U.S. patent application number 12/722902 was filed with the patent office on 2010-09-16 for lithographic apparatus and device manufacturing method.
This patent application is currently assigned to ASML Netherlands B.V.. Invention is credited to Arie Jeffrey DEN BOEF, Laurent Khuat Duy, Dimitra Sarri.
Application Number | 20100231881 12/722902 |
Document ID | / |
Family ID | 42730434 |
Filed Date | 2010-09-16 |
United States Patent
Application |
20100231881 |
Kind Code |
A1 |
DEN BOEF; Arie Jeffrey ; et
al. |
September 16, 2010 |
LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD
Abstract
The invention provides a level sensor configured to measure a
height level of a substrate comprising: a projection unit to
project a measurement beam on the substrate, a detection unit to
receive the measurement beam after reflection on the substrate, a
processing unit to calculate a height level on the basis of the
reflected measurement beam received by the detection unit, wherein
the level sensor further comprises a tilt measuring device, wherein
the tilt measuring device is arranged to receive at least partially
the reflected measurement beam, and configured to provide a tilt
signal representative for a tilt of the substrate with respect to a
nominal plane.
Inventors: |
DEN BOEF; Arie Jeffrey;
(Waalre, NL) ; Khuat Duy; Laurent; (Veldhoven,
NL) ; Sarri; Dimitra; (Eindhoven, NL) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
42730434 |
Appl. No.: |
12/722902 |
Filed: |
March 12, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61160126 |
Mar 13, 2009 |
|
|
|
Current U.S.
Class: |
355/67 ;
250/214R; 356/602 |
Current CPC
Class: |
G03F 9/7034 20130101;
G01B 11/0608 20130101 |
Class at
Publication: |
355/67 ;
250/214.R; 356/602 |
International
Class: |
G03B 27/54 20060101
G03B027/54; G01B 11/14 20060101 G01B011/14; G01B 11/24 20060101
G01B011/24 |
Claims
1. A level sensor configured to measure a height level of a
substrate, the level sensor comprising: a projection unit to
project a measurement beam on the substrate; a detection unit to
receive the measurement beam after reflection on the substrate; a
processing unit to calculate a height level on the basis of the
reflected measurement beam received by the detection unit; and a
tilt measuring device arranged to receive at least part of the
reflected measurement beam, and configured to provide a tilt signal
representative for a tilt of the substrate with respect to a
nominal plane.
2. The level sensor of claim 1, wherein the tilt measuring device
is connected to the processing unit to feed the tilt signal to the
processing unit in order to correct the height level on the basis
of the tilt of the substrate.
3. The level sensor of claim 1, wherein the tilt measuring device
comprises at least two tilt detectors to receive the at least part
of the reflected measurement beam, wherein the at least two tilt
detectors are spaced in a tilt sensitive direction, and wherein a
comparison of a ratio between relative amounts of radiation
received by the at least two tilt detectors is representative for
the tilt of the substrate.
4. The level sensor of claim 3, wherein the processing unit is
configured to make the comparison.
5. The level sensor of claim 1, wherein the tilt measuring device
comprises a beam splitting device to split the at least part of the
measurement beam into at least two parts each part directed to one
of the at least two tilt detectors, wherein a comparison of a ratio
between relative amounts of radiation received by the at least two
tilt detectors is representative for the tilt of the substrate.
6. The level sensor of claim 5, wherein the processing unit is
configured to make the comparison.
7. The level sensor of claim 1, comprising a partially reflective
mirror arranged to receive the reflected measurement beam and to
split the reflected measurement beam in a first part to be received
by the detection unit and a second part to be received by the tilt
measuring device.
8. The level sensor of claim 7, wherein the partially reflective
mirror is more reflective than transmissive.
9. The level sensor of claim 1, wherein the projection unit
comprises a radiation output to provide a measurement beam, and a
projection grating arranged to receive the measurement beam and to
impart the measurement beam with a periodic radiation intensity,
and wherein the detection unit comprises a detection grating
arranged to receive the reflected measurement beam, and at least
one detector arranged to receive the measurement beam.
10. The level sensor of claim 1, wherein the tilt measuring device
is configured to determine a tilt of the substrate about an axis
perpendicular to the plane in which the measurement beam runs.
11. The level sensor of claim 1, wherein the measurement beam runs
in an y-z plane and the tilt measuring device is configured to
determine a tilt of the substrate about the x-axis (Rx).
