Process for producing a structured mask

A process is disclosed for producing a structured mask for use in reproducing structures of that mask on an object with the aid of electromagnetic or particulate radiation, in particular for ion beam lithography. A flat smooth substrate of more than 20 .mu.m in thickness is selected and a thin diaphragm is produced from that substrate by etching one of the sections removed from the edging to a depth of c. 0.5-20 .mu.m, the tensile stress within the diaphragm being greater than 5 MPa. Lithographic structures are then formed on a central region of the diaphragm with a tensile stress of more than 5 MPa; apertures are etched into the diaphragm to form the mask structures and the effective thickness inside a diaphragm region substantially enclosing the mask structures is reduced, causing the central region containing the structures to be joined to the substrate edging elastically in such a way that the mean tensile stress within this central region is reduced to below 5 MPa. The region with reduced effective thickness preferably takes the form of a peripheral channel or at least one perforation.

The present invention relates to a method for producing a structured mask 
for the purpose of imaging structures of this mask onto an object by means 
of electromagnetic radiation or particle radiation, especially for use in 
ion beam lithography. 
Masks for use in lithography are large surface-area, generally disc shaped 
structures consisting of a membrane of a thickness ranging from approx. 1 
.mu.m to 20 .mu.m which are held by an outer frame encompassing the 
membrane. In the case of so-called stencil masks, holes are created in the 
membrane within a central mask design field and these holes form the 
structures of the mask. Upon radiating a stencil mask with electromagnetic 
radiation or particle radiation the beams pass unhindered through the 
holes of the membrane, whereas the beams are absorbed in all other regions 
of the membrane. 
Such masks are mainly produced by etch thinning a substrate, e.g. a silicon 
wafer, by means of a suitable etch-stop method, to the desired thickness, 
wherein an outer edge remains untreated, so that a portative solid edge is 
formed, by means of which solid edge the mask can later be clamped into a 
frame. The structures of mask are applied lithographically, e.g. by means 
of electron beam lithography, onto the upper side of the membrane and 
holes are produced in the membrane by means of etching. 
It follows from this, that in order to produce a stencil mask a series of 
processing steps are necessary in which the membrane is subjected to a 
mechanical loading. It is therefore necessary that the membrane is under a 
certain amount of tensile stress. Such tensile stress can be stress which 
is caused by a thermic reaction or even stress caused by the doping 
process. Tensile stress caused by doping occurs if the substrate or the 
membrane is doped with a doping substance whose atom diameter is less than 
that of the substrate atoms, e.g. if silicon is doped with boron or 
phosphorus. This type of doping procedure is necessary in many cases in 
order to create an effective (electrochemical or wet-chemical) etching 
stop for the thin etching of the substrate since a membrane of uniform 
thickness can be produced in this manner. Methods of thinning silicon 
substrates by means of etching are described in detail inter alia in 
Extended Abstracts, vol 82-1 (1992) in abstract no. 122 entitled "Studies 
of Boron and Boron-Germanium doped Epitaxial Silicon Films for Ultrathin 
Silicon Diaphragms" (Black et. al). and in The Bell System Technical 
Journal, March 1970 on pages 473 ff entitled "Electrochemically Controlled 
Thinning of Silicon". 
A method for producing a thin membrane for use as a stencil mask is also 
evident from EP-A-367 750 held by the applicant. In the case of the method 
disclosed therein, the resulting stress in the mask is taken into 
consideration at the time of manufacturing the membrane, in that for the 
process of doping the substrate in order to produce an etching stop a 
predetermined doping material is used in a predetermined doping 
concentration. In dependence upon the density of holes the localized 
stress in the structured mask is changed with respect to the stress in the 
membrane. The density of holes in conventional masks ranges from a few 
percent to approx. 50%. 
