Processing of photomask

A laser beam of a pulse width 10 to 20 nanoseconds is irradiated and scanned relatively and two-dimensionally on a metal thin film formed on a transparent substrate from the transparent substrate side and through the transparent substrate with a superposition number of laser beam spots set up to at most 15 and to thereby remove a predetermined region of the metal thin film. The irradiated metal film begins to melt from the substrate side and is removed away explosively from the substrate, leaving the substrate substantially unaffected.

This invention relates to the removal processing of a film deposited on a 
transparent substrate and more particularly to a method and apparatus for 
removal-processing or removal-machining a photomask film on a transparent 
substrate in a predetermined region. 
Conventionally, use of laser beam has been tried for removing part of a 
metal thin film of a photomask. The photomask generally consists of a 
transparent substrate such as a glass plate and an opaque thin film of 
metal such as chromium deposited on the substrate. The laser beam has been 
irradiated directly on the metal thin film. When the laser beam is 
irradiated on the exposed surface of the metal thin film, the metal thin 
film begins to melt from the outer (exposed) surface to the inside. Along 
with the evaporation of the metal, the melted metal thin film mixes with 
part of the glass substrate due to the thermal conduction. As a result, 
part of the metal thin film remains on the glass substrate, i.e., on the 
transparent substrate. Therefore, according to the conventionally tried 
laser processing method of photomasks, it has been impossible to remove 
undesired portions of the metal thin film perfectly from the substrate and 
leave only a predetermined pattern of the metal thin film. 
A technique of irradiating a YAG laser beam on a chromium film through a 
glass substrate is proposed in Japanese publication entitled "Irradiation 
Characteristic of Focused Q-switched YAG Laser Beam" in "Journal of the 
Japan Society of Precision Engineering", October, 1970, pages 690-696, 
particularly page 693. However, it is suggested merely as a solution to 
prevent damage of a beam focusing lens due to explosion or dispersion of 
chromium in the machining. 
An object of this invention is to provide a method of processing a 
photomask capable of perfectly removing undesired portions of a thin film 
constituting a photomask and also of removing portions of a thin film 
larger than the processing spot size of a laser beam. 
Another object of this invention is to provide a method of processing a 
photomask capable of perfectly removing undesired regions of a thin film 
and of removing portions of the thin film larger than the processing spot 
size of a laser beam so as to leave a predetermined pattern of the thin 
film and also of suppressing the roughness and the hollow or pit depth of 
the processed glass surface below 0.1 .mu.m and as low as possible. 
Another object of this invention is to provide an apparatus for processing 
a photomask capable of automatically removing undesired portions of a thin 
film and of removing portions of the thin film larger than the processing 
spot size of a laser beam. 
Another object of this invention is to provide an apparatus for processing 
a photomask capable of automatically removing undesired portions of a thin 
film and of removing portions of the thin film larger than the processing 
spot size of a laser beam while suppressing the roughness and collapse or 
pit depth of the processed glass substrate surface below 0.1 .mu.m and as 
low as possible. 
According to one aspect of this invention, there is provided a method of 
processing a photomask comprising the steps of preparing a photomask 
consisting of a transparent substrate and a thin film deposited thereon, 
irradiating a pulsed laser beam spot on the thin film of the photomask in 
a predetermined region from the transparent substrate side and through the 
transparent substrate and scanning the relative position of the pulsed 
laser beam spot on said photomask in two-dimensional directions, thereby 
removing the thin film of the photomask in the predetermined region. 
According to another aspect of this invention, there is provided a method 
of processing a photomask comprising the steps of preparing a photomask 
comprising a transparent substrate and a thin film formed thereon, 
irradiating a pulsed laser beam having a pulse width of at most 20 
nanoseconds on the thin film in a predetermined region from the 
transparent substrate side of the photomask and through the transparent 
substrate and relatively scanning the pulsed laser beam with respect to 
the photomask in two-dimensional directions, and controlling the 
oscillation frequency of the pulsed laser beam and the scanning speed of 
the laser beam on the thin film so as to set the superposition number of 
the laser beam spots in a range of up to 15, thereby removing the thin 
film in the predetermined region. 
