Laser was first demonstrated in 1960. Since then it has been developed into a powerful tool in many application. It is especially useful in material processing. Laser possesses many unique properties such as narrow frequency bandwidth, coherence and high power density. Often the light beam is intense enough to vaporize the hardest and most heat resistant materials. Besides, due to its high precision, reliability and spatial resolution, it is widely used in the industry for machining of thin films, modification of materials, material surface heat treatment, welding and micro patterning. Apart from these, polycomponent materials can be ablated and deposited onto substrates to form stoichiometric thin films. This last mentioned application of laser is the so-called pulsed laser deposition (PLD).

In general, the idea of PLD is simple. A pulsed laser beam is focused onto the surface of a solid target. The strong absorption of the electromagnetic radiation by the solid surface leads to rapid evaporation of the target materials. The evaporated materials consist of highly excited and ionized species. They presented themselves as a glowing plasma plume immediately in front of the target surface if the ablation is carried out in vacuum. Figure 1 shows some typical plasma plumes produced during PLD process.

(Fig.1. Some typical plasma plumes produced during PLD process.)

Indeed, PLD is so straightforward that only a few parameters, such as laser energy density and pulse repetition rate, need to be controlled during the process. The targets used in PLD are small compared with the large size required for other sputtering techniques. It is quite easy to produce multi-layered films of different materials by sequential ablation of assorted targets. Besides, by controlling the number of pulses, a fine control of film thickness down to atomic monolayer can be achieved. The most important feature of PLD is that the stoichiometry of the target can be retained in the deposited films. This is the result of the extremely high heating rate of the target surface (K/s) due to pulsed laser irradiation. It leads to the congruent evaporation of the target irrespective of the evaporating point of the constituent elements or compounds of the target. And because of the high heating rate of ablated materials, laser deposition of crystalline film demands a much lower substrate temperature than other film growth techniques. For this reason the semiconductor and the underlying integrated circuit can refrain from thermal degradation.

In spite of the said advantages of PLD, some shortcomings have been identified in using this deposition technique. One of the major problems is the splashing or the particulates deposition on the films. The physical mechanisms leading to splashing include the sub-surface boiling, expulsion of the liquid layer by shock wave recoil pressure and exfoliation. The size of particulates may be as large as a few micrometers. Such particulates will greatly affect the growth of the subsequent layers as well as the electrical properties of the films. Another problem of PLD is the narrow angular distribution of the ablated species, which is generated by the adiabatic expansion of laser, produced plasma plume and the pitting on the target surface. These features limit the usefulness of PLD in producing large area uniform thin films, and PLD has not been fully deployed in industry. Recently remedial measures have been proposed. Inserting a shadow mask is effective to block off the large particulates. Rotating both the target and substrate can help to produce larger uniform films.

Albert Einstein postulated the stimulated emission process in as early as 1916. The first optical master using a rod of ruby as the lasing medium was, however, constructed in 1960 by Theodore H. Maiman at Hughes Research Laboratories, a lapse of 44 years. Using laser to ablate material has to be traced back to 1962 when Breech and Cross, used ruby laser to vaporize and excite atoms from a solid surface. Three years later, Smith and Turner used ruby laser to deposit thin films. This marked the very beginning of the development of the pulsed laser deposition technique.

However, the development and investigations of pulsed laser deposition did not gather the expected momentum. In fact, the laser technology was immature at that time. The availability of the types of laser was limited; the stability output was poor and the laser repetition rate was too low for any realistic film growth processes. Thus the development of PLD in thin film fabrication was slow comparing with other techniques such as MBE, which can produce much better thin film quality.

The rapid progress of the laser technology, however, enhanced the competitiveness of PLD in the following decade. The lasers having a higher repetition rate than the early ruby lasers made the thin film growth possible. Subsequently, reliable electronic Q-switches lasers became available for generation of very short optical pulses. For this reason PLD can be used to achieve congruent evaporation of the target and to deposit stoichiometric thin films. The absorption depth is shallower for UV radiation. Subsequent development led to lasers with high efficient harmonic generator and excimer lasers delivering powerful UV radiation. From then on, non-thermal laser ablation of the target material became highly efficient.

