Wing P. Leung

Laser‐cleaning techniques for removal of surface particulates

Flash laser heating using short‐pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm. This is very useful because micron‐ and submicron‐sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal. Several varieties of the new laser‐cleaning techniques have been developed by us as well as by others. For example, the pulsed laser irradiation can be used with or without the simultaneous deposition of a thin liquid film on the surface to be laser cleaned. The laser wavelength can also be chosen so that absorption occurs mainly at the sample surface, or in the liquid, or in the particulate, or in a combination of these. In this paper, we discuss and compare examples of these different approaches. We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laserirradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest ultraviolet laser fluence of ∼0.1 J/cm2.

Techniques of Flash Radiometry.

We analyze in detail flash radiometry techniques for the remote sensing of spectroscopic and physical properties of thin condensed matter samples. Such techniques rely on the transient infrared radiation from the sample heated by a short‐duration pulsed radiation. Exact analytical solutions for the conventional transmission radiometry technique (in which the excitation source and the infrared detector are on opposite sides of the sample) as well as the new backscattering radiometry technique (in which the excitation source and the detector are on the same side of the sample) are presented with the effect of heat loss neglected. The analysis allows the determination of the thermal diffusivity or thickness, as well as the absorption coefficients at the excitation wavelength and at the detecting wavelength of the sample from the experimental radiometry profile. The effects of excitation pulse duration and finite rise time of the detection system are discussed. Experiments with pulsed radiometry measurements ...

Noncontact monitoring of laser ablation using a miniature piezoelectric probe to detect photoacoustic pulses in air

We show for the first time the use of pulsed photoacoustic detection using a miniature piezoelectric transducer to study and monitor photoablation in air by an ultraviolet KrF excimer laser. At incident laser fluences below the ablation threshold, the photoacoustic signal is produced by the thermal expansion of the air near the irradiated surface (‘‘thermal‐piston signal’’) and propagates at the speed of sound. Above the ablation threshold, the photoacoustic signal contains an additional component due to the ejection of ablated materials (‘‘ablative‐piston signal’’); this propagates initially at supersonic speeds. The amplitude of the ablative‐piston signal is found to be a monotonic function of the etch‐depth per pulse. Hence, by accumulating the amplitude of the ablative acoustic pulse, the total etch depth can be monitored in real time.

Thermal diffusivity in thin films measured by noncontact single-ended pulsed-laser-induced thermal radiometry.

A pulsed nitrogen laser is used to induce a sharp thermal gradient in a thin film, and the infrared thermal radiation from the irradiated region is monitored from the same side as the excitation beam (i.e., single-ended detection). We show that the profile of this pulsed photothermal radiometry signal can be analyzed to provide the thermal diffusivity or thickness of the sample as well as information on subsurface modifications or the degree of thermal contact with a substrate. We present data for several important classes of film, including metal, polymer, and paper (e.g., in currency) and show the important features of the present technique for thin-film characterization, namely, nondestructive, fast, and remote sensing.

“Laser cleaning” removes particles from surfaces

Abstract Laser-induced removal of particles from surfaces was demonstrated. Such “laser cleaning” can be performed on the bare contaminated surface or with additional micronthickness liquid film coverage on the surface. With the latter “steam laser cleaning” we achieved removal of epoxy, alumina, silicon, and gold particles of size 0.1 μm to 10 μm from silicon surfaces. The proposed mechanism of particle ejection is described.

Photodeflection probing of the explosion of a liquid film in contact with a solid heated by pulsed excimer laser irradiation

The explosion or rapid vaporization of a liquid film on an opaque surface by a pulsed laser is studied experimentally. Using a probe‐beam deflection sensing (PDS) scheme, together with a previously developed transmission monitor, the distortion of the PDS signal due to the generation of shock waves by the exploding liquid is investigated. Various liquids, including alcohols and pure water in contact with substrates such as polyimide, amorphous carbon, and silicon, are studied for a wide range of excimer laser fluences. It is concluded that the present PDS technique is highly sensitive to the explosion threshold.

Thermal conduction at a contact interface measured by pulsed photothermal radiometry

We have used pulsed photothermal radiometry to measure the thermal contact resistance at the interface of a smooth polymer film and a polished metal substrate. This method relies on the heating of the film surface by a short light pulse and detecting the subsequent infrared thermal radiation from the surface. An analytical solution to the heat diffusion equation shows that in a suitable delayed time interval, the infrared signal decays exponentially in time with a time constant related to the thermal contact resistance of the interface. By changing gases in the interface at constant pressures, we are able to separate the thermal conductance into two components: that due to solid contacts and that due to gas conduction. The thermal conductance due to gas conduction in the interface is proportional to the thermal conductivity of the gas found in continuum fluid theory, except for He which is more than 30% lower. We believe that the discrepancy in He is partly due to the fact that the mean free path of He ga...

Effect of intense and prolonged 248 nm pulsed‐laser irradiation on the properties of ultraviolet‐grade fused silica

We have studied changes in the absorption, temperature, ultraviolet (UV) spectrum, birefringence, and surface profile of various high‐purity ‘‘UV‐grade’’ fused silica samples during KrF excimer laser irradiation at 400 mJ/cm2 and 300 Hz and up to a total of 60 million pulses. All samples exhibit permanent and irreversible changes in the spectroscopic and physical properties. Some samples show a partial ‘‘self‐annealing’’ behavior during the irradiation in which the absorption first increases to a peak value of ∼10%/cm, and then decreases and levels off at a lower value. This may indicate the formation of a new stable state. The radiation‐induced effects are significantly affected by the laser repetition rate, annealing, and ambient temperature of the sample. For comparison, crystalline quartz is tested under similar conditions, and is found to be essentially unchanged by the radiation.

Temperature dependence of optical constants for amorphous silicon

The temperature dependence of the optical constants for amorphous silicon (a‐Si) is studied for two different sample thicknesses at two infrared wavelengths. It is observed that the extinction coefficient of a‐Si can increase significantly with temperature in the strong absorption regime. In addition, using the Mott–Davis formula, results are obtained for the variation of the optical gap energy for a‐Si with temperature, with similar feature observed for both amorphous and crystal silicon.

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 liquids studied include water, ethanol, methanol, 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 of sound in the liquid.