Andrew C. Tam

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.

Efficient pulsed laser removal of 0.2 μm sized particles from a solid surface

Laser cleaning with pulsed ultraviolet and infrared lasers is successfully employed to remove particulate contamination from silicon wafer surfaces and from delicate lithography membrane masks. Particulate material investigated include latex, alumina, silicon, and gold. Gold particles as small as 0.2 μm can be effectively removed. This new and highly efficient laser cleaning is achieved by choosing a pulsed laser with short pulse duration (without causing substrate damage), and a wavelength that is strongly absorbed by the surface; the removal efficiency is further enhanced by depositing a liquid film of thickness on the order of micron on the surface just before the pulsed laser irradiation.

Remote sensing applications of pulsed photothermal radiometry

The technique of pulsed photothermal radiometry (PPTR) (i.e., detection of the transient infrared thermal radiation from a condensed matter sample heated by a short‐pulsed radiation) is useful for single‐ended remote sensing applications. We demonstrate for the first time such applications for spectroscopic detection, measurement of absolute absorption coefficients in opaque materials, and sensing of dimensions or thermal properties in inhomogeneous materials. Theory of the PPTR technique in the simple case of a semi‐infinite homogeneous material is described.

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 ...

Pressure generation and measurement in the rapid vaporization of water on a pulsed-laser-heated surface

The transient pressure generated by the interaction of short‐pulsed laser light with the liquid–solid interface is studied quantitatively. A KrF excimer laser beam of tens of nanoseconds pulse duration irradiates water on a solid surface and induces rapid thermal expansion and explosive vaporization. The pressure pulses launched into water by such processes are detected experimentally by the photoacoustic probe beam deflection method and a broadband piezoelectric transducer. The peak intensities of the traveling pressure wave measured by these two methods are compared with the theoretical thermoelastic predictions. The measurements show that a compressional pressure wave packet is radiated from the water‐solid interface with the peak intensity of the order of 1 MPa at laser fluences up to about 100 mJ/cm2. Simultaneous monitoring of the bubble growth kinetics by the optical specular reflectance probe has been performed. It is observed that the pressure generation is enhanced by the bubble expansion in the...

A new high-precision optical technique to measure magnetostriction of a thin magnetic film deposited on a substrate

The drive in data storage technology towards utilizing magnetic films with lower magnetostriction (to reduce the magnetoelastic energy term) and reduced thickness has resulted in the requirement for more sensitive, reliable, and easy-to-use tools to monitor magnetostriction. A measurement tool based on an in-plane rotating and saturating magnetic field and laser-beam-deflection technique, which is able to meet these requirements, is described. The tool developed offers high accuracy, large dynamic range, long-term stability, simple sample insertion, and a fast, easy measurement procedure. With this tool, the measurement of small magnetostriction coefficients of thin soft-magnetic films can become a simple, fast, and reliable procedure, thus helping the development of magnetic thin-film production processes and routine composition control. >

Marangoni mechanism in pulsed laser texturing of magnetic disk substrates

This paper proposes a mechanism for topographical features formed during pulsed laser texturing of Ni-P magnetic disk substrates. A salient feature of the process is the ability to raise a central peak in the irradiated spot, providing a low contact area bearing for the slider-head of a computer hard drive. Formation of topography is believed to involve gradient capillary forces acting at the surface of the molten pool (Marangoni effect). However, the central peak cannot be explained with thermo-capillary forces alone. Therefore, it is suggested that a compositional gradient due to the depletion of a surfactant at the molten surface provides the necessary condition to reverse the capillary force in the central region. This perspective is investigated using finite element modeling of the Lagrangian fluid mechanics coupled with heat and mass diffusion.

Optoacoustic determination of photocarrier generation efficiencies of dye films

An optoacoustic method is used for the first time to study photoconductive efficiencies of thin dye films coated on a substrate. A piezoelectric transducer is attached to the substrate, and the dye film is excited by a modulated cw laser beam, of intensity less than 0.1 mW/cm2. Strong signal enhancement occurs when the laser modulation frequency is equal to a fundamental mechanical resonance frequency of the sample‐transducer assembly (about 40 kHz in our experiment). The quantum efficiency for photocarrier generation is derived by measuring the decrease in optoacoustic signal when an electric field is applied to the sample.

Photoacoustic generation and detection of 10‐ns acoustic pulses in solids

We describe the generation and detection of short acoustic pulses (∼10‐ns widths) in opaque plates, using weak laser pulses (less than 1‐mJ energy) for excitation. The longitudinal, shear, and surface acoustic waves generated by a single laser pulse can be time resolved by using a thin‐film piezoelectric transducer, and acoustic mode conversions at surfaces are clearly observable. This new photoacoustic material testing technique is useful for fast ultrasonic measurements or for detecting deep subsurface flaws.

Optical probing of the temperature transients during pulsed-laser induced boiling of liquids

The thermodynamics of the rapid boiling of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The transient temperature field is measured by monitoring the reflectance of a thin film with calibrated optical properties. The metastability behavior of the liquid and the criterion for the liquid‐vapor phase transition in nanosecond time scale are obtained for the pressure from 1 atmosphere to 3.3 MPa.