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Description  |
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FIELD OF THE INVENTION
This invention relates to a method for forming a pattern in a photoresist
layer having openings with predetermined shapes and to an apparatus that
can be applied in practicing such method.
BACKGROUND OF THE INVENTION
Owing to the small structures (order of magnitude 1 .mu.m) currently
required in semiconductor technology, photoresist structures are generated
usually by high-resolution projection exposure systems. The positive
resists which are used for their favorable characteristics are highly
transparent to the radiation (.lambda.=435 nm) generally employed for
projection irradiation, i.e., the photoresist in the irradiated regions is
disintegrated relatively uniformly across its entire thickness. After the
irradiated photoresist layers have been developed, which--to effectively
control this step--is performed with diluted basic developer solutions,
the holes generated in the photoresist have vertical walls. When the photo
resist patterns thus produced are used as etch masks in a subsequent dry
etch step, the shape of the holes in the photoresist pattern is accurately
transferred to the etched material, i.e., the holes in the etched material
have vertical walls, as well. When, in still further process steps, a
layer is vapor-deposited on the etched structure, problems arise because
the thickness of the deposited layer is non-uniform and too thin,
particularly in the region of the hole walls and the hole edges, which
leads to poor step coverage and tearing. A typical example of such
phenomena are metal lines with weak spots along the edges of contact holes
in isolating layers. The described defects are responsible for an
increased failure rate of components containing such structures.
Measures are known that are taken to prevent holes in vertical walls in
etched structures. It is known, for example, to use multi-step etching,
wherein by stepwise expansion of the etch mask, steps are etched into the
hole walls (contour etching). In practice, this leads to inclined hole
walls which prevent the above-described difficulties during the deposition
of a layer. However, this known process is time-consuming and complicated,
and thus expensive and not readily reproducible.
In another known process, the walls of the holes in the photoresist are
inclined by heating the photoresist structure after development, which
softens the photoresist, causing it to flow. As a result, the photoresist
edges are rounded and the hole walls are inclined, meaning that the
dimensions of the hole at the bottom, where it opens towards the material
to be etched, are smaller than at the top. However, this process has the
disadvantge that the edge angles of the walls depend on the hole diameter
and the packing density in the vicinity of the hole. The inclination of
the hole walls in the photoresist pattern is transferred to the holes
resulting in the etched material. Although much simpler than the
previously described known process, the latter process is not sufficiently
accurate for forming holes with very small dimensions that must be
reproducible within narrow tolerances.
DE-OS 2 645 081 described a further process for producing photoresist
patterns with inclined hole walls, in which
1. the UV-light used for irradiation defocuses or decollimates, and
2. a small spacing has to be kept between the mask and the photoresist
layer, and/or
3. a thick (>2 .mu.m) photoresist layer is used, and
4. a disperse (non-collimated) light source is employed.
This process, however, has the disadvantage that the transferred pattern is
poorly defined and not readily reproducible.
In EP application 0 227 851, another process for forming a photoresist
pattern having holes with inclined walls is described, wherein regions of
a layer of a positive photoresist are image-irradiated in a projection
exposure system and, additionally, at least the photoresist regions
adjoining the image-irradiated layer regions are irradiated, using a
radiation at which the photoresist is highly transparent, and wherein the
irradiated regions are developed by means of a basic developer. The
additional irradiation may cover the entire photoresist layer or may be
limited to the regions adjoining the image-irradiated regions. This
process avoids the disadvantages described above, however, in its
preferred embodiment the thickess of the photoresist layer is
substantially reduced, which can only be avoided by using an additional
mask which makes the process more complicated and more expensive.
A further method for influencing the wall profile of openings in
photoresist layers is described in the article, "A Novel Method for
Submicron Structurization Using Optical Projection Lithography," by K.
Ismail, published in Microelectronic Engineering 1 (1983), page 295. In
this method, called the "double exposure technique," the photoresist is
exposed through a mask with the exposure time relatively short, such that
the incident energy would not be high enough for developing the gate area.
The mask is then shifted, for example, by using a micrometer screw, and
exposed again for the same period where the shift is such that the two
exposures overlap. The overlapping region is doubly exposed so that by
developing the resist only this region is completely developed. The
exposed opening has inclined walls. However, the angle range achievable of
the wall inclination is restricted, especially when the ratio of the
thickness of the resist layer to the dimension of the mask opening is
large.
