U.S. patent application number 10/990335 was filed with the patent office on 2006-05-18 for alignment strategy optimization method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Bart Swinnen, Franciscus Bernardus Maria Van Bilsen, Roy Werkman.
Application Number | 20060103822 10/990335 |
Document ID | / |
Family ID | 36272307 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060103822 |
Kind Code |
A1 |
Werkman; Roy ; et
al. |
May 18, 2006 |
ALIGNMENT STRATEGY OPTIMIZATION METHOD
Abstract
The invention relates to a method of optimizing an alignment
strategy for processing batches of substrates in a lithographic
projection apparatus. First, all substrates in a plurality of
batches of substrates in the lithographic projection apparatus are
sequentially aligned and exposed using a predefined alignment
strategy. Then, alignment data is determined for each substrate in
the plurality of batches of substrates. Next, at least one
substrate in each batch of substrates is selected to render a set
of selected substrates comprising at least one substrate in each
batch. In a metrology tool, overlay data for each of the selected
substrates is determined. Then, overlay indicator values for a
predefined overlay indicator are calculated for the predefined
alignment strategy and for other possible alignment strategies. In
this calculation, the alignment data and the overlay data of the
selected substrates is used. Finally, an optimal alignment strategy
is determined, the strategy being defined as alignment strategy
among the predefined alignment strategy and the other possible
alignment strategies with a lowest overlay indicator value.
Inventors: |
Werkman; Roy; (Waalre,
NL) ; Van Bilsen; Franciscus Bernardus Maria;
(Eindhoven, NL) ; Swinnen; Bart; (Holsbeek,
BE) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
IMEC
Leuven
BE
|
Family ID: |
36272307 |
Appl. No.: |
10/990335 |
Filed: |
November 17, 2004 |
Current U.S.
Class: |
355/55 ;
355/53 |
Current CPC
Class: |
G03F 9/7046 20130101;
G03F 7/70633 20130101 |
Class at
Publication: |
355/055 ;
355/053 |
International
Class: |
G03B 27/52 20060101
G03B027/52 |
Claims
1. A method of optimizing an alignment strategy for processing
batches of substrates in a lithographic projection apparatus, said
method comprising: sequentially aligning and exposing substrates in
a plurality of batches of substrates in accordance with a
predetermined alignment strategy; determining alignment data for
each substrate in said plurality of batches of substrates;
selecting at least one substrate from each batch of substrates to
render a set of selected substrates; determining overlay data for
each of said selected substrates; calculating overlay indicator
values of a predefined overlay indicator for said predetermined
alignment strategy and for additional alignment strategies based on
said alignment data and said overlay data of said set of selected
substrates; and determining an optimal alignment strategy from said
predetermined alignment strategy and additional alignment
strategies based on a lowest overlay indicator value.
2. The method of claim 1, wherein determining overlay data
comprises: measuring position errors for a plurality of overlay
targets present on each of said selected substrates to produce
measured overlay data; and determining said overlay data by
applying a least squares model to said measured overlay data.
3. The method of claim 1, wherein said calculating of said overlay
indicator values comprises: calculating, for all selected
substrates, derived overlay data using said overlay data
corresponding to said predetermined alignment strategy and said
alignment data corresponding to said additional alignment
strategies; calculating said overlay indicator values for said
additional alignment strategies using said derived overlay
data.
4. The method of claim 1, wherein the at least one predetermined
alignment strategy comprises one or more of: (a) using a predefined
alignment mark type, (b) using a predefined number of alignment
marks on a substrate, (c) using predefined positions of alignment
marks on a substrate, (d) using a measurement beam with a
predefined wavelength in an alignment system or combination of
wavelengths, (e) using a predefined diffraction order in said
alignment system or combination of diffraction orders, (f) using a
predefined number of wafer model parameters, (g) using a predefined
position detection algorithm, or (h) using a predefined alignment
system.
5. The method of claim 1, wherein said alignment data and said
overlay data are represented by wafer model parameters.
6. The method of claim 5, wherein more than one substrate in each
batch is selected to be measured and wherein said method further
comprises: calculating, for each selected batch, an average value
of all wafer model parameters over all substrates; subtracting for
each selected batch, and for each wafer model parameter, said
average value from said wafer model parameter values to render
corrected wafer model parameter values; and calculating the overlay
indicator using said corrected wafer model parameter values.
7. A system comprising: a lithographic projection apparatus
configured to align substrates in a plurality of batches of
substrates, transfer a pattern from a patterning device onto the
substrates and to produce alignment data in accordance with a
predetermined alignment strategy; a metrology tool arranged to
measure overlay of substrates selected from each batch; and a
processor arranged to receive overlay data of selected substrates
from said metrology tool and alignment data from said lithographic
projection apparatus, wherein said processor calculates overlay
indicator values of a predefined overlay indicator for said
predetermined alignment strategy and for additional alignment
strategies based on said alignment data and said overlay data and
determines an optimal alignment strategy from said predetermined
alignment strategy and additional alignment strategies based on a
lowest overlay indicator value.
