an acoustic pulse is observed to change upon reflection at the liquid solid interface if bubbles are present, providing a direct proof for laser-induced bubbles

Optical probing of the temperature and pressure transients at a liquid/solid interface due to pulsed laser-induced vaporization

The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependant optical properties, is embeeded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat trader modeling results. The backward reflectance probe is not affected by the mation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambrent pressures using a high-pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure

Steam laser cleaning of silicon surfaces : laser-induced gas bubble nucleation and efficiency measurements

The removal of dust particles from semiconductor surfaces requires new cleaning strategies such as Steam Laser Cleaning (SLC). It is based on laser-induced explosive evaporation of a liquid layer applied on the surface. We have investigated the laser-induced nucleation and growth of gas bubbles at silicon/water, silicon/isopropanol and silver-film/water - interfaces by light scattering and surface plasmon spectroscopy. The achieved superheating of the liquid before bubble nucleation sets in strongly depends on the substrate roughness. On rough metal films it is only about 30 K in water, compared to about 150 K on smooth silicon surfaces. Isopropanol (IPA) on smooth silicon surfaces could be heated to 116° C, corresponding to a superheating of 36 K. In combination with numerical calculations it was possible to determine the heat transfer coefficients silicon-water (x = 3 ·107 W/m2 K) and silicon IPA (x = 1 ·107 W/m2 K). Using optical techniques we have measured the pressure wave created by the growing bubbles and the bubble growth velocities. For a quantitative study of the efficiency of SLC we deposited spherical colloidal particles on industrial silicon wafers. We observed a sharp threshold for particle removal at 110 mJ/cm2 (laser l = 532 nm, FWHM = 8 ns) which is independent of the size (diameter 800 nm down to 60 nm) and material of the particles and efficiencies above 90% for particle removal. On the basis of our results we discuss the validity of the existing SLC models and the perspective of the application of SLC as an industrial cleaning tool

Enhanced acoustic cavitation following laser-induced bubble formation : long-term memory effect

The enhancement of acoustic caviation at a liquid-solid interface following laser-induced bubble formation is studied. The experiment results indicate that metastable ultramicroscopic bubbles formed on the solid surface cause a long-term memory effect on acoustic cavitation. By performing a double-pulse experiment using two excimer lasers, the temporal decay of this memory effect is determined for two different liquids on a chromium surface. An explanation of the observed decay mode by a diffusion model is presented

Memory-Effect on Acoustic Cavitation

The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it wiIl be shown that following laser-induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as memory effect, decays in a few hundred microseconds. By performiag a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and sdt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion madel is presented

Optical and acoustic study of nucleation and growth of bubbles at a liquid-solid interface induced by nanosecond-pulsed-laser heating

The dynamics of liquid-vapor phase-change in the nanosecond time-scale induced by pulsed-laser heating of a liquid on a solid sample is studied by means of optical reflectance and scattering measurements, and the piezoelectric detection technique. The Iiquids studied include water, ethanol, merhanol, IsoproPropyl Alcohol (IPA), and mixtures of water and IPA. The threshold fluence for nucleation is determined with high accuracy using the optical and acoustic signals. Heat diffusion calculations performed for the threshold fluences indicate that the liquids are sufficiently superheated before nucleation sets on. The transient optical reflectance signal is analyzed by an effective-medium theory to provide bubble-growth kinetics, so that the bubble-growth velocity for the test liquids could be estimated. In addition, it is observed that, following the thermally induced nucleation, repetitive acoustic cavitation at the surface of the solid sample occurs, with a time interval related to the speed ofsound in the liquid