All masks produced in the manner described above thus comprise owing to the 
manufacturing process a certain amount of tensile stress which causes the 
holes in the stencil mask to be offset with respect to their desired 
position. As a result, displacements can be great if one area with a high 
density of holes is immediately adjacent to an area with a low density of 
holes. Furthermore, the deviation of the structures from the desired 
position generally increases towards the outside. For these reasons 
efforts have been made for some time to reduce the stress in the membranes 
manufactured in this manner to a lowest possible value in order to 
minimize the displacements of the structures. More precise tests regarding 
distortions of masks can be found inter alia under the title "Pattern 
Distortions in EBP Stencil Masks" (Keyser and Kulcke), published in 
Microelectronic Engineering, 11 (1990) 363-366 and the publication 
entitled "Stress Induced Pattern-Placement Errors in Thin Membrane Masks" 
(Liddle and Volkert, AT&T Bell Laboratories), J. Vac. Sci. Technol. B 
12(6), Nov/Dec 1994, 3528 ff. It is maintained in the latter article that 
it is not possible using stencil masks to maintain the predetermined error 
tolerances in the case of a layout in the sub-0.25 .mu.m range. 
It has become apparent that it is technologically difficult at the moment 
to reduce the stress in the original membrane to a value below 10 MPa in 
order to produce the mask, since otherwise the membrane is unable to 
withstand the mechanical loads of the mask manufacturing process, e.g. 
spinning photoresists. Thus, conventional manufacturing methods at present 
produce a lower limit for the stress which results in a certain amount of 
displacements of the mask structures which are unacceptable for electronic 
circuits of particularly small dimensions. A method for producing such a 
mask according to the prior art (10 MPa stress) is for example described 
in detail in J.Vac. Sci. Technol B 10(6), Nov/Dec 1992, 2819 ff under the 
title "Silicon Stencil Masks for Lithography below 0.25 .mu.m by 
Ion-Projection Exposure". 
U.S. Pat. No. 780,382 held by the applicant discloses one possible solution 
wherein the entire mask design field is provided with a supporting grating 
structure so that the structures of the mask can be held by this grating 
and cannot be displaced in their position. Furthermore, it is proposed to 
adapt the thickness of the grating structure to suit the local hole 
density so that a constant effective thickness is created within the 
entire mask design field. EP-A-330 330 (=U.S. Pat. No. 4,827,138) proposes 
as a solution to the above mentioned problem to produce the entire mask as 
an extremely fine grating structure made from a carrier material in which 
predetermined holes are filled with a different material in order to 
produce the opaque regions of the mask. A disadvantage of these solutions 
resides inter alia in the fact that the carrier structure imposes 
undesired limitations on the layout design of the electric circuit and 
considerably more procedural steps are involved in the manufacture of such 
masks. Furthermore, when using these masks for lithography purposes the 
grating structures are also imaged onto the substrate in a disadvantageous 
manner. This condition must be remedied as the image becomes defocussed or 
the beam path becomes tilted and both lead to an increase in the line 
width and thus to a reduction in the resolution. 
In an as yet unpublished Austrian Patent Application held by the applicant 
under the file reference A1585/94 it is proposed to solve the above 
mentioned problem of tensile stress causing displacements of mask 
structures to vary the thickness and/or the doping concentration in 
dependence upon the local hole density of the mask in such a manner that a 
homogenous average stress is created within the mask design field after 
production of the masks. For example, areas which have a high hole density 
are formed slightly thicker than areas with low hole density in order to 
obtain a substantially homogenous effective thickness. Likewise, it is 
also possible to achieve a homogeneous average stress with a constant 
thickness by the fact that areas with high hole density are doped to a 
greater extent that areas with a low hole density. Also, in order to 
produce such a mask it is, however, necessary to achieve a predetermined 
minimum stress value in order to avoid the mask being destroyed during the 
manufacturing process. There exists between the mask design field, which 
is generally square or rectangular, and the edge of the membrane, which is 
generally circular, crescent-shaped membrane segments whose tensile stress 
is unchanged, e.g. amounts to 10 MPa, whereas in the middle of the mask 
design field the average tensile stress is reduced depending upon the 
material reduction to a somewhat lower value owing to the holes. As a 
consequence, stress is exerted on the mask design field which results in a 
displacement of the mask structures in the direction of this edge. 
A disclosure different to attempting to minimize the stress within the mask 
is evident in AT-PS 383 438, which describes the manufacture of a mask 
wherein an inner sheeting, which contains the mask design field, is 
connected to a reinforcing ring whose thermal coefficient of expansion is 
greater than that of the mask material, so that during operation at higher 
temperatures the mask is maintained stressed by virtue of the differing 
coefficients of expansion. However, this is associated with considerable 
distortions which nowadays are no longer acceptable. In order that the 
stressing procedure of the inner sheeting by means of the reinforcing ring 
is not impaired by the stiff edge of the mask, a thin outer sheeting is 
likewise provided outside the reinforcing ring, serpentine-like webs are 
etched into the said outer sheeting and absorb the expansion of the ring 
and the inner sheeting. 
