NL2014093A - Lithographic apparatus and device manufacturing method. - Google Patents

Description

LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD FIELD

[0001] The present invention relates to a lithographic apparatus and a method for manufacturing a device.

BACKGROUND

[0001] Extreme ultraviolet (EUV) radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, and may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0002] One application of an EUV radiation source is in lithography. A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] In order to reduce the minimum printable size, imaging may be performed using radiation having a short wavelength. It has therefore been proposed to use an EUV radiation source providing EUV radiation within the range of 13-14 nm, for example. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.

[0004] In addition to the desired EUV radiation, the EUV radiation source generates non-EUV, out-of-band (such as deep ultraviolet (DUV)) radiation. This out-of-band radiation may be transmitted to the substrate, and can negatively affect the resultant image as the resist may be sensitive to this out-of-band radiation.

SUMMARY

[0005] It is desirable to mitigate for the effect of the out-of-band source radiation during a lithographic process.

[0006] The invention in a first aspect provides a lithographic apparatus, comprising an illumination system configured to condition a radiation beam, said radiation beam comprising both EUV radiation and non-EUV radiation; wherein said lithographic apparatus is operable to: obtain a measurement of the non-EUV radiation content comprised within said radiation beam; model imaging effects, said imaging effects comprising the resultant effects of said non-EUV radiation on an image formed on a substrate by said lithographic apparatus as a result of a lithographic process; and determine optimized illumination system settings which mitigate some or all of said imaging effects.

[0007] The invention in a second aspect provides for a method of manufacturing a device, comprising: forming an image on a substrate using a lithographic process, said lithographic process using a radiation beam comprising both EUV radiation and non-EUV radiation; obtaining a measurement of the magnitude of non-EUV radiation comprised within said radiation beam; modelling imaging effects, said imaging effects comprising the resultant effects of said non-EUV radiation on an image formed on a substrate by said lithographic apparatus as a result of a lithographic process; determining optimized illumination system settings which mitigate some or all of said imaging effects; and using the optimized illumination system settings in a subsequent lithographic process [0008] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0009] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts schematically a lithographic apparatus having reflective projection optics;

Figure 2 is a more detailed view of the apparatus of Figure 1; and Figure 3 shows an alternative source arrangement usable in the apparatus of Figure 2; and

Figure 4 is a flowchart depicting a method according to an embodiment of the invention.

[0010] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

PET ATT F.D DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0011] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation). a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0012] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0013] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support stmcture can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0014] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0015] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0016] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0017] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0018] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines 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 exposure.

[0019] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0020] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0021] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0022] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0023] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. 3. In another mode, the support stmcture (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0024] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0025] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using CO2 laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0026] The radiation system 42 embodies the function of source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0027] As part of an LPP radiation source, a laser system 61 is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector 50. Also, the radiation system includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a desired plasma formation position 73.

[0028] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a highly ionized plasma with electron temperatures of several 105 K. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector mirror 50 onto the intermediate focus point IF.

[0029] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS.

For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.

[0030] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source module (apparatus) 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0031] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR (infrared) radiation present from the laser beam 63. The non-EUY wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non-EUV radiation. As schematically depicted in Figure 2, a transmissive SPF may be applied upstream of the virtual source point IF. Alternatively or in addition to such a filter, filtering functions can be integrated into other optics. For example a diffractive filter can be integrated in collector 50 and/or mirrors 53, 54 etc., by provision of a grating structure tuned to divert the longer, IR radiation away from the virtual source point IF. Filters for IR, DUV and other unwanted wavelengths may thus be provided at one or more locations along the paths of beams 55, 56, 57, within source module (radiation system 42), the illumination system IL and/or projection system PS.

[0032] To deliver the fuel, which for example is liquid tin, a droplet generator or target material supply 71 is arranged within the source chamber 47, to fire a stream of droplets towards the plasma formation position 73. In operation, laser beam 63 may be delivered in a synchronism with the operation of target material supply 71, to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, or even several tens or hundreds of kilohertz. In practice, laser beam 63 may be delivered by a laser system 61 in at least two pulses: a pre pulse PP with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse MP of laser energy is delivered to the cloud at the desired location, to generate the plasma. In a typical example, the diameter of the plasma is about 2-3 mm. A trap 72 is provided on the opposite side of the enclosing structure 47, to capture fuel that is not, for whatever reason, turned into plasma.

