Information
-
Patent Grant
-
6542220
-
Patent Number
6,542,220
-
Date Filed
Friday, November 3, 200023 years ago
-
Date Issued
Tuesday, April 1, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 355 30
- 355 53
- 355 55
- 355 72
- 355 75
- 355 76
- 355 77
- 430 5
- 430 3
- 430 20
- 310 10
- 310 12
- 378 34
- 378 35
- 318 649
-
International Classifications
- G03B2742
- G03B2758
- G03B2762
-
Abstract
A lithographic apparatus has at least one compartment closely surrounding at least one of the mask and substrate holders but not either of the illumination or projection systems so as to reduce the volume that must be purged with gas transparent to the projection radiation. In a scanner, the compartment surrounding the mask holder preferably moves with the mask table and may be formed by a combination of a frame-shaped mask table driven in the scanning operation and stationary plates fixed relative to the projection and illumination systems.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to purge gas systems in a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate.
2. Description of the Related Art
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 illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) of a substrate (silicon wafer) which 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 which are successively irradiated via the mask, one at a time. 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—which is 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 gleaned, for example, from International Patent Application WO97/33205, which is incorporated herein by reference.
In general, lithographic apparatus contain a single mask table and a single substrate table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination radiation. Wavelengths of less than 180 nm are therefore currently being contemplated, for example 157 nm or 126 nm. However, such wavelengths are strongly absorbed by normal atmospheric air, leading to unacceptable loss of intensity as the beam traverses the apparatus. Furthermore, contaminants—which may be introduced by, for example, outgassing of the photoresist layer on the substrate—may adsorb onto certain optical elements, such as that lens element (of the projection system) that is nearest to the substrate. The undesirable adsorption of such contaminants will, in general, also lead to detrimental intensity loss. In order to solve these problems, it has been proposed to flush the apparatus with a flow of gas, the gas being substantially transparent to the illumination wavelength, e.g. nitrogen (N
2
). However, nitrogen gas of the purity necessary to avoid absorption of the exposure radiation, and in the quantities necessary for a flush of the whole apparatus, is expensive.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lithographic projection apparatus, especially one using radiation substantially absorbed by atmospheric air, in which the consumption of purge gas is reduced.
According to the invention there is provided a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; characterized by:
a compartment closely surrounding at least one of one of said first and second object tables but not surrounding either said illumination system or said projection system, said compartment, in use, being supplied with a purge gas more transparent than air to the radiation of said projection beam.
According to the invention there is also provided a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; characterized by:
a purge compartment provided between said projection system and said second object table and fixed relative to said projection system.
By providing a compartment closely surrounding either one of said object tables or in the space between the projection system and the substrate table, the volume that must be purged can be substantially reduced, as compared to purging the whole apparatus. As well as the direct saving in purge gas consumption as a result of the reduction in the volume being purged, there are further reductions, since contamination of the purge gas can be reduced, allowing additional reuse, as can leakage of the purge gas. Additionally, the time taken to purge the system back to a sufficiently clean state of operation after the apparatus has been shut down or opened, e.g. for maintenance, is reduced.
Particular additional advantages can be achieved in step-and-scan apparatus where the compartment can be arranged to surround and move with the object rather than surrounding all of the substrate or mask table, drive arrangements and associated components such as sensors. This can be achieved using a combination of a frame, formed as part of the object table, moving between fixed parallel plates, or by forming the object table into a box substantially surrounding the object. Where the compartment is to be formed between the projection system and the substrate (wafer) these items can themselves form opposite sides of the compartment, which may then be defined by ducts fixed relative to the projection lens and forming a frame around the space traversed by the projection beam. A preferential embodiment employs gas flow velocities which are sufficient to completely or partially prevent contaminants (e.g. as introduced by resist outgassing) from adsorbing onto optical elements in the apparatus. Such velocities may, for example, be or the order of about 1 m/s.
The invention also provides a method of manufacturing a device using a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging irradiated portions of said mask onto target portions of said substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterized by the step of:
providing purge gas to a compartment closely surrounding at least one of said first and second object tables but not surrounding either said illumination system or said projection system, said purge gas being more transparent than air to the radiation of said projection beam.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, 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. Eventually, an array of devices (dies) 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 0-07-0672504.
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. The skilled artisan 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 “exposure area” or “target portion”, respectively.