It is the object of the invention to improve the process of manufacturing a 
structured mask in such a manner that the resulting mask is obtained with 
particularly low stress, that its distortion is negligibly low and thus 
that there are no negative effects on the image. One possible solution to 
this problem would be to develop new manufacturing technology, e.g. new 
etching methods, which during the manufacturing process cause a slight 
mechanical loading of the mask. However, this is encumbered with 
relatively high costs which cannot be justified for cost reasons for such 
a manufacturing process. The aforementioned object should therefore, if 
possible, be achieved using established manufacturing technology in order 
to guarantee a reliable and economic process of manufacturing masks. 
This object is achieved on the one hand by means of a method for producing 
a structured mask for the imaging of structures of this mask onto an 
object by means of electromagnetic radiation or particle radiation, in 
particular for ion beam lithography, which comprises the following steps: 
a) Select a two-dimensional, planar substrate of a thickness greater than 
20 .mu.m, 
b) Produce a thin membrane by etching a portion of the substrate remote 
from the edge to a thickness of approx. 0.5 to 20 .mu.m, wherein the 
tensile stress within the membrane is greater than 5 MPa, 
c) Form the structures using lithography onto a central portion of the 
membrane at a tensile stress of greater than 5 MPa, 
d) Etch holes in the membrane which form the structures of the mask, 
e) Reduce the effective thickness within an area of the membrane 
substantially encompassing the structures of the mask, so that central 
portion containing the structures is coupled in a resilient-elastic manner 
to the edge of the substrate in such a manner that the average tensile 
stress is reduced within this central portion to a value below 5 MPa. 
In the case of this method it has proven itself to be particularly 
advantageous if the steps d) and e) are carried out simultaneously and 
upon performing step c) simultaneously with the structures of the mask 
structures of the area having reduced effective thickness are also 
provided in the membrane. 
As an alternative thereto the said object is also achieved by means of a 
method for the aforementioned type which comprises the following steps: 
a') Select a two-dimensional, planar substrate of a thickness greater than 
20 .mu.m, 
b') Form the structures of the mask using lithography on an upper side of 
the substrate wherein the tensile stress is greater than 5 MPa at least 
within a layer adjacent to the upper side and provided for producing a 
membrane, 
c') Etch depressions into the upper side of the substrate to a depth of 0.5 
to 20 .mu.m which form the structures of the mask, 
d') Reduce the effective thickness within an area, on the upper side of the 
substrate, substantially encompassing the structures of the mask. 
e') Produce a thin membrane by etching a portion of the substrate, remote 
from the edge, on its lower side to a thickness of approx. 0.5 to 20 
.mu.m, after which the central portion of the membrane containing the 
structures of the mask is connected by way of the area having reduced 
effective thickness in a resilient-elastic manner to the edge of the 
substrate and the average tensile stress of the mask within the area 
having reduced effective thickness is reduced to a value below 5 MPa. 
It has also proven to be particularly advantageous in this method if the 
steps c') and d') are performed simultaneously and that upon performing 
the step b') simultaneously with the structures of the mask structures of 
the area having reduced effective thickness are also provided in the 
substrate. 
All the above mentioned methods and their preferred embodiments have in 
common that outside the mask design field, i.e. in the area between the 
structures of the mask and the rigid edge of the membrane a closed area is 
provided in which the effective thickness is reduced to the extent that 
this functions in a resilient-elastic manner, so that the area having the 
structures is virtually decoupled in a resilient-elastic manner from the 
edge. As a consequence, the average tensile stress within the mask design 
field can be reduced to a predetermined value below 5 MPa. Since in the 
case of the methods in accordance with the invention the effective 
thickness is only reduced after the lithographic forming of the mask 
structures, it is possible to use, for forming the mask structures, all 
hitherto applied and established manufacturing techniques for which a 
sufficiently prestressed membrane (having a tensile stress of more than 5 
MPa, in a normal case approx. 10 MPa) is required. In the case of a mask 
produced in accordance with the invention only extremely slight 
distortions now occur. Distortions, which are of a linear character, i.e. 
