Abstract:
A CD-pitch dependency for a lithographic pattern printing process is related to the spectral intensity distribution of radiation used for projecting the pattern. A CD-pitch dependency can vary from one system to another. This can result in an iso-dense bias mismatch between systems. The invention addresses this problem by providing a lithographic apparatus including an illumination system for providing a projection beam of radiation, a projection system for projecting a patterned beam onto a target portion of a substrate, and a substrate table for holding the substrate, with a controller to provide an adjustment of the spectral distribution of radiant intensity of the projection beam. The adjustment of the spectral intensity distribution is based on data relating to an iso dense bias, and includes a broadening of the spectral bandwidth or a change of shape of the spectral intensity distribution.

Description:
RELATED APPLICATIONS 
   This is a continuation in part of U.S. patent application Ser. No. 11/019,531, filed Dec. 23, 2004 which is now U.S. Pat. No. 7,534,552, hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a lithographic apparatus, an excimer laser and a device manufacturing method. This invention also relates to a device manufactured thereby. 
   2. Description of the Related Art 
   A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. 
   Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of radiation. The projection system may, for example, be a refractive optical system, or a reflective optical system, or a catadioptric optical system, respectively including refractive optical elements, reflective optical elements, and both refractive and reflective optical elements. 
   Generally, the projection system comprises a device to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system. 
   An illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The illumination system of the apparatus typically comprises adjustable optical elements for setting an outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of an intensity distribution upstream of the mask, in a pupil of the illumination system. A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate. 
   Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is referred to as the critical dimension (“CD”). 
   With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for both semi-dense and isolated features may limit the process latitude (i.e., the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask having the same nominal critical dimensions will print differently depending on the amount of defocus (out of a plane of best focus) of the part of the target portion where the feature is imaged, due to, for example, substrate topography, image curvature or substrate unflatness. 
   A difference in printed CD between two similar features such as contact holes at two respective, different locations on a substrate having corresponding different focal positions will be referred to hereinafter as a CD-focus error. For example, a contact hole having a particular contact hole size and disposed at a first position in the pattern, will print differently from the same feature having the same size and disposed at a second position, when at respective conjugate first and second positions at substrate level the respective exposed substrate areas are disposed at respective different focus positions. Hence, when both contact holes are to be printed simultaneously, a position dependent variation of printed CD is observed. Data describing a specific CD-focus dependency are generally represented by a plot of CD versus focus for a constant exposure dose, and referred to as a Bossung curve. The phenomenon CD-focus error is a particular problem in photolithographic techniques. 
   Conventional lithographic apparatus do not directly address the problem of CD-focus error. Conventionally, it is the responsibility of the users of conventional lithographic apparatus to attempt to maintain CD-focus error within tolerance by compensating for a defocus (for example by adjusting or varying the focus position of the substrate during an exposure), or by optimizing the setting of apparatus optical parameters, such as the NA of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that focus dependency of dimensions of printed isolated and semi-dense features is reduced. However, such a measure to reduce CD-focus error may lead to an increase of other lithographic prosess errors or sensitivities, and may therefore still adversely affect the process latitude. 
   Given a pattern to be provided by a patterning device, and to be printed using a specific lithographic projection apparatus including a specific radiation source, one can identify data relating to CD-focus error which are characteristic for that process, when executed on that lithographic system. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is a problem of mutually matching the corresponding different CD-focus dependencies, such as to reduce CD variations occurring in the manufacturing process. 
   An actual CD-focus dependency as described above may be varying in time. For example, due to lens heating the aberration of the projection system may vary, and or due to heating and other instabilities properties such as illumination settings, and exposure dose of radiation energy may vary in time. Therefore there is the problem of controlling and keeping within tolerance a desired CD-focus dependency. 
   A lithographic process is generally characterized by a process latitude or process window such as, for example, an Exposure-Defocus window, also referred to as an ED-window. An ED-window indicates the available depth of focus in combination with the allowed amount of residual error in the exposure dose of irradiated target portions for a given tolerance on CD. 
   With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD due to focus variations and exposure dose variations may limit the process latitude. In general the useable focus-range of an ED-window is asymmetric around a focus position referred to as the Best Focus or BF. At best focus, a change of CD as a function of a change of focus position of the substrate is smallest or even zero (in the latter case the Bossung curve is locally “iso-focal”, i.e., parallel to the focus axis of the Bossung plot). Typically the available range of focus for contact holes tends to be located asymmetrically around the position BF of best focus, and the CD-focus dependency is then asymmetric with respect to best focus. This corresponds to a process whereby printed contact holes close earlier in one defocus direction, as compared to the other defocus direction. A corresponding Bossung curve is typically shaped as a tilted and curved line segment. An asymmetric CD-focus dependency is limiting the lithographic process latitude, in particular for processes applied to wafers having a considerable topography. 
   SUMMARY OF THE INVENTION 
   The present inventors have identified the following. Techniques are known to enhance the depth of focus for a projection lithographic process by manipulating the spectral distribution of radiant intensity of the projection beam. Generally, radiation used for exposure is provided by an excimer laser; for example, a KrF excimer laser operating at 248 nm wavelength or an ArF excimer laser operating at 193 nm wavelength may be used. The spectral distribution of radiant intensity provided by such lasers comprises a spectral intensity peak having a symmetric shape with respect to a peak wavelength λ p . The bandwidth of the spectral peak may be expressed as a full-width half-maximum bandwidth (referred to as FWHM bandwidth) or alternatively as the bandwidth within which 95% of the total output power of the laser is contained (referred to as the E95 bandwidth), with the peak wavelength λ p  typically centered within said bandwidths. 
   The finite magnitude of the bandwidth introduces a “smear out” of the image of a feature over a focus range around a best focus position BF. Said smear out is represented by a plurality of images displaced along the optical axis of the projection system, in accordance with a plurality of radiation wavelengths (in a range of wavelengths centered at λ p ). The plurality of axially displaced images is formed by the projection system due to the presence of residual axial chromatic aberration of the projection system. If F is the distance between the plane of best focus corresponding to the radiation wavelength λ p  and an image plane corresponding to the radiation wavelength λ, the effect of axial chromatic aberration is described by dF/dλ=AC, where AC is a constant. Therefore, to a good approximation the effect of a constant defocus of the substrate over a distance F, during exposure, is the same as the effect of a change of wavelength Δλ given by Δλ=F/AC and exposing with radiation of this changed wavelength with the substrate held in the best-focus focal plane. 
   The effects of finite spectral bandwidth of the laser radiation can be modeled by linearly converting a symmetric laser spectral distribution of exposure intensity into a symmetric focus distribution using the lens property AC defined by dF/dλ=AC. Over a fairly wide range of wavelengths the laser spectrum can be converted linearly into a focus spectrum using this lens dependency dF/dλ (see FIG. 1a U.S. Patent Application Publication No. 2002/0048288 A1). 
   A finite laser bandwidth results in the re-distribution of the aerial image through focus. The total aerial image will be a sum of the aerial images, each aerial image defocused in accordance with F=AC Δλ, and weighted by the relative exposure intensity at the wavelength λ=λ p +Δλ. 
   This addition of (generally defocused) images has an effect on the image contrast at wafer level. A relatively large laser bandwidth introduces a relatively low image contrast at wafer level, but at the same time the available depth of focus is increased. However, above mentioned problem of contact holes closing earlier in one defocus direction, as compared to the other defocus direction, limiting the lithographic process latitude, remains. 
