Method and apparatus for controlling iso-dense bias in lithography

The present invention relates to a lithographic apparatus and a method of using the apparatus in the manufacture of a device such as an integrated circuit (IC). In particular, the present invention relates to a lithographic apparatus wherein iso-dense bias in a printed pattern on a substrate is capable of being controlled by using radiation with a spectral distribution comprising two or more spectral peaks.

FIELD

The present invention relates to a lithographic apparatus and a method of using the apparatus in the manufacture of a device such as an integrated circuit (IC). In particular, the present invention relates to a lithographic apparatus wherein iso-dense bias in printed features occurring at different pitches is capable of being controlled by using a spectral energy distribution comprising two or more discrete spectral peaks.

BACKGROUND

An illumination system is configured to condition a radiation beam and between a reticle and a substrate there is a projection system for imaging an irradiated portion of a reticle onto a target portion of the substrate. The illumination system includes components for directing, shaping or controlling a projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.

Generally, the projection system comprises an optical system 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. The illumination system typically comprises adjustable elements for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of an intensity distribution upstream of a 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 used to improve the processing parameters when an image of an 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. The smallest space and/or smallest width is referred to as the critical dimension (“CD”). In general, a single substrate will contain a network of adjacent target portions that are successively exposed.

With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for isolated features and dense features may limit the process latitude (i.e. the available depth of focus in combination with an 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 CD will print differently depending on their pitch on the mask (i.e. the separation between adjacent features) due to pitch dependent diffraction effects. For example, a feature consisting of a line having a particular line width when in isolation (i.e. having a large pitch), will print differently from the same feature having the same line width when together with other lines of the same line width in a dense arrangement on the mask (i.e. having small pitch). Hence, when both dense and isolated features of CD are to be printed simultaneously, a pitch dependent variation of printed CD is observed. This phenomenon is called “iso-dense bias”, and is a particular problem in photolithographic techniques. Iso-dense bias is measured in nanometers and represents an important measure for practical characterization of lithography processes.

Conventional lithographic apparatus typically do not directly address the problem of iso-dense bias. It is the responsibility of users of the lithographic apparatus to attempt to compensate for the iso-dense bias by either changing optical parameters of the apparatus, such as the numerical aperture of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that differences in dimensions of printed isolated and dense features are minimized.

Generally, in a high volume manufacturing site different lithographic apparatus will be used for the same lithographic manufacturing process step to insure optimal exploitation of the machines, and consequently (in view of, for example, machine-to-machine differences) a variance and/or errors in CD may occur in the manufacturing process. Generally, the actual pitch dependency of such errors depends on the specific layout of the pattern and the features, aberrations occurring in the projection system of the lithographic apparatus in use, properties of the radiation sensitive layer on the substrate, and radiation beam properties such as illumination settings, and exposure dose of radiation energy. Therefore, given a pattern to be provided by a patterning device, and to be printed using a specific lithographic apparatus including a specific radiation source, one can identify data relating to iso-dense bias 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 the problem of mutually matching the corresponding different iso-dense bias characteristics, such as to reduce CD variations occurring in the manufacturing process.

A known technique to match an iso-dense bias characteristic of a machine (for a process whereby an annular illumination mode is used) to an iso-dense bias characteristic of another machine is to change the σ-outer and σ-inner settings, while maintaining the difference between the σ-outer and σ-inner settings (i.e. whilst maintaining the annular ring width of the illumination mode) of one of the two machines. The nominal σ-settings are chosen so as to optimize the process latitude (in particular, the depth of focus and the exposure latitude). In this approach, when the σ-settings are changed, the process latitude becomes smaller and may even become too small for practical use.

US Patent Application No. 2002/0048288A1 relates to an integrated circuit lithographic technique for bandwidth control of an electric discharge laser. The laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in resist. US Patent Application No. 2002/0048288A1 is incorporated herein by reference.

U.S. Pat. No. 5,303,002 relates to a method and apparatus for patterning a resist layer wherein a plurality of bands of radiation are used to provide an enhanced depth of focus. U.S. Pat. No. 5,303,002 is incorporated herein by reference.