displacements of mask structures, which increase in a linear manner 
outwards, can furthermore be compensated by a corresponding adjustment to 
the magnification. More details regarding this are evident inter alia from 
the U.S. patent U.S. Pat. No. 4,985,634 held by the applicant. This 
correction method can be applied both in the case of projection optics 
(IPL, Ion Projection Lithography) and also in the case of a shadow-casting 
installation (MIBL, Masked Ion Beam Lithography). In other words, when 
manufacturing and using the mask in the proper manner there remains only 
extremely slight non-linear distortions or displacements, which in any 
case, in contrast to the publication mentioned in the introduction "Stress 
Induced Pattern-Placement Errors . . ." can guarantee the use of such 
masks in the 0.1 .mu.m range. 
The area having reduced effective thickness can be produced within the 
scope of the present invention in different ways. On the one hand, this 
area can be produced by etching the membrane in the region of this area to 
a reduced thickness, e.g. in the form of a circumferential groove. 
Likewise, the area having reduced effective thickness can be produced by 
etching holes into the membrane within this area. 
The effective thickness within this area preferably amounts to between 1 
and 40% of the thickness of the unstructured membrane. Good results can be 
achieved with areas whose effective thickness is reduced within the area 
to a value below 20%. 
The subject matter of the present invention is furthermore a projection 
mask which is produced by means of one of the two above mentioned methods. 
In the case of such a mask the area having reduced effective thickness is 
formed advantageously as at least one perforation encircling the 
structured region. In the event that two or more perforations are 
provided, it is of advantage that these are disposed substantially 
parallel or concentric and the perforation slots are disposed offset with 
respect to each other in the tangential direction, rendering possible a 
bending load on the portion lying between them. Owing to the fact that in 
the case of this embodiment a bending load also occurs in addition to the 
tensile load, the resilient-elastic effect of this area increases 
considerably and it is therefore possible even in the case of a 
substantially smaller reduction in the effective thickness to achieve a 
sufficient reduction in the tensile stress within the mask design field. 
In the case of a preferred exemplified embodiment the area comprises holes 
which are disposed in a row and between said holes the structured region 
of the mask and the edge of the membrane are mutually connected in each 
case by way of a web. If the webs are formed radially, then although they 
are subjected only to a tensile load, the said webs are to be produced 
simply in corresponding dimensions. If the webs comprise not only radial 
but also tangential portions, then the case of a mixture of load (tensile 
and bending load) occurs so that a particularly strong resilient-elastic 
effect of the area can be expected.

Firstly, the essential steps of the method will again be discussed with 
reference to a concrete exemplified embodiment, wherein reference is made 
to FIGS. 5a, 5b and 5c in order to explain the manufacture of a large 
surface-area silicon mask for use in ion beam lithography. 
The first step of the method is to select a suitable substrate, e.g. a 
commercially available silicon wafer (n- or p-type). In order to be able 
to perform an electrochemical etching stop method, it is necessary to 
produce a pn- or np-transition on the desired membrane thickness for the 
mask by doping the silicon wafer. In so doing care must be taken that the 
atoms of the doping substance have a smaller diameter than the silicon 
atoms (&lt;1.10 Angstrom) in order to produce a tensile stress in the 
membrane. For a silicon wafer it is preferable to use for this purpose 
boron or phosphorous, namely boron as the p-dopant of an n-type wafer and 
phosphorus as an n-dopant for a p-type wafer. The wafer is preferably 
doped using ion implantation, wherein the dose and the energy are such 
that the pn-transition responsible for the etching stop is formed in a 
depth of 2-3 .mu.m and the dopant concentration produces a tensile stress 
of approx. 10 MPa, as such a membrane can be further processed 
comfortably. This stress is, however, adapted in the individual case to 
suit the requirements of the subsequent method steps and can therefore 
also lie above or below this value. 
The mathematical relationship between dose and resulting stress is 
explained in detail in EP-A-367 750 held by the applicant, so that the 
dopant concentration necessary for a stress of 10 MPa can be determined 
precisely in advance. As mentioned above an electrochemical etching stop 
method is used for etch thinning the wafer, which method is described in 
detail in the publication, mentioned in the introduction, in The Bell 
System Technical Journal, March 1970 on pages 473 ff under the title 
"Electrochemically Controlled Thinning of Silicon" and is not explained in 
detail here. 