   It is an object of the present invention to obviate or mitigate one or more of the aforementioned problems in the prior art. In particular, it is an object of the invention to provide improved control over CD-focus dependency. 
   The present inventors have identified that providing an asymmetric spectral intensity distribution of the laser radiation can be used to compensate an asymmetry of imaging parameters (such as printed CD) with respect to focus and/or best focus, and in particular to reduce the asymmetry of an asymmetric CD-focus dependency. 
   According to an aspect of the invention there is provided a lithographic apparatus comprising a radiation system for providing a beam of electromagnetic radiation having a spectral distribution of radiant intensity, a support structure for supporting a patterning device, the patterning device serving to impart the beam of radiation with a pattern in its cross-section, a substrate table for holding a substrate, a projection system for projecting the beam of radiation after it has been patterned onto a target portion of the substrate, and a controller configured and arranged to provide an adjustment of said spectral distribution of radiant intensity based on data relating to exposure at a first focal position and exposure at a second focal position and representing a corresponding first printed size and second printed size of the feature. 
   The present invention provides an advantage of adjustment of the spectral distribution of radiant intensity I(λ): using data relating to exposure at a first focal position and exposure at a second focal position, it is possible to either match system to system focus behavior or to enhance the symmetry of focus behaviour with respect to a position of best focus. 
   According to a further aspect of the invention there is provided an excimer laser having a bandwidth-controller arranged to control a bandwidth of the spectral distribution of radiant intensity, and whereby the bandwidth-controller is constructed and arranged to spectrally shift the bandwidth in reaction to a user supplied signal representative for a selected bandwidth shift of the spectral distribution. 
   According to an aspect of the invention there is provided a device manufacturing method comprising providing a beam of electromagnetic radiation having a spectral distribution of radiant intensity, patterning the beam of radiation with a pattern in its cross-section using a patterning device, projecting the patterned beam of radiation onto a target portion of a substrate, and adjusting of said spectral distribution of radiant intensity in accordance with data relating to exposure at a first focal position and exposure at a second focal position and representing a corresponding first printed size and second printed size of the feature. 
   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, 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) or a metrology or 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. 
   The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm). 
   The term “patterning device” used herein should be broadly interpreted as referring to devices that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
   Patterning devices 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; in this manner, the reflected beam is patterned. The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a way depending 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 can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” 
   The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.” 
   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. 
   The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
       FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention; 
       FIG. 2  depicts a lithographic apparatus according to a further embodiment of the invention; 
       FIG. 3(   a ) illustrates an example of an asymmetric spectral intensity distribution with a location of a peak wavelength, a center wavelength, and E95 wavelengths; 
       FIG. 3(   b ) illustrates examples of symmetric spectral intensity distributions; 
       FIG. 3(   c ) examples of asymmetric spectral intensity distributions; 
       FIG. 4(   a ) is an example of a lithographic printing process characterized by an asymmetric Bossung curve with indication of a useable focus range; 
       FIG. 4(   b ) is an example of a symmetric Bossung curve obtained with the method according to the invention, by using a radiation beam with an asymmetric spectral intensity distribution in the process represented by  FIG. 4(   a ); 
       FIG. 5  illustrates a symmetric spectral intensity distribution as a superposition of two spectrally overlapping intensity distributions and an asymmetric spectral intensity distribution as a superposition of two mutually displaced spectral intensity distributions; 
       FIG. 6  shows a schematic representation of Bossung curves for dense and isolated features; 
       FIG. 7  illustrates a symmetric spectral intensity distribution and a weight factor representing the intensity distribution; 
       FIG. 8  shows an example of Bossung curves for dense and isolated features, and the effect of an increase of spectral bandwidth; 
       FIG. 9  illustrates an asymmetric laser spectral intensity distribution and an representative weight function consisting of two adjacent block shaped sections; 
       FIG. 10  shows an example of Bossung curves for dense and isolated features, and the effect of an increase of asymmetry of a spectral intensity distribution; 
       FIG. 11  schematically depicts the effect of a transition from a relatively narrow, symmetric spectral intensity distribution to a broader symmetric and to a broader asymmetric spectral intensity distribution on a Bossung curve for an isolated feature; 
       FIG. 12  depicts a simulated effect of symmetric increase of FWHM (Full Width Half Maximum) for nominal 65 nm isolated lines; 
       FIG. 13  depicts symmetric laser spectral intensity distributions distribution as used for the simulations of  FIG. 12 ; 
       FIG. 14  illustrates four intensity distributions for use with a simulation of the effect of increased intensity distribution asymmetry at a constant FWHM (Full Width Half Maximum); 
       FIG. 15  shows the Bossung curves for dense and isolated features corresponding to the spectra of  FIG. 14 ; 
       FIG. 16  depicts Bossung curves for dense and isolated features corresponding to the spectra of  FIG. 14  after correction for focus shift; 
       FIG. 17  is a schematic representation of introduction of a wafer tilt due to rotation around an x-axis; 
       FIG. 18  is a schematic representation illustrating an analogy of effect of wafer tilt on focus history seen by a part of the wafer during scan and a normal exposure in the absense of tilt but now with a laser spectral intensity distribution having a finite bandwidth. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises:
         an illumination system (illuminator) IL for providing a projection beam PB of radiation (e.g., UV radiation or EUV radiation).   a first support structure (e.g., a mask table) MT for supporting a patterning device (e.g., a mask) MA and connected to first positioning actuator PM for accurately positioning the patterning device with respect to item PL;   a substrate table (e.g., a wafer table) WT for holding a substrate (e.g., a resist-coated wafer) W and connected to second positioning actuator PW for accurately positioning the substrate with respect to item PL; and   a projection system (e.g., a refractive projection lens) PL for imaging a pattern imparted to the projection beam PB by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.       
   As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above). 
   The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. 
   The illuminator IL may comprise adjustable optical elements AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as π-outer and π-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation, referred to as the projection beam PB, having a desired uniformity and intensity distribution in its cross-section. 
   The projection beam PB is incident on the mask MA, which is held on the mask table MT. Having traversed the mask MA, the projection beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning actuator PW and position sensor IF (e.g., an interferometric device), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the beam PB. Similarly, the first positioning actuator PM and another position sensor (which is not explicitly depicted in  FIG. 1 ) can be used to accurately position the mask MA with respect to the path of the beam PB, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning actuator PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 . 
   The depicted apparatus can be used in the following example modes: 
   In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the projection beam is projected onto a target portion C at once (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. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. 
   In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the projection 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 mask table MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. 
   In another mode, the 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 projection 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 devices, such as a programmable mirror array of a type as referred to above. 
   A controller CON is provided to provide an adjustment of the spectral distribution of radiant intensity of the beam PB based on data relating to exposure at a first focal position and exposure at a second focal position and representing a corresponding first printed size and second printed size of the feature. 
   Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
     FIG. 2  schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus of  FIG. 2 , in contrast to the apparatus in  FIG. 1 , is of a reflective type (e.g., employing a reflective mask). 
   The apparatus of  FIG. 2  comprises:
         an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or EUV radiation);   a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;   a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.       

