U.S. patent application number 12/232742 was filed with the patent office on 2009-04-23 for method for error reduction in lithography.
Invention is credited to Peter Ekberg, Torbjorn Sandstrom.
Application Number | 20090104549 12/232742 |
Document ID | / |
Family ID | 20415701 |
Filed Date | 2009-04-23 |
United States Patent
Application |
20090104549 |
Kind Code |
A1 |
Sandstrom; Torbjorn ; et
al. |
April 23, 2009 |
Method for error reduction in lithography
Abstract
The present invention relates to a method and a system for
predicting and/or measuring and correcting geometrical errors in
lithography using masks, such as large-area photomasks or reticles,
and exposure stations, such as wafer steppers or projection
aligners, printing the pattern of said masks on a workpiece, such
as a display panel or a semiconductor wafer. A method to compensate
for process variations when printing a pattern on a workpiece,
including determining a two-dimensional CD profile in said pattern
printed on said workpiece, generating a two-dimensional
compensation file to equalize fluctuations in said two-dimensional
CD-profile, and patterning a workpiece with said two-dimensional
compensation file.
Inventors: |
Sandstrom; Torbjorn; (Pixbo,
SE) ; Ekberg; Peter; (Lindingo, SE) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 8910
RESTON
VA
20195
US
|
Family ID: |
20415701 |
Appl. No.: |
12/232742 |
Filed: |
September 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10827530 |
Apr 20, 2004 |
7444616 |
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12232742 |
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09979148 |
Nov 20, 2001 |
6883158 |
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PCT/SE00/01030 |
May 22, 2000 |
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10827530 |
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Current U.S.
Class: |
430/30 ;
355/53 |
Current CPC
Class: |
G03F 7/704 20130101;
G03F 1/72 20130101; G03F 7/705 20130101; G03F 7/70433 20130101;
G03F 7/706 20130101; G03F 7/70616 20130101; G03F 7/70783
20130101 |
Class at
Publication: |
430/30 ;
355/53 |
International
Class: |
G03C 5/00 20060101
G03C005/00; G03B 27/42 20060101 G03B027/42 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 1999 |
SE |
9901866-5 |
Claims
1.-7. (canceled)
8. A method to compensate for process variations when printing a
pattern on a workpiece, said method comprising: determining a
two-dimensional CD profile in said pattern printed on said
workpiece, generating a two-dimensional dose compensation profile
to equalize fluctuations in said two-dimensional CD-profile, and
patterning a workpiece with said two-dimensional dose compensation
profile.
9. A method to compensate for process variations when printing a
pattern on a workpiece, said method comprising: predicting a
two-dimensional CD profile in said pattern to be printed on said
workpiece, generating a two-dimensional dose compensation profile
to equalize fluctuations in said 2-dim CD-profile, patterning the
workpiece with said 2-dim dose compensation profile.
10. An apparatus for process variation compensation when printing a
pattern on a workpiece, said apparatus comprising: determining a
two-dimensional CD distribution in said pattern printed on said
workpiece, generating a two-dimensional compensation file to
equalize variations in said two-dimensional CD distribution, and
patterning a workpiece with said two-dimensional compensation
file.
11. A method of compensating for CD variations on a workpiece while
printing a pattern on said workpiece, said method comprising:
determining two-dimensional critical dimension (CD) variations
associated with said pattern being printed; estimating future CD
variations of said patterning process; compensating the dose while
patterning said workpiece by pro-actively equalizing for said
estimated CD variations; and patterning said workpiece using said
compensation of the exposure dose.
12. An apparatus for compensating for CD variations on a workpiece
while printing a pattern on said workpiece, said apparatus
comprising: means for determining two-dimensional critical
dimension (CD) variations associated with said pattern being
printed; means for estimating future CD variations of said
patterning process; means for compensating the dose while
patterning said workpiece by pro-actively equalizing for said
estimated CD variations; and means for patterning said workpiece
using said compensation of the dose.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of copending
application Ser. No. 09/979,148, filed on Nov. 20, 2001 and for
which priority is claimed under 35 U.S.C. .sctn. 120. Application
Ser. No. 09/979,148 is the national phase of PCT International
Application No. PCT/SE00/01030 filed on May 22, 2000 under 35
U.S.C. .sctn. 371. This application also claims priority of
Application No. 9901866-5 filed in Sweden on May 20, 1999 under 35
U.S.C. .sctn. 119. The entire contents of each of the
above-identified applications are hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] The invention relates to production and precision patterning
of photomasks and use of such photomasks in microlithography, for
example in the production of flat panel displays and semiconductor
circuits. Errors in the pattern on the display panel or
semiconductor chip can be separated into errors from systematic
sources, from the interaction of maskblank and pattern with the
equipment, and from random fluctuations. The invention relates to
the reduction of these errors. In a different sense the invention
relates to the characterization of the photomask substrate and the
equipment and processes used with a photomask, the storage and
retrieval of information obtained by such characterization, and the
generation of corrections to be applied at the time of writing the
photomask in order to reduce imperfections in lithography using
photomasks.
BACKGROUND
[0003] The development of semiconductor lithography has been
exponential since the early 60ies and the produced features are
getting smaller every second or third year, at the same time as the
circuits get faster and more complex. FIG. 1 shows an industry
projection of the development for some years forward. Of course the
predictions are less certain the farther into the future we look
and nobody knows if the electronics industry will still be using
transistors in the year 2020. For the next 10 years the projections
are more certain and the main uncertainty relates not to "How
small?" but to "Exactly when?".
[0004] The errors in lithography can broadly be classified as
placement and size errors, or "registration" and "critical
dimension", ("CD") in the jargon of the trade. There is a more or
less fixed relation between the errors that can be allowed in the
pattern and the size of the smallest features in the pattern. A
rule of thumb is that on the mask the placement of figures has to
be within 5% of the design rule and the size of the features should
be within 2.5%. These are surprisingly small numbers, but have been
justified by both theory and experiments. FIG. 1 also shows the
necessary registration and CD (size) control on the mask for each
year, assuming that 4.times. masks will continue to be used. It is
seen that the errors are now 1999 in the low tens of nanometers and
will in less than 15 years be ten times smaller. At the same time
then chips will be larger which means either larger masks or less
reduction. Either way it will be difficult to achieve the needed
pattern fidelity.
[0005] The invention devises a new general method to reduce errors
in the lithography in order to achieve total errors that are
consistent with the projected lithography development. An important
application is for the reduction of clamping errors, being both an
important error source and a good example of errors caused by the
interaction of several factors.
[0006] Clamping Errors
[0007] When a glass plate is held it is deformed by the holding
device and by its own weight. Furthermore, it can also be distorted
by the built-in stress in surface films deposited on it and by the
patterning of said films. Semiconductor masks are typically
152.times.152.times.6.25 mm and the patterned area may be
127.times.127 mm. FIG. 2b shows how the bending of a plate 201
under the force of gravitation causes the upper area of the glass
to contract. When the plate is released, e.g. held vertically, it
springs back to its natural shape, shown in FIG. 2a, and
contraction disappears. If a pattern was written on the plate while
it was bent, the pattern will be stretched after relief. FIG. 3
shows a diagram with the resulting maximum error when a plate is
supported along two opposite edges, as in FIG. 2. Here, the
expected lateral position error is shown as a function of the
thickness of the plate and the size, i.e. the distance between the
two supported sides. The interesting conclusion from FIG. 3 is that
the magnitude of errors that can result from inappropriate support
of a glass plate is in order of magnitude larger than what is
allowable in a high-end mask. Point A shows a standard
semiconductor reticle 152.times.152.times.6.25 mm and the maximum
deviation is around 400 nm. Point B is the new standardised mask
format 225.times.225.times.9 mm and, despite the fact that the
glass plate is thicker, the deviation is above 1 .mu.m. Point C
finally illustrates that for large-area masks the problem is even
worse: an 800 mm plate 8 mm gives a possible error of 60 .mu.m. It
is also seen from FIG. 3 that increasing the thickness of the glass
plate is a weak remedy. It is impossible to increase the thickness
of the 800 mm plate to bring the error down to 0.1 .mu.m. The same
is valid for the 225 mm mask in B: even a glass cube with the side
225 mm has deviations larger than 10 nm.
