Monitoring the Formation of Thin Films by Photothermal Technique

Abstract

Evaporation is a widely used technique for developing thin films. In this study the formation of a film on top of a substrate is investigated using a photothermal technique. The configuration implies the illumination of an opaque substrate by modulated laser radiation and the detection of the heat at the illuminated zone. The system is considered as a two-layer system in which the non-illuminated layer presents a dynamic response, the thickness and thermophysical properties, which change as a function of time. Results of modeling such a system, which depend on the evaporation rate and concentration changes are presented. It was found that for the process in which evaporation constitutes a dominant contribution, one observes a stable long time process, and at a certain instant, the amplitude of the photothermal signal suddenly grows. On the other hand, additional contributions (such as the increase of concentration) involve processes that cause the amplitude of the photothermal signal to grow smoothly with time. The contributions of different mechanisms involved during the film formation are discussed.

Photothermal Study of the Formation Dynamics of Fumed Silica Thin Films

“…For the second stage, after 500 s, a characteristic minimum appears when the thermal diffusion length of the sample is comparable with the thickness of the sample. This behavior has been predicted theoretically, and its interpretation has to do with the interference of thermal waves [8]. In the last stage, the PA signal increases, strongly reaching a final constant value that corresponds to the end of the process of the film formation.…”

“…The time evolution of a similar system to the one analyzed here has been studied theoretically [8]. It has been shown that the main characteristics of the curves can be understood considering a simple two-layer system as shown in Fig.…”

“…Furthermore, during the monitoring of film formation by thermal waves, it could happen, as a consequence of the evaporation process, that the thickness of the sample becomes of the order of the thermal diffusion length μ = (α/π f ) 1/2 , which is related with the thermal diffusivity (α) and the modulation frequency ( f ) of the heat source. In this case, thermal wave interference effects are expected to be observed [8,21] when the thickness is close to the thermal diffusion length. These effects and ulterior processes occurring after them deserve to be investigated and can be the clue to develop useful methodologies for PT monitoring of thin film formation by casting.…”

“…Recently, it has been of great interest in studying the dynamic evolution of different systems using PT techniques [5][6][7][8]. A useful procedure in the study of dynamical systems consists of the measurement of the PT signal during the time evolution of the physical system, maintaining a fixed modulation frequency of the light source.…”

Diffusion of Methylene Blue in Phantoms of Agar Using a Photoacoustic Technique

“…PAs have been used recently in the study of the evolution of dynamic systems, such as oxygen release in plants, blood sedimentation, evaporation of liquids, etc. [8][9][10][11]. The PA signal is not only directly related to the time evolution of the optical and thermal properties, but also with various physical processes leading to modulated heat and additional changes in the geometry of the sample [12].…”