   A difference between the spectral bandwidth of lasers which are part of respective lithographic projection apparatus result in differences between a CD-focus dependency of printed CD for these respective apparatus. Further, an asymmetric CD-focus dependency with respect to best focus may result in a too small process latitude. The present invention seeks to address these problems by providing an apparatus which is equipped with a controller CON configured and arranged to provide an adjustment of the symmetry of the spectral distribution of the laser radiation whereby the adjustment is aimed at affecting the CD-focus dependency of the lithographic apparatus. The adjustment may be a dynamic adjustment to compensate for variations in time of a CD-focus error. Such variations in time may, for example, be caused by lens heating due to absorption of laser beam radiation during exposure. The CD-focus dependency is specific for the apparatus in combination with the layout of the mask pattern and other process parameters and properties such as for example the illumination mode and setting, the exposure time, the resist type, the specific lens aberrations, as well as settings for the pre-exposure and post exposure processing steps. 
   An asymmetric CD-focus dependency can be affected, according to the present invention, by adjusting the shape of the spectral intensity distribution of the laser beam, and in particular, by introducing an asymmetry into the shape of the spectral intensity distribution. An excimer laser generally is provided with means to control and adjust the spectral distribution of the emitted laser radiation. For example U.S. Patent Application Publication No. 2002/0048288A1 relates to an excimer laser provided with a controller of a line-narrowing device for controlling a the spectral distribution of the laser beam. The controller is arranged to adjust the bandwidth of the spectral distribution by dithering a wavelength tuning mirror in phase with the repetition rate of the laser. The line narrowing unit comprises a grating and a fast tuning mechanism, and the controller controls a monitoring of the laser beam to determine bandwidth of individual pulses laser pulses, and a periodically adjusting of the tuning mechanism during a series of pulses so that the wavelengths of some pulses in the series of pulses are slightly longer than a target wavelength and the wavelengths of some pulses in the series of pulses are slightly shorter than the target wavelength in order to produce for the series of pulses an effective laser beam spectrum having at least two spectral peaks. In the latter case, the spectral distribution of radiant intensity may for example be a superposition of a first and a second peaked spectral intensity distribution having a respective equal first and second full-width half-maximum bandwidth, and a respective equal first and second intensity. The spectral peaks feature a respective first and second peak wavelength, and the difference Δλ p  between the first and second peak wavelength is adjustable. 
   Similarly, U.S. Pat. No. 5,303,002 relates to an excimer laser which generates a beam of radiation whereby the spectral distribution of radiant intensity of the laser beam of radiation comprises a plurality of narrow spectral bands of radiation. A line narrowing device is arranged to select one or more line narrowed outputs to be used for the lithographic process. Each of the outputs may have an attenuator which can adjust the intensity of each spectral band independently so that an asymmetric spectral intensity distribution can be provided. The corresponding radiation beams pass through a gain generator and are combined to produce a beam of radiation with the desired spectral distribution. The spectral intensity distribution may be an asymmetric distribution, i.e. a distribution with a spectral shape deviating from a symmetric shape with respect to a center wavelength λ c . 
   As mentioned above, generally an ED-window is not symmetric around best focus (e.g., contact holes generally close earlier in one defocus position than the other). Also, a Bossung curve may be a curved, tilted line segment. In the present invention a spectral intensity distribution which is asymmetric with respect to a center wavelength λ c  is used to correct for such a tilt of a Bossung curve, and to make the Useable Focus Range (viable process/production window) substantially symmetric around best focus BF. Thus the CD-focus dependency of features is made symmetric with respect to best focus using a projection beam having an asymmetric spectral intensity distribution. 
   Referring to  FIG. 3(   a ), there is shown an example of an asymmetric spectral distribution  300 . The wavelengths λ 1  and λ 2  in  FIG. 3(   a ) define the E95 bandwidth represented by the arrow  301 . The wavelength λ c  is the center wave length, i.e. the wavelength at the center of the range [λ 1 , λ 2 ]. The curve  300  represents the spectral intensity distribution I(λ), which is peaked at a peak wavelength λ p . In general, an asymmetric intensity distribution is characterized by the inequality I(λ−λ c )≠I(λ c −λ). A measure for asymmetry may be expressed in terms of the moments of intensity MI left  and MI right  defined as 
   