In the event that the method, likewise already mentioned above, in 
accordance with the Austrian Patent Applicant A1585/94 held by the 
applicant is used, then the dose and/or the depth of the dopant of the 
wafer substrate must be varied locally accordingly within the mask design 
field. This occurs by means of a two- or multiple-stage doping process, 
wherein prior to each step a predetermined structuring of the wafer 
surface is achieved by lithography. This method can be used without any 
problem for masks of the type in accordance with the invention, it is, 
however, not absolutely necessary. The content with respect to the masks 
of the said A1585/94 is in any case specifically to be regarded as part of 
the current disclosure. 
Upon termination of the etch thinning the wafer substantially has the 
appearance as illustrated in FIG. 5a. Its thickness on its outer edge 2 
remains unchanged, whereas the entire region 3 within this edge is etched 
to a predetermined thickness, i.e. approx. 2-3 .mu.m. Based on the fact 
that the tensile stress during the production of the masks can rise 
readily to a higher value, e.g. 20 MPa, it is possible to hold 
considerably thinner masks stable during the processing, so that the 
thickness can be reduced, for example, to 1 .mu.m. 
The next step is the lithographic structuring of the membrane surface in 
order to provide the structures in the mask. For this purpose, the etched 
thin wafer is first provided with a thin oxide layer. In order not to 
change the stress characteristics of the membrane considerably, the oxide 
layer should be as homogenous and as thin as possible. Particularly good 
oxide layers can be produced, for example, by LPCVD (Low Pressure Chemical 
Vapour Deposition). Furthermore, the oxide layer can be treated in a 
subsequent step with nitrogen at a predetermined temperature in order to 
reduce the tensile stress of this oxide layer precisely to the value for 
the silicon membrane, e.g. 10 MPa. In the event that the production 
process of the mask is drawn out over a longer period of time, it is 
recommended to store the wafer in a nitrogen atmosphere in order to hold 
the stress precisely at the predetermined value. 
Generally, the wafer is clamped at this point in time into a frame 2' in 
order for the subsequent lithography process to be more easily manageable. 
Owing to the thermal coefficients of expansion this frame is normally 
produced from the same material as the membrane, i.e. in the present case 
from silicon. In the case of silicon wafers Pyrex rings can also be used 
if necessary. FIG. 5a shows the mask in this state. 
The subsequent lithographic structuring of the membrane upper side is 
performed in a known manner by applying a suitable photoresist material in 
a layer which is free of stress and as thin and homogeneous as possible 
and by subsequently exposing the photoresist layer to an electron beam e" 
with the aid of a commercially available electron beam plotter, e.g. MEBES 
III. 
In the event that the area, encompassing the mask design field, in which 
the effective thickness is reduced in accordance with the invention to the 
extent that a resilient-elastic coupling is produced between the edge and 
the mask design field, comprises holes which are mutually separated by 
webs (cf. FIG. 1a, 2 and 3) not only the structures of the mask itself are 
plotted during the step of electron beam plotting, the structures of this 
area are also plotted simultaneously. 
The exposed resist layer is initially developed, after which the oxide 
layer on the exposed or non-exposed sites--depending upon whether a 
positive or negative resist is used--is etched away as far as the membrane 
surface and the still remaining resist is removed. As the next step, the 
holes forming the structures of the mask are etched. This procedure is 
performed in an advantageous manner by means of reactive ion beam etching 
(RIE, Reactive Ion Etching). The technological parameters for the method 
steps just described are sufficiently known to the person skilled in the 
art and are not explained in detail here. In the case of a preferred 
variant, all steps after the creation of the resist until the etching of 
the holes can be performed in a single procedural step by means of a 
multi-step process in a reactive ion beam etching reactor. Precise details 
regarding an etching reactor suitable for this purpose and its operating 
conditions can be found inter alia in the publication in J.Vac. Sci. 
Technol. B 10(6), Nov/Dec 1992 2716 ff under the title "Magnetically 
enhanced triode etching of large area silicon membranes in a molecular 
bromine plasma". 
