U.S. patent application number 10/956053 was filed with the patent office on 2005-02-24 for substrate holder and device manufacturing method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Krikhaar, Johannes Wilhelmus Maria, Lof, Joeri, Minnaert, Arthur Winfried Eduardus, Teuwen, Maurice Anton Jaques, Van Beijsterveldt, Hubertus Jacobus Maria.
Application Number | 20050041234 10/956053 |
Document ID | / |
Family ID | 31985133 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050041234 |
Kind Code |
A1 |
Krikhaar, Johannes Wilhelmus Maria
; et al. |
February 24, 2005 |
Substrate holder and device manufacturing method
Abstract
A substrate holder to adapt a small wafer to a wafer table of a
lithographic apparatus adapted to receive a larger wafer includes a
plate member with a burl pattern on which the small wafer is to be
placed, positioning pins to locate the small wafer and a clamp
formed by a clamp ring and magnets attached to the plate
member.
Inventors: |
Krikhaar, Johannes Wilhelmus
Maria; (Veldhoven, NL) ; Van Beijsterveldt, Hubertus
Jacobus Maria; (Breda, NL) ; Minnaert, Arthur
Winfried Eduardus; (Veldhoven, NL) ; Teuwen, Maurice
Anton Jaques; (Heel, NL) ; Lof, Joeri;
(Eindhoven, NL) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
31985133 |
Appl. No.: |
10/956053 |
Filed: |
October 4, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10956053 |
Oct 4, 2004 |
|
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10615598 |
Jul 9, 2003 |
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6822730 |
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Current U.S.
Class: |
355/72 ; 451/287;
451/402; 451/41 |
Current CPC
Class: |
G03F 7/70691 20130101;
G03F 7/707 20130101 |
Class at
Publication: |
355/072 ;
451/041; 451/287; 451/402 |
International
Class: |
B24B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2002 |
EP |
02254878.8 |
Claims
What is claimed is:
1. A substrate holder, comprising: a plate member having a first
nominal size receivable by a lithographic apparatus, said plate
member having a receiving surface on which a substrate of a second
nominal size is disposed, and said plate member being configured to
substantially entirely support a lower surface of said substrate;
and a clamp constructed and arranged to hold said substrate of a
second nominal size on the plate member, the second nominal size
being smaller than the first nominal size, wherein said
lithographic apparatus is originally configured to expose
substrates having substantially said first nominal size, and
wherein the clamp comprises a ring of magnetic material having an
inner contour similar to but smaller than the outer contour of the
substrate of a second nominal size and a plurality of magnets are
fixed to the plate member.
2. A substrate holder according to claim 1, wherein the clamp is
adapted to hold the substrate of a second nominal size around
substantially an entire periphery of the substrate.
3. A substrate holder according to claim 1, wherein a portion of
the substrate of a second nominal size that is not imaged during
exposure with said lithographic apparatus is a peripheral portion
of about 3 mm wide.
4. A substrate holder according to claim 1, wherein the plate
member comprises a silicon wafer to which the clamp is
attached.
5. A substrate holder according to claim 1, wherein the plate
member is substantially circular and includes one or more flats or
notches.
6. A substrate holder according to claim 1, wherein the first
nominal size is 150 mm or larger and the second nominal size is 100
mm or smaller.
7. A substrate holder according to claim 1, wherein the plate
member has one or more positioning pins located such that when the
substrate is abutted against the one or more positioning pins the
substrate is located at a predetermined position and orientation on
the plate member.
8. A substrate holder according to claim 7, for use with a
substrate having one or more flats or notches, wherein the plate
member is provided with one or more flats or notches and the one or
more positioning pins are located such that the one or more flats
or notches of the substrate are in a predetermined, corresponding
orientation to the one or more flats or notches of the plate
member.
9. A substrate holder according to claim 1, wherein the plate
member includes a burl pattern in a region on which the substrate
is to be held.
10. A method of operating a substrate holder according to claim 1,
the method comprising: locating the plate member on a platform;
placing the substrate on the plate member in a correct orientation;
locating the clamp on a chuck; and lowering the chuck onto the
platform to locate the clamp over the substrate to thereby clamp
the substrate to the plate member.
11. A method according to claim 11, wherein the lowering comprises:
locating holes in the chuck with pins on the platform to align the
clamp, the substrate and the plate member.