     
       
         
           
             
               
                 
                   
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   and the spectrum may be referred to as asymmetric when MI left  is different from MI right . For example, the spectrum may be referred to as asymmetric when the spectral intensity distribution I(λ) is an asymmetric distribution whereby the moments of intensity, as defined in equation (1), satisfy the inequality 
   
     
       
         
           
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     FIG. 3(   b ) shows an example of symmetric spectral radiation distributions  302 , 304  and  306 . 
     FIG. 3(   c ) shows an example of asymmetric spectral radiation distributions  303 ,  305  and  307 . 
   Referring now to  FIG. 4(   a ), there is shown an example of an asymmetric Bossung curve with indication of the useable focus range. The Bossung curve corresponds to a lithographic process which in the presense of a symmetric spectral intensity distribution has an asymmetric CD-focus dependency. Note that in negative defocus, the process window is not limited by the indicated lower process limit. by e.g., pattern collapse. In positive defocus the CD is limited by the preselected range of allowable CD values. 
   Referring to  FIG. 4(   b ), there is shown an example of a symmetric Bossung curve obtained after applying an asymmetric laser intensity distribution to the lithographic process represented by the Bossung curve of  FIG. 4(   a ). Again, an indication of the useable process window is given. Note that in negative defocus, the process window is now limited by the preselected range of allowable CD values. 
   According to an aspect of the invention the source SO in  FIG. 1  is an excimer laser providing a pulsed beam of laser radiation. The laser comprises bandwidth monitoring equipment and wavelength tuning equipment permitting bandwidth control of the laser beam by a bandwidth-controller of the laser. The bandwidth-controller of the laser is generally used to maintain a preselected bandwidth (compensating, for example, changes in the laser-gain medium over the life of the laser), in accordance with a selection made by the laser manufacturer. According to the present invention, however, the bandwidth-controller of the laser is provided with an input channel arranged for receiving a signal representative for a selected asymmetry of the spectral distribution in accordance with a selection made by the user of the laser. For example, the signal can be provided by the controller CON of the lithographic apparatus according to the present invention. With an eximer laser featuring a user-selectable spectral bandwidth asymmetry the adjustmet of CD-fucus dependency according to the present invention can be provided dynamically, for example, during a sanning exposure of a target portion C or during a plurality of exposures of target portions C covering a substrate. Both intra-die and inter die controll of CD-focus dependency is obtained this way. Similarly, an eximer laser provided with user-selectable spectral asymmetry setting can be used to obtain CD-focus dependency matching between different apparatus, in accordance with the present invention. 
     FIG. 5  illustrates a spectral distribution of radiant intensity  500  as a superposition of a first peaked spectral intensity distribution  501  and a second peaked spectral intensity distribution  502  having a respective first bandwidth  503  and second bandwidth  504 . The first and second peak wavelengths λ p1  and λ p2  are equal, but the second peak intensity is lower than the first peak intensity.  FIG. 5  further illustrates the effect of providing, through control of the line width narrowing device of the pulsed excimer laser, an adjustment comprising a change Δλ p  of the difference between the first and second peak wavelength. The adjustment is (the difference λ p1 −λ p2  in  FIG. 5  being initially zero) in the present example equal to the difference λ p2 −λ p1 . The resulting intensity distribution  506  has an asymmtery with respect to a center wavelength λ c . 
   Referring to  FIG. 6  there is shown a schematic representation of a Bossung curve  600  typical for an isolated feature and a Bossung curve  601  typical for the feature in dense arrangement, i.e., arranged at a duty cycle 1:1. The Bossung curve  600  represents a plot of printed critical dimension for the feature in isolated arrangement, and the corresponding CD is denoted by CD iso , as it would be obtained with exposure in different focal positions. The exposure energy is a constant along the plots  600  and  601 . The different focal positions are given by the focal coordinate F (above referred to as a “defocus”), which defines the position of the substrate with respect to a position of best focus BF. 
   Typically, the printed critical dimension CD dense  of the dense feature does not depend (to a first approximation) on focal position, because of the extended depth of focus resulting from two beam imaging. Generally, imaging of dense features is arranged such that only two diffracted orders of radiation, as emerging from the pattern, are captured by the imaging projection lens. 
   The printed critical dimension CD iso  may be modelled as a polynomial of F according to
 
