U.S. patent application number 15/791703 was filed with the patent office on 2018-02-15 for inspection apparatus, inspection method, lithographic apparatus, patterning device and manufacturing method.
This patent application is currently assigned to ASML Netherlands B.V.. The applicant listed for this patent is ASML Netherlands B.V.. Invention is credited to Robertus Cornelis Martinus DE KRUIF, Paul Christiaan HINNEN, Robert John SOCHA, Anton Bernhard VAN OOSTEN.
Application Number | 20180046091 15/791703 |
Document ID | / |
Family ID | 53397926 |
Filed Date | 2018-02-15 |
United States Patent
Application |
20180046091 |
Kind Code |
A1 |
VAN OOSTEN; Anton Bernhard ;
et al. |
February 15, 2018 |
Inspection Apparatus, Inspection Method, Lithographic Apparatus,
Patterning Device and Manufacturing Method
Abstract
Disclosed is a patterning device configured to pattern a beam of
radiation according to a desired pattern during a lithographic
process. The patterning device comprises first features configured
to form a first target on a substrate during the lithographic
process and second features configured to form a second target on
the substrate during the lithographic process. The second features
are taller, in a direction transverse to the plane of the first and
second targets, than the first features, such that the first and
second targets have a relative best focus offset.
Inventors: |
VAN OOSTEN; Anton Bernhard;
(Lommel, BE) ; HINNEN; Paul Christiaan;
(Veldhoven, NL) ; DE KRUIF; Robertus Cornelis
Martinus; (Eindhoven, NL) ; SOCHA; Robert John;
(Campbell, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Netherlands B.V. |
Veldhoven |
|
NL |
|
|
Assignee: |
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
53397926 |
Appl. No.: |
15/791703 |
Filed: |
October 24, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15179509 |
Jun 10, 2016 |
|
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15791703 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70525 20130101;
G03F 7/70625 20130101; G03F 7/70558 20130101; G03F 1/44 20130101;
G03F 7/70516 20130101; G03F 7/70641 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G03F 1/44 20120101 G03F001/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2015 |
EP |
15171970.5 |
Claims
1. A patterning device comprising: first features configured to
form a first target on a substrate during a lithographic process,
and second features configured to form a second target on the
substrate during the lithographic process; wherein the second
features are taller, in a direction transverse to the plane of the
first and second targets, than the first features, such that the
first and second targets have a relative best focus offset.
2. The patterning device of claim 1, wherein the relative best
focus offset results from astigmatism during formation of the first
and second targets.
3. The patterning device of claim 1, wherein the first and second
targets comprise line-space grating structures.
4. The patterning device of claim 1, wherein: design rules
constrain target features to a set critical dimension; and on a
grid of set pitch, the first and second targets are each formed
from rows of the grid, such that each of the first and second
features are formed from one or more adjacent, corresponding target
features formed on the grid.
5. The patterning device of claim 1, wherein the first and second
targets comprise corresponding line and space targets having a same
pitch and inverse duty cycles.
6. The patterning device of claim 1, wherein: the first features
are configured to form the first target with line features having a
substantially focus independent side wall angle; and the second
features are configured to form the second target with line
features having a focus dependent side wall angle.
7. The patterning device of claim 1, wherein the second features
comprise segmented line features, wherein each of the segmented
line features has a pitch below an imaging resolution of the
lithographic process.
8. A patterning device comprising: first features configured to
form a first target on a substrate during a lithographic process,
and second features configured to form a second target on the
substrate during the lithographic process; wherein the first
features are configured to form the first target with line features
having a substantially focus independent side wall angle, and
wherein the second features are configured to form the second
target with line features having a focus dependent side wall
angle.
9. The patterning device of claim 8, wherein the first and second
targets comprise line-space grating structures.
10. The patterning device of claim 8, wherein: the second target
with the line features comprises segmented line features, and each
of the segmented line features has a pitch below an imaging
resolution of the lithographic process.
11. The patterning device of claim 8, wherein the first and second
targets comprise corresponding line and space targets having a same
pitch and inverse duty cycles.
12. The patterning device of claim 8, wherein the first and second
targets have a relative best focus offset.
13. The patterning device of claim 12, wherein the relative best
focus offset results from the focus dependent side wall angle of
the second target.
14. The patterning device of claim 8, wherein the second features
are taller, in a direction transverse to the plane of the first and
second targets, than the first features, such that the first and
second targets have a relative best focus offset
15. A patterning device comprising: first features configured to
form a first target on a substrate during a lithographic process,
and second features configured to forming a second target on the
substrate during the lithographic process; wherein design rules
constrain target features to a set critical dimension, and wherein
on a grid of set pitch, the first target and the second target are
each formed from rows of the grid, such that each of the first
features and the second features are formed from one or more
adjacent, corresponding target features formed on the grid.
16. The patterning device of claim 15, wherein the first and second
targets comprise line-space grating structures.
17. The patterning device of claim 15, wherein the second features
comprise segmented line features, such that each of the segmented
line features has a pitch below an imaging resolution of the
lithographic process.
18. The patterning device of claim 15, wherein the first and second
targets have a relative best focus offset.
19. The patterning device of claim 18, wherein the relative best
focus offset results from astigmatism during formation of the first
and second targets.
20. The patterning device of claim 15, wherein the first features
and the second features do not introduce focus dependent asymmetry
in the first and second targets.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/179,509 filed on Jun. 10, 2016, which is incorporated
by reference herein in its entirety
BACKGROUND
Field of the Invention
[0002] The present invention relates to inspection apparatus and
methods usable, for example, to perform metrology in the
manufacture of devices by lithographic techniques. The invention
further relates to such methods for monitoring a focus and/or dose
parameter in a lithographic process.
Background Art
[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g., including part of, one, or several
dies) on a substrate (e.g., a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned.
[0004] In lithographic processes, it is desirable frequently to
make measurements of the structures created, e.g., for process
control and verification. Various tools for making such
measurements are known, including scanning electron microscopes,
which are often used to measure critical dimension (CD), and
specialized tools to measure overlay, the accuracy of alignment of
two layers in a device. Recently, various forms of scatterometers
have been developed for use in the lithographic field. These
devices direct a beam of radiation onto a target and measure one or
more properties of the scattered radiation--e.g., intensity at a
single angle of reflection as a function of wavelength; intensity
at one or more wavelengths as a function of reflected angle; or
polarization as a function of reflected angle--to obtain a
diffraction "spectrum" from which a property of interest of the
target can be determined.
[0005] Examples of known scatterometers include angle-resolved
scatterometers of the type described in US2006033921A1 and
US2010201963A1. The targets used by such scatterometers are
relatively large, e.g., 40 .mu.m by 40 .mu.m, gratings and the
measurement beam generates a spot that is smaller than the grating
(i.e., the grating is underfilled). Examples of dark field imaging
metrology can be found in international patent applications
US20100328655A1 and US2011069292A1 which documents are hereby
incorporated by reference in their entirety. Further developments
of the technique have been described in published patent
publications US20110027704A, US20110043791A, US2011102753A1,
US20120044470A, US20120123581A, US20130258310A, US20130271740A and
WO2013178422A1. These targets can be smaller than the illumination
spot and may be surrounded by product structures on a wafer.
Multiple gratings can be measured in one image, using a composite
grating target. The contents of all these applications are also
incorporated herein by reference.
[0006] One important parameter of a lithographic process which
requires monitoring is focus. There is a desire to integrate an
ever-increasing number of electronic components in an IC. To
realize this, it is necessary to decrease the size of the
components and therefore to increase the resolution of the
projection system, so that increasingly smaller details, or line
widths, can be projected on a target portion of the substrate. As
the critical dimension (CD) in lithography shrinks, consistency of
focus, both across a substrate and between substrates, becomes
increasingly important. CD is the dimension of a feature or
features (such as the gate width of a transistor) for which
variations will cause undesirable variation in physical properties
of the feature. Traditionally, optimal settings were determined by
"send-ahead wafers" i.e. substrates that are exposed, developed and
measured in advance of a production run. In the send-ahead wafers,
test structures were exposed in a so-called focus-energy matrix
(FEM) and the best focus and energy settings were determined from
examination of those test structures.
[0007] Current test structure designs and focus measuring methods
have a number of drawbacks. Many test structures require
subresolution features or grating structures with large pitches.