12. A method according to claim 11, wherein locating the clamp on a
chuck includes applying a vacuum to the chuck.
13. A device manufacturing method, comprising: disposing a
substrate on a receiving surface of a plate member, said plate
member being configured to substantially entirely support a lower
surface of said substrate; clamping the substrate to the plate
member, said plate member having a first nominal size larger than a
second nominal size of the substrate; loading the plate member
having the substrate clamped thereto in a lithographic apparatus;
and projecting a patterned beam of radiation onto a target portion
of a layer of radiation-sensitive material disposed on an upper
surface of said substrate wherein said lithographic apparatus is
originally configured to expose substrates having substantially
said first nominal size, and wherein the substrate is clamped to
the plate member magnetically.
14. A device manufacturing method according to claim 14, wherein
clamping the substrate comprises clamping the substrate around
substantially an entire periphery of the substrate.
15. A device manufacturing method according to claim 14, wherein
providing the substrate further comprises: abutting the substrate
against one or more positioning pins on the plate member such that
when the substrate is abutted against the one or more positioning
pins the substrate is located at a predetermined position and
orientation on the plate member.
16. A device manufacturing method according to claim 16, wherein
the plate member is provided with one or more flats or notches and
the one or more positioning pins are located such that the one or
more flats or notches of the substrate are in a predetermined,
corresponding orientation to the one or more flats or notches of
the plate member.
17. A device manufacturing method according to claim 14, wherein
providing the substrate further comprises locating the substrate on
a burl pattern on the plate member.
18. A device manufacturing method according to claim 14, wherein
loading the plate member having the substrate clamped thereto in a
lithographic apparatus comprises: locating the plate member on a
platform; placing the substrate on the plate member in a correct
orientation; locating the clamp on a chuck; and lowering the chuck
onto the platform to locate the clamp over the substrate to thereby
clamp the substrate to the plate member.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/615,598, filed Jul. 9, 2003, which also
claims priority from European Patent Application EP 02254878.8,
filed Jul. 11, 2002, both of which are herein incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a lithographic projection
apparatus, a substrate holder and a device manufacturing
method.
[0004] 2. Description of the Related Art
[0005] The term "patterning device" as here employed should be
broadly interpreted as referring to device that can be used to
endow an incoming radiation beam with a patterned cross-section,
corresponding to a pattern that is to be created in a target
portion of the substrate. The term "light valve" can also be used
in this context. Generally, the pattern will correspond to a
particular functional layer in a device being created in the target
portion, such as an integrated circuit or other device (see below).
An example of such a patterning device is a mask. The concept of a
mask is well known in lithography, and it includes mask types such
as binary, alternating phase-shift, and attenuated phase-shift, as
well as various hybrid mask types. Placement of such a mask in the
radiation beam causes selective transmission (in the case of a
transmissive mask) or reflection (in the case of a reflective mask)
of the radiation impinging on the mask, according to the pattern on
the mask. In the case of a mask, the support structure will
generally be a mask table, which ensures that the mask can be held
at a desired position in the incoming radiation beam, and that it
can be moved relative to the beam if so desired.
[0006] Another example of a patterning device is a programmable
mirror array. One example of such an array is a matrix-addressable
surface having a viscoelastic control layer and a reflective
surface. The basic principle behind such an apparatus is that, for
example, addressed areas of the reflective surface reflect incident
light as diffracted light, whereas unaddressed areas reflect
incident light as undiffracted light. Using an appropriate filter,
the undiffracted light can be filtered out of the reflected beam,
leaving only the diffracted light behind. In this manner, the beam
becomes patterned according to the addressing pattern of the
matrix-addressable surface. An alternative embodiment of a
programmable mirror array employs a matrix arrangement of tiny
mirrors, each of which can be individually tilted about an axis by
applying a suitable localized electric field, or by employing
piezoelectric actuators. Once again, the mirrors are
matrix-addressable, such that addressed mirrors will reflect an
incoming radiation beam in a different direction to unaddressed
mirrors. In this manner, the reflected beam is patterned according
to the addressing pattern of the matrix-addressable mirrors. The
required matrix addressing can be performed using suitable
electronics. In both of the situations described hereabove, the
patterning device can comprise one or more programmable mirror
arrays. More information on mirror arrays as here referred to can
be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193,
and PCT publications WO 98/38597 and WO 98/33096. In the case of a
programmable mirror array, the support structure may be embodied as
a frame or table, for example, which may be fixed or movable as
required.