[0033] Laser system 61 is may be for example of the ΜΟΡΑ (Master Oscillator Power Amplifier) type. Such a laser system 61 includes a “master” laser or “seed” laser, followed by a power amplifier system PA, for firing a main pulse of laser energy towards an expanded droplet cloud, and a pre pulse laser for firing a pre pulse of laser energy towards a droplet. A beam delivery system 65 is provided to deliver the laser energy 63 into the source chamber 47. In practice, the pre-pulse element of the laser energy may be delivered by a separate laser. Laser system 61, target material supply 71 and other components can be controlled by a controller (not shown separately). The controller performs many control functions, and has sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of radiation system 42, and optionally elsewhere in the lithographic apparatus. In some embodiments of the present invention, the main pulse and the pre pulse are derived from a same laser. In other embodiment of the present invention, the main pulse and the pre-pulse are derived from different lasers which are independent from each other but controlled to operate synchronously. A problem that can arise in the LPP source apparatus is that optical elements of the laser beam delivery system 65 will become contaminated with debris from the plasma. In particular a final optical element, be it a lens or a mirror, is directly exposed to particles of fuel ejected from the plasma. A refractive (transmissive) element will quickly become obscured by tin deposits, leading to reduced transmission of the laser radiation and undesired heating. A reflective final element, such as a copper mirror, may be more tolerant of Sn deposits for time, but will need cleaning eventually to maintain efficiency of reflection and focusing.

[0034] In order to block as much contamination as possible, a contamination trap 80 of some sort may be provided between the plasma formation site 73 and optical elements of the beam delivery system 65.

[0035] Figure 3 shows an alternative LPP source arrangement which may be used in place of that illustrated in Figure 2. A main difference is that the main pulse laser beam is directed onto the fuel droplet from the direction of the intermediate focus point IF, such that the collected EUV radiation is that which is emitted generally in the direction from which the main laser pulse was received.

[0036] Figure 3 shows the main laser beam delivery system 130 emitting a main pulse beam 131 delivered to a plasma formation position 132. At least one optical element of the beam delivery system, in this case a folding mirror 133 is located on the optical axis between plasma position 132 and the intermediate focus. (The term “folding” here refers to folding of the beam, not folding of the mirror.) The EUV radiation 134 emitted by a plasma at position 132, or at least the major portion that is not directed back along the optical axis O into the folding mirror 133 is collected by a grazing incidence collector 135. This type of collector is known already, but is generally used in discharge produced plasma (DPP) sources, not LPP sources. Also shown is a debris trap 136. A pre-pulse laser 137 is provided to deliver a prepulse laser beam 138 to fuel droplets. In this example, the pre-pulse energy is delivered to the side of the fuel droplet that faces away from the intermediate focus point IF. It should be understood that the elements shown in this schematic diagram are not to scale.

[0037] Both laser produced plasma (LPP) and discharge produced plasma (DPP) extreme ultraviolet lithography (EUVL) sources emit a broad spectrum of wavelengths, comprising the desired EUV radiation (at 13.6 nm) and other out-of-band wavelengths. Out-of-band wavelengths in this context may comprise deep ultraviolet (DUV) radiation (at around 300 nm) and beyond. The DUV portion of the emitted light is a by-product of the EUV plasma emission and can, in principle, be transferred through the illuminator and the projection optics to the wafer and affect imaging performance by contributing to the exposure in photoresist. This is because the photoresist at the wafer is not only sensitive for the 13.6 nm EUV light, but also for the other out-of-band wavelengths.

[0038] The actual EUV imaging performance is deteriorated by the non-EUV out-of-band content in the spectrum. This part of the spectrum contains only wavelengths that are far too long to be able to resolve the features of interest at the mask (MA) on the wafer (W), and therefore only reduces the image contrast. As consequence, the imaging performance (e.g., critical dimension uniformity (CDU), Image placement) is affected, and imaging and optical process correction (OPC) (e.g. for matching between two different lithographic tools) is compromised.