Such structures may contravene design rules of the users of
lithographic apparatuses. Focus measuring techniques may comprise
measuring asymmetry in opposite higher (e.g., first) order
radiation scattered by special, focus dependent, target structures
and determining focus from this asymmetry. For EUV lithography,
resist thickness, and therefore the thickness of target structures,
is smaller (for example, half as thick). Therefore focus
sensitivity and signal strength may be insufficient to use such
asymmetry methods in EUV lithography. In addition, asymmetry based
techniques may require careful selection of target geometries to
ensure a desired relationship (e.g., linear) between asymmetry and
focus. This selection process can be complex and require
significant effort to find a suitable target geometry. It may even
be the case that no suitable target geometry exists.
SUMMARY OF THE INVENTION
[0008] The present invention aims to address one or more of the
above identified drawbacks.
[0009] The invention in a first aspect provides method of
monitoring a focus parameter during a lithographic process, said
method comprising:
[0010] acquiring a first measurement value, said first measurement
value having been obtained from inspection of a first target;
[0011] acquiring a second measurement value, said second
measurement value having been obtained from inspection of a second
target, wherein said first target and second target have been
exposed with a relative best focus offset;
[0012] determining the focus parameter from said first measurement
value and said second measurement value.
[0013] The invention yet further provides a method of manufacturing
devices wherein a device pattern is applied to a series of
substrates using a lithographic process, the method including:
[0014] using the method of the first aspect to monitor said focus
parameter, and controlling the lithographic process for later
substrates in accordance with the determined focus parameter.
[0015] The invention yet further provides a computer program
product comprising machine-readable instructions for causing a
processor to perform the method of the first aspect.
[0016] The invention yet further provides a patterning device
configured to pattern a beam of radiation in a lithographic process
according to a desired pattern, said patterning device comprising
first features for forming a first target on substrate during the
lithographic process, and second features for forming a second
target on substrate during the lithographic process; wherein said
second features are taller than said first features, in a direction
transverse to the plane of the target and such that said first
target and said second target have a relative best focus
offset.
[0017] The invention yet further provides a method of monitoring a
dose parameter during a lithographic process, said method
comprising: acquiring a first measurement value, said first
measurement value having been obtained from inspection of a first
target; acquiring a second measurement value, said second
measurement value having been obtained from inspection of a second
target, determining the dose parameter from said first measurement
value and said second measurement value; wherein said first and
second targets comprise corresponding line and space targets having
the same pitch and inverse duty cycles.
[0018] The invention yet further provides a method of manufacturing
devices wherein a device pattern is applied to a series of
substrates using a lithographic process, the method including:
using the method of the aspect immediately above to monitor said
dose parameter, and controlling the lithographic process for later
substrates in accordance with the determined dose parameter.
[0019] The invention yet further provides a patterning device
configured to pattern a beam of radiation in a lithographic process
according to a desired pattern, said patterning device comprising
first features for forming a first target on a substrate during the
lithographic process, and second features for forming a second
target on the substrate during the lithographic process; wherein
said first features are configured to form said first target with
line features having a substantially focus independent side wall
angle and said second features are configured to form said second
target with line features having a focus dependent side wall
angle.
[0020] The invention yet further provides a patterning device
configured to pattern a beam of radiation in a lithographic process
according to a desired pattern, said patterning device comprising
first features for forming a first target on a substrate during the
lithographic process, and second features for forming a second
target on a substrate during the lithographic process; wherein
design rules constrain target features to a set critical dimension
and on a grid of set pitch, wherein said first target and said
second target are each formed from rows of said grids such that
each of said first features and said second features are formed
from one, or plural adjacent, corresponding target features formed
on said grids.
[0021] Further features and advantages of the invention, as well as
the structure and operation of various embodiments of the
invention, are described in detail below with reference to the
accompanying drawings. It is noted that the invention is not
limited to the specific embodiments described herein. Such
embodiments are presented herein for illustrative purposes only.
Additional embodiments will be apparent to persons skilled in the
relevant art(s) based on the teachings contained herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] 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:
[0023] FIG. 1 depicts a lithographic apparatus;
[0024] FIG. 2 depicts a lithographic cell or cluster in which an
inspection apparatus according to the present invention may be
used;
[0025] FIG. 3 illustrates the principles of operation of a
spectroscopic scatterometer as a first example of an inspection
apparatus;
[0026] FIG. 4 illustrates in schematic form an angle-resolved
scatterometer as another example of an inspection apparatus;
[0027] FIGS. 5(a) to 5(b) illustrate schematically an inspection
apparatus adapted to perform angle-resolved scatterometry and
dark-field imaging inspection methods;
[0028] FIG. 6 illustrates target forming elements on a reticle
suitable for forming a grating on a substrate having focus
dependent asymmetry;
[0029] FIGS. 7(a) to 7(b) shows (a) a plot of a measured value for
a target parameter (y-axis) against focus for two targets having a
relative best focus offset; and (b) a plot of the difference
between measured values for a target parameter from a first target
and a second target (y-axis) against focus (x-axis);
[0030] FIGS. 8(a) to 8(d) show schematically, in cross section,
possible target forming designs on a reticle;
[0031] FIGS. 9(a) to 9(b) shows schematically, in cross section,
(a) a reticle blank according to an embodiment and (b) a further
possible target forming design on a reticle;
[0032] FIG. 10 show schematically, in plan view, yet further
possible target forming designs on a reticle;
[0033] FIG. 11 is a flowchart of a method of monitoring focus
according to an embodiment of the invention;
[0034] FIG. 12 show schematically, in plan view, possible target
forming designs on a reticle for performing dose measurements;
[0035] FIG. 13 shows a plot of measured intensity (y-axis) against
CD (x-axis) for targets such as those illustrated in FIG. 12;
and
[0036] FIGS. 14(a) to 14(f) each show an example of a possible
target feature forming design when constrained by an exemplary
grid-based design rule.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0037] Before describing embodiments of the invention in detail, it
is instructive to present an example environment in which
embodiments of the present invention may be implemented.
[0038] FIG. 1 schematically depicts a lithographic apparatus LA.
The apparatus includes an illumination system (illuminator) IL
configured to condition a radiation beam B (e.g., UV radiation or
DUV radiation), a patterning device support or support structure
(e.g., a mask table) MT constructed to support a patterning device
(e.g., a mask) MA and connected to a first positioner PM configured
to accurately position the patterning device in accordance with
certain parameters; two substrate tables (e.g., a wafer table) WTa
and WTb each constructed to hold a substrate (e.g., a resist coated
wafer) W and each connected to a second positioner PW configured to
accurately position the substrate in accordance with certain
parameters; and a projection system (e.g., a refractive projection
lens system) PS configured to project a pattern imparted to the
radiation beam B by patterning device MA onto a target portion C
(e.g., including one or more dies) of the substrate W. A reference
frame RF connects the various components, and serves as a reference
for setting and measuring positions of the patterning device and
substrate and of features on them.
[0039] 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.
[0040] The patterning device support 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 patterning device support can take many
forms, The patterning device support may ensure that the patterning
device is at a desired position, for example with respect to the
projection system.
[0041] 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.
[0042] As here depicted, the apparatus is of a transmissive type
(e.g., employing a transmissive patterning device). 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). Examples of patterning devices
include masks, programmable mirror arrays, and programmable LCD
panels. Any use of the terms "reticle" or "mask" herein may be
considered synonymous with the more general term "patterning
device." The term "patterning device" can also be interpreted as
referring to a device storing in digital form pattern information
for use in controlling such a programmable patterning device.
[0043] 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".
[0044] 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.
[0045] In operation, 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 including, 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.
[0046] The illuminator IL may for example include an adjuster AD
for adjusting the angular intensity distribution of the radiation
beam, 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.
[0047] The radiation beam B is incident on the patterning device
MA, which is held on the patterning device support MT, and is
patterned by the patterning device. Having traversed the patterning
device (e.g., 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, 2-D encoder or capacitive sensor), the substrate table WTa
or WTb 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. 1) can be used to accurately position
the patterning device (e.g., reticle/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.
[0048] Patterning device (e.g., reticle/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 patterning device (e.g., mask) MA, the mask
alignment marks may be located between the dies. Small alignment
mark may also be included within dies, in amongst the device
features, in which case it is desirable that the markers be as
small as possible and not require any different imaging or process
conditions than adjacent features. The alignment system, which
detects the alignment markers is described further below.
[0049] The depicted apparatus could be used in a variety of modes.
In a scan mode, the patterning device support (e.g., 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 speed and
direction of the substrate table WT relative to the patterning
device support (e.g., 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. Other types of lithographic
apparatus and modes of operation are possible, as is well-known in
the art. For example, a step mode is known. In so-called "maskless"
lithography, a programmable patterning device is held stationary
but with a changing pattern, and the substrate table WT is moved or
scanned.