 CD   iso   =A   0   +A   1   F+A   2   F   2   +A   4   F   4 ,  (2)
 
   whereby the coefficient A 0  represents the printed CD at best focus. Further, the coordinate F may be expressed in terms of an absolute focus coordinate f defined by 
   F=f−f BF , where the coordinate f BF  is the absolute coordinate, along the z-axis, of the best focus position BF. 
   In the absence of a so-called linear focus term, i.e. when A 1 =0, the resulting second order approximation denoted by CD iso  (0,2; f) of CD iso  is then given by
 
 CD   iso (0,2 ;f )= A   0   +A   2 ( f−f   BF ) 2 .  (3)
 
   In contrast, the Bossung curve for the dense feature may simply be modeled as CD dense =B 0 . Thus, at best focus BF, the dense features are printed at a width B 0 , and the isolated features at a width A 0 , and the iso-dense bias between thee features would be A 0 -B 0  nm. 
   In accordance with the present invention, the effects of finite spectral bandwidth on the Bossung curve can be modeled by linearly converting a symmetric spectral intensity distribution of the laser beam into a symmetric focus distribution using the lens property AC defined by dF/dλ=AC. Since F=f−f BF , also df/dλ=AC at or near best focus position. The laser bandwidth results in the re-distribution of the aerial image through focus. The total aerial image will be a sum of the aerial images, each aerial image defocused in accordance with F=AC Δλ, and weighted by the relative exposure intensity at each wavelength λ. The weighting may be expressed by a weight-function W in accordance with the spectral distribution of radiant intensity I(λ) of the laser radiation. 
   The resulting printed CD incorporating the effect of the addition of the (generally defocused) images may be represented by CD av , and can be approximated by the following averaging: 
   
     
       
         
           
             
               
                 
                   
                     
                       CD 
                       av 
                     
                     ⁡ 
                     
                       ( 
                       f 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ∫ 
                         
                           f 
                           - 
                           
                             
                               1 
                               2 
                             
                             ⁢ 
                             
                               F 
                               BW 
                             
                           
                         
                         
                           f 
                           + 
                           
                             
                               1 
                               2 
                             
                             ⁢ 
                             
                               F 
                               BW 
                             
                           
                         
                       
                       ⁢ 
                       
                         
                           
                             CD 
                             ⁢ 
                             
                                 
                             
                           
                           iso 
                         
                         ⁢ 
                         
                           ( 
                           
                             f 
                             ′ 
                           
                           ) 
                         
                         ⁢ 
                         
                           W 
                           ⁡ 
                           
                             ( 
                             
                               
                                 f 
                                 ′ 
                               
                               - 
                               f 
                             
                             ) 
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           
                             f 
                             ′ 
                           
                         
                       
                     
                     
                       
                         ∫ 
                         
                           f 
                           - 
                           
                             
                               1 
                               2 
                             
                             ⁢ 
                             
                               F 
                               BW 
                             
                           
                         
                         
                           f 
                           + 
                           
                             
                               1 
                               2 
                             
                             ⁢ 
                             
                               F 
                               BW 
                             
                           
                         
                       
                       ⁢ 
                       
                         
                           W 
                           ⁡ 
                           
                             ( 
                             
                               
                                 f 
                                 ′ 
                               
                               - 
                               f 
                             
                             ) 
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           
                             f 
                             ′ 
                           
                         
                       
                     
                   
                 
                 , 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   where the “bandwidth” F BW  represents the focus range equivalent to the bandwidth of the spectral intensity distribution. For example, with λ 1  and λ 2  being the E95 bandwidth wavelengths, F BW  can be defined as F BW =AC(λ 1 −λ 2 ). The weight function W(f) is proportional to the spectral distribution of radiant intensity I(λ) and can be obtained from I(λ) by expressing I(λ) as a function of (λ−λ c ), and writing λ−λ c  as an equivalent focal coordinate f with (λ−λ c )=f/AC, in view of the lens property df/dλ=AC. 
   For simplicity it will be assumed that the weight function W(f) in accordance with the symmetric intensity distribution  302  of  FIG. 3  can be approached by a block function  700 , as illustrated in  FIG. 7 . 
   Combination of this approximation with the approximation CD iso  (0,2; f) for the printed CD of an isolated feature, results in the following prediction for the average CD (at best focus) of a feature due to the introduction of a finite laser bandwidth (resulting in the re-distribution of the aerial image over a focus range of from −½F BW  to ½F BW ): 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           CD 
                           av 
                         
                         ⁡ 
                         
                           ( 
                           
                             f 
                             BF 
                           
                           ) 
                         
                       
                       = 
                         
                       ⁢ 
                       
                         
                           
                             ∫ 
                             
                               
                                 f 
                                 BF 
                               
                               - 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                             
                               
                                 f 
                                 BF 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                           
                           ⁢ 
                           
                             
                               
                                 CD 
                                 iso 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   0 
                                   , 
                                   
                                     2 
                                     ; 
                                     
                                       f 
                                       ′ 
                                     
                                   
                                 
                                 ) 
                               
                             
                             ⁢ 
                             
                               ⅆ 
                               
                                 f 
                                 ′ 
                               
                             
                           
                         
                         
                           
                             ∫ 
                             
                               
                                 f 
                                 BF 
                               
                               - 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                             
                               
                                 f 
                                 BF 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                           
                           ⁢ 
                           
                             ⅆ 
                             
                               f 
                               ′ 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           A 
                           0 
                         
                         + 
                         
                           
                             1 
                             12 
                           
                           ⁢ 
                           
                             A 
                             2 
                           
                           ⁢ 
                           
                             F 
                             BW 
                             2 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   From the above equation it is clear that the change ΔCD iso  in printed critical dimension at best focus (due to a change from ideal monochromatic radiation to the introduction of a certain laser bandwidth resulting in a through focus re-distribution of the image over a focus range from −½F BW  to ½F BW ) is given by 
   