12. A lithographic apparatus comprising: an illumination system
configured to condition a radiation beam; a support constructed to
support a patterning device, the patterning device being capable of
imparting the radiation beam with a pattern in its cross-section to
form a patterned radiation beam; a substrate table constructed to
hold a substrate; a projection system configured to project the
patterned radiation beam onto a target portion of the substrate;
and a level sensor configured to measure a height level of a
substrate on the substrate table, the level sensor comprising: a
projection unit to project a measurement beam on the substrate, a
detection unit to receive the measurement beam after reflection on
the substrate, a processing unit to calculate a height level on the
basis of the reflected measurement beam received by the detection
unit, and a tilt measuring device arranged to receive at least part
of the reflected measurement beam, and configured to provide a tilt
signal representative for a tilt of the substrate with respect to a
nominal plane.
13. The lithographic apparatus of claim 12, wherein the tilt
measuring device is connected to the processing unit to feed the
tilt signal to the processing unit in order to correct the height
level on the basis of the tilt of the substrate.
14. A method to determine a height map of a substrate, the method
comprising: measuring a height of the substrate at a measurement
location by projecting a measurement beam on the substrate using a
projection unit, receiving the reflected beam from the substrate
using a detection unit, determining a tilt of the substrate, and
determining a height level on the basis of the reflected
measurement beam and the determined tilt of the substrate;
repeating the measuring for multiple measurement locations on the
substrate; and determining a height map of the substrate on the
basis of the height levels.
Description
[0001] This application claims priority and benefit under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application No. 61/160,126,
entitled "LEVEL SENSOR ARRANGEMENT FOR LITHOGRAPHIC APPARATUS AND
DEVICE MANUFACTURING METHOD", filed on Mar. 13, 2009. The content
of that application is incorporated herein in its entirety by
reference.
FIELD
[0002] The present invention relates to a level sensor, a
lithographic apparatus and a method for determining a height map of
a substrate for use in a lithographic process.
BACKGROUND
[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 such a case, 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. Conventional
lithographic apparatus include so-called steppers, in which each
target portion is irradiated by exposing an entire pattern onto the
target portion at once, and so-called scanners, in which each
target portion is irradiated by scanning the pattern through a
radiation beam in a given direction (the "scanning"-direction)
while synchronously scanning the substrate parallel or
anti-parallel to this direction. It is also possible to transfer
the pattern from the patterning device to the substrate by
imprinting the pattern onto the substrate.
[0004] The surface of a substrate on which a pattern should be
projected is usually not completely flat. Moreover, a substrate can
show thickness variation of several microns. This flatness and/or
thickness variation of the substrate surface may result in
incorrect projection of the pattern, for instance due to focus
errors or imaging errors.
[0005] To correct for unflatness and/or thickness variation of a
substrate, it has been proposed to provide a level sensor,
desirably integrated in the lithographic apparatus. Such a level
sensor may be used to determine a height map of a substrate, before
a pattern is transferred, e.g., projected, on the substrate. This
height map may subsequently be used to correct the position of the
substrate during transfer of the pattern on the substrate.
SUMMARY
[0006] A multiple spot level sensor using optical triangulation may
be provided to determine a height map of the substrate. The
substrate is moved with respect to the level sensor to determine a
height level at different measurement locations across the surface
of the substrate to obtain a height map of the substrate.
[0007] A potential drawback of this level sensor is that the
measurement range of the level sensor wherein reliable measurement
results can be obtained is relatively small, typically in a height
range of about 1 .mu.m. This measurement range is smaller than the
normal variation in flatness and/or height of a substrate.
[0008] Therefore, a closed-loop height control may be used during
measurement of the height level at different measurement locations
on the substrate to maintain the respective measurement locations
within the reliable measurement range of the level sensor. In this
way the small height range in which the level sensor provides
accurate results, is optimally used. However, closed-loop height
control limits the measurement speed of the level sensor, since the
substrate has to be continuously repositioned to maintain the
subsequent measurement locations within the reliable measurement
range of the level sensor.
[0009] Moreover, closed-loop height control makes simultaneous
height measurement at multiple measurement location practically
difficult, if not impossible, since the multiple measurement
locations cannot be arranged simultaneously within the same limited
measurement range of the level sensor.
[0010] It is possible to use linearization techniques to increase
the measurement range of a level sensor. Also, other types of level
sensors and level sensing techniques having a larger measurement
range may be proposed to obviate the need for closed-loop height
control of the substrate. However, increasing the measurement range
may introduce a potential drawback in height measurement of the
substrate. For measurement of a height level of a substrate,
typically a measurement beam is used which is reflected on the
substrate. When no closed-loop height control is used, the focus
height of the level sensor is adjusted to the expected or mean
substrate height, i.e. the height of a perfect flat and even
substrate. Due to the variations in thickness and/or flatness of
the substrate, this expected or mean height level does not
correspond at all measurement locations with the actual substrate
height. In case the height difference between the expected or mean
substrate height and the actual substrate height increases, the
focus of the measurement beam with respect to the substrate surface
may become increasingly important for a correct measurement.
[0011] It is desirable, for example, to provide a level sensor for
use in a lithographic apparatus which level sensor is desirably
less sensitive to effects due to focus height of the measurement
beam of the level sensor.
