(12) United States Patent
`van der Veen et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,455,862 B1
`Sep. 24, 2002
`
`US006455862B1
`
`(54) LITHOGRAPHIC PROJECTION APPARATUS
`
`6,208,707 B1 * 3/2001 Oshino ...................... .. 378/34
`6,218,057 B1 * 4/2001 Cirelli et al. ................ .. 430/5
`
`(75) Inventors: Paul van der Veen, Eindhoven (NL);
`
`6226346 B1 * 5/2001 Hudyma - - - - - -
`
`- - - - -- 378/34
`
`Oscar E J_ Noordman Eindhoven
`(NL)
`’
`
`. . . .. 359/208
`6,262,826 B1 * 7/2001 Shafer . . . . . .
`6,280,906 B1 * 8/2001 Braat et a1. ............... .. 430/296
`
`(73) Assignee: ASML Netherlands B.V., Veldhoven
`(NL)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) APPL NO; 09/461,275
`(22) Filed:
`Dec. 14, 1999
`
`(30)
`
`Foreign Application Priority Data
`
`Dec. 16, 1998
`
`(EP) .......................................... .. 98204268
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`1 014 429 A1
`
`6/2000
`
`OTHER PUBLICATIONS
`
`Leclerc et al, “Luminescence and transient absorption bands
`in fused SiO2 induced by KrF laser radiation at various
`temperatures”, reprinted from Journal of Non—crystalline
`Solids, 149 (1992), pp. 115—121.
`* Cited by examiner
`
`Lee _
`Primary Examiner—‘lohn
`Assistant Examzner—Johnme L Smith, II
`(74) Attorney) Agent) or Firm—P?lsbury Winthrop LLP
`
`(51) Int. Cl.7 ........................... .. A61L 5/00; G21G 5/00;
`
`(57)
`
`ABSTRACT
`
`(52) us CL
`(58) Field of
`
`G03B 27/42
`
`~~~~~~~~~~~~~~~~~~~~ "
`430/5
`’
`
`’
`
`’
`
`’
`
`250/4922. 355/53
`355/53_’430/30
`3 6 41/ 468 28?
`250/49'2 2’
`'
`
`(56)
`
`References Cited
`
`US. PATENT DOCUMENTS
`
`gear? I? Crete/i1 """"""""" " 355/53
`2 i
`5,828,573 A * 10/1998 Hayashi
`....... .. 364/468.28
`5:973:826 A * 10/1999 Chapman et a1_ _________ __ 359/355
`
`_
`
`_
`
`_
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`_
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`_
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`_
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`_
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`Al1thograph1c projection apparatus havmg a radiation sys
`tem LA, EX, IN, CO for supplying a projection beam PB of
`electromagnetic radiation; a mask table MT provided With a
`mask holder for holding a mask MA; a substrate table WT
`provided With a substrate holder for holding a substrate W;
`a projection system PL for imaging an irradiated portion of
`the mask MA onto a target portion C of the substrate W. The
`electromagnetic radiation has a Wavelength less than 200
`nm, and the apparatus also has a control device for main
`taining the energy dose D5 at substrate level at a substantially
`Constant Va1ue> by Substantially Compensating for
`irradiation-induced drift in the intensity IS at substrate level.