8. A device manufacturing method comprising: conditioning a beam of
radiation; configuring the conditioned beam of radiation with a
desired pattern in its cross-section; sequentially aligning and
projecting the patterned beam of radiation onto substrates in a
plurality of batches in accordance with a predetermined alignment
strategy; determining alignment data for each of the substrates in
said plurality of batches of substrates; selecting at least one
substrate from each batch of substrates to render a set of selected
substrates; determining overlay data for each of said selected
substrates; calculating overlay indicator values of a predefined
overlay indicator for said predetermined alignment strategy and for
additional alignment strategies based on said alignment data and
said overlay data of said set of selected substrates; and
determining an optimal alignment strategy from said predetermined
alignment strategy and additional alignment strategies based on the
lowest overlay indicator value.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an alignment strategy
optimization method for finding an optimal alignment strategy to
process substrates in a lithographic manufacturing process.
[0003] 2. Description of the Related Art
[0004] A lithographic apparatus is a machine that applies a desired
pattern onto a target portion of a substrate. Lithographic
apparatus can be used, for example, in the manufacture of
integrated circuits (ICs). In that circumstance, a lithographic
patterning device, which is alternatively referred to as a "mask"
or "reticle," may be used to generate a circuit pattern
corresponding to an individual layer of the IC, and this pattern
can be imaged onto a target portion (e.g., comprising part of, one
or several dies) on a substrate (e.g., a silicon wafer) that has a
layer of radiation-sensitive material (i.e., resist).
[0005] In general, a single substrate will contain a network of
adjacent target portions that are successively exposed. Known
lithographic apparatus include so-called steppers, in which each
target portion is irradiated by exposing an entire pattern onto the
target portion in one go, while in so-called scanners, each target
portion is irradiated by scanning the pattern through the
projection beam in a given direction (the "scanning"-direction)
while synchronously scanning the substrate parallel or
anti-parallel to this direction.
[0006] During the manufacturing process, a number of patterned
layers may be created on a substrate. In order to create an
operating device or to provide optimal performance, it may be
desirable or even necessary for the patterns of layers positioned
on top of each other to be well aligned with respect to each other.
Such a condition may be accomplished by accurately positioning the
substrate with respect to the mask and the projection beam. In the
first place, it may be desirable or necessary for the substrate to
be in the focal plane of the patterned beam, in order to obtain a
sharp image of the patterning structure (a process also known as
"focus and leveling"). The direction associated with this distance
is called the Z-direction.
[0007] Secondly, it may be desirable or necessary to accurately set
the position of the substrate in the directions perpendicular to
the Z-direction, i.e. the X- and Y-direction, in order to position
the different layers correctly on top of each other (a process also
known as "aligning"). Accurate aligning is generally done by
accurately determining the position of the substrate relative to a
substrate table which holds the substrate and determining the
position of the substrate table with respect to the mask and
projection beam. Alignment may be done using an alignment system,
as described, for instance in U.S. Pat. No. 6,297,876, which is
incorporated herein by reference.
[0008] Alignment is performed using the alignment system which is
arranged to find the position of alignment markers on the
substrate. The performance of the alignment system is one of the
elements of a lithographic system that influences the overlay
accuracy to a large extent. During alignment, multiple marks on the
substrate are measured to obtain a coordinate system. Some advanced
IC processes alter the geometry of the alignment marks, which may
compromise the coordinate system. ASML's ATHENA.TM. Phase-Grating
Alignment system offers extensive operational flexibility to cope
with most advanced IC processes, because of its dual-wavelength
operation and its simultaneous detection of up to the seventh
diffraction order. A more extensive overview of the ATHENA
alignment sensor system and its basic operation is provide in F.
Bornebroek et al., "Overlay Performance in Advanced Processes",
Proc. SPIE Microlithography, Vol. 4000 (2000) pp. 520-531, which is
herein incorporated by reference. The ATHENA system offers great
flexibility in applying an optimal alignment strategy, see, e.g.,
P. Hinnen et al., "Advances in Process Overlay", Proc. SPIE
Microlithography, Vol. 4344 (2001) pp 114-125.
[0009] A diffraction pattern (e.g. as generated by an alignment
beam projected to an alignment mark) may comprise a number of
diffraction orders, and some number (for instance, seven) of the
diffraction orders may be measured. Each diffraction order
comprises positional information about the alignment mark. In many
cases, a position of the alignment mark can be determined based on
the determined position of a single diffraction order, but more
accurate results may be obtained when more diffraction orders are
taken into account.