In the case of this procedure step, the average stress of the mask now 
reduces from its starting value, e.g. 10 MPa, upon correctly dimensioning 
the area to a value below 5 MPa, preferably down to approx. 2 MPa, so that 
the mask design field of the membrane now does not comprise any disturbing 
distortions. The stress in the material within the area increases 
considerably and can be in the order of a value above the tensile stress 
in the mask design field. However, this does not represent any problems 
for the subsequent use of the membrane as the breaking point, e.g. of the 
silicon material, lies in the range of 100 MPa. Even in the case of a 
reduction in the effective thickness to 1% of the original membrane 
thickness this critical value is not achieved. In comparison to when 
producing the mask, no substantial mechanical loads occur when using the 
mask in a projection or shadow-casting lithography installation, so that 
there is a great deal of leeway for the dimensioning of the area having 
reduced effective thickness. A lower limit for the tensile stress within 
the mask design field of a stencil mask of the type described resides 
substantially in the fact that owing to its interaction with the electric 
field of the imaging system the membrane can no longer be held level in 
one plane. However, calculations have shown that in the case of masks 
currently used having a mask design field of 100.times.100 mm this limit 
lies in the range below 2 MPa of average effective tensile stress. This is 
evident inter alia from the international application under the file 
reference PCT/AT/00003 held by the applicant. 
After the step of the reactive ion beam etching process the remaining oxide 
layer must merely be removed, e.g. as usual, by means of oxide stripping 
using HF. Thus the manufacture of the mask is substantially completed for 
the event that the area having reduced effective thickness is formed by 
holes and webs. 
If on the other hand, the area having reduced effective thickness is to be 
produced by reducing the membrane thickness in this area, as illustrated 
for example in FIGS. 1 and 4, then during the reactive ion beam etching 
process the structures of the mask are initially only produced within the 
mask design field. The average tensile stress drops within the mask design 
field to a lower value, so that the mask structures at this point in time 
are subjected to a certain amount of displacement. In this case after the 
reactive ion beam etching process a further lithographic step is performed 
in order to produce the structures of the area having reduced effective 
thickness. Since the structures of the area in contrast to the structures 
of the mask itself do not require high resolution, then this lithographic 
process can be performed in an optical manner or with a simple shadow 
mask. During the subsequent process of etching the membrane to a reduced 
thickness various methods can be used, for example reactive ion bean 
etching, electrochemical etching with etching stop, wet-chemical etching 
or sputter etching. In the case of sputter etching by means of a directed 
ion beam the lithography step can also be omitted as the ion beam can be 
directed to the area without it. It is likewise possible to limit the 
etching process by means of a simple shadow mask to the area to be etched. 
In so doing, the tensile stress within the mask design field is reduced to 
an average value below 5 MPa, so that the aforementioned displacements of 
the mask structures can be eliminated again. Naturally, the area of 
reduced thickness of the membrane can comprise a particularly high 
distortion and stress. As mentioned above, however, this does not have any 
influence on the quality of the imaging, since this region is not involved 
in the imaging process. (cf. FIG. 5c). 
It is possible in the case of a further embodiment to combine the two above 
mentioned method types, i.e. the area of reduced effective thickness can 
be an etched thin region of the membrane in which holes are also provided 
which are mutually separated by means of webs. 
In the case of an alternative embodiment of the method in accordance with 
the invention, although the same method steps as explained above are used, 
the sequence of these method steps is however changed. In the case of this 
embodiment the structures of the mask are first provided in the relatively 
thick wafer and etched to a depth which corresponds at least to the 
thickness of the subsequently produced membrane. If necessary, the 
structures of the area having reduced effective thickness are also 
produced at this point in time. The final method step to be performed is 
then the etching thin of the wafer to the thickness of the membrane, 
wherein the tensile stress is only reduced to the desired value below 5 
MPa upon completion of the etching method, e.g. by means of an 
electrochemical etching stop method. The area having reduced effective 
thickness is again responsible for this in the above mentioned manner. The 
alternative method can, however, also be performed in the manner in which 
the area having reduced effective thickness is only produced after the 
etching thin of the membrane. As a consequence, although two lithographic 
method steps are required, the advantage is however achieved that the 
membrane during the entire etching thin process and also thereafter 
comprises the predetermined tensile stress, e.g. 10 MPa, and is thus 
particularly stable. At the end of the manufacturing method, the mask is 
substantially of the appearance illustrated in FIG. 5b, wherein the 
average tensile stress within the mask design field now only amounts to 
approx. 2 MPa. This mask can now be used without any further processing 
measures in a lithography installation, of which a detailed sectional view 
is illustrated schematically in FIG. 5c, where it can be seen that above 
the mask a so-called cooling cylinder 20 is disposed which is provided for 
the radiant cooling of the mask, so that this can be held at a constant 
temperature. Furthermore, an aperture stop 21 is evident in FIG. 5c by 
means of which the impinging beam i is limited to the mask design field 
and if necessary adjusting apertures. This aperture stop thus prevents the 
structures of the area 7, e.g. the perforation, being imaged onto the 
object lying below. It is evident from FIG. 5c that this mask is equally 
suitable for use both for projection apparatus e.g. IPL, and also for a 
shadow-casting device, e.g. MIBL. 