[0012] A level sensor having a measurement beam which is out of
focus at the measurement location with respect to the local
substrate height may become more sensitive to tilt of the
substrate. Therefore, it is desirable to provide a level sensor
that is less sensitive to tilt of the substrate.
[0013] According to an embodiment of the invention, there is
provided a level sensor configured to measure a height level of a
substrate, the level sensor comprising:
[0014] a projection unit to project a measurement beam on the
substrate,
[0015] a detection unit to receive the measurement beam after
reflection on the substrate,
[0016] a processing unit to calculate a height level on the basis
of the reflected measurement beam received by the detection unit,
and
[0017] a tilt measuring device arranged to receive at least part of
the reflected measurement beam, and configured to provide a tilt
signal representative for a tilt of the substrate with respect to a
nominal plane.
[0018] According to an embodiment of the invention, there is
provided a lithographic apparatus comprising:
[0019] an illumination system configured to condition a radiation
beam;
[0020] a support constructed to support a patterning device, the
patterning device being capable of imparting the radiation beam
with a pattern in its cross-section to form a patterned radiation
beam;
[0021] a substrate table constructed to hold a substrate;
[0022] a projection system configured to project the patterned
radiation beam onto a target portion of the substrate; and
[0023] a level sensor configured to measure a height level of a
substrate on the substrate table, the level sensor comprising:
[0024] a projection unit to project a measurement beam on the
substrate, [0025] a detection unit to receive the measurement beam
after reflection on the substrate, [0026] a processing unit to
calculate a height level on the basis of the reflected measurement
beam received by the detection unit, and [0027] a tilt measuring
device arranged to receive at least part of the reflected
measurement beam, and configured to provide a tilt signal
representative for a tilt of the substrate with respect to a
nominal plane.
[0028] According to an embodiment of the invention, there is
provided a method to determine a height map of a substrate, the
method comprising:
[0029] measuring a height of the substrate at a measurement
location by projecting a measurement beam on the substrate using a
projection unit, receiving the reflected beam from the substrate
using a detection unit, determining a tilt of the substrate, and
determining a height level on the basis of the reflected
measurement beam and the determined tilt of the substrate;
[0030] repeating the measuring for multiple measurement locations
on the substrate; and
[0031] determining a height map of the substrate on the basis of
the height levels.
[0032] According to an embodiment of the invention, there is
provided a method for determining a height map of a substrate for
use in a lithographic process, the method comprising: [0033]
providing a level sensor configured to measure a height level of a
substrate, the level sensor comprising: [0034] a projection unit to
project a measurement beam on the substrate, [0035] a detection
unit to receive the measurement beam after reflection on the
substrate, [0036] a processing unit to calculate a height level on
the basis of the reflected measurement beam received by the
detection unit, and [0037] a tilt measuring device arranged to
receive at least part of the reflected measurement beam, and
configured to provide a tilt signal representative for a tilt of
the substrate with respect to a nominal plane, [0038] measuring the
height of a substrate at a measurement location by projecting a
measurement beam on the substrate using the projection unit,
receiving the reflected beam using the detection unit, and
determining a height level in the processing unit, [0039] repeating
the measuring step for multiple measurement locations on the
substrate, and [0040] determining a height map of the substrate on
the basis of the height levels,
[0041] wherein the method further comprises determining a tilt of
the substrate by the tilt measuring device, and wherein determining
the height level comprises correcting the height level on the basis
of the determined tilt of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] 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:
[0043] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0044] FIG. 2 depicts a side view of a level sensor according to an
embodiment of the invention;
[0045] FIG. 3 depicts a schematic view of a projection grating
(left side) and resulting image (right side);
[0046] FIG. 4 depicts a side view of an embodiment of a detection
grating and a detail of the detection grating;
[0047] FIG. 5 depicts a top view of an embodiment of a level sensor
having multiple measurement locations;
[0048] FIG. 6 depicts a side view of an embodiment of a level
sensor comprising a tilt measuring device;
[0049] FIG. 7 depicts a top view of an embodiment of a beam
delivery system for a multiple measurement location level sensor;
and
[0050] FIGS. 8, 9 and 10 depict a top view, a cross section and a
side view of a further embodiment of a beam delivery system for a
multiple measurement location level sensor.
DETAILED DESCRIPTION
[0051] FIG. 1 schematically depicts a lithographic apparatus
according to one embodiment of the invention. The apparatus
includes an illumination system (illuminator) IL configured to
condition a radiation beam B (e.g. UV radiation or any other
suitable radiation), a patterning device support structure (e.g. a
mask table) MT constructed to support a patterning device (e.g. a
mask) MA and connected to a first positioning device PM configured
to accurately position the patterning device in accordance with
certain parameters. The apparatus also includes a substrate support
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioning device
PW configured to accurately position the substrate in accordance
with certain parameters. The apparatus further includes 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.
