Method of stabilizing a mask

Our invention is a process for stabilizing a projection mask which is put in operation at an elevated temperature. The frame containing the mask foil is heated to a temperature which is higher than the temperature of the mask foil. The mask foil is thus kept under tension by controlling the temperature of the frame it is held in and distortions like the distortions which would otherwise occur in long time operation and as conditioned by the mask foil hanging through it are avoided. The effect of the expansion of the mask foil can be compensated in the image forming unit by correction of the image formation scale.

FIELD OF THE INVENTION 
Our present invention relates to a method of stabilizing a mask. 
BACKGROUND OF THE INVENTION 
A projection screen can have a mask in the form of a mask foil and a 
retaining frame for this mask foil which are subjected to a thermal stress 
in operation by a high-energy image forming medium or beam, for example, 
ions, electrons or X-rays. Regions permeable to the image-forming 
radiation, e.g. openings, are present in the mask foil. 
The mask foil is advantageously combined with a metallic retaining frame. 
Considerable distortions in the mask foil can be generated by a thermal 
stress applied to the mask foil, e.g. by the image-forming beam 
particularly when the mask foil does not have the same effective thickness 
at all points. Thus the thermal stressing of a mask foil of uniform 
thickness can lead to distortions of the openings in the foil so that the 
methods of thermal stressing can be used generally only for test masks in 
1:1 ion or electron shadowography and/or in ion or electron beam image 
demagnifying projection. 
OBJECTS OF THE INVENTION 
It is an object of our invention to provide an improved process for 
stabilizing a mask which avoids these drawbacks. 
It is also an object of our invention to provide an improved process for 
stabilizing a mask which permits application to the mask of a greater 
thermal load by an image-forming medium than has been possible up to now 
and simultaneously avoids an intolerable amount of distortion when a mask 
with structural openings is used. 
SUMMARY OF THE INVENTION 
These objects and others which will become more readily apparent 
hereinafter are attained in accordance with our invention in a process for 
stabilizing a mask comprising a mask foil and a retaining frame for it 
which are subjected to a thermal stress by an image-forming medium, for 
example a high-energy beam of ions, electrons or X-rays, which passes 
through a plurality of permeable regions in the mask foil. 
According to our invention the mask, whose mask foil is mounted free of 
tension or with such a small tension in the retaining frame that a 
distortion associated with the tension is not present or is negligible, is 
clamped in the retaining or mounting frame and the retaining frame is 
heated in operation to a temperature which exceeds the temperature of the 
mask foil. 
The mask foil can be combined with an anchoring piece, (e.g. bonded frame) 
by a standard manufacturing process. It is insured that the mask foil is 
only under a slight tension so that associated distortions of structure in 
or on the mask foil are not present during the image-forming process. 
Advantageously silicon or glass can be used as the material for the 
anchoring piece. 
Methods for making mask foils from silicon (doped with quantities of boron 
and germanium) or from silicon nitride and mounting them in with a frame 
of silicon are already known. In these methods the thermal expansion 
coefficient of the anchoring frame can be the same or similar to that of 
the mask foil. The stabilizing method according to our invention can be 
used advantageously with such projection screens. 
The effect achievable by the process of our invention is based on the 
phenomenon that the expansion that occurs upon heating the mask foil faces 
no significant resistance and thus the mask foil not only remains planar 
but also all structures in the mask foil are enlarged with the same linear 
factor (i.e. coefficient of thermal expansion) so that the distortion of 
the mask structure which is only slight at the beginning undergoes no 
increase and even tends to decrease. 
In the process according to our invention, since the retaining frame is in 
contact with the mask foil, the retaining frame is heated in a controlled 
way and thus the mask foil heated by the image-forming medium can assume 
the required size unimpeded by the heating. In this way we achieve a 
completely stress free mask which can take a significantly higher thermal 
from load by the image forming medium. 
Advantageously the mask foil is held in the retainer with a pretension 
corresponding to a stretch of 10.sup.-5 to 10.sup.-6 (ratio of elongation 
in a particular direction to the linear dimension in this direction), so 
that in the propagation of the image-forming medium through the mask, the 
structures of the mask formed in the substate manifest only slight 
distortions which are negligible. In practice the holder is heated 
typically by about 10.degree. C. above the temperature of the mask to hold 
it exactly planar. 
In this way only a very slight tension exists in the mask foil so that 
mechanical vibrations are not transferable to the stress-free mask and 
changes in the load on the mask foil by the image-forming medium can be 
allowed. 
In order to guarantee that the mask foil does not undergo a plastic 
deformation by a temperature drop when the image-forming medium is absent, 
according to a further feature of our invention, on shut off of the 
image-forming medium the mask foil is brought to a temperature which is at 
least equal to the temperature of the mask foil at the time of shut off by 
a heating medium. The heating medium can be a fluid, an infrared emitting 
source or another particle beam source. In so far as the mask foil sags 
under the effect of heating this is not significant for further use of the 
foil since after a temperature drop the foil is again put under tension. 
The measurement of the mask temperature can be performed pyrometrically. 
Upon a tendency toward temperature drop, the additional heat source can be 
switched on. The additional source can be operated continuously with 
reduced intensity and can have its intensity increased when the 
image-forming beam is not detected. 
On shut off of the image-forming medium, therefore, the intensity is 
correspondingly increased. 
According to another aspect of the invention, we provide a method of 
operating a projection screen through which a high-energy beam is passed, 
comprising the steps of: 
bonding a planar frame all around the periphery of a projection mask to 
form a projection screen, the frame and the mask having substantially the 
same coefficients of thermal expansion; 
securing the projection screen in a mounting frame so that the screen at 
ambient temperature is subjected to an elongation of at most 10.sup.-5 
measured as the ratio of the change in length of a unit length of the mask 
to the unit length caused by tension in the securing of the projection 
screen in the mounting frame; and 
heating the mounting frame to a temperature in excess of that developed in 
the mask upon the passage of the high-energy beam therethrough to maintain 
the mask in a planar state; and 
passing the high-energy beam through the screen. 
Advantageously, the projection screen is subjected to tension in the 
mounting frame so as to generate in the mask an elongation of 10.sup.-5 to 
10.sup.-6 therein. 
Preferably the mounting frame is heated to a temperature of 10.degree. C. 
to 20.degree. C. above that required to maintain planarity of the mask. 
The method also can comprise the step of detecting a termination of passage 
of the high-energy beam through the mask and at least concurrently with 
the detection supplying thermal energy to the mounting frame to prevent 
loss of planarity of the mask.