The radiation used as the projection beam should not be seen as being restricted to the cited examples of radiation having a wavelength of 157 nm or 126 nm; it is conceivable that other wavelengths or types may also be used in the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and its attendant advantages will be further described below with reference to exemplary embodiments and the accompanying schematic drawings, in which:
FIG. 1
depicts a lithographic projection apparatus according to a first embodiment of the present invention;
FIG. 2
is a plan view of the mask stage of the lithographic apparatus of
FIG. 1
, showing the major components;
FIG. 3
is a cross-sectional view of the mask stage of the lithographic apparatus of
FIG. 1
, showing the major components;
FIG. 4
is a flow diagram of the purge gas system of the first embodiment of the present invention;
FIG. 5
is a flow diagram of the purge gas system according to a variation of the first embodiment of the present invention;
FIG. 6
is a view in cross-section parallel to the ZX-plane of the mask stage of a lithographic projection apparatus according to a second embodiment of the present invention;
FIG. 7
is a view in cross-section parallel to the ZY-plane of the mask stage of
FIG. 6
;
FIG. 8
is a partial, cross-sectional view of the mask stage of
FIG. 6
showing the mounting of the upper purge plate;
FIG. 9
is a cross-sectional view of the mask table of the mask stage of
FIG. 6
showing the cover plate;
FIG. 10
is a cross-sectional view of a variation of the mask table of the mask stage of
FIG. 6
showing a modified cover plate;
FIG. 11
is a cross-sectional view of the mask table of the mask stage of
FIG. 6
showing the shape of the bottom of the table;
FIG. 12
is a partial, cross-sectional view of the mask table of
FIG. 6
showing exhaust arrangements;
FIG. 13
is a cross-sectional view of an arrangement for conditioning the beam path of an interferometer in the mask stage of
FIG. 6
;
FIG. 14
is a cross-sectional view of the mask stage of
FIG. 6
showing a “letter-box” arrangement for passing an interferometer beam into the purge compartment;
FIG. 15
is a partial, cross-sectional view of the mask stage of
FIG. 6
showing the mounting of the lower purge plate to the projection lens;
FIG. 16
is a partial, cross-sectional view of the mask stage of
FIG. 6
showing an alternative arrangement for mounting the lower purge plate to the projection lens;
FIG. 17
is a partial, cross-sectional view of the mask stage of
FIG. 6
showing the arrangements for mask exchange;
FIG. 18
is a partial plan view of the upper purge plate of the mask stage of
FIG. 6
showing the mask exchange opening;
FIG. 19
is a cross-sectional view of the mask stage of a lithographic projection apparatus according to a third embodiment of the present invention;
FIG. 20
shows the mask stage of
FIG. 19
in plan with the mask table in two extreme positions of its scanning motion;
FIG. 21
is a cross-sectional view of the mask stage of a lithographic projection apparatus according to a fourth embodiment of the present invention;
FIG. 22
is a cross-sectional view, taken perpendicularly to
FIG. 21
, of the mask stage of
FIG. 21
;
FIG. 23
is a cross-sectional view of a bearing arrangement in the mask stage of
FIG. 21
;
FIGS. 24
to
26
are cross-sectional views of the mask stage of a variant of the fourth embodiment with the mask table in different positions.
FIG. 27
is a horizontal cross-section of a purge gas system in the substrate stage of a lithographic projection apparatus according to a fifth embodiment of the present invention;
FIG. 28
is a cross-sectional view along the line Q—Q in
FIG. 27
of the substrate stage of the fifth embodiment of the present invention; and
FIG. 29
is a side view of the purge gas system of FIG.
27
.
DETAILED DESCRIPTION OF THE INVENTION
In the drawings, like parts are identified by like reference numerals.
FIG. 1
schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises:
a radiation system comprising radiation source LA, and illumination system IL (Ex, IN, CO) for supplying a projection beam PB of radiation (e.g. UV radiation with a wavelength of 157 nm or 126 nm);
a first object table (mask table) MT provided with a mask, or first object, holder for holding a mask MA (e.g. a reticle), and connected to first positioning means for accurately positioning the mask with respect to item PL;
a second object table (substrate or wafer table) WT provided with a substrate, or second object, holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means for accurately positioning the substrate with respect to item PL;
a projection system (“lens”) PL (e.g. a refractive or catadioptric system or a mirror group) for imaging an irradiated portion of the mask MA onto an exposure area C (target portion) of a substrate W held in the substrate table WT.
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, for example.
The radiation system includes a source LA (e.g. an Hg lamp or an excimer laser) which produces a beam of UV radiation. This beam is caused to traverse various optical components comprised in the illumination system IL—e.g. beam shaping optics Ex, an integrator IN and a condenser CO—so that the resultant beam PB has a desired shape and intensity distribution in its cross-section. The beam PB subsequently intercepts the mask MA, which is held in a mask holder on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto an exposure area C of the substrate W. With the aid of the interferometric displacement measuring means IF, the substrate table WT can be moved accurately by the second positioning means, e.g. so as to position different exposure areas C in the path of the beam PB. Similarly, the first positioning means 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 scanning motion of the mask. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG.