`
`
`
`6,015,644 A * 1/2000 Cirelli et al. 6,103,428 A * 8/2000 Hatai et al. .................. .. 430/5
`
`12 Claims, 4 Drawing Sheets
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`
`Nikon Exhibit 1003 Page 1
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`

`

`U.S. Patent
`US. Patent
`
`Sep. 24, 2002
`Sep. 24, 2002
`
`Sheet 1 0f 4
`Sheet 1 0f 4
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`US 6,455,862 B1
`US 6,455,862 B1
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`
`
`Nikon
`
`Exhibit1003
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`PageZ
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`Nikon Exhibit 1003 Page 2
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`

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`U.S. Patent
`
`Sep. 24, 2002
`
`Sheet 2 0f 4
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`US 6,455,862 B1
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`Nikon Exhibit 1003 Page 3
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`U.S. Patent
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`Sep. 24, 2002
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`Sheet 3 0f 4
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`US 6,455,862 B1
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`

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`US. Patent
`
`Sep.24,2002
`
`Sheet4 0f4
`
`US 6,455,862 B1
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`Nikon Exhibit 1003 Page 5
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`
`
`
`

`

`US 6,455,862 B1
`
`1
`LITHOGRAPHIC PROJECTION APPARATUS
`
`BACKGROUND OF THE INVENTION
`
`2
`nm, 157 nm and 126 nm, and researchers hope that such
`lasers can be re?ned so as to produce suf?cient intensity for
`lithography purposes (so as to guarantee adequate
`throughput). In this context, it should be noted that currently
`available i-line equipment generally employs a mercury
`lamp With a poWer of the order of about 3—5 kW, Whereas
`DUV apparatus typically uses excimer lasers With a poWer
`of the order of about 5—10 W, or even higher. The intensity
`demands on the neW-Wavelength excimer lasers are there
`fore very high.
`The assignee of the current patent application recently
`announced the successful development of the World’s ?rst
`fully functional, Wide-?eld, production-level lithographic
`projection apparatus operating at 193 nm; up to that point,
`only relatively primitive test tools operating at 193 nm had
`been available. The introduction of this apparatus Was
`preceded by intense research efforts into source
`development, illuminator design, and lens materials. During
`this research, an important difference Was observed betWeen
`the neW 193-nm machine and existing 248-nm devices, as
`Will noW be discussed.
`In experiments leading to the invention, the inventors
`observed that intense radiative ?uxes of 193-nm light caused
`transient changes in the characteristics of refractive materi
`als placed in their paths (for example, quartZ or CaF2 lens
`elements). Moreover, the same effect Was observed by the
`inventors to occur in various optical coatings present on
`lenses or mirrors located in the optical path. These changes
`Were observed to affect, for example, the transmissivity of
`the projection system, thus altering the radiation intensity
`received at the substrate, even if the intensity delivered by
`the radiation system (excimer laser) Was kept constant;
`consequently, such effects could cause serious exposure
`errors on the substrate (e.g. under-exposure of a resist layer).
`To make matters Worse, the inventors observed that these
`transmissivity changes demonstrated a complex temporal
`dependence.
`Typically, an apparatus as described in the opening para
`graph Will additionally comprise one or more intensity
`(energy) sensors. For example, at a test position prior to the
`mask, it is possible to divert a small portion of the radiation
`in the projection beam out of the main path of the beam and
`onto an intensity sensor, thus alloWing continual monitoring
`of the intensity produced by the radiation system. Similarly,
`it is possible to provide the upper surface of the substrate
`table With an intensity sensor, located outside the perimeter
`of the substrate; such a sensor can then be used to calibrate
`the apparatus on a regular basis, by alloWing periodic
`comparisons of the intensity produced by the radiation
`system and the actual intensity IS received at the substrate.
`In analogy to the effects described in the previous paragraph,
`the inventors discovered that the sensitivity of such sensors
`could demonstrate a signi?cant temporal drift as a result of
`irradiation With 193-nm radiation, resulting in intrinsic
`errors in the intensity measured at substrate level. Needless
`to say, if there is a (variable) intrinsic error in IS as a result
`of such sensitivity drift, this Will result in a miscalibration of
`the apparatus, With the attendant risk of exposure errors.
`In the case of radiation Wavelengths at or above 248 nm,
`the effects described in the previous tWo paragraphs have
`hitherto not been observed. HoWever, in the case of
`machines operating at 193 nm, these effects can be very
`serious. For example, in investigative experiments, the
`inventors observed that, in the case of a step-and-scan test
`apparatus employing a 5W ArF laser (193 nm) and various
`optical components comprising quartZ and/or CaF2 elements
`(inter alia a ?y-eye lens or light mixing rod, lenses near the
`
`1. Field of the Invention
`The invention relates to a lithographic projection appa
`ratus having a radiation system for supplying a projection
`beam of electromagnetic radiation; a mask table provided
`With a mask holder for holding a mask; a substrate table
`provided With a substrate holder for holding a substrate; a
`projection system for imaging an irradiated portion of the
`mask onto a target portion of the substrate.