U.S. Pat. No. 5,978,394 relates to a feedback arrangement for controlling a wavelength of a radiation beam. U.S. Pat. No. 5,978,394 is incorporated herein by reference.

One aspect of embodiments of the present invention provides a lithographic apparatus which allows iso-dense bias to be controlled.

A further aspect of embodiments of the present invention provides a method of controlling iso-dense bias in a lithographic apparatus.

SUMMARY

According to an aspect of the present invention, there is provided a lithographic apparatus including an illumination system configured to condition a radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the lithographic apparatus is capable of controlling and substantially maintaining iso-dense bias at a pre-set nominal value when forming a pattern on the substrate by being capable of generating the radiation beam to have a spectral energy distribution comprising two or more discrete spectral peaks.

By iso-dense bias herein is meant a pitch dependent variation of printed critical dimension (CD) when both dense and isolated features are printed substantially simultaneously

Using the present invention, it is possible to control iso-dense bias in a lithographic apparatus and thereby have the apparatus operating at a substantially constant value of iso-dense bias throughout the whole period of a processing step. This allows a lithographic apparatus to be initially set up and optimised to operate at a set iso-dense bias (i.e. a nominal setting). This may provide improved pattern resolution.

Instabilities in the lithographic apparatus causing iso-dense bias variations may be a variety of parameters selected from any of the following: drifting of laser bandwidth during use; changes in properties of an illuminator (such as adjustment, set-up and alignment of a projection and optical lens); or changes in photo resist properties (such as processing conditions including bake time and temperature gradient). By measuring the associated iso-dense bias at any specific time, the effect of parameters causing iso-dense bias variations in the lithographic apparatus may be controlled and compensated for by altering a separation between two or more discrete spectral peaks in the radiation beam

The procedure of controlling iso-dense bias according to the present invention may be used to control iso-dense bias in a number of different lithographic apparatus. For example, different lithographic apparatus in a high volume manufacturing site may therefore be controlled to operate in the same lithographic manufacturing process step with substantially equivalent values for iso-dense bias. This allows maximum exploitation of different lithographic apparatus which may be present at a specific manufacturing site.

For example, the iso-dense bias in a lithographic apparatus may be controlled and substantially maintained at a substantially constant value of anywhere between about −0.1 to −10 nm, between about −1 to −8 nm and preferably about −5 nm. The pre-set value for the iso-dense bias may be referred to as the nominal setting for the lithographic apparatus.

The iso-dense bias may, for example, be measured by taking a printed wafer and measuring the iso-dense bias in an offline metrology tool such as a CD-SEM (i.e. a Critical Dimension-Scanning Electron Microscope) or a scatterometer. A further method may be to monitor the iso-dense bias inline using techniques such as optical scatterometry. The iso-dense bias may be measured at any pre-determined frequency such as daily, weekly, batch per batch, wafer per wafer or die per die. The frequency of measuring the iso-dense bias may be determined by how much and how often the iso-dense bias may be expected to, or be found to change.

An advantage of being able to control iso-dense bias by using a spectral energy distribution comprising two or more discrete spectral peaks is that the control can be performed during actual operation of the lithographic apparatus and is also quick to effect. In contrast, previous methods to control iso-dense bias involve, for example, changing bias on a reticle. This is time-consuming and may require a considerable amount of ‘down-time’ for the lithographic apparatus.

Preferably, the spectral energy distribution may comprise two discrete spectral peaks i.e. the radiation beam may be of dual wavelength. Alternatively, the spectral energy distribution may comprise any number of discrete spectral peaks more than two.

Typically, the two or more discrete spectral peaks in the spectral energy distribution may be separated from one another and said separation may be adapted to control iso-dense bias. Therefore, in the event that iso-dense bias in a lithographic apparatus changes from the initially pre-set value (i.e. a nominal setting), then the separation of the two or more discrete spectral peaks may be changed and adapted to compensate for an internal change in the iso-dense bias of the apparatus such as drifting of laser bandwidth. The iso-dense bias may therefore be substantially maintained and controlled at a set value.

