Coating processes – Direct application of electrical – magnetic – wave – or... – Electromagnetic or particulate radiation utilized
Reexamination Certificate
2000-06-21
2001-11-06
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electromagnetic or particulate radiation utilized
C427S249100, C427S255600, C427S294000, C427S492000, C427S554000, C427S561000
Reexamination Certificate
active
06312768
ABSTRACT:
TECHNICAL FIELD
This invention relates to the ultrafast laser deposition of various materials to form thin film of amorphous and crystalline structures, including nanometric structures.
BACKGROUND TO THE INVENTION
Pulsed laser deposition (PLD) in which pulses of laser radiation are used to evaporate material from a target which is then deposited on a substrate represents a breakthrough in methods for the production of technologically important thin films. Versatility and simplicity are essential features of pulsed laser deposition as a technique for depositing thin films of complex materials. Virtually any material, from pure elements to multicomponent and organic compounds can be deposited; and the stoichiometry of the target material is faithfully reproduced in the film.
Advantages of PLD stem from the fact that laser beams, unlike ion beams or electron beams, are easy to transport and manipulate, and the dynamic range of delivered energy is the largest compared to virtually any deposition process. As a result, PLD has the highest instantaneous deposition rate, up to 100 times higher than in other thin film deposition methods such as Chemical Vapour Deposition. Molecular Beam Epitaxy. Plasma Processing, Magnetron and RF Sputtering, and others. Nevertheless, pulsed laser deposition has not become widely used as a thin film production method for important technologies such as the semiconductor electronics industry or photonics, because of the creation of particulates during PLD which prevent the formation of suitably high quality films.
This major disadvantage is well known using conventional PLD methods, where it is normal to employ low repetition rate, powerful nanosecond-range laser pulses to evaporate the target. In this situation large numbers of macroscopic particles and droplets, having a typical size from a fraction of a micron to a few microns, are ejected from the target during the evaporation process. As a consequence, the convention PLD process cannot provide good surface quality or uniformity of the deposited films since these particles and droplets become embedded in the resulting film. The particulate problem severely limits the commercial applications of the existing pulsed laser deposition technique. In high-performance electronic and in optical applications such as optical thin film devices with sophisticated architecture, stringent constrains exist for surface smoothness; therefore the tolerance of particulate density and size is generally very low, in order<1 particle per mm
2
.
DISCLOSURE OF THE INVENTION
It is an object of this invention to provide a laser deposition method to form a film with a surface substantially free of particulates and with exceptional surface quality.
In one broad form this invention provides a method of deposition of thin amorphous and structural films including the steps of sequentially evaporating small amounts of material to be deposited from a target of the material with each pulse of a laser irradiating the target, each pulse having an energy less than that required to evaporate sufficient of the material to result in a significant number of particles in the evaporated plume, and depositing the evaporated material on a substrate to form the film.
Preferably, the laser pulses have a repetition rate selected to produce a substantially continuous flow of evaporated material at the substrate.
This invention provides a method of laser deposition of amorphous or structural films with almost complete elimination of particles from the vapour plume thus resulting in deposition of thin films with very high surface quality. The method of this invention differs from the conventional PLD by the use of shorter laser pulses, typically picosecond and femtosecond pulse-range instead of nanosecond and much higher repetition rates, typically from kilohertz (kHz) to hundreds of megahertz (MHz) instead of tens of hertz (Hz). Furthermore the target is preferably evaporated by very low energy laser pulses and at an optimal laser intensity which depends on thermodynamic parameters of the target thus improving the efficiency of the evaporation process.
The invention provides a method of ultrafast laser ablation and deposition of thin amorphous or structural films by applying a succession of short laser pulses at high repetition rate for heating the target and generating successive bursts of atoms and/or ions in a vapour plume. Furthermore, as the laser repetition rate used increases, it is possible to reach a condition where a continuous beam of atoms strikes the substrate surface because the spread of atom velocities in the laser evaporated plume allows the slow atoms from one pulse to arrive at the same time as the faster atoms from later pulses. As a result, the film grows on the substrate surface from a continuous flow of atomic vapours with regulated atom flux density. This aids the creation of structured films, for example epitaxial growth. The method of this invention differs substantially from conventional PLD processes where the bursts of material from successive pulses arrive separately at the substrate.
A continuous flow can be established we recognize because of a spread of velocities exists for the material evaporated by a laser. The temperature at the evaporation surface is typically 5·10
3
K, depending on the target material. For graphite the average velocity &ngr;
c
for evaporated carbon atoms in the vapour flow will be &ngr;
c
=2·10
5
cm/s, but the velocity of the expansion front is greater and is typically ~3&ngr;
c
=6·10
5
cm/s. This means that continuous flow of evaporated carbon atoms is formed at a distance d=1.5·&ngr;
c
/R
rep
, here R
rep
is the minimum laser repetition rate. Thus, for typical target to substrate distance of 10 cm the minimum laser repetition rate for formation of continuous vapour is R
rep
=30 kHz. In order to create a homogeneous continuous vapour flow with low temporal and spatial modulation in vapour density near the substrate surface the actual laser repetition rate should be several times R
rep
, suggesting MHz or greater rates are desirable.
Selection of an appropriate high repetition rate>>R
rep
then allows the flux of the atoms on the substrate surface to be fine tuned via small variations in the laser repetition rate, for example, permitting the deposition of single atomic layers of material. In convention PLD, where the number of evaporated atoms per pulse is up to ten orders of magnitude higher than for the present invention, such fine deposition control is not possible. Thus this invention overcomes prior art limitations in controlling the accuracy of the thickness of the deposited film.
The relationship between laser pulse characteristics such as time duration to, wavelength, laser intensity on the target surface and pulse repetition rate R
rep
from the one hand, and the target material characteristics such as density, specific heat, heat conduction, and heat of vaporisation from the other hand, allows selection of the optimal level of absorbed laser intensity I
a
for the most efficient evaporation of the target. The laser pulse length and the laser intensity are preferably selected according to the following relationship:
I
a
t
P
1
2
=
a
1
2
ρ
0
Ω
;
where
a
=
K
C
p
ρ
0
;
(
1
)
here a is the thermal diffusion coefficient in [cm
2
/s]; K the heat conduction coefficient in [J/(s·cm·K)], C
p
is specific heat [J/(g·K)], &rgr;
0
is target material density in [g/cm
3
]; &OHgr;=&egr;
b
/M[J/g] is the heat of vaporisation, &egr;
b
is the binding energy in [J], and M is the atomic mass of the target material in [g], and I
a
is in [W/cm
2
]. Keeping the laser intensity close to the practical optimal intensity allows the most effective regime of evaporation to be achieved with minimal thermal loss into the target.
By reducing the laser pulse duration from the conventional nanosecond range to the picosecond or shorter time-
Gamaly Eugene
Luther-Davies Barry
Rode Andrei
Carlson David V.
Pianalto Bernard
Seed IP Law Group PLLC
The Australian National University
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