[0007] Another example of a patterning device is a programmable LCD
array. An example of such a construction is given in U.S. Pat. No.
5,229,872. As above, the support structure in this case may be
embodied as a frame or table, for example, which may be fixed or
movable as required.
[0008] For purposes of simplicity, the rest of this text may, at
certain locations, specifically direct itself to examples involving
a mask and mask table. However, the general principles discussed in
such instances should be seen in the broader context of the
patterning device as hereabove set forth.
[0009] Lithographic projection apparatus can be used, for example,
in the manufacture of integrated circuits (IC's). In such a case,
the patterning device may generate a circuit pattern corresponding
to an individual layer of the IC, and this pattern can be imaged
onto a target portion (e.g. comprising one or more dies) on a
substrate (silicon wafer) that has been coated with a layer of
radiation-sensitive material (resist). In general, a single wafer
will contain a whole network of adjacent target portions that are
successively irradiated via the projection system, one at a time.
In current apparatus, employing patterning by a mask on a mask
table, a distinction can be made between two different types of
machine. In one type of lithographic projection apparatus, each
target portion is irradiated by exposing the entire mask pattern
onto the target portion at once. Such an apparatus is commonly
referred to as a wafer stepper. In an alternative apparatus,
commonly referred to as a step-and-scan apparatus, each target
portion is irradiated by progressively scanning the mask pattern
under the projection beam in a given reference direction (the
"scanning" direction) while synchronously scanning the substrate
table parallel or anti-parallel to this direction. Since, in
general, the projection system will have a magnification factor M
(generally <1), the speed V at which the substrate table is
scanned will be a factor M times that at which the mask table is
scanned. More information with regard to lithographic devices as
here described can be seen, for example, from U.S. Pat. No.
6,046,792.
[0010] In a known manufacturing process using a lithographic
projection apparatus, a pattern (e.g. in a mask) is imaged onto a
substrate that is at least partially covered by a layer of
radiation-sensitive material (resist). Prior to this imaging, the
substrate may undergo various procedures, such as priming, resist
coating and a soft bake. After exposure, the substrate may be
subjected to other procedures, such as a post-exposure bake (PEB),
development, a hard bake and measurement/inspection of the imaged
features. This array of procedures is used as a basis to pattern an
individual layer of a device, e.g. an IC. Such a patterned layer
may then undergo various processes such as etching,
ion-implantation (doping), metallization, oxidation,
chemo-mechanical polishing, etc., all intended to finish off an
individual layer. If several layers are required, then the whole
procedure, or a variant thereof, will have to be repeated for each
new layer. It is important to ensure that the overlay
Ouxtaposition) of the various stacked layers is as accurate as
possible. For this purpose, a small reference mark is provided at
one or more positions on the wafer, thus defining the origin of a
coordinate system on the wafer. Using optical and electronic
devices in combination with the substrate holder positioning device
(referred to hereinafter as "alignment system"), this mark can then
be relocated each time a new layer has to be juxtaposed on an
existing layer, and can be used as an alignment reference.
Eventually, an array of devices will be present on the substrate
(wafer). These devices are then separated from one another by a
technique such as dicing or sawing, whence the individual devices
can be mounted on a carrier, connected to pins, etc. Further
information regarding such processes can be obtained, for example,
from the book "Microchip Fabrication: A Practical Guide to
Semiconductor Processing", Third Edition, by Peter van Zant, McGraw
Hill Publishing Co., 1997, ISBN 0-07-067250-4.
[0011] For the sake of simplicity, the projection system may
hereinafter be referred to as the "lens." However, this term should
be broadly interpreted as encompassing various types of projection
system, including refractive optics, reflective optics, and
catadioptric systems, for example. The radiation system may also
include components operating according to any of these design types
for directing, shaping or controlling the projection beam of
radiation, and such components may also be referred to below,
collectively or singularly, as a "lens". Further, the lithographic
apparatus may be of a type having two or more substrate tables
(and/or two or more mask tables). In such "multiple stage" devices
the additional tables may be used in parallel or preparatory steps
may be carried out on one or more tables while one or more other
tables are being used for exposures. Dual stage lithographic
apparatus are described, for example, in U.S. Pat. No. 5,969,441
and WO 98/40791.
