Laser machining apparatus and method of the same

A laser machining apparatus includes a laser beam source, such as of excimer laser, which produces a laser beam to be projected on a work piece or a sample, first and second illumination light sources which have wavelengths substantially equal to the wavelength of the laser beam and illuminate the entire image and the laser beam, respectively, a first beam splitter which guides the image produced by the illumination light to an observation unit, a second beam splitter which guides the laser beam from the laser beam source to an objective lens, and a controller which controls the machining condition including the relative positioning between the sample and the laser beam depending on the result of observation. The laser beam guide path structure from the laser beam source to the sample has its interior wall made of laser-transparent material such as glass, and the transparent material is enclosed by a laser blocking material such as a metal or water.

BACKGROUND OF THE INVENTION 
The present invention relates to the technology of high-power laser 
application apparatus such as laser machining apparatus and a method of 
the same. More particularly, the invention relates to laser machining 
apparatus, laser lithography apparatus, and a method thereof using an 
excimer laser beam with a wavelength of invisible vacuum ultraviolet rays. 
A conventional laser machining apparatus has the observation wavelength 
made different from the wavelength of the machining laser beam, as 
described in a Japanese literature "Laser Machining", pp. 84-85, by Susumu 
Nanba, et al., published on Nov. 30, 1972 by Nikkan Kogyo Shinbun. FIG. 2 
is the most simplified illustration of the conventional laser machining 
apparatus. In the figure, a near infrared light beam with a 1.06 .mu.m 
wavelength produced by a laser oscillator 31 is reflected by a mirror 32 
which reflects the laser and transmits the visible light, and guided to an 
objective lens 33 by which the laser beam is focused on a work piece 34 
placed on a table 35 so that it is machined For the observation of 
machining, the image of the work piece 34 received by the objective lens 
33 is transmitted through the mirror 32 and focused on the operator's eye 
by an objective lens 36 so that the operator adjusts the laser beam to 
maintain the machining accuracy. 
However, the conventional technique uses different wavelengths for the 
machining laser beam and observation light, and therefore color matching 
is needed for the objective lens or a compensation for color matching by 
some means is required for accurate observation. 
Various application techniques of the excimer laser, which include excimer 
doping of semiconductor, thin film formation and laser fabrication, 
pertaining to this invention are disclosed in Japanese publication 
"Machine Tool Series, Laser Machining", pp. 135-154, which is a separate 
issue of "Applied Mechanics", published on Sep. 10, 1990. The frontier 
technology of excimer laser is introduced in publication "Precision 
Engineering" (JSPE), by Ueda, No. 5, pp. 837-840, published in 1989, and 
the current topics and prospect of submicron lithography based on excimer 
laser is introduced in "Applied Physics", Vol. 56, No. 9, pp. 44-48, 
published in 1987. 
FIGS. 12 and 13 show examples of conventional laser application apparatus 
that are illustrated on pages 32 and 33 of "The 5th Laser School Text, B3, 
Safety of Laser", sponsored by the Optical Industrial Promotion Associates 
in Japan and the Ministry of Commerce and Industry. 
In FIG. 12, a laser beam produced by a CO.sub.2 laser source 81 is guided 
through a metallic conduit 82 with a thickness of 3 mm or more, reflected 
in a mirror box 84, and focused on a work piece 85 for machining. A 
machining chamber 83 is made of acrylic resin with a thickness of 10 mm or 
more and is designed so that the laser beam does not leak out of the 
chamber. In FIG. 13, the laser beam produced by a YAG laser source 91 is 
guided through a metallic conduit with a thickness of 1 mm or more, 
reflected in a mirror box 94, and focused on a work piece 95 for 
machining. A machining chamber 93 is made of metal and its interior wall 
is painted in black. An industrial television camera 96 is provided on the 
mirror box 94 so that machining is observed on a TV monitor 97. 