`2. Description of Related Art
`An apparatus of this type 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 then be
`imaged onto a target area (die) on a substrate (silicon Wafer)
`Which has been coated With a layer of photosensitive mate
`rial (resist). In general, a single Wafer Will contain a Whole
`netWork of adjacent dies that are successively irradiated
`through the reticle, one at a time. In one type of lithographic
`projection apparatus, each die is irradiated by exposing the
`entire reticle pattern onto the die in one go; such an
`apparatus is commonly referred to as a Waferstepper. In an
`alternative apparatus—Which is commonly referred to as a
`step-and-scan apparatus—each die is irradiated by progres
`sively scanning the reticle pattern under the projection beam
`in a given reference direction (the “scanning” direction)
`While synchronously scanning the Wafer table parallel or
`anti-parallel to this direction; since, in general, the projec
`tion system Will have a magni?cation factor M
`(generally<1), the speed v at Which the Wafer table is
`scanned Will be a factor M times that at Which the reticle
`table is scanned. More information With regard to litho
`graphic devices as here described can be gleaned from
`International Patent Application WO 97/33205.
`Up to very recently, apparatuses of this type contained a
`single mask table and a single substrate table. HoWever,
`machines are noW becoming available in Which there are at
`least tWo independently movable substrate tables; see, for
`example, the multi-stage apparatus described in Interna
`tional Patent Applications WO 98/28665 and WO 98/40791.
`The basic operating principle behind such multi-stage appa
`ratus is that, While a ?rst substrate table is underneath the
`projection system so as to alloW exposure of a ?rst substrate
`located on that table, a second substrate table can run to a
`loading position, discharge an exposed substrate, pick up a
`neW substrate, perform some initial alignment measure
`ments on the neW substrate, and then stand by to transfer this
`neW substrate to the exposure position underneath the pro
`jection system as soon as exposure of the ?rst substrate is
`completed, Whence the cycle repeats itself; in this manner, it
`is possible to achieve a substantially increased machine
`throughput, Which in turn improves the cost of oWnership of
`the machine
`The lithographic projection equipment most commonly
`used today operates at an exposure Wavelength of 365 nm
`(so-called i-line apparatus) or 248 nm (so-called DUV
`apparatus). HoWever, the ever-decreasing design rules in
`integrated circuitry have created a demand for even smaller
`exposure Wavelengths 2», since the resolution that can be
`attained With lithographic equipment scales inversely With
`2». Consequently, much research has been devoted to ?nding
`neW light sources operating at Wavelengths shorter than 248
`nm. Currently, attention is being focused on neW Wave
`lengths that can be produced by excimer lasers, such as 193
`
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`US 6,455,862 B1
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`3
`reticle masking blades, the main projection lens, etc.) the
`transmission T along the path of the radiation (betWeen the
`laser and the substrate table) decreased by as much as 5—7%
`Within 2—3 minutes of initiating irradiation, and then slowly
`relaxed upWard once more (Within a time of the order of
`about 5 minutes) When irradiation Was interrupted (or set to
`another level). Moreover, differences in amplitude and tem
`poral behavior Were observed for different optical materials
`and material combinations. Such large transmission changes
`can cause serious dose errors at substrate level, With the
`possibility of large numbers of substrate rejects (particularly
`in IC manufacture).
`SUMMARY OF THE INVENTION
`It is an object of the invention to alleviate these problems.
`This and other objects are achieved in an apparatus as
`speci?ed in the opening paragraph, characteriZed in that the
`electromagnetic radiation has a Wavelength less than 200
`nm, and that the apparatus further comprises means for
`maintaining the energy dose D2 at substrate level at a
`substantially constant value, by substantially compensating
`for irradiation-induced drift in the intensity IS at substrate
`level.
