Jon Opsal

Detection of thermal waves through optical reflectance

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.

Thermal-wave depth profiling: Theory

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.

Thermal and plasma wave depth profiling in silicon

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.

Thin‐film thickness measurements with thermal waves

We have developed a method for measuring the thickness of thin films that is nondestructive, noncontact and that can make measurements with 2‐μm spatial resolution (i.e., 2‐μm spot size) on both optically opaque as well as optically transparent films. With this method, which is based on the use of high‐frequency thermal waves, thicknesses of Al and SiO2 films on Si substrates have been measured in the 500–25 000‐A range.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.

Temporal behavior of modulated optical reflectance in silicon

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.

Beam profile reflectometry: A new technique for dielectric film measurements

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.

Multiparameter measurements of thin films using beam-profile reflectometry

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.

Detection of thermal waves through modulated optical transmittance and modulated optical scattering

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.

Thermal Wave Imaging with Thermoacoustic Detection

Thermoacoustic imaging is a new technique that permits the detection and imaging of microscopic surface and subsurface fea- tures in a sample. We present a detailed theoretical treatment of the thermoacoustic process with particular emphasis on the roles of ther- mal and elastic parameters in both one and three dimensions. Exam- ples are presented of thermoacoustic images obtained with a scanning electron microscope. These include images of subsurface mechanical defects, metallic grains and grain boundaries, and of dislocation net- works and dopant regions in semiconductor crystals.

Nondestructive analysis of ultrashallow junctions using thermal wave technology

It is shown that the thermal wave (TW) nondestructive technology widely used in semiconductor industry for ion-implant monitoring can also be used for characterization of ultrashallow junctions created as a result of thermal annealing of ion implanted wafers. A set of Si wafers implanted with boron at energies 0.2–0.5 keV and implantation doses in the range of 1014–1015 cm−2 thermally annealed at different temperatures (950–1100 °C) has been studied. For all samples, the TW signal is found to vary linearly with junction depth and is shown to exhibit a very good correlation with secondary ion mass spectrometry data. A special processing of experimental data using both the TW quadrature and in-phase signal components allowing for resolution of effects introduced by different implantation doses, energies, and annealing temperatures is discussed.