The conventional techniques shown in FIGS. 12 and 13 have their optical 
systems enclosed by metallic or acrylic material which does not transmit 
the light so that the laser beam does not leak out. This arrangement, 
however, imposes such a problem that the laser blocking wall material 
which is exposed to the laser beam due to diffraction, scattering or other 
reason is melted, evaporated and removed and consequently deposited on 
some important optical part, causing it to be damaged or destroyed when it 
is hit by the laser beam. This problem is especially serious in the fields 
of high-power laser machining, laser nuclear fusion and laser energy 
transmission. 
SUMMARY OF THE INVENTION 
The present invention is intended to solve the foregoing problem, and its 
primary object is to provide a laser machining apparatus and a method 
thereof which enable the observation of machining even in the case of 
using a laser source with a wavelength far from the visible wavelength 
range, while accomplishing high-accuracy machining. In order to achieve 
the object, a first feature of the present invention is to use optical 
systems tuned to substantially the same wavelength for the machining laser 
beam and the observation light. 
Another object of the present invention is to provide a laser application 
apparatus which is operative continuously without the emergence of 
substance from the laser beam enclosing structure which can be exposed to 
a high-power laser beam. In order to achieve the object, a second feature 
of the present invention is to provide a laser beam enclosure with having 
an interior wall formed of a material that transmits the laser beam, and 
an exterior wall of the exclosure formed of a laser beam blocking 
material. 
In the first feature of the present invention, the machining wavelength and 
observation wavelength are made equal so that there is no displacement 
between the machining focal position and the observation image position, 
and consequently accurate positioning and accurate machining are possible. 
Because of the use of the same wavelength, color matching for the 
machining objective lens is not needed, which facilitates the design of 
optical system, particularly for invisible ultraviolet and infrared rays, 
and it becomes possible to accomplish a high-precision machining optical 
system in the range of vacuum ultraviolet rays that has scarcely been 
attempted in the past due to the severe restriction in the availability of 
material for the optical system. Through proximity color matching, instead 
of the equal one, a laser machining optical system that has been 
difficult, if not impossible, in the past can be accomplished. 
In the second feature of the present invention, the laser beam enclosing 
structure, i.e., portions facing the laser beam path, are formed of 
material which transmits the laser beam, and consequently even if it is 
exposed to the laser beam by some reasons, the laser beam merely goes 
through the structure and does not machine it. The transparent enclosure 
is provided on its exterior wall with a member which blocks the laser 
beam, and accordingly the laser beam does not go out of the enclosure. 
Even if the laser beam blocking member is machined by the laser beam, the 
resulting product stays in the outside of the enclosure and such events as 
the contamination of optical parts on the beam path and the subsequent 
damage or destruction of these portions by being exposed to the laser beam 
do not occur. Consequently, a high-output laser application apparatus can 
be operated stably for a long time. 
Through the provision of a jacket containing circulating liquid as the 
laser beam blocking member, diffraction energy can be absorbed in the 
exterior of the transparent laser beam enclosure, and such instability 
causing factors an ambient temperature rise can be eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described with reference to 
FIG. 1. A vacuum ultraviolet (VUV) laser beam 2 with a wavelength of 193 
nm produced by an excimer laser oscillator 1 is shaped to have a 
rectangular cross section by a beam shaper 3, adjusted for its 
transmission power by a transmittance filter 4, adjusted for its power 
density through enlargement or reduction of the beam size by a zoom 
optical system 5, uniformed for the beam intensity distortion by a beam 
integrator 6, reflected in its 85% proportion by a first splitter 7, and 
directed to a rectangular aperture slit 8. 
The laser beam 9 shaped by the rectangular aperture slit 8 has its 85% 
component transmitted by a second split filter 10 and is incident on an 
objective lens 11 which is tuned to a 193 nm wavelength. The objective 
lens 11 projects the laser beam with the shape of the rectangular aperture 
slit 8 on a work piece 13 placed on an XYZ fine movement table set 12, and 
the work piece 13 is machined. 
The 15% laser beam component which transmits the first splitter 7 is 
absorbed by a first beam damper 14. The 15% laser beam component reflected 
by the second splitter 10, out of the laser beam 9, is absorbed by a 
second beam damper 15. 