Pulsed laser deposition as a film growth technique has attained its reputed fame and has attracted wide spread interest after it has been used successfully to grow high-temperature superconducting films in 1987. During the last decade, pulsed laser deposition has been employed to fabricate crystalline thin films with epitaxy quality. Ceramic oxide, nitride films, metallic multilayers, and various superlattices grown by PLD have been demonstrated. Recently, using PLD to synthesis nanotubes, nanopowders and quantum dots have also been reported. Production-related issues concerning reproducibility, large-area scale-up and multiple-level have begun to be addressed. It may start up another era of thin film fabrication in industry.

The principle of pulsed laser deposition, in contrast to the simplicity of the system set-up, is a very complex physical phenomenon. It involves all the physical processes of laser-material interaction during the impact of the high-power pulsed radiation on a solid target. It also includes the formation of the plasma plume with high energetic species, the subsequent transfer of the ablated material through the plasma plume onto the heated substrate surface and the final film growth process. Thus PLD generally can be divided into the following four stages.

1. Laser radiation interaction with the target
2. Dynamic of the ablation materials
3. Decomposition of the ablation materials onto the substrate
4. Nucleation and growth of a thin film on the substrate surface

In the first stage, the laser beam is focused onto the surface of the target. At sufficiently high energy density and short pulse duration, all elements in the target surface are rapidly heated up to their evaporation temperature. Materials are dissociated from the target and ablated out with stoichiometry as in the target. The instantaneous ablation rate is highly dependent on the fluences of the laser irradiating on the target. The ablation mechanisms involve many complex physical phenomena such as collisional, thermal and electronic excitation, exfoliation and hydrodynamics.

During the second stage the emitted materials tend to move towards the substrate according to the laws of gas-dynamic and show the forward peaking phenomenon. R.K. Singh reported that the spatial thickness varied as a function of , where n>>1. The laser spot size and the plasma temperature have significant effects on the deposited film uniformity. The target-to-substrate distance is another parameter that governs the angular spread of the ablated materials. Hanabusa also found that a mask placed close to the substrate could reduce the spreading.

The third stage is important to determine the quality of thin film. The ejected high-energy species impinge onto the substrate surface and may induce various type of damage to the substrate. The mechanism of the interaction is illustrated in the following figure. These energetic species sputter some of the surface atoms and a collision region is established between the incident flow and the sputtered atoms. Film grows immediately after this thermalized region (collision region) is formed. The region serves as a source for condensation of particles. When the condensation rate is higher than the rate of particles supplied by the sputtering, thermal equilibrium condition can be reached quickly and film grows on the substrate surface at the expense of the direct flow of the ablation particles.

Nucleation-and-growth of crystalline films depends on many factors such as the density, energy, degree of ionization, and the type of the condensing material, as well as the temperature and the physical-chemical properties of the substrate. The two main thermodynamic parameters for the growth mechanism are the substrate temperature T and the supersaturation m. They can be related by the following equation:

where k is the Boltzmann constant, R is the actual deposition rate, and Re is the equilibrium value at temperature T.

The nucleation process depends on the interfacial energies between the three phases present – substrate, the condensing material and the vapour. The minimum-energy shape of a nucleus is like a cap. The critical size of the nucleus depends on the driving force, i.e. the deposition rate and the substrate temperature. For the large nuclei, a characteristic of small supersaturation, they create isolate patches (islands) of the film on the substrates, which subsequently grow and coalesce together. As the supersaturation increases, the critical nucleus shrinks until its height reaches an atomic diameter and its shape is that of a two-dimensional layer. For large supersaturation, the layer-by-layer nucleation will happen for incompletely wetted foreign substrates.

The crystalline film growth depends on the surface mobility of the adatom (vapour atoms). Normally, the adatom will diffuse through several atomic distances before sticking to a stable position within the newly formed film. The surface temperature of the substrate determines the adatom’s surface diffusion ability. High temperature favours rapid and defect free crystal growth, whereas low temperature or large supersaturation crystal growth may be overwhelmed by energetic particle impingement, resulting in disordered or even amorphous structures.

Metev and Veilo suggested that the , the mean thickness at which the growing, thin and discontinuous film reaching continuity is given by the formula:

where R is the deposition rate (supersaturation related) and T is the temperature of the substrate and A is a constant related to the materials.

In the PLD process, due to the short laser pulsed duration (~10 ns) and the small temporal spread (<10) of the ablated materials, the deposition rate can be enormous (~10). Consequently a layer-by-layer nucleation is favoured and ultra-thin and smooth film can be produced. In addition the rapid deposition of the energetic ablation species helps to raise the substrate surface temperature. In this respect PLD tends to demand a lower substrate temperature for crystalline film growth.