[0012] Silicon wafers used for manufacture of semiconductor for
devices come in standard sizes, 150 mm, 200 mm, 300 mm (often
referred to by their approximate size in inches--6", 8" and 12")
and the larger sizes are becoming increasingly popular because more
devices can be manufactured on a single wafer, reducing the number
wafer exchange steps and increasing throughput. Lithographic
apparatus, and other apparatus used in semiconductor manufacture,
are designed to work with these standard wafer sizes. In general, a
lithographic apparatus will accept wafers of only one standard
size. In spite of the general preference for larger wafer sizes, in
some specialized fields it is desirable to be able to manufacture
on wafers that are smaller and thinner than the standard sizes
mentioned above. It is, however, uneconomical to manufacture or
adapt lithographic apparatus to accommodate such small and thin
wafers.
[0013] Silicon wafers of the type discussed are usually very thin
(for example 100 to 350 .mu.m, e.g. 140 .mu.m) and because of this
tend to curve substantially under their own weight. When making
exposures on a stepper tool, the wafer needs to be flat within one
micrometer. Further, it is necessary for the wafer to be flat
during handling so that the wafer can be moved through tight spaces
without collision. Furthermore, curved wafers give rise to other
problems when handled by a robot, for example because they are
fragile and it is sometimes difficult to obtain good contact with a
vacuum holder when a wafer is curved. It is therefore desirable to
keep the wafer flat at all times that the wafer is inside the
lithographic apparatus.
SUMMARY OF THE INVENTION
[0014] It is an aspect of the present invention to provide a
substrate holder that constructed and arranged to hold a relatively
small substrate and allow that wafer to be imaged on in a
lithographic apparatus having a substrate table adapted to receive
a larger substrate and/or to be processed in other substrate
processing apparatus adapted for larger substrates.
[0015] It is a further aspect of the present invention to provide a
substrate holder constructed and arranged to maintain the wafer
flat.
[0016] The holder of the present invention can be used with
substrate, such as silicon wafers, and substrates of other
non-magnetic materials.
[0017] This and other aspects are achieved according to the present
invention in a substrate holder comprising a plate member having a
first nominal, or predetermined, size receiveable by a lithographic
apparatus, and a clamp constructed and arranged to hold a substrate
of a second nominal, or predetermined, size, the second nominal
size being smaller than the first nominal size.
[0018] Preferably the clamp is adapted to hold the substrate around
substantially all of its periphery. This allows the wafer to be
kept substantially flat and ensures that good contact with the
vacuum system is achieved when holes are provided in the plate
member for transmitting the vacuum to the wafer.
[0019] By clamping the smaller substrate to a larger plate member,
which is of a nominal size, such that it can be processed by a
lithographic apparatus, the smaller substrate can be processed by
the lithographic apparatus, or other substrate processing
apparatus, without modification thereof.
[0020] Preferably the plate member is substantially circular in
plan and may have one or more flats or notches. The plate member
may comprise a silicon wafer of standard dimensions to which the
clamp is attached. Alternatively, the plate member may be made of
Zerodur (.TM.) or another non-magnetic material of low thermal
expansivity.
[0021] The first nominal size is preferably 150 mm, 200 or 300 mm
or larger. The second nominal size may be 100 mm or smaller. The
nominal size in each case is the nominal diameter of the plate
member or substrate, disregarding any flats or notches. The
substrate and holder need not be round but may be of another shape,
e.g. square. In that case, the nominal size is the largest
dimension.
[0022] The plate member preferably has one or more positioning pins
located such that when the substrate is abutted thereagainst, the
substrate is located at a predetermined position and/or orientation
on the plate member. Where the plate member is provided with one or
more flats or notches and the holder is to be used with a substrate
having one or more flats or notches, the positioning pins are
preferably located such that the flats or notches of the substrate
are in a predetermined, preferably corresponding, orientation to
the flats or notches of the plate member.
[0023] The clamp preferably comprises a ring of magnetic material
having an inner contour similar to, but smaller than, the outer
contour of the substrate and a plurality of magnets fixed to the
plate member. This arrangement ensures even forces are applied to
the substrate, helping to keep the substrate flat while enabling
the total thickness of the substrate holder to be kept small.