[0050] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0051] Lithographic apparatus LA is of a so-called dual stage type
which has two substrate tables WTa, WTb and two stations--an
exposure station EXP and a measurement station MEA--between which
the substrate tables can be exchanged. While one substrate on one
substrate table is being exposed at the exposure station, another
substrate can be loaded onto the other substrate table at the
measurement station and various preparatory steps carried out. This
enables a substantial increase in the throughput of the apparatus.
The preparatory steps may include mapping the surface height
contours of the substrate using a level sensor LS and measuring the
position of alignment markers on the substrate using an alignment
sensor AS. If the position sensor IF is not capable of measuring
the position of the substrate table while it is at the measurement
station as well as at the exposure station, a second position
sensor may be provided to enable the positions of the substrate
table to be tracked at both stations, relative to reference frame
RF. Other arrangements are known and usable instead of the
dual-stage arrangement shown. For example, other lithographic
apparatuses are known in which a substrate table and a measurement
table are provided. These are docked together when performing
preparatory measurements, and then undocked while the substrate
table undergoes exposure.
[0052] As shown in FIG. 2, the lithographic apparatus LA forms part
of a lithographic cell LC, also sometimes referred to a lithocell
or cluster, which also includes apparatus to perform pre- and
post-exposure processes on a substrate. Conventionally these
include spin coaters SC to deposit resist layers, developers DE to
develop exposed resist, chill plates CH bake plates BK. A substrate
handler, or robot, RO picks up substrates from input/output ports
1/O1, 1/O2, moves them between the different process apparatus and
delivers then to the loading bay LB of the lithographic apparatus.
These devices, which are often collectively referred to as the
track, are under the control of a track control unit TCU which is
itself controlled by the supervisory control system SCS, which also
controls the lithographic apparatus via lithography control unit
LACU. Thus, the different apparatus can be operated to maximize
throughput and processing efficiency.
[0053] In order that the substrates that are exposed by the
lithographic apparatus are exposed correctly and consistently, it
is desirable to inspect exposed substrates to measure properties
such as overlay errors between subsequent layers, line thicknesses,
critical dimensions (CD), etc. Accordingly a manufacturing facility
in which lithocell LC is located also includes metrology system MET
which receives some or all of the substrates W that have been
processed in the lithocell. Metrology results are provided directly
or indirectly to the supervisory control system SCS. If errors are
detected, adjustments may be made to exposures of subsequent
substrates, especially if the inspection can be done soon and fast
enough that other substrates of the same batch are still to be
exposed. Also, already exposed substrates may be stripped and
reworked to improve yield, or discarded, thereby avoiding
performing further processing on substrates that are known to be
faulty. In a case where only some target portions of a substrate
are faulty, further exposures can be performed only on those target
portions which are good.
[0054] Within metrology system MET, an inspection apparatus is used
to determine the properties of the substrates, and in particular,
how the properties of different substrates or different layers of
the same substrate vary from layer to layer. The inspection
apparatus may be integrated into the lithographic apparatus LA or
the lithocell LC or may be a stand-alone device. To enable most
rapid measurements, it is desirable that the inspection apparatus
measure properties in the exposed resist layer immediately after
the exposure. However, the latent image in the resist has a very
low contrast--there is only a very small difference in refractive
index between the parts of the resist which have been exposed to
radiation and those which have not--and not all inspection
apparatus have sufficient sensitivity to make useful measurements
of the latent image. Therefore measurements may be taken after the
post-exposure bake step (PEB) which is customarily the first step
carried out on exposed substrates and increases the contrast
between exposed and unexposed parts of the resist. At this stage,
the image in the resist may be referred to as semi-latent. It is
also possible to make measurements of the developed resist
image--at which point either the exposed or unexposed parts of the
resist have been removed--or after a pattern transfer step such as
etching. The latter possibility limits the possibilities for rework
of faulty substrates but may still provide useful information.
[0055] FIG. 3 depicts a known spectroscopic scatterometer which may
be used as an inspection apparatus in a metrology system of the
type described above. It comprises a broadband (white light)
radiation projector 2 which projects radiation onto a substrate W.
The reflected radiation is passed to a spectrometer 4, which
measures a spectrum 6 (intensity as a function of wavelength) of
the specular reflected radiation. From this data, the structure or
profile 8 giving rise to the detected spectrum may be reconstructed
by calculation within processing unit PU. The reconstruction can be
performed for example by Rigorous Coupled Wave Analysis and
non-linear regression, or comparison with a library of pre-measured
spectra or pre-computed simulated spectra. In general, for the
reconstruction the general form of the structure is known and some
parameters are assumed from knowledge of the process by which the
structure was made, leaving only a few parameters of the structure
to be determined from the scatterometry data. Such a scatterometer
may be configured as a normal-incidence scatterometer or an
oblique-incidence scatterometer.
[0056] FIG. 4 shows the basic elements of a known angle-resolved
scatterometer that may be used instead of or in addition to a
spectroscopic scatterometer. In this type of inspection apparatus,
radiation emitted by a radiation source 11 is conditioned by an
illumination system 12. For example, illumination system 12 may
include a collimating using lens system 12a, a color filter 12b, a
polarizer 12c and an aperture device 13. The conditioned radiation
follows an illumination path IP, in which it is reflected by
partially reflecting surface 15 and focused into a spot S on
substrate W via a microscope objective lens 16. A metrology target
T may be formed on substrate W. Lens 16, has a high numerical
aperture (NA), preferably at least 0.9 and more preferably at least
0.95. Immersion fluid can be used to obtain with numerical
apertures over 1 if desired.
[0057] As in the lithographic apparatus LA, one or more substrate
tables may be provided to hold the substrate W during measurement
operations. The substrate tables may be similar or identical in
form to the substrate tables WTa, WTb of FIG. 1. (In an example
where the inspection apparatus is integrated with the lithographic
apparatus, they may even be the same substrate tables.) Coarse and
fine positioners may be configured to accurately position the
substrate in relation to a measurement optical system. Various
sensors and actuators are provided for example to acquire the
position of a target of interest, and to bring it into position
under the objective lens 16. Typically many measurements will be
made on targets at different locations across substrate W. The
substrate support can be moved in X and Y directions to acquire
different targets, and in the Z direction to obtain a desired
focusing of the optical system on the target. It is convenient to
think and describe operations as if the objective lens and optical
system being brought to different locations on the substrate, when
in practice the optical system remains substantially stationary and
only the substrate moves. Provided the relative position of the
substrate and the optical system is correct, it does not matter in
principle whether one or both of those is moving in the real
world.
[0058] When the radiation beam is incident on the beam splitter 16
part of it is transmitted through the beam splitter and follows a
reference path RP towards a reference mirror 14.
[0059] Radiation reflected by the substrate, including radiation
diffracted by any metrology target T, is collected by lens 16 and
follows a collection path CP in which it passes through partially
reflecting surface 15 into a detector 19. The detector may be
located in the back-projected pupil plane P, which is at the focal
length F of the lens 16. In practice, the pupil plane itself may be
inaccessible, and may instead be re-imaged with auxiliary optics
(not shown) onto the detector located in a so-called conjugate
pupil plane P'. The detector is preferably a two-dimensional
detector so that a two-dimensional angular scatter spectrum or
diffraction spectrum of a substrate target 30 can be measured. In
the pupil plane or conjugate pupil plane, the radial position of
radiation defines the angle of incidence/departure of the radiation
in the plane of focused spot S, and the angular position around an
optical axis O defines azimuth angle of the radiation. The detector
19 may be, for example, an array of CCD or CMOS sensors, and may
use an integration time of, for example, 40 milliseconds per
frame.
[0060] Radiation in reference path RP is projected onto a different
part of the same detector 19 or alternatively on to a different
detector (not shown). A reference beam is often used for example to
measure the intensity of the incident radiation, to allow
normalization of the intensity values measured in the scatter
spectrum.
[0061] The various components of illumination system 12 can be
adjustable to implement different metrology `recipes` within the
same apparatus. Color filter 12b may be implemented for example by
a set of interference filters to select different wavelengths of
interest in the range of, say, 405-790 nm or even lower, such as
200-300 nm. An interference filter may be tunable rather than
comprising a set of different filters. A grating could be used
instead of interference filters. Polarizer 12c may be rotatable or
swappable so as to implement different polarization states in the
radiation spot S. Aperture device 13 can be adjusted to implement
different illumination profiles. Aperture device 13 is located in a
plane P'' conjugate with pupil plane P of objective lens 16 and the
plane of the detector 19. In this way, an illumination profile
defined by the aperture device defines the angular distribution of
light incident on substrate radiation passing through different
locations on aperture device 13.