1
. In the case of a waferstepper (as opposed to a step-and-scan apparatus) the mask table may be connected only to a short-stroke positioning device, to make fine adjustments in mask orientation and position, or it may just be fixed. Most components of the apparatus, including all vibration generating components, are mounted on or from the base plate BP and base frame BF. However, the projection lens, as well as necessary components of the interferometric displacement measuring means and other sensors, are mounted on the reference, or metrology, frame RF, which is mechanically isolated from the rest of the apparatus to provide a stable reference.
The depicted apparatus can be used in two different modes:
1. In step-and-repeat (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 an exposure area C. The substrate table WT is then shifted in the X and/or Y directions so that a different exposure area C can be irradiated by the beam PB;
2. In step-and-scan (scan) mode, essentially the same scenario applies, except that a given exposure area C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given reference 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 moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large exposure area C can be exposed, without having to compromise on resolution.
FIGS. 2 and 3
show in more detail the mask stage, including mask table MT, of the lithographic apparatus according to the first embodiment.
As shown in
FIGS. 2 and 3
, the whole of the mask stage of the first embodiment is enclosed in purge compartment
100
which is flushed with a purge gas transparent to the radiation of the projection beam. Suitable compositions for the purge gas are discussed at the end of the description. The mask table MT is connected by long-stroke drive arrangement to balance masses BM
1
and BM
2
which are encased in respective compartments
110
. The long-stroke drive arrangements drive the mask table to scan in the Y direction underneath illumination system IL, and to a mask-exchange position underneath mask handler MH. The position of the mask table is continuously monitored by interferometer IF which directs measurement beams onto mirrors (not shown) mounted on the side face of mask table MT. An air shower AS is provided above the area traversed by the measurement beams for interferometer conditioning. The air shower provides gas of known and constant refractive index, for example the purge gas, at constant temperature to ensure interferometer measurements are not rendered inaccurate by variations in refractive index.
As shown in
FIG. 3
, the mask stage compartment
100
has openings in the upper surface around the illumination system IL and in the lower surface around the projection lens PL. Gaps in the balance mass compartments
110
are provided for the interface between the long-stroke motors and the mask table MT.
To close the lower part of the mask stage compartment
100
, an additional plate
105
is mounted to the projection system PL with labyrinthine seals
120
provided between plate
105
and compartment
100
. The labyrinthine seals
120
provide a sufficient seal on the mask stage compartment
100
but do not involve actual contact between the plate
105
and the mask stage compartment
100
, to prevent transmission of vibrations to the projection system PL. Nevertheless, leakage of purge gas from the mask stage compartment can occur around labyrinthine seals
120
and so purge gas exhausts
130
are provided nearby to collect any such leaking purge gas.
The purge gas for the light path in the vicinity of the mask table MT is provided through purge gas outlets
101
provided close to the mask table MT. Gas to purge the remainder of the mask stage compartment
100
is introduced by the air showers for the Y-interferometer beams. The major purge gas exhaust (not shown) is provided at a convenient point in the lower part of the compartment.
To avoid excessive consumption of purge gas, this is largely reused. The supply and reuse system is shown in FIG.
4
. Clean purge gas at the required rate is supplied from purge gas supply
150
and is primarily used in the areas most sensitive to absorption, e.g. in the vicinity of the projection beam, and so is directed to the appropriate outlets
101
in the mask stage compartment
100
. The less critical gas used in the mask stage compartment
100
, such as gas bearings (air bearings) in the long- and short-stroke drive arrangements and gravity compensators (vertical actuators) in the mask table, can be supplied by recycled purge gas with slightly impaired purity. This is exhausted from the mask stage compartment
100
at a relatively high rate by pump/fan
141
. The majority of the exhausted purge gas is passed through a filter
142
and heat exchanger
143
before being returned to the mask stage compartment
100
. A proportion of the exhausted, “polluted” purge gas, to match the supply of clean purge gas, is vented, or diverted to tanks for off-site reconditioning. The filter
142
removes contaminants from the purge gas while the heat exchanger ensures that it is returned to the mask stage compartment at the correct temperature. The filter
142
and ratio of fresh-to-recycled purge gas are chosen to ensure that the concentration of oxygen and water contamination in the mask stage compartment is kept within acceptable limits, e.g. less than 200 ppm, or preferably less than 20 ppm.