[0024] In a preferred embodiment, the plate member is provided with
a fine burl pattern in the region on which the substrate is to be
placed. The burl pattern prevents short range height variations in
the clamped substrate.
[0025] According to a further aspect of the invention there, is
provided a device manufacturing method including providing a
substrate that is at least partially covered by a layer of
radiation-sensitive material; projecting a patterned beam of
radiation onto a target portion of the layer of radiation-sensitive
material, wherein providing the substrate includes clamping the
substrate to a plate member having a larger nominal size than the
substrate and loading the plate member having the substrate clamped
thereto into the lithographic apparatus.
[0026] Although specific reference may be made in this text to the
use of the apparatus according to the invention in the manufacture
of ICs, it should be explicitly understood that such an apparatus
has many other possible applications. For example, it may be
employed in the manufacture of integrated optical systems, guidance
and detection patterns for magnetic domain memories, liquid-crystal
display panels, thin-film magnetic heads, etc. One of ordinary
skill in the art will appreciate that, in the context of such
alternative applications, any use of the terms "reticle", "wafer"
or "die" in this text should be considered as being replaced by the
more general terms "mask", "substrate" and "target portion",
respectively.
[0027] In the present document, the terms "radiation" and "beam"
are used to encompass all types of electromagnetic radiation,
including ultraviolet radiation (e.g. with a wavelength of 365,
248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation,
e.g. having a wavelength in the range 5-20 nm), as well as particle
beams, such as ion beams or electron beams.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying
schematic drawings in which:
[0029] FIG. 1 depicts a lithographic projection apparatus in which
embodiments of the present invention may be used;
[0030] FIG. 2 is an exploded view of a substrate holder according
to an embodiment of the present invention together with a
substrate;
[0031] FIG. 3 is a cross-sectional view of part of the substrate
holder of FIG. 2 with a substrate clamped therein;
[0032] FIG. 4 is a plan view of a substrate showing the areas that
can be imaged and measured when the substrate is held in the
substrate holder of FIG. 2;
[0033] FIG. 5 is a perspective view of a tool for mounting
substrates onto the substrate holder of FIG. 2; and
[0034] FIG. 6 is a perspective view of a wafer handling tool, as
viewed from underneath.
[0035] In the Figures, corresponding reference symbols indicate
corresponding parts.
DETAILED DESCRIPTION
[0036] FIG. 1 schematically depicts a lithographic projection
apparatus 1 according to an embodiment of the invention. The
apparatus includes a radiation system Ex, IL constructed and
arranged to supply a projection beam PB of radiation (e.g. UV or
EUV radiation, such as, for example, generated by an excimer laser
operating at a wavelength of 248 nm, 193 nm or 157 nm, or by a
laser-fired plasma source operating at 13.6 nm). In this
embodiment, the radiation system also comprises a radiation source
LA. The apparatus also includes a first object (mask) table MT
provided with a mask holder constructed and arranged to hold a mask
MA (e.g. a reticle), and connected to a first positioning device PM
to accurately position the mask with respect to a projection system
or lens PL; a second object (substrate) table WT provided with a
substrate holder constructed and arranged to hold a substrate W
(e.g. a resist-coated silicon wafer), and connected to a second
positioning device PW to accurately position the substrate with
respect to the projection system or lens PL. The projection system
or lens PL (e.g. a quartz and/or CaF.sub.2 lens system or a
refractive or catadioptric system, a mirror group or an array of
field deflectors) is constructed and arranged to image an
irradiated portion of the mask MA onto a target portion C (e.g.
comprising one or more dies) of the substrate W. The projection
system PL is supported on a reference frame RF. As here depicted,
the apparatus is of a transmissive type (i.e. has a transmissive
mask). However, in general, it may also be of a reflective type,
(e.g. with a reflective mask). Alternatively, the apparatus may
employ another kind of patterning device, such as a programmable
mirror array of a type as referred to above.
[0037] The source LA (e.g. a UV excimer laser, an undulator or
wiggler provided around the path of an electron beam in a storage
ring or synchrotron, a laser-produced plasma source, a discharge
source or an electron or ion beam source) produces a beam PB of
radiation. The beam PB is fed into an illumination system
(illuminator) IL, either directly or after having traversed a
conditioner, such as a beam expander Ex, for example. The
illuminator IL may comprise an adjusting device AM for setting the
outer and/or inner radial extent (commonly referred to as 6-outer
and 6-inner, respectively) of the intensity distribution in the
beam. In addition, it will generally comprise various other
components, such as an integrator IN and a condenser CO. In this
way, the beam PB impinging on the mask MA has a desired uniformity
and intensity distribution in its cross-section.