[0052] 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.
[0053] The patterning device support structure 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 structure can use mechanical, vacuum, electrostatic or
other clamping techniques to hold the patterning device. The
patterning device support structure may be a frame or a table, for
example, which may be fixed or movable as required. The patterning
device support structure may ensure that the patterning device is
at a desired position, for example with respect to the projection
system. Any use of the terms "reticle" or "mask" herein may be
considered synonymous with the more general term "patterning
device."
[0054] 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 so as to create
a pattern in a target portion of the substrate or any device to
imprint a pattern into imprintable medium. 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.
[0055] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0056] 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".
[0057] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0058] The lithographic apparatus may be of a type having two (dual
stage) or more substrate supports (and/or two or more patterning
device supports). In such "multiple stage" machines the additional
supports may be used in parallel, or preparatory steps may be
carried out on one or more supports while one or more other
supports are being used for exposure.
[0059] 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 can be used to increase the numerical aperture
of projection systems. The term "immersion" as used herein does not
mean that a structure, such as a substrate, must be submerged in
liquid, but rather only means that a liquid is located between the
projection system and the substrate during exposure.
[0060] Referring to FIG. 1, 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
minors 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.
[0061] The illuminator IL may include an adjuster AD configured to
adjust the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may include various
other components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0062] The radiation beam B is incident on the patterning device
(e.g., mask) MA, which is held on the patterning device support
structure (e.g., mask table) MT, and is patterned by the patterning
device. Having traversed the patterning device 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 positioning device PW and position sensor IF (e.g. an
interferometric device, linear encoder or capacitive sensor), the
substrate table WT can be moved accurately, e.g. so as to position
different target portions C in the path of the radiation beam B.
Similarly, the first positioning device PM and another position
sensor (which is not explicitly depicted in FIG. 1) can be used to
accurately position the patterning device MA with respect to the
path of the radiation beam B, e.g. after mechanical retrieval from
a mask library, or during a scan. In general, movement of the
patterning device table MT may be realized with the aid of a
long-stroke module (coarse positioning) and a short-stroke module
(fine positioning), which form part of the first positioning device
PM. Similarly, movement of the substrate support WT may be realized
using a long-stroke module and a short-stroke module, which form
part of the second positioner PW. In the case of a stepper (as
opposed to a scanner) the patterning device table MT may be
connected to a short-stroke actuator only, or may be fixed.
Patterning device MA and substrate W may be aligned using
patterning device 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 MA, the patterning device
alignment marks may be located between the dies.
[0063] The depicted apparatus could be used in at least one of the
following modes:
1. In step mode, the patterning device support MT and the substrate
support WT are kept essentially stationary, while an entire pattern
imparted to the radiation beam is projected onto a target portion C
at one time (i.e. a single static exposure). The substrate support
WT is then shifted in the X and/or Y direction so that a different
target portion C can be exposed. In step mode, the maximum size of
the exposure field limits the size of the target portion C imaged
in a single static exposure. 2. In scan mode, the patterning device
support MT and the substrate support WT are scanned synchronously
while a pattern imparted to the radiation beam is projected onto a
target portion C (i.e. a single dynamic exposure). The velocity and
direction of the substrate support WT relative to the patterning
device support 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. 3. In another mode, the patterning device support MT is
kept essentially stationary holding a programmable patterning
device, and the substrate support WT is moved or scanned while a
pattern imparted to the radiation beam is projected onto a target
portion C. In this mode, generally a pulsed radiation source is
employed and the programmable patterning device is updated as
required after each movement of the substrate support WT or in
between successive radiation pulses during a scan. This mode of
operation can be readily applied to maskless lithography that
utilizes programmable patterning device, such as a programmable
mirror array of a type as referred to above.
[0064] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0065] In FIG. 1 the possible location of a level sensor 1
according to an embodiment of the invention in a lithographic
apparatus is shown. The substrate support WT and substrate W
supported thereon are shown in dashed lines in a measurement
location of the substrate. In this measurement location a height
level of the substrate W may be determined.
Level Sensor
[0066] FIG. 2 shows a level sensor generally indicated by reference
numeral 1. The level sensor 1 is configured to determine a height
map of a substrate 2. This height map may be used to correct the
position of a substrate during projection of a pattern on the
substrate 2. The level sensor may be arranged in a stand-alone
device, but is desirably integrated in a lithographic apparatus as
shown in FIG. 1.
[0067] The level sensor 1 comprises a projection unit 3, a
detection unit 4, and a processing unit 5. The projection unit 3
comprises a radiation output 6 (e.g., a radiation source or an
outlet connected to a radiation source elsewhere) and a projection
grating 7. The radiation output 6 may be, or connected to, any
suitable radiation source. Desirably, it is, or is connected to, a
broadband light source but a polarized or non-polarized laser beam
can also be used. The radiation output 6 provides a measurement
beam which is directed to the projection grating 7.
