Protective device for photographic masks

A protective device for photographic masks or reticles used in the photolithographic operation for fabricating semiconductor devices. The protective device includes a frame having a protective pellicle attached thereover and the frame includes temporary fastening means for detachably fastening the frame to a mask substrate. The temporary fastening is effected by using for example vacuum chucking, magnetic attractive force or spring force. The frame may be formed with vent holes each having a filter so that the internal pressure of the space enclosed by the pellicle and the frame on the mask is made equal to the outside air pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improvements in and relating to protective 
devices for photographic masks or reticles used in the fabrication of 
integrated circuits. 
2. Description of the Prior Art 
In the past, in order to protect the surface of photographic masks or 
reticles from dust, devices have been known in which, for example, a 
transparent thin film referred to as a pellicle is attached over one open 
surface of a frame of a size sufficient to enclose the pattern area on a 
substrate of a mask and the other open surface of the frame is made fast 
with an adhesive or adhesive tape to the substrate in such a manner that 
the pattern area is included within the frame. In this device, the 
pellicle is supported by the frame at a predetermined distance from the 
pattern on the surface of the substrate. 
Such protective pellicle has the function of preventing the deposition of 
dust on the substrate of a mask and protecting the mask and it contributes 
toward improving the productivity of integrated circuits, etc. 
For instance, if dust deposits on a photographic mask for semiconductor 
fabrication purposes, there is the danger of the pattern of the dust being 
projected onto a semiconductor wafer during the photolithographic 
operation and thereby deteriorating the reliability of the resulting 
semi-conductor devices due to operational malfunctions, etc. Then, by 
using the previously mentioned protective device, it is possible to shut 
off the atmosphere on the mask from the outside air and thereby alleviate 
the deficiencies. 
On the other hand, if any foreign particles such as dust deposit on the 
surface of a mask substrate having a pellicle already mounted thereon or 
to the inner surface of the pellicle, the pellicle must be removed from 
the substrate to clean the substrate or the pellicle again. However, the 
pellicle mounted on the mask substrate by the frame cannot be easily 
removed from the mask substrate, with the resulting inconveniences that 
the surface of the substrate is damaged during the removing, that the 
removed pellicle becomes unsuitable for reuse and so on. 
Also, while the frame of the pellicle must be aligned with the mask 
substrate so as to enclose the pattern area, if the position of the frame 
is beyond a predetermined error range, it is also necessary to remove the 
frame and to again adhere the frame in the proper position. In such a 
case, the fixedly mounted pellicle frame is also inconvenient. 
Further, the space enclosed by the mask substrate, the frame and the 
pellicle is in very excellently sealed condition hermetically. As a 
result, when the mask is transported by aircraft or transferred to a high 
altitude, the pressure difference between the internal pressure and the 
external pressure causes the pellicle attached to the frame to bulge or 
become depressed. Such deformation of the pellicle is disadvantageous in 
that not only the optical characteristics of the pellicle itself are 
changed, but also, depending on the degree of the deformation, the 
pellicle is caused to contact with the surface of the mask substrate or 
the pellicle is damaged by contacting with any metal part in the 
surroundings during the transport of the mask within a projection 
apparatus and so on. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a protective device for 
photographic masks which overcomes the foregoing deficiencies in the prior 
art and which includes a protective pellicle or pellicles that can be 
easily removed from a mask substrate as occasion demands. 
It is another object of the invention to provide such protective device for 
photographic masks which prevents any deformation of a protective pellicle 
due to a change in the atmospheric pressure. 
In accordance with the invention, a pellicle-equipped frame or frames 
include temporary fastening means for detachably mounting the frames on 
the surfaces of a mask substrate. 
In accordance with a preferred embodiment of the invention, vacuum chucking 
is utilized for temporary fastening purposes. 
In accordance with another preferred embodiment of the invention, the 
attractive force of magnets or the pressing force of springs is utilized 
for temporary fastening purposes. 
In accordance with one aspect of the invention, each frame is formed with 
one or more vent holes so as to prevent the occurrence of any pressure 
difference between the space enclosed by the pellicle and the frame and 
the outside air. 
In accordance with still another embodiment of the invention, a filter 
element for preventing the passage of dust is provided for each vent hole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some embodiments of the invention utilizing vacuum chucking for temporarily 
fastening a pellicle or pellicles to a substrate will now be described. 
Referring to FIG. 1, arranged on the upper surface of a glass substrate 100 
of a photographic mask is a frame 110 having a pellicle 112 horizontally 
attached over the upper edge thereof and arranged on the lower surface of 
the glass substrate 100 is a frame 120 having a pellicle 122 horizontally 
attached over the lower edge thereof. The other edges of the frames 110 
and 120 facing the glass substrate 100 are respectively provided with 
buffer or cushioning materials 114 and 124 made of silicone rubber or the 
like as shown in FIG. 2 and each of the frames is held in contact with the 
glass substrate 100 through its cushioning material so as to enclose a 
pattern area 12 consisting of opaque and transparent areas and formed on 
the lower surface of the glass substrate 100. 
