Exposure process and apparatus

Abstract

Disclosed is a method for exposing a substrate in an exposure process. The method comprises determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; and deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field. A respective dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location and / or a predicted imaging performance of the field(s) within that subset.

Description

EXPOSURE PROCESS AND APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24183461.3 which was filed on 20 June 2024 and EP application 24212658.9 which was filed on 13 November 2024 which are incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Such a lithographic apparatus works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. Moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan does this. The wafer stage is then asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.

[0006] Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, onto an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer. The reticle may be transmissive or reflective depending on the wavelengthof light used; EUV systems for example use a reflective reticle within a vacuum chamber to prevent absorption of the EUV radiation.

[0007] Exposures are repeated over multiple fields on a substrate. Some peripheral fields overlap the substrate edge, such that they do not comprise full dies on the substrate and therefore can never be functional. However, simply not exposing these peripheral fields or non-product fields can adversely impact the functionality of adjacent product fields.

[0008] It is desirable to improve lithographic methods for exposure of such (e.g., peripheral) nonproduct fields.SUMMARY

[0009] In the first aspect of the invention there is provided a method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; and deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field; wherein a respective dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location, a dose error duration over which the dose error occurs and / or a predicted imaging performance of the field(s) within that subset.

[0010] In a second aspect of the invention there is provided a method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures, the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; deciding whether to perform a second exposure on each product field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a first dose error threshold value; and deciding whether to perform a second exposure on each non-product field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a second dose error threshold value, the second dose error threshold value being greater than the first dose error threshold value.

[0011] In a third aspect of the invention there is provided a method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate, the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; wherein: said first exposures for said product fields are performed at a nominal power level; said first exposures for said non-product fields are performed at a second power level being greater than the nominal power level; said power levels describing a power level of an illumination source used in said first exposures; and a target dose to be received in said first exposures is the same for said product fields and said non-product fields.

[0012] In a fourth aspect of the invention there is provided a method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; determining, for each of said plurality of fields except a final field exposed in said plurality of first exposures, whether each of these fields received a sufficient dose with respect to a target dose in the first exposure; and deciding whether to perform a second exposure on each field except the final field only if it is evaluated to have received an insufficient dose; wherein said determining and deciding steps are not performed on said final field regardless of the dose it received, said final field comprising a non-product field.

[0013] In a fifth aspect of the invention there is provided a method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field; and determining an exposure routing for the second exposure, where required, based on said dose error threshold of at least one field requiring the second exposure.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2(a) illustrates a method for determining a dose evaluation profile for use in methods disclosed herein;Figure 2(b) illustrates exposure routing strategies for a first exposure pass and a second exposure pass;Figure 3(a) illustrates an example field arrangement on a substrate, where the fields comprise product fields (yielding fields) and non-product fields (non-yielding fields);Figure 3(b) illustrates an example field arrangement on a substrate, where the fields are divided into a number of subsets, each subset being allocated a respective different dose threshold value;Figure 4(a) is an example graphical user interface for implementing different dose thresholds per field according to examples described herein;Figure 4(b) is a block diagram illustrating an arrangement for setting different dose thresholds per field or subsets thereof;Figure 5(a) illustrates a dose error profile comprising a short-duration dose error and a resulting exposed feature;Figure 5(b) illustrates a dose error profile comprising a long-duration dose error and a resulting exposed feature;Figure 6(a) is a plot of re-expose rate RR (% of substrates re-exposed on a logarithmic scale) against relative source power for exposure of both product fields and non-product fields; andFigure 6(b) is a plot of relative exposure time RET against relative source power for exposure of both product fields and non-product fields.DETAILED DESCRIPTION

[0015] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask or reticle), a projection system PS and a substrate table WT configured to support a substrate W.

[0016] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0017] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B ’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0018] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0019] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0020] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0021] In an exposure process or lithographic process, a substrate is exposed repeatedly in a plurality of individual fields, e.g., with repetitions of a die exposed in respective different regions or target areas on the substrate. A die may correspond to a target area or field (i.e., one die per field), where a target area or field (the two terms may be used interchangeably) is defined by the area exposed in each individual exposure. However, this is not always the case, and there may be two or more dies per field, or else there may be dies comprised of two or more fields stitched together (i.e., formed in multiple adjacent exposures). It is also known to form each field in multiple overlapping exposures (e.g., doublepatterning techniques). All such techniques and die / field arrangements may benefit from the concepts disclosed herein.