[0008] In conclusion FIG. 3 shows the magnitude of the
gravitational deformation and it shows how everything becomes more
difficult for larger mask sizes.
[0009] Other Errors
[0010] Clamping deformation gives a placement or registration
error. Another large source of placement errors in the finished
product is the distorsion in the exposure tool, be it a wafer
stepper for semiconductors or a projection aligner for display
panels. The maskwriter has stage errors, but these are usually well
controlled after calibration to a xy metrology system, such as
those made commercially available by Nikon and Leica. The placement
can also be affected by processing, both because films deposited on
or removed from a workpiece have built-in stress and deform the
workpiece, and because some process steps cause a shrinkage or
warping of the workpiece, e.g. high temperature annealing steps. An
important source of errors is the pellicle used on masks. Fitting
the frame to the mask plate causes the mask plate to bend.
[0011] Other effects make the size of the pattern features come out
differently at different location on a mask or on a chip or display
panel. There are several possible mechanisms behind this: uneven
focus, non-uniform developer agitation, non-uniform photoresist
thickness, uneven chrome properties on the mask and uneven film
thickness on a wafer or panel, exposure dose variations in the
exposure tools, effects of the time between the exposure and
development or between resist coating and exposure and effect from
non-perfect pre-exposure and post-exposure baking procedures. Size
errors also occur because of the basic imaging properties of mask
writer and the exposure station. In particular small features tend
to come out too small due to finite resolution and features are
affected by the presence of other features in the neighbourhood due
to stray exposure. These types of size errors are also intimately
coupled to shape errors, such as shortening of line ends and
rounding of corners. The exact details of the mask and wafer
exposure tools also interact with the pattern and create for
example grid snap effects, and spurious pattern features.
[0012] Mix and Match
[0013] The term used in the mask industry is "registration" which
really means misregistration from a reference grid, normally an
ideal mathematical grid. In the past registration of the finished
product to an ideal mathematical grid has not been necessary. If
all layers (approximately 25 in a semiconductor chip and 6 in a
TFT) are printed using the same type of exposure station systematic
and equal behaviour of the exposure stations will cancel, since
every layer is distorted in the same way.
[0014] However, when resolution is pushed in order to achieve
circuit speed and packing density the cost of lithography is rising
rapidly, both because of higher tool cost and because of more
expensive masks. To make production economical the display and chip
manufacturers are trying not to use more sophisticated technology
than needed for each layer, so called mix-and-match. Different
layers can be printed using different types of exposure tools with
different error characteristics. Furthermore the masks may be of
different type, e.g. phase-shifting masks for one layer and
standard binary masks on another layer. The different types of
masks may require them to be written on different maskwriters.
[0015] The management of errors is made more complicated by the
fact that the exposure tools for critical and non-critical layers
may not even have the same exposure field. FIG. 4 shows dies formed
on a semiconductor wafer using tools with different fields. The
critical layers like the transistor layers are printed with a tool
that has a field that accommodates only one die, FIG. 4a. Less
critical layers, such as the top metal layers, are printed with a
different stepper having a larger field and possibly a different
mask reduction factor, FIG. 4b. One or both of the layers can be
exposed by a step-and-scan tool, and if both use step and scan they
may very well have the scan direction at right angle to each other.
Furthermore, it is anticipated that in the future a single die will
be exposed in two or more scanning strokes using a so called
stitching scanner, FIG. 4c.
[0016] In the past the thinking has been that the masks should be
as close to perfect, i.e. as close to the ideal mathematical grid,
as possible. Then the masks for different layers can be written on
different types of maskwriters or even by different mask making
companies for best economy and logistics. When all masks are
exposed on the same stepper, or on steppers of the same type, the
systematic errors in the stepper will largely cancel. This breaks
down in the mix-and-match scenario of FIG. 4a-c. The only
straightforward way to resolve the complicated overlay properties
of layers printed in different ways is to make each layer print an
image that is close to the mathematically ideal.
[0017] The invention gives a method to predict the printing errors
in a specific exposure station and correct it beforehand by
predistorting and prebiasing the pattern in the opposite sense. In
this application is described a practical method to manage the
lithographic errors al the way to the finished product, and also in
the presence of non-ideal mask blanks and clamping structures.
OBJECT OF THE INVENTION
[0018] It is therefore an object of the present invention to
provide a method for predicting and correcting geometrical errors
in lithography in order to achieve an improved precision.
[0019] This object is achieved with a method according to the
appended claims.
BRIEF SUMMARY OF THE DRAWINGS
[0020] FIG. 1 gives a projection of future lithography and the
requirements on placement (registration) and size (CD).
[0021] FIG. 2a shows a plate, e.g. a mask substrate. FIG. 2b shows
the same plate supported at two sides and the contractive stress
above and the tensile stress below the neutral layer which keeps
its length when the plate is bent. FIG. 2c shows the lateral
displacement resulting from a bent plate and how it is related to
the dz/dx of the surface.
[0022] FIG. 3 shows the maximum lateral displacement in the
arrangement of FIG. 1b as a function of plate size and thickness.
The shaded areas show the resulting error.
[0023] FIG. 4a-c shows dies formed on a semiconductor wafer using
tools with different fields, so called mix-and-match
lithography.
[0024] FIG. 5 shows schematically a control method according to the
invention.
[0025] FIG. 6 shows flow chart describing the development of models
and how the models are used to predict and correct errors.
[0026] FIG. 7 shows the control method according to FIG. 5 more in
detail.
[0027] FIG. 8 shows how the plate and/or the pattern is affected by
different types of errors.
[0028] FIG. 9 shows a typical implementation of an error correction
system suitable for the invention using both pattern modification
and writing hardware control.
[0029] FIG. 10 is a schematic view of a system according to one
embodiment of the invention.
[0030] FIG. 11 is a schematic flow chart of the method used in the
system according to FIG. 10.
[0031] FIG. 12 is a schematic view illustrating the correction in
the embodiment in FIG. 5 in more detail.
[0032] FIG. 13 illustrates an SLM writer and a DUV stepper with a
programmable mask according to another embodiment of the
invention.
[0033] FIG. 14a illustrates an example of a two-dimensional (2D)
cylindrical CD distribution according to another embodiment of the
invention.
[0034] FIG. 14b illustrates an example of cylindrical CD
distribution according to another embodiment of the invention.
[0035] FIG. 14c illustrates an example of cylindrical CD
distribution using global dose compensation according to another
embodiment of the invention.
[0036] FIG. 14d illustrates an example of a pre-programmed global
dose map implementable using gray levels of an SLM or by utilizing
dose control in a laser according to another embodiment of the
invention.
