Heating Of Thin Films Research Articles

The Microstructural Response of a Multilayer Thin Film to Local Laser Heating

ABSTRACTThe microstructures associated with pulsed-laser heating of multilayer thin films used for phase-change optical data storage are studied in three related films. Two employ the multilayer structure SiOx/TeGeSn/SiOx where the chalcogenide layer is either amorphous or crystalline. The third uses the same trilayer structure with a crystalline chalcogenide layer but with an additional layer of aluminum. Pulsed-laser melting of micron-sized spots leads to solidification in the non-aluminized specimens with a morphology dependent on the preexisting structure of the chalcogenide layer and to amorphization in the aluminized film. Heat-flow modelling shows that the maximum temperature reached is lower and the cooling rate through the glass-forming temperature range higher in the aluminized structure. Discussion centers on the thermal and optical properties of the different films and the impact of the pre-existing chalcogenide structure on the subsequent phase transformations.

The importance of thermal stresses and strains induced in laser processing with focused Gaussian beams

The thermoelastic equations are solved for laser heating of a semi-infinite elastic medium by a focused TEM00 Gaussian beam. Single integral expressions are derived for stresses and strains, which are analytically evaluated on axis at the surface and far from the laser-heated region (for a laser spot size≫absorption depth), and numerically evaluated for the example of laser heating of a silicon substrate. This analysis is extended to the case of laser heating of thin films on substrates. Use of these stress and strain profiles suggests that dislocations may form at the surface during high-temperature laser processing of silicon at scan speeds typically used in direct laser writing. Also the elastic strains induced during laser heating shift phonon frequencies from their thermal equilibrium values, thereby complicating the use of Raman scattering as an optical probe of temperature. This is shown to be particularly significant for laser heating of silicon thin films on fused silica substrates and not very important for laser processing of silicon substrates or silicon thin films on sapphire.

A study of laser marking of thin films

Some observations of laser marking of thin films are presented. Time-resolved reflectivity measurements performed during laser marking of thin organic films indicate that the pit-formation processes start in less than 10 ns after the onset of the laser pulse and that some reversal of the process occurs following exposure termination. Temperature calculations for laser heating of thin films with focused scanning beams explain some of the mark (pit) morphology and material flow behavior that is observed. Scanning electron microscopy (SEM) studies of the marks indicate that at high laser powers, pit formation occurs by ablation and at low laser powers, pits can be formed without ablation. Pits were also formed in a 100nm thin film by resistive heating where the maximum temperature was less than 300 °C.

Mullite formation during heating of thin films of eutectic composition in aluminosilicate systems

Mullite formation during heating of thin films of eutectic composition in aluminosilicate systems

Laser heating and melting of thin films with time-dependent absorptance: An exact solution for time intervals less than or equal to the transit time

Laser heating and melting is studied considering the laser-induced reflectivity variations R(τ). The study is made for time intervals less than or equal to the transit time using the Fourier series expansion technique together with the differential and integral forms of the heat diffusion equation. The equations obtained for the thickness of the molten layer Z(τ) and the rate of melting Z′(τ) are no longer quadratic. Computations for a thin silver film of thickness 5×10−5 m, subjected to constant laser irradiance of value q0=9×1010 W/m2 are carried out. Smaller threshold times required to initiate melting (i.e., damage) than those for the case of constant reflectivity are obtained. It is also found that melting occurs with nearly a constant rate.

Thermal and plasma models of pulsed heating of thin films : L. N. Aleksandrov. Vacuum36(7–9), 455 (1986)

Thermal and plasma models of pulsed heating of thin films : L. N. Aleksandrov. Vacuum36(7–9), 455 (1986)

Thermal and plasma models of pulsed heating of thin films

Abstract The main assumptions of thermal and plasma models of pulsed heating are discussed. With the effective time of energy dissipation of the electron-hole plasma in a crystal of ∼10 −8 s and in an amorphous phase of ∼10 −10 s for pulses longer than 10 −8 s, phase and structural transformations are described on the basis of a heat conduction equation. The participation of plasma processes (formation of electron-hole pairs and their recombination) is conditioned by delay of the plasma relaxation by structural defects and continuous formation of plasma and ionized atoms in the process of pulse action. It is possible to observe acceleration of diffusion processes and crystallization both in the field of mechanical stresses and in the electric field, photostimulation and weakening of atomic bonds in the plasma field. A possibility of kinetic overheating of a crystal without melting above T m has been demonstrated. The results of simulation of the crystallization by a thermal model within the pulse duration of 1 ps-10 s are presented. Agreement between theory and experiment is discussed.

