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A quantitative analysis based on a new three-dimensional nonlinear theoretical model of the experimental fundamental nonlinear photothermal response from multilayer structures containing a tungsten layer is presented. Two sets of wafers, one with a very rough tungsten overlayer surface and another with a smooth polished tungsten surface have been studied. It is shown that the rough tungsten surface gives rise to a very strong nonlinear behavior of the thermal-wave field.

A general three-dimensional theoretical model for fundamental and harmonic response generation as a result of periodic heating of a system consisting of a nonlinear layer with temperature-dependent thermal conductivity and specific heat and a linear substrate is developed. Analysis of the fundamental component of the surface temperature shows that the nonlinear thermal conductivity alone does not affect the phase of the thermal-wave field. The efficiency of the thin nonlinear layer as an energy conversion filter that drives the harmonic response of the substrate is shown by the analysis of the limiting cases of the theoretical model.

Beam‐profile reflectometry is a new technique for measuring the thickness and optical constants of dielectric, semiconducting, and thin metal films. The technique consists of measuring the intensity profile of a highly focused beam reflected from the sample. By using a linearly polarized light source and a tightly focussed beam, the S‐ and P‐polarization reflectivities of the film are simultaneously obtained over a wide range of angles. The focusing also provides a submicrometer spot, thus allowing these measurements to be performed in very small geometries. How the information present in the reflectivity profiles can reveal information about as many as three unknown film parameters simultaneously is described, and results from film samples for which the multiparameter fitting capability was essential to the success of the measurement are also presented.

We describe a new technique for measuring the thickness and optical constants of dielectric, semiconducting, and thin metal films. Beam profile reflectometry provides excellent precision for films as thin as 30 A and as thick as 20 000 A. The technique is also capable of simultaneous 2 and 3 parameter measurements and it performs all measurements with a submicron spot size.

Thermal features on and beneath the surface of a sample can be detected and imaged with a thermal-wave microscope. Various methodologies for the excitation and detection of thermal waves are discussed, and several applications, primarily in microelectronics, are presented.

The exact form of the Rosencwaig‐Gersho theory for the photoacoustic effect in solids is used to depict how the magnitude and phase of the photoacoustic signal vary with chopping frequency and normalized length. Applications of the theory to studies of film‐substrate systems are discussed.

There are a great many substances, both organic and inorganic, that, because of their physical state, cannot be readily studied by conventional absorption or reflection techniques. In photoacoustic spectroscopy, light absorbed by the sample is converted into a measurable acoustic signal, and spectra closely corresponding to optical absorption spectra, can be obtained on any type of solid or semisolid material, whether it be crystalline, powder, smear, gel, etc. The physical processes underlying the photoacoustic effect in solids, and a mathematical analysis of the production of the photoacoustic signal will be presented. The methodology of photoacoustic spectroscopy will also be described. Various applications of this technique in the fields of physics, chemistry, biology, and medicine will be discussed. These applications will be treated under three main headings: bulk studies, surface studies, and de‐excitation studies.

We report on the results of a study of the temporal behavior of the laser‐induced modulated optical reflectance from the surfaces of crystalline silicon wafers, epitaxial silicon films, and ion implanted but unannealed silicon wafers. The observed temporal behavior of this signal appears to be associated with the presence and temporal evolution of electronic surface states.

We have developed a one‐dimensional model for thermal‐wave depth profiling that provides expressions for the temperature at the surface of the sample and for the thermoelastic response beneath the surface. The model shows that elastic wave interference effects produce significant differences between samples with mechanically free and constrained surfaces, and that thermal‐ wave images of thermal conductivity variations are obtainable from the thermoelastic signal only if the front surface is mechanically free. We have also considered the case of subsurface heating and found that for heating occurring at depths of more than a few thermal diffusion lengths, the thermoelastic signal becomes independent of thermal conductivity variations. This has important implications for thermal‐wave image range and resolution.

We have developed two photoacoustic methods to perform low absorption measurements on optical components. One method can make measurements in the 10−4–10−5 absorption range, while the other can be used down to the 10−6 range.

The scanning photoacoustic microscope (SPAM) is used in both the conventional and phase‐contrast modes to detect a well‐characterized subsurface flaw in Al. The physical mechanism is that of thermal diffusion, with a subsurface probe depth and flaw resolution length of approximately one thermal‐diffusion length. Comparison of the dependences of the photoacoustic signal upon chopping frequency from the different regions of the sample confirm that the differential signal from the flaw corresponds to a transition from thermally thick to thermally thin boundary conditions. Experimental results are in good agreement with calculations based upon a three‐dimensional thermal‐diffusion model.