[0062] The detector 19 may measure the intensity of scattered light
at a single wavelength (or narrow wavelength range), the intensity
separately at multiple wavelengths or integrated over a wavelength
range. Furthermore, the detector may separately measure the
intensity of transverse magnetic- and transverse electric-polarized
light and/or the phase difference between the transverse magnetic-
and transverse electric-polarized light.
[0063] Where a metrology target T is provided on substrate W, this
may be a 1-D grating, which is printed such that after development,
the bars are formed of solid resist lines. The target may be a 2-D
grating, which is printed such that after development, the grating
is formed of solid resist pillars or vias in the resist. The bars,
pillars or vias may alternatively be etched into the substrate.
This pattern is sensitive to chromatic aberrations in the
lithographic projection apparatus, particularly the projection
system PS. Illumination symmetry and the presence of such
aberrations will manifest themselves in a variation in the printed
grating. Accordingly, the scatterometry data of the printed
gratings is used to reconstruct the gratings. The parameters of the
1-D grating, such as line widths and shapes, or parameters of the
2-D grating, such as pillar or via widths or lengths or shapes, may
be input to the reconstruction process, performed by processing
unit PU, from knowledge of the printing step and/or other
scatterometry processes.
[0064] In addition to measurement of parameters by reconstruction,
angle resolved scatterometry is useful in the measurement of
asymmetry of features in product and/or resist patterns. A
particular application of asymmetry measurement is for the
measurement of a focus parameter (for example, the focus setting
during exposure of the target) from targets which print with a
focus dependent asymmetry. The concepts of asymmetry measurement
using the instrument of FIG. 3 or 4 are described for example in
published patent application US2006066855A1 cited above. Simply
stated, while the positions of the diffraction orders in the
diffraction spectrum of the target are determined only by the
periodicity of the target, asymmetry of intensity levels in the
diffraction spectrum is indicative of asymmetry in the individual
features which make up the target. In the instrument of FIG. 4,
where detector 19 may be an image sensor, such asymmetry in the
diffraction orders appears directly as asymmetry in the pupil image
recorded by detector 19. This asymmetry can be measured by digital
image processing in unit PU, and from this, focus can be
determined.
[0065] FIG. 5(a) shows in more detail an inspection apparatus
implementing angle-resolved scatterometry by the same principles as
the apparatus of FIG. 4, with additional adaptations for performing
so-called dark field imaging. The apparatus may be a stand-alone
device or incorporated in either the lithographic apparatus LA,
e.g., at the measurement station, or the lithographic cell LC. An
optical axis, which has several branches throughout the apparatus,
is represented by a dotted line O. A target grating T and
diffracted rays are illustrated in more detail in FIG. 5(b).
[0066] The same reference numbers are used for components described
already in the FIG. 4 apparatus. The illumination path is labeled
IP as before. The reference path RP is omitted, for clarity.
Compared with that apparatus, a second beam splitter 17 divides the
collection path into two branches. In a first measurement branch,
detector 19 records a scatter spectrum or diffraction spectrum of
the target exactly as described above. This detector 19 may be
referred to as the pupil image detector.
[0067] In the second measurement branch, imaging optical system 22
forms an image of the target on the substrate W on sensor 23 (e.g.
a CCD or CMOS sensor). An aperture stop 21 is provided in a plane
that is in the collection path in a plane conjugate to the
pupil-plane (it may also be called a pupil stop). Aperture stop 21
can take different forms, just as the illumination aperture can
take different forms. Typically, aperture stop 21 functions to
block the zeroth order diffracted beam so that the image of the
target formed on sensor 23 is formed only from the first order
beam(s). This is the so-called dark field image, equivalent to dark
field microscopy. The images captured by sensors 19 and 23 are
output to image processor and controller PU, the function of which
will depend on the particular type of measurements being
performed.
[0068] In the illumination path in this example, additional optics
are shown such that a field stop 13' can be placed in a plane
conjugate with the plane of the target and the image sensor 23.
This plane may be referred to as a field plane, or conjugate image
plane, and has the property that each spatial position across the
field plane corresponds to a position across the target. This field
stop may be used for example to shape the illumination spot for a
particular purpose, or simply to avoid illuminating features that
are within the field of view of the apparatus but not part of the
target of interest. The following drawings and discussion refer, by
way of example, to techniques for implementation of the function of
aperture device 13, but the present disclosure also encompasses use
of the same techniques to implement the function of field stop
13'.
[0069] As shown in more detail in FIG. 5(b), target grating T is
placed with substrate W normal to the optical axis O of objective
lens 16. In the case of an off-axis illumination profile, A ray of
illumination I impinging on grating T from an angle off the axis O
gives rise to a zeroth order ray (solid line 0) and two first order
rays (dot-chain line +1 and double dot-chain line -1). It should be
remembered that with an overfilled small target grating, these rays
are just one of many parallel rays covering the area of the
substrate including metrology target grating T and other features.
Since the aperture in plate 13 has a finite width (necessary to
admit a useful quantity of light, the incident rays I will in fact
occupy a range of angles, and the diffracted rays 0 and +1/-1 will
be spread out somewhat. According to the point spread function of a
small target, each order +1 and -1 will be further spread over a
range of angles, not a single ideal ray as shown.
[0070] Different modes of illumination are possible by using
different apertures. Apertures 13N (`north`) and 13S (`south`) each
provide off-axis illumination from a specific narrow range of
angles only. Returning to FIG. 5(a), this is illustrated by
designating diametrically opposite portions of the annular aperture
as north (N) and south (S). The +1 diffracted rays from the north
portion of the cone of illumination, which are labeled +1(13N),
enter the objective lens 16, and so do the -1 diffracted rays from
the south portion of the cone (labeled -1(13S)). As described in
the prior applications mentioned in the introduction, using the
dark-field imaging sensor 23 while switching between apertures 13N,
13S of this type is one way of obtaining asymmetry measurements
from multiple small targets. Aperture stop 21a can be used to block
the zeroth order radiation when using off-axis illumination.
[0071] While off-axis illumination is shown, on-axis illumination
of the targets may instead be used and an aperture stop with an
off-axis aperture is used to pass substantially only one first
order of diffracted light to the sensor. In one example, prisms 21b
are used in place of aperture stop 21 which have the effect of
diverting the +1 and -1 orders to different locations on sensor 23
so that they can be detected and compared without making two
images. This technique, is disclosed in the above-mentioned
published patent application US2011102753A1, the contents of which
are hereby incorporated by reference. 2nd, 3rd and higher order
beams (not shown in FIG. 5) can be used in measurements, instead of
or in addition to the first order beams.
[0072] When monitoring a lithographic process, it is desirable to
monitor focus of the lithography beam on the substrate. One known
method of determining the focus setting from a printed structure is
by measuring the critical dimension (CD) of the printed structure.
CD is a measure of the smallest feature (e.g., line width of an
element). The printed structure may be a target, such as a
line-space grating, formed specifically for focus monitoring. It is
known that CD usually displays 2nd order response to focus, forming
what is known as a "Bossung curve" on a plot of CD (y-axis) against
focus (x-axis). A Bossung curve is a substantially symmetrical
curve which is substantially symmetrical around a peak representing
the best focus. The Bossung curve may be substantially parabolic in
shape. There are several drawbacks to this approach. One drawback
is that the method shows low sensitivity near best focus (due to
the parabolic shape of the curve). Another drawback is that the
method is insensitive to the sign of any defocus (as the curve is
largely symmetrical around best focus). Also this method is
sensitive to inter alia dose and process variation (crosstalk).
[0073] To address these issues, diffraction based focus (DBF) was
devised. Diffraction based focus may use target forming features on
the reticle which print targets having a degree of asymmetry which
is dependent on the focus setting during printing. This degree of
asymmetry can then be measured using a scatterometery based
inspection method, for example by measuring the intensity asymmetry
between the intensities of +1st and -1st order radiation diffracted
from the target, to obtain a measure of the focus setting.
[0074] FIG. 6 illustrates DBF target forming design 615 configured
for diffraction based focus measurements. It comprises plural DBF
structures 620, each of which comprises high resolution
substructures 625. The high resolution substructures 625 on top of
a base pitch creates an asymmetric resist profile for each DBF
structure 620, with the degree of asymmetry being dependent upon
focus. Consequently a metrology tool can measure the degree of
asymmetry from a target formed using DBF target forming design 615
and translate this into the scanner focus.