DESCRIPTION OF THE INVENTION
[0037] The invention is best described as a control system, such as
is shown in FIG. 5. The pattern picks up errors 501, 502 of
different kinds when it is converted from a design data file 503
and a mask blank 504 to a mask 505 during a mask writing procedure
506, i.e. exposure, by means of a mask writer 507. The mask is
thereafter used to produce an electronic device 508. The pattern
file describes what the chip or panel designer wants to see printed
and any deviation is to him an error. One part of these errors is
that systematic and other errors are different from one time to
another. The invention is based on the identification of different
types of errors and the appropriate way to reduce each type. Errors
that can be found in the output from the system and identified to
be recurring in a systematic fashion are reduced by feeding an
inverse error 509 back to the writing of the mask. This is the
feed-back loop 510 in FIG. 5. The feed-back can be
pseudo-continuous, i.e. corrections are made after each written
mask, or intermittently. The error is measured by a measuring means
511, and a filter 512 is useful to keep the feed-back from
fluctuation with the noise component of the measured error. The
low-pass filter 512 can use a very simple procedure: use the
average of five measurements over five days for setting the first
feed-back, then change it only when the running average over five
consecutive measurement are outside of predetermined tolerance
interval. On the other hand it can be based on more sophisticated
statistics such as time series estimations and feed back an
appropriately filtered correction for each measurement of the
output errors. More elaborate statistics can be used to cancel slow
changes in the properties of the total system without ever
producing a rejected part, or to extract error components with
different characteristics. A statistician skilled in the art can
set up statistical process control (SPC) procedures that tunes the
system without interrupting the production flow.
[0038] In FIG. 12 the correction system provided in the embodiment
described above with reference to FIG. 5 is illustrated in more
detail. Input data is transferred to a data collection unit 1201 in
the error reduction system 506. As will be described more in detail
in the following, such input data could be one, or preferably
several, and most preferably all, of the following: pattern
(design) data, blank data mask writer data, exposure tool data,
process data and metrology tool data. The input data is then
forwarded to a data validation unit 1202, where the data is
validated. The validated data is then transferred to error
prediction unit 1203 comprising a model, for a statistical
estimation of model parameters. Correction data is then output as a
position correction map 1204 and/or a correction size map 1203, for
correction of pattern element position and pattern element size,
respectively.
[0039] The correction maps are forwarded to the mask writer 507.
One or both of the correction maps could be forwarded to the data
path 1206 in the mask writer 507 for correction of the data
provided from the pattern data file, and thus correcting said
distorsion. The altering of the input design data could hereby
preferably be made in at least one processor, and preferably in
several such processors. Alternatively, or as a complement, the
position correction map could be forwarded to the position servos
1205 controlling e.g. the support table supporting the mask
substrate during the writing process. In this case, the correction
map implies a correction of the position control for the servo
system, and thereby the position of the pattern on the substrate.
Alternatively, or as a complement, the size correction map could be
forwarded to an exposure dose control 1207, such as a dose
modulator. Hereby, a correction of the dose according to the
predicted correction map could be provided during the exposure.
[0040] There are in principle four driving forces for errors: the
physical properties of the substrates on which the patterns are
printed, i.e. the mask blanks and the wafers or panels, the
position on the substrate, the exposing equipment (which can be
using electromagnetic radiation or particle beams as the exposing
medium) including exposing sequence and environment, and the
pattern itself. These driving forces interact with each other
directly of via the production processes to create errors. An
important feature of the invention is the use of models to
translate measured or previously known physical parameters to a
placement, size or shape error to be corrected. An example is a
plasma etcher. The plasma is non-uniform towards the edges of the
substrate and in local areas where the surface exposed to the
plasma is different from other places in the pattern. This creates
a position- and pattern-dependent size error. It can however be
characterised with a small number of parameters, such as an edge
fall-off magnitude and typical length, and a sensitivity and
disturbance length for pattern variations. Using this model with
four parameters it is possible not only to precompensate for the
edge fall-off, which is equal from plate to plate, but also for the
local variation which vary with the patterns.
[0041] Using a model-based error prediction makes it possible to
account for a large number of different error mechanisms with a
manageable amount of empirical data collection. Designing sampling
and measurement plans that effectively fit the parameters of a
model from a limited number of measurements is known in the art,
and can be found in textbooks on experimental design. It is also
known how to design plans to separate between different driving
forces. The ideal situation is that measurement are made
non-destructively on production masks and wafers, but for a
specific model it may be more efficient to use a special monitor
substrate, i.e. to be able to distribute a matrix of test
structures over the entire surface.
[0042] The model-based error prediction is further described in
FIG. 6. Two different mechanisms for generation of errors are set
up, e.g. edge falloff and pattern dependent etch activity in an
etch step. A measurement is designed which can find the parameters
for each and separate between them. In this example it can be
measuring features in areas with three densities each at three
locations, inside, intermediate, and close to the edge of the
substrate. The measurement is done and the parameters extracted.
Before writing and correcting a pattern relevant information has to
be collected in this case the pattern density in different areas of
the pattern and the distance of pattern areas to the edge of the
substrate. The total correction is generated, in most cases by
superposition of the corrections for the two mechanisms, other
times by a more complicated summation.
[0043] Another aspect of model-based error correction is shown in
FIG. 8, namely decomposition of complicated error behaviour into a
set of independent and computable error mechanisms. The general
geometrical error in a mask writer is very complicated, but it can
be decomposed into isotropic expansion, built-in shape, gravitation
sag, clamping deformation due to non-ideal geometry of the clamping
structure, and interaction between the built-in shape and the
clamping geometry. If each error is small, which is the case for
lithographic substrates, the contributions can be superposed. On
top of it come the stage errors, i.e. errors in the coordinate
system of the writer.
[0044] An important case of model-based error correction is when a
chip has a CD error towards the edge, for example due to stray
light. If a 0.18 feature is 3% percent too small one would expect
that a +3% size correction in the mask would compensate it
perfectly. However, due to the finite resolution of a stepper there
is a more complicated function relating size on the mask to size on
the wafer, the so-called "Mask Error Enhancement Factor", MEEF. The
enhancement factor is size dependent and depends on the details of
the tools and process. Therefor a model need to be used that takes
the MEEF into account, and the corrections will not be correct
until the MEEF model has been verified a couple of turns around the
feedback loop.
[0045] Process errors 502, if they can be held constant, can in
principle be corrected with the feedback loop. Other errors 501 are
impossible to correct by feed-back, because they are not constant
in time. In the invention one important such error has been
identified as the clamping distortion of the mask in the mask
writer, the metrology system and in the exposure station that uses
the masks. At first sight the clamping errors seem uncontrollable,
but we have found that they can be predicted from accurate
geometrical data for the mask blank itself and the clamping
structure of equipment using or operating on the mask. Another
seemingly random error source that is controlled in another
embodiment of the invention is variations in linewidth due to
variations in resist and chrome properties over the mask substrate.
Using the invention it is possible to set up and apply models for
how the resist thickness and chrome properties affect the feature
size of the pattern and correct for the errors created.
[0046] Since these properties are possible to measure by measuring
means 513, 514 prior to the writing of the mask, it is possible to
predict the errors and correct them at the time of writing the
pattern, the feed-forward correction loop 515 in FIG. 5.