According to another aspect of this invention, there is provided an 
apparatus for processing a photomask comprising a transparent substrate 
and a thin film deposited thereon, comprising a laser oscillator for 
intermittently generating a laser beam of pulse form, scanning means for 
relatively scanning the pulsed laser beam generated from the laser 
oscillator with respect to said photomask in two-dimensional directions, 
condensing or focusing optical means for condensing and irradiating the 
pulsed laser beam on the thin film through the transparent substrate of 
the photomask, and controller means for controlling the scanning speed of 
the pulsed laser beam on the thin film through said scanning means and the 
oscillation repetition frequency of the pulsed laser beam generated from 
said laser oscillator so as to set the superposition number of the laser 
beam spots in a range of up to 15. 
According to another aspect of this invention, there is provided an 
apparatus of processing a photomask comprising a transparent substrate and 
a thin film deposited thereon, comprising means for generating a laser 
beam of pulse form having a pulse width of at most 20 nanoseconds, means 
for scanning the laser beam of pulse form generated from the laser 
generating means relatively and two-dimensionally with respect to the 
metal thin film deposited on the transparent substrate, condensing optical 
means for condensing the laser beam of pulse form scanned by the scanning 
means on a predetermined region of the thin film through the transparent 
substrate, means for disposing the photomask so that the thin film 
irradiated by the condensing optical means is on the back side of the 
transparent substrate with respect to the condensing optical means, and 
control means for controlling the generation of the laser beam of pulse 
form from the laser generating means in synchronism with the scanning of 
the laser beam of pulse form scanned by the scanning means so as to set 
the superposition number of the laser spots in a range of up to 15. 
Other objects, features and advantages of this invention will become 
apparent from the following detailed description of the embodiments of 
this invention when taken in conjunction with the accompanying drawings.

There are two types of processing a chromium photomask for the IC 
manufacture; correcting defective portions of a predetermined circuit 
pattern (erroneously deposited portions) and removing away a predetermined 
pattern from a uniform chromium film to leave a circuit pattern. 
Hereinbelow, embodiments of correcting defective portions of a chromium 
photomask for the IC manufacture will be described. A chromium photomask 3 
generally consists of a glass substrate 1 and a chromium thin film 2 
deposited on the glass substrate 1 by vacuum evaporation and having a film 
thickness of about 700 A and a pattern corresponding to a negative circuit 
pattern, as shown in FIG. 1. Such a metal thin film 2 may include 
projecting defectives 4, island defectives 5 and sand-like dispersed 
defectives 6, as shown in FIG. 2. These defectives 4, 5 and 6 should be 
perfectly removed from the substrate 1. An embodiment of the apparatus for 
correcting these defectives will be described referring to FIG. 3. 
In FIG. 3, an Ar ion laser source 7 generates a pulsed laser beam 10 of a 
short pulse width, i.e. the pulse width at the half of the peak intensity 
is in a range from 10 to 20 nanoseconds. The laser oscillator 7 is 
acousto-optically driven by a driver 8 which is driven by a pulse 
generator 9. The laser beam 10 is scanned by a scanning system 11 in the 
direction perpendicular to the drawing sheet. The scanning system may be 
formed of a rotating prism or a rotating mirror. An optical system 12 
formed, for example, of convex lenses etc. condenses the laser beam 10 
generated from the laser source 7 and scanned by the scanning system 11 on 
a predetermined plane. A photomask 3 carrying a chromium thin film 2 is 
mounted and fixed on an evacuated chuck 13 with the chromium film 2 
directed toward the evacuated chuck 13. The evacuated chuck 13 is fixed on 
the upper surface of a resilient or elastic table 14 which is fixed on one 
ends of resilient thin metal bars 14a fixed on a Y table 17a at the other 
ends. The resilient table 14 can be finely moved in the horizontal 
direction in the figure by a feed motor 16 through a fine feed mechanism 
15 formed, for example, of a screw mechanism of a fine pitch. The feed 