`For the sake of clarity, the folloWing de?nitions Will be
`adhered to throughout this text:
`1. Intensity I5 is the energy ES per unit time t received at
`substrate level (ES=IS><t). This Will generally be a mea
`sured or derived value.
`2. Dose D5 is the amount of radiative energy transferred by
`the projection beam at substrate level in a speci?c time
`interval t5 (DS=IS><tS). Unless otherWise stated, t5 Will be
`taken to be the exposure time te, ie the length of time for
`Which a single target area (die) on the substrate is
`(planned to be) exposed to a radiative ?ux during a given
`batch of exposures.
`In experiments leading to the invention, the inventors
`?red a pulsed ArF laser beam through optical elements
`comprising quartZ and/or CaF2. It Was found that, as the duty
`cycle, energy and/or frequency of the laser pulses Was
`varied, the radiative intensity I transmitted through the
`optical elements also varied. Alternatively, if pulses of a
`constant duty cycle Were ?red through the elements for an
`extended period of time (minutes), then the value of I Was
`seen to undergo a gradual decay toWards an asymptotic
`value Which Was about 5—7% beloW the starting value I0.
`This behavior appeared to be a complicated function of
`many parameters, such as time, the energy, length and
`frequency of the laser pulses, and the previous “irradiation
`history” of the optical elements (a sort of hysteresis effect).
`HoWever, after much analysis, the inventors Were able to
`model this behavior on the basis of a set of equations (see
`Embodiment 2, for example). Accordingly, it became pos
`sible to predict the ratio I/IO that Would be observed at a
`particular point in an irradiation cycle, on the basis of the
`previous “history” of that irradiation cycle.
`Once such a prediction could be made With relatively
`good accuracy, the possibility of correcting such transient
`changes in 1/10 became tangible. Since a reliable prediction
`Was noW available, the inventors chose a feedforWard cor
`rection (anticipatory measure) instead of a feedback correc
`tion (reactive measure), inter alia because the latter Would
`necessarily incur a greater time penalty than the former.
`According to the invention, the inventors have devised
`several different Ways of achieving the correction according
`to the invention, Which can be used individually or in
`combination. These can be further elucidated as folloWs:
`(a) It is possible to adjust the intensity output of the
`radiation system, eg by altering the amplitude of the
`
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`pulses produced by a pulsed laser source, or by adjust
`ing the pulse frequency of that source.
`(b) It is possible to dispose a variable ?lter at some point
`betWeen the radiation system and the substrate (eg in
`the illuminator, or above the mask), and to use this ?lter
`to vary the intensity reaching the substrate. Such a ?lter
`may, for example, take the form of a partially trans
`missive optical element, Whose transmissivity Tis a
`function of the angle of incidence 6 of incoming
`radiation; by varying 6, it is then possible to vary T
`(c) It is possible to adjust the exposure time te. A drift
`tendency in I5 is then counterbalanced by imposing an
`inverse tendency on te, so as to keep D5 substantially
`constant.
`(d) In the case of a step-and-scan apparatus (as opposed
`to a conventional Waferstepper), there is yet another
`manner in Which to perform the correction according to
`the invention. Such a step-and-scan apparatus addition
`ally comprises:
`a ?rst driving unit for moving the mask table in a given
`reference direction parallel to the plane of the table;
`a second driving unit for moving the substrate table
`With a speed v (the so-called scanning speed) parallel
`to the reference direction so as to be synchronous
`With the motion of the mask table. The corrective
`method is then characteriZed in that irradiation
`induced drift in I5 is counteracted by appropriate
`variation of the scanning speed v, so as to keep D5
`substantially constant.
`A great advantage of methods (c) and (d) With respect to
`method (a), for example, is that methods (c) and (d) gener
`ally alloW correction of a Wider range of ?uctuations in IS,
`Without having to disturb the laser from its optimum oper
`ating state. In accordance With the invention, the loWer the
`value of IS, the higher the value of te (method
`or the
`loWer the value of v (method
`Which has to be chosen,
`and vice versa; in this Way, although the intensity IS may
`change at substrate level, the radiative dose D5 at that level
`Will remain substantially constant.