     
       
         
           
             Δ 
             ⁢ 
             
                 
             
             ⁢ 
             
               CD 
               iso 
             
           
           = 
           
             
               
                 
                   A 
                   2 
                 
                 · 
                 
                   1 
                   12 
                 
               
               ⁢ 
               
                 F 
                 BW 
                 2 
               
             
             ∼ 
             
               F 
               BW 
               2 
             
           
         
       
     
   
   In contrast, no such change occurs for the size of the dense features, since in the present approximation CD dense  is a constant value, independent of focus position: CD dense =B 0 , in accordance with the iso-dense characteristics as illustrated in  FIG. 4 . 
     FIG. 8  schematically illustrates the effect of the change from an ideal practically monochromatic radiation spectrum of the laser beam to the introduction of a finite laser bandwidth in accordance with the present approximation. The arrow  800  represents the (focus independent) shift ΔCD iso  of the Bossung curve  600  representing the printed CD as obtained with the exposure process using a practically monochromatic (not bandwidth broadened) laser radiation spectrum, and the curve  810  is the Bossung curve for the increased laser bandwidth. Since generally the Bossung curve for the feature in dense arrangement is less or not sensitive to a change of spectral bandwidth, the adjustment of spectral bandwidth can be used for adjusting the CD-pitch dependency. 
   Assuming that the energy dependence of the CD is focus independent an undesired residual impact of laser bandwidth on printed CD could be easily compensated in order to maintain the CD of a reference feature (such as for example the dense lines in the present embodiment) unaltered. 
   The same approximation as described above can be generalized for an arbitrary defocus position F (and using F=f−f BF ) as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             CD 
                             av 
                           
                           ⁡ 
                           
                             ( 
                             F 
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ∫ 
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 - 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 + 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                             
                             ⁢ 
                             
                               
                                 
                                   CD 
                                   iso 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     0 
                                     , 
                                     
                                       2 
                                       ; 
                                       
                                         f 
                                         ′ 
                                       
                                     
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 
                                   f 
                                   ′ 
                                 
                               
                             
                           
                           
                             
                               ∫ 
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 - 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 + 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                             
                             ⁢ 
                             
                               ⅆ 
                               
                                 f 
                                 ′ 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             A 
                             0 
                           
                           + 
                           
                             
                               A 
                               2 
                             
                             ⁢ 
                             
                               F 
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               12 
                             
                             ⁢ 
                             
                               A 
                               2 
                             
                             ⁢ 
                             
                               F 
                               BW 
                               2 
                             
                           
                         
                       
                     
                   
                 
                 , 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   The change in CD induced by the re-distribution of the aerial image over a focus range from −½F BW  to ½F BW  is independent of the focus position F and is proportional with F BW   2 . 
   For the fourth order focus term in Equation (2) can be derived the following contribution CDav(4) to CDav: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             CD 
                             av 
                           
                           ⁡ 
                           
                             ( 
                             
                               4 
                               , 
                               F 
                             
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ∫ 
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 - 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 + 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                             
                             ⁢ 
                             
                               
                                 
                                   
                                     A 
                                     4 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         f 
                                         ′ 
                                       
                                       - 
                                       
                                         f 
                                         BF 
                                       
                                     
                                     ) 
                                   
                                 
                                 4 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 
                                   f 
                                   ′ 
                                 
                               
                             
                           
                           
                             
                               ∫ 
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 - 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                               
                                 F 
                                 + 
                                 
                                   f 
                                   BF 
                                 
                                 + 
                                 
                                   
                                     1 
                                     2 
                                   
                                   ⁢ 
                                   
                                     F 
                                     BW 
                                   
                                 
                               
                             
                             ⁢ 
                             
                               ⅆ 
                               
                                 f 
                                 ′ 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             A 
                             4 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 F 
                                 4 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   1 
                                   80 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                   4 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 . 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   Equation 7 shows that there is now a defocus-dependent shift as well as a constant shift of the Bossung curve. 
   Similarly, for a first order focus term in Equation (2) can be derived the contribution CDav(1) to CDav: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           CD 
                           av 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             , 
                             F 
                           
                           ) 
                         
                       
                       = 
                         
                       ⁢ 
                       
                         
                           
                             ∫ 
                             
                               F 
                               + 
                               
                                 f 
                                 BF 
                               
                               - 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                             
                               F 
                               + 
                               
                                 f 
                                 BF 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                           
                           ⁢ 
                           
                             
                               
                                 A 
                                 1 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     f 
                                     ′ 
                                   
                                   - 
                                   
                                     f 
                                     B 
                                   
                                 
                                 ) 
                               
                             
                             ⁢ 
                             
                               ⅆ 
                               
                                 f 
                                 ′ 
                               
                             
                           
                         
                         
                           
                             ∫ 
                             
                               F 
                               + 
                               
                                 f 
                                 BF 
                               
                               - 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                             
                               F 
                               + 
                               
                                 f 
                                 BF 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                 ⁢ 
                                 
                                   F 
                                   BW 
                                 
                               
                             
                           
                           ⁢ 
                           
                             ⅆ 
                             
                               f 
                               ′ 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           A 
                           1 
                         
                         ⁢ 
                         F 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
         
       
     