motor 16 is mounted on an upper surface of a projecting portion of the Y 
table 17a. The Y table 17a is allowed to slide in y-direction 
perpendicular to the drawing sheet by a drive motor 18a fixed on an X 
table 17b which is allowed to slide in the horizontal direction of the 
figure. The X table 17b is driven by a drive motor 18b fixed on a base 
plate 20. A controller 19 applies a signal for determining the oscillation 
frequency of the pulse generator 9 to be pulse generator in synchronism 
with the sweep signal obtained from the scanning system 11 in accordance 
with the y-directional size of the defect of the chromium film, while 
giving command signal to the drive motor 16 in accordance with the 
x-directional size of the defect of the chromium film. The controller 19 
also applies a signal for stopping the oscillation of the pulse generator 
9 to the pulse generator 9 after finely feeding the resilient table 14 by 
a predetermined amount. Here, the location of the defective portion of the 
chromium thin film 2 of the photomask 3 is preliminarily detected by a 
detector (not shown), and the controller 19 drives the drive motors 18a 
and 18b and hence locates the Y table 17a and the X table 17b on the basis 
of the positional detection signal to bring the defective portion of the 
chromium thin film into the scanning area of the scanning system 11 and in 
the fine feed region of the resilient table 14. The scanning system may be 
as disclosed in copending U.S. patent application Ser. No. 627,279 filed 
Oct. 30, 1975 assigned to the same assignee. 
According to the above structure, the high power laser beam 10 generated 
from the laser oscillator 7 is condensed by the condensing optical system 
12 and irradiated as a small spot on the defective portion of the chromium 
thin film 2 from the substrate side and through the transparent substrate 
1. The irradiated chromium thin film 2 deposited on the glass substrate 
begins to melt and evaporate from the inner surface portion at the 
intersurface between the substrate 1 and the thin film 2. The inner 
surface portion, however, is isolated from the ambient atmosphere and 
hence the temperature rise increases the inner pressure extremely high. 
The chromium thin film 2 is forced to melt from the inner surface to the 
outer surface exposed to the outer atmosphere by the irradiation of the 
high power laser beam 10. At some critical point, the melted and 
evaporated chromium undergoes explosion by the increased inner pressure 
and scatters away from the glass substrate to leave only the glass plate. 
There is no remainder of chromium or alloy of chromium and the glass on 
the glass substrate. 
Next, description will be made of the case where the defective portion of 
the chromium thin film 2 is considerably larger than the spot size of the 
laser beam condensed by the condensing optical system. 
A defective portion of a chromium thin film 2 of a photomask 3 fixed on the 
vacuum chuck 13 with the chromium thin film 2 directed downwards is 
detected in its position by a detector (not shown) and the positional 
detection signal is sent to the controller 19. The controller 19 supplies 
command signals to the drive motors and locates the defective portion of 
the photomask 3 in the scanning region of the optical scanning system 11 
and in the fine feed region of the resilient table 14 by moving the Y 
table 17a and the X table 17b with respect to the laser beam 10. Here, the 
laser beam 10 is arranged to condense or focus on the defective portion of 
the chromium thin film 2 through the glass substrate 1, as shown in FIG. 
4. 
Then, the real scanning width Wy of the pulsed laser beam 10 in the maximum 
scanning width Sy limited by the scanning system 11 is determined 
according to the y-directional size of the defective portion of the 
chromium film 2, as shown in FIG. 5. The pulse generator 9 is oscillated 
by the command signal from the controller 19 in synchronism with the 
scanning of the scanning system so as to irradiate a pulsed laser beam 10 
from the laser source 7 on the chromium thin film in the width Wy. Along 
with the scanning of the laser beam 10 by the scanning system 11, the 
controller 19 generates a drive command for the drive motor 16 to shift 
the resilient table 14 through the fine feed mechanism 15 thereby to shift 
the photomask 3 for a width of Wx corresponding to the x-directional size 
of the defective portion of the chromium thin film 2 as shown in FIG. 5. 