`In general, the inventors have found that, in exposing a Si
`Wafer (eg a 20-cm Wafer) With a plurality of dies (eg of the
`order of about 100—200 dies), a signi?cant change in IS (eg
`of the order of a feW percent) can occur betWeen exposure
`of the ?rst and last die. HoWever, during exposure of any
`single die, the variation in I5 is typically small (eg of the
`order of about 0.1—0.5%) and may be neglected in many
`cases Without causing serious dose errors. In general, this
`means that it Will usually be suf?cient to assess IS and take
`corrective measures (as in methods (a)—(d) above) just
`before exposure of each die or (small) group of dies, the
`value of the correction remaining constant during that par
`ticular exposure. Nevertheless, if it is necessary or desirable
`to further limit the effect of irradiation-induced drift during
`a single exposure, then the invention also alloWs adjust
`ments to the degree of correction during the course of any
`given exposure (“intra-die” correction).
`According to the invention, a distinction can be made
`betWeen a basic feedforWard correction method and a num
`ber of possible extensions that can help to further improve
`the performance of the inventive apparatus. For example:
`A basic method can be completely based on a model that
`describes the transient effects. In such a case, there is no
`attempt to update correction parameters (eg by using
`intermediate auto-calibrations against a reference) so as
`to take the actual momentary transmission of the optics
`into account. This can be referred to as a “static
`method”;
`
`Nikon Exhibit 1003 Page 7
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`US 6,455,862 B1
`
`5
`An extension to such a basic method is to use a regular
`auto-calibration to make adjustments for deviations
`betWeen the outcome of the said model and the mea
`sured (actual) transmission status. This can be referred
`to as a “dynamic method”;
`In a further extension of this dynamic method, the result
`of the auto-calibration is used to ?ne tune one or more
`parameters of the said model. Consequently, sloW
`changes in the behavior of the transient effects during
`use of the apparatus (e.g. caused by deteriorations in
`the optical materials) can be automatically corrected by
`appropriate adjustment of model parameters. This can
`be referred to as a “dynamic method With learning
`effect”.
`Due to the transient transmission variation described
`above, it Will be desirable from time to time to perform a
`relative calibration of the energy sensors E1 and E2. If such
`a calibration is performed at Zero order, it Will have to be
`done in the absence of a reticle on the mask table. In current
`machines, this Would entail removal of the reticle from the
`mask table, Which is time-consuming and therefore incurs a
`throughput penalty. A more elegant approach proposed by
`the inventors is the provision of a small through-hole in the
`mask table, outside the area of the mask; in this scenario, one
`only has to move the mask table so that the through-hole is
`positioned in the projection beam, thus alloWing radiation to
`reach the sensor E2 Without traversing a reticle. in this Way,
`it becomes unnecessary to remove the reticle from the mask
`table in order to perform a Zero-order calibration.
`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 energy-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, eg 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
`?nish 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 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: APractical
`Guide to Semiconductor Processing”, Third Edition, by
`Peter van Zant, McGraW Hill Publishing Co., 1997, ISBN
`0-07-067250-4.
`It should be noted that an article by N. Leclerc et al. in J.
`Non-Crystalline Solids 149 (1992), pp 115—121, reports the
`occurrence of transient transmission degradation in high-OH
`fused silica cubes When irradiated With 215-nm radiation.
`The article does not, hoWever, report similar effects in CaF2,
`or in optical coatings on optical elements, or in energy
`sensors, it does not report any speci?c Work on the complex
`optical systems used in lithographic devices, and does not
`recogniZe the potentially grave consequences for dose con
`trol and product quality in the use of such equipment for IC
`manufacture using high-intensity 193-nm radiation. Neither
`does the article seek to model the observed effects for the
`
`6
`purpose of performing a correction, nor suggest a corrective
`feedforWard as here elucidated.
`The extensive research performed by the inventors at 193
`nm has led them to postulate that similar trouble With
`transient effects Will occur in lithographic projection appa
`ratuses operating at 157 nm or 126 nm.