All the methods described above have in common that it is rather difficult,
if not impossible, to shape the topography of the inclined walls in a
predetermined manner. This option is very desirable. For example, if the
photoresist pattern is used as a mask for doping a substrate by ion
implantation, the doping profile in the substrate can be formed by shaping
the wall profile in the photoresist pattern accordingly.
Sometimes it is of interest to tailor the openings in the photoresist layer
with respect to their wall profile and/or with respect to their lateral
shape, independently of the shape of the respective openings in the
irradiation mask used. (In this context, lateral shape of an opening means
the shape of the projection of the opening onto a surface aligned in
parallel with the surface of the photoresist layer.) This applies
especially to methods with which the reduction of the size of an opening
in a mask can be achieved to a size below the resolution limit of the
light used. The above cited article, published in Microelectronic
Engineering, describes the formation of an opening in a photoresist layer
which is narrower than the respective opening in the mask. However, the
width of the opening is coupled to the inclination of its wall in the
sense that with decreasing width, the inclination angle also decreases.
Another method for achieving narrow openings in photoresist layers uses a
bake cycle which causes the unexposed photoresist to flow into the
developed opening whereby its size is reduced. However, this method is not
sufficiently reproducible for application in VLSI technology.
SUMMARY OF THE INVENTION
It is, therefore, one object of the invention to provide a method for
forming a photoresist pattern with openings having inclined walls where
the inclination angle can be varied in a wide range independently of the
photo resist thickness.
It is another object of the invention to provide a method for shaping the
topography of the inclined walls in a predetermined way.
It is a further object of the invention to provide a method to form
reproducibly predetermined wall angles and or predetermined wall
topographies.
It is still another object of the invention to provide a method for forming
a photoresist pattern with elements having lateral shapes differing from
the shapes of the respective elements in the mask pattern as projected on
the photoresist surface.
It is a still further object of the invention to provide a method which is
applicable to a manufacturing environment.
It is also an object of the invention to provide an apparatus that can be
applied for executing the above methods.
In accordance with the invention, there is provided a method of forming a
photoresist pattern having openings with predetermined wall profiles,
which involves shifting a substrate supporting the photoresist from a
first position in which an irradiation mask and the substrate are aligned
with each other laterally in a continuous mode or in at least two steps,
relative to the irradiation mask or the mask projection during exposure,
the substrate being displaced during at least part of the exposure.
In another aspect of the invention, there is provided a method of forming a
photoresist pattern, which involves shifting a substrate supporting the
photoresist from a first position in which an irradiation mask and the
substrate are aligned with each other laterally in a continuous mode or in
at least one step, relative to the irradiation mask or the mask projection
during exposure, and displacing the substrate during at least part of the
exposure, the shifting being performed in such a way that the lateral
shape of the pattern elements is changed with respect to the respective
mask pattern elements.
The invention also provides an apparatus for shifting laterally a substrate
relative to a mask or the projection of the mask on the substrate surface
relative to the substrate, which includes means for shifting the substrate
into the x- and/or y-direction or means between the mask and the substrate
for shifting the path of the beam relative to the substrate, and means for
controlling the shifting means.
The shifting of the substrate relative to the mask or the mask projection
includes the cases of shifting the mask with respect to the substrate and
vice versa and shifting the mask projection with respect to the substrate.
"Shifting in steps" in this context means a fast shift of short duration
relative to the exposure time. It is also possible to switch off the light
during the step.
Despite its simplicity, the method of the invention is capable of producing
wall profiles of great varietiy and, if necessary, complexity without
being dependent on or being influenced by the other parameters of the
photolithographic process. Also, the application of the method of the
invention is not dependent on a special mask pattern or on the thickness
or the characteristics of the photoresist, i.e., the method is applicable
to positive, as well as negative, photoresists and, also, in connection
with the reversal process, using a positive resist to form a negative
pattern.
The method of the invention allows formation of non-symmetric wall
profiles. For example, when inclined walls are needed on only one side of
the opening, by using the method of the invention, it is possible to form
substantially vertical walls on the other sides of the opening. By this,
remarkable savings of space can be achieved.
The method of the invention also allows modification of the shape of the
pattern elements in the photoresist with respect to the respective mask
pattern elements in a predetermined way. These changes can include an
enlargement or a shrinkage of the pattern element in all or in one or
several privileged directions, and also a complete new shaping of its
contours. The walls of these openings in the photoresist pattern are
essentially vertical, provided that the exposure dose reaching the exposed
area is sufficient at least for full exposure everywhere in the whole
area.