[0022] It is desirable in an exposure process or lithographic process to maximize the number of yielding (i.e., functional or non-defective) dies. It is also desirable to maximize the speed in exposing each die and / or substrate and therefore throughput (substrates exposed per hour). These two aspects typically require balancing, as improving one often comes at the expense to the other.

[0023] Some regions of the substrate never yield, because they are not sufficiently large for a full die. Such regions are at the periphery of the substrate, where the exposure field extends beyond the substrate edge and therefore only a portion of the field is exposed on the substrate. These peripheral fields, which do not comprise any full dies, are referred to herein as non-product fields. As such, non-product fields are to be understood in the context of this disclosure to comprise fields which are always non-yielding (have no potential to yield) because they do not comprise a complete die; e.g., due to their location on the substrate. Potentially yielding fields, but which happen not to yield for any other reason are not “non-product fields” for the purpose of this disclosure.

[0024] Figure 3(a) illustrates an example die arrangement on a substrate, the extent of which is represented by a substrate edge 200. The non-shaded fields 210 may be expected to yield, e.g., in normal circumstances. However the shaded fields 220 are non-product field which will never yield because they do not comprise a full die within the substrate area within the substrate edge 200. Depending on the actual die size, some of the other non-shaded peripheral fields (i.e., those which also overlap the substrate edge 200) may also be non-product fields. Note that this is only an exemplary die layout and the number and / or size of the dies and their arrangement on the substrate may differ from that shown.

[0025] A radiation source (e.g., the source SO) in a lithographic apparatus (e.g., the lithographic apparatus LA) may have intrinsic instability (i.e., temporal fluctuations in power output). For example, there may be some pulse-to-pulse power variation. In principle, said instability may lead to under exposure or over-exposure of photoresist on a substrate. Under-exposure may be corrected for by providing a second exposure (a re-exposure or “top-up” exposure). Over-exposure cannot be correctedfor. In practice, the output power of the radiation source may be controlled using a control loop that substantially prevents over-exposure but which may still suffer from under-exposure.

[0026] On some lithographic apparatuses (for example, but not exclusively, EUV lithographic apparatuses), a field re-exposing strategy is used to remedy any dose errors which are detected during exposure of a substrate. This may comprise evaluating the dose received by each field during each exposure (a received dose value per field) in an initial exposure sequence or exposure pass. If any fields are evaluated to have received a dose below a threshold dose (e.g., purely for example, a dose error greater than 1 % below a requested dose or target dose), then these field(s) are re-exposed in a second pass. The cumulative (i.e., combined) dose of the two passes for the re-exposed fields should be within a threshold margin of the requested dose, e.g., within + / - 1% of the requested dose.

[0027] Because the radiation source is controlled at the energy level, i.e., to maintain a constant energy output (e.g., an energy setpoint / dose target) at mini-burst level (where a mini-burst may comprise on the order of 10 pulses), there is no information on dose fluctuation at wafer level. This is because the exposure is performed at a finite scan speed with an illumination slit having a finite length. Therefore, the effective impact of source energy fluctuation should be evaluated only after convoluting the energy profile with the illumination slit profile.

[0028] Referring to Figure 2(a), a dose evaluation may comprise obtaining energy data emeas as measured (e.g., at the source). Plot 100 of energy e against time t shows an exemplary measured energy trace emeaswith respect to the set target energy etarg. This measured energy trace emeasis binned to obtain binned measured energy data e’measas shown by plot 110 (binned measured energy data e’measagainst time bin value t_bin). This binned measured energy data e’measmay be convolved, for every exposure, with the illumination slit profile ISP (plot 120) to obtain the dose evaluation profile DEVP (plot 130 of dose against time bin t_bin).

[0029] The resultant dose evaluation profile DEVP can then be compared to a reference dose profile to provide a dose error profile along the scan direction. If this dose error profile exceeds a threshold dose, a re-exposure will be triggered.

[0030] A control algorithm can be used to assess if the dose error is within the allowed dose error threshold everywhere (ok, no re-expose necessary) or if there is an under-dose somewhere which will trigger the need for a re-exposure. More specifically, the dose evaluation may comprise the following steps:1) The energy sensor of the lithographic apparatus measures energy of all EUV pulses during an exposure to obtain EUV energy data.2) The lithographic apparatus calculates the dose error by convolution of the EUV energy data with the exposure slit and comparison to a reference dose profile.3) For fields for which the dose error is above a threshold, the lithographic apparatus converts the energies measured during the original exposure to a high resolution ‘requested relative energy profile’ for the re-exposure and sends it to the source.4) The energy controller on source follows the requested energy profile during re-expose as requested by lithographic apparatus.5) The lithographic apparatus calculates the dose delivered in the re-exposure.6) The lithographic apparatus calculates the dose error of the sum of the dose of the original exposure and the dose in the re-exposure.Steps 3) to 6) to may be performed by a suitable re -exposure algorithm or re-expose controller, for example.

[0031] During the re-exposure, typically the same wafer routing is followed as was used during the first pass exposure. As such, even when the (only) repair field is at the far end of the wafer, the wafer is still cycled in the same meander manner to reach the field to be repaired (the exposure route may be truncated after the final repair field however). The reason for this is to ensure that the wafer experiences the same heating history such that the impact of heat on e.g., overlay is minimized.

[0032] Figure 2(b) illustrates this present method of second exposure routing. A substrate comprises (in this simplified example) nine fields, of which after a first exposure along first exposure path EPl, seven fields 150 (only one labeled) do not require re-exposure and two fields 160 do require reexposure. The second exposure path EP2 follows the same path to re-expose fields 160, at least till the final field to be re-exposed.

[0033] It is known to control an effective source power (ESP) of a radiation source in a lithographic apparatus. A suitable control loop may be used to maintain a nominal (average) operating power of the radiation source at a level which is below the maximum available output power by a chosen amount. This chosen difference between a nominal or average maximum available output power of the radiation source (also referred to as open-loop power) and an average or nominal operating power of the radiation source may be referred to as a dose margin. An ESP is lower than a maximum achievable source power, and, as a result, instances of over-exposure can be avoided. A reduction in ESP (i.e. an increase in dose margin) generally corresponds to a reduction in source instability. In addition, the dose of radiation is spread over a greater number of pulses (because at lower source power the scanner will scan at a lower speed resulting in more pulses in the slit), reducing the effect of an individual pulse which has a significant fluctuation away from the nominal operating power. A relatively high ESP may result in a relatively short required exposure time, but may also increase the need for top-up exposures due to higher instability of the radiation source. This may have a detrimental effect on lithographic throughout. However, a relatively low ESP may result in a relatively long exposure time (to the detriment of throughput). In practice, a dose margin may be selected to maintain a relatively quick exposure without requiring too many top-up exposures or re-exposures.

[0034] As such, to maximize productivity, the source may be operated at a low dose margin such that an occasional dose error >1% is accepted and repaired via re-exposure as has been described. To optimize or maximize throughput, a method may be employed which adjusts the source power such that the number or percentage of fields needing re-exposure is balanced against the time per exposure so asto maximize productivity. Briefly, the greater the source power, the smaller the exposure time. However, control of the overall dose received per field is more difficult with high source powers. As such, the optimized source power may balance the number of underexposed fields (receiving a dose below a threshold and requiring re-exposure) against exposure time per field so as to minimize overall time to expose a substrate / maximize substrates per hour. For example, it has been demonstrated that a maximum throughput may be achieved when approximately 0.5% (e.g., between 0.2% and 0.8%, 0.3% and 0.7%, between 0.4% and 0.6% or between O..45% and 0.55%) of exposures have a dose error requiring re-exposure.

[0035] The aforementioned re-exposure algorithm or re-expose controller may monitor the actual number of re-exposures required and compare it to a number of re -exposures which result in maximum productivity or throughput. This can be implemented as a control loop such that the lithographic apparatus can continuously adjust the source power to the value that results in maximum throughput based on the comparison.

[0036] One issue with the current dose evaluation is that it does not distinguish between one or more of different die types, different die / field positions on wafer and / or different quality requirements (e.g., process window requirements or specification requirements for one or more imaging metrics such as overlay, edge placement error, critical dimension etc.). The same dose threshold (often 1%) is set regardless of any of these factors.

[0037] The inventors appreciate that not all fields and / or dies require the same dose threshold. For example, the focus and overlay performance will be better for dies located close to the center of a wafer than for dies located at or near to the edge. Therefore, to achieve the same edge placement error (EPE) requirement, the dose requirement for these center dies can be less strict than for the edge dies, and hence can tolerate higher dose threshold (e.g. greater than 1 %) to trigger re-expose. By contrast, in some cases, e.g., for more complex logic circuitry having tight process requirements (a small process window) it may be preferable to use a lower (e.g., less than 1%) dose threshold to achieve the best imaging performance, particularly nearer the substrate edge.

