Fig 1 Schematic diagram of a typical pulsed laser deposition system

Laser. The laser system and focusing optics are located outside the chamber. The focal length needs to be matched to the chamber size so that the required energy densities can be achieved at the target surface. Many different laser wavelengths have been successfully used. The excimer is the most commonly used laser because of its high peak power (> 40 MW) in the ultraviolet and ease of operation. In an excimer laser, the lasing medium is a rare gas/halogen mixture. A summary of rare gas mixtures and the output laser wavelengths is presented in Table 2. KrF (248 nm) is probably the most common excimer used for thin film deposition due to its stability and high peak power. In addition to excimer lasers, other gas laser systems (CO2) and solid state laser systems (Nd:YAG) have been used to deposit thin films. The output wavelengths of these lasers can vary from 0.2 to 10 pm. Film quality as a function of laser wavelength has been compared (Ref 43), and it is generally accepted that film quality is significantly better for ultraviolet (UV) lasers (200 to 300 nm) than for infrared (IR) lasers (1 to 10 pm). This is due to a decreasing penetration depth of the laser radiation into the bulk material as the wavelength is reduced. The shallower penetration depth results in more energy being deposited per unit volume for comparable UV and IR pulse energies. As a result, UV lasers produce higher surface temperatures and more efficient evaporation of the solid target. IR lasers are often associated with thin films having rough surface morphologies, as a result of the incomplete vaporization of the target and the ejection of micron-size particles. Attenuation of the radiation by oxygen (in addition to the photochemical production of ozone) limits the use of laser wavelengths below 200 nm.

Table 2 Excimer laser wavelengths

Wavelength, nm

Rare gas/halogen mixture

157

F2

293

Ar-F2

222

Kr-Cl2

248

Kr-F2

308

Xe-Cl2

351

Xe-F2

An individual laser pulse transfers a large amount of material from the target to the substrate (~1016 atoms per pulse), resulting in average deposition rates of ~1 A/laser pulse (deposition rates are a strong function of the target composition and phase and the deposition conditions). To achieve a coating thickness on the order of 1.0 pm requires 5000 to 20,000 laser pulses. Commercial excimer systems operate at repetition rates as high as 150 Hz, and high-quality films 0.5 pm thick have been deposited at average growth rates as high as 150 A/s.

Substrate Heater. For many materials, it is desirable to deposit at elevated substrate temperatures. Two problems are often encountered when trying to achieve uniform substrate temperatures < 900 °C in oxidizing environments. First, the heating element must be inert to the ambient. Second, the typical deposition conditions (10-6 to 1 torr) do not allow for efficient gas-phase heat conduction from the radiating element to the substrate. Projection lamps serve as efficient heating elements because the filament is encapsulated in a quartz envelope. The lamps can be used to heat a small steel block to temperatures that approach the softening temperature of the quartz. The substrate is mounted onto the steel heater block. Transfer of the heat from the steel block to the substrate is usually done through a conductive bonding agent such as silver paint. This type of heater can be used for small substrates (up to ~1 in. diameter) and temperatures up to ~950 °C.

Alternatively, substrates can be heated using a pseudo-blackbody heater (Ref 42). Here the substrate is encased in a well-shielded heater with only a small opening for the depositing vapor. The substrate is not required to be in intimate thermal contact with the heater block because the temperature everywhere inside the blackbody is the same. Substrate heaters of this type have been successfully developed for the deposition of large-area films (>1 in. diameter). Rotation of the substrate inside the blackbody heater improves the temperature uniformity.

Target Manipulation. The target must be continuously manipulated to avoid having the laser strike the same spot on the target with successive shots. This is done exclusively by rotating the target during deposition. This minimizes problems associated with erosion at the target surface. Commercial systems are available that hold up to six rotating targets, 1 in. in diameter, that can be selected under computer control to be positioned in front of the laser focus. With this type of system, multilayer structures can easily be formed.

Laser Beam Rastering. Target rotation ensures uniform erosion. The laser beam can also be rastered across the target. In addition to minimizing problems associated with erosion, rastering of the laser beams increases the area of a substrate that can be coated. In PLD, the plume of evaporated material is highly forward-directed. If the position of the laser beam remains fixed in space, the plume of evaporated material is directed at a relatively small area on the substrate. For most deposition systems, a stationary laser beam produces a uniform coating over 1 cm2. In its simplest form, the laser beam is moved by a motorized mirror mount across the target. The substrate and target are mounted so that their centers are on axis. The plume of evaporated material is swept across the substrate. The main limitation to this geometry is that the coated area is determined by the size of the target. Off-axis geometries have also been demonstrated (Ref 44). Offsetting the target center from the substrate center allows the same aerial coverage with a target that is only half the size of the substrate. Another scale-up technique is to raster the substrate in two dimensions while holding the laser position fixed on the target (Ref 45). This scheme has the advantage of not requiring large-area targets but suffers from the complications of having to move the heating elements.

Research efforts continue on the scale-up of PLD to large areas, although it is not yet clear what technological application will be the first to implement large-scale PLD in an industrial environment. Both ferroelectric and HTS thin films have been deposited over large areas as demonstrations. Uniform coatings have been successfully demonstrated on planar surfaces up to 5 in. in diameter. However, there is still a great deal of work to be done on the device structures based on these materials, and the demands for large-area coatings is relatively small. The versatility of the technique is simply not yet great enough to generate industrial acceptance.

Deposition Parameters. Typical deposition conditions are:

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