It is, however, possible to combine the tailoring of the wall inclination
and/or the wall topography of the openings in the photoresist layer with
the modification of their lateral shape with respect to the shape of the
respective mask openings. In most applications, the modifications of the
shape of the pattern elements is in no way coupled to the change of the
wall profile or the wall topography. These modifications can include an
enlargement or a shrinkage of the pattern element, and also a complete new
shaping of its contours. This is one advantageous embodiment of the method
of the invention. Another is a method where only a predetermined wall
profile of the openings in the photoresist is formed, whereas their
lateral shape remains unchanged with respect to the respective openings in
the mask. The different results are achieved by controlling the shifts of
the substrate relative to the mask in an appropriate and predetermined way
and in adaptation to the thickness of the photoresist, its sensitivity at
the wavelength of the light applied and the intensity of the exposure.
It is advantageous to use the method of the invention not only in
connection with projection printing, but also in connection with the
application of x-rays and e-beams. These applications are especially
valuable, therefore, because, thus far, no other method is known for
forming inclined walls by using x-ray or e-beam lithography.
The concept of achieving the relative shift in the inventive apparatus by
shifting the substrate and having the mask in a fixed position is
desirable, because this technique is highly developed for wafer steppers
used for transferring mask patterns into photoresist layers and because
this concept allows incorporation of the apparatus for shifting into such
a wafer stepper. The shifting of the substrate into the x- and/or the
y-direction can be accomplished by piezo translators or liner motors. In a
preferred embodiment of the apparatus of the invention, the means used in
wafer steppers to adjust the substrate relative to the mask can
simultaneously be used for the shift in connection with the present
invention. With the described means for shifting, accuracies of .+-.0.1
.mu.m can be achieved. Means capable of shifting the path of the beam
relative to the mask, like a plain parallel glass plate suspended in such
a way that it can be tilted in a controlled manner, are known.
In a preferred embodiment of the apparatus of the invention, the shifting
of the substrate or the movement of the means between the mask and the
substrate are program controlled.
The foregoing and other objects, features and advantages of the invention
will be apparent from the following more particular description of the
preferred embodiments of the invention, as illustrated in the accompanying
drawings.
FIG. 1 is a schematic cross-sectional detail representation of a projection
mask with an opening and a photoresist layer with inclined walls formed
using the method of the invention on a substrate, where--plotted against
the exposure time--the respective lateral shifts of the substrate relative
to the mask are indicated,
FIG. 2A is a representation, as in FIG. 1, where the opening formed at the
lower surface of the photoresist has a smaller dimension than the
corresponding opening in the mask,
FIG. 2B is a representation, as in FIG. 1, where the opening formed differs
from the one shown in FIG. 2A by an additional enlargement of the
dimension of the opening at the upper surface of the photoresist,
FIG. 3 is a representation, as in FIG. 1, where the walls are shaped in a
special way in order to use the pattern in the photoresist layer as ion
implantation mask,
FIG. 4 is a representation, as in FIG. 1, with the difference being that
part of a dielectric layer underneath the photoresist layer is also shown,
where the opening formed is used to form in the dielectric layer a via
hole to a conductor which is very close to adjacent conductors,
FIG. 5 is a schematic cross-sectional detail representation of a mask
having two openings and a structured photoresist layer which includes a
spacer having unsymmetrical sides with the dimension at the lower surface
of the photoresist spacer being smaller than the respective dimension of
the mask,
FIG. 6 illustrates in a schematic representation the shifting of an
x-y-table relative to a mask and optical means into the x- and/or
y-direction where the table supports a substrate coated with a photoresist
layer to be exposed according to the invention, and
FIG. 7 is a schematic representation of one embodiment of the apparatus of
the invention where the shifting into the x- and/or y-direction is program
controlled.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the invention is embodied in a conventional photolithographic
process. In this process, a substrate is coated with a photoresist layer
in a predetermined thickness usually by spin coating. This photoresist
layer is--optionally after and/or prior to a baking cycle--patternwise
irradiated where, when a negative photoresist or a positive resist in
reverse mode is used, the irradiated resist becomes insoluble, and when a
positive photoresist is used, the irradiated resist becomes soluble. In
the context of this application, only irradiation through a mask having
opaque and transparent regions is considered. The following description
deals only with irradiation using UV-light where, when small structures
(order of magnitude 1 .mu.m) are to be generated, high-resolution
projection exposure systems are used. (During projection exposure, the
pattern in the projection mask is usually transferred to the photoresist
layer, reduced by a factor of 5, which is not reflected in FIGS. 1 to 5.)