Residual Errors
[0047] By the feed-forward or feed-back correction 509, or
especially a combination of the two, a large portion of the total
errors can be controlled and corrected. The residual errors are due
to random errors during maskwriting, exposure and processing, and
have to be addressed as such, i.e. by better temperature control,
vibration-insulation, process automation, etc. They are also due to
incomplete error models and uncertainty in the model parameters. We
believe that when the framework of the invention is established
there will be a development of better and better models until
eventually all but the genuine noise errors are removed. The
development of models and software for their characterisation and
use could well become the mission of independent commercial
companies.
[0048] A flow chart describing the development of models and how
the models are used to predict and correct errors is given in FIG.
6.
[0049] A Comprehensive List of Error Sources
[0050] FIG. 7 shows the work flow from glass block and CAD file to
a finished chip or TFT display, and important error mechanisms that
can be modelled and corrected. The work flow is divided in three
separate parts of making the mask blank, writing the mask on the
blank, and finally using the mask for lithographic production, and
each part is divided in several different steps. For each step is
further indicated different types of errors possible to occur
during said step. These errors are however merely examples of
errors possible to occur during each step, and many other errors
are probably possible as well. As is described above, errors in at
least some of the steps are measured and used either in a feedback
or a feed-forward loop to predict the error in the written pattern,
and to generate corrections to compensate for said predicted errors
during the exposure step in the mask making.
[0051] Correction of Clamping Errors
[0052] If is held in exactly the same way during writing and use
there is no error. Until now it has been possible to treat the
semiconductor mask as a stiff plate with no deformation induced by
clamping, provided that the clamping has been done carefully. There
are two developments that make the clamping-induced deformation
more critical in the future: increased mask size and dramatically
tightened precision requirements. The new mask format of
225.times.225.times.9 mm has been defined and the allowable
geometrical errors in a mask will be 30 nm in 2001 and below 10 nm
a few years later. The registration error allowed is typically 5%
of the features on the photomask and the error of critical
dimensions (CD) must be less than 2.5%. Current plans for
lithography predict that the features will be around 25 nm in the
year 2010. With a reduction ration of 4 the features are 100 nm on
the mask and the registration requirement is then 5 nm and the CD
tolerance 2.5 nm. A few years later the requirements are predicted
to be sub-nanometric if the march toward smaller scale were to
continue. Mask production following what is known in prior art
cannot produce masks to these requirements. In the invention
methods are devised that can reduce many systematic errors by an
order of magnitude or more.
[0053] It is known in prior art to support the glass plate at three
points. With three supports there is no bending induced by the
supports. Therefore it deforms only under gravity. It is also known
to correct the pattern geometry for the computed deformation due to
the gravitational sag when it is supported on three supports. The
deformation depends only on the plate size and its material
properties and can be computed beforehand.
[0054] In flat panel production the masks may be
600.times.800.times.12 mm. It is not possible to support such a
large plate on three points and get a sufficiently flat surface.
The plate needs to be supported at more than three points and
becomes kinematically over-constrained. The supporting structure
will introduce deformations if the points are not in a perfect
plane. In this case the deformation is a combination of
gravitational sag and deformation due to the support structure. The
same applies to a co-ordinate measuring machine where the plate
needs to be measured without distorsion.
[0055] The basic problem is that due to technical constraints
different machines producing and using a patterned workpiece hold
the workpiece by different methods, For example a reflection-type
metrology system for semiconductor reticles normally uses a
three-point support with the support points chosen for minimum
deformation, but the stepper using the reticle must support it
along the edges to keep the patterned area unobstructed for the
exposing light.
[0056] If the workpiece or the clamping structure is non-ideal,
i.e. non-flat, the different types of clamping in different types
of equipment give an uncertainty in the geometry of the
workpiece.
[0057] A numerical example: assume that a standard semiconductor
reticle (152.times.152.times.6.25 mm) is supported at the four
corners. One of the corners is 1 .mu.m out of the plane of the
other three corners, either because the glass is non-flat or the
supporting points are skewed. The bending of the glass causes a
line along one diagonal of the plate to be stretched and the other
diagonal to be compressed. This is the same as an orthogonality
error in the pattern on the mask with a maximum lateral position
error on the plate of 0.5 .mu.m*6.25/2 mm/152 mm=20 nm. The error
from the clamping must be added to other error sources such as
drift, scale errors and effects from the process. Therefore 20 nm
is unacceptable as an error from the clamping alone. And if the
points were located closer together or if there were more than four
constraining points the errors would be even larger. We have found
that supporting a plate on four well chosen points is exceptionally
good for large plates, giving a deflection that is 20 smaller than
with three support points. The invention makes it possible to use
four points and compute the effect of the plate being
over-constrained. If the flatness in force-free state is known and
the height of the upper surface is measured at the four point one
has all necessary information to correct the pattern for bending
and gravitation.
[0058] The invention devises a method for complete prediction of
the clamping errors as well as partial correction for
stress-induced errors in a multi-machine environment with equipment
for writing masks, printing panels or wafers from masks and
metrology systems for measuring masks, wafers and panels.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0059] The mask blanks are cut and polished by a glassmaking
company. The surface figure is controlled to a maximum error
corresponding to a quality class of the glass product. The glass
plate is coated with a sputtered film of chromium and a photoresist
coating is spun on. In the preferred embodiment of the invention
the flatness of the front and backside of the glass is measured
before and after the coating with chrome and after the resist
coating. A flatness map is generated for each side together with
other auxiliary information such as the exact thickness and the
Young's modulus of the glass material. Each glass plate has a
serial number engraved at the perimeter of the chrome surface
before the chrome coating so that the identity of the plate can be
tracked through-out it's lifetime. The serial number is engraved in
the glass surface or chrome film in clear text and machine-readable
format, e.g. by laser ablation. It is also possible to laser
engrave an identity mark inside the volume of the glass or use
other marking methods such as magnetic recording in the chrome
coating or embedding of a memory devise in the glass plate.
Identification of semiconductor wafers by an engraved marking is
standard in the semiconductor business, and in the invention the
same would be applied to mask blanks. Any other secure
identification system can be used, for example storing and shipping
the mask blanks in marked and bar-code labelled boxes.
[0060] The mask blank maker stores the flatness data for each blank
on a computer and the data is published on a network accessible to
the mask makers at a later time, e.g. on an Internet server.
Alternatively the data could follow the blank, e.g. on a diskette
shipped with the blank or on the embedded memory device.
[0061] How to measure the flatness and other properties of the mask
blank is known in the art. Flatness is often measured by
interferometers made by for example the companies Moller-Wedel,
Zygo and Tropel. The resist thickness can be measured with
spectroscopic reflectometry, ellipsometry and other optical
methods. The chrome thickness can be measured by optical
transmission or inductive methods and the reflectivity of the
chrome layer by reflectometry. The exact thickness can be measured
with mechanical or optical methods.
[0062] When a mask or reticle is ordered by a semiconductor company
the specification includes the serial number of the stepper for
which the mask is written. The clamping system of each stepper is
characterised by a file of geometrical data describing the method
of clamping, but also and most important the individual
imperfections of the clamping system. This information is stored on
a network accessible to the mask maker or sent to the mask maker
together with the order. The mask maker can also pull information
about the process, e.g. uniformity data for a etching step from the
semiconductor manufacturers computer. Alternatively the data can be
attached to the order document as embedded data or as separate
documents.
