Laser ablation apparatus

A laser ablation apparatus includes a laser beam source for ablation for emitting a first laser beam, a lens for focusing the first laser beam, a vacuum chamber having a window on its wall through which the first laser beam is injected into the vacuum chamber, a target holder arranged in the vacuum chamber to hold a target onto which the first laser beam is irradiated, a substrate holder arranged opposing to the target holder in the vacuum chamber to hold a substrate, and a laser beam source for particle decomposition for emitting a second laser beam in the vacuum chamber to decompose particles turned out from the target by the first laser beam in the vacuum chamber. The apparatus can further include a pair of mirrors, arranged on both sides of the target holder, for multiply reflecting the second laser beam over the target holder.

BACKGROUND OF THE INVENTION 
The present invention relates to a laser ablation apparatus used for film 
formation of a superconductor or a ferroelectrics utilized in a thin film 
device. 
One example of a conventional laser ablation apparatus will be explained 
below. 
In the conventional laser ablation apparatus, as shown in FIG. 2, a laser 
beam 2 having the energy density not less than a threshold value is 
irradiated on an object 7 to be processed located in a vacuum chamber 5 to 
fly substance such as particles 19 out from the object 7 and then adhere 
the substance 19 to a substrate 15 supported by a substrate holder 14. The 
laser beam 2 is normally irradiated on the object 7 by focusing a 
short-wavelength pulsed laser beam with high energy density. The laser 
beam is focused by a lens 3 through a vacuum sealing window 4 onto the 
object 7. The air in the vacuum chamber 5 is exhausted by a vacuum pump 6. 
In such an apparatus, however, parts of the object 7 is heated to high 
temperature in a short period of time, and then clusters 20 not more than 
several micron (which is normally called as droplets) are flown out from 
the object 7. Then, since such clusters 20 are mingled in a thin film 
formed on the substrate 15, it is difficult to make the thin film flat 
which is basically required in a thin film device. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a laser 
ablation apparatus capable of forming a flat thin film. 
In accomplishing these and other objects, according to one aspect of the 
present invention, there is provided a laser ablation apparatus 
comprising: 
a laser beam source for ablation for emitting a first laser beam; 
a lens for focusing the first laser beam; 
a vacuum chamber having a window on its wall through which the first laser 
beam is injected into the vacuum chamber; 
a target holder arranged in the vacuum chamber to hold a target onto which 
the first laser beam is irradiated; 
a substrate holder arranged opposing to the target holder in the vacuum 
chamber to hold a substrate; and 
a laser beam source for particle decomposition for emitting a second laser 
beam in the vacuum chamber to decompose particles turned out from the 
target by the first laser beam in the vacuum chamber. 
By the above construction of the present invention, by injecting the laser 
beam into the vacuum chamber, the laser beam can be effectively absorbed 
in the ablation particles including the droplets turned out from the 
target, and the droplets can be heated and evaporated effectively, and 
then the droplets can be sufficiently decomposed. Thus, the sizes of the 
droplets mingled in the thin film adhered onto the substrate can become 
smaller, resulting in forming a flat thin film. 
According to another aspect of the present invention, there is provided a 
laser ablation apparatus comprising: 
a laser beam source for ablation for emitting a first laser beam; 
a lens for focusing the first laser beam; 
a vacuum chamber having a window on its wall through which the first laser 
beam is injected into the vacuum chamber; 
a target holder arranged in the vacuum chamber to hold a target onto which 
the first laser beam is irradiated; 
a substrate holder arranged opposing to the target holder in the vacuum 
chamber to hold a substrate; 
a laser beam source for particle decomposition for emitting a second laser 
beam in the vacuum chamber decompose particles turned out from the target 
by the first laser beam in the vacuum chamber; and 
a pair of mirrors, arranged on both sides of the target holder, for 
multiply reflecting the second laser beam over the target holder. 