   
   So the re-distribution of the aerial image over a focus range from −½F BW  to ½F BW  does not impact the linear focus term. 
   According to an embodiment of the invention, the spectral distribution of radiant intensity comprises a spectral intensity peak having, with respect to a center wavelength, a symmetric shape and wherein said adjustment comprises a change of the symmetric shape into an asymmetric shape with respect to the center wavelength. 
   An asymmetric spectral distribution of radiant intensity of the laser beam can be provided, for example, by differently attenuating each of a plurality of narrow spectral bands of radiation in a line narrowing device which is arranged to select a plurality of line narrowed outputs to be used for the lithographic process. In  FIG. 9  a asymmetric intensity distribution I(λ) is represented by the plot  300 . Similar to the embodiment described above, the intensity distribution may be approximated by adjacent, block shaped intensity distributions. In particular, as is illustrated in  FIG. 9 , in the present embodiment the intensity distribution is modelled as two adjacent block functions  910  and  920 , of equal area, and different width. The E95 wavelengths λ 1  and λ 2  define a total bandwidth equivalent to the focus range  901  with a magnitude denoted by 
             3   2     ⁢     F     BW   ,             
and the spectrum is approximated by the left block function  910  of width
 
             1   2     ⁢     F   BW           
and the right block function  920  of bandwidth F BW . As described above for a symmetric intensity distribution, the present asymmetric spectral radiant intensity distribution may be converted into a weight function W(f) proportional to the spectral distribution of radiant intensity I(λ) by expressing I(λ), or in this embodiment by expressing the block functions representing I(λ)) as a function of (λ−λ c ), and writing λ−λ c  as an equivalent focal coordinate f with (λ−λ c )=f/AC, in view of the lens property df/dλ=AC. Since the block functions  910  and  920  are of equal area, the exposure dose in the corresponding focus ranges is equal.
 
   The effect of a change of the spectral intensity distribution which initially is representing a quasi monochromatic laser line into an asymmetric spectral intensity distribution on a Bossung curve can be estimated using the procedure as described above. 
   A combination of the present approximation for the intensity distribution I(λ) (resulting in to adjacent block-shaped weight functions) with the approximation CD iso  (0,2; f) for the printed CD of an isolated feature, results in the following prediction for the average critical dimension CD av  (at arbitrary defocus F): 
   
     
       
         
           
             
               
                 
                   
                     CD 
                     av 
                   
                   ⁡ 
                   
                     ( 
                     F 
                     ) 
                   
                 
                 = 
                   
                 ⁢ 
                 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       
                         
                           ∫ 
                           
                             F 
                             + 
                             
                               f 
                               BF 
                             
                             - 
                             
                               
                                 1 
                                 2 
                               
                               ⁢ 
                               
                                 F 
                                 BW 
                               
                             
                           
                           
                             F 
                             + 
                             
                               f 
                               BF 
                             
                           
                         
                         ⁢ 
                         
                           
                             
                               CD 
                               iso 
                             
                             ⁡ 
                             
                               ( 
                               
                                 0 
                                 , 
                                 
                                   2 
                                   ; 
                                   
                                     f 
                                     ′ 
                                   
                                 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ⅆ 
                             
                               f 
                               ′ 
                             
                           
                         
                       
                       
                         
                           ∫ 
                           
                             F 
                             + 
                             
                               f 
                               BF 
                             
                             - 
                             
                               
                                 1 
                                 2 
                               
                               ⁢ 
                               
                                 F 
                                 BW 
                               
                             
                           
                           
                             F 
                             + 
                             
                               f 
                               BF 
                             
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           
                             f 
                             ′ 
                           
                         
                       
                     
                   
                   + 
                 
               
             
           
           
             
               
                   
                 ⁢ 
                 
                   
                     1 
                     2 
                   
                   ⁢ 
                   
                     
                       
                         ∫ 
                         
                           F 
                           + 
                           
                             f 
                             BF 
                           
                         
                         
                           F 
                           + 
                           
                             f 
                             BF 
                           
                           + 
                           
                             F 
                             BW 
                           
                         
                       
                       ⁢ 
                       
                         
                           
                             CD 
                             iso 
                           
                           ⁡ 
                           
                             ( 
                             
                               0 
                               , 
                               
                                 2 
                                 ; 
                                 
                                   f 
                                   ′ 
                                 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ⅆ 
                           
                             f 
                             ′ 
                           
                         
                       
                     
                     
                       
                         ∫ 
                         
                           F 
                           + 
                           
                             f 
                             BF 
                           
                         
                         
                           F 
                           + 
                           
                             f 
                             BF 
                           
                           + 
                           
                             F 
                             BW 
                           
                         
                       
                       ⁢ 
                       
                         ⅆ 
                         
                           f 
                           ′ 
                         
                       
                     
                   
                 
               
             
           
           
             
               
                 = 
                   
                 ⁢ 
                 
                   
                     A 
                     0 
                   
                   + 
                   
                     
                       A 
                       2 
                     
                     ⁢ 
                     
                       F 
                       2 
                     
                   
                   + 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       A 
                       2 
                     
                     ⁢ 
                     
                       FF 
                       BW 
                     
                   
                   + 
                   
                     
                       1 
                       4 
                     
                     ⁢ 
                     
                       A 
                       2 
                     
                     ⁢ 
                     
                       F 
                       BW 
                       2 
                     
                   
                 
               
             
           
         
       
     
   
   As schematically indicated in  FIG. 10 , not only an offset  900  with magnitude 
             1   4     ⁢     A   2     ⁢     F   BW   2           
is introduced (similar to the situation whereby an increase of bandwidth of a symmetric spectral distribution is applied) but also a linear term
 
             1   2     ⁢     A   2     ⁢     FF   BW           
is introduced. The presence of these two contributions results in a shifted and counter-clockwise tilted Bossung curve ( 910 ), as schematically indicated in  FIG. 10 . Further, the focus position along the optical axis where a change of critical dimension as a function of a change of focal position is zero, is now located at a defocus position F iso  slightly defocused from the best focus position f BF .
 