To minimize use of clean purge gas, the free space in the mask stage compartment
100
is kept as small as possible, and to prevent leakage from the compartment into the rest of the machine or into the clean room within which it is used, the compartment
100
may be made double-walled. Within the compartment, sources of contaminants, such as plastic components, adhesives, electronic components, etc. are minimized and significant contamination sources may be separately enclosed and purged. At start-up of the machine, and after any opening of the mask stage compartment
100
for maintenance, a complete flush with fresh purge gas is performed to bring the level of contaminants down to required levels. During this process, the purge gas is not reused due to its relatively high level of contaminants.
In a variant of the purge gas supply system, shown in
FIG. 5
, a supply of fresh purge gas is avoided by purifying the exhausted purge gas. In this case, a purifier
144
is provided in the air-control cabinet
140
provided in/with the lithographic apparatus and all of the exhausted purge gas extracted by pump/fan
141
is put through this, the filter
142
and heat exchanger
143
. The purge gas output from heat exchanger
143
is however not returned directly to the apparatus but passed to a further purifier
151
and heat exchanger
152
provided near to the lithographic apparatus. The purifiers
144
,
151
and fan
141
can generate large amounts of heat, electrical noise and vibration and so are preferably isolated from the remainder of the lithographic apparatus as far as possible.
Embodiment 2
A second embodiment of the invention, which may be the same as the first embodiment save as described below, is shown in
FIGS. 6
to
18
. In the second embodiment, the mask stage compartment is closed by fixed top and bottom purge plates
210
,
220
which closely conform to the shape of the mask table MT. Only the projection beam path, and not the interferometer beam paths, is purged.
As shown in
FIG. 6
, which is a cross-sectional view looking in the scanning, Y, direction, the mask table comprises a short-stroke frame
230
, which is driven by the long-stroke drive arrangement (not shown), and chuck
240
. The chuck
240
comprises the mask holder for the mask MA and is driven relative to the short-stroke frame
230
by the short-stroke positioning means (not shown). The upper purge plate
210
is fixed to the base frame BF (shown in
FIG. 1
) and has an aperture around the illumination system IL. The lower purge plate
220
is fixed to the projection system support structure and includes an aperture around the top element in the projection system PL. Both top and bottom purge plates
210
,
220
may be hollow or may include conduits (not shown) for the supply of purge gas. This is supplied to the mask stage compartment via orifices
211
,
221
provided in the apertures around the illumination system IL and projection system PL. Thus, the purge gas is vented directly into the central well
247
in chuck
240
, which is the region traversed by the projection beam. Exhausts
251
,
252
,
253
,
254
are provided at convenient locations, e.g. between the chuck
240
and short-stroke frame
230
to ensure a flow of purge gas away from the mask MA. This arrangement ensures that exhaust gas can only diffuse into the purge compartment through a narrow slit between short-stroke frame
230
and the purge plates
210
,
220
and will be evacuated through exhausts
251
-
254
before reaching the area of the projection beam.
FIG. 7
is a cross-sectional view of the mask stage of the second embodiment viewed perpendicular to the scanning direction and shows that the top and bottom purge plates
210
,
220
extend along both sides of the illumination and projection systems IL, PL to ensure that the mask table is covered throughout its range of motion. This view also shows the level sensors
260
mounted on the projection lens PL.
FIG. 8
shows how the upper purge plate
210
is mounted to compartments
271
,
272
for the balance masses BM
1
, BM
2
and the long-stroke drive arrangements, via flanges
273
,
274
. It can also be seen that the upper purge plate
210
follows the contour of the short-stroke frame
230
with a predetermined gap, e.g. 1 mm. The connections between top purge plate
210
and flanges
273
,
274
incorporate arrangements to allow this gap to be adjusted.
The upper and lower purge plates
210
,
220
may be made of, for example, honeycomb stainless steel plates. This material is non-magnetic to prevent disturbance forces caused by the motor magnets in the long- and short-stroke drive arrangements.
To reduce dead spaces and improve gas flow as the mask table is scanned, the upper surface of the chuck
240
is preferably made as smooth as possible.
FIGS. 9 and 10
illustrate alternative options for arranging this. In
FIG. 9
, the whole of the top of the chuck
240
, except the mask aperture, is covered by cover plate
241
. The cover plate
241
may be formed of a composite material plated with aluminum. In the variants of
FIG. 10
, only the area over the vertical short-stroke actuator
242
is covered by a cover plate
243
, which may again be of composite material.