[0068] The projection grating 7 comprises a periodic grating, i.e.
a pattern having a periodic structure resulting in a measurement
beam having a periodic structure in radiation intensity. The left
side of FIG. 3 shows an example of such a projection grating having
a periodic structure. The measurement beam with periodic radiation
intensity is directed towards a measurement location 8 via an
optical reflector 9. The substrate is located at this measurement
location 8. Further optical elements may be provided to guide the
measurement beam towards the substrate 2. At the measurement
location 8 the measurement beam is reflected on the substrate 2 and
runs via a second optical reflector 10, and possibly further
optical elements to the detection unit 4. The detection unit 4
comprises a detection grating 11 and three detectors 12a, 12b,
12c.
[0069] The detection grating 11 comprises a periodic structure as
shown in FIG. 4. This periodic structure comprises for each period
an array of three segments 13a, 13b, 13c. The top surface of each
segment within the array of segments has a different angle with
respect to the angle of incidence of the measurement beam 14. As a
result, the measurement beam is split by the three segments into
three measurement beam parts 14a, 14b, 14c, each directed to one of
the three detectors 12a, 12b, 12c, for instance photodiodes or
other elements capable of measuring radiation intensity.
[0070] Since the structure of the segments of the arrays of
segments is periodic, each respective segment 13a, 13b, 13c per
array of segments directs the part of the measurement beam received
by that segment towards the associated detector 12a, 12b, and 12c.
Thus all first segments 13a direct radiation of the measurement
beam towards the first detector 12a, second segments 13b of the
periodic structure towards the second detector 12b, and third
segments 13c of the periodic structure towards the third detector
12c.
[0071] The measured radiation intensities are received by a
processing unit 5, wherein on the basis of the radiation
intensities received by the different detectors, a height level of
the substrate 2 can be deducted as now will be explained for an
exemplary embodiment.
[0072] Again referring to FIG. 3, the periodic structure shown at
the left side of FIG. 3 is configured from rhombic shapes having a
length L of about 30 .mu.m and a width W of about 4 .mu.m. Due to
the small NA of the imaging optics used in the level sensor, the
periodicity in the width direction of the projection grating is not
resolved, while the periodicity in the length direction L of the
projection grating 7 is resolved. It is remarked that in an
alternative embodiment the periodicity may also be resolved in the
width direction.
[0073] At the right side of FIG. 3, the resulting projection image
of this periodic structure on the substrate 2 is shown. The image
of FIG. 3 shows that the projection image has a periodicity in the
length direction of the projection grating 7. This image is
reflected on the top surface of the substrate 2 towards the
detection unit 4. The intensity distribution received by the
detection grating 11 of the detection unit 4 can be approximated by
a sinusoidal intensity distribution, wherein the intensity is
dependent on the length direction variable x and the shift s of the
image caused by the substrate height.
I=A+B cos(x+s)
In this equation the pitch of the sinusoidal variation is chosen
equal to 2.pi.. The shift of the image s is determined by the
substrate height. This shift s is determined to calculate the
height of the substrate at the respective measurement location 8,
while taking into account the parameters A and B which are also
unknown and variable.
[0074] FIG. 4 shows, for illustration, an aerial image AI of the
sinusoidal intensity distribution above the three segments 13a,
13b, and 13e. Each of the segments 13a, 13b, 13c receives another
part of the intensity distribution. Due to the different angles of
the segments 13a, 13b, 13c each part of the intensity distribution
is guided to the respective one of the detectors 12a, 12b, 12c. The
radiation intensities received by the detectors 12a, 12b, 12c are
guided to the processing unit 5 to determine the height level of
the substrate at the measurement location 8.
[0075] The radiation intensities D1, D2 and D3, received by each of
the detectors 12a, 12b, 12c can be described by the following
relations:
D 1 = 2 .pi. 3 A + B [ sin ( - .pi. 3 + s ) - sin ( - .pi. + s ) ]
##EQU00001## D 2 = 2 .pi. 3 A + B [ sin ( .pi. 3 + s ) - sin ( -
.pi. 3 + s ) ] ##EQU00001.2## D 3 = 2 .pi. 3 A + B [ sin ( .pi. + s
) - sin ( .pi. 3 + s ) ] ##EQU00001.3##
From these three equations with 3 unknown variables, two quadrature
signals can be derived.