[0063] The mask maker who typically have several mask writers of
different kind have a similar database of data for his writing
systems and processes. He also has empirical data of how the mask
is distorted by the application of a pellicle, the stretched film
that acts as a dust protection on the finished mask.
[0064] When the chrome and resist were applied at the mask blank
manufacturer the blank bent due to stress built into the films.
When the resist is removed during processing in the mask shop the
stress from the resist disappears and the blank goes back to the
state it was before the resist was applied. More important is that
when a pattern is formed by partial removal of the chrome the
stress from the chrome film is partially relieved. With knowledge
of the bending caused by the application of the chrome film during
manufacture and the pattern to be written it is possible to predict
the deformation of the plate by patterning of the chrome.
[0065] During the planning of the writing job the mask maker
fetches the applicable information for the stepper or exposure
station in which the mask will be used, for the maskwriting
equipment, for the blank and for the pattern. Using the error model
a total combined error can be computed and corrected for,
optionally using the MEEF factor or a similar function as a
transfer function from correction to result in the finished
pattern. In another application of the invention the collected
information can be used to select among mask blanks, writing
systems and processes. A simple example is the selection of blanks
for uniformity for patterns that occupy different areas on the
mask. Of course, the principle of the invention is the same whether
the end result ("the output") is a chip, a display panel or just
the mask.
[0066] This contains geometrical information about the clamping
geometry, imperfections of the clamping structure and also other
known image distorsion created by the tool and subsequent
processing.
[0067] The Maskwriter
[0068] In a preferred embodiment the mask writer has several
provisions for correction of distortions caused by the processes
chain from patterning to use. First it has a precision stage
controlled by laser interferometers with a adjustable scales in x
and y, corresponding to uniform shrinkage or expansion plus uniform
bending across each axis, such as may result from built-in film
stress. Secondly it has adjustments for orthogonality and
trapezoidal distorsion, by modification of the driving of the
servos and by software. For higher order errors such as barrel
distorsion, mirror bow and irregularity of the coordinate system,
the maskwriting machine has an xy correction map that sends
position-dependent offsets to the servo systems. The information
collection and error prediction system sends a uniform scale and an
xy map to the maskwriter for each mask to be written, said map
being the pattern correction necessary to correct for all known
errors. These of course include the maskwriter's own stage
errors.
[0069] Furthermore the maskwriter has a clamping structure that is
adapted to the particular type of mask blank to be written. In one
embodiment the plate is placed on three supports so that the
deformation of the plate in the maskwriter is independent of the
plate shape and can be computed easily. For the most accurate
correction the supports are placed identically to similar supports
in the equipment using the mask, e.g. near two adjacent corners and
at the center of the opposite side.
[0070] In many cases it is not possible to support the mask plate
at three points. Especially for larger-size masks it is necessary
to have more than three supports. In this case it is impossible to
hold a mask without introducing bending moments, if not both the
support structure and the plate are perfectly flat. In one
embodiment the forces exerted on the supports from an ideal
plate/support structure combination are derived theoretically and
the geometry of the supports is adjusted until the force on each
support matches the theoretical force. In another embodiment the
non-flatness of the plate is known beforehand and the geometry of
the clamping structure is modified so as to minimise bending
forces. Other deformations such as that from gravitational sag and
sub-sequent process distortion are computed and corrected for.
[0071] In another embodiment the support structure is not
adjustable but geometrically characterised, so that it is possible
to compute the bending of a particular plate resulting from the
combination of plate non-flatness and clamping geometry.
[0072] In another embodiment the maskwriting system itself is
adapted to measure parameters that are needed for the correction,
such as the flatness of the substrate. If the flatness is measured
after clamping in the maskwriter, the measured data can be used in
several ways. First it can be used to check the model of the
clamping structure. Secondly it can replace the detailed knowledge
of the stage flatness, since the exact form and the clamping
deformation can be calculated from the surface flatness combined
with flatness data of the substrate. In the case that there is no
flatness data available and the plate is known to be flatter than
the stage and/or the gravitational sag, the measured deviation from
flatness indicates a real deformation and can be used for an
approximate correction.
[0073] A measurement of other parameters such as resist thickness
can also be integrated with the maskwriter, to provide information
necessary for the correction independent of the mask blank maker or
to be used with in-house spun plates.
[0074] The invention can be used in different types of maskwriters
using scanning laser beams, spatial light modulators or particle
beams.
[0075] Exposure Station
[0076] Like the maskwriting equipment the exposure station or
stepper that is going to use the mask to print the pattern on a
workpiece has a number of errors, such as image distorsion in the
exposure step and warpage in the subsequent processes. In a
preferred embodiment the mask is supported by three supports and
the clamping gives no additional bending forces beside the
gravitational sag. In another preferred embodiment other design
constraints makes it necessary to clamp it kinematically
over-restrained, i.e with more than three support points. The
pattern distorsion due to the bending forces resulting from
imperfect geometry can be cancelled by modification of the clamping
structure as in the maskwriter or by pre-diction and precorrection
of the mask.
[0077] The geometry and the errors of the exposure station are
characterised and stored as a machine parameter file, which
contains enough information to compute the errors of a real
physical mask and how it is printed. The file may, apart from the
identity and bookkeeping information, contain, the number of
supports, their xyz coordinates and compliances and optional
springloading, further distorsions of the machine's coordninate
system etc.
[0078] It is also valuable to have a process distorsion file which
contains an error map for how an exposed workpiece is distorted by
subsequent processing. An example is a glass panel for a TFT-LCD
display that may shrink by several tens of ppm in high-temperature
steps.
[0079] Metrology Tools
[0080] The masks and the exposed workpieces are measured in
coordinate measuring system, such as those commercially available
from the companies Leica and Nikon. The metrology tools also have a
clamping geometry and built-in errors that can be described in a
machine geometry file. Even though the metrology tool does not
impose its errors directly to the mask, it does so indirectly by
being the reference against which all other systems are
calibrated.
[0081] Computing the Bending Forces
[0082] A mask blank has a simple geometry and is made from
high-quality quarts or glass. Therefore, it can be represented by a
simple finite element model as is well known to a person skilled in
mechanical design. All errors are small compared to the size of the
plate and the resulting errors can be computed by linear
superposition of distorsions from different sources, e.g.
gravitational sag and bending due to clamping are additive. This
allows for simplified methods of analysing the elastic glass plate
by decomposition of different bending modes. This is advantageous
for real-time correction on an embedded computer in the
mask-writer, but it is equally possible to run a full finite
element simulation on an embedded computer with adequate memory and
power, or at an offline workstation.
[0083] Given a machine parameter file, with the geometry of the
supports and the orientation of the force of gravity relative to
the plate, and a plate parameter file with the geometry, flatness
and elastic properties of the plate, the computer can use the
finite element model to find the shape of the plate, the distorsion
of a pattern on its surface, and the contact forces at the support
points.
[0084] If there are many support points it is not known beforehand
that a particular non-flat plate really makes contact to all
supports. It is, however, possible to find which points make
contact. In principle the solution has to be self-consistent. The
plate makes contact to a support point if there is a positive
contact force, including the effect of possible spring-loading. By
this method it is also possible to find the approximate area of
contact between the mask blank and a flat surface, e.g. a flat
stage top, by representing the flat surface by an array of contact
points.