[0075] While the DBF target forming design 615 enables diffraction
based focus measurements, it is not suitable for use in all
situations. EUV resist film thicknesses are significantly lower
than those used in immersion lithography, which makes it difficult
to extract accurate asymmetry information from the asymmetric
profile of the structures forming part of a target. In addition
such structures may not comply with the strict design constraints
applicable to certain product structures. During the chip making
process all features on the reticle must print and stand up to
subsequent processing steps. Semiconductor manufacturers use design
rules as a means to restrict the feature designs to ensure the
printed features conform to their process requirements. An example
of such a design rule relates to the allowable size of structures
or pitches. Another example design rule relates to pattern density,
which may restrict the density of a resulting resist pattern to be
within a particular range.
[0076] It is therefore proposed to monitor focus using at least a
first target and a second target which have been formed with a best
focus offset df between the two targets. As before, the focus
response with a measurement value for a target parameter (for
example CD or other measurements as will be described below) takes
the form of a Bossung curve for each of the first and second
targets. Focus is a function of a first measurement value of a
target parameter obtained from measurement of the first target and
a second measurement value of a target parameter obtained from
measurement of the second target. Therefore, it is proposed that
measured values of a parameter from the first and second targets be
obtained and a value for focus derived from these measured values.
A specific example of how focus may be derived is described below,
with reference to FIG. 7. However, the skilled person will realise
that there are many alternative methods that allow focus to be
extracted from the measured values obtained from the first and
second targets. While the description below specifically discusses
using a difference of the two measurement values (whether they be
intensity values or otherwise), other mathematical operations and
methods may be used to extract a focus value. For example, it is
possible to divide one of the measurement values (from one of the
first and second targets) into the other measurement value (from
the other of the first and second targets).
[0077] FIG. 7(a) shows a plot of a target parameter Pt against
focus f for both the first target and second target. It shows a
first Bossung curve 700 corresponding to the first target and a
second Bossung curve 710 corresponding to the second target. Also
shown is the best focus offset df (the focus offset between the two
peaks of Bossung curves 700, 710). Where the targets overlap 720 is
the focus range through which a difference of the target parameter
for the first target and the target parameter for the second target
has an essentially linear relationship with focus. This is
illustrated in FIG. 7(b), which is a plot of this difference
Pt2-Pt1 (where Pt1 is the target parameter for the first target and
Pt2 is the target parameter for the second target) against focus.
As can be seen, the relationship 740 is linear. The Pt2-Pt1 metric
may be sensitive to crosstalk, e.g. by dose and/or process. A more
robust metric may be Pt2-Pt1/PtAV, where PtAV, is the average of
Pt2 and Pt1. The relationship 750 (dotted) of Pt2-Pt1/PtAV with
focus is also shown in FIG. 7(b). This relationship is still
sufficiently linear while being more robust against crosstalk.
[0078] In the specific example illustrated, the slope of
relationship 740 or relationship 750 can be described by 2*df*a
where df is the best focus offset and a is the Bossung curvature.
Accordingly, focus can be obtained from the following equation
(Equation 1):
f = P t 2 - P t 1 2 df a P tAV ##EQU00001##
where P.sub.tAv in the denominator is optional.
[0079] To increase focus sensitivity, it is possible to increase
the best focus offset, thereby increasing the slope of relationship
740 or relationship 750.
[0080] In the above discussion, it should be appreciated that any
target parameter Pt can be used provided it has a Bossung curve
response with focus. While CD may be used, a new diffraction based
focus methodology, is proposed which has significant advantages
over prior methods. This method comprises using intensity signals
obtained from diffraction orders of radiation scattered by the
first target and second target to determine focus. In particular,
it is proposed to use intensity values of a single diffraction
order from each of the first and second targets to determine focus.
The proposed method may use intensity values from corresponding
diffraction orders of the first and second targets. For example,
the intensity values could be that of the +1st (or -1st)
diffraction orders from the first and second targets.
Alternatively, or in combination, the intensity values could be
that of the zeroth diffraction orders from the first and second
targets.
[0081] In a specific example of such a method, it is proposed to
use the difference dI in the measured intensity of a diffraction
order of radiation scattered by the first target and the measured
intensity of the corresponding diffraction order of radiation
scattered by the second target. This difference is hereafter
referred to as the dI metric. However, the dI metric may be any
metric derived from diffraction intensity values from the first and
second targets (e.g., by dividing one of the intensity values into
the other).
[0082] In an embodiment, the dI metric may be the difference in
corresponding first diffraction orders (or higher diffraction
orders), e.g., the +1st diffraction order of radiation scattered by
the first target and the +1st diffraction order of radiation
scattered by the second target (clearly minus orders could equally
be used). In another embodiment, the dI metric may comprise the
difference between intensity measurements of zeroth orders from the
first and second targets.
[0083] As before, the dI metric may be divided by an average of the
intensity measurements Iav, to reduce the effect of crosstalk.
However, the Bossung curvature for the dI metric is only weakly
dependent on dose, such that the dI metric may already exhibit a
sufficiently low dose crosstalk.
[0084] Using the dI metric in this way provides good signal
strength and signal-to noise response, even when the targets
comprise shallow gratings (e.g., for use in EUV lithography).
[0085] As mentioned above, the dI metric may comprise the
difference of the zeroth orders of radiation scattered by the first
and second targets. In this way, targets with smaller pitches can
be used. Consequently, target pitch for the first and second
targets can be chosen to agree with any customer design rules.
Also, smaller target pitches means that the overall target size can
be reduced. Multiple pitches are also possible. Using the zeroth
order radiation means that the diffracted radiation does not need
to be captured, and the dI metric describes differences in light
absorbed by targets having a relative best focus offset.
Measurement of zeroth orders may also increase signal strength and
signal-noise characteristics.
[0086] Where first order diffracted radiation is used, and since
only a single first order is required per measurement, the pitch
required to use first order radiation light is reduced to .lamda./2
(where .lamda. is the detection wavelength) in the limit of
numerical aperture NA=1. At present this limit is .lamda.. This
will mean that the linear target dimension can be reduced by a
factor of 2 and the real estate by a factor of 4.
[0087] The best focus offset between the first target and second
target can be introduced in a number of ways. In one embodiment,
the lithographic apparatus used to print the targets may have a
deliberate, controlled astigmatism. The astigmatism may be
introduced to the projection optics via a number of manipulators
included within the projection optics. The projection lenses in
many lithographic apparatuses enable a sufficiently large
astigmatism offset to create a best focus offset, without unwanted
wavefront effects. In an embodiment, the astigmatism may introduce
a best focus offset between horizontal and vertical features. To
exploit this, the first and second targets may comprise
respectively a horizontal grating and a vertical grating (or vice
versa).
[0088] In an embodiment, the best focus offset can be introduced by
the reticle (also referred to as a patterning device or mask). It
is proposed to have pairs of targets (e.g., line-space gratings)
incorporated on a reticle. The reticle may contain locations
approximately the size of a target (for example 20.times.20 .mu.m,
8.times.8 .mu.m or 5.times.5 .mu.m) and a border zone where the
substrate is etched to a depth d. One of the first and second
targets pair is deposited at normal mask level, the other at a
(preferably adjacent) etched position.
[0089] FIG. 8 illustrates a number of alternative reticle
arrangements for achieving such an arrangement. FIG. 8a shows, in
cross section, a reference target which may be used to print one of
said first target and said second target. This is a conventional
target feature on a reticle, comprising radiation blocking
structures 800 on a transparent reticle substrate 810. The reticle
may be of any structure or material. For example, the transparent
reticle substrate 810 may comprise quartz glass, and the radiation
blocking structures 800 may comprise chromium, molybdenum silicide
(any opacity) or tantalum boron nitride.
[0090] It is proposed that this reference target is used with one
of the target arrangements of FIG. 8(b), 8(c) or 8(d). However, any
combination of two (or more) of any of the targets shown in FIG. 8
may be used provided they result in a relative best focus
offset.
[0091] FIG. 8(b) shows a transparent reticle substrate 810 which,
in the region of the target, has been etched to a depth d before
the addition of radiation blocking structures 800. Such an
arrangement provides a simple target, but manufacture is
complicated by not being able to do this in the "mask shop". FIG.
8(c) shows an arrangement similar to that of FIG. 8(a), but where
the reticle substrate 810 has been etched through to a depth d
after deposition of the radiation blocking structures 800. FIG.
8(d) shows an arrangement where the radiation blocking structures
800 are topped with additional metal (e.g., chromium) caps 820.
This is similar to a TIS (transmission image sensor) target. Such
an arrangement is not possible in EUV lithography.
[0092] In the etched examples above, the depth d may be for example
0.1 .mu.m or more, more specifically in the region of 0.1 .mu.m to
5 .mu.m, or 0.5 .mu.m to 5 .mu.m, and for example 0.5 .mu.m to 3
.mu.m. In an embodiment, depth d may be in the region of 1
.mu.m.