B cos ( s ) = 2 D 2 - D 1 - D 3 3 3 ##EQU00002## B sin ( s ) = D 1
- D 3 3 ##EQU00002.2##
These two quadrature signals allow the value s to be found for any
value of s so there are no linearity errors and no dead-zones with
zero sensitivity for substrate height changes. As a result, the
level sensor 1 is suitable to determine the height of the substrate
in a relatively large height range of more than +/-5 .mu.m, or even
+/-10 .mu.m. Therefore, the need of closed-loop height control
during measuring of the height level may be obviated.
[0076] In an further embodiment, the detection grating may comprise
four or more segments for each period of the measurement beam. In
the embodiment shown in FIG. 4, the segments 13a, 13b, 13c each
have the same length ls. In an alternative embodiment, the segments
may have different lengths, as long as the length of the complete
array of segments corresponds to a period of the image of the
measurement beam projected on the detection grating 11.
[0077] The angles of the top surfaces of the segments are about
-15.degree., 0.degree. and 15.degree. with respect to the main
plane of the detection grating 11. Any other suitable angle may be
applied for each of the segments. The difference between the angles
should be large enough to split the measurement beam in three
distinguishable beam parts that can be directed to the three
distinct detectors 12a, 12b, 12c, so that intensity differences
between the three beam parts can be determined.
[0078] Since the measurement of a height level of the substrate 2
can be measured open-loop, and in a relatively large height range,
the level sensor 1 is suitable for measuring simultaneously the
height level at multiple measurement locations 8. To locate the
measurement location 8 of the level sensor 1 at different positions
on the substrate 2, different methods may be applied.
[0079] In an embodiment, the substrate 2 may be moved along the
level sensor 1 in a scanning movement. Since no closed loop height
control may be required, this movement can be made at a constant
speed therewith avoiding the need of accelerations of the substrate
support supporting the substrate 2 during determining a height
level of a substrate. In an further embodiment, the level sensor 1
may be moved over the substrate 2 while the substrate is
stationary. In a further embodiment, both the level sensor 1 and
the substrate may be moved to obtain an optimum path to move the
measurement locations over the surface of the substrate.
[0080] In an embodiment, the level sensor may be provided with a
movable radiation guiding device configured to direct the
measurement beam at different locations on the substrate without
moving the complete level sensor and/or substrate. With such a
level sensor, at least the height of a part of a substrate, for
instance a line of measurement locations, may be measured without
movement of the complete level sensor and/or substrate being
required. In such an embodiment, less movement or less complex
movement of the substrate with respect to the level sensor may be
necessary to obtain a height map of a complete substrate.
[0081] FIG. 5 shows a further embodiment of a level sensor 101. In
the level sensor 101 of FIG. 5, the same or similar features of the
embodiment of FIG. 2 are indicated with the same reference
numerals.
[0082] The level sensor 101 is configured to measure a height of a
substrate at different measurement locations 108 spaced on a line
crossing the width of the substrate 2. For each measurement
location 108, the level sensor 101 comprises the components of the
level sensor shown in FIG. 2. Thus each measurement location 108 is
associated with a projection unit comprising a radiation output 6
and a projection grating 7, and a detection unit comprising a
detection grating 11 and a number of detectors (not shown).
Furthermore, optical elements 9 and 10 are provided to guide the
measurement beam from the projection unit to the measurement
location and, after reflection on substrate 2, from the measurement
location to the detection unit.
[0083] In the depicted embodiment, for each measurement location
108, one radiation output 6, one detection grating 11 and a set of
three or more detectors is provided. There is one projection
grating 7 and one set of optical elements 9 and 10 for all the
measurement locations. The projection grating 7 and the optical
elements 9,10 extend for this reason along the measurement
locations 108.
[0084] In an alternative embodiment, one detection grating 11 may
be provided to be used for all measurement locations 8. Also or
alternatively, one or more parts, for instance projection grating,
detection grating, etc. may be provided for multiple, but not all,
measurement locations 8.
[0085] The level sensor shown in FIG. 5 is configured to measure
the height of a substrate at twelve measurement locations 8. The
full width of the substrate can be covered by this line of
measurement locations 8. When desired more or less measurement
locations 8 may be provided. The measurement locations 8 may also
be arranged in other configurations than along one line as shown in
the embodiment of FIG. 5.
[0086] The construction of the level sensor 1 as shown in FIG. 2 is
in particular suitable to be used in a multiple measurement
location level sensor since the level sensor is capable of
measuring the height of the substrate 2 surface in a relatively
large height range. Typically, this larger height range for
measurement is larger than normal variations in thickness and/or
flatness of a substrate. This larger measurement range may obviate
closed-loop height control. As a result, the level sensor of FIG. 5
may be used to measure the substrate height simultaneously at
multiple measurement locations 8 without the requirement of
correcting the position of the substrate due to height differences
between the heights of the substrate at the different measurement
locations.