It is, however, emphasized that the method of the invention is equally
suitable for irradiation with x-rays and electrons, where the mask pattern
is transferred to the photoresist layer in the ratio of 1:1. If UV-light
is used, the natural resolution limit is on the order of 0,5 to 1 .mu.m.
It is also important to mention that the radiation is diffracted along the
edges of the mask pattern, which influences to a certain degree the region
exposed to the UV-light. The latent image produced in the photoresist
layer by the irradiation is developed using a developer appropriate for
the photoresist where the photoresist layer is structured in dependency to
the transferred pattern. In the description which follows, the invention
is described with reference to FIGS. 1 to 7.
The generated photoresist pattern can serve very different purposes. Among
these, are application as an etching mask for the selective etching of the
underlying material, application in a lift-off process which is an
additive process for forming conductors, and application as an ion
implantation mask for the selective doping of the underlying material.
It is the irradiation part of the photolithographic process with which the
present invention is concerned.
FIG. 1 shows in a schematic cross-sectional view a photoresist layer 1
overlying a substrate 8 and which has been irradiated through a mask 2
according to the method of the invention. For ease of illustration, mask 2
has only one opening 4, but it should be understood that in practice the
mask has multiple openings of different shapes. Also, mask 2 does not
have--as usual in projection photolithograhy--five times the size, but
rather the same size as the pattern to be transferred into the photoresist
layer 1. Photoresist layer 1 is shown after development of an opening 3
having inclined walls 5 and 6. To achieve the wall profiles of opening 3
as shown, the photoresist layer was shifted relative to the mask in the
x-direction according to the diagram shown in which the amplitude of the
shift is plotted against the exposure time. The exposure cycle is as
follows: In the first section, the exposure takes place without shifting
the substrate relative to the mask. This exposure time is long enough for
the full development of the region of the photoresist layer underneath the
mask opening. In the second section, the substrate is shifted into the
+x-direction, with the amplitude being larger than the dimension of the
mask opening. Subsequently, the substrate is shifted back with the light
switched off (switching off light is not necessary, if the time for shift
back<<exposure time) towards the original position, however, not
completely, since in order to achieve the steady slope of wall 6, it is
necessary to additionally irradiate the lower part of wall 6, slightly.
This is because the amplitude of the shift in the second section was
larger than the dimension of the mask opening 4, with the consequence that
at the end of the shift into the +x-direction, the lower part of the wall
6 was shaded. Therefore, it is necessary to make in the next section of
the irradiation a small shift (indicated in the diagram) into the
-x-direction. When the substrate is again in its original position, the
bottom part of the opening 3 and the wall 6 are irradiated. It remains now
only to repeat the indicated shifts in a mirror-symmetrical way in order
to irradiate the region of the wall 5. The inclined walls are achieved
because the areas of the photoresist closer to the center of the opening
are exposed for longer times than the areas at its periphery.
By varying the shift amplitude, the inclination angles of walls 5 and 6 can
be made steeper or shallower. Naturally, it is also possible to generate
differently inclined walls 5 and 6, including the case where wall 5 is
inclined and wall 6 vertical. While for ease of illustration in FIG. 1,
only shifts in the x-direction are contemplated, it is also possible to
make a shift in the y-direction or simultaneously into the x- and
y-directions, including shifts causing the walls all around to be
inclined. It must be emphasized that the function x(t) can be different
from the one indicated in FIG. 1 as long as the local dose to which the
different areas of the photoresist surface are exposed remains the same.
These statements apply to the description of the embodiments illustrated
by FIGS. 2A to 5, as well.
