Lena Nicolaides

Novel dental dynamic depth profilometric imaging using simultaneous frequency-domain infrared photothermal radiometry and laser luminescence

A high-spatial-resolution dynamic experimental imaging setup, which can provide simultaneous measurements of laser-induced frequency-domain infrared photothermal radiometric and luminescence signals from defects in teeth, has been developed for the first time. The major findings of this work are (i) radiometric images are complementary to (anticorrelated with) luminescence images, as a result of the nature of the two physical signal generation processes; (ii) the radiometric amplitude exhibits much superior dynamic (signal resolution) range to luminescence in distinguishing between intact and cracked sub-surface structures in the enamel; (iii) the radiometric signal (amplitude and phase) produces dental images with much better defect localization, delineation, and resolution; (iv) radiometric images (amplitude and phase) at a fixed modulation frequency are depth profilometric, whereas luminescence images are not; and (v) luminescence frequency responses from enamel and hydroxyapatite exhibit two relaxation lifetimes, the longer of which (approximately ms) is common to all and is not sensitive to the defect state and overall quality of the enamel. Simultaneous radiometric and luminescence frequency scans for the purpose of depth profiling were performed and a quantitative theoretical two-lifetime rate model of dental luminescence was advanced.

Computational Aspects of Laser Radiometric Multiparameter Fit for Carrier Transport Property Measurements in Si Wafers

A computational multiparameter fitting methodology that uses a three-dimensional laser photothermal radiometric model for semiconductors is presented in this study. One- and three-dimensional models of the free-carrier plasma-wave generation and response to laser photothermal (PT) excitation in a semiconductor have been reported in the literature. 1,2 The amplitude of PT response in these models has been used to measure carrier transport properties of electronic materials. The total radiation emitted from a silicon sample illuminated with a modulated laser beam arises from two sources: emission of IR radiation from the photoexcited carrier plasma-wave (injected excess carrier density) and from direct lattice photon absorption and optical-to-thermal (nonradiative) power conversion leading to temperature rise (a thermal wave). 1,3 Sheard and co-workers 1,2 observed experimentally that under infrared photothermal radiometric (PTR) detection, carrier emission dominates and the thermal-wave contribution can be neglected for some Si samples. This observation was addressed theoretically by Salnick et al. 4,5 These authors generated a composite plasma- and thermal-wave PTR model of semiconductors and showed that the plasma-wave signal component can dominate in high-quality materials virtually at all modulation frequencies. However, in this model the radial spatial variation of laser-generated excess carriers and of the temperature rise was not considered. Ikari et al. 6 have recently presented a general theoretical model for the laser-induced PTR signal from a semiconductor wafer of finite thickness using a three-dimensional geometry. In this model, carrier diffusion and recombination, as well as heat conduction, along the radial and axial directions in the sample were taken into account using cylindrical coordinates. A pair of conventional coupled plasma- and heat diffusion-wave equations were written and solved in Hankel space. In this theoretical framework, the plasma and thermal components can be written as follows

Quantitative dental measurements by use of simultaneous frequency-domain laser infrared photothermal radiometry and luminescence

Modulated (frequency-domain) infrared photothermal radiometry (PTR) is used as a dynamic quantitative dental inspection tool complementary to modulated luminescence (LM) to quantify sound enamel or dentin. A dynamic high-spatial-resolution experimental imaging setup, which can provide simultaneous measurements of laser-induced modulated PTR and LM signals from defects in teeth, has been developed. Following optical absorption of laser photons, the experimental setup can monitor simultaneously and independently the nonradiative (optical-to-thermal) energy conversion by infrared PTR and the radiative deexcitation by LM emission. The relaxation lifetimes (tau1, tau2) and optical absorption, scattering, and spectrally averaged infrared emission coefficients (mu(alpha), mu(s), mu(IR)) of enamel are then determined with realistic three-dimensional LM and photothermal models for turbid media followed by multiparameter fits to the data. A quantitative band of values for healthy enamel with respect to these parameters can be generated so as to provide an explicit criterion for the assessment of healthy enamel and, in a future extension, to facilitate the diagnosis of the onset of demineralization in carious enamel.