A heavy hydrogen lamp 16, which illuminates the rectangular aperture slit 8 
to produce a projection image, has its output light conducted through the 
first splitter 7, rectangular aperture slit 8 and second splitter 10, and 
converged by the objective lens 11 and projected on the work piece 13. A 
heavy hydrogen lamp 17 for general illumination has its 50% component of 
light output reflected by a third splitter 18, and is further reflected 
for its 15% component by the second splitter 10 and projected on to the 
work piece 13 through the objective lens 11. 
These images of work piece and slit are collected by the objective lens 11, 
reflected by the second splitter 10, transmitted by the third splitter 18, 
and focused by a focusing lens 19 on a light-sensitive screen 21 of a 
ultraviolet image pickup tube 20. The image pickup tube 20 converts the 
image into an electrical signal, which is processed by an image signal 
processor 21, and a resultant observation image is displayed on a display 
unit 22. 
In this manner, the laser beam for machining and the light for observation 
have the same wavelength, and accordingly there arises no displacement 
between their images and the work piece is machined accurately. The 
objective lens 11 is tuned to a single color of 193 nm, which eliminates 
the difficulty of color matching between the vacuum ultraviolet rays (193 
nm) and another color such as color of a visible light and facilitates the 
development of high-performance lenses. In FIG. 1, the structure and 
component parts enclosed by the dot-dash line are accommodated in a 
hermetically sealed enclosure which will be explained in detail in the 
following embodiments. 
Although the embodiment shown in FIG. 1 is pertinent to a vacuum 
ultraviolet laser machining apparatus, it is apparently applicable also to 
a visible laser and infrared laser machining apparatus. By choosing the 
lens system and observation system appropriately, it is apparently 
applicable also to a soft X-ray machining apparatus. In the case of focal 
plane machining such as steel plate machining for example, the 
illumination lamp 16 and aperture slot 8 in the foregoing arrangement are 
unnecessary. 
FIG. 3 shows another embodiment of the present invention pertinent to the 
observation system of the laser machining apparatus. The image of the work 
piece provided by the focusing lens 19 is received by the light sensitive 
screen 24 of a ultraviolet image intensifier 23 so that the resulting 
visible light image can be observed directly by the eye 26 of the operator 
through an eyepiece lens 25. In this case, the controller and display unit 
can be eliminated. Owing to a large electrical amplification, even a faint 
observation image can fairly be seen. 
FIG. 4 shows another embodiment of the observation system. The image of the 
work piece provided by the focusing lens is received by the light 
sensitive screen 24 of a ultraviolet image intensifier 23 having a 60 dB 
amplification, and the resultant visible light image is focused by a relay 
lens 27 on the light sensitive screen 29 of an image pickup tube 28, which 
converts the image into an electrical signal. The image signal is 
processed by an image signal processor 21, which displays a resultant 
observation image on a display unit 22. This system is capable of 
eliminating the background noise, and a faint image of work piece can be 
seen at high resolution. By providing an image emphasizing module 30 in 
the image signal processor 21, the quality of image can further be 
improved. 
FIGS. 5A-5C show embodiments of individual sections of the inventive 
apparatus. FIG. 5A shows the beam shaper 3 and transmittance filter 4. The 
beam shaper 3 is made up of a convex bar lens and a concave bar lens. It 
receives a laser beam with a 30-by-10 mm rectangular cross section to 
produce a laser beam with a 10-by-10 mm square cross section, so that the 
rear-stage optical system can be simplified. The transmittance filter is 
an alignment of mirrors 43 of dielectric materials with transmittances of 
5%, 10%, 20%, 30%, 50% and 100% tuned to the 193 nm wavelength. The filter 
can be set to use a section of an intended transmittance thereby to adjust 
the intensity of laser beam so that a wide range of machining condition 
can be covered. The reflected unwanted laser beam is directed to a beam 
damper 44 and absorbed by it. 
FIG. 5B shows the zoom optical system 5. The system operates to vary the 
magnification from 1/2 to 2 for the incident image by moving a lens suite 
45, allowing arbitrary setting of the laser power density from 1/4 to 4 
fold, and the range of machining condition can further be expanded. 