[0093] FIG. 9 illustrates a further reticle arrangement for
achieving a best focus offset in a first (reference) target. This
arrangement is suited (by way of example) for OMOG (Opaque MoSi on
Glass) and for attPSM (attenuated phase-shifting mask) reticle
types. In particular, the trend for attPSM reticles is for a
reduction in the Cr thickness. This will reduce the Cr topping
effect of FIG. 8(d) embodiment described above. The reticle
production comprises depositing one (or more) extra absorber stacks
are on the blank; where the blank comprises the reticle substrate
on which is deposited a single absorber stack. The absorber stack
may comprise an opaque layer (e.g., a MoSi layer) topped with a
metal layer (e.g., a Cr layer).
[0094] FIG. 9(a) shows the new reticle blank. It comprises a
reticle substrate 910 topped with two absorber stacks. A first
absorber stack comprises first layer 920a (e.g., a MoSi layer) and
second layer 920b (e.g., a Cr layer). The second absorber layer
also comprises two layers: third layer 920c (e.g., a MoSi layer)
and fourth layer 920d (e.g., a Cr layer).
[0095] FIG. 9(b) shows the final reticle arrangement. It shows
first target 930 and second target 940. First target 930 is
conventional in that it comprises a single opaque layer (e.g.
formed from first layer material 920a) of blocking structures 900.
The second target 940 comprises blocking structures 950, each
having three layers: a first layer 950a, a second layer 950b and a
third layer 950c, formed respectively from first layer material
920a, second layer material 920b and third layer material 920c. The
fourth layer 920d is removed completely.
[0096] The process for producing such a reticle may comprise the
following steps:
[0097] Deposit one or more extra absorber stacks on the blank (this
may be performed by the provider of the blank);
[0098] Etch the target layer (containing the targets 930, 940)
through both stacks to the depth of substrate 910.
[0099] Remove the extra stack for the first target and its
corresponding layer. This layer contains the product and the
reference targets (the first targets), but not the second targets.
Resist covers the second targets during this step;
[0100] Etch the reference layer in a conventional manner. Resist
covers the second targets during this step
[0101] A further method for obtaining two targets with a best focus
offset between them comprises providing a first target comprising a
line-space target with a focus insensitive side wall angle (SWA),
such that the SWA of the individual structures of the first target
is focus insensitive, and a second target with focus sensitive SWA.
The second target may comprise a segmented line, the segmentation
being sub-resolution with respect to the lithography apparatus.
[0102] FIG. 10 illustrates a reticle arrangement for producing such
first and second targets. The first target 1000 (shown in part)
comprises a line-space target having structures 1010 which produce
corresponding target structures on a substrate with an SWA that is
focus insensitive. In an embodiment, the SWA is small (i.e., close
to vertical). The second target 1020 (shown in part) comprises a
line-space target having segmented line structures 1030. The
segmented line structures 1030 comprise high resolution
substructures 1040, which may be similar to high resolution
substructures 625 of FIG. 6. The second target 1020 is such that
the resulting target exposed on a substrate has a focus dependent
SWA.
[0103] The first target 1000 and second target 1020 each have a
target parameter response with focus which describe Bossung curves
having a best focus offset, similar to the response illustrated in
FIG. 7. This best focus offset is as a result of the focus
dependent SWA of only one of the targets. SWA varies linearly with
focus, which causes the shift in the Bossung peak. By this method,
an asymmetric target (such as that illustrated in FIG. 6) can be
separated into two separate symmetric targets having similar
performance. This enables more efficient target selection and the
use of the full pitch for parameter values.
[0104] An advantage of introducing the best focus offset in the
reticle (rather than via astigmatism in the projection optics) is
this allows both on-product and off-product focus monitoring. The
requirement of astigmatism in the projection optics means that such
methods can only be used for off-product monitoring.
[0105] FIG. 11 is a flowchart of the steps of a method for
monitoring a focus parameter during a lithographic process
according to an exemplary embodiment. The steps are as follows, and
are then described in greater detail thereafter:
[0106] 1100--Start.
[0107] 1110--Print first and second targets with a relative best
focus offset;
[0108] 1120--Perform first measurement from inspection of the first
target to obtain first measurement value;
[0109] 1130--Perform second measurement from inspection of second
target to obtain second measurement value;
[0110] 1140--Calculate focus from difference of first measurement
value and second measurement value;
[0111] 1150--Use calculated focus measurement in focus setting for
subsequent exposures.
[0112] 1160--End.
[0113] At step 1110, first and second targets (at least) are
printed with a relative best focus offset as already described. The
relative best focus offset may be introduced, for example, via a
relative depth offset on the reticle between the target forming
structures which form the first and second targets. Alternatively,
relative best focus offset may be introduced via astigmatism in the
projection optics of the lithographic system. By way of a further
alternative example, the reticle arrangement illustrated in FIG. 10
may be used. Other methods of introducing a relative best focus
offset between two targets are also possible and are envisaged
within the scope of this disclosure.
[0114] At step 1120, a first measurement is performed from
inspection of the first target to obtain a first measurement value
for a target parameter. In an embodiment, this first measurement
may be of the intensity (or related parameter) of one of the
diffraction orders of radiation scattered by the first target. This
first measurement may be obtained using any of the scatterometer
devices described herein, for example. It is also contemplated
within the scope of this disclosure that the first measurement be a
CD measurement (whether obtained using a scatterometer, scanning
electron microscope or other suitable device) or any other
measurement of a parameter which has a Bossung curve relationship
with focus.
[0115] At step 1130, a second measurement is performed from
inspection of the second target to obtain a second measurement
value for a target parameter. This second measurement should be
performed using the same method as the first measurement. Where the
first measurement is of the intensity (or related parameter) of one
of the diffraction orders of radiation scattered by the first
target, the second measurement should be of the same diffraction
order of radiation scattered by the second target. The diffraction
order may be either of the first diffraction orders or the zeroth
diffraction order. However, higher diffraction orders can also be
used and are within the scope of the disclosure. Further
measurements may be made if there are more than two targets
printed. These additional targets may each comprise best focus
offsets that are different to that of said first target and/or said
second target.
[0116] It should be appreciated that step 1120 and step 1130 may be
performed as a single step such that the first measurement value
and second measurement value are obtained in a single acquisition.
In addition, where there are more than two targets being measured,
all the targets may be measured in a single acquisition to obtain a
corresponding number of measurement values. In a specific example,
a measurement device, such as that illustrated in FIG. 5, can be
used to measure a composite target comprising multiple individual
targets (individual periodic structures or gratings). The gratings
of the composite target may be positioned closely together so that
they will all be within an image field or measurement spot formed
by the illumination beam of the metrology apparatus. In this way,
the gratings can be all simultaneously illuminated and
simultaneously imaged on the detector. These images can then be
processed to identify the separate images of the gratings. This can
be done by pattern matching techniques, so that the images do not
have to be aligned very precisely at a specific location within the
sensor frame. Once the separate images of the gratings have been
identified, the intensities of those individual images can be
measured, e.g., by averaging or summing selected pixel intensity
values within the identified areas. In another embodiment, the
first and second targets may be comprised within a composite
target, but measured separately in two separate acquisitions.
[0117] At step 1140, the focus is calculated from the first and
second measurement values, for example from the difference of the
first and second measurement values. This calculation can be
performed using Equation 1 or other suitable equation or
method.
[0118] At step 1150, the calculated focus can then be used in focus
parameter monitoring during subsequent lithographic processes, so
as to maintain focus accuracy and consistency during exposure.
[0119] The above discussion describes methods for determining
focus. However, also disclosed is a method for measurement of dose.
Current diffraction based dose metrology is based on simulation of
the diffraction pattern of a parametrized resist pattern. The
parameters are then adjusted so that the resulting zero order
diffraction efficiency, notably its angular dependency, agrees with
measurements. Line/space (LS) targets with CD and pitch in a range
of interest are used. This method is referred to as CD
reconstruction (CDR), and depends on the correctness of the
parametrized model. The model is necessarily a schematic
approximation to the resist pattern with a limited number of
parameters. The model requires knowledge of the geometry and
optical parameters of the stack. This is commonly proprietary
information and therefore difficult to obtain, and may be
inaccurate.