To even out pressure variations between the top and the bottom of the chuck
240
, through-holes
244
, shown in
FIG. 10
, are provided. These should be sufficient in number and size to allow rapid equalization of any pressure variations that may occur. Additionally, as shown in
FIG. 11
, a recess
245
can be milled in the lower surface of the chuck
240
so that it has a shape corresponding to the upper surface as far as possible. This serves to equalize any flow, and hence pressure, variations that may occur during movement of the chuck
240
.
FIG. 12
shows in more detail the placement of exhausts
252
,
254
. As will be seen, these are situated in the oblique parts of the cover plates
210
,
220
ensuring that there is sufficient volume in their vicinity to even out pressure variations. This is the case even in the vicinity of the mounting members
246
which span between the short-stroke frame
230
and the gravity compensator (vertical actuator)
242
mounted on the chuck
240
.
As mentioned above, it is necessary to ensure that the space traversed by the interferometer beams which measure the position of the chuck
240
is occupied by a gas of constant refractive index. As shown in
FIG. 13
, an extension pipe
281
is attached to the short-stroke frame
230
. The Y-interferometer beam Y-IF passes through the extension pipe
281
and a bore in the short-stroke frame
230
and is reflected by a mirror (not shown) or retro-reflector mounted on the side of the chuck
240
. The beam path within the bore in the short-stroke frame
230
and the extension pipe
281
is conditioned by clean purge gas flowing out from the central compartment of the mask stage whilst the greater part of the beam path outside the short-stroke frame is conditioned by the air shower
282
. The air shower
282
may direct clean, dry purge gas at an angle towards the scanning range of the short-stroke stage
230
so as to condition a region
283
underneath the purge plate
210
. This, and the extension pipe
281
, minimizes the unconditioned length
284
of the Y-interferometer beam path.
For the X-interferometer beams, a different arrangement is necessitated by the need to measure the X-position of the chuck
240
throughout its range of scanning movement in the Y direction. As shown in
FIG. 14
, which is a view similar to
FIG. 6
but with certain components removed for clarity, a rectangular opening
285
is provided in the short-stroke frame
230
to allow the beam from X-interferometer X-IF to reach chuck
240
. Opening
285
defines a narrow slit in the short-stroke frame
230
extending in the Y-direction for the whole length of the movement range of the short-stroke frame
230
. Conditioning air for the X-interferometer beam passes through the opening
285
but this flow is minimized by making the letterbox
285
narrow and extend close to the chuck
240
. Conditioning air for the X-interferometer beam mixes with the purge gas in the space between chuck
240
and short-stroke frame
230
but is prevented by pressure differentials from flowing into the inner space where the mask is provided.
Where the top of the projection lens PL presents a flat surface
222
, as in
FIG. 15
, this itself can form the lower boundary of the inner purge compartment. In such an arrangement Z and other sensors
260
are inset into the top of the projection lens compartment. The lower purge plate
220
is then divided into separate parts either side of the projection system PL. The two parts of the lower purge plate
220
are arranged not to contact the projection system PL to prevent transmission of vibrations to the projection system PL. Additional exhausts
255
are provided to remove purge gas leaking through the resultant gap. These can be situated between the main casing of the projection lens PL and an outer lens cooler
226
, for example.
In an alternative arrangement, shown in
FIG. 16
, where the Z and other sensors
260
extend above the top of the projection lens, the lower purge plate
223
is formed to extend over the top of the projection lens with a central aperture (not shown) for the projection beam and additional apertures
225
for the sensors
260
. Cut-aways
224
in the lower part of the purge plate
223
can be provided to enable a close fit to the projection system PL whilst avoiding actual contact.
To avoid the need to re-flush the complete mask stage compartment with clean purge gas after mask exchange, the mask exchange is arranged to occur in purge gas. To effect this, the upper and lower purge plates
210
,
220
extend underneath the mask handler
290
, shown in
FIGS. 17 and 18
.
FIG. 17
is a cross-sectional view of the mask handler
290
and
FIG. 18
is a plan view of the upper purge plate
210
in this area. The mask handler
290
includes a closed compartment
291
in which the replacement mask MA′ can be provided in advance of the exchange procedure. The chamber
291
can be separately flushed with purge gas and a motorized door
292
is provided in the upper purge plate
210
to allow for exchange when the mask table is positioned underneath it. As shown in
FIG. 18
, additional purge gas outlets
293
are provided in the vicinity of the motorized door
292
to provide positive pressure differential of purge gas during mask exchange and thereby prevent any contamination reaching the purge compartment.
Embodiment 3
A third embodiment of the invention, which may be the same as the first or second embodiments save as described below, is shown in
FIGS. 19 and 20
. In the third embodiment, the short-stroke frame
330
is formed into a largely closed box surrounding the chuck
340
.