SPECIFIC DESCRIPTION 
The mask foil 1 shown in the drawing is anchored in the anchoring frame 2 
which, for example is composed of silicon. The thickness of the screen 
mask foil 1 amounts to about 1 to 5 micrometers. 
The mask foil 1 is advantageously held together with the anchoring piece 2 
in a retaining frame which comprises an upper retaining frame member 3 and 
a lower retaining frame member 4. 
Between the upper retaining frame member 3 and the mask foil 1, 
particularly however between the anchoring piece 2 and the lower frame 
member 4, a good heat conducting elastic layer 8, 9 for example of an 
elastomer is applied. The elastic layer is a protection for the mask foil 
1 and the anchoring piece 2 on assembly of the upper frame member 3 and 
the lower frame member 4 which is frequently composed of brittle material. 
The elastic layers 8, 9 compensate for the unevenness of the anchoring 
piece 2 and as protection against breaking. 
The mask foil 1 is held in the retaining frame 3, 4 free of tension or with 
hardly any tension. The upper frame member 3 and the lower frame member 4 
can be attached to each other by screws, one of which can be seen at 5. 
The retaining frame for the mask foil 1 of the anchoring piece 2 can be 
formed by the upper retaining frame member 3 and the lower retaining frame 
member 4. The planar retaining frame 2 bonded to the mask however can be 
used as the sole frame structure, the members 3 and 4 being eliminated. 
The retaining frame facilitates, of course, the manipulation of the mask. 
The mask foil 1 can be so stressed that its elongation S1/l.sub.o (ratio 
of a change in a unit length to that unit length) is between 10.sup.-5 and 
10.sup.-6. 
The increment Sl represents the length change with respect to the initial 
length l.sub.o, the initial length l.sub.o corresponding to the length at 
the reference temperature, that is at room temperature. 
With a mask foil diameter of 50 mm an extension or elongation of about 0.5 
micrometer over the entire planar state results. 
The retaining frame 3, 4 of a good heat conducting material is held in the 
image forming unit between the plate like clamps 6, 7 of a clamping 
device. 
Between the clamps 6, 7 and the retaining frame 3, 4 heat conducting 
flexible (elastic) layers 8, 9, for example formed by an elastomer, are 
disposed. 
The clamps 6, 7, composed of a sufficiently good heat conducting material 
can be provided with channels 10 through which a heating medium--or as 
need requires a cooling medium--is conducted. Similarly heating wires 10a 
if necessary in addition to the channels 10 can be provided in the clamps 
6, 7 of the clamp device. The mask foil 1 and the retaining frame 3, 4 
have advantageously nearly the same thermal expansion coefficient. 
In operation, that is, when the image forming medium acts on the mask foil 
1, the frame 3, 4 and/or the clamps 6, 7 are heated to a temperature 
exceeding the temperature of the mask foil 1, for example by conduction of 
a suitably heated heat carrier through a channel 10 of the clamps 6, 7 
and/or by connection of the heated wire built into the clamps 6, 7 to a 
source of EMF. 
Operating temperatures of 100.degree.-200.degree. C. are usual. When one 
has an operating temperature of 100.degree. C., the frame 3, 4 and/or the 
clamps 6, 7 are heated to a temperature of for example 110.degree. C. In 
an INVAR-frame which contains a mask foil of 50 mm diameter that means for 
example an enlargement of 0.5 micrometer. 
From FIGS. 2 and 3 the condition of a mask used in various projection units 
is apparent; namely in FIG. 2 in a unit in which heating of the retaining 
frame 3, 4 is not provided and in FIG. 3 with heating of the retaining 
frame 3, 4 above the temperature of the mask foil 1 according to our 
invention. 
In the upper portion of FIG. 2 a mask is indicated comprising a mask foil 1 
and an anchoring piece 2. On the mask foil an extent s is indicated. When 
the mask is now heated by an image forming medium 11, for example an ion 
beam, which--conditioned by an aperature 12--strikes only the mask foil 1 
the mask expands and hangs. This leads to a distortion of the image 
formed. In order to prevent this the frame 3, 4 is heated above the 
temperature of the mask foil 1, whereby the mask foil 1 is put under 
tension. The stretch s of the mask foil 1 has increased by approximately 
Ss. 
This expansion can, by changing the image formation conditions be optically 
corrected. Distortions as occur with a sagging mask foil 1 are prevented 
since the mask foil 1 is planar. 
The mask foil 1 is advantageously provided with permeable regions 1a 
through which the image forming medium 11 passes. These permeable regions 
1a can of course be openings in the structure. 
The high energy beam 20 is detected pyrometrically, e.g. by a senior 21 
which can operate heating controller 22 to regulate the heating fluid flow 
at 10 or the heating current through heating elements 10a when the high 
energy beam is cut off, thereby maintaining planarity of the mask.