[0039] In practice the following variations (between lithographic tools or over time) occur: • The actual source spectrum will depend on the operating conditions of the source; • The resist sensitivity to out-of-band radiation is different for the various resist types/manufacturers; and • The spectral transmission of the illuminator (IL) and projection optics (PS) is dependent on the coatings applied to the mirrors.

[0040] Related to these variations, the following problems have been observed: • The “dose to clear” measurement sequence (based, for example, on an aluminium coated reticle as described below) is a time consuming, off-line calibration test which provides only a “snapshot” measurement, which is only truly valid for the specific test conditions; • The calibration test does not yield detailed spectral information, which could be used to improve correction for the actual resist spectral sensitivity. Consequently the test might have to be repeated for every new resist applied by the customer; • The calibration test furthermore does not reveal the imaging impact on local pattern-density variations of the layout features on the mask (MA); and • As the source and scanner optics (illuminator, projection optics) will not be identical for every lithographic apparatus, the calibration data taken from one lithographic apparatus cannot be applied to another lithographic apparatus. Therefore, the calibration test must be repeated at every lithographic apparatus.

[0041] To address the above issues, it is proposed to model the imaging effects caused by the out-of-band, non-EUV radiation; and to change the illumination characteristics during the lithographic process so as to mitigate these imaging effects.

[0042] Figure 4 is a flowchart illustrating a method according to an embodiment of the invention. At step 400, the magnitude (and in some embodiments, the effect) of the out-of-band source spectrum is determined. This may be done in a manner which takes into account the contribution attributable to some or all of: the source spectrum, the optics (Illumination and/or projection optics), any transmission effects and the resist sensitivity. Two exemplary embodiments for performing this step are described in detail below. The first method uses a partially aluminium-coated test mask. A second method uses a sensor which is capable of measuring the out-of-band spectrum. In the latter case, effects resultant from the photoresist type are not included in the determination.

[0043] At step 410, an imaging simulation model is applied, which calculates the imaging effects (for example, defects in critical dimension (CD), CDU, Proximity errors) of the non-EUV out-of-band part of spectmm as is measured/estimated during step 400. This imaging simulation model may, but does not exclusively need to, make use of the fact that dark and bright features on the mask reflect the out-of-band light with different intensities. The local pattern density variations present on the mask can therefore be used to model these effects.

[0044] At step 420, modified illumination characteristics are calculated/determined, which compensate for the imaging effects calculated at step 410 caused by the non-EUV, out-of-band part of the spectrum. This calculation can also take into account the actual customer patterns (clips) to be imaged. An example method of correction for the imaging effects caused by the out-of-band light, is to modify the illumination shape using (for example) flexible mirrors present in the illuminator. The best shape of this illuminator can be calculated by computational lithography techniques that inter alia include the layout features present on the mask. For multiple lithographic apparatuses with variations in out-of-band response, differences in illumination settings can be calculated. In this way, a user who wants to use an identical mask on different lithographic apparatuses can do so by tuning the illumination settings of each lithographic apparatus to compensate for the impact of out-of-band radiation variation, such that the imaging quality is satisfactory in each case. Consequently, a mask which has had OPC fine-tuned (out-of band radiation compensated) for one machine can he used on another.

[0045] At step 430, the actual illumination characteristics are adapted in accordance with the calculation of step 420. This may be done by altering/tuning the illumination pupil of the lithographic tool such that the negative imaging effects are mitigated. The adapted illumination characteristics can then be used in subsequent lithographic operations.

[0046] A sensor operable to directly sense the spectral content of the out-of-band radiation may be provided on the lithographic tool. Such a sensor can then be used to directly perform the spectral measurement of step 400. Preferably, such a sensor may be employed at wafer level (for example on wafer table WT), so that any spectral measurement takes into account the illumination and projection optics and any transmission effects before the wafer stage. However, the sensor may be placed elsewhere. Such a sensor may be part of the lithographic apparatus, or may be a stand-alone sensor which is inserted (preferably close to the wafer table WT) only when a test is to be executed at a location. The sensor may operate in the spectral range of 10 to 200 nm, but is not restricted to this range. The illumination characteristics can then be adapted based on the spectral data recorded using the sensor.