The short-stroke frame
330
is driven by the long-stroke motors (not shown) and thus makes large scanning moves in the Y-direction. Within the short-stroke frame
330
, the chuck
340
is suspended by vertical actuators (not shown). Such actuators might require the supply of compressed gas, which should then be supplied with purge gas. The purge gas supplies
360
,
361
to the main mask stage compartment are mounted to the illumination system IL and projection system PL and are therefore stationary. The exhausts
351
,
352
are also fixed relative to the illumination and projection systems IL, PL and are situated towards the lateral edge of the closed box formed by the short-stroke frame
330
. The exhausts
351
,
352
take purge gas, through orifices (not shown) in the short-stroke frame, from the space between the edges of the chuck
340
and the short-stroke
330
so that there is a flow of purge gas from the inner area of the closed compartment, the well
347
in the chuck
340
, outwards. Note that exhausts are also provided on the other side of the short-stroke frame from exhausts
351
,
352
shown in
FIG. 19
but these exhausts have been omitted from the Figure for clarity. As the purge gas supplies
360
,
361
and exhausts
351
,
352
are stationary whilst the short-stroke frame
330
scans in the Y-direction, gas bearings
362
,
355
are provided between the supplies
360
,
361
and short-stroke stage
330
and between the exhausts
351
,
352
and the short-stroke frame
330
. At least the gas bearings
362
are provided with purge gas to prevent leakage into the inner compartment. The short-stroke frame
330
can move vertically to a small degree and this is accommodated by the gas bearings
362
,
355
.
FIG. 20
shows the short-stroke frame
330
, chuck
340
and mask MA in the extremes of their scanning motion relative to the purge gas supply
360
and exhausts
351
,
352
. As can there be seen, the apertures in the closed box formed by the short-stroke stage
330
, which closely surround the footprint of the mask MA, passes under the purge gas supply in the middle portion of its scan, when the exposure is effected, but is closed at the extremes.
To enable measurement of the position of the chuck
340
, a membrane
331
forms a window in one side of the closed box formed by the short-stroke stage
330
to allow the X-interferometer measurement beam to be incident on a mirror (not shown) provided on the side of the chuck. The membrane
331
extends in the Y-direction sufficiently to allow the X-position of the chuck
340
to be measured throughout the scanning motion of the chuck
340
and short-stroke frame
330
. Smaller windows can be provided to allow in the beams from the Y-interferometers Y-IF since these need only accommodate the relatively small range of movement of the chuck
340
in the X-direction. The air shower
382
for conditioning the Y-interferometer beam extends above the short-stroke frame
330
and can condition the beams from the Y-interferometer Y-IF throughout the scanning range of the short-stroke frame
330
and chuck
340
.
Embodiment 4
A fourth embodiment, which may be the same as the first to third embodiments save as described below, is shown in
FIGS. 21
to
23
. In the fourth embodiment, the short-stroke frame
430
forms an open, moving purge-box which is closed by fixed supply and exhaust bins
411
,
412
.
FIG. 21
is a cross-sectional view of the mask stage of embodiment
4
viewed in the scanning, Y, direction. The short-stroke frame
430
forms a partially-open box surrounding the chuck
440
. The short-stroke frame
430
is driven for the scanning motion by the long-stroke drive (not shown) whilst the chuck
440
is supported from the short-stroke frame
430
by vertical actuators (not shown) and makes small movements in all degrees of freedom. The upper exhaust bin
411
is fixed with respect to the illumination system IL and provides purge gas to the inner compartment above the mask MA. This gas flows outwardly past restrictions between the short-stroke frame
430
and chuck
440
and is exhausted upwardly from above the sides of the chuck
440
by exhausts
450
. Lower exhaust bin
412
is similarly fixed relative to projection lens PL and exhausts gas from around the first element of the projection system PL so as to purge the space below the mask MA.
As shown in
FIG. 21
, the partially-open box provided by the short-stroke frame
430
leaves one side of the chuck
440
free so that it can be measured directly by the beams from the X-interferometer X-IF. As shown in
FIG. 22
, which is a cross-sectional view in the X-direction, a bore
431
is provided in one side of the short-stroke frame
430
to allow the beam from the Y-interferometer Y-IF through to the chuck
440
.
Since the short-stroke frame
430
will move relative to the upper and lower supply and exhaust bins
411
,
412
, gas bearings
413
are provided in the upper and lower supply and exhaust bins
411
,
412
. One of these gas bearings is shown in greater detail in FIG.