Image-enhanced thermal-wave slice diffraction tomography with numerically simulated reconstructions

Thermal-wave slice diffraction tomography (TSDT) is a photothermal imaging technique for non-destructive evaluation (NDE), leading to the detection of subsurface cross sectional defects in opaque solids in the very-near-surface region ( - mm). An exact, Greens-function based mathematical model that represents the behaviour of a three-dimensional thermal wave is developed and correlated with a numerical reconstruction technique. The computational technique utilizes the well known Tikhonov regularization method to invert almost singular matrices resulting from the ill-posedness of the inverse thermal-wave problem for the reconstruction of thermal diffusivity cross sectional images in materials. Numerical calculations of the inverse problem are carried out using the Born approximation and simulated reconstructions in back scattering and transmission are presented.

Physical mechanisms of thermal-diffusivity depth-profile generation in a hardened low-alloy Mn, Si, Cr, Mo steel reconstructed by photothermal radiometry

It is well established that in hardened steels thermal-diffusivity broadly anticorrelates with microhardness, allowing thermal-wave depth profilometry to be used as a tool to measure microhardness profiles. Nevertheless, the physical mechanisms for this anticorrelation have not been well understood. In this work, the thermal-diffusivity profiles of rough, hardened industrial steels were reconstructed after the elimination of roughness effects from the experimental data. Carburizing and quenching are widely used for the heat treatment of steel components, and it is important to understand their effects on thermal-diffusivity profiles. A thorough examination of the actual mechanism by which thermal-diffusivity depth profiles are affected by first carburizing and then quenching AISI-8620 steels was performed. It was concluded that the variation of thermal diffusivity with depth is dominated by the carbon concentration profile, whereas the absolute value of the thermal diffusivity is a function of microstructure.

Nonlinear inverse scattering methods for thermal-wave slice tomography: a wavelet domain approach

A wavelet domain, nonlinear inverse scattering approach is presented for imaging subsurface defects in a material sample, given observations of scattered thermal waves. Unlike methods using the Born linearization, our inversion scheme is based on the full wave-field model describing the propagation of thermal waves. Multiresolution techniques are employed to regularize and to lower the computational burden of this ill-posed imaging problem. We use newly developed wavelet-based regularization methods to resolve better the edge structures of defects relative to reconstructions obtained with smoothness-type regularizers. A nonlinear approximation to the exact forward-scattering model is introduced to simplify the inversion with little loss in accuracy. We demonstrate this approach on cross-section imaging problems by using synthetically generated scattering data from transmission and backprojection geometries.

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.

Laser infrared photothermal radiometric depth profilometry of steels and its potential in rail track evaluation

Laser Infrared Photothermal Radiometry has been utilized for several thermal-wave inverse-problem NDE applications. These include depth profilometry of steels and rails and scanning tomography of sub-surface defects in steels. Further, a computational algorithm has been refined to address the ill-posedness of the thermal-wave inverse problem. As a result, depth profiles of case-hardened steels, railway track heads from the field, and machined sub-surface hole thermal-diffusivity images have been reconstructed successfully using this emerging NDT technology.

Methods for surface roughness elimination from thermal-wave frequency scans in thermally inhomogeneous solids

Two approaches for eliminating surface roughness in the thermal-wave frequency response of inhomogeneous solids are developed. The first approach is based on the theoretical formulation of roughness as an effective homogeneous overlayer and is adequate for eliminating low roughness levels from experimental data. The second approach models roughness as random spatial white noise resulting in a linear superposition of logarithmic-Gaussian distributions representing roughness scales in the spatial frequency spectrum and in the modulation frequency domain. Two scales of roughness on the surface of hardened AISI 8620 steel with the same hardness depth profiles are found and the experimental data are reconstructed to retrieve similar inhomogeneous thermal diffusivity depth profiles.

Simultaneous determination of ultra-shallow junction depth and abruptness using thermal wave technique

Thermal wave (TW) studies of ultra-shallow junctions (USJ) formed by ion implantation into a semiconductor wafer followed by rapid thermal annealing (RTP) are described. It is shown that using the TW technique allows for a simultaneous determination of the most important USJ parameters—depth and profile abruptness. In a TW-based system, the USJ depth is obtained using the quadrature component of the TW signal while determination of USJ profile abruptness is based on the analysis of the TW quadrature and in-phase components measured at two different pump-probe beam offsets. Experimental results for junction depth and abruptness obtained on a set of B+-implanted, RTP-annealed USJ samples show better than 0.99 correlations to the corresponding secondary ion mass spectroscopy data.