Wavelength-dependent resonant surface heating and desorption with IR lasers: A new spectroscopic tool

Wavelength-dependent resonant heating of thin films by vibrational excitation of internal modes with a CO 2 laser is studied by measuring the time-of-flight distributions of desorbed molecules. The spectroscopic features of previously published results for CCl 4and C 6H 5CHO, and of data reported here for CH 2ClCH 2Cl, are discussed. The translational temperatures determined from Maxwellian distributions and the desorption yield possess the same spectral behavior as the IR absorption band.

Heating of thin films by laser radiation with allowance for the temperature dependence of the reflection coefficient

We solve the thermophysical problem of the heating of thin metal films on semiconductor substrates by laser radiation for linear and stepwise changes of the absorptivity as a function of the surface temperature.

Some reflection high energy electron diffraction and depth-selective Mössbauer spectroscopy investigations on the structures in evaporated thin tin films before and after their heating in vacuum

The structures and the structural transformations in four types of thin tin films, as determined from their depth-selective Mössbauer spectroscopy and reflection high energy electron diffraction (RHEED) characteristics, were investigated before and after they were heated in a vacuum of approximately 5×10 −6 Torr over the temperature interval 260–600(800)°C. It is shown that a form of tin with a diamond cubic structure and lattice parameters almost identical with those of silicon, here designated α 2-Sn, which shows an isomeric shift δ of +4.4±0.2 mms −1, and with the supposed participation of d electrons in its chemical bonding, spontaneously transforms into other tin structures with higher isomeric shifts. It has been found that α 2-Sn can transform itself into three-dimensional incommensurate states (3DISs) of β-Sn, which are long-range-ordered structures detected in RHEED photographs. The long-range-ordered 3DISs of β-Sn can also be obtained as the initial structure in some thin tin films. It was found that α-Sn is a stable structure in the thin film state and can exist as long-range-ordered 3DISs up to 500°C. Above this temperature α-Sn transforms into β-Sn. It has been shown that heating of thin films of α-Sn and β-Sn generates tin structures with δ⩾+4.4mm s −1, an indication of d electron participation in their chemical bonding. The existence and structural transformations of a tin structure with δ=+1.00±0.2 mm s −1, considered to be an Sn-Sn intermetallic compound, is also reported.

Influence of adhesion on processes of laser heating and damage to thin absorbing films

A phenomenological approach is proposed to allow for the influence of adhesion on the rate of heating of thin films exposed to laser radiation. It is found that the adhesion determines the mechanism of damage to thin films in the advanced damage threshold regime. Conditions for which the melt rolling mechanism of damage to thin films predominates in the advanced damage threshold regime are determined.

The frequency dependence of phonon interactions at Terahertz-frequencies studied by time-resolved phonon spectroscopy

Phonon properties concerning propagation, frequency spectrum and lifetime in the Terahertz range have been investigated. The phonons have been produced by square wave Joule heating of thin constantan films evaporated onto a strongly doped ruby. Cross sections for phonon scattering at the Cr3+ impurity ions of the ruby were determined as a function of frequency. The theoretical estimate for mass defect scattering given by Klemens [9] is found to agree reasonably with the experimental results. Lifetime measurements indicate that phonons in ruby preferentially propagate in the transversal mode. Phonon frequency spectra in the constantan films were measured under experimental conditions, where the strength of electron phonon coupling is significant; thus electron-phonon relaxation times could be obtained as a function of frequency.

Laser heating and melting of thin films on low-conductivity substrates

A laser beam, incident on a highly absorbent thin film supported by a poorly conductive substrate, causes that film to heat and melt. The time required to reach the melting point and that required to complete the melting process are calculated as a function of the incident laser flux. The calculations neglect heat losses arising from lateral diffusion, convection, and thermal radiation, but they account for a possible reflectivity change at the melting point. They yield a criterion for the minimal absorbed flux necessary to maintain stable monotonic melting.

Estimation of temperature rise in electron beam heating of thin films : Tung-Po Lin, IBM J., September (1967), p. 527

Estimation of temperature rise in electron beam heating of thin films : Tung-Po Lin, IBM J., September (1967), p. 527

The heating of thin molybdenum films in a high-frequency electric field

Molybdenum films, formed by evaporation of molybdenum on to a glass substrate, are investigated by measurement of the heating which occurs on the application of a high-frequency electric field. The effects of varying glass temperature during deposition, and of contaminating the glass with oxygen, hydrogen, and with layers of metallic oxides or fluorides, are observed. The stability of the films is examined and related to the processes of agglomeration and recrystallization. The effect of bombarding the films with electrons is investigated.