FIG. 5C shows an embodiment of the structure of the beam integrator 6. The 
arrangement includes an alignment of seven small convex bar lenses 46, 
which is followed by a pair of large convex bar lenses 47, so that a 
uniformed laser beam is collimated. The resulting beam intensity 
distribution is within a range of plus/minus several percent, and it 
contributes significantly to uniform machining. 
FIG. 6 shows, as a first example, the arrangement of the laser beam shaper 
which is used for the function of the aperture slit 8 in the foregoing 
embodiment. The figure shows by model a cross section of the device, with 
the light path being illustrated in it. A parallel laser beam 51 is 
projected downwardly in the Figure along the optical axis Z--Z. A slit 
member 52 is disposed perpendicularly to the optical axis Z--Z. The slit 
member 52 has an aperture 521 having an intended shape, with its 
peripheral section 52b being formed to function as a prism. The prism 
section 52b has an upper surface (the side exposed to the laser beam 51) 
which is a plane orthogonal to the optical axis Z--Z, and has a lower 
surface which is tapered to form a knife edge around the aperture 52a. The 
slit member 52 is made of a material which is transparent for the laser 
beam. Specifically, a material is chosen depending on the wavelength of 
the laser beam, and it is preferably an optical glass, fusion quartz or 
synthesized quartz in dealing with a laser beam ranging between near 
infrared and ultraviolet rays. 
A beam component, out of the laser beam 51, which enters the aperture 52a 
of the slit member 52 is formed into a laser beam 53 having the same 
cross-sectional shape as the aperture 52a. The remaining portion of the 
laser beam 51, which is incident on the prism section 52c, is refracted by 
the prism and steered away from the optical axis Z--Z as a refracted laser 
beam 54. 
Since the slit member 52 does not block the laser beam, but it merely 
refracts or transmits the laser beam, it scarcely absorbs energy of the 
laser beam, and its wear caused by the laser beam is little enough to be 
neglected practically. This embodiment produces a laser beam 53 having an 
intended cross-sectional shape, and yet prevents the beam shaping device 
from wearing. 
FIG. 7 shows a modification of the embodiment shown in FIG. 6. Like 
component parts in these Figures are referred to by the common symbols. 
The modified arrangement differs from the counterpart of FIG. 6 as 
follows. The slit member 52 has its upper surface (the side exposed to the 
laser beam 51) tapered to form a prism section 52c. This means that the 
tapered section has a certain angle with the imaginary plane which is 
orthogonal to the optical axis Z--Z of the laser beam 51. Accordingly, 
part of the laser beam 51 reflected on the upper surface 52c-1 of the 
prism does not go back to the laser source along the optical axis Z--Z, 
and it does not adversely affect the operation of the laser sourcing 
oscillator. 
FIG. 8 shows another modification. A slit member 52 is disposed on the 
light path of the laser beam 51 which is directed downwardly in the Figure 
along the optical axis Z--Z. The main light path of the laser beam 51 is 
enclosed by a cylindrical enclosure or casing 55. The Figure shows only 
the right half portion of the structure which is symmetrical with respect 
to the optical axis Z--Z. 
The casing includes a window 56 at the position where the laser beam 54 
refracted by the prism section 52c hits, with a laser absorber 57 being 
attached to the exterior wall to cover the window 56. The laser absorber 
57 is provided with heat dissipation fins 58 in this embodiment. The laser 
beam 54 refracted by the prism section 52c enters the laser absorber 57 
through the window 56, and energy possessed by the laser beam is 
transformed into heat. 
The slit member in this modification arranged as explained above is 
transparent for the laser beam and it lets the laser beam transmit through 
it. Accordingly, the slit member scarcely absorbs energy of the laser beam 
and it does not wear. The laser beam which enters the aperture of the slit 
member is shaped to have the same cross section as the aperture, and the 
rest of the laser beam which is incident on the peripheral prism section 
is refracted and steered away from the main light path. It becomes 
possible to shape, as intended, the laser beam with a high power density, 
and this scheme is advantageous for LSI wiring and mask modification 
machining which necessitate the projection machining optical system. 