[0085] Because of the linearity of the elastic plate and the simple
geometry it is also possible to derive a simplified set of
simultaneous linear equations describing the deformation of the
workpiece due to forces at a number of predefined positions on the
workpiece. Other positions can be treated by interpolation between
the basic computational points. The system of equation is solved
for the geometrical constraints given in the geometry files and the
bending forces are derived. These are added to the gravitational
sag that depends only on where the support points are placed.
[0086] Correction of Other Mask Blank Related Errors
[0087] Other properties of the mask blank affect the quality of the
image, such as the resist thickness, the chrome thickness and the
reflectivity of the chrome. These can be measured in the same way
as the flatness or in fact in the same equipment.
[0088] Implementation of the Correction
[0089] Once the errors are predicted and the appropriate
corrections are computed there are two principally different
methods to apply them. Either the pattern data is modified, e.g.
the corner points of each feature are moved, or else the
corrections are fed to the writing hardware, such as position
servos for placement and modulator or light source for intensity
control. The former method is more general and can in principle
give arbitrary large corrections. It is also the only possible
alternative for shape corrections involving serifs and similar
features. For small and slowly varying corrections the second
method gives a smoother correction, since it has the resolution of
the interferometer while modification of the data only has the
resolution of the address grid. A second advantage is that the data
can be prepared offline without knowledge of the mask blank and
machine specifics and the corrections applied at the time of
writing. But for very dense patterns the data preparation has to be
done in real time anyway, so there is little difference in
logistics whether the correction is applied to the pattern or to
the writing hardware. Correcting the pattern is perfectly flexible:
corrections can be applied for placement, size and shape and there
is no limit to the size or complexity of the corrections.
[0090] Real-Time Pattern Correction
[0091] With a real-time data path with high processing capacity it
is possible and advantageous to apply corrections for beam size or
resolution effects and stray exposure together with all other
corrections at the time of writing. In a typical implementation
there is a bank of parallel processors, possibly organised in
groups doing different steps of the data preparation, with up to
several hundred CPUs and ASIC:s. The slowly varying corrections,
e.g. for clamping distorsion, will not effect the work load or data
flow appreciably, but corrections for resolution, beam size and
stray effects generate immense amounts of extra data and require
processing capacity to match the data flow. However, there is an
big advantage to real-time correction and for exactly this reason.
The data volumes are difficult to handle with off-line correaction.
In a commercial mask shop where a maskwriter is running essentially
24 hours a day, there is also no cost benefit of doing the
correction off-line, since an equally powerful computer is needed
to furnish corrected data to the maskwriter at full writing
capacity.
[0092] The general concept of pattern correction to compensate for
the limited resolution is known in the prior art. In the invention
the correction is done in real time as a part of the real-time
processing. For real-time correaction of resolution, beam size and
stray effects the algorithm is preferably running on the
maskwriter's embedded processor bank. The control system supplies
only the parameters for the interaction between the image formation
and the pattern, e.g. a series of superposed gaussian profiles
representing the point-spread function or the beam of the writer
and exposure system, or a system of rules for the correction. A
typical rule is to add a 0.16 .quadrature.m serif to all outer
corners in the pattern.
[0093] Correction of the imaging properties of the maskwriter is
preferably done transparently, with parameters that are fitted to
that particular maskwriter and process but otherwise not changing.
Once the parameters are set up the user will not need to care, or
even know, about the correction, but will only see a mask pattern
with a more accurate representation of the input data.
[0094] Pattern correction for the exposure station is preferably
done in collaboration between the mask shop and the chip or panel
manufacturer. The manufacturer prints some test patterns that are
designed for extraction of the model parameters. Once the model
parameters are set the correction could be transparent and
automatically applied to all designs. The information system of the
invention pulls the parameters together with clamping data and
image distorsion when the writing job is set up. Or alternatively
the manufacturer would explicitly provide another set of correction
parameters, or correct the data directly using his own correction
model. However, we believe that the design of image correction
models, design of test patterns and extraction software will be the
mission of specialist consulting companies, and that the
manufacturer will use the system in the transparent mode. The
information system of the invention provides the framework, which
makes such transparent operation possible and convenient.
[0095] FIG. 9 shows a typical implementation of an error correction
system suitable for the invention using both pattern modification
and writing hardware control. The writing hardware 901 prints a
pattern 902 on the mask blank 903 using a low-level representation
of the pattern 904, e.g. a bitmap or a decomposition into small
area elementary forms such as trapezoids. The low-level format is
created from a geometrical database with a high-level input 905,
containing a geometrical description of the pattern, such as a list
of filled polygons. Since the input format can contain the
geometrical features in any order and they can have any shape they
are pre-processed into an intermediate format 906 after having been
distributed 907 from the external file interface 913 on a number of
parallel processors 908. For each conversion step the data volume
expands and the necessary processing power increases. Therefore the
final processing needs more parallel processing units 909 than the
preprocessing. The error correction system has a control unit that
predicts the errors from the collected error data and computes an
appropriate pre-compensation based on its set of models and rules.
The correction of small slowly varying size and placement errors is
sent 912 to the writing hardware, especially to the dose control
and the position servos. Pattern corrections are sent in the form
the correction rules to the datapath interface 913 and run on the
embedded computer banks 908 and 909. Even the pattern corrections
can take different forms and several correction algorithms can be
run at different conversion steps. For example, correction for the
beam size effects in the mask generator is suitable to run at the
last stage of the conversion, while the larger serifs needed to
compensate for the imaging effects in the stepper are added to the
pattern during the preprocessing step.
[0096] For efficiency it is suitable for the different processors
to work on separate areas of the pattern, but to apply corrections
for non-local imaging effect the processor needs to know the
pattern in an area around the actual point it is working in.
Therefore the pattern is cut into partly overlapping computation
fields. The redundant information is only used for the correction
and discarded after use. For faster processing of non-local
information, e.g. for correction of stray exposure, a temporary
representation of the pattern at a lower resolution is created and
used to compute background exposure. It is possible to use more
than one low-resolution representation at a single time to
represent phenomena at different length scales.
[0097] In one embodiment the two processing steps are complemented
with a third step that runs on a separate bank of processors
essentially dedicated to pattern correction.
[0098] In a preferred embodiment of the invention two correaction
maps are computed, one for position and one size.
[0099] The position map has the form of a table with x and y
deviations given at a grid of points. For a given stage position
the corresponding s and y correction is computed by interpolation
in the correction map and the result is fed as an offset to the
position servos. The same method is used for finding the correction
of CD errors by interpolation in the CD map.
[0100] In a different embodiment the correction maps are computed
and made available to the datapath. During the conversion from
pattern database to hardware-driving signals both position and size
is corrected. In the preferred embodiment, which has a
stripe-organised writing strategy, it is done at the fracture step,
just before the pattern is divided into strips. The vortex points
of the pattern elements are simply moved according to the
interpolated correction maps. Most pattern elements have edges
parallel to the axises, and would after the correction have slanted
lines, but since the scale of the correactions is normally of the
order of a part per million or less, all but a few edges are still
parallel to the axises after their coordinates have been truncated
to the resolution of the data preparation software, e.g. 0.1 nm.
Before writing they are further truncated to the address grid of
the writer, e.g. 4 nm for semiconductor reticle at a specific
specification level. The written pattern will have the smooth
curves of the correction map snapping to the address grid, but with
an appropriately chosen address grid this will give a negligible
contribution to the error statistics.
[0101] The Information System
[0102] An information system is built to manage the lithography
errors in the chain from mask blank maker to user of the final
products. The hub in the information system is the mask maker where
during job planning all information is collected and used to
predict errors and pre-compensate them.