`Although speci?c reference has been made hereabove 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 inte
`grated optical systems, guidance and detection patterns for
`magnetic domain memories, liquid-crystal display panels,
`thin-?lm magnetic heads, etc. The skilled artisan Will appre
`ciate 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 area”, respectively.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention and its attendant advantages Will be further
`elucidated With the aid of exemplary Embodiments and the
`accompanying schematic draWings, Whereby:
`FIG. 1 schematically depicts a lithographic projection
`apparatus according to the invention;
`FIGS. 2A and 2B give a graphical rendition of the
`irradiation-induced drift in the intensity at substrate level
`resulting from various irradiation sessions performed during
`test experiments;
`FIG. 3 corresponds substantially to the upper portion of
`FIG. 2, and additionally shoWs a ?t to the experimental data
`on the basis of a predictive model developed by the inven
`tors;
`FIG. 4 shoWs a control schematic for performing the
`current invention.
`FIG. 5 shoWs the results of exposure sessions performed
`With the aid of the current invention, together With corre
`sponding results performed Without the aid of the current
`invention.
`In the Figures, corresponding reference symbols refer to
`corresponding parts.
`
`10
`
`15
`
`25
`
`35
`
`45
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`55
`
`Embodiment 1
`FIG. 1 schematically depicts a lithographic projection
`apparatus according to the invention. The apparatus com
`prises:
`a radiation system LA, Ex, IN, CO for supplying a
`projection beam PB of radiation (e.g. ultraviolet light
`With a Wavelength of 193 nm, 157 nm or 126 nm);
`a mask table MT provided With a mask holder for holding
`a mask MA (eg a reticle);
`a substrate table WT provided With a substrate holder for
`holding a substrate W (eg a resist-coated silicon
`Wafer);
`a projection system PL (eg a lens or catadioptric system,
`or a mirror group) for imaging an irradiated portion of
`the mask MA onto a target portion C (die) of the
`substrate W.
`The radiation system comprises a source LA (eg an
`excimer laser) Which produces a beam of radiation.-This
`beam is passed along various optical components,—e.g.
`beam shaping optics Ex, an integrator IN and a condenser
`
`Nikon Exhibit 1003 Page 8
`
`

`

`US 6,455,862 B1
`
`7
`CO—so that the resultant beam PB has a desired shape and
`intensity distribution throughout its cross-section. For
`example, the beam cross-section may take the form of a
`uniform disc or annulus, or a multipole con?guration (such
`as quadrupole or dipole).
`The beam PB subsequently intercepts the mask MA Which
`is held in a mask holder on a mask table MT. Having passed
`through the mask MA, the beam PB passes through the
`projection system PL, Which focuses the beam PB onto a
`target area C of the substrate W. With the aid of the
`interferometrically controlled displacement and measuring
`means IF, the substrate table WT can be moved accurately,
`e.g. so as to position different target areas C in the path of
`the beam PB.
`The apparatus is further provided With tWo energy sensors
`E1, E2. In the case of sensor E1, de?ecting means D (such as
`a partially re?ective optical component, for example) are
`used to divert a portion of the radiation in the projection
`beam out of the main path of that beam and toWards the
`off-axis sensor E1; accordingly, this sensor E1 can be
`employed as a continual monitor of the intensity being
`emitted by the source LA. On the other hand, sensor E2 is
`mounted on the side of the substrate table WT Which faces
`the beam PB, outside the perimeter of the substrate W itself;
`With the aid of the means IF, this energy sensor E2 can be
`moved from time to time so that it intercepts the beam PB,
`thus alloWing a regular calibration measurement of the
`actual value of IS (or E) at the level of the substrate W.
`The depicted apparatus can be used in different modes:
`In step mode, the mask table MT is ?xed, and an entire
`mask image is projected in one go (i.e. a single “?ash”)
`onto a target area C. The substrate table WT is then
`shifted in the X and/or y directions so that a different
`target area C can be irradiated by the (stationary) beam
`PB;
`In scan mode, essentially the same scenario applies,
`except that a given target area C is not exposed in a
`single “?ash”. Instead, the mask table MT is movable
`in a given direction (the so-called “scan direction”, eg
`the X 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 magni?cation of the projec
`tion system PL (typically, M=1A1 or
`In this manner,
`a relatively large target area C can be exposed, Without
`having to compromise on resolution.