   This tilt could be used to compensate for spherical aberration of the projection lens, characterized by for example the Z9 Zernike coefficient (resulting in a Bossung tilt of a Bossung curve, which in the absence of sherical aberration would be untilted). Thus, if the initial Bossung curve  600  would have been tilted clockwise, the counter-clockwise tilt of curve  910  would then result in the Bossung curve  910  being untilted, i.e., symmetric with respect to best focus. Also this tilting behavior could be used to influence side wall angles and pattern collapse behavior. 
   Referring to  FIG. 11 , examples of Bossung curves  140 ,  141 ,  142  show the impact of a transition from a conventional relatively narrow and symmetrical spectral intensity distribution ( 143 ) to a symmetrical bandwidth-broadened distribution ( 144 ) and to an asymmetrical spectral intensity distribution ( 145 ). The dashed lines indicate the approximation used for the weight function W(f). The Bossung curve for dense lines is not shown, and is unaffected, thereby providing two independent parameters for adjusting an iso dense bias characteristic of an apparatus. Note for both the symmetrical and asymmetrical case the total focal range  146  is the same. 
   Turning now to  FIG. 12  there is shown the simulated effect of symmetric increase of FWHM (Full Width Halve Maximum) for nominal 65 nm isolated lines (Prolith 5 pass calculation, NA 0.93 and sigma 0.94/0.74, binary reticle, calibrated resist model). As expected from the calculations, the constant decrease in CD through focus by introduction of an enlarged laser bandwidth. Note all calculation are performed using the same exposure dose. 
   Referring to  FIG. 13  there is shown a symmetric laser bandwidth distribution as used for the simulations in  FIG. 12 . For the simulations these laser bandwidth distributions were approximated. 
   The impact of varying the asymmetry of a spectral intensity distribution I(λ) is shown by way of simulations and illustrated in  FIG. 14  and  FIG. 15 .  FIG. 14  shows different asymmetric spectral intensity distributions  111 ,  112 ,  113 , and  114 . For the simulations these spectral intensity distributions were approximated.  FIG. 15  shows the simulated effect of increased spectral asymmetry for constant FWHM (Full Width Half Maximum=0.2 pm), and for nominal 65 nm dense and isolated lines (Prolith 5 pass calculation, NA 0.93 and sigma 0.94/0.74, binary reticle, calibrated resist model). The Bossung curves  111 ′,  112 ′,  113 ′, and  114 ′ correspond to the respective spectra  111 ,  112 ,  113 , and  114 . As expected from the calculations, the effect is a shift of the Bossung curve along the focus-axis and change of the tilt of the Bossung curve at a fixed focus. Note that all calculations were performed using the same exposure dose. 
   Referring finally to  FIG. 16 , there are shown the same results as depicted in  FIG. 16  after correcting for the focus shift showing that, as expected from the calculations, the Bossung curve has tilted due to the introduction of an asymmetric laser bandwidth for constant FWHM (Full Width Halve Maximum=0.2 pm) for nominal 65 nm isolated lines (Prolith 5 pass calculation, NA 0.93 and sigma 0.94/0.74, binary reticle, calibrated resist model). Note all calculations were performed using the same exposure dose. 
   Image smear-out due to a symmetric spectral intensity distribution, and approximated by a block shaped weight function W(f) as illustrated in  FIG. 7 , is fully analogous to image smear-out due to a contiuous z-movement of the substrate during exposure of a target portion. This analogy is schematically illustrated in  FIGS. 17 and 18 . Referring to  FIG. 17 , there is shown a schematic representation of introduction of a wafer Rx tilt (a roation around an axis parallel to the x-axis) showing that when exposing the wafer with Rx tilt, the wafer is exposed at different focus positions (−F to +F) during the scan. In scan direction each point of the wafer sees a through focus behavior ranging from −a·R x  to a·R x . (2a is slit width). 
   The imaging situation as is presented in  FIG. 17  can be also achieved by using a finite laser bandwidth. In general wavelength has an impact on best focus position, so by stretching the laser bandwidth has a similar result on the aerial image of a structure as wafer tilt. 
   Referring to  FIG. 18 , there is shown a schematic representation of effect of wafer-Rx tilt and laser bandwidth stretching on focus and dose seen by the structure to be imaged as compared to normal exposure. 
   An effect on average CD due to a continuous change of focus position of the substrate during exposure can be modeled by the equations 4-8, by replacing 
             1   2     ⁢     F   BW           
with half the range of focus displacement of the substrate. Thus, to describe the effect on printed CD in case a substrate is tilted and a scanning movement is executed along the tilted direction, as illustrated in  FIG. 17 ,
 
             1   2     ⁢     F   BW           
is to be replaced by a·R x  in equations 4 to 8. In view of this analogy, and according to an aspect of the present invention, residual tilt and tilt-variations of the substrate, present during exposure, can be supplemented with a complementary constant as well as variable portion of bandwidth of the spectral intensity distribution I(λ), so that the added effect of residual tilt and finite spectral bandwidth is constant during exposure, thereby improving CD uniformity.
 
   According to an aspect of the invention, a control interface which is part of the controller CON in  FIG. 1  may be provided between a step-and-scan lithographic apparatus and an excimer laser which is part of the radiation system of the apparatus, and which provides pulsed radiation. The interface is used by the apparatus to send wavelength setpoints to the laser on a pulse to pulse basis. The laser has an internal module which measures the wavelength per pulse. The difference between the actual measured wavelength and the wavelength setpoint is used by the lithographic appararatus as an error-signal in a pulse to pulse feedback control loop. This enables the controller to adjust the laser wavelength during exposures. For instance wavelength profiles can be used during exposures to compensate focus profiles during the exposure. Thus, the radiation peak wavelength λ p  can be varied during an exposure, resulting in a spectral shift of the bandwidth of the spectral intensity distribution I(λ), and the bandwidth shift can be used as an extra manipulator to optimize the lithographic printing process. For example, an effect on printed CD of pressure variations and wafer unflatness (as part of a dynamic Field Curvature correction) can be compensated in accordance with the present invention. 
   While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention. It will also be appreciated that the disclosed embodiments may include any of the features herein claimed.