[0047] In addition to the spectral distribution of the radiation when it arrives at wafer level; it is also desirable to know the sensitivity ratio of EUV radiation versus out-of-band radiation for the specific resist being used. This ratio can vary for different resists. One method for calculating this ratio is to measure the “dose to clear” of the resist exposed using EUV (13.6 nm) radiation and to measure the “dose to clear” of the resist using radiation at one or more out-of-band wavelengths. A “dose to clear” measurement is a measurement of the amount of exposure energy (dose) required to just clear the resist in a large clear area for a given process. A comparison of the EUV “dose to clear” measurement with one or more of the other out-of-band “dose to clear” measurements provides quantitative information on the DUV/EUV sensitivity ratio. The sensitivity ratio at a certain out-of-band wavelength (or spectrum) is defined as the ratio between the “dose to clear” measurement for that out-of-band wavelength (or spectrum) and the dose to clear measurement for EUV radiation. Once calculated, the sensitivity ratio can be used in step 420 when calculating illumination modifications. In this way, illumination modifications can be calculated which take into account the sensitivity ratio for a particular resist. For a new layer with a different resist, this calibration (and illumination modification) should be performed again.

[0048] In an embodiment, the sensitivity ratios of a number of different resists can be calculated in advance and stored in a sensitivity calibration table. Then, depending on the resist that is employed for a specific layer, an illumination modification can be calculated using the appropriate sensitivity ratio from the sensitivity calibration table, which can be applied in the illuminator.

[0049] It may not always be possible, depending on the lithographic tool specification, to directly sense the spectral distribution of the illumination radiation using a sensor. Where this is so, the spectral distribution of the illumination radiation can instead be estimated ίη-silu using a resist covered wafer. One method for calculating the spectral distribution is to measure the “dose to clear” of the resist using a normal EUV reticle and to measure the “dose to clear” of the resist using a reticle which reflects only the DUV part of the spectrum (for example an aluminium coated reticle). Such a method is disclosed in the paper “Deep Ultraviolet Out-of-Band Contribution in Extreme Ultraviolet Lithography: Predictions and Experiments” (Lorusso et al, Proc. of SPIE Vol. 7969 796920-1) which is hereby incorporated by reference. At a first approximation, the ratio of the dose measured using the normal EUV reticle and the dose measured using the aluminium reticle can be taken to be the ratio of EUV radiation to out-of-band radiation. Better approximations can be made by taking into account the resist specific sensitivity for EUV and DUV, as described.

[0050] In a similar manner to the sensitivity calibration table, where the method of the previous paragraph is employed, a calibration table can be produced for the spectral distribution measurements made using this method. Such a calibration table may comprise spectral distribution values, and/or corresponding illumination settings (which mitigate for the effects of the spectral distribution values), for a number of different resists. In this way, illumination modification can be determined and applied depending on the specific resist being employed.

[0051] The off-line calibration method can be used further to capture non-uniform intra-field (and intra-wafer) DUV/EUV effects caused by (for example) non-uniformities of reflections or transmissions from mask, scanner, or optics. This non-uniformity, along with local non-uniformities at the below 100 micron level caused by variations in local pattern density at the mask, can then be used as an input to calculate a balanced, trade-off solution of illuminator and dose settings.