A new method, based on thermal wave technology, is used to monitor the ion implantation process in silicon. It is a noncontact, nondestructive technique that requires no special sample preparation or processing, has high sensitivity even at low dose, and provides a one‐micron spatial resolution capability. This method allows, for the first time, the ability to monitor the critical ion implantation process directly on the patterned product integrated circuit wafers as well as on the usual test wafers.

A method and apparatus for thin film thickness measurements with thermal waves in which heating and detection laser beams are focused onto the film, normal to the surface of the film, with the two beams parallel and non-coaxial.

We have developed a photoacoustic method that is capable of detecting the occurrence of laser damage in optical thin films and of providing new data about energy transfer processes in the presence of damage.

Experiments on photoacoustic thermal‐wave microscopy are presented, for the first time, at high resolution. Using piezoelectric detection and a modulation frequency of 185 kHz, we obtain a thermal‐wave resolution of ∼7 μm. We show that the photoacoustic phase signal provides true thermal‐wave imaging even in the presence of surface features with strong optical contrast.

We describe a depth‐profiling concept using the critically damped plasma wave corresponding to the propagation of the free‐carrier plasma density generated by a modulated laser in a semiconductor.

Thermal‐wave electron microscopy has been used at 640‐kHz modulation frequency to detect and image P‐doped Si regions in a bare Si wafer.

We show that variations in local optical constants induced by the presence of thermal waves can be used for thermal wave detection and analysis through measurements of thermal wave‐induced modulated transmittance and scattering.

We have performed high‐resolution thermal‐wave microscopy of surface and subsurface features of an opaque solid using, for the first time, electron beams to generate the signal.

We show that thermal wave detection and analysis can be performed, in a noncontact and highly sensitive manner, through the dependence of sample optical reflectance on temperature. Applications to the study of microelectronic materials are illustrated by an example of measuring the thickness of thin metal films.

Conventional spectroscopic studies on opaque membranes are difficult to pursue because of excessive light scattering and complications arising from the specimen's surface. Recently, a new spectroscopic technique, photoacoustic spectroscopy, unlike conventional optical spectroscopy, has been demonstrated to be an informative technique amenable to spectroscopic studies of solids and membrane-like samples. We have applied photoacoustic spectroscopy to the study of hydration and maturation of newborn rat stratum corneum, and have obtained clean spectra in the 220 to 450 nm region indicative of a change in thermal diffusivity with increased hydration, and biochemical changes associated with the initial maturation period.

The dependence on the thermal properties of the gas in a photoacoustic cell predicted by the sample‐gas thermal resistance theory is also predicted by the Rosencwaig‐Gersho theory for very thin samples.

A new technique for performing optical spectroscopy on solids has been developed. Photoacoustic spectra of cytochrome c and hemoglobin show how this technique can be used to obtain information about optical absorptions and subsequent de-excitations in solid biological materials, particularly those which cannot readily be studied by conventional means.

There presently exists the need to measure the postanneal thickness of silicide films deposited on Si. In addition, there exists the need to monitor the deposition parameters of silicide films, since variations in the deposition process may alter the film stoichiometry, density, and preanneal electrical resistivity. In this paper we describe results of efforts to satisfy these needs using a thermal‐wave Therma‐Probe 100.

The physical processes in photoacoustic microscopy are described with an emphasis on the unique capability for microscopic thermal wave visualization of the sample.

Chopped light impinging on a solid sample in an enclosed cell produces an acoustic signal within the cell, A theoretical explanation for this effect is presented, providing a quantitative foundation for photoacoustic spectroscopy. Nonradiative de‐excitation of the absorbed light within the solid sample produces a periodically varying heat flow from the sample to the surrounding gas and backing material. The solution to the thermal diffusion equations shows that only a thin boundary layer of gas adjacent to the surface of the sample responds thermally to this periodic heat flow. The boundary layer can then be thought of as acting as an acoustic piston creating the acoustic signal detected in the cell. An explicit formula gives the. magnitude and phase of the acoustic pressure in terms of the optical, thermal, and geometric parameters of the system. Simple approximations for the pressure are given according to the relative magnitudes of the optical absorption length, thermal diffusion length, and thickness ...