[0103] A convenient way to organise the information when several
parties are involved, e.g. mask blank makers, mask shops and mask
users, is to have each party maintain its own information and store
it on its own computer system. The computers are accessible from
the mask maker's computer by remote access over phone line, ISDN,
high-speed link or the Internet. In the latter case it is important
that the integrity of the information is validated, so that the
user of information knows it is complete, unaltered and issued by
the correct sender. The parties involved may also want to have it
confidential. All this can be assured in modern communication links
by appropriate use of passwords, encryption, check figures and
digital signatures.
[0104] Workflow
[0105] It is the responsibility of the mask shop to find the
necessary corrections that will make a particular reticle print
without systematic registration errors. The customer, i.e. the mask
user, provides one or more pattern files and the order document
requests any special treatment, metrology etc. In a preferred
embodiment of the invention the order document specifies the
exposure station where the mask will be used and optionally also a
process error description. During job planning and set-up the mask
maker's computer accesses the data from the mask user, data which
specifies the geometry of the exposure station, including known
imperfections, and image distorsions. Optionally the machine file
also includes a map of size errors created by the exposure step. If
a process file has been specified in the order document, it
specifies warping of the workpiece during subsequent processing and
optionally also size errors created by the process. For example a
plasma etching step is sensitive to the "loading", i.e. the local
density of pattern area exposed to the plasma, and etches
differently along the edges of a chip. Since this is a systematic
behaviour that is repeated more or less in every design it can be
partly corrected for in the mask using information about said
mask.
[0106] When the writing job is planned a mask blank is assigned to
the mask. The computer accesses the data storage at the mask blank
maker or a local storage with information files for the blanks that
are in stock. The information file fetched corresponds to the
particular mask blank that will be used for writing the mask, and
holds information about the exact size and thickness as well as
relevant physical material properties. It also contains flatness
data, resist thickness, chrome thickness and reflectivity and data
relating to the bending force created by built-in stress in the
chrome and resist. The scheme is, of course, independent of the
exact materials used, and other types of existing or future mask
blanks can be handled in a similar fashion.
[0107] Data File Formats
[0108] The data file formats are designed to be extendable by
having the data fields tagged with keywords. A new feature can be
included after definition of a new keyword and old data files will
still be compatible. For simplicity the data files in the preferred
embodiment are ASCII character files. This allows for simple
debugging and files can be modified or created in any text editor
or spread-sheet program. To avoid the risk of inadvertent change of
a file it is locked by a checksum and validated by a digital
signature. For machine-generated files these are generated
automatically, but hand edited files need to get the checksum and
signature added by a special validation program. This gives a
reasonable trade-off between security and flexible engineering and
debugging. For encryption any commercial encryption program can be
used.
Example of a System Using an Embodiment of the Inventive Method
[0109] A system for producing large-area displays with distortion
control according to an embodiment of the invention should now be
described more thoroughly.
[0110] Referring to FIG. 10, an embodiment of the system for
producing large area display panels according to the invention is
shown. The system could be used for producing shadow masks for
conventional CRT (Cathode Ray Tube) displays, but is especially
useful for producing TFT (Thin Film Transistor), CF (Color Filter),
PDP (Plasma Display Panel) or PALC (Plasma-addressed liquid
crystal) displays.
[0111] The system comprises a first mask producing means 1001 for
producing a mask with a predetermined pattern according to input
data. The mask producing means is preferably a microlithographic
writing device for writing with high precision on photosensitive
substrates. The term writing should be understood in a broad sense,
meaning exposure of photoresist and photographic emulsion, but also
the action of light on other light sensitive media such as
dry-process paper, by ablation or chemical processes activated by
light or heat. Light is not limited to mean visible light, but a
wide range of wavelengths from infrared to extreme UV. Such a mask
producing apparatus is previously known from e.g. EP 0 467 076 by
the same applicant. In general the apparatus comprises a light
source, such as a laser, a first lens to contract the light beams,
a modulator to produce the desired pattern to be written, the
modulator being controlled according to input data, a reflecting
mirror to direct the beams towards the substrate, and a lens to
contract the beams before it reaches the substrate. The mirror is
used for the scanning operation to sweep the beam along scan lines
on the substrate. Instead of a mirror, other scanning means may be
used, such as a rotating polygon, rotating prism, rotating
hologram, an acousto-optic deflector, an electro-optic deflector, a
galvanometer or any similar device. It is also possible to use
raster scanning or spatial light modulators. Further, the substrate
is preferably arranged on an object table which has a motion in two
orthogonal directions relative to the optical writing system, by
means of two electrical servo motors.
[0112] The system according to the invention further comprises
microlithographic exposing means 1002 for exposing a photosensitive
panel substrate with light and with use of the mask to impose the
pattern of the mask on the substrate, whereby said substrate has a
layer being sensitive to said light. Several such exposing means
are also previously known in the art. The exposing means could be
of the contact copy type, proximity exposure type, or a projection
aligner. The system according to the invention could also be used
in a direct writer, whereby the compensation is not made in a
physical mask, but in a data mapping controlling the writing beam.
For TFT and CF display panels projection aligners are usually used,
and for PDP and PALC the contact or proximity type are frequently
used.
[0113] Furthermore, the system comprises measuring means 1003 for
measuring the pattern on the substrate and detecting deviations
relative to the intended pattern as given by the input data. This
could be done by measuring the geometrical position of the pattern,
preferably at some reference positions, to get a so called
registration mapping, and compare it with the intended pattern
which is deducible from the input data. Further, the width of lines
in the pattern, the so called CD (Critical dimension), could be
measured. Measuring equipment is commercially available, and for
example the equipment could comprise a CCD-camera or be based on
interferometry.
[0114] From the measuring means 1003 a distortion control signal is
sent to a second mask producing means 1004. This second mask
producing means could be a separate apparatus, but is preferably
the same as the first mask producing means 1001. This second mask
producing means is fed with input data describing the intended mask
pattern to be written, and is also fed with the distortion control
signal from the measuring means 1003, whereby the writing process
for producing the second mask is controlled to modify the pattern
to compensate for the measured deviations, and thus compensate for
production distortions. The measurement is preferably made after
the subsequent processing steps of the panel as well, i.e. the
development, blasting and/or etching, whereby systematic errors
from theses processes are taken care of in the compensation as
well.
[0115] The compensation in the mask writer could be accomplished in
different ways. In a writer of the type described above, with an
object table continuously moving in a slow strip direction and a
scanner sweeping in a fast scanning direction, the compensation
could be made according to a surface mapping. According to this
mapping the compensation in the scanning direction could be
accomplished by e.g. offsetting the starting time of the beam
during the scanning. In the stripe direction the compensation could
also be made by time offsets, either directly or indirectly by
means of different ramp functions. There are also other possible
way to accomplish such compensation. For example the compensation
could be made by controlling the servo motors for the object table,
by adjustment of the time dependent angle of the scanners, by
changing the input data or by controlling an internal control unit
such as piezoelectrically controlled mirrors.
[0116] However, if a direct writer is used, the same type of
compensation could be made in real time.
[0117] Compensation for deviations in the line width, CD, could be
accomplished in the same way as deviations in the registration.
However, this compensation could also be made by changes in the
power of the writing beam, i.e. the exposing dose, by changing the
laser output or having an analog modulator. This compensation could
be accomplished by means of a herefore adapted dose mapping to
control the dose.