[0038] It should be noted with regard to FIG. 1 that the source LA
may be within the housing of the lithographic projection apparatus
(as is often the case when the source LA is a mercury lamp, for
example), but that it may also be remote from the lithographic
projection apparatus, the radiation beam which it produces being
led into the apparatus (e.g. with the aid of suitable directing
mirrors). The latter scenario is often the case when the source LA
is an excimer laser. The present invention encompasses both of
these scenarios. In particular, the present invention encompasses
embodiments wherein the radiation system Ex, IL is adapted to
supply a projection beam of radiation having a wavelength of less
than about 170 nm, such as with wavelengths of 157 nm, 126 nm and
13.6 nm, for example.
[0039] The beam PB subsequently intercepts the mask MA, which is
held on the mask table MT. Having traversed the mask MA, the beam
PB passes through the lens PL, which focuses the beam PB onto a
target portion C of the substrate W. With the aid of the second
positioning device PW and interferometer IF, the substrate table WT
can be moved accurately, e.g. so as to position different target
portions C in the path of the beam PB. Similarly, the first
positioning device PM can be used to accurately position the mask
MA with respect to the path of the beam PB, e.g. after mechanical
retrieval of the mask MA from a mask library, or during a scan. In
general, movement of the object tables MT, WT will be realized with
the aid of a long-stroke module (coarse positioning) and a
short-stroke module (fine positioning). However, in the case of a
wafer stepper (as opposed to a step and scan apparatus) the mask
table MT may just be connected to a short stroke actuator, or may
be fixed. The mask MA and the substrate W may be aligned using mask
alignment marks M.sub.1, M.sub.2 and substrate alignment marks
P.sub.1, P.sub.2.
[0040] The depicted apparatus can be used in two different
modes:
[0041] 1. In step mode, the mask table MT is kept essentially
stationary, and an entire mask image is projected at once, i.e. a
single "flash," onto a target portion C. The substrate table WT is
then shifted in the X and/or Y directions so that a different
target portion C can be irradiated by the beam PB;
[0042] 2. In scan mode, essentially the same scenario applies,
except that a given target portion C is not exposed in a single
"flash." Instead, the mask table MT is movable in a given direction
(the so-called "scan direction", e.g. the Y direction) with a speed
v, so that the projection beam PB is caused to scan over a mask
image. Concurrently, the substrate table WT is simultaneously moved
in the same or opposite direction at a speed V=Mv, in which M is
the magnification of the lens PL (typically, M=1/4 or 1/5). In this
manner, a relatively large target portion C can be exposed, without
having to compromise on resolution.
[0043] Such a lithographic apparatus described above may be adapted
for use with a substrate (wafer) of particular size, e.g. of
nominal diameter 150 mm, 200 mm or 300 mm, and shape, e.g. circular
with one or two flats or a notch. In particular, the substrate
table will be adapted to receive such a wafer and have a burl
pattern of appropriate size and shape and locating pins to fit the
notches or flats of the intended substrates. In an apparatus in
which the substrate is held on the substrate table by vacuum, the
vacuum nozzles will cover the whole area of the substrate to ensure
an even clamping force to avoid distorting the substrate. Also, the
focal planes of the projection system and various sensors, such as
alignment and level sensors, are set at a particular level and the
substrate table, which will generally have only a limited range of
movement perpendicular to the plane of the substrate, is adapted to
position the top surface of a substrate of given thickness at that
particular level. If a substrate that is too thin or too thick is
used, the substrate table may not be able to adjust its position
sufficiently to put the top surface of the substrate in focus of
the projection system and the various sensors.
[0044] FIG. 2 is an exploded view of a substrate holder according
to an embodiment of the present invention and a substrate. The
substrate holder 10 includes a plate member 11, to which are fixed
positioning pins 12,13 and permanent magnets 14 which, together
with clamp ring 15, form a clamp to hold substrate W firmly in
place. A burl pattern 16 is provided to support the substrate
W.
