Ultrashort pulse laser deposition of thin films

Details Ultrashort pulse laser deposition of thin films Ultrashort pulse laser deposition of thin films

2000-05-10

2002-04-16

Mills, Gregory (Department: 1763)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298030, C204S192110, C378S120000

Reexamination Certificate

active

06372103

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the production of diamond-like carbon films, and more specifically, it relates to the use of short-pulse lasers to produce amorphous films of diamond-like carbon with properties typical of PLD-produced DLC at deposition rates much higher than even many CVD methods.
2. Description of Related Art
The use of diamond thin films has the potential for major impact in many industrial and scientific applications. These include heat sinks for electronics, broadband optical sensors, windows, cutting tools, optical coatings, laser diodes, cold cathodes, and field emission displays. Attractive properties of natural diamond consist of physical hardness, high tensile yield strength, chemical inertness, low coefficient of friction, high thermal conductivity, and low electrical conductivity. Unfortunately, these properties are not completely realized in currently produced diamond thin films.
Chemical vapor deposition, in its many forms, has been the most successful to this point in producing crystalline diamond films microns to millimeters in thickness which are made up of closely packed diamond crystals microns in physical dimension. However, high purity films are difficult to realize due to the use of hydrogen in the growth process which becomes included in the film matrix. These impurities are manifest in film physical properties which are inferior to those of pure crystalline diamond. In addition, the large density of grain boundaries due to the polycrystalline nature of the films reduce the films diamond-like character. Finally, substrates must be heated to several hundred degrees Celsius, which is not suitable for many materials.
Pulsed laser deposition(PLD) is attractive due to its ability to produce high purity films—limited only by the purity of the target. For diamond film production, high purity carbon can be ablated directly by lasers and deposited as thin films at ambient temperatures. However, lasers currently in use generally deliver long (>10 ns) pulses, and the generally explosive nature of laser ablation, in addition to the desired single-atom or single-ion carbon, liberates significant amounts of carbon clusters (C
n
where n=2-30) and macroscopic particles (>1-10 &mgr;m) of carbon. These carbon particles interrupt the ordered deposition of crystalline diamond, forming undesirable grain boundaries and rough surfaces that are difficult to polish. In addition, PLD generated films tend to be “amorphous” or nanocrystalline with no observable long-range order, but still possessing physical properties which are diamond-like in some approximation. This has given rise to the term “diamond-like carbon” (DLC)when referring to these PLD-produced, amorphous carbon films. Growth rates for PLD have been prohibitively slow until recently with the advent of high average power, high rep-rate lasers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide techniques for production of high quality films with properties very dose to that of crystalline diamond.
There has been evidence that increasing laser intensity, and thus particle kinetics, leads to DLC film which are increasingly more diamond-like in character. Given that short pulse (100 picoseconds or less) lasers can reach intensities much higher than those achievable with conventional lasers, combined with the fact that laser ablation using short pulses is relatively gentle, short pulse PLD is a viable technique of producing high quality films with properties very dose to that of crystalline diamond. The plasma generated using femtosecond lasers is composed of single atom ions with no clusters producing films with high sp
3
/sp
2
ratios. Using a high average power femtosecond laser system, the present invention dramatically increases deposition rates to up to 25 &mgr;m/hr (which exceeds many CVD processes) while growing particulate-free films. In the present invention, deposition rates is a function of laser wavelength, laser fluence, laser spot size, and target/substrate separation. The relevant laser parameters are shown to ensure particulate-free growth, and characterizations of the films grown are made using several diagnostic techniques including electron energy loss spectroscopy (EELS) and Raman spectroscopy. The nonthermal processes involved in femtosecond laser ablation prove ideal for laser machining of high explosives, and some pellets of explosive materials have been successfully cut.


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