To produce the opening 3 shown in FIG. 2A, in principle, the same
irradiation procedure has to be followed as described in connection with
FIG. 1. Different only are the variation of the amplitude with the
exposure time and the absolute value of the amplitude. The consequence is,
that an opening is formed having a dimension at the lower surface of the
photoresist--i.e. the surface adjacent substrate 8--which is smaller than
the dimension of the opening 4 in the mask 2. Using this embodiment of the
method of the invention, where it is intended to form an opening having a
smaller dimension than the respective opening in the mask, one is not so
free in the determination of the inclination angle because the inclination
angle is coupled with the dimension of the opening at the lower surface
and, to a lesser degree, with the dimension of the opening at the upper
surface of the photoresist. The smallest dimension of the opening at the
lower surface of the photoresist layer for a given shift is indicated in
FIG. 2A by the wall profile, shown with solid lines. By increasing the
exposure time, the dimension of the opening at the bottom surface of the
photoresist layer is increased, and with it, the inclination angle. This
is indicated in FIG. 2A by the wall profile and the curve in the diagram,
shown with dotted lines.
A cross-section of an opening as shown in FIG. 2B is generated by
performing the same exposure cycle as applied for generating the opening
shown in FIG. 2A, and by subsequently superimposing the exposure cycle as
shown in FIG. 1, but with a shorter exposure time.
The wall profile shown in FIG. 3 is formed by applying the exposure cycle
indicated in the respective diagram. Photoresist patterns of the kind
displayed in FIG. 3, i.e. with a stepped topography, can, for example, be
used as ion implantation masks for doping the underlying substrate in a
predetermined way. Using the photoresist pattern shown in FIG. 3 as an
implantation mask, the doping profile formed in the substrate will be
equally stepped. The possibility of shaping doping zones reproducibly, not
only laterally, but also vertically, in a predetermined way, becomes more
and more important with the increasing density of integrated circuits.
FIG. 4 shows in a cross-sectional view a structure comprising a photoresist
layer 1 on a dielectric layer 7 overlying substrate 8. On substrate 8,
three conductive lines are applied which are parallel and in close
proximity to each other. In order to connect conductive line 9 to a second
level metallization, a via hole 12 is to be etched into dielectric layer
7. The pattern formed in photoresist layer 1 by means of the present
method serves as an etching mask for producing the via hole. The opening 3
in the photoresist layer 1 was formed by performing the exposure cycle
indicated in the diagram. In the etch process, the via hole 12, having the
same wall profile as the opening 3, will be formed in the dielectric layer
7. A via hole with such a wall profile serves two purposes. The inclined
wall profile in the upper part of via hole 12 guarantees--as explained in
detail at the beginning of this specification--a good step coverage at the
edge of the via hole when metal is applied to it by evaporation. If the
wall inclination would already start in the lower part of the via hole,
the danger that a short circuit could occur between conductive lines 10
and 11 and the evaporated metal could not be excluded. By shaping the via
hole, so that it shows the wall topography as represented in FIG. 4, a
good step coverage is guaranteed and the danger of a short circuit is
minimized, even when the density of the conductive lines is high.
FIG. 5 illustrates the formation of a spacer 13 of photoresist on substrate
8. In the mask 2, two openings 4 are shown having a dimension of, for
example, 1 .mu.m and a distance about equal to such dimension (it is
assumed that the mask pattern is not reduced upon exposure). In using the
exposure cycles indicated in the respective diagram, the resulting two
openings 3 have vertical walls 5, inclined walls 6 and a dimension of the
opening at the lower surface of the photoresist layer that is larger than
the dimension of the openings 4 in mask 2. Consequently, the spacer 13 has
a vertical side and an inclined side, and the dimension of the spacer at
the lower surface of the photoresist layer is--in the present
example--clearly smaller than 1 .mu.m, i.e., below the resolution limit. A
spacer of this kind can be included in an implantation mask to form doped
regions having an extremely small distance.
In addition to tailoring the wall profile and the dimensions of the pattern
elements transferred into the photoresist layer, their shape can be
changed by using the present method. For example, a mask element having a
circular shape may be transformed into a straight or curved line in the
photoresist layer by performing the appropriate shifts of the substrate.
This variation of the method of the invention is valuable when, for
example, a special conductive pattern is needed only once or in a few
cases so that the generation of a suitable irradiation mask would be very
uneconomical. The above description of various embodiments of the method
of the invention shows its flexibility and effectiveness in forming very
differently shaped photoresist patterns and its usefulness and adaptation
capability for many applications.