Here, for the purpose of preventing the thermal effect to the glass 
substrate due to the irradiation of the laser beam spots on the same 
location, the scanning pitch Py of the pulsed laser beam spots 10 is 
adjusted by the scanning rate of the pulsed laser beam 10 scanned by the 
scanning system 11 and the oscillation frequency (e.g., in the range of 
from 10 to 1000 Hz) of the pulse generator 9 which determines the 
frequency of the pulsed laser beam 10 generated from the laser oscillator 
7. Namely, the laser beam 10 generated from the laser source 7 is 
irradiated on the defective portion of the chromium thin film 2 as a spot 
of a diameter d ranging from 2 to 5 .mu.m through the condensing optical 
system 12 and the superposition number ny=d/Py (where d is the processing 
spot diameter and Py is the pitch at which the pulsed laser spots are 
scanned) is arranged to be 1 to 10 in the scanning direction 
(y-direction). Similarly, the rotation of the drive motor 16 is so 
adjusted that in the direction perpendicular to the scanning direction, 
the superposition number of the laser beam spots on the defective portion 
of the chromium thin film 2, i.e., nx=d/Px (wherein d is the processing 
spot diameter and Px is the pitch of feeding the laser beam), is in the 
range of from 1 to 20. 
In this way, the pulsed laser beam 10 having the oscillation frequency and 
the oscillation region determined by the pulse generator 9 is generated 
from the laser oscillator 7 and condensed and irradiated on the defective 
portion of the chromium thin film 2 in a processing spot size of 2 to 5 
.mu.m through the condensing optical system 12. While the pulsed laser 
beam 10 is scanned by the scanning system 11 in the scanning direction 
(y-direction) at a high rate, the defective portion of the chromium thin 
film 2 is fed by the drive motor 16 through the fine feed mechanism 15 in 
the feeding direction (x-direction) at a constant rate for a short 
distance Wx. As a result, the region of the defective chromium thin film 
defined by Wx and Wy is irradiated by the pulsed laser beam 10 and the 
defective portion of the chromium thin film 2 in this region is perfectly 
removed away. 
Next, description will be made of the roughness Rmax and the pit depth of 
the processed glass surface from which the chromium thin film has been 
removed by the irradiation of the pulsed laser beam. 
In order to produce a trace of overlapping spots the above laser machining 
system may also scan a laser spot on the same line to be scanned twice or 
more. For example, the spot may be at first scanned on a line to be 
scanned with a superposition number nx=0.8, and then may be scanned once 
more on the same line from the start point of scanning shifted by a half 
the pitch, i.e., 1/2 Px. As a result, the superposition number on the 
given line of x-direction totally becomes 1.6. Thus, it will be understood 
that the lower limit of the superposition number nx or ny may be more than 
1 as a result of total scanning. 
When a pulsed laser beam having a repetition frequency f=250 Hz is 
condensed to a processing spot diameter d=2 .mu.m and irradiated on a 
defective portion of the chromium thin film 2 with a scanning rate vy=250 
.mu.m/sec given by the scanning system 11, the y-pitch Py of the spots of 
the laser beam 21 becomes Py=vy/f=1 .mu.m and the superposition number ny 
of the spots 21 in the y-direction becomes ny=d/Py=2. Further, when the 
feed speed of the chromium thin film 2 vx by the resilient table 14 is set 
at vx=0.6 .mu.m/sec and the scanning of the pulsed laser beam is arranged 
to reciprocate once in one second, the average pitch px of the spots 
becomes px.apprxeq.vx/2.apprxeq.0.3 .mu.m and the superposition number nx 
of the spots 21 in the x-direction becomes nx=d/px=7. The superposition of 
the laser spots 21 in the feed direction (x-direction), however, is only 
realized in the reciprocating action of the scanning of the pulsed laser 
beam 10. Namely, there is a time lag for the laser beam to traverse the 
same spot again and the thermal effect generated by the laser beam 
irradiation does not remain to the time when the laser beam comes back to 
the same spot again. Thus, comparing the cases when the superposition 
number ny of the spots 21 in the y-direction is 2 and when the 
superposition number nx of the spots 21 in the x-direction is 7, the 
roughness and the pit depth of the processed surface is superior in the 
case of x-direction and the problem is limited to the case of y-direction. 
