Jose A. Garcia

Thermal-wave resonator cavity design and measurements of the thermal diffusivity of liquids

A liquid-ambient-compatible thermal wave resonant cavity (TWRC) has been constructed for the measurement of the thermal diffusivity of liquids. The thermal diffusivities of distilled water, glycerol, ethylene glycol, and olive oil were determined at room temperature (25 °C), with four-significant-figure precision as follows: (0.1445±0.0002)×10−2 cm2/s (distilled water); (0.0922±0.0002)×10−2 cm2/s (glycerol); (0.0918±0.0002)×10−2 cm2/s (ethylene glycol); and (0.0881±0.0004)×10−2 cm2/s (olive oil). The liquid-state TWRC sensor was found to be highly sensitive to various mixtures of methanol and salt in distilled water with sensitivity limits 0.5% (v/v) and 0.03% (w/v), respectively. The use of the TWRC to measure gas evolution from liquids and its potential for environmental applications has also been demonstrated.

Pd/PVDF thin film hydrogen sensor based on laser-amplitude-modulated optical-transmittance: dependence on H2 concentration and device physics

Abstract A new all-optical laser-intensity-amplitude-modulated hydrogen sensor has been developed as a next-generation device to the earlier photopyroelectric hydrogen sensor. When modulated light is incident on a palladium thin film coated on a polymeric membrane, optical reflectance and transmittance signals are generated in photodiode detectors. When hydrogen gas comes into contact with the thin palladium film the gas is absorbed, altering the optical properties of the palladium and producing a signal output dependent on hydrogen concentration in the ambient. The detection range of this sensor is between 0.2 and 100% [H2] by volume and is intended for sensitive monitoring of the explosive range, 4% by volume H2 in air. This sensor is very durable and robust; no palladium delamination or blistering occurs even after repeated exposures to pure hydrogen. The signal dependencies on increasing hydrogen concentration were found to be consistent with increased occupation of empty electronic states by electrons associated with absorbed hydrogen atoms, leading to upward shifts of the Pd Fermi level, EF, and to decreasing optical transition probabilities.

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

Normalized photoacoustic techniques for thermal diffusivity measurements of buried layers in multilayered systems

The one-dimensional heat diffusion problem for a three-layer system is solved assuming the surface absorption model. The analytical solution is shown to be suitable for the implementation of normalized depth-profilometric photoacoustic methodologies involving the open photoacoustic-cell configuration for thermal diffusivity measurements in buried underlayers within a three-layer stack. Our normalization procedures eliminate the frequency-dependent instrumental electronic contribution (transfer function) and some thermophysically nonrelevant proportionality factors in the theoretical equations, thus making the depth-profilometric analysis feasible. The measurement methodology is achieved by normalizing the theoretical photoacoustic signal from the three layers with the corresponding signal from the uppermost two layers, involving linear fits to measure the thermal diffusivity of the third underlayer. Three different multilayered materials were examined using the proposed methodologies. High reproducibility of the thermal diffusivity measurements and good agreement with values reported in literature were found. Besides the foregoing procedures, a lumped photoacoustic model was developed, which yields the effective thermal-diffusivity value of the multilayer stack.

Thermophysical Properties of Thermal Sprayed Coatings on Carbon Steel Substrates by Photothermal Radiometry

Laser infrared photothermal radiometry (PTR) was used to measure the thermophysical properties (thermal diffusivity and conductivity) of various thermal sprayed coatings on carbon steel. A one-dimensional photothermal model of a three-layered system in the backscattered mode was introduced and compared with experimental measurements. The uppermost layer was used to represent a roughness-equivalent layer, a second layer represented the thermal sprayed coating, and the third layer represented the substrate. The thermophysical parameters of thermal sprayed coatings examined in this work were obtained when a multiparameter-fit optimization algorithm was used with the backscattered PTR experimental results. The results also suggested a good method to determine the thickness of tungsten carbide and stainless-steel thermal spray coatings once the thermophysical properties are known. The ability of PTR to measure the thermophysical properties and the coating thickness has a strong potential as a method for in situ characterization of thermal spray coatings.