[0118] When the second mask is used in the same exposing means 1002
all systematic errors depending on different temperature
conditions, errors in the exposing means etc., are compensated for,
and the pattern precision of the produced display panels are
greatly improved.
[0119] The first mask could either contain the same pattern as the
intended pattern for the second mask, i.e. the pattern not being
compensated, or contain a reference pattern, intended for deviation
measure only.
[0120] Further, error data could be accumulated, and a rolling
means value could be used for the compensation. The error
compensation could also be a combination of several different part
error compensations. Those part compensations could be based on the
premises for the process, e.g. which stepper and type of glass that
is being used. Hereby the total error compensation could be a
combination of one or several error compensations for each process
step.
[0121] Above, a system for passive distortion control has been
described. In this system compensation is made for the processes
and equipment being used in the system. However, the compensation
is not adapted for different panel substrates. In this passive
system a measurement to alter the distortion compensation is
preferably made once for every new batch of substrates, and
thereafter the same mask is used for producing all the panels in
the batch. This passive distortion control is specifically useful
for production of TFT or CF displays. The requested precision for
the patterns on the mask for this production is extremely high, and
the masks are very difficult, and thereby expensive, to
manufacture. On the other hand the masks last for a long time in
this production.
[0122] The system according to the invention could also comprise
second measuring means 1004 for measuring the thickness of the
light sensitive layer on the substrate prior to the exposure,
whereby said measurement is also used for said compensation. Hereby
the compensation is adapted for varying resist layers between
different batches of substrates. Such batch wise compensation could
also be accomplished with use of data specified by the
manufacturer.
[0123] This second measuring means 1004 could also be used for
measuring each and every panel substrate that is going to be
exposed, and thereafter adapt the process for each individual
panel. Hereby the system could compensate for varying glass quality
in different panels, varying thickness and quality of the resist or
emulsion of the substrate area, different form variations etc. This
active distortion control is especially useful for production of
PDP or PALC display panels, where the masks are comparably easy and
inexpensive to produce. This method could also be used for direct
writers.
[0124] In the active distortion control the panel is initially
measured, regarding e.g. resist thickness. Many such measuring
methods are available for someone skilled in the art, e.g. a test
exposure, dosimetry, of the substrate with different doses, by
profilometry, interferometry, confocal microscopy, by an
interferometric method or the like. The shape of the substrate
could also be initially measured, and this could be accomplished by
known methods such as moire interferometry, projected fringes,
laser triangulation, ordinary interferometry etc. Preferably
already existing patterns are also initially measured, whereas such
exists. Display panels are usually exposed in several separate
steps, typically 3-7 exposing steps, and normally the same exposing
station is used for all the exposures. By writing masks with
compensation for individual errors in different station the display
producer could schedule the production more freely, independent of
which stations that is used. This is of great importance for making
the production more efficient and the utilisation of the stations
better.
[0125] Referring now to FIG. 11, a method for producing large area
display panels according to the invention, and with use of the
above-mentioned system will be described.
[0126] The method according to the invention comprises a first step
S1 in which a mask with a predetermined pattern according to input
data is produced. Thereafter the mask is used for
microlithographically exposing a photosensitive substrate with
light to impose the pattern of the mask on the substrate, whereby
said substrate has a layer being sensitive to said light, in step
S2. The exposed pattern is then measured, possibly after several
subsequent processing steps, or even in the finished product, in
S3, to detect deviations of the exposed pattern relative to the
intended pattern as given by the input data. In step S4 a
distortion control mapping is then produced, to be used in step S5
during production of a second mask having a pattern according to
input data and modified to diminish the measured deviations, and
thus to compensate for production distortions. In the last step S6
the second modified mask is then used in a photolithographic
fabrication of display panels. Similar compensation may be used in
a direct writer, where the compensation could be made in a data
mapping.
[0127] Dose Compensation
[0128] CD uniformity may be an important specification for photo
masks. A final result may be a combination of pattern generator
equipment uniformity and process equipment uniformity.
[0129] After exposure of a mask in a pattern generator the resist
covered plate may be processed in different process systems, e.g.,
hot plate, development equipment for resist, and etching equipment
for chrome. The process equipment steps may introduce systematic CD
variations on the plate, e.g., cylindrical symmetry, due to the
physical design of process equipment.
[0130] If the CD contribution from process equipment is stable over
time the overall result of CD uniformity on the final mask may be
improved by introducing a CD correaction of opposite sign, (process
compensation), in the pattern generator while printing the
plate.
[0131] As described above in detail above, process compensation in
a pattern generator may be generated directly in the data path used
for pattern generation by modifying the printed pattern itself. As
also described above, alternatively, process compensation in a
pattern generator may be generated indirectly by changing other
global parameters as, e.g., exposure dose while printing the plate.
For example, CD may be a function of dose.
[0132] Writing Principle, SLM Writer
[0133] An SLM writer may operate as a DUV stepper with a
programmable mask. Hence, a full image may be generated as series
of 2D projected SLM stamps, where each SLM stamp may be printed
with a separate laser flash of short duration. Multi pass printing
including offset between passes may be used that substantially
reduce systematic CD errors over the SLM stamp.
[0134] Global CD, may be corrected with a dose system, using a dose
map, as each individual SLM stamp is small compared to the full 6''
plate.
[0135] As shown in FIG. 13, a dose map as a function of xy-position
on the mask may be generated, e.g., by using the gray levels of SLM
or by using a pre-programmed global dose profile utilizing the dose
control in the laser. During the exposure cycle the x-stage may
move at constant speed. As shown in FIG. 13, one or more
interferometers may generate trigger pulses at correct positions
and act as a master trigger in the system.
[0136] Both the data path/SLM and the laser may be controlled by
this master trigger. Process compensation using a dose map(x,y) may
be implemented using the SLM or laser dose control.
[0137] FIG. 14a illustrates an example of a (2D) cylindrical CD
distribution according to an embodiment of the invention. As shown
in FIG. 14a, no dose compensation is provided.
[0138] FIG. 14b illustrates an example of cylindrical CD
distribution. As shown in FIG. 14b, dose compensation is provided
and CD is compensated.
[0139] FIG. 14c illustrates an example of cylindrical CD
distribution using global dose compensation. As shown in FIG. 14b,
dose compensation is provided and CD is compensated.
[0140] FIG. 14d illustrates an example of a pre-programmed global
dose map implementable using gray levels of the SLM or by utilizing
dose control in the laser.
[0141] As set forth above, example embodiments of the present
invention may improve precision by predicting and/or measuring
errors and correcting the errors. As also set forth above, example
embodiments of the present invention may improve precision by
compensating directly in the data path used for pattern generation
and/or compensating indirectly by changing other global parameters,
such as, e.g., exposure dose. As also set forth above, example
embodiments of the present invention may improve precision by
maintaining correction information in a correction map or
correction profile.
[0142] The invention shall not be limited to the embodiments
described above, which are only examples of implementation. For
example what is said above about chrome-on-glass masks is equally
applicable to masks made from other materials, e.g. with coatings
of other metals, iron oxide, diamond-like carbon, multilayer
coatings etc., and with other substrate materials such as calcium
fluoride, Zerodur, silicon etc, as well as for both transmissive
and reflecting masks and masks which work by scattering. It is also
applicable to stencil masks and membrane masks for particle-beam
and x-ray lithography.
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