Laser sputter deposition is a thin film deposition technique for silicon wafers, which uses high-energy pulsed laser radiation to ablate or knock-off target molecules and deposit them as a thin film layer on a wafer substrate. This technique was first tried by Smith and Turner in 1965. Three years later, Breech and Cross wrote a paper on laser vaporization and atom excitation.
At that time, quality of laser sputter deposition was far behind than that of the layers deposited by other techniques like molecular beam epitaxy and chemical vapor deposition. In 1987 laser sputter deposition became an established technique when Dijkkamp and his group used laser deposition on high-temperature superconductors (YBa2Cu3O7). Their work marked the starting point for the industry to utilize laser sputter deposition for layers of high quality crystals.
Since then, all types of oxides, nitrides, or even the metallic systems, and polymers have been deposited through laser.
How does laser sputter deposition work?
In laser sputter deposition, the target material undergoes a number of energy conversions before it is deposited on the substrate. First, when the radiated laser pulse hits the target, the energy is transferred as an electronic excitation, and then converted to thermal, chemical and mechanical energy.
All these energy conversions cause the target to evaporate, ablate, create plasma, and exfoliate.
The whole process can be divided in the following stages:
a. Laser Sputtering of the Target Material
Laser sputtering uses a high-energy excimer laser pulse with very short wavelength like the KrF laser at 248nm. As the high-energy laser pulse is directed towards the target material causing energy transfer between the free electrons and the atoms. This will result in an extreme increase in temperature and in the evaporation of a part of the target.
b. Expansion of Material through Plasma
The vaporized material will expand towards the substrate in the plasma located in parallel to the target material. This expansion is dependent on the background pressure. An increase in background pressure slows down the excited particles, providing a higher chance for the deposited material to resputter. Thus, the deposition rate will decrease and material’s stoichiometry will be altered.
c. Deposition of the Sputtered Particles
Once the sputtered particles start bombarding the substrate surface, they will cause some of the substrate’s atoms to be knocked-off of its surface. Deposition begins when the condensation rate, caused by the collision of the sputtered particles of the target material and the substrate, is high enough to obtain thermal equilibrium. It is important to have of continuous flow of sputtered particles to maintain thermal equilibrium.
d. Growth of the Deposited Material
There are three growth techniques that can be used in laser sputter deposition: step flow, layer by layer, and 3D growth. In step flow growth, once the atoms of the target material reach the substrate surface, they will diffuse. Step flow growth is usually used on substrates with high crystal miscuts since the atomic steps on the substrate surface are formed from miscuts on the crystal.
Layer-by-layer technique, on the other hand, continuously grows islands of material until the islands overlap each other, a phenomenon known as coalescence. During coalescence, there is a huge density of material on the crater of the substrate surface. This whole process is performed for every additional layer.
In 3D growth mode, islands of material are formed on top of each other. This technique is somehow similar to layer-by-layer except for the fact that the deposition does not appear in a layered fashion because materials roughen every time it is added on top of the island.
The Characteristics of Laser Sputter Deposition
In comparison with other deposition techniques, laser sputter deposition is much more flexible as shown by the following parameters:
Since the target size of the laser beam on laser sputter deposition is small, it is possible for this technique to deposit complex compounds. Such capability is usually useful for research purposes, especially when the preparation for the target material using other techniques would be expensive, like in the case of superconductor films.
Target Material Type
A deposition process with different types of target material must be operated in an ultra high vacuum chamber with the laser source located outside the chamber. Some of the materials that can be prepared using laser sputter deposition are listed in Table 1.
|Ferroelectric materials||Pb(Zr,Ti)O3||Kidoh (1991)|
|Diamond-like carbon||C||Martin (1990)|
|Buckminster fullerene||C60||Curl and Smalley (1991)|
|Polymers||Polyethylene, PMMA||Hansen and Robitaille (1988)|
Krebs and Bremert (1993)
Table 1. Some of the materials prepared for the first time by laser sputter deposition.
The Pulsed Nature of Laser
Some complex compounds such as polymer-metals require different multilayered conditions for the deposition of each component. One example is the deposition of polycarbonate silver. This kind of deposition is not possible with other techniques because of the big gap between the energy requirements of the two components. But with laser sputter deposition, the pulsed nature of the laser allows the polycarbonates to be deposited at low laser fluence of 60mJ/cm2 and the silver crystals at much higher laser fluence, about 80 times higher at 5J/cm2.
With the continuous advancements in laser technology, laser sputter deposition has emerged in the industry as a competitive technique for depositing thin films of complex stoichiometry but because of the small target size usable, it is not practical for large-scale layer formation.