[0120] Therefore, a simpler method for determining dose is
proposed, which comprises forming a first line-space target and a
second line-space target having resist patterns with inverse duty
cycles, or in pairs of resist patterns with matched properties such
that cross talk, both offset and scaling, is minimised. In an
embodiment, the first and second targets each have the same pitch
but, where the first target is a line target, the second target
will be a corresponding space target, such that the width of the
lines of the first target is equal to the width of the spaces of
the second target. FIG. 12 illustrates such a target arrangement
having a first target 1200 with individual resist features 1210
having a CD a, and a second target 1220 with individual resist
features 1230 having a CD b, each target having the same pitch.
[0121] FIG. 13 shows a graph 1310 of Resist (line) CD against first
order intensity I for a line space target of pitch 600 nm. It can
be seen that the measured intensity for corresponding line and
space targets (e.g., a first target with resist CD of 150 nm and a
second target with resist CD of 450 nm) should be approximately the
same. However resist CD is dependent on dose, such that an increase
in dose results in a decrease in resist CD and vice versa.
Consequently, an increase in dose (for example), will cause a
decrease in resist CD for both the first target and second target.
This will cause a decrease in the first order intensity for the
first target and an increase in the first order intensity for the
second target. The dose sensitivity for the first and second
targets can therefore be seen to be opposite. The difference of the
intensity measurements for the first and second targets can
therefore be a target parameter used as a dose metric.
[0122] It can be noted that any process variation, to a certain
extent, may have the same effect as dose variation. Process
variations such as post exposure bake (PEB) and secondary electron
blur (SEB) variation, as well as bottom anti-reflective coating
(BARC) and resist thickness variation, alter the exposure intensity
in the thin resist film. However, BARC and resist thickness
variation affect the measured diffraction intensity: thicker BARC
and resist results in an increase in the measured first order
intensity. This effect does not depend on CD and the resulting
intensity change is equal for both targets. If the targets have a
matching nominal first order intensity response, then any crosstalk
induced signal offset will cancel when taking the difference of
measurements from the two targets. Any crosstalk induced scaling
will be equal for the signal difference and the signal average.
Therefore it is expected that the ratio of difference and average
measured intensities will be robust against process crosstalk, but
sensitive to dose including process induced dose effects.
[0123] It is proposed to measure dose calibration curves as a
function of focus. The actual dose can be inferred from the dose
calibration, assuming that focus is known. Focus can be determined
using any of the methods disclosed herein, by earlier diffraction
based focus methods (e.g., using structures of a form shown in FIG.
6), or any other suitable method.
[0124] While the first and second targets are described as
line-space targets, they can comprise any dose dependent resist
targets which produce suitable first order responses. In an
embodiment, a correction may be determined to account for any
difference in response to process induced dose effects between the
target and product.
[0125] The proposed method enables first order intensity
measurements to be used to determine dose without the need for
model simulation or prediction. The method suppresses sensitivity
to process variations on the target performance. The impact of
illumination and dose conditions on target performance is small.
The method is sensitive to scanner dose and dose-like process
effects. The use of first order signals are more accurate than
using zeroth order signals as they have better shot noise
performance.
[0126] The targets are described above as line-space grating
targets, as these are simple to produce and measure. However, the
targets may comprise any structure which results in a Bossung curve
response between a measurable target parameter and focus. For
example, the targets may comprise combined horizontal and vertical
line-space gratings, forming a "contact hole" arrangement. Such a
target may enable more diffraction orders to be captured. The
target arrangements may comprise more than two targets.
Consequently, the methods described herein may comprise performing
measurements on more than two targets.
[0127] In some circumstances, design rules are imposed which result
in constraints on certain parameters of reticle features. An
example of such design rules are the provision of design grids
which have (and therefore impose on the target) a fixed pitch
and/or CD for a line-space target. It may be that many of the
targets described herein would infringe such design rules.
[0128] By way of specific example, a design grid based design rule
may impose a target pitch of 100 nm and a CD of 40 nm; that is
lines can only be formed with a CD of 40 nm and on a grid having a
100 nm pitch in the direction of the line-space grating. However,
it may be desirable that a target actually has a pitch of 600 nm,
such that first order signals can be detected and measured. It is
proposed that such a line-type target can be obtained by providing
1 or 2 such lines on corresponding grid locations in a row of such
grids. Each grid therefore will define a single target feature. An
analogous space-type target can be obtained by providing 4 or 5
such lines on corresponding grid locations in a row of such
grids.
[0129] It is desirable to image these patterns such that they have
the designed dimensions and a sufficiently large depth of focus on
the substrate. Therefore, the lines may be biased (e.g., in a
manner similar to optical proximity correction methods) and (for
example 20 nm) assist features may optionally be placed on the
empty locations on each grid. In this way, the target may be made
more stable and more symmetrical (e.g., in terms of SWA).
[0130] FIGS. 14(a) to 14(f) each comprise an example of possible
target features based upon a grid. In particular, where the CD is
40 nm and grid pitch 100 nm as per the example previously given,
the 2 line example of FIGS. 14(e) and 4 line example of FIG. 14(c)
emulate target features for respectively line and space targets
having a CD of about 100-150 nm and pitch of 600 nm.
[0131] A method of manufacturing devices using the lithographic
process can be improved by providing an inspection apparatus as
disclosed herein, using it to measure processed substrates to
measure parameters of performance of the lithographic process, and
adjusting parameters of the process (particularly focus) to improve
or maintain performance of the lithographic process for the
processing of subsequent substrates.
[0132] It should be understood that the particular parameters used
in the above examples are not the only ones that may be defined.
Additional and/or alternative parameters can be used in a real
design process, according to limitations of the lithographic
apparatus and the inspection apparatus to be used for the
metrology. While the target structures described above are
metrology targets specifically designed and formed for the purposes
of measurement, in other embodiments, properties may be measured on
targets which are functional parts of devices formed on the
substrate. Many devices have regular, grating-like structures. The
terms `target grating` and `target structure` as used herein do not
require that the structure has been provided specifically for the
measurement being performed.
[0133] In association with the physical grating structures of the
targets as realized on substrates and patterning devices, an
embodiment may include a computer program containing one or more
sequences of machine-readable instructions describing a methods of
designing metrology recipes and/or controlling the inspection
apparatus to implement the illumination modes and other aspects of
those metrology recipes. This computer program may be executed for
example in a separate computer system employed for the
design/control process. Alternatively, the design process may be
wholly or partly performed within unit PU in the apparatus of FIG.
3, 4 or 5 and/or the control unit LACU of FIG. 2. There may also be
provided a data storage medium (e.g., semiconductor memory,
magnetic or optical disk) having such a computer program stored
therein.
[0134] Further embodiments according to the invention are provided
in below numbered clauses:
[0135] 1. A method of monitoring a focus parameter during a
lithographic process, said method comprising:
[0136] acquiring a first measurement value, said first measurement
value having been obtained from inspection of a first target;
[0137] acquiring a second measurement value, said second
measurement value having been obtained from inspection of a second
target,
[0138] wherein said first target and second target have been
exposed with a relative best focus offset;
[0139] determining the focus parameter from said first measurement
value and said second measurement value.
[0140] 2. A method according to clause 1 wherein said first
measurement value has been obtained from a first measurement of
radiation scattered from said first target and said second
measurement value has been obtained from a second measurement of
radiation scattered from said second target.
[0141] 3. A method according to clause 2 wherein said first
measurement is an intensity measurement of a diffraction order of
radiation scattered from said first target and said second
measurement is an intensity measurement of a corresponding
diffraction order of radiation scattered from said second
target.
[0142] 4. A method according to clause 3 wherein said diffraction
order is the zeroth diffraction order.
[0143] 5. A method according to clause 3 wherein said diffraction
order is a non-zeroth diffraction order.
[0144] 6. A method according to any preceding clause comprising the
steps of performing said first measurement to obtain said first
measurement value and performing said second measurement to obtain
said second measurement value.
[0145] 7. A method according to any of clauses 1 to 5 comprising
inspecting at least said first target and said second target in a
single measurement to obtain said first measurement value and said
second measurement value.
[0146] 8. A method according to any preceding clause wherein said
step of determining the focus parameter comprises determining the
focus parameter from the difference of said first measurement value
and said second measurement value.
[0147] 9. A method according to clause 8 wherein said difference of
said first measurement value and said second measurement value is
divided by an average of said first measurement value and said
second measurement value.
[0148] 10. A method according to clause 8 or 9 wherein the
variation of said first measurement value with focus and the
variation of said second measurement value with focus each define
corresponding Bossung curves having a relative focus offset, and
the variation of said difference of said first measurement value
and said second measurement value with focus is substantially
linear.
[0149] 11. A method according to clause 10 wherein the variation of
said difference of said first measurement value and said second
measurement value with focus is defined by a line having a slope
dependent upon the curvature of said Bossung curves and the
relative best focus offset.
[0150] 12. A method according to any preceding clause wherein the
relative best focus offset results from astigmatism in the
lithographic apparatus during formation of said first target and
second target.
[0151] 13. A method according to clause 12 wherein said astigmatism
results in a relative best focus offset between horizontal and
vertical structures, and wherein said first target comprises
substantially horizontal structures and said second target
comprises substantially vertical structures.
[0152] 14. A method according to any of clauses 1 to 12 wherein the
relative best focus offset is resultant from the patterning device
which defines said first target and second target.
[0153] 15. A method according to clause 14 wherein there is an
offset in the depth of the patterns in said patterning device which
define said first target and second target.
[0154] 16. A method according to clause 14 or 15 wherein one of the
patterns for defining one said first target or said second target
comprises line features which are taller, in a direction transverse
to the plane of the target when compared to the other of said first
target or said second target.
[0155] 17. A method according to clause 16, having at least one
additional layer of absorber material.
[0156] 18. A method according to clause 17 wherein the additional
layer comprises one of: a metal, molybdenum silicide or tantalum
boron nitride.
[0157] 19. A method according to clause 17 wherein the additional
layer comprises an additional stack of absorber material.
[0158] 20. A method according to clause 14 wherein said first
target comprises line features having a substantially focus
independent side wall angle and said second target comprises line
features having a focus dependent side wall angle.
[0159] 21. A method according to clause 20 wherein each of said
first target and said second target comprise line-space grating
structures, and wherein the pattern on the patterning device which
defines said second targets comprise segmented line features, each
segmented line feature having a pitch below the imaging resolution
of the lithographic process.
[0160] 22. A method according to any preceding clause comprising
forming said first target and said second target on a substrate
with said relative best focus offset.
[0161] 23. A method according to any preceding clause wherein said
first target and said second target each comprise line-space
grating structures.
[0162] 24. A method according to any preceding clauses wherein
design rules constrain target features to a set critical dimension
and on a grid of set pitch, wherein said first target and said
second target are each formed from rows of said grids such that
each line feature of said first target and said second target are
formed from one, or plural adjacent, corresponding target features
formed on said grids.
[0163] 25. A method according to any preceding clause wherein
patterns defining said first target and said second target do not
comprise features which purposely introduce focus dependent
asymmetry in said first target and said second target.
[0164] 26. A method according to any preceding clause
comprising:
[0165] making additional measurements of additional targets, said
additional targets being additional to said first target and said
second target, each of said additional targets having a different
best focus to that of said first target and/or said second target;
and using said additional measurements in said step of determining
the focus parameter.
[0166] 27. A method of monitoring a dose parameter during a
lithographic process, said method comprising:
[0167] acquiring a first measurement value, said first measurement
value having been obtained from inspection of a first target;
[0168] acquiring a second measurement value, said second
measurement value having been obtained from inspection of a second
target,
[0169] determining the dose parameter from said first measurement
value and said second measurement value;
[0170] wherein said first and second targets comprise corresponding
line and space targets having the same pitch and inverse duty
cycles.
[0171] 28. A method according to clause 26 or 27 wherein said step
of determining the dose parameter comprises determining the dose
parameter from the difference of said first measurement value and
said second measurement value.
[0172] 29. A method according to any of clauses 26 to 28 wherein
said step of determining the dose parameter comprises:
[0173] determining a focus parameter of said lithographic process;
and
[0174] referring to a dose calibration curve corresponding to said
determined focus parameter.
[0175] 30. A method according to clause 29 wherein said step of
determining the dose parameter comprises performing the method of
any of clauses 1 to 26.
[0176] 31. A metrology apparatus for measuring a parameter of a
lithographic process, the metrology apparatus being operable to
perform the method of any of clauses 1 to 30.
[0177] 32. A metrology apparatus according to clause 31
comprising:
[0178] a support for said substrate having a plurality of targets
thereon;
[0179] an optical system for measuring each target; and
[0180] a processor.
[0181] 33. A lithographic system comprising:
[0182] a lithographic apparatus comprising:
[0183] an illumination optical system arranged to illuminate a
pattern;
[0184] a projection optical system arranged to project an image of
the pattern onto a substrate; and
[0185] a metrology apparatus according to clause 31 or 32,
[0186] wherein the lithographic apparatus is arranged to use the
determined focus parameter and/or dose parameter calculated by the
metrology apparatus in applying the pattern to further
substrates.
[0187] 34. A computer program comprising processor readable
instructions which, when run on suitable processor controlled
apparatus, cause the processor controlled apparatus to perform the
method of any one of clauses 1 to 26.
[0188] 35. A computer program carrier comprising the computer
program of clause 34.
[0189] 36. A method of manufacturing devices wherein a device
pattern is applied to a series of substrates using a lithographic
process, the method including: [0190] using the method of any of
clauses 1 to 26 to monitor said focus parameter, and [0191]
controlling the lithographic process for later substrates in
accordance with the determined focus parameter.
[0192] 37. A method of manufacturing devices wherein a device
pattern is applied to a series of substrates using a lithographic
process, the method including: [0193] using the method of any of
clauses 27 to 30 to monitor said dose parameter, and [0194]
controlling the lithographic process for later substrates in
accordance with the determined dose parameter.
[0195] 38. A patterning device configured to pattern a beam of
radiation in a lithographic process according to a desired pattern,
said patterning device comprising first features for forming a
first target on a substrate during the lithographic process, and
second features for forming a second target on the substrate during
the lithographic process; wherein said second features are taller
than said first features, in a direction transverse to the plane of
the target and such that said first target and said second target
have a relative best focus offset.
[0196] 39. A patterning device according to clause 38 wherein said
first features and second features are deposited on a reticle
substrate and said second features are deposited on a section of
the reticle substrate which has been etched to a different level,
in a direction transverse to the plane of the target, compared to a
section of the reticle substrate on which said first features have
been deposited.
[0197] 40. A patterning device according to clause 38 or 39 wherein
said second features comprise one or more additional layers than
said first features.
[0198] 41. A patterning device according to clause 40 wherein the
one or more additional layers comprises a layer of absorber
material.
[0199] 42. A patterning device according to clause 41 wherein the
additional layer of absorber material comprises one of: a metal,
molybdenum silicide or tantalum boron nitride.
[0200] 43. A patterning device according to clause 40 wherein the
one or more additional layers comprises an additional stack of
absorber material, said additional stack of absorber material
comprising a first layer of absorber material and a second layer of
absorber material.
[0201] 44. A patterning device according to clause 43 wherein the
first layer of absorber material comprises molybdenum silicide or
tantalum boron nitride, and said second layer of absorber material
comprises a metal.
[0202] 45. A patterning device configured to pattern a beam of
radiation in a lithographic process according to a desired pattern,
said patterning device comprising first features for forming a
first target on a substrate during the lithographic process, and
second features for forming a second target on the substrate during
the lithographic process; wherein said first features are
configured to form said first target with line features having a
substantially focus independent side wall angle and said second
features are configured to form said second target with line
features having a focus dependent side wall angle.
[0203] 46. A patterning device according to clause 45 wherein each
of said first target and said second target comprise line-space
grating structures, and wherein the second features comprise
segmented line features, each segmented line feature having a pitch
below the imaging resolution of the lithographic process.
[0204] 47. A patterning device configured to pattern a beam of
radiation in a lithographic process according to a desired pattern,
said patterning device comprising first features for forming a
first target on a substrate during the lithographic process, and
second features for forming a second target on a substrate during
the lithographic process; wherein design rules constrain target
features to a set critical dimension and on a grid of set pitch,
wherein said first target and said second target are each formed
from rows of said grids such that each of said first features and
said second features are formed from one, or plural adjacent,
corresponding target features formed on said grids.
[0205] 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, 355, 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.
[0206] 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.
[0207] The foregoing description of the specific embodiments will
so fully reveal the general nature of the invention that others
can, by applying knowledge within the skill of the art, readily
modify and/or adapt for various applications such specific
embodiments, without undue experimentation, without departing from
the general concept of the present invention. Therefore, such
adaptations and modifications are intended to be within the meaning
and range of equivalents of the disclosed embodiments, based on the
teaching and guidance presented herein. It is to be understood that
the phraseology or terminology herein is for the purpose of
description by example, and not of limitation, such that the
terminology or phraseology of the present specification is to be
interpreted by the skilled artisan in light of the teachings and
guidance.
[0208] The breadth and scope of the present invention should not be
limited by any of the above-described exemplary embodiments, but
should be defined only in accordance with the following claims and
their equivalents.
* * * * *