[0010] The position of the substrate may be expressed by wafer
model parameters such as a translation T, a rotation R, and an
magnification M. The translation may be in the X-direction Tx
and/or in the Y-direction Ty. The rotation may be a rotation of the
x-axis about the z-axis Rzx and/or a rotation of the y-axis about
the z-axis Rzy. The magnification may in the X-, Mx, and/or in the
Y-direction. The wafer model parameters (Tx, Ty, Rzx, Rzy, Mx, My)
can be used to compute the position, magnification and/or
orientation of a substrate based on the measured positions of the
diffraction orders. The wafer model parameters can be used to find
the optimal alignment strategy.
[0011] Alignment strategies may consist of choice of mark type and
location as well as the choice of the diffraction order and
wavelength to be used. The appropriate selection procedure is
chosen depending on the environment (i.e. research or production).
For any alignment strategy, it is possible to automate the
wavelength selection during the lithographic process. For every
mark, the optimal wavelength is selected based on the signal
strength of each diffraction order.
[0012] Choosing the optimal strategy is important in obtaining
optimal overlay. Different procedures for selecting alignment
strategy have been developed to comply with different applications.
Depending on the application, either a comprehensive technique to
determine the ultimate strategy or a fast and adequate
strategy-optimization technique is recommended. These procedures
for selecting an alignment strategy calculate an `overlay
indicator` for every possible alignment strategy. The strategy with
the lowest indicator is recommended, since it corresponds to a
minimum process-induced overlay variance over a batch of
substrates. Overlay indicators can be insensitive to processing
effects that are constant over the batch, since process corrections
are used to correct for these effects during production.
[0013] To select an alignment strategy, multiple alignment mark
types are measured. Each measurement gives for example 14 positions
from seven diffraction orders at two wavelengths. Various
procedures for selecting an alignment strategy are known, such as,
for example, the Overlay Metrology Tool Feedback (OVFB) procedure.
For this procedure, multiple processed marks on multiple substrates
are measured in a process free coordinate system. The principle of
the overlay metrology tool feedback analysis is explained with
reference to FIGS. 1A, 1B, 1C, 1D. In this example, only the
Y-direction is discussed for reasons of simplicity.
[0014] FIG. 1A shows a substrate 1 comprising four alignment marks
2, 3, 4, 5 of a first mark type and four alignment marks 6, 7, 8, 9
of a second mark type. During production, the alignment marks 2, 3,
4, 5 are used for alignment when exposing the substrate 1. Vectors
in FIG. 1A indicate a relative position of the alignment marks 2,
3, 4, 5 with respect to an active (i.e. used) grid. In addition,
the alignment marks 6, 7, 8, 9 of the second mark-type are measured
as well. The substrate 1 is developed and overlay is measured on an
offline metrology tool.
[0015] FIG. 1B shows the substrate 1 and four overlay targets 10,
11, 12, 13 together with vectors indicating the measured overlay
error. A model is applied to the measured overlay values (i.e. the
length of the vectors) in order to determine the (in this example
only one) wafer model parameter(s) (Ty). The(se) wafer model
parameter(s) are used to calculate alignment errors of the
alignment marks 2, 3, 4, 5 used for exposure and of the alternative
alignment marks 6, 7, 8, 9, (see vectors in FIG. 1C).
[0016] Based on the alignment errors in FIG. 1C, a mark-type is
chosen. This choice can be made in different ways. For example, as
will be explained below, the value of an "Overlay Performance
Indicator" (OPI) may be used. Alternatively, the alignment errors
in FIG. 1C may be used to calculate the possible overlay when
switching alignment marks, and the mark-type which causes the
lowest overlay will then be chosen. In this example, a switch could
be made from using alignment marks 2, 3, 4, 5 of the first mark
type to the alignment marks 6, 7, 8, 9 of the second mark type. In
FIG. 1D, the overlay errors in that case are indicated by vectors,
which apparently are smaller than those in FIG. 1C.
[0017] The calculation of OPI will now be discussed. The first
action is to determine the wafer model parameters for a certain
alignment strategy. The batch averages are subtracted, because
process correction can compensate for these. The OPI is defined as
the mean plus 3 times the standard deviation of the maximum
expected overlay error of each substrate. The maximum expected
overlay error Max_err for a 4 parameter wafer model is given by:
Max_err= {square root over
(T.sub.x.sup.2+T.sub.y.sup.2)}+wafer_radius {square root over
(R.sup.2+M.sup.2)}, (1) with R=(Rzx+Rzy)/2, M=(Mx+My)/2
[0018] where Tx is a translation in the x-direction, Ty a
translation in the y-direction, Rzx a rotation of the x-axis about
the z-axis, Rzy a rotation of the y-axis about the z-axis, Mx an
magnification in the direction of the x-axis, My a magnification in
the direction of the y-axis, wafer_radius the radius of a
substrate.
[0019] Now, the OPI is given by:
OPI=<Max_err>+3.sigma.(Max_err) (2) where .sigma. is a
standard deviation over all substrates.