The concept of the present invention illustrated in the foregoing 
embodiments can be applied to a copending U.S. patent application (and 
European patent application) based on the Japanese Patent Application No. 
02-126691 filed on May 18, 1990 assigned to the same assignee as of the 
present invention, the content of which is herein incorporated by 
reference. 
Another embodiment of the present invention will be described with respect 
to FIG. 9, FIG. 10 and FIG. 11. FIG. 9 is a cross-sectional diagram of the 
laser machining apparatus which reflects the second feature of this 
invention. A laser beam 66 produced by a laser source 61, such as of YAG 
laser or excimer laser, is guided inside a light path cover 62 made of 
optical glass, quartz glass or the like, referenced in a mirror box 64 
made of similar glass, and focused for machining on a work piece 65 which 
is placed in a machining chamber 63. The machining chamber 63 has its 
interior wall made of glass. The glass cover 62 and enclosure 63 are 
provided on their exterior wall with metallic covers 67 which blocks the 
laser beam. 
Part of the laser beam which hits the interior wall of the light path cover 
62 or enclosure 63 by some reasons, such as diffraction or scattering, 
goes through the glass wall, but is blocked by the metallic cover 67, and 
it does not go out of the apparatus. Even if such a substance as metallic 
vapor is produced when the metallic cover 67 is hit by the laser beam, it 
is prevented from entering into the optical path by being blocked by the 
interior glass wall of the cover 62 and enclosure 63. Accordingly, the 
performance of the optical parts is not impaired by such emerging 
substance, and the apparatus can be operated continuously. 
FIG. 10 is a cross-sectional diagram of another embodiment. A ultraviolet 
laser beam 66 produced by a laser source 61, such as of excimer laser, is 
guided inside an optical path cover 62 made of glass such as quartz glass 
which transmits ultraviolet rays, reflected in a mirror box 64 made of 
similar glass material, and focused for machining on a work piece 65 which 
is placed in an enclosure 63 made of the same material. 
The light path cover 62, enclosure 63 and mirror box 64 are provided on 
their exterior walls with a light blocking jacket 68 which is filled with 
liquid, e.g., water, controlled to have a temperature slightly higher than 
the room temperature and circulated at a constant flow rate. A sensor 69 
for detecting the presence or absence of the liquid is provided at the 
highest portion of the liquid, so that the laser source 61 produces a 
laser beam only when the presence of liquid is confirmed. 
In this arrangement, even if part of the laser beam hits the interior wall 
of the light path cover 62, enclosure 63 or mirror box 64 by diffraction, 
scattering or some other reasons, it transmits the glass wall and enters 
the exterior light blocking jacket 68, by which the ultraviolet rays are 
absorbed progressively and its energy is carried by the liquid to a 
thermal exchanger 70 and discharged. The liquid, with its temperature 
adjusted slightly higher than the room temperature, is circulated back to 
the light blocking jacket 68, thereby also preventing dew condensation in 
the interior wall of light path cover 62. Consequently, even if part of 
the laser beam hits a peripheral member of the light path, it can be taken 
out harmlessly and the high-power laser application apparatus can be 
operated stably and continuously. 
FIG. 11 shows a cross section of the mirror section in the above 
embodiment. The mirror box 64 incorporates a mirror holder 75 which is 
made of glass material for the surface exposed to the laser beam. The 
laser beam 66 has its light path varied by 90.degree. by being reflected 
by the mirror 71. The mirror 71 is supported by a frame 72 made of glass, 
and the frame 72 is fixed on a mirror receptacle 74 by means of latches 
73. The mirror receptacle 74 is mounted on the mirror holder 75 through 
adjusting screws 75. The mirror holder 75 has one side exposed to the 
laser beam 66 provided with a guide plate 77 made of glass, so that no 
contaminant is produced by the irradiation of the laser beam. 
Consequently, even if part of the laser beam hits a peripheral member of 
the light path, it can be taken out harmlessly and the high-power laser 
application apparatus can be operated stably and continuously.