23
. As can there be seen, purge gas is supplied through supply conduit
414
to form gas bearing
413
in the space between the supply and exhaust bin
411
and the short-stroke frame
430
. The purge gas from the gas bearing will leak inwardly towards the exhausts
450
and outwardly towards ambient air. An additional exhaust conduit
415
is provided between the supply conduit
414
and ambient air to exhaust leaking purge gas for recycling and to prevent inward leaks of air. The gas bearings
413
can be pre-stressed by a pressure differential between ambient air and the inner purge compartment, magnetically, by additional vacuum areas or using additional mass, for example.
The fourth embodiment minimizes the volume that must be purged. Additional purge gas outlets on the short-stroke frame
430
itself can be provided, with nitrogen supplied to the short-stroke frame
430
by a simple, or double-walled, pipe. The Z-sensors
460
mounted to the projection lens can be repositioned outwardly so as to measure the position of the chuck
440
outside the lower supply and exhaust bin
412
and if necessary through a window
432
provided in the short-stroke frame
430
.
A variant of the fourth embodiment is shown in
FIGS. 24
,
25
and
26
. In the variant of the fourth embodiment, the principal change is that the gas bearings
413
′ are moved to be on the short-stroke frame
430
′, rather than the purge plate. The gas bearings
413
′ therefore bear against the flat inner surfaces of the purge plates
411
′,
412
′. Purge gas can be supplied to the short-stroke frame
430
for the bearings by a flexible single- or double-walled pipe. In the fourth embodiment, a flat surface equal in length to the scanning motion of the mask table in the Y direction must be provided for the gas bearings
413
or
413
′ to bear against. In the variant of FIGS.
24
to
25
, this flat surface is provided on the fixed purge plates allowing the size of the moving wafer table to be reduced, reducing the moving mass.
In other variations of the fourth embodiment, the lower supply and exhaust bin
412
can be integrated into the casing for the projection system PL or as an extension of the IL structure. Additionally, the exhaust can be taken only from the upper purge plate with through-holes provided in the chuck
440
to ensure no pressure differential between the sides of the chuck
440
arises.
Embodiment 5
A fifth embodiment is shown in
FIGS. 27
to
29
. The fifth embodiment provides a purge box arrangement for the substrate (wafer) stage of the lithographic projection apparatus and may be combined with any of the embodiments described above.
FIG. 27
shows a horizontal cross-section of the substrate stage purge box
500
which comprises first and second duct enclosures
510
,
520
surrounding central area
501
which is situated underneath the final element of the projection lens PL.
FIG. 28
is a vertical cross-section along the line Q—Q in
FIG. 27
with the vertical scale exaggerated for clarity. First duct member
510
provides the major supply of purge gas to the central area
501
through main supply conduit
511
. Either side of main supply channel
511
are exhaust channels
512
and
513
. The main supply channel
511
ends in two arms
514
,
515
extending along the first and second sides of the rectangular central area
501
. The side walls of arms
514
,
515
are provided with an array of orifices
516
(shown in
FIG. 28
) through which the purge gas is provided to the central area
501
. The bottom surfaces of arm channels
514
,
515
are also provided with orifices
517
(shown in
FIG. 28
) through which the purge gas exits to form gas bearings
518
to keep duct member
510
off the substrate W. The exhaust channels
512
,
513
have an array of larger orifices
519
in their lower surfaces for exhausting purge gas from the gas bearings
518
and any air leaking in from the outside.
The second duct member
520
carries the major exhaust channel
521
which extends along the third side of central area
501
and is used to remove the bulk of the purge gas from one side of central area
501
. Secondary supply channels
522
,
523
are provided either side of main exhaust channel
521
. Secondary supply channels
522
,
523
have an array of orifices
527
in their lower surfaces so as to form gas bearings
528
to keep the second duct member
520
off substrate W. Outermost in second duct member
520
are secondary exhaust channels
524
,
525
which have orifices
529
in their lower surfaces, similarly to exhaust channels
512
,
513
in the first duct member, for exhausting purge gas from the gas bearings
528
and preventing any air that may leak under the second duct member
520
from reaching the central area
501
.
FIG. 25
also shows an outer skirt
540
which is provided around the periphery of first and second duct members
510
,
520
. This serves to limit the flow of air leaking underneath the first and second duct members
510
,
520
to the central area
501
.
As can be seen in
FIG. 27
, the first and second duct members
510
,
520
do not meet but leave a clear diagonal channel
530
, closed adjacent the central area
501
by windows
531
, for sensors, e.g. level sensors, which need to observe the surface of the wafer immediately underneath the projection lens PL. If no such sensors are necessary in the apparatus, the clear channel
530
may be omitted.
FIG. 28
is a side view showing that the first and second duct members
510
,
520
are mounted to the reference, or metrology, frame RF.