[0020] For a 6 parameter model, it is not possible to calculate the
OPI analytically. The 6 parameter OPI value has to be calculated
numerically. Once a value for the OPI for all strategies is
calculated, the alignment strategy with the lowest OPI is selected
since it is believed that that particular strategy results in
minimal overlay.
[0021] Nowadays, overlay indicator values are calculated on single
batches. Overlay indicators can be based on alignment or alignment
plus overlay data. The confidence level of the indicators based on
both data sources is significantly higher than the indicator based
on alignment data only. Based on the values of the indicators for
different alignment strategies, a decision is made on which
strategy to use to expose future batches.
[0022] Alignment strategy optimization is done by calculating the
values of overlay indicators on single batches that need to be
fully measured on an offline metrology tool in order to get high
confidence values. During regular operations, however, only a few
substrates out of a batch are measured, therefore the high
confidence level indicators can not be used and the alternative
indicator is used which has a very low confidence level. Measuring
of extra substrates on the offline metrology tool costs extra
effort and time.
SUMMARY OF THE INVENTION
[0023] It is desirable to get overlay indicators with high
confidence values without the need for measuring complete batches
on an offline metrology tool.
[0024] As such, the principles of the present invention, as
embodied and broadly described herein, provide a method of
optimizing an alignment strategy for processing batches of
substrates in a lithographic projection apparatus. In one
embodiment, the method includes sequentially aligning and exposing
substrates in a plurality of batches of substrates in accordance
with a predetermined alignment strategy, determining alignment data
for each substrate in the plurality of batches of substrates,
selecting at least one substrate from each batch of substrates to
render a set of selected substrates, and determining overlay data
for each of the selected substrates. The method further includes
calculating overlay indicator values of a predefined overlay
indicator for the predetermined alignment strategy and for
additional alignment strategies based on the alignment data and the
overlay data of the set of selected substrates, and determining an
optimal alignment strategy from the predetermined alignment
strategy and additional alignment strategies based on the lowest
overlay indicator value.
[0025] In accordance with another embodiment, there is provided a
system that optimizes an alignment strategy for processing batches
of substrates. The system includes a lithographic projection
apparatus configured to align substrates in a plurality of batches
of substrates, transfer a pattern from a patterning device onto the
substrates and to produce alignment data in accordance with a
predetermined alignment strategy, a metrology tool arranged to
measure overlay of substrates selected from each batch and a
processor arranged to receive overlay data of selected substrates
from the metrology tool and alignment data from the lithographic
projection apparatus. The processor calculates overlay indicator
values of a predefined overlay indicator for the predetermined
alignment strategy and for additional alignment strategies based on
the alignment data and the overlay data and determines an optimal
alignment strategy from the predetermined alignment strategy and
additional alignment strategies based on the lowest overlay
indicator value.
[0026] In yet a further embodiment, there is provided a device
manufacturing method that operates to optimize an alignment
strategy for processing batches of substrates in a lithographic
projection apparatus. The device manufacturing method includes
conditioning a beam of radiation, configuring the conditioned beam
of radiation with a desired pattern in its cross-section,
sequentially aligning and projecting the patterned beam of
radiation onto substrates in a plurality of batches in accordance
with a predetermined alignment strategy, and determining alignment
data for each of the substrates in the plurality of batches of
substrates. The method also includes selecting at least one
substrate from each batch of substrates to render a set of selected
substrates, determining overlay data for each of the selected
substrates, calculating overlay indicator values of a predefined
overlay indicator for the predetermined alignment strategy and for
additional alignment strategies based on the alignment data and the
overlay data of the set of selected substrates, and determining an
optimal alignment strategy from the predetermined alignment
strategy and additional alignment strategies based on the lowest
overlay indicator value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0028] FIGS. 1A-1D show the principle of the off-line overlay
metrology tool feedback analysis;
[0029] FIG. 2A depicts a lithographic apparatus as used in a system
according to an aspect of the invention;
[0030] FIG. 2B depicts a system according to an aspect of the
invention;
[0031] FIG. 3 shows an example of alignment data for a batch of
substrates;
[0032] FIG. 4 shows an example of overlay data related to the batch
of which the alignment data is shown in FIG. 3;
[0033] FIG. 5 schematically shows the composition of an artificial
batch;
[0034] FIG. 6 shows an example of alignment and overlay data of
substrates in 3 batches;
[0035] FIG. 7 shows an example of alignment and overlay data of the
substrates in an artificial batch;
[0036] FIG. 8 shows a simple example on how to calculate an overlay
indicator, and
[0037] FIG. 9 is a flow chart showing an embodiment of a
manufacturing method according to the invention.
DETAILED DESCRIPTION
[0038] FIG. 2A schematically depicts a lithographic apparatus as
may be used in a system according to an aspect of the invention.