[0052] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0053] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0054] 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 descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses. 1. A lithographic apparatus, comprising an illumination system configured to condition a radiation beam, said radiation beam comprising both EUV radiation and non-EUV radiation; wherein said lithographic apparatus is operable to: obtain a measurement of the non-EUV radiation content comprised within said radiation beam; model imaging effects, said imaging effects comprising the resultant effects of said non-EUV radiation on an image formed on a substrate by said lithographic apparatus as a result of a lithographic process; and determine optimized illumination system settings which mitigate some or all of said imaging effects. 2. The lithographic apparatus of clause 1 being operable to use the optimized illumination system settings in a subsequent lithographic process. 3. The lithographic apparatus of any preceding clause comprising a sensor operable to measure the spectral distribution of said radiation beam. 4. The lithographic apparatus of clause 3 wherein said sensor is located at the same level as said substrate. 5. The lithographic apparatus of clause 4 comprising a substrate table constructed to hold said substrate, said sensor being comprised within the substrate table. 6. The lithographic apparatus of any preceding clause operable such that said image is formed in photoresist on said substrate and said lithographic apparatus is operable to take into account sensitivity of the photoresist to the non-EUV radiation when determining said optimized illumination system settings. 7. The lithographic apparatus of clause 6 being operable such that sensitivity measurements to determine the sensitivity of the photoresist to the non-EUV radiation are performed in advance for a number of different photoresists, and the results, and/or resultant imaging effects and/or optimized illumination settings are stored, and used during subsequent lithographic processes such that appropriate settings are selected in dependence to the photoresist used. 8. The lithographic apparatus of clause 1 or 2 being operable such that said image is formed in photoresist on said substrate and the lithographic apparatus is operable to measure the sensitivity of the photoresist to said non-EUV radiation, and to use this sensitivity measurement to determine said measurement of the magnitude of non-EUV radiation. 9. The lithographic apparatus of clause 8 wherein, in performing said sensitivity measurement, said lithographic apparatus is operable to: measure the exposure energy required to clear said resist using a normal EUV reticle; measure the exposure energy required to clear said resist using a reticle which reflects only a non-EUV radiation band; and determine said measurement of the magnitude of non-EUV radiation by comparing the two exposure energy measurements. 10. The lithographic apparatus of clause 8 or 9 being operable such that said sensitivity measurements are performed in advance for a number of different photoresists and the results, and/or resultant imaging effects and/or optimized illumination settings are stored, and used during subsequent lithographic processes such that appropriate settings are selected in dependence to the photoresist used. 11. The lithographic apparatus of any preceding clause being operable such that the local pattern density variations present on the mask are used in said modelling of imaging effects. 12. The lithographic apparatus of any preceding clause being operable such that said determination of optimized illumination settings comprises using computational lithography techniques that take into account the layout features present on a reticle used during said lithographic process. 13. A method of manufacturing a device, comprising: forming an image on a substrate using a lithographic process, said lithographic process comprising using a radiation beam comprising both EUV radiation and non-EUV radiation; obtaining a measurement of the magnitude of non-EUV radiation comprised within said radiation beam; modelling imaging effects, said imaging effects comprising the resultant effects of said non-EUV radiation on an image formed on a substrate as a result of a lithographic process; determining optimized illumination system settings which mitigate some or all of said imaging effects; and using the optimized illumination system settings in a subsequent lithographic process. 14. The method of clause 13 comprising using a sensor to measure the spectral distribution of said radiation beam. 15. The method of clause 14 wherein said measurement of the spectral distribution is performed in the vicinity of said substrate. 16. The method of any of clauses 13 to 15 wherein said image is formed in photoresist on said substrate and said method comprises taking into account sensitivity of the photoresist to the non-EUV radiation when determining said optimized illumination system settings. 17. The method of clause 16 wherein sensitivity measurements to determine the sensitivity of the photoresist to the non-EUV radiation are performed in advance for a number of different photoresists, and the results, and/or resultant imaging effects and/or optimized illumination settings are stored, and used during subsequent lithographic processes such that appropriate settings are selected in dependence to the photoresist used. 18. The method of clause 13 wherein said image is formed in photoresist on said substrate and the method comprises measuring the sensitivity of the photoresist to said non-EUV radiation, and to use this sensitivity measurement to determine said measurement of the magnitude of non-EUV radiation. 19. The method of clause 18 wherein, in performing said sensitivity measurement, said method comprises: measuring the exposure energy required to clear said resist using a normal EUV reticle; measuring the exposure energy required to clear said resist using a reticle which reflects only a non-EUV radiation band; and determining said measurement of the magnitude of non-EUV radiation by comparing the two exposure energy measurements. 20. The method of clause 18 or 19 wherein said sensitivity measurements are performed in advance for a number of different photoresists and the results, and/or resultant imaging effects and/or optimized illumination settings are stored, and used during subsequent lithographic processes such that appropriate settings are selected in dependence to the photoresist used. 21. The method of any of clauses 13 to 20 wherein the local pattern density variations present on the mask are used in said modelling of imaging effects, 22. The method of any of clauses 13 to 21 wherein said determination of optimized illumination settings comprises using computational lithography techniques that take into account the layout features present on a reticle used during said lithographic process.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.