Provided inside the frames 110 and 120 are grooves or a series of holes 116 
and 126 which are formed along the shape of the frames 110 and 120 and 
opened to the glass substrate 100. The grooves or series of holes 116 and 
126 are respectively communicated by air intake ports surrounding the 
interiors of the frames 110 and 120 and the intake ports are connected to 
a vacuum pump 103 through pipes 118 and 128 from one or more openings 
formed in each of the frames 110 and 120. 
After the absence of defects, flaws and deposited foreign particles on the 
glass substrate 100 and the frames 110 and 120 has been confirmed, the 
frames 110 and 120 are mounted on the substrate 100 through the cushioning 
materials 114 and 124 and they are then chucked by the vacuum pump 103. 
Then, the presence of any foreign particles is again checked so that if 
the check detects the presence of any defects or foreign particles which 
cause trouble in the manufacturing operation of integrated circuits, the 
vacuum chucking is immediately stopped and the frames are demounted, 
thereby cleaning the photographic mask and removing the foreign particles. 
On the contrary, if the check determines that the condition is proper, the 
vacuum chucking may be continued as such or alternatively it is possible 
to fasten the frames to the glass substrate by applying an adhesive to and 
around their boundaries and then stop the vacuum chucking. 
FIGS. 3 and 4 show a second embodiment of the invention so constructed that 
a pellicle is automatically mounted on a photographic mask by means of 
vacuum chucking More specifically, a frame 210 fitted with a pellicle is 
supported inside a frame member 230 by protruding elements 231, 232 and 
233 which are secured to the frame member 230 and movable protruding 
elements 234, 235 and 236 which are each movable in the directions 
indicated by an arrow relative to the frame member 230 and which are each 
biased against the frame 210 by a spring 237. Attached to the frame member 
230 is a slider 241 which is movable in the direction of x along an 
L-shaped member 242 and the L-shaped member 242 is also movable along a 
guide 243 in the direction of y. Thus, the frame 210 can be aligned with a 
photographic mask 200 in the directions of x and y. The photographic mask 
200 is aligned and mounted on an x-y stage 24 by means of pins 245. 
Each of the protruding elements 231 to 236 is fitted with pipes 251 and 252 
which are connected to a vacuum pump which is not shown. The pipes 252 are 
provided to fasten the frame 210 to the frame member 230 through holes 254 
and the pipes 251 are provided to hold the frame 210 and the photographic 
mask 200 together through holes 253. The frame 210 includes valves 211 so 
that when the vacuum pump is operated with the frame 210 placed on the 
photographic mask 200 through a cushioning material 214, the valves 211 
are opened and the pressure in grooves or a series of holes 216 is 
reduced. Thereafter, even if the operation of the vacuum pump is stopped, 
the valves 211 are closed and the reduced pressure condition is 
maintained. 
The actual operation will now be described in greater detail. In order to 
fasten the frame 210 to the frame member 230, the movable protruding 
elements 234, 235 and 236 are moved backward and the frame 210 is brought 
into contact with the fixed protruding elements 231, 232 and 233. Then, 
the movable protruding elements 234 to 236 are moved forward and the frame 
210 is supported by the six protruding elements. 
Then, after the photographic mask 200 has been mounted on the stage 244 and 
transferred to below the lower surface of the frame member 210, the stage 
244 is raised or lowered until it reaches a position where the distance 
between the upper surface of the mask 200 and the lower surface of the 
frame 210 is several mm. At this time, only the pipes 252 are vacuum 
sucked. In this condition, the frame member 230 is aligned in the x and y 
directions and the frame 210 is positioned in place relative to the mask 
200. After the alignment has been made, the stage 244 is raised so that 
the cushioning material 214 is brought into contact with the mask 200 and 
vacuum sucking is effected through the pipes 251. This vacuum sucking need 
not be effected continuously and it is necessary to effect the vacuum 
sucking for a moment owing to the provision of the valves 211. As a 
result, the frame 210 is chucked to the mask 200. Then, after the vacuum 
sucking through the pipes 252 has been released, the movable protruding 
elements 234, 235 and 236 are moved backward and the stage 244 is lowered, 
thereby mounting the frame 210 on the upper surface of the mask 200 in its 
temporarily fastened condition. Since this operation can be entirely 
performed automatically, it is possible to apply this operation to a 
pellicle automatic attaching apparatus to perform the following sequence 
of operations fully automatically. Namely, the operations include a mask 
cleaning step by a cleaning machine, a frame tacking step, a foreign 
particle inspection step, an adhesion step for fastening the frame to the 
mask with an adhesive if the result of the inspection is normal, and a 
step of removing the frame and performing the operation again if any 
abnormal condition is found as a result of the inspection. 
It is to be noted that a device for forcing the valves 211 to open may be 
employed to remove the temporarily fastened frame from the mask. Of 
course, the above-mentioned embodiment can be realized without the valves. 
Referring to FIGS. 5 to 7, there is illustrated a third embodiment of the 
invention which temporarily fastens pellicles to a substrate by means of 
magnets. 