FIG. 6 is basically a schematic representation of a conventional wafer
stepper used for repeatedly transferring a chip mask five times reduced on
a wafer coated with a photoresist layer. Such a wafer stepper can--with
modifications--advantageously be used for practicing the method of the
invention. The wafer 18 onto which the chip pattern 19 is to be
transferred is supported by an x-y-table 17 which is movable into the x-
and the y directions with an accuracy of .+-.0.1 .mu.m. The UV-light 25
passes the mask 22 having five times the enlarged chip pattern 19' and the
optical means 24 providing the image reduction, and impinges on wafer 18.
The wafer stepper is operated in a step and repeat mode which means, that
after each exposure the table 18 is moved (naturally this movement has to
be extremely accurate) for a chip length, then the exposure is repeated
and so on until the whole wafer surface is exposed to the irradiation. The
means for moving table 18 can be used for shifting the photoresist layer
relative to the mask as described in detail above. It is, however, also
possible--if for any reasons individual means for shifting are more
appropriate--to install such shifting means additionally in the wafer
stepper. In another embodiment that might also be preferred--instead of
shifting the table 18--means are brought into the path of the beam between
the mask and the photoresist layer, such means being capable of shifting
the beam laterally in a controlled manner.
In practicing the method of the invention, the shift of the substrate or
the beam has to be controlled in a predetermined way. FIG. 7 shows
schematically one suitable arrangement to accomplish this for the case
where the substrate is shifted. It is preferred--especially in a
manufacturing environment--that the shift be computer controlled. In FIG.
7, numeral 30 indicates a computer, numeral 31 a digital analog converter
(DAC), numerals 32 and 33 shifting means into the x- or y-direction,
respectively, numeral 18 the x-y-table and numeral 34 the shutter for
switching on and off the light. The software controlled instructions from
the computer 30 are transformed in the DAC into machine readable form and
then fed into shifting means 32 and 33. These perform the necessary
shifts.
It depends on the chosen arrangement whether the modification necessary for
the adaptation of the pattern generator to practicing the method of the
invention is only software related, or whether the control means (computer
and DAC) and the shifting means or only the latter have to be additionally
installed.
In the following, the formation of an opening as shown in FIG. 1 is
described in even greater detail.
Five samples were processed in the same way. The samples were photoresist
coated silicon wafers. The photoresist employed was a positive photoresist
marketed under the trade designation S1400-31 by Shipley. (This
photoresist consists basically of a novolac resin and a diazonaphtoquinone
sensitizer.) The thickness of the resist layer was about 2 .mu.m. The
samples were in each case heated prior to and after irradiation--prior to
irradiation for 20 minutes to 85.degree. C. and after irradiation for 10
minutes to 95.degree. C. For the irradiation, the projection exposure
system distributed by GCA under the name DSW6300 was used at a wave length
of 436 nm and an energy of 110 mj/cm.sup.2. The projection exposure system
has a numeric aperture of 0.3 and the mask image is transferred to the
photoresist layer five times reduced. The mask used for the irradiation
had multiple square pattern elements with an edge length of 5 .mu.m, i.e.,
if the pattern elements are transferred to the photoresist layer without
practicing the present invention, the transferred pattern elements have an
edge length of 1 .mu.m. The resist coated wafers were brought into the
projection exposure system, and the center of the wafer was positioned
under the mask pattern. Subsequently, the wafer was exposed where, during
part of the exposure, the wafer was shifted according to the diagram shown
in FIG. 1. For the shifts, the x- and y-stage drivers of the projection
exposure system were used. The modification of the exposure system
provided for this experiment was exclusively software related. During the
first section of the exposure cycle (this section took 1.3 seconds), the
substrate was not shifted. In the second section that had the same
duration, the substrate was shifted 1.25 .mu.m; then the light was
switched off and the substrate was shifted back until the shift, as
compared with the original position, was 0.25 .mu.m. Subsequently, the
light was switched on again, and the substrate was shifted back into the
original position within about 0.7 seconds. Subsequently, the shifts
performed from the second section on were repeated, but in the opposite
direction. The development of the irradiated and postbaked samples was
effected at 25.degree. C. using a 0.1 n KOH solution for 3 minutes in a
tank.
From 10 openings of each of the five samples, the dimensions were measured.
It was found that the dimension of the openings at the upper surface of
the photoresist layer was 3.5 .mu.m and the dimension of the openings at
the lower surface was 1 .mu.m. The reproducibility of these dimensions was
.+-.60 nm (3 .sigma.), which was mainly caused by the variations in the
development process.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the spirit and scope of the invention.
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