[0045] The substrate holder 10 enables, for example, a substrate of
nominal diameter 100 mm (4") and thickness 140 .mu.m to be
processed in a lithographic projection apparatus, or other
substrate processing device, that is adapted to handle substrates
of nominal diameter 150 mm (6") and thickness 500 .mu.m. In
addition to being dimensioned to fit the substrate table WT of the
lithographic projection apparatus and present the substrate W at a
suitable height, the substrate holder 10 ensures that the substrate
W, which may be relatively flexible and highly curved due to its
thinness, is kept flat for exposure. This is achieved by the fact
that the substrate W is clamped by the clamp ring 15 around all, or
substantially all, its periphery and by the burl pattern which is
sufficiently fine to prevent short range height variations. The
clamp ring prevents the substrate W from moving during handling of
the substrate holder in the wafer stepper. On the exposure chuck,
the system vacuum can be used to clamp both the substrate holder 10
and the substrate W. This is achieved by using holes through the
bottom of the plate member 11 which transmit the vacuum to the
underside of the substrate W from substrate holder 10.
[0046] As can be more clearly seen in FIG. 4, the substrate W has a
primary flat F1 and can have a secondary flat F2. The positioning
pin 12 is located so that the flat F1 abuts against two ends 12a,
12b of the positioning pin 12 to correctly locate the substrate W
on the substrate holder 10, which in turn has a flat 17 for
location of the substrate holder 10 on the substrate table WT. The
positioning pin 13 abuts a point on the wafer edge of another flat
F2 to achieve correct positioning. The clamp ring 15 may be
provided with recesses in its underside to accommodate the
positioning pins 12, 13 and/or the magnets 14.
[0047] It will be appreciated that the positioning pins 12, 13 and
flats F1, F2, 17 do not have to ensure positioning of the substrate
to within the overlap requirements of the lithographic process to
be performed but rather need only locate the substrate W
sufficiently that alignment markers provided on it are within the
capture range of the alignment system of the lithographic
apparatus. In other embodiments, the substrate W may have different
a number and/or arrangement of flats and/or notches, while the
lithographic apparatus may be adapted to receive a substrate with a
different a number and/or arrangement of flats and/or notches. In
that case, the positioning pins 12,13 on the substrate holder
should be so located as to engage with whatever flats and notches
are provided on the substrate in order to correctly position the
substrate while the plate member 11 should have flats and or
notches to engage whatever positioning device or structure is
provided on the substrate table of the lithographic projection
apparatus. The positioning pins 12, 13 locate the wafer W in the
plane of the plate member 11 while the burl pattern locates it in
the out of plane direction.
[0048] In this embodiment, the plate member 11 comprises a standard
150 mm (6") silicon wafer, the location pins 12, 13 are of ferrite
stainless steel and attached to the plate member 11 by an epoxy
resin. The magnets 14 are NdFeB permanent magnets plated with NiP
and the clamp ring is made of two layers of ferrite stainless
steel. The use of two layers enables recesses to receive the
magnets and/or positioning pins to be easily provided.
Through-holes or cut-outs are made in the clamp ring to form the
lower layer only. This is much easier than forming blind shots in a
thin ring. Also, one layer can be made of a steel selected for its
magnetic properties and the other of a steel selected for strength.
Of course, a single layer can be used for the clamp to ensure a
minimum height of the clamp.
[0049] As can be seen from FIG. 3, the height T of the clamp ring
15 will obstruct the projection beam PB if it is attempted to
project an image within a distance di of the inner periphery of the
clamp ring. The exact value of distance di will depend on the
thickness T and the numeric aperture (NA) of the projection lens
PL, which determines the angle of the outermost rays of the
projection beam. The width of the annulus that cannot be imaged on
will be equal to di plus the maximum width O.sub.max of the overlap
between clamp ring 15 and wafer W. By suitable choice of T and
O.sub.max, the width of the unimageable portion of the substrate,
which may be regarded as the shadow S.sub.1 of the clamp ring to
the projection beam, can be made comparable to the normal edge bead
of 3 mm. In a lithographic apparatus with on-axis leveling, the
leveling sensor beam LS-B will have a shallower angle of
inclination than the projection beam. This means that the shadow
S.sub.LB of the clamp ring to the leveling sensor beam LS-B will be
wider and the vertical position of a larger portion of the wafer
surface cannot be measured. This problem can be avoided by
extrapolating from leveling measurements made in the inner portion
of the substrate W. In FIG. 4, the imageable area is shown
cross-hatched and the boundary of the shadow of the level sensor
beam is shown as a dashed line.