It goes without saying that the above described method in accordance with 
the invention is by no means limited to the use of a silicon mask. 
Likewise, it is possible in the case of the method in accordance with the 
invention also to use other materials, for example SiO.sub.2, Poly-Si, 
Si.sub.3 N.sub.4, SiC or a suitable carbon modification, e.g. a diamond 
sheeting. 
Depending upon the average hole density within the mask design field the 
reduction in the effective thickness in the area encompassing the mask 
structures can have the following effect. In the case of a small reduction 
in the effective thickness, e.g. to 70%, the mask structures can always 
still be displaced outwards in the direction of the area. In the case of a 
particularly large reduction in the effective thickness, e.g. to 1%, then 
the mask structures can be displaced inwards. Thus, between the two values 
there is a point for the reduction of the effective thickness, at which 
the structures are neither displaced outwards nor inwards. The average 
tensile stress is reduced, in dependence upon the average hole density and 
the starting value of the tensile stress, to a predeterminable value. It 
is, however, also possible within the scope of the present invention to 
reduce the average tensile stress to a value lying somewhat below or above 
this predetermined value, so that the mask structures are displaced by a 
certain amount with respect to their desired position. The extent of this 
displacement increases however in a linear manner with the radius and can 
therefore either be taken into consideration when designing the layout or 
corrected during the subsequent use of the mask by means of the lens 
system, in that the device is corresponding enlarged or reduced. 
FIG. 1 illustrates schematically an exemplified embodiment for a mask 1 
(non-scale) which was produced according to the method in accordance with 
the invention. This mask 1 produced from a circular wafer substrate 
comprises a relatively thick edge 2 which is necessary for handling the 
mask. Within the edge there is located a membrane 3 which was produced by 
etching thin the wafer substrate. A transition region 4 which is somewhat 
thicker than the membrane, e.g. twice as thick, is disposed between the 
membrane 3 and the edge 2. This said transition region prevents the 
membrane from fracturing owing to the notch effect occurring here. 
A mask design field 5, edged by the broken line, is provided in the central 
region of the membrane and the structures of the mask, illustrated 
schematically, are formed as holes 6a, 6b, 6c, 6d within said mask design 
field. In accordance with the invention the mask 1 comprises an area 7, 
indicated by the broken line, which comprises a reduced effective 
thickness and which encompasses the mask design field 5. The said area 7 
connects the middle of the membrane 3 with the edge 2 in a 
resilient-elastic manner so that the tensile stress in the middle of the 
mask design field reduces to an average value of less than 5 MPa and does 
not cause any unacceptable displacements or distortions of the mask 
structures. As is evident in FIG. 1, the area 7 in the case of this 
exemplified embodiment is formed as a circumferential groove 8 which is 
formed on the lower side of the mask 1 and concentric to the mask design 
field. However, such a groove can be provided within the scope of the 
present invention also on the upper side or both on the upper side and 
also on the lower side. 
In the case of the mask illustrated in FIG. 1a which is extremely similar 
to that of FIG. 1, the area 7 is formed as a row of identical holes 9 
disposed on the periphery of a circle. The said holes are separated from 
each other by a web 10 respectively, wherein the radially extending webs 
10 represent the single connection between the edge 2 and the membrane 3, 
whereby in turn the resilient-elastic effect occurs. This design of the 
area 7 is designated as a perforation ring. The exemplified embodiments 
illustrated in FIGS. 1 and 1a have in common that the material remaining 
in the area 7 is tensile loaded. 