Typically, the two or more discrete spectral peaks in the spectral energy distribution may have a separation of less than about 5 pm, less than about 3 pm or less than about 1 pm. Preferably, the two or more discrete spectral peaks in the spectral energy distribution may have a separation of less than about 0.5 pm, less than about 0.25 pm or less than about 0.10 pm. During operation of the lithographic apparatus, the separation of the two or more discrete spectral peaks may be varied within any of these ranges to maintain and control the iso-dense bias at the pre-set nominal value. The separation may be determined by a peak-to-peak separation of the two or more discrete spectral peaks which is sometimes referred to as a modal separation.

Typically, the two or more discrete spectral peaks in the spectral energy distribution may have a spectral bandwidth of about 0.05-0.30 pm and preferably about 0.15 pm.

Preferably, the two or more discrete spectral peaks in the spectral energy distribution may have the same intensity. For example, the two or more discrete spectral peaks may have an energy of about 10-70 mJ/cm2.

In a preferred embodiment, the spectral energy distribution may comprise two discrete spectral peaks having a peak-to-peak separation of about 0.3 pm, a spectral bandwidth of about 0.15 pm and the same intensity in each peak. Operating with a dose of 20 mJ/cm2, a scanspeed of 400 mm/sec and a repetition rate of 4,000 Hz for a laser radiation source, the two separate discrete spectral peaks may each have an energy of about 0.05 mJ per pulse.

The spectral energy distribution comprising two or more spectral peaks may be formed using any suitable technique. For example, a technique referred to as RELAX as described in US Patent Application No. 2002/0048188A1, incorporated herein by reference, may be used. RELAX is an acronym for Resolution Enhancement by Laser-Spectrum Adjusted Exposure and relates to a fast responding tuning mechanism used to adjust center wavelength of laser pulses to achieve a desired spectral distribution. This technique basically involves utilising a piezoelectric drive operating at a suitably high frequency such as about 100-6,000 Hz to oscillate (i.e. vibrate) a tuning mirror. This provides extremely fast movement of the tuning mirror which may be used to form the spectral energy distribution comprising two or more discrete spectral peaks. The piezoelectric drive may operate at substantially the same frequency as a radiation source such an excimer laser. For example, as a KrF excimer laser may operate at about 120 Hz and an ArF excimer laser may operate at between 4,000 Hz to 6,000 Hz, the piezoelectric drive may operate at any of these frequencies. In particular embodiments, the laser may have an operational frequency of about 4,000 Hz or 6,000 Hz

The piezoelectric drive may operate substantially in-phase with pulses emitted from a radiation source such as an excimer laser. In alternative embodiments, the piezoelectric drive may operate at substantially a half or substantially a quarter of a frequency of a radiation source

The piezoelectric drive may be computer controlled and may respond to updated signals at a frequency of up to about 6,000 Hz. A suitable piezoelectric drive may, for example, be piezo stack Model P-840.10 supplied by Physik Instrumente GmbH in Waldbronn, Germany. In this regard we also refer to U.S. Pat. No. 5,978,394, incorporated herein by reference, which describes extremely fast movement of a tuning mirror.

Oscillating the tuning mirror with the piezoelectric drive, may have the effect of invoking small changes in the wavelength of pulses from a radiation source such as an excimer laser. It may be found that, for example, moving the tuning mirror by about 75 nm has the effect of changing the wavelength between two discrete spectral peaks by about 0.1 pm.

The oscillation of the tuning mirror creates the spectral energy distribution with two or more discrete spectral peaks which may be used to control iso-dense bias. Changing the amplitude of oscillation of the tuning mirror, determines the separation of the two or more discrete spectral peaks and hence the iso-dense bias contribution from the two or more discrete spectral peaks. This may be used to substantially maintain and control the iso-dense bias at an initially pre-set nominal value.

In use, centre wavelengths of the two or more spectral peaks may be measured using a wavemeter unit which allows very fast calculation of the wavelengths. The RELAX technique as discussed in US Patent Application No. 2002/0048288A, incorporated herein by reference, uses a photo diode array and computationally efficient algorithms to allow the wavelength of each pulse of energy from a radiation source to be measured. This calculation procedure may be performed at up to 6,000 times per second.