Therefore, the roughness and the pit depth of the processed surface with 
respect to the superposition of the spots in the y-direction (scanning 
direction) will be described next. 
Namely, when the superposition number ny of the spots was set at 2, the 
half width of a pulse output was set in the range of from 10 to 20 
nanoseconds and the peak power density of the pulsed laser beam was varied 
in the range of 2.times.10.sup.8 to 8.times.10.sup.8 watts/cm.sup.2 by the 
adjustment of the power source current and the output of the driver 8, the 
roughness Rmax of the processed surface from which the chromium thin film 
was removed away was constant at 0.02 .mu.m as shown in FIG. 7b. Thus, it 
was found that the peak power density (watts/cm.sup.2) had no relation to 
the roughness of the processed surface in this case. On the other hand, 
when the peak power density of the pulsed laser beam was set at 
4.times.10.sup.8 watts/cm.sup.2 and the superposition number ny of the 
laser spots in the y-direction was varied from 2 to 20, it was found that 
the roughness Rmax of the processed surface rapidly decreased from the 
value of 0.1 .mu.m as the superposition number ny decreased less than 15. 
Here, the roughness of the processed surface was 0.02 .mu.m when the 
superposition number ny of the spots was 2, 0.04 .mu.m when the 
superposition number ny was 4, 0.08 .mu.m when the superposition number ny 
was 8, and almost constant at 0.10 .mu.m when the superposition number was 
no less than 15. Further, when the peak power density of a pulsed laser 
beam was set at 4.times.10.sup.8 watts/cm.sup.2 and the superposition 
number ny of the spots in the y-direction was varied in the range of from 
2 to 20, it was found that the pit depth of the processed surface, i.e. 
the pit depth of the glass substrate, rapidly decreased below 0.1 .mu.m as 
the superposition number ny decreased below 15. Here, the pit depth of the 
processed surface was 0.01 .mu.m when the superposition number ny of the 
spots was 2, 0.07 .mu.m when the superposition number ny was 4, 0.09 .mu.m 
when the superposition number ny was 8, and almost constant at 0.10 .mu.m 
when the superposition number ny was no less than 15. On the other hand, 
when the superposition number of the spots ny was set at 2 and the peak 
power density was varied from 2.times.10.sup.8 to 8.times.10.sup.8 
watts/cm.sup.2, the pit depth of the processed surface was about 0.01 
.mu.m for the peak power density not larger than 4.times.10.sup.8 
watts/cm.sup.2 and increased with the increase of the peak power density 
above 4.times.10.sup.8 watts/cm.sup.2, as seen from FIG. 8b. From the 
above results, it is seen that the roughness and the pit depth of the 
processed surface from which a chromium thin film has been removed is 
influenced by the superposition number ny of the pulse laser spots and can 
be suppressed below 0.10 .mu.m by setting the superposition number ny 
below 15. 
As exposure light for printing a circuit pattern formed in a photomask, 
light emitted from a mercury lamp and having a wavelength .lambda.=3800 A 
is usually used. In the exposure and printing the surface roughness and 
the edges of possible pits or recesses existing in the transparent 
substrate, particularly at positions corresponding to the holes forming 
the circuit pattern, should be made to an extent in which the exposure 
light is not diffracted but can travel straight through the substrate. 
Accordingly, in order to prevent such unwanted diffraction, the surface 
roughness and the maximum pit depth optically may be smaller than 1/4 the 
wavelength .lambda. of exposure light, i.e., 0.095 .mu.m. In the present 
embodiments the limit values of the surface roughness and the pit depth 
are shown by 0.10 .mu.m which is slightly greater than 0.095 .mu.m. 
However, the above limit values have no problem in practical use. 
Particularly, by setting the superposition number ny of the spots below 5 
the roughness and the pit depth of the processed surface can be reduced 
below 0.05 .mu.m and 0.08 .mu.m, respectively. 