[0038] Another specific example are the aforementioned non-product fields. With speed being an important factor, it would seem intuitive to simply not expose the non-product fields; i.e., the fields at the edge of the wafer that do not comprise full dies. However, it is known that not exposing these nonyielding die regions has a negative impact on the quality of the processed adjacent product fields. Because of this, the non-product fields are exposed to ensure a uniform wafer coverage. In particular, not exposing the non-product fields can cause undesired effects on adjacent product fields during certain processing steps such as, for example, development and / or etch (e.g. may result in undesirable loading effects during etching). As such, presently the non-product fields are exposed in the same manner as the yielding dies, resulting in a lower productivity compared to not exposing the non-product fields. However, the inventors have appreciated that the non-product fields do not require the same imaging performance (linewidth / CD control) as the product fields. Therefore, the exposure for non-productfields may be performed subject to a larger dose error threshold than for other fields which may be expected (or at least have potential) to yield.

[0039] Therefore, according to a first concept disclosed herein, a method for exposing a substrate in an exposure process is provided which comprises: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; and deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error (i.e., with respect to a target dose) which is above a respective dose error threshold value for that field; wherein a respective dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location and / or a predicted imaging performance of the field(s) within that subset.

[0040] For the avoidance of doubt, the term subset should be construed with its normal meaning and therefore may comprise a single field or the full set of fields (i.e., all fields on the wafer). As such, one or more of the subsets may comprise a single field, and at one extreme it is possible that each field is assigned a different error threshold (each subset comprises a respective different single field). At the other extreme, all of the fields on the substrate may be assigned the same error threshold (as is the case now), however in this case the single error threshold is based upon at least one of a property, location and / or predicted imaging performance of the fields. Of course, all intermediate examples are possible, e.g., where a plurality of different (proper) subsets, each (or at least some) comprising multiple fields, are assigned respective different dose error thresholds such that each field within reach respective subset has the same error threshold value.

[0041] Such a method may comprise an initial error threshold setting step which comprises setting the error threshold value for each subset (where each subset may comprise a single field, multiple fields or all fields of a substrate) based on one or more of: a property, location and / or predicted imaging performance of the field(s) within the subset.

[0042] In one example, the error threshold may be set based on location of the field(s) within a subset. For example, fields may be grouped into subsets, with associated error thresholds assigned, based on an approximate distance of the fields from a center (or edge) of the substrate.

[0043] Alternatively or in addition, a property of the field(s) on which the error threshold may be based, may comprise whether the fields are product or non-product fields. For example, a non-product field subset may comprise (one or more) fields identified as non-product fields. A (significantly) higher value for the dose error may be set for this non-product field subset.

[0044] Alternatively or in addition, computational lithography or a similar process may be performed to estimate imaging performance (e.g., one or more of overlay, focus, edge placement error) of the exposure of one or more of the fields on the substrate. The inputs of the computational lithography are from scanner and wafer metrology tools. The performance deviation from the optimum is computed. The output is a performance prediction matrix that can be used to determine the process window. For example, if the imaging performance is estimated to be very good for at least some fields, a less stringent(higher) exposure threshold may be sufficient. Therefore, subset grouping and / or error threshold setting decisions may be made based this estimated imaging performance.

[0045] Alternatively or in addition, a property of the field(s) on which the error threshold may be based, may comprise the type of die(s) within the field and / or a required imaging performance, e.g., whether the dies have an associated small process window / stringent imaging specification, such as logic circuitry, or a larger process window / less stringent imaging specification, such as for some types of memory circuitry and / or some specific layers.

[0046] Figure 3(b) illustrates an example of a substrate where the fields are divided into four subsets SSI, SS2, SS3, SS4. Each of these subsets has assigned thereto a respective different dose error threshold value. Subsets SSI, SS2, SS3 comprise fields which have been grouped based on the field location (approximate distance to center). Subset SS4 comprise the non-product fields (i.e., non-product fields 220 of Figure 3(a)).

[0047] In an embodiment, the center fields subset SS 1 may be allocated the smallest error threshold value (e.g., 1% as present, although this may be any value). The outer fields subset SS2 may comprise an error threshold between that of the center fields subset SSI and edge fields subset SS3. For example The error threshold value of subset SS2 may be between 1 and 100% greater or between 1 and 50% greater than that of subset SSI. Similarly, the error threshold value of subset SS3 may be between 1 and 100% greater or between 1 and