By the above construction of the present invention, by multiply reflecting 
the laser beam over the target holder, the laser beam can be effectively 
absorbed in the ablation particles including the droplets turned out from 
the target, and the droplets can be heated and evaporated effectively, and 
then the droplets can be sufficiently decomposed. Thus, the sizes of the 
droplets mingled in the thin film adhered onto the substrate can become 
one tenth or less, resulting in forming a flat thin film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before the description of the present invention proceeds, it is to be noted 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
A laser ablation apparatus according to one preferred embodiment of the 
present invention is shown in FIG. 1. A laser beam 2 for ablation 
oscillated from an excimer laser is focused by a lens 3 through a vacuum 
sealing window 4 provided on the vacuum chamber wall onto a target 7 in a 
vacuum chamber 5. The laser beam 2 is irradiated on the target 7 movably 
supported by an X-Y table 17 in the vacuum chamber 5. The target 7 can 
move in an x-direction (a traverse direction in FIG. 1) and a y-direction 
(a direction perpendicular to a sheet of the drawing in FIG. 1) by the X-Y 
table 17 in the vacuum chamber 5. The vacuum chamber 5 has a vacuum pump 6 
for exhausting air in the chamber 5. A laser beam 9 for decomposing 
particles oscillated from a YAG laser 8 passes through a vacuum sealing 
window 10 provided on the vacuum chamber wall to inject in the vacuum 
chamber 5 passes over the target 7 in parallel with the surface of the 
target 7, and then passes through a vacuum sealing window 11 opposed to 
the vacuum sealing window 10 and provided on the vacuum chamber wall to 
travel to the outside of the vacuum chamber 5. A laser reflection mirror 
12 is arranged outside the vacuum sealing window 11 to reflect the laser 
beam 9. The laser reflection mirror 12 is slightly, e.g. by 0.5 degree, 
inclined to a vertical direction in FIG. 1, that is, to a direction 
perpendicular to the surface of the target. Another laser reflection 
mirror 13 is arranged outside the vacuum sealing window 10 to reflect the 
laser beam 9 reflected by the mirror 12. Therefore, the laser beam 9 is 
multiply reflected over the target 7 between the mirrors 12 and 13 through 
the vacuum sealing windows 11 and 10 so as to decompose the droplets flown 
out from the target 7. An initial path of the laser beam 9 injected in the 
vacuum chamber 5 is, preferably, below multiple paths reflected by the 
mirrors 12 and 13 and over the target 7. A substrate holder 14 is arranged 
opposing to the target 7 in the vacuum chamber 5 and supports a substrate 
15. 
According to the construction, the substance of the target 7 is turned out 
therefrom by a 20 nsec-pulsed laser beam 2, for example and then the 
substance fly as particles 16 to the substrate 15 to form a thin film on 
the substrate 15. At that time, a 300 nsec-pulsed laser beam 9 as one 
example is injected in a space apart from the target 7 by 5 mm in the 
vacuum chamber 5 in 1 .mu.sec after the oscillation of the laser beam 2. 
When the distance between the mirrors 12 and 13 is 300 mm and the inclined 
angle (.theta.) of the mirror 12 is 0.5 degree as one example, the laser 
beam 9 multiply reflected between the mirrors 12 and 13 is absorbed by the 
particles 16 and droplets during the flight paths of the turned-out 
particles 16 and droplets. Therefore, heat is supplied to the flying 
particles 16 and droplets from the laser beam 9 and then the evaporation 
speed of the droplets becomes faster and the droplets can be sufficiently 
decomposed to make the sizes of the droplets one tenth or less which will 
be mingled in a thin film. 
Instead of the pulse oscillation, the laser beam source 8 can be performed 
by the continuous oscillation to have the same advantages. 
According to the apparatus of the embodiment, by multiply reflecting the 
laser beam 9 over the target 7, the laser beam 9 can be effectively 
absorbed in the ablation particles 16 including the droplets turned out 
from the target 7, and the droplets can be heated and evaporated 
effectively, and then the droplets can be sufficiently decomposed. Thus, 
the sizes of the droplets mingled in the thin film adhered onto the 
substrate 15 can become one tenth or less, resulting in forming a flat 
thin film. 
In the above construction of the embodiment, if the laser beam 9 is 
injected over the target 7 without the reflection of the mirrors 12 and 
13, by injecting the laser beam 9 into the vacuum chamber 5, the laser 
beam 9 can be effectively absorbed in the ablation particles 16 including 
the droplets turned out from the target 7, and the droplets can be heated 
and evaporated effectively, and then the droplets can be sufficiently 
decomposed. Thus, the sizes of the droplets mingled in the thin film 
adhered onto the substrate 15 can become smaller, resulting in forming a 
flat thin film. 
FIGS. 3A, 3B, 4A, and 4B show experiments according to the embodiment in 
which 27 sec-pulsed laser beams emitted from 248 nm-KrF excimer lasers are 
absorbed in particles over targets. In FIG. 3A, the inclined angle of the 
laser beam to the surface of the target is 45 degree as shown in FIG. 3B. 
In FIG. 4A, the inclined angle of the laser beam to the surface of the 
target is 90 degree as shown in FIG. 4B. As is clear from the experiments, 
the sizes of the droplets in FIGS. 4A and 4B can become smaller than that 
in FIGS. 3A and 3B because the laser beam is effectively absorbed in 
particles in FIGS. 4A and 4B compared with in FIGS. 3A and 3B. Thus, it is 
preferably to multiply reflect the laser beam 9 over the target 7 by the 
mirrors 12 and 13 for effective absorption. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as included within the scope of the present invention as 
defined by the appended claims unless they depart therefrom.