[0050] A loading tool 20 for loading wafers W onto the substrate
holder 10 is shown in FIG. 5. The loading tool has a platform 21
onto which the plate member 11 is placed, e.g. by a wafer handling
robot (not shown in FIG. 5) or manually. The platform 21 has a
locating bar 22 which engages the flat 17 of the plate member 11 to
ensure correct location thereof. Once the plate member 11 is
correctly located the wafer W is placed onto the burl pattern and
against the positioning pins. Again this can be performed by a
wafer handling robot or manually. The wafer W is thereafter clamped
by a vacuum to keep it in place. Meanwhile, the clamp ring 15 is
held on an annular vacuum chuck 23 by vacuum. The annular vacuum
chuck 23 has locator holes 24 on projections that engage with
locator pins 25 provided on platform 21 when the annular vacuum
chuck 23 is inverted and lowered onto the platform 21 so that the
clamp ring is correctly located over the substrate W. The vacuum is
then released and the annular vacuum chuck removed, leaving the
clamp ring 15 held by magnets 14 to the plate member 11. The
substrate holder 11 and held substrate W can then be delivered to
the lithographic apparatus, e.g. by wafer handling robot.
[0051] The wafer handling robot (known as a wafer handling tool)
referred to above can be used to position the plate member 11
correctly on the platform 21 and can also be used to position the
substrate W correctly onto plate member 11. A wafer handling robot
is preferred to manual handling because manual handling can cause
contamination of the wafer W and can result in less precise
positioning than is possible using a robot.
[0052] FIG. 6 shows a bottom perspective view of an embodiment of
the wafer handling tool 30. The tool comprises a central planar
recessed area 31 and a peripheral region 32 that extends out of the
plane of the central recessed area 31. The peripheral region 32 is
in this embodiment split into three sections A, B, C and is capable
of presenting a vacuum to the edge of the wafer. In this
embodiment, the vacuum is presented using a circumferential slot 33
formed in the peripheral region 32. Sections A and B of the
peripheral region 32 are smaller than Section C. In this
embodiment, Sections A and B each span a circumferential range of
about 30.degree. and Section C spans a circumferential range of
about 160.degree..
[0053] Each of the three peripheral regions A, B, C shown in FIG. 6
can be connected by holes inside the tool to a vacuum connector 34
which in turn is connected to a vacuum pump system or the like.
[0054] The vacuum slots, or chambers, 33 in the peripheral region
32 are preferably located such that, in use, only the outer edge of
the wafer is in contact with the tool. The recessed central area 31
prevents the wafer touching any other part of the tool. As will be
seen in FIG. 6, there are gaps 35, 36 at certain positions around
the circumference of the tool and this is to allow the wafer to be
pressed against fixed points on the wafer holder surface. For
example, for a wafer having flats F1 and F2, the tool 30 can pick
the wafer up with the flat F1 being exposed in gap 36 and with flat
F2 being exposed in one of the gaps 35 and this aids in setting the
wafer down again onto the plate member 11. The gaps 35 and 36 also
help to visually verify that the wafer is positioned correctly
against the positioning pins 12, 13.
[0055] The wafer handling tool 30 is preferably made from a
plastics material because such materials are easy to manufacture
and do not damage the wafer surface. Polycetals, eg.
polyoxymethylene (POM), are particularly suitable.
[0056] The wafer handling tool 30 allows the wafer to be handled
from the top, instead of the bottom. This is made possible by the
use of the peripheral vacuum chambers 33 that do not interact with
the useable central portion of the wafer. The tool can be used in
various situations where it is not possible or desirable to handle
the wafer from below. For example, a wafer needs to be positioned
on a surface in which there is no room for a backside tool; the
wafer has to be removed from the surface it has stuck to or where
there is no room for the standard tool; there are devices or other
sensitive components on the backside of the wafer.
[0057] The peripheral vacuum chamber design of the tool allows very
fragile and thin wafers to be handled in a reliable way.
Furthermore, it has been found that the tool itself helps to
flatten curved wafers which makes later positioning of the wafer
much easier and more reliable.
[0058] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. The description is not
intended to limit the invention.
* * * * *