The iso-dense bias in the lithographic apparatus may be continually monitored and measured so that the iso-dense bias may be known at any specific time. As discussed above, the iso-dense bias may be measured using offline metrology tools such as a CD-SEM or a scatterometer. The measured iso-dense bias may be compared with a pre-set nominal value for a lithographic apparatus for which the apparatus is optimised to operate at. In the event that the measured iso-dense bias changes from the pre-set nominal value, a separation between the two or more spectral peaks of the spectral energy distribution may be adapted (i.e. changed) so as to alter the iso-dense bias until it returns to the pre-set nominal value. On measuring the iso-dense bias, the measured value and the change in value required may be compared with, for example, a computer modelled graph showing how a difference in the separation of the two or more discrete spectral peaks of the spectral energy distribution may be adapted to control and maintain the iso-dense bias at the pre-set nominal value. The computer modelled graph may be created using standard modelling packages such as PROLITH (Trade Mark). PROLITH is available from KLA Tencor in Austin, Tex., United States. It may be found that by plotting separation of two discrete spectral peaks against iso-dense bias produces a substantially parabolic curve.

In embodiments, where iso-dense bias variations occur between different lithographic apparatus, the variations may be controlled by using a spectral energy distribution comprising two or more spectral peaks in the different lithographic apparatus. The iso-dense variations between the different lithographic apparatus may be caused by a variety of parameters such as drifting of the laser during use, changes in properties of an illuminator or changes in photo resist properties. The different lithographic apparatus may therefore be controlled to operate with substantially the same iso-dense bias. This allows maximum usage of different lithographic apparatus at a manufacturing site.

Typically, the radiation beam forming the spectral energy distribution may have a wavelength corresponding to that of Deep Ultra-Violet (DUV). Alternatively, the radiation beam forming the spectral energy distribution used may have a wavelength of between about 50-500 nm or about 100-400 nm. Preferred wavelengths for the radiation beam forming the spectral energy distribution may be about 157 nm, 193 nm, 248 nm or 365 nm.

In alternative embodiments, the radiation beam forming the spectral energy distribution may be Extreme Ultra-Violet (EUV) radiation having a wavelength in the range of about 5-20 nm. Preferably, EUV radiation may have a wavelength of about 13.5 nm.

According to a further aspect of the present invention, there is provided a lithographic apparatus wherein the lithographic apparatus is capable of controlling and substantially maintaining iso-dense bias in a printed pattern on a substrate at a pre-set nominal value by being capable of generating a radiation beam having a spectral energy distribution comprising two or more discrete spectral peaks.

According to a yet further aspect of the present invention, there is provided a device manufacturing method including providing a substrate, providing a conditioned radiation beam using an illumination system, imparting a pattern to the radiation beam, and projecting the patterned beam of radiation onto a target portion of the substrate, wherein iso-dense bias in a printed pattern on the substrate is capable of being controlled and substantially maintained at a pre-set nominal value by being capable of generating the radiation beam to have a spectral energy distribution comprising two or more discrete spectral peaks.

According to a further aspect of the present invention, there is provided a device manufacturing method including projecting a patterned beam of radiation onto a substrate, wherein iso-dense bias in a printed pattern on the substrate is capable of being controlled and substantially maintained at a pre-set nominal value by being capable of generating a radiation beam to have a spectral energy distribution comprising two or more discrete spectral peaks.

According to a further aspect of the present invention, there is provided a device manufactured according to the method of projecting a patterned beam of radiation onto a substrate, wherein iso-dense bias in a printed pattern on the substrate is capable of being controlled and substantially maintained at a pre-set nominal value by being capable of generating a radiation beam to have a spectral energy distribution comprising two or more discrete spectral peaks.

The manufactured device may, for example, be integrated circuits (ICs), integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal displays (LCDs) and thin-film magnetic heads.

According to a further aspect of the present invention, there is provided a lithographic apparatus for controlling the effect of iso-dense bias when forming a pattern on a substrate, said apparatus including a radiation source capable of forming a radiation beam, spectral energy distribution processing system capable of processing the radiation beam to create a spectral energy distribution comprising at least two discrete spectral peaks at different wavelengths and a patterning projection device capable of applying the at least two discrete spectral peaks of the spectral energy distribution substantially simultaneously to create a pattern on a substrate.