Frames 310 and 320 each fitted with a pellicle are arranged on both sides 
of a photographic mask glass substrate 300 in the like manner as FIG. 1. 
The frames 310 and 320 are respectively provided with sheet materials 314 
and 324 at their edges which contact with the glass substrate 300. The 
sheets 314 and 324 are made of a soft material such as rubber or synthetic 
resin and are formed to have a thickness of about 0.1 to 2.0 mm. FIG. 7 
shows in an enlarged form a part of the joint between the frame 310 and 
the sheet material 314. As shown in the Figure, the sheet 314 is formed 
with a notch 315 at a plurality of places as occasion demands. This is the 
same with the sheet material 324. 
The frames 310 and 320 are respectively provided with magnets 317 and 327 
at their corners. Each of the magnets 317 has a polarity selected such 
that an attractive force acts between its and the opposing magnet 327. 
Thus, when the frames 310 and 320 are aligned in such a manner that a mask 
pattern area 302 is not covered by them and the pellicles are mounted to 
hold the glass substrate 300 therebetween from the sides thereof, an 
attractive force due to the magnets 317 and 327 acts so as to temporarily 
fasten the pellicles to the glass substrate 300. 
After the completion of the temporary fastening of the pellicles in the 
proper positions, the notches 315 of the sheet materials 314 and 324 are 
filled with an adhesive to reinforce the joint. However, this fastening 
with an adhesive is not always required. 
It is to be noted that one or the other of the magnets 317 and 318 may be 
replaced with a magnetic material such as iron or nickel. 
When it is desired to fasten a pellicle only to one surface of a 
photographic mask, as shown in FIG. 8, a frame 410 is placed on one 
surface of a glass substrate 400 on which a master pattern is formed. In 
this case, magnets 417 are fixedly mounted at several places on the 
outside of the frame 410. A plate element 421 made of a magnetic material 
is arranged on the opposite surface of the glass substrate 400. A sheet 
material is placed between the frame 410 and the magnets 417 and between 
the plate element 421 and the glass substrate 400. 
FIG. 9 shows a fifth embodiment of the invention in which the mechanical 
force of springs or the like is utilized to temporarily fasten a pellicle 
to a photographic mask. 
Slits 511 are formed in the outer sidewalls of a frame 510. A supporting 
block 530 made of a synthetic resin is mounted on the side edge of a glass 
substrate 500 near to each of the slits 511. Each block 530 is formed with 
a notch for fastening it to the glass substrate 500. An L-shaped plate 
spring 531 is attached to the upper part of the block 530 so as to rotate 
about a fastening pin 532. When the plate springs 531 are each rotated to 
the illustrated position, the forward end of each plate spring 531 enters 
into the slit 511 and the frame 510 is biased against the glass substrate 
500. 
With a pellicle fastened temporarily or completely with an adhesive to a 
photographic mask in accordance with each of the previously mentioned 
embodiments, the construction of the below-mentioned embodiment is 
effective in preventing the pellicle from being deformed and damaging its 
flatness due to the pressure difference between the inside and outside the 
pellicle. 
Referring to FIGS. 10 and 11, a vent hole 611 is formed in each of the 
sides of a frame 610 supporting a pellicle 612. Each vent hole 611 has a 
diameter of about 0.5 to 2 mm, for example, and it is connected with a 
large-diameter portion 613 in the outer side of the frame 610. Also, a 
stepped portion is formed between the vent hole 611 and the large-diameter 
portion 613. From the outside of the frame 610 a filter 615 is fastened 
with an adhesive 617 to the stepped portion so as to be mounted on the 
vent hole 611. The filters 615 are substantially the same in diameter with 
the large-diameter portions 613 and in this way consideration is given to 
prevent each filter 615 from entering into the vent hole 611 even if it 
falls off the stepped portion. 
The filters 615 of the type which allows the passage of air and prevents 
the passage of dust are used and membrane-type filters are preferred. For 
example, MF Millipore filters (trade name) or Cellotate filters (trade 
name) sold by Millipore Corporation may be used. Of these, MF Millipore 
filters are permeable filters made of cellulose-mixed ester as a raw 
material and the Cellotate filters are permeable filters made of cellulose 
acetate as a raw material. It is to be noted that in a reduction 
projection-type exposure apparatus and the like, the focal depth of a 
projection lens is very small and therefore foreign particles of less than 
a predetermined size, e.g., about 0.5 to 1.0 mm, are not resolved. Thus, 
the use of filters which would pass dust of such size along with air 
presents no difficulty from a practical point of view. 
A closed space enclosed by the glass substrate 600, the frame 610 and the 
pellicle 612 is communicated with the external space through the filters 
615. Thus, there is no danger of causing any unbalance between the 
pressure in the closed space and the pressure in the external space. 
Various modifications of the vent holes 611 and the filters 615 may be 
conceived as shown in FIG. 12 in which filter 715 is adhered to the outer 
side of each vent hole 711 of a frame 710 or in FIG. 13 in which a filter 
810 is inserted in a frame 810 so as to traverse each vent hole 811.