[0087] The multiple measurement locations 8 may be moved along the
substrate to obtain height information of a large number of
locations on the substrate 2. This movement may be obtained by
movement of the level sensor 101, movement of the substrate 2,
and/or movement of a radiation guiding element of the level sensor
101, for instance movement of the optical elements 9 and 10. A
combination of these movements may be used to obtain a height map
of the substrate 2 to be used for correctional movements of the
substrate during the actual lithographic process.
Tilt Measuring Device
[0088] FIG. 2 shows a level sensor 1 capable of measuring the
height of a substrate with relatively high accuracy. The level
sensor may provide a reliable measurement within a height range of
+/-5 .mu.m, or even +/-10 .mu.m or possibly even larger. However,
when the image of the projection grating becomes defocused relative
to the detection grating, the level sensor may become sensitive to
tilt of the substrate about the x-axis. Although this effect is
small, the tilt of the substrate 2 may be measured and taken into
account when the height map of a substrate is determined. The
correction of the determined height by taking the tilt of the
substrate into account may improve the accuracy of the height map
even further. An example of a level sensor capable of measuring the
tilt of the substrate, will now be discussed.
[0089] FIG. 6 shows the level sensor of FIG. 2, further comprising
a tilt measuring device 20. The tilt measuring device 20 is
arranged to receive at least part of the reflected measurement
beam, and configured to provide a tilt signal representative for a
tilt of the substrate about the x-axis (Rx) with respect to a
nominal plane 21. The tilt measuring device 20 is connected to the
processing unit 5 to feed the tilt signal to the processing unit 5
in order to correct, when needed, the height level measured by the
level sensor 1.
[0090] In the level sensor 1, the measurement beam reflected on the
substrate 2 is received by the optical element 10 which is a
partially reflective mirror which reflects the major part of the
measurement beam, for example 80% of the radiation intensity
towards the detection unit 4.
[0091] The rest of the measurement beam is transmitted through the
partially reflective mirror and is used to detect the tilt of the
substrate in the tilt measuring device 20. This rest of the
measurement beam is indicated in FIG. 6 as tilt measurement beam
22.
[0092] The tilt measurement beam 22 is received by a beam splitting
device 23. The beam splitting device 23 is configured to divide the
tilt measurement beam in two parts, each part directed to one of
two tilt detectors 24, 25. The beam splitting device 23 comprises
two wedges placed against each other. The part of the tilt
measurement beam received by the lower wedge is guided to the tilt
detector 24, and the part of the tilt measurement beam received by
the upper wedge is guided to the tilt detector 25. The two wedges
of the beam splitting device 23 are spaced in a tilt sensitive
direction.
[0093] Comparison of a ratio between relative amounts of radiation
received by the two tilt detectors 24, 25 is representative for the
tilt of the substrate. For instance, when the substrate is tilted
as indicated by the arrow T in FIG. 6, the part of the tilt
measurement beam 22 received by the upper wedge will increase,
while the part received by the lower wedge will decrease. The
differences between the amounts of radiation received by the two
tilt detectors 24, 25 can be normalized to determine small
intensity differences between the radiation received by the tilt
detectors 24, 25. This comparison of the amount of radiation
received by the respective tilt detectors 24, 25 results in
detection of small tilt variation of the substrate 2.
[0094] The tilt measuring device of FIG. 6 is only an example of a
tilt sensor that can be used to measure the tilt of the substrate
to correct the height determined by the tilt measurement device 20.
Any other tilt measuring device 20 capable of measuring the tilt of
a substrate may be applied. Generally it will be desirable to
measure the tilt about an axis perpendicular to the plane in which
the measurement beam of the level sensor runs. In the shown
embodiment, the measurement beam runs in the y-z plane; thus the
tilt to be measured is tilt of the substrate about the x-axis
(Rx).
[0095] The tilt measuring device 20 as shown in FIG. 6 can be
constructed to be very compact, and is therefore suitable to be
applied in a level sensor system for multiple measurement locations
as shown in FIG. 5. In such a level sensor, for each measurement
location of multiple measurement locations, a tilt measuring device
20 may be provided to measure for each measurement location 8 a
tilt variable of the substrate so that for each measurement
location 8 the height determined by the level sensor 101 may be
corrected for a tilt of the substrate 2.
Measurement Beam Delivery System
[0096] In FIG. 5, a top view of a level sensor 101 is shown
configured to measure a height level of a substrate 2 at multiple
measurement locations 8. This level sensor 101 provides the
possibility to measure the height of the substrate 2 simultaneously
at the different measurement locations 8. This level sensor further
provides the possibility for open loop scanning of the measurement
locations along the surface of the substrate 2. When desired, the
tilt of the substrate 2 may be measured by one or more tilt
measuring devices to correct the height measured by the level
sensor 1 to correct the measured height for any effect due to a
tilt of the substrate 2.