In the case of the exemplified embodiment illustrated in FIG. 2, this is a 
mask I in accordance with the invention of which only a segment-shaped 
portion is illustrated which demonstrates the area 7. In the case of this 
exemplified embodiment the area 7 consists of three concentric perforation 
rings 11a, 11b, 11c having holes 12a, 12b, 12c and radial webs 13a, 13b, 
13c, wherein the two webs 13a, 13b and 13b 13c of two adjacent 
perforations rings 11a, 11b or 11b, 11c are displaced by a predetermined 
angle, so that these webs 13a, 13b and 13b, 13c are mutually connected in 
each case by way of a tangential cross-piece 14a, 14b. After forming the 
holes 12a, 12b, 12c in the area 7, in the case of this exemplified 
embodiment not only the radial webs 13a, 13b, 13c are under tensile load 
but a bending stress also builds up in the tangential cross-pieces 14a, 
14b by means of which the effective elasticity of the area is 
substantially increased. As a consequence, in contrast to the exemplified 
embodiments described above, it is possible in an advantageous manner to 
achieve sufficient reduction in the tensile stress within the mask design 
field even when the reduction in the effective thickness is less great. 
Such an effect can naturally be achieved with all shapes of webs which 
comprise at least in portions a tangential component. An advantage of the 
embodiment illustrated in FIG. 2 resides inter alia in the fact that the 
contact points of the webs 13a, 13c at the outer edge and at the membrane 
are aligned radially, so that when loading the area no tangential forces 
occur, which could cause the mask design field to be distorted. 
A further exemplified embodiment of this type is illustrated in FIG. 3 
which illustrates a plan view of the area 7 having the reduced effective 
thickness. This area comprises only a single perforation ring, in which 
however the webs 15a, 15b are formed in an angular shape. The tangential 
components of these webs in the case of this exemplified embodiment also 
create a bending stress which guarantees increased elasticity. The greater 
the angles in the webs 15a, 15b, the more the the bending stress portion 
increases, until the characteristics of the area 7 are determined with 
respect to the elasticity mainly by virtue of the bending stress 
component. In the case of the exemplified embodiment illustrated, in each 
case two adjacent webs 15a, 15b are formed symmetrically to a radially 
extending straight line, so that in turn disturbing tangential forces 
cannot occur. 
Based on the simple manufacturing process and the good resilient effect 
which occurs even in the case of a small reduction in the material, 
embodiments of the type as illustrated in FIGS. 2 and 3, are to be 
regarded as preferred embodiments. However, it goes without saying that 
the invention is not limited thereto. 
FIG. 4 illustrates a further exemplified embodiment for an area 7 having 
reduced effective thickness, wherein based on the exemplified embodiment 
as shown in FIG. 1 a circumferential groove 16 is provided, which is, 
however, provided here on the upper side of the membrane. Different to 
FIG. 1, in the case of the exemplified embodiment as shown in FIG. 4 two 
further grooves 17a, 17b are provided on the lower side of the membrane 
and the said further grooves are provided on both sides of the groove 16, 
so that between the grooves 16 and 17a or 16 and 17b thin vertical walls 
18a, 18b are formed which are connected at their lower ends to the bases 
19a of the groove 16. At their upper ends the walls 18a, 18b are connected 
on the one side by way of membrane sections 19b 19c to the mask design 
field and on the other side to the edge. This structure is also subjected 
to tensile load (sections 19a, 19b, 19c) and also to bending stress (walls 
18a, 18b) when exerting a force, so that a particularly high level of 
elasticity can be achieved. 
A considerable advantage of the device in accordance With the invention in 
addition to the particularly low tensile stress within the mask design 
field resides inter alia in the fact that the stress within this field 
also changes very little owing to the elastic coupling if influenced by 
external forces, e.g. in the case of non-uniform, thermal loading of the 
edge or in the case of a mechanical, non-uniform loading of the frame. 
The area having reduced effective thickness does not necessarily, as 
illustrated, have to be circular in shape. Within the scope of the present 
invention, this area can be of any shape whatsoever, e.g. a square shape. 
Both the width and also the effective thickness of the area need not be 
constant, but rather can be adapted to suit the local required elasticity 
between the edge and the mask design field. It is naturally also possible 
to form the entire area between the mask design field and the edge as an 
area having reduced effective thickness.