According to a yet further aspect of the present invention, there is provided a method of controlling iso-dense bias in a lithographic apparatus, the method including generating a beam of radiation having a spectral energy distribution, processing the spectral energy distribution to create at least two discrete spectral peaks, and projecting the spectral energy distribution with the at least two discrete spectral peaks to form a pattern on a substrate.

According to a further aspect of the present invention there is provided a process for providing lithographic exposures including modelling with a computer program iso-dense bias lithographic parameters to determine a desired laser spectrum needed to produce a desired lithographic result, and utilizing a fast responding tuning mechanism to adjust center wavelength of laser pulses in a burst of pulses to achieve an integrated spectrum for the burst of pulses approximating the desired laser spectrum.

DETAILED DESCRIPTION

FIG. 2relates to a spectral energy distribution comprising two spectral peaks wherein a separation between the spectral peaks is identified by the delta sign. Using a dose of 20 mJ/cm2, a scanspeed of 400 mm/sec and a repetition rate of 4,000 Hz for a laser radiation source, each peak has an energy of 0.05 mJ per pulse.

The spectral energy distribution shown inFIG. 2with the two discrete spectral peaks is formed using the RELAX technique as described in US Patent Application No. 2002/0048288A1, incorporated herein by reference. Basically, the technique involves mounting a tuning mirror on a mirror mount. Large changes in the position of the tuning mirror are controlled by a stepper motor through a lever arm. A piezoelectric drive which is computer controlled is used to oscillate the tuning mirror and to control small changes in the movement of the tuning mirror which results in the small changes in the wavelength of the spectral peaks necessary to control the iso-dense bias. The piezoelectric drive operates substantially in-phase with the frequency of a laser acting as a radiation source.

In a particular embodiment using the RELAX technique, a diamond pad at an end of the piezoelectric drive is used to contact a spherical tooling ball at a fulcrum of the lever arm. The mirror mount is attached to the lever arm through a dowel pin on the lever arm and four spherical ball bearings are mounted on the mirror mount. The piezoelectric drive is mounted on a frame of a line narrowing package (LNP). The tuning mirror is mounted in the mirror mount using a three-point mount with aluminium spheres. Springs are used to hold the tuning mirror against the aluminium spheres. In a further alternative embodiment, bellows are used to isolate the piezoelectric drive from the environment inside the LNP. This has the advantage of preventing UV damage to the piezoelectric drive and avoiding possible contamination caused by out-gassing from piezoelectric material.

A preferred embodiment of the present invention is to use an ArF excimer laser system capable of operating in the range of 4,000 Hz to 6,000 Hz. The piezoelectric drive used is piezo stack Model P-840.10 supplied by Physik Instrumente GmbH in Waldbronn, Germany. The piezoelectric drive operates substantially in-phase and at substantially the same frequency as the excimer laser (i.e. 4,000 Hz to 6,000 Hz). Parameters of the laser beam such as pulse energy and center wavelength are measured and controlled at a fast rate by a computer controlled system. The parameters may be measured and controlled as set out in US Patent Application No. 2002/00482288A1, incorporated herein by reference. However, in summary the process involves using a wavemeter processor and photo diode array which is capable of measuring the wavelength of individual pulses occurring in the range of 4,000 Hz to 6,000 Hz. A computer controller using a special algorithm may be used to measure and control pulse energy and wavelength from the excimer laser. As the calculations must be done in real time, the algorithms are specifically chosen to facilitate the fast calculations required.

By changing the amplitude over which the tuning mirror vibrates, affects the separation of the spectral peaks. It is found that by moving the tuning mirror in increments of 75 nm alters the separation of the spectral peaks by 0.1 pm. The movement of the tuning mirror is dependent on the piezoelectric drive which is itself computer controlled

FIG. 3relates to the effect on iso-dense bias of varying the separation between two discrete spectral peaks in a spectral energy distribution as shown inFIG. 2. It is clear fromFIG. 3that as the separation between the spectral peaks increases, the iso-dense bias increases (i.e. the negative value increases). The values shown inFIG. 3are calculated for 100 nm dense lines and 150 nm isolated lines with an attenuated phase-shift mask wherein the numerical aperture (NA) is 0.75. The σ-outer setting is 0.85 and the σ-inner setting is 0.55. The graph inFIG. 3is generated using industry standard modeling packages such as PROLITH (Trade Mark)

InFIG. 3, the plot of separation between the two spectral peaks versus iso-dense bias is substantially parabolic in shape. This means that a nominal working point (i.e. wavelength split) for the two spectral peaks should be chosen in such a way that the separation is not too close to zero, otherwise not enough control may be obtainable.