[0097] Although this level sensor provides possibilities for fast
and accurate determination of a height map of a substrate 2, the
level sensor as shown in FIG. 5 may require substantial space above
the substrate 2. Such space may not always be available above the
substrate in a lithographic apparatus.
[0098] To use the space available in a lithographic apparatus more
efficiently, FIG. 7 shows an alternative beam delivery method to
provide measurement beams at multiple measurement locations at the
substrate which multiple measurement locations 8 are arranged on a
line. The substrate 2 is arranged in a measurement position, i.e.
at least one of the measurement locations 8 of the level sensor is
located somewhere on the substrate 2 so that height measurement of
the substrate 2 can be carried out.
[0099] For each measurement location 8, a radiation output 6, a
projection grating 7, a detection grating 11 and three detectors
(not shown) are provided. These components of the level sensor as
such correspond to the components used in the level sensor shown in
FIGS. 2 and 5. However, the radiation output 6 and the projection
grating 7 are provided at one side next to the substrate 2, while
the detection grating 11 and the detectors are provided at the
other side next to the substrate 2. The different components may be
located at different heights with respect to the substrate 2, but
in the shown embodiment all components are arranged at a height
level higher than the substrate 2.
[0100] An advantage of this arrangement is that no components are
arranged above the substrate 2, and all components at one side of
the substrate 2 are arranged relative closely to each other
resulting in a relatively small space required for all components
in a lithographic apparatus.
[0101] It is remarked that the term "next to" is used to indicate a
location that when projected perpendicularly from the main plane of
the substrate lies outside the substrate. The terms "above" or
"below" indicate a location that when projected perpendicularly
from the main plane of the substrate lies within the substrate.
[0102] FIGS. 8, 9 and 10 show a further embodiment in which the
space required for the measurement beams of the level sensor can be
further reduced. In the embodiment of FIGS. 8, 9 and 10, reflection
bars 30, 31 are provided above the substrate 2. The reflection bars
30, 31 are each provided with an oblique reflective surface 30a,
31a (see FIG. 9 which shows a view along line A-A in FIG. 8).
[0103] The angles of the oblique surfaces 30a, 31a are desirably
equal with respect to the horizontal plane and are directed towards
the surface of the substrate. The angles are chosen such that a
measurement beam received by the reflection bar 30 in a
substantially horizontal plane is reflected towards a measurement
location 8 on the substrate 2. At the measurement location 8 the
measurement beam is reflected back towards the reflection bar 31.
The reflected beam is received on the reflection bar 31 at
substantially the same height as where it was reflected on the
reflection bar 30. From the reflection bar 31 the measurement beam
is guided in a substantially horizontal direction to the detection
unit 4.
[0104] In the detection unit 4 further optical elements may be
provided to detect signals representative for the height of the
substrate 2. These signals may be used to determine the height of
the substrate 2 at the measurement location in a processing unit
which may be arranged in the detection unit 4 or at any other
suitable location. The components of the level sensor may
correspond to the components of the above described embodiments,
but any other suitable level sensor device may also be used.
[0105] The sensor components are depicted by a projection unit 3
and a detection unit 4. The projection unit 4 is configured to
provide multiple measurement beams to measure a height level at
different measurement locations 8 on the substrate 2. All
measurement beams are provided at substantially the same height and
directed to the reflection bar 30. All reflected measurement beams
are received at substantially the same height by the reflection bar
31, and directed towards the detection unit 4.
[0106] An advantage of the measurement beam delivery device of the
embodiment shown in FIGS. 8, 9, and 10 is that the measurement beam
can be kept at a relatively low height level with respect to the
substrate 2, while at the same time the projection unit 3 and the
detection unit 4 are arranged next to the substrate where more
space may be available for the provision of these units 3, 4.
[0107] In the embodiment shown in FIGS. 8, 9, and 10, only three
measurement locations 8 are shown. In practice, many more
measurement locations 8, for instance over 50 measurement
locations, may be located at a line or in any other suitable
configuration. Furthermore, the substrate 2 may be moved to move
the measurement locations 8 over the substrate surface to determine
the height level of the substrate over the complete surface of the
substrate to determine a height map of the substrate. The movement
may be realized by movements of the level sensor and/or movements
of the substrate, or by any other suitable method.
[0108] It is remarked that the projection unit 3 and detection unit
4, when using the reflection bars 30, 31 may also be located
partially or completely above the substrate in the measurement
location.
[0109] Further, it is remarked that the projection unit 3 and/or
detection unit 4 of the embodiments of FIGS. 7, 8, 9, and 10 may
comprise compensation devices to compensate, when necessary, for
any difference in measurement beam path length between the
different measurement beams.
[0110] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
[0111] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0112] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 248, 193, 157
or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a
wavelength in the range of 5-20 nm), as well as particle beams,
such as ion beams or electron beams.
[0113] 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.
[0114] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0115] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *