We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been found to result from anisotropic expansion velocities of the plasma edges arising due to the density gradients in the gaseous plasma.The physical process of the laser ablation technique for deposition of thin films can be classified into three separate interaction regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation and initial isothermal expansion, and (iii) adiabatic expansion leading to deposition of thin films. The first two regimes occur during the time interval of the laser pulse, while the last regime initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The expansion velocities of the plasma edges are related to the initial dimensions and temperature of the plasma, and the atomic weight of the respective species present in it. Preliminary calculations have been carried out on spatial thickness variations as a function of various parameters in PLE deposited thin films. The effects of the various beam and substrate parameters including energy density and substrate‐target distance affecting the nature of deposition of superconducting thin films have been theoretically examined. Experimental results have been obtained from thin films deposited on silicon substrates by XeCl pulsed excimer laser (λ=308 nm, τ=45×10−9 s) irradiation. The spatial thickness and compositional variations in thin films have been determined using Rutherford backscattering technique and the results compared with the theoretical calculations.

Theoretical model for deposition of superconducting thin films using pulsed laser evaporation technique

The gas flow dynamics of laser ablation plumes is investigated experimentally and theoretically. Experimentally, angular‐resolved time‐of‐flight (ARTOF) measurements are performed on a model system (laser etching of copper in a chlorine environment). The TOF spectra obtained can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Theoretically, an analytical model is constructed, based on the hydrodynamical problem of an expanding elliptical gas cloud. The model allows semiquantitative prediction of ARTOF distributions and angular intensity distributions. Observed trends in laser ablation deposition such as independence of the angular intensity distribution on mass of the atom and laser fluence, and dependence of the angular distribution on spot dimensions are explained.

Gas flow dynamics in laser ablation deposition

Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

A model is proposed for the origin of spatial nonuniformities in the composition of pulsed laser‐deposited films derived from multicomponent targets. It is based on the idea that the forward peaking of each species in the plume depends on the species mass via the ratio of two mass‐dependent velocities: the flow velocity, which characterizes the directed forward motion, and the mean random thermal velocity, which characterizes lateral motion. The on‐axis enrichment of light‐mass species observed in films deposited at moderate laser fluences (e.g., Cu in depositions of YBa2Cu3O7) is attributed to the dual effects of a mass‐dependent collision rate and collision effectiveness. In weak expansions, these effects leave the lower mass species with a lower temperature, a higher flow velocity, and a relatively more forward‐peaked distribution than their high‐mass counterparts. The improved compositional uniformity observed for depositions at higher laser fluences is attributed to an incomplete transition to a stro...

On the origin of spatial nonuniformities in the composition of pulsed-laser-deposited films

This article presents a systematic overview of the work carried out in the area of pulsed laser deposition (PLD), characterization and device application aspects of high temperature superconducting YBCO thin films. The theoretical and experimental aspects of the pulsed laser deposition process for the synthesis of YBCO thin films have been described in detail. The deposition technique has emerged as a very powerful method to make composition and microstructure controlled superconducting YBCO films. Some unique features of this process are due to the rapid heating and evaporation of the target and the interaction of the laser beam with the evaporated materials leading to the formation of a high temperature plasma. Major advantages of PLD process are congruent evaporation and crystallinity due to the presence of high energy evaporants and fast response time. It has the potential to encompass a wide scope of physical vapor-deposition techniques from thermal evaporation to sputtering and MBE. The electrical and structural studies performed on laser deposited YBCO films have shown that films produced by PLD are superior than films produced by other thin film growth techniques. The major technical obstacle of particulates emission, encountered initially with PLD, has now greatly been solved. Some new developments with PLD technology such as large-area and nonplanar substrates coatings and growth of novel superconductors as oriented films have been demonstrated.

Pulsed laser deposition and characterization of high-Tc YBa2Cu3O7 − x superconducting thin films

Excimer laser ablation was used to prepare thin films from a bulk target of SmBa2Cu3O7. The systematic variation in film thickness, stoichiometry, and microstructure, as a function of laser fluence and angle from the target surface normal, was determined. We show that films which are stoichiometric over wide solid angles can be rapidly prepared from this material by use of high laser fluence. Films prepared from various Bi‐Sr‐Ca‐Cu‐O targets at high laser fluence also reproduce bulk stoichiometry. In contrast, films prepared by this technique from a YBa2Cu3O7 target are deficient in Ba and Cu for laser fluences required to produce compositional homogeneity over wide areas.

Systematics of thin films formed by excimer laser ablation: Results on SmBa2Cu3O7

Thin films were prepared by excimer laser ablation from Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O targets. The films were prepared upon amorphous quartz and crystalline Si (111) substrates in partial pressures of oxygen. The substrate-film interaction, crystallization, composition, and microstructure of these films verst_s the deposition condition variables were studied including, substrate temperature, oxygen partial pressure, laser fluence and repetition rate. Additional deposition variables include the substrate/substrate holder and plasma generation loop DC and RE bias conditions and the target which can be DC biased. With the various biasing/plasma conditions we have studied to date, we have been unable to lower the substrate deposition temperature required for crystallization of Y-Ba-Cu-O films by more than 50 degrees C. Films of Y-Ba-Cu-O produced upon Si (111) are shown to be superconducting.

Preparation of High Temperature Superconductor Thin Films on Si Based Substrates by Excimer Laser Ablation, Film-Substrate Interactions, and Plasma Processing Effects on Crystallinity

D. Simons

Papers

Systematics of thin films formed by excimer laser ablation: Results on SmBa2Cu3O7

Preparation of High Temperature Superconductor Thin Films on Si Based Substrates by Excimer Laser Ablation, Film-Substrate Interactions, and Plasma Processing Effects on Crystallinity