ADVANCED APPLICATIONS: LASER DEPOSITION: Pulsed lasers target industrial coatings

Pulsed laser deposition can produce diamond-like coatings that make a surface nearly diamond-hard, or the process can apply high-temperature superconducting thin films to pave the way for practical superconducting devices. The technique also has the potential to radically enhance other devices, including electroluminescent displays and micro-optics. Only recently, though, have researchers mounted a strong attack on the technical problems holding pulsed laser deposition back from routine production use.

Simple process, complex control

Pulsed laser deposition is basically a simple process. A high-intensity pulsed laser beam is focused on a target in a chamber that is either evacuated or filled with a specific gas such as oxygen or nitrogen. The pulse ablates the target material, generating a high-pressure plasma region that expands perpendicularly. When the substrate to be coated is placed in the path of the plasma plume, the ablated material adheres to the surface. The repetitive pulsing lays down a thin film of the material.

The process may be simple, but process control can be quite sensitive to many parameters. When producing a diamond-like coating, for example, the target is graphite, and the chamber is evacuated to about 10^-5 Pa. This allows the plasma ions to achieve energies up to 1.5 keV and leads to their adherence to the surface in a diamond-like crystal with a hardness of 60-70 GPa. Creation of these high energies requires minimum laser intensity of roughly 10¹¹ W/cm² for 1064-nm infrared light and 10⁸ W/cm² for 193- or 248-nm ultraviolet (UV) radiation. The UV light has an advantage in its ability to reduce splashing of microparticles ejected from the target surface, because light in this wavelength range ablates in large part by photon sputtering instead of thermalization.

For diamond growth, the substrate surface also must stay cool, which means coating rates are limited to about 0.01-0.02 nm per pulse, with substrate distances of 5 or 10 cm. In addition, when an ambient gas such as oxygen is used to deposit oxides, process complexity grows because contact with the gas rapidly decelerates the plasma flow. The ionization energy of the plasma transfers to the kinetic energy of the gas, which reaches supersonic velocity and develops into a shock wave. The interaction of this shock wave with the substrate limits deposition rates for oxides to about 0.1 nm per pulse, or approximately one complete atomic layer.¹

Putting deposition to work

Diamond-film deposition is still one of the leading applications for pulsed laser deposition. Such thin films confer extraordinary hardness on surfaces and also can have several remarkable electrical properties. For instance, diamond is normally a powerful insulator, able to hold back potentials of hundreds of kilovolts. When irradiated with UV light, however, the material becomes photoconductive. The result is that diamond-film-based switches can potentially switch tens of kiloamps in a few nanoseconds.

A second major application area for the deposition technique involves the generation of high-temperature superconducting films. Films can be deposited on substrates to produce superconductors considered flexible and sturdy enough for practical use. Such oxides are deposited in an oxygen atmosphere, as are the newly developed colossal magnetoresistive films finding use in magnetic detectors and magnetic recording media. Other metal-oxide films that can be deposited by lasers include optical coatings, piezoelectric and ferroelectric films for use in memories, electroluminescent coatings for flat-panel displays, and ionic conductors used in fuel cells.

Thin films are not the only products of pulsed laser deposition. When carbon-nickel-cobalt targets are irradiated with a pulsed laser such as Nd:YAG, the carbon is redeposited in the form of single-walled nanotubes.² While such nanotubes are still far from commercialization, they have interesting characteristics, such as the ability to form conductors, semiconductors, and insulators, despite their tiny size.

Exploring obstacles

Several technical obstacles still stand between pulsed laser deposition and widespread commercialization. For instance, once a coating is applied to a substrate, it must adhere well throughout prolonged use of the product. This requirement is most demanding for the diamond-like coatings used to increase the hardness of heavy-duty parts such as bearings.

Simply applying diamond coatings to steel is impractical because the films delaminate in only a few rotations of a bearing. The problem is that the soft steel deforms under the hard film, breaking the adhesion. To cure this problem, researchers developed the concept of functionally graded coatings that blend gradually from pure metal to pure diamond; when coating titanium, a layer of the coating consists of a gradual shifting of titanium carbide composite to a pure diamond outer layer. Producing such coatings requires a hybrid device that uses pulsed laser deposition for the diamond and magnetron sputtering for the metal.

Another option involves multilayer coatings of materials such as titanium, titanium carbide, and diamond. While some hardness is lost, recent research has shown that multilayer graded coatings lasted more than a million cycles of wear against a sapphire ball, while a single layer of functionally graded coating lasted 100,000 cycles, and the simple diamond coating lasted only a few cycles.

One way to improve the hardness of diamond-like coatings in hybrid systems is to use a magnetic field. According to reports from the University of Hong Kong, the current in the plasma plume increases in magnetic fields of a few hundred to a couple of thousand Gauss.³ This result shows that more carbon ions are produced as electrons spiral around the magnetic-field lines, colliding with carbon atoms and stripping more electrons from them. With a higher current and more carbon ions, the binding of the carbon to the diamond-like films becomes more effective, and thus the hardness increases.

Generating a coating that offers the required hardness is a particular problem when pulsed laser deposition occurs in gas, as with the deposition of carbon nitride coatings. This is due to the plasma stream slowing down greatly as it moves through the gas. University of Liverpool (England) scientists, however, have found a way around this difficulty by injecting a jet of nitrogen gas while the laser is pulsing on the carbonate target.⁴ The chemical reactions between carbon and nitrogen occur in the jet, which then expands without further collisions in the vacuum chamber and arrives at the substrate at high velocity, thus ensuring good hardness.

Another complication for pulsed laser deposition involves the production of doped thin films, which are often essential for high-temperature superconducting applications. Just premixing the materials makes changes in doping rates difficult, and doping can become nonuniform during deposition.

At the National University of Singapore, researchers have designed a dual-beam approach to flexible and uniform doping.⁵ A laser beam is split into two and focused on two adjacent targets—one for the film and the other for the dopant. A chopper placed in the way of the dopant beam allows continuous variation of the concentration of the dopant. When a different dopant is needed, the beam can simply be shifted to another nearby target.

Despite progress in overcoming technical obstacles to industrial application of pulsed laser deposition, the process is still constrained by economic barriers that face almost any new high-technology process. Producing reliable, consistent results requires process-monitoring techniques that are often beyond the resources of small company laboratories and university research teams. In addition, there is still no application with such clear near-term market potential that large companies are willing to invest substantial development funds. One promising possibility might be thin-film gallium nitride for blue-light-emitting laser diodes and LEDs, but, for now, industrial applications for pulsed laser deposition still lie in the future.