Minority carrier lifetime and iron concentration measurements on p-Si wafers by infrared photothermal radiometry and microwave photoconductance decay

A comparative study of electronic transport properties of p-Si wafers intentionally contaminated with Fe was performed using infrared photothermal radiometry (PTR) and microwave photoconductance decay (μ-PCD). Strong correlations were found between PTR and μ-PCD lifetimes in a lightly contaminated wafer with no significant PTR transient behavior. The absolute PTR lifetime values were larger than the local averaged μ-PCD values, due to the different excitation wavelengths and probe depths. In a heavily contaminated wafer the μ-PCD and PTR lifetime correlation was poorer. PTR measurements were highly sensitive to iron concentration, most likely due to the dependence of the bulk recombination lifetime on it. Rapid-scanned (nonsteady-state) PTR images of the wafer surface exhibited strong correlations with both μ-PCD lifetime and [Fe] concentration images in both heavily and lightly contaminated wafers. For the lightly and uniformly contaminated wafer, PTR scanning imaging was found to be more sensitive to ir...

Pd/PVDF thin film hydrogen sensor system based on photopyroelectric purely-thermal-wave interference

Abstract A novel sensitive solid-state sensor system for trace hydrogen gas detection has been developed as a next generation device to earlier photopyroelectric (PPE) hydrogen sensors. The basic principle of the sensor is based on the technique of PPE purely-thermal-wave interferometry recently developed in this Laboratory. The active element of the sensor is a thin polyvinylidene fluoride (PVDF) pyroelectric film, sputter-coated with Pd on one surface and with a Ni–Al alloy electrode on the other surface. Unlike the conventional PPE hydrogen sensors, this new sensor produces a coherent differential PPE signal in a single detector, rather than using two detectors (one active, the other reference) and complicated electronics. The measurement results show that the signal noise level, the detectivity and the signal dynamic range are improved by more than one order of magnitude compared with the conventional single-beam method. The operating characteristics have been examined for three different thicknesses of Pd coating on the same thickness PVDF-film detector. The signal generating mechanism, attributed to the change of the optical absorptance of the Pd coating when exposed to hydrogen, and/or the shift in the Pd work function, is also discussed.

Study of the thin‐film palladium/hydrogen system by an optical transmittance method

The thin‐film palladium/hydrogen system was studied using a novel all optical method. Isotherms at room temperature (25 °C) were obtained for palladium films with different thicknesses. The measured isotherms included the α, α‐to‐β, and β phase regions. A decrease of the phase transition region was observed as the palladium film thickness was decreased. This optical method has good potential for use in studying the equilibrium and kinetic aspects of any thin‐film/gas system.

Kinetics of surface-state laser annealing in Si by frequency-swept infrared photothermal radiometry

Frequency-swept (“chirped”) infrared photothermal radiometry was combined with conventional single-frequency modulation of an Ar ion laser beam to yield a quantitative study of the surface-state annealing processes induced by the low-fluence laser beam on n- and p-type Si wafers. The appearance of a signal transient was found to be strongly dependent on the electronic quality of the wafer surface and was absent in the thermally oxidized p-Si wafer. The low-injection minority-carrier lifetimes and diffusion coefficients were not affected by the laser-surface interaction, but the surface recombination velocity strongly decreased with time of exposure. A two-trap rate model was advanced to explain the transient behavior in terms of surface-state annealing and carrier ejection.

Microelectronic circuit characterization via photothermal radiometry of scribeline recombination lifetime

Abstract Three-dimensional (3D) photothermal radiometric microscopic imaging and laser-intensity-modulation frequency scans have been used for the non-contact, non-intrusive measurement of electronic transport properties of integrated circuits in patterned 4″ Si wafers. The experimental data showed that carrier recombination lifetimes along each scribeline remain constant. However, variations in surface recombination velocities and carrier diffusion coefficients were found. It was further found that such variations are related to the presence of highly doped poly-Si structures adjacent to the scribeline. As a result of these measurements, it is concluded that scribeline photothermal radiometric probing can be used effectively for monitoring local values of the carrier recombination lifetime and, through those, wafer contamination and damage during device fabrication processing.