Correlation between plasma dynamics and thin film properties in pulsed laser deposition

doi:10.1016/S0169-4332(96)00640-X

Applied Surface Science

Volumes 109–110, 1 February 1997, Pages 595-600

https://doi.org/10.1016/S0169-4332(96)00640-X Get rights and content

Abstract

The pressure–distance scaling law for pulsed laser deposition is examined for several thin film systems. This scaling law is due to the plasma dynamics occurring within the laser plasma plume near the location of the substrate. Time-of-flight studies of both ions and neutrals confirm the existence of an optimal velocity distribution for optimal film deposition. Fast ions play a major role in determining the quality of the films deposited. They may provide surface activation of the film or induce damage to the film depending on their kinetic energies.

Introduction

Pulsed laser deposition (PLD) is now an established method for thin film deposition. Its advantages have been amply demonstrated in many instances where PLD films compare favorably to films made by other methods (see for example Ref. [1]). This includes the much heralded high temperature superconducting films, ferroelectric ceramic films, semiconducting films of all combinations, and transparent conducting films 2, 3, 4. Obviously, many studies have been devoted to the understanding of the fundamental mechanisms of the thin film deposition process, especially the correlation between the quality of the thin film and the deposition parameters 5, 6, 7, 8, 9.

One difficult problem with such studies is the complex nature of the laser plasma plume. This plasma plume is extremely dynamic in characteristics, especially for reactive deposition where an ambient reactive gas is present. There are many studies of the plasma plume generated by the excimer laser pulse. Methods ranging from optical spectroscopy to ion mass spectroscopy have been employed 5, 6, 7. The properties of the plasma have been measured as a function of time, distance and for specific ionic and atomic species. Temporal resolutions ranging from nanoseconds to milliseconds have been used in an attempt to extract different types of information on the plasma.

However, many studies are aimed at examining the physics of the laser generated plasma without considering the thin film formation process itself. Hence considerable plasma studies were carried out at conditions not appropriate for good film deposition. While there are plenty of interesting physics to be learnt, however, it is ultimately the properties of the thin film deposited that count. Some films may be easy to make, while some are more sensitive to the plasma conditions. Therefore it is important that plasma dynamics be correlated to thin film properties.

In this study, we attempt to correlate the properties of the film and the laser generated plasma. Time-of-flight characterization of the laser plume for both ionic and atomic species were performed. Ion probe and emission spectroscopy were both employed. We also monitor simultaneously the properties of the thin films deposited as a function of the plasma parameters. It was found that the most important plasma parameter is the velocity of the various ionic and atomic species at the location of the substrate. This is manifested as a target distance–background pressure correlation law in PLD. We shall call it the PD scaling law [10]. This PD scaling law was found to be present for superconducting films as well as semiconducting film deposition [11].

Section snippets

Pressure–distance scaling law

There are a few important parameters that can be varied in PLD: the substrate temperature T_s, the ambient pressure P, the ambient gas flow rate, the target–substrate distance D, and the laser fluence J. While it is generally true that the deposition parameters can be varied independently to optimize the quality of the thin films, some of these parameters are related. The most obvious is the correlation between P and T_s. It is well-known from thermodynamic considerations that for each substrate

Optical time-of-flight spectroscopy

The optical emission time-of-flight measurement arrangement is similar to our earlier publication [7]. Basically, the emission from the laser plume was measured as a function of time and wavelength. The spatial resolution was achieved with an imaging lens in combination with a slit at the entrance to the spectrometer. It is usually about 0.5 mm along the propagation direction of the plume, and much longer in the transverse direction. The wavelength resolution is needed to identify emissions

Ion time-of-flight spectroscopy

Ions can be detected easily by electrical probes. It is easier to perform this measurement than optical TOF. But the data is more difficult to interpret because of the disrupting nature of the probe. There are space charge effects and capacitive coupling problems. Moreover the extraction voltage interferes with the plasma. Nevertheless, despite these problems, ion probe measurements are very useful and essential for plasma diagnostics.

Fig. 7 shows typical ion probe signals as seen by an

Summary

In summary, we have presented some results on characterizing the properties of the laser plasma plume. At the same time we also monitored the properties of the thin films. A strong correlation was observed between the ambient pressure during deposition and the velocity of the neutral atoms and the ions. It was found that there is always a bimodal velocity distribution for both atoms and ions. While the fast atoms are damped away rapidly, the fast ions still survive at the substrate surface

Acknowledgements

This research was supported partially by the Research Grants Council of Hong Kong.

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