The apparatus comprises: [0039] an illumination system
(illuminator) IL: for providing a projection beam PB of radiation
(e.g., UV or EUV radiation). [0040] a first support structure
(e.g., a mask table/holder) MT: for supporting patterning device
(e.g., a mask) MA and coupled to first positioning mechanism PM for
accurately positioning the patterning device with respect to item
PL; [0041] a substrate table (e.g., a wafer table/holder) WT: for
holding a substrate (e.g., a resist-coated wafer) W and coupled to
second positioning mechanism PW for accurately positioning the
substrate with respect to item PL; and [0042] a projection system
(e.g., a reflective projection lens) PL: for imaging a pattern
imparted to the projection beam PB by patterning device MA onto a
target portion C (e.g., comprising one or more dies) of the
substrate W.
[0043] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
[0044] The support structure supports, i.e. bears the weight of,
the patterning device. It holds the patterning device in a manner
that depends on the orientation of the patterning device, the
design of the lithographic apparatus, and other conditions, such
as, for example whether or not the patterning device is held in a
vacuum environment. The support structure can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure may be a frame or a table,
for example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
[0045] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0046] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0047] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0048] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0049] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more mask tables).
In such "multiple stage" machines the additional tables may be used
in parallel, or preparatory actions may be carried out on one or
more tables while one or more other tables are being used for
exposure.
[0050] The lithographic apparatus may also be of a type wherein at
least a portion of the substrate may be covered by a liquid having
a relatively high refractive index, e.g. water, so as to fill a
space between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
[0051] Referring to FIG. 2A, the illuminator IL receives a
radiation beam from a radiation source SO. The source and the
lithographic apparatus may be separate entities, for example when
the source is an excimer laser. In such cases, the source is not
considered to form part of the lithographic apparatus and the
radiation beam is passed from the source SO to the illuminator IL
with the aid of a beam delivery system BD comprising, for example,
suitable directing mirrors and/or a beam expander. In other cases
the source may be an integral part of the lithographic apparatus,
for example when the source is a mercury lamp. The source SO and
the illuminator IL, together with the beam delivery system BD if
required, may be referred to as a radiation system.
[0052] The illuminator IL may comprise an adjuster AD for adjusting
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0053] The radiation beam B is incident on the patterning device
(e.g., mask MA), which is held on the support structure (e.g., mask
table MT), and is patterned by the patterning device. Having
traversed the mask MA, the radiation beam B passes through the
projection system PS, which focuses the beam onto a target portion
C of the substrate W. With the aid of the second positioner PW and
position sensor IF (e.g. an interferometric device, linear encoder
or capacitive sensor), the substrate table WT can be moved
accurately, e.g. so as to position different target portions C in
the path of the radiation beam B. Similarly, the first positioner
PM and another position sensor (which is not explicitly depicted in
FIG. 2A) can be used to accurately position the mask MA with
respect to the path of the radiation beam B, e.g. after mechanical
retrieval from a mask library, or during a scan.
[0054] In general, movement of the mask table MT may be realized
with the aid of a long-stroke module and a short-stroke module,
which form part of the first positioner PM. Similarly, movement of
the substrate table WT may be realized using a long-stroke module
and a short-stroke module, which form part of the second positioner
PW. In the case of a stepper (as opposed to a scanner) the mask
table MT may be connected to a short-stroke actuator only, or may
be fixed. Mask MA and substrate W may be aligned using mask
alignment marks M1, M2 and substrate alignment marks P1, P2.
Although the substrate alignment marks as illustrated occupy
dedicated target portions, they may be located in spaces between
target portions (these are known as scribe-lane alignment marks).
Similarly, in situations in which more than one die is provided on
the mask MA, the mask alignment marks may be located between the
dies.
[0055] The depicted apparatus could be used in at least one of the
following modes: [0056] 1. step mode: the mask table MT and the
substrate table WT are kept essentially stationary, while an entire
pattern imparted to the radiation beam is projected onto a target
portion C at one time (i.e. a single static exposure). The
substrate table WT is then shifted in the X and/or Y direction so
that a different target portion C can be exposed. In step mode, the
maximum size of the exposure field limits the size of the target
portion C imaged in a single static exposure. [0057] 2. scan mode:
the mask table MT and the substrate table WT are scanned
synchronously while a pattern imparted to the radiation beam is
projected onto a target portion C (i.e. a single dynamic exposure).
The velocity and direction of the substrate table WT relative to
the mask table MT may be determined by the (de-)magnification and
image reversal characteristics of the projection system PS. In scan
mode, the maximum size of the exposure field limits the width (in
the non-scanning direction) of the target portion in a single
dynamic exposure, whereas the length of the scanning motion
determines the height (in the scanning direction) of the target
portion. [0058] 3. other mode: the mask table MT is kept
essentially stationary holding a programmable patterning device,
and the substrate table WT is moved or scanned while a pattern
imparted to the radiation beam is projected onto a target portion
C. In this mode, generally a pulsed radiation source is employed
and the programmable patterning device is updated as required after
each movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
[0059] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0060] FIG. 2B depicts an example of a system, in accordance with
an embodiment of the present invention. The system comprises a
lithographic projection apparatus 32 arranged to transfer a pattern
from a patterning device onto a substrate, not shown. Furthermore,
the system comprises an overlay metrology tool 33 and a processor
34.
[0061] The metrology tool 33 is arranged to measure overlay on
substrates, not shown. The processor 34 is arranged to receive
overlay data from the metrology tool 33, and alignment data from
the lithographic projection apparatus 32. The processor 34 may be
part of the lithographic projection apparatus, but other
configurations are possible. The overlay data may be received by
the processor 34 directly from the overlay metrology tool 33 or
from an attached software application, loaded on another processor,
not shown or on the same processor 34. Preferably, the system is
arranged in a computer network such as to communicate with other
apparatus and/or applications.
[0062] During manufacturing processes, substrates are grouped in a
box to form a particular batch. Substrates in a batch stay together
throughout the entire manufacturing process. The batches pass
several manufacturing actions. Two main manufacturing actions which
are of interest for this invention, are the lithographic exposure
action in the lithographic apparatus and an overlay inspection
action in an overlay inspection station, i.e. the offline metrology
tool.
[0063] In the lithographic apparatus 32, each individual substrate
in a batch follows the following sequence: [0064] 1. Place a
substrate on the substrate table WT, [0065] 2. Measure alignment
marks and position the substrate under the lens PS, [0066] 3.
Expose the substrate, [0067] 4. Place the substrate back in the
box.
[0068] During sub-sequence (2), multiple alignment marks present on
a substrate, are measured. To that end, a subset of all available
markers may be used. Next, a wafer model is used in which wafer
model parameters, such as (Tx, Ty, Rzx, Rzy, Mx, My), are
determined using a least squares optimization method. In order to
accurately determine the wafer's coordinate system, the position
data of a number of alignment marks need to be measured. Based on
the values of the wafer model parameters, a substrate will be
repositioned and the magnification of the lithographic apparatus
will be adjusted.
[0069] During sub-sequence (2), the position of the alignment marks
may be measured with a specified diffraction order (i.e.
diffraction order of the ATHENA sensor) but also with all other
diffraction orders. For every diffraction order, the wafer model
parameters will be determined and stored in an alignment report. A
simplified example of such a report is shown in FIG. 3. For clarity
reasons, only 4 diffraction orders are represented (normally 7
diffraction orders * 2 colors are collected per substrate). It
should be noted that instead of using diffraction orders only,
other "alignment strategies" may be used.
[0070] FIG. 3 illustrates an example of a table with the values of
wafer model parameters (Tx, Ty, Rzx, Rzy, Mx, My) for several
substrates of one batch. The values indicated are nanometers,
microradians, and part per million for Tx/Ty, Rzx/Rzy and Mx/My
respectively. The wafer model parameters are calculated for 4
diffraction orders, so the table of FIG. 3 actually comprises 100
records. In column 2 of the table, the diffraction order is listed.
The results of a single diffraction order is enough to position the
substrate, the results of all other orders is then for diagnostics
only. Sometimes a combination of orders is used. Note that this
data file enables one to determine the adjustment of the
lithographic projection apparatus in case another diffraction order
would have been used.
[0071] At present, software algorithms are used which request a
manufacturer to provide both data files (alignment and overlay)
from a batch. Having both data files, the software performs some
calculations and determines, for example, the OPI values. To enable
this calculation, a manufacturer is required to verify overlay on
the metrology tool on all substrates of a batch. This is because
when using the above mentioned OVFB method, the OPI calculation
requires that for every substrate both the alignment data and the
overlay data is available.
[0072] FIG. 4 depicts an example of overlay data expressed in wafer
model parameters related to the batch of which the alignment data
was shown in FIG. 3. In the table of FIG. 4, only data (i.e. the
wafer model parameters) of some substrates is shown. In order to
utilize the overlay indicators with the high confidence levels, all
substrates in a batch need to be measured in the offline metrology
tool. Measuring all the substrates is very time consuming.
[0073] In one embodiment of the invention, a plurality of batches
is aligned and processed in the lithographic apparatus. This
plurality of batches comprises a predefined number of batches.
Then, at least one substrate of every batch is selected to be
measured for determining overlay data. The set of selected
substrates is referred to as an "artificial batch". After having
processed this set of batches, alignment data and the overlay data
of the artificial batch are combined into one data-set. Preferably,
to limit the measurement time, only the overlay on one or two
substrates in every batch is measured.
[0074] FIG. 5 schematically depicts the composition of the
artificial batch. In this example, the set of batches comprise five
batches, i.e. batch 100, 200, 300,400, 500, only three of them are
shown. Batch 100 comprises substrates 101-1n, batch 200 comprises
substrates 201-2n, etc. (where n is an integer, typically 25). From
each processed batch only a few substrates will be measured on the
metrology tool. In this example, it is always the second and fifth
substrate. Artificial batch 600 is composed of substrates 102, 105,
202, 205, . . . , 502, 505, see FIG. 5.
[0075] From the available alignment data, only the data belonging
to the selected substrates 102, 105, 202, 205, . . . , 502, 505
will be used. The data from the other substrates will be neglected.
FIG. 6 shows the selected substrates and their possible alignment
and overlay data. FIG. 7 shows an example of alignment and overlay
data of the substrates in the artificial batch 600, i.e. substrate
102, 105, 202, 205, 302, . . . , 505. According to the invention,
the alignment and overlay data of the artificial batch 600 is used
to calculate values for an overlay indicator, such as the OPI
indicator. For each possible alignment strategy, e.g. diffraction
order, a value of the overlay indicator is calculated.
[0076] Finally, an optimal alignment strategy is determined by
choosing the particular alignment strategy among the current
alignment strategy and the other possible alignment strategies,
that has the lowest overlay indicator value. In this way, an
optimal alignment strategy can be chosen, measuring only a subset
(i.e. the artificial batch) of all the substrate processed.
[0077] The overlay data may be determined by measuring position
errors for a plurality of overlay targets present on each of the
selected substrates. This will result in so-called measured overlay
data. Next, the overlay data is calculated by applying a least
squares model to the measured overlay data. By modeling the
measured overlay data using a least squares model, only the wafer
model parameter values are processed and not all the raw data
coming from the metrology tool and the alignment system. This
reduces the amount of data to be processed.
[0078] Possible alignment strategies may comprise one or more of
the following: (a) using a predefined alignment mark type; (b)
using a predefined number of alignment marks on a substrate; (c)
using predefined positions of alignment marks on a substrate; (d)
using a measurement beam with a predefined wavelength in an
alignment system or combination of wavelengths; (e) using a
predefined diffraction order in said alignment system or
combination of diffraction orders; (f) using a predefined number of
wafer model parameters; (g) using a predefined position detection
algorithm; and (h) using a predefined alignment system.
[0079] A position detection algorithm is an algorithm which is used
to determine a position from a mark image captured by for example,
a CCD element. The alignment system used may vary as well. In such
a case, the alignment system may be equipped with multiple
alignment sensors, as is described in K. Ota et al. "New Alignment
Sensors For Wafer Stepper", Proc. SPIE Optical/Laser
Microlithography IV, Vol. 1463 (1991) pp 304-314, which is herein
incorporated by reference.
[0080] In another embodiment of the present invention, more than
one substrate in each batch is selected to be measured on the
metrology tool. The overlay indicators using compound batches
concerns both inter- and intra-batch variation of wafer model
parameters. In order to separate the inter- and intra-batch terms,
the wafer model parameters are corrected by calculating the average
parameter values per batch and then subtracting the averages from
the wafer model parameters. The overlay indicator is then
calculated using the corrected wafer model parameter values. In
this case the overlay indicator mainly concerns the intra-batch
term. An alternative overlay indicator can than be calculated using
the average parameter values. In that case the overlay indicator
mainly concerns the inter-batch term.
[0081] To explain the method according to the invention, a simple
example is described in more detail with reference to tables in
FIG. 8. The example is based on a simplified wafer model (Tx) i.e.
translation X, on 4 substrates and with 4 diffraction orders.
Optimization is performed using a comparison based on an average
overlay per diffraction order. FIG. 8 shows alignment data, which
is collected on the lithographic apparatus and overlay data as
collected by the metrology tool. Note that in the third column of
the alignment data table, the relative translation values compared
to the first diffraction order data are shown, i.e.
.DELTA.Tx=Tx(order i)-Tx(order 1), where i is 2, 3 or 4. It is
assumed that the substrates have been exposed with the first
diffraction order of the ATHENA alignment system. This means that
the overlay data is valid for the first diffraction order only. In
case the first order is used for alignment the 4 substrates will
have an overlay of 18, 12, 6 and 24 nm respectively, see, e.g., the
second column of the second table.
[0082] According to an embodiment of the present invention, the
overlay indicator values for the other possible alignment
strategies are calculated, for all selected substrates, using
so-called "derived" overlay data. This "derived" overlay data is
derived from the overlay data corresponding to the predefined
alignment strategy, and from the alignment data corresponding to
the other possible (i.e. not used) alignment strategies. In this
way, the overlay data is derived that would have been achieved when
exposing the substrates by another possible alignment strategy.
[0083] An example is given in the third table of FIG. 8. The third
table of FIG. 8 presents an overlay error Tx, which would have been
induced in case an alternative diffraction order would have been
used. The first substrate would have been positioned 6 nm to the
left in case the second diffraction order would have been active.
In that case, the overlay measured with the metrology tool would
have been 12 nm (assuming identical metrology errors). In case the
third diffraction order would have been used, the overlay measured
by the metrology tool would have been 18-15=3 nm. This calculation
can be performed for all order/substrate combinations resulting in
a derived overlay table. Based on the derived overlay scenario, one
can select the best diffraction order, by calculating the average
overlay for all strategies (i.e. diffraction orders). For this
example, a simple averaging has been applied which leads to a
minimal overlay for the third order (the third order results in the
lowest indicator). So in this case, the strategy corresponding to
the third order will be used for aligning future batches.
[0084] In another embodiment, a six parameter model (Tx, Ty, Rzx,
Rzy, Mx, My) is used for both the alignment and overlay data and an
OPI calculation is used to select the optimum alignment strategy.
The OPI indicator can be used to quantify variations in the
substrate systematic, and thus is proportional to the attainable
overlay. The averages of the model parameters, i.e. <Tx>,
<Ty>, <Rzx>, <Rzy>, <Mx>, <My> per
batch are calculated in addition to the differences between the
model parameters per substrate (i) and the batch averages, i.e.
Txi-<Tx>, Tyi-<Ty>, Rzxi-<Rzx>, Rzyi-<Rzy>,
Mxi-<Mx>, Myi-<My>.
[0085] Because the average batch parameters can be corrected with
process corrections, it is the differences from substrate to
substrate that define the attainable overlay. These differences in
substrate parameters (caused by the process) indicate a maximum
overlay error at the edge of each substrate since that is were the
largest systematic induced error will be. This maximum error
(caused by the process) is also calculated, see, e.g., formula (1).
For a processed substrate, one can calculate the maximum error
(caused by the process) that is remaining after the average model
parameters have been removed. For k processed substrates (k is an
integer), k maximum errors (caused by the process) are calculated.
The lower the OPI, the better the overlay. Because this analysis
is, in effect, a simulation of an actual substrate alignment in
relation to a stable grid, the OPI gives the best correlation to
actual overlay stability. The OPI has been shown to correlate very
well with actual overlay.
[0086] According to another embodiment of the invention, there is
provided a device manufacturing method comprising transferring a
patterned beam of radiation onto a substrate, wherein an alignment
strategy is optimized according to the method described above. The
alignment strategy corresponding with the lowest overlay indicator,
is used to align future batches in the lithographic apparatus (i.e.
exposure tool).
[0087] FIG. 9 shows an example of a flow chart of the manufacturing
method according to an embodiment of the invention. A manufacturing
method 800 starts at block 801. Then in block 802, an initial
strategy is selected for aligning batches b.sub.m, where m=1 . . .
M.
[0088] In block 803, a counter n is set to 1. Next, in block 804,
substrates of batch n are aligned using `current` strategy P and
then exposed on the lithographic apparatus. Then in block 805, at
least one substrate out of batch n is measured in the metrology
tool, resulting in overlay data for that substrate.
[0089] In block 806, variable m is compared to the maximum number
of batches used for optimization, i.e. M. If in block 806, m is not
yet equal to M, then it is tested in block 807 to determined
whether all batches are processed. If this is not true, block 808
follows in which m in increased by 1 and block 804 is executed
again. This loop is executed until in block 806 m>=M.
[0090] In block 809, an artificial batch is composed. If in block
807 all batches are already processed, the manufacturing process
ends, pursuant to block 813. The artificial batch is used to
calculate overlay indicator values for all possible strategies in
block 810. In block 811, the strategy corresponding to the
indicator with the lowest value, is selected to be the new
`current` strategy P. If not all batches are processed yet, the
result of the test 812 is NO and block 803 follows. If the result
of test 812 is YES, block 813 follows which means the end of the
manufacturing process.
[0091] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc.
[0092] The skilled artisan will appreciate that, in the context of
such alternative applications, any use of the terms "wafer" or
"die" herein may be considered as synonymous with the more general
terms "substrate" or "target portion", respectively. The substrate
referred to herein may be processed, before or after exposure, in
for example a track (a tool that typically applies a layer of
resist to a substrate and develops the exposed resist), a metrology
tool and/or an inspection tool. Where applicable, the disclosure
herein may be applied to such and other substrate processing tools.
Further, the substrate may be processed more than once, for example
in order to create a multi-layer IC, so that the term substrate
used herein may also refer to a substrate that already contains
multiple processed layers.
[0093] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0094] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 248, 193, 157
or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a
wavelength in the range of 5-20 nm), as well as particle beams,
such as ion beams or electron beams.
[0095] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive, reflective, magnetic, electromagnetic and
electrostatic optical components.
[0096] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein. Furthermore, it is possible to directly use (raw)
measured data from the metrology tool in order to calculate an
overlay indicator and to find the optimal alignment strategy. For
example, it is possible to use the average of the raw measured data
plus three times the standard deviation of the raw measured data as
an indication of overlay.
[0097] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the invention as defined by the
appended claims.
* * * * *