FIG. 4represents a simple flowchart representing steps involved in controlling the iso-dense bias in a lithographic apparatus.

The first step of the flowchart shown inFIG. 4, is to determine how frequently the iso-dense bias should be measured. For example, the iso-dense bias can be measured daily, weekly, batch per batch, wafer per wafer or die per die.

The second step in the flowchart ofFIG. 4is to measure the iso-dense bias. The iso-dense bias may be measured using an offline metrology tool such as CD-SEM, a scatterometer or any other suitable metrology tool. The iso-dense bias may also be measured inline using an optical scatterometer. It should be noted that measuring the iso-dense bias wafer per wafer or die per die is only possible in combination with certain types of metrology tools such as inline metrology tools incorporated into the lithographic apparatus.

The third step is to compare the actual measured iso-dense bias with a reference value which represents a nominal working iso-dense bias for which the lithographic apparatus has been optimized during set up. For example, the reference value for the iso-dense bias may be −5 nm. On comparison of the actual measured iso-dense bias with the reference value (i.e. the nominal working iso-dense bias), if these values are equivalent, then no action needs to be taken and the separation of the spectral peaks therefore does not need to be changed. The iso-dense bias is then measured again which is determined by the frequency of measurements and the process is repeated.

However in the third step, if the actual measured iso-dense bias is not equivalent to the reference value for the nominal working iso-dense bias, then the separation of the spectral peaks needs to be changed. The amount of change in the separation of the spectral peaks is determined by a computer simulation representing separation of spectral peaks versus iso-dense bias as shown inFIG. 3. The separation of the spectral peaks is therefore changed using the parabolic curve of spectral peak separation versus iso-dense bias to control and maintain the iso-dense bias of lithographic apparatus at the nominal working iso-dense bias for the lithographic apparatus i.e. the reference value. The above process is then repeated with further measurements taken for the iso-dense bias and further changes to the spectral peak separation effected if necessary.

If a significant change is required in the separation of the spectral peaks, then the frequency of measuring the iso-dense bias should be made more frequent.

FIG. 5relates to a change in iso-dense bias as a separation between two discrete spectral peaks of a spectral energy distribution is varied. The graph was created using the PROLITH (Trade Mark) modeling package.

In calculating the graph shown inFIG. 5, the nominal setting for the separation of the two discrete spectral peaks is set at 0.30 pm. In addition, the nominal setting for the iso-dense bias of the lithographic apparatus is −5 nm. The object is therefore to change, if necessary, separation of the discrete spectral peaks to maintain and control the iso-dense bias of the lithographic apparatus at −5 nm.

During the use of the lithographic apparatus, the iso-dense bias is measured by any kind of metrology tool such as CD-SEM or scatterometry. In this Example, the iso-dense bias is measured to be −5 nm which corresponds with the nominal setting (see the arrow).

When the iso-dense bias changes from −5 nm, the difference between the spectral peaks of the spectral energy distribution is changed to maintain the iso-dense bias at −5 nm. For example, if the iso-dense bias increases to more than −5 nm (i.e. more negative) in the lithographic apparatus, then the separation of the spectral peaks should be decreased so that the contribution towards the iso-dense bias in the lithographic apparatus from the separation of the spectral peaks is decreased. This allows the iso-dense bias of the lithographic apparatus to be maintained and controlled at −5 nm. On the other hand, if the iso-dense bias decreases from −5 nm (i.e. is less negative), then the separation of the spectral peaks should be increased to compensate for the reduced iso-dense bias. This once again allows the iso-dense bias to be maintained at the nominal value of −5 nm.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications not limited to optical lithography.