`During operation of the depicted apparatus, the radiation
`intensity IS at substrate level demonstrates an irradiation
`induced drift. This is due, for example, to transient changes
`in the transmissivity of one or more of the optical elements
`Ex, IN, CO, PL, caused by intrinsic changes in the refractive
`material (main body) of these elements and/or the optical
`coatings With Which they are generally provided; alterna
`tively or concurrently, the sensitivity of one or both of the
`energy sensors E1, E2 can undergo a transient change,
`causing inaccurate correlation of IS With the intensity pro
`duced by the source LA (since the derived value of IS no
`longer corresponds to the actual value of IS). During expo
`sure of a given substrate W, or a batch of substrates, such
`drift can lead to serious exposure errors (dose errors),
`Whereby one or more target areas C may undergo substantial
`under- or over-exposure.
`The invention circumvents these effects by using a model
`to predict the drift in IS at any time t during use of the
`apparatus, and automatically compensating for this drift, eg
`using one or more of the methods (a)—(d) elucidated above.
`
`8
`Embodiment 2
`
`The considerations Which folloW describe the basic struc
`ture of a model Which can be used to predict the transmission
`T of (the components of) an optical system (eg the appa
`ratus depicted in FIG. 1) as a function of time t.
`The transient model used Will in general satisfy the
`folloWing de?ning equations:
`
`10
`
`61m) _ Tim)
`
`6;
`
`_ _ T
`
`15
`
`ATitr=const.Ep
`
`(3.2)
`
`(3.3)
`
`The transmission T is described by a ?xed part TO and a
`transient term Titr, as in equation (3.1). The in?uence of
`time t on Titr is given by equation (3.2), in Which I is a
`temporal constant. The impact of a single laser pulse With
`energy Ep incident on the optics is given by equation (3.3).
`Performing a calculation of the transient part of the
`transmission can be done as in the folloWing example.
`It is assumed that the last knoWn value for Titr Was
`calculated at t=t0, and had a value TitriO. If a burst (train)
`of N pulses With energy Ep is incident on the optics, starting
`at t=t1, then the folloWing calculation steps must be per
`formed in order to calculate the average transmission during
`the burst.
`1) From t=tO to t=t1: no light Was passed through the
`optics. TitriO then evolves according to equation
`(3.2) only. This differential equation must be solved
`numerically from tO to t1. This can done by taking
`discrete steps in time At, so that the differential equa
`tion becomes a difference equation.
`
`Titrn
`Titrn+1 : Titrn — T -A[
`
`(3-4)
`
`2) From t=t1 to t=t2: N pulses are passed through the
`optics. The transient effect Titril at the beginning of
`the burst noW evolves according to both equations (3.2)
`and (3.3). Solving equation (3.3) for each laser pulse is
`generally not necessary; calculations can be performed
`With AN pulses at the same time. The calculations must
`be repeated until the effect of all N pulses has been
`determined. In addition to calculating the in?uence of
`the laser pulses, the in?uence of time must also be
`taken into account. If AN pulses correspond to a time
`interval At (At=AN/f, Where f is the pulse frequency of
`the laser), then both effects can be calculated
`simultaneously, as in equation (3.5).
`
`T trn m
`Titrn+1 : Titrn + const- Ep -AN — T
`
`(3.5)
`
`The result of the calculations is the value of the transient part
`of the transmission at the beginning (Titril) and the end
`(Titri2) of the burst of N pulses. The resulting output
`variables of the calculations are calculated/de?ned as fol
`loWs
`
`T tr
`* average
`
`Titril + Titri2
`: i
`2
`
`(3-6)
`
`25
`
`35
`
`45
`
`55
`
`a O
`
`65
`
`Nikon Exhibit 1003 Page 9
`
`

`

`9
`-continued
`