Purge Gas Compositions
In all of the embodiments described above, the purge gas may, for example, comprise very pure nitrogen, N
2
, or a gas selected from the group He, Ne, Ar, Kr and Xe, or a mixture of two or more of any of these gases. The gas composition used is one which is substantially transparent to UV radiation of the wavelength of the projection beam and preferably has a refractive index which is substantially the same as that of air, when measured under the same conditions of temperature and pressure (e.g. standard clean room conditions) and using radiation of the same wavelength. The refractive index should preferably be the same as that of air at the wavelength of a radiation beam used in the interferometric displacement measuring means IF. The pressure of the purge gas in the mask and/or substrate stages may be atmospheric pressure, or it may be above atmospheric pressure so that any leak results in an outflow of gas rather than contaminating the system with incoming air. Further details of suitable purge gasses can be found in co-pending European patent application number 00306022.5 (Applicant's ref P-0197.000-EP). Preferred mixtures of gases include:
97.3 vol. % N
2
and 2.7 vol. % He
97.0 vol. % N
2
and 3.0 vol. % Ne
59.0 vol. % N
2
and 41.0 vol. % Ar
97.5 vol. % Ar and 2.5 vol. % Xe
92.9 vol. % Ar and 7.1 vol. % Kr.
Whilst we have described above specific embodiments of the invention it will be appreciated that the invention may be practiced otherwise than described. The description is not intended to limit the invention.
Claims
- 1. A lithographic projection apparatus comprising:an illumination system constructed and arranged to supply a projection beam of radiation; a first object table for holding a mask; a second object table for holding a substrate; a projection system constructed and arranged to image an irradiated portion of said mask onto a target portion of said substrate; a compartment supplied with a purge gas, and at least one of said first and second object tables being inside the compartment, and said illumination system and said projection system being outside of the compartment, the purge gas being more transparent than air to the radiation of said projection beam, wherein said object table which is disposed within said compartment is moveable relative to said projection system and said compartment is arranged to move with said object table.
- 2. Apparatus according to claim 1 wherein said compartment surrounds said first object table.
- 3. Apparatus according to claim 2 wherein said compartment is mounted to a long-stroke positioner constructed and arranged to position said first object table.
- 4. Apparatus according to claim 1 wherein first object table comprises a frame surrounding said object table, and said compartment comprises said frame and first and second purge plates closely conforming to upper and lower surfaces of said first object table respectively.
- 5. Apparatus according to claim 4 wherein said first and second purge plates have apertures respectively corresponding to the illumination and projection systems and purge gas supply orifices surrounding said apertures.
- 6. Apparatus according to claim 4 wherein said first and second purge plates extend to a region occupied by a mask exchange device.
- 7. Apparatus according to claim 1 wherein said first object table comprises a box generally surrounding said first object table to provide said compartment.
- 8. Apparatus according to claim 1, wherein windows are provided in said first object table to allow measurement beams to pass through said first object table.
- 9. Apparatus according to claim 1, further comprising:upper and lower plates, fixed relative to said illumination and projection systems respectively, and forming said compartment; and gas bearings provided between said upper and lower plates and said first object table.
- 10. Apparatus according to claim 1 wherein the purge gas comprises one or more gases selected from the group consisting of:N2, He, Ar, Kr, Ne and Xe.
- 11. Apparatus according to claim 1 wherein said radiation of said projection beam has a wavelength less than 180 nm.
- 12. Apparatus according to claim 11 wherein said radiation of said projection beam has a wavelength selected from the group consisting of: between 152 nm and 162 nm and between 121 nm and 131 nm.
- 13. Apparatus according to claim 1 wherein said compartment surrounds second object table.
- 14. A method of manufacturing a device comprising:providing a substrate provided with a radiation-sensitive layer to a second object table; irradiating portions of a mask bearing a pattern and positioned on a first object table and imaging said irradiated portions of the mask onto said target portions of said substrate; providing purge gas to a compartment in which at least one of said first and second object tables is contained, said compartment being movable with said object table which is contained within, and wherein said illumination system and said projection system are disposed outside of said compartment, and wherein said purge gas is more transparent than air to the radiation of said projection beam.
- 15. A semiconductor device manufactured using a lithographic projection apparatus according to the method of claim 14.
Priority Claims (2)
Number |
Date |
Country |
Kind |
99203670 |
Nov 1999 |
EP |
|
00203675 |
Oct 2000 |
EP |
|
US Referenced Citations (12)
Foreign Referenced Citations (1)
Number |
Date |
Country |
WO9957607 |
Nov 1999 |
WO |