Here, setting the superposition number ny of the spots at 1 means d=Py, 
i.e., processing spots of a diameter d are circumscribed, hence there 
remain un-processed regions at the circumferential portions and a 
two-dimensional pattern cannot be processed continuously. When the 
superposition number ny is set at 2, Py=1/2 d, hence the laser beam spots 
are scanned in y-direction at the pitch of 1/2 of the processing spot 
diameter and a linear pattern can be processed leaving only projections of 
0.1 .mu.m when d=2 .mu.m. Thus, when the superposition number ny of the 
laser spots is set above 1.2 and the laser beam is scanned in y-direction 
while feeding the substrate in x-direction, the roughness or the pit depth 
of the processed surface is limited to those minute projections below 0.5 
.mu.m which may be formed in one scanning of the laser beam in y-direction 
and a linear pattern of chromium thin film can be removed perfectly from 
the substrate with the left-behind processed surface having surface 
imperfections giving substantially no influence in the exposure and 
development. Preferably the superposition number ny is set above 1.5. In 
this case, the possible projections in the left-behind surface is below 
0.3 .mu.m and the accuracy of exposure and development can be improved. It 
is to be noted that the above problem of the surface unevenness occurs 
only in the periphery of the processed region. 
In the above embodiment, the laser beam pulses had a pulse width (half 
width of the light intensity) of 10 to 20 nanoseconds. When laser beam 
pulses of a pulse width of the order of 100 to 200 nanoseconds are 
irradiated, the glass substrate may be easily melted, evaporated and 
solidified in an undesirable shape with some mixture of the chromium film 
which gives bad effects to the glass substrate and to the exposure 
characteristics with a possibility of leaving deep pits in the substrate 
surface. Therefore, at least at the present stage, the laser beam pulses 
of a pulse width of 10 to 20 nanoseconds seem superior to those of a 
longer pulse width for processing a metal film on a transparent substrate. 
Further, the photomask was a chromium mask consisting of a chromium thin 
film evaporated on a glass substrate, but it may be an AR-coated chromium 
mask consisting of a chromium thin film provided with semi-transparent 
chromium oxide thin films on the both sides. In this case also, the 
composite structure may be deposited on a glass substrate with effects 
much than the use of the chromium film only. Similar effects can be 
obtained also in the case of using silicon or Fe.sub.2 O.sub.3 thin film 
in place of the chromium thin film mask. 
Further, in the above embodiments, the fine feed in x-direction was carried 
out by deflecting thin resilient metal bars 14a by the fine feed mechanism 
15. Similar effects, can also be obtained by providing a scanning system 
similar to the scanning system 11 as disclosed in the above-mentioned 
copending U.S. application, for scanning the laser beam in x-direction. 
Thus, this invention is not limited to the above embodiment. 
Although the description has been made on the case of correcting the 
defective portions of a metal thin film of a photomask, this invention can 
be equally applied to the case of forming a predetermined circuit pattern 
in a uniform metal film evaporated on a transparent substrate to form a 
photomask. 
As has been described above, according to this invention, a photomask 
deposited with a metal thin film is prepared, and a pulsed laser beam is 
irradiated on a predetermined region of the thin film through the glass 
substrate from the substrate side and scanned relatively to the thin film 
in two-dimensional directions to remove the thin film in the predetermined 
region. Therefore, the thin film can be precisely removed away from the 
transparent substrate without causing any welding of the thin film to the 
transparent substrate in the periphery of the irradiated region. Further, 
there is no need for varying the spot size of the pulsed laser beam nor 
adjusting the laser output according to the area to be removed but only 
the scanning width is arbitrarily adjusted to remove a region larger than 
the spot size. 
Yet further, a pulsed laser beam of a pulse width of up to at most 20 
nanoseconds is irradiated and scanned with the superposition number of 
spots set in the range of up to 15. Therefore, the roughness and the pit 
depth of the processed surface from which a thin film has been removed 
away can be suppressed below 0.1 .mu.m. Since the size and the location of 
a region of metal thin film to be removed away is preliminarily detected, 
a pulse generator for driving a laser source and a scanning system can be 
controlled by the detected information of size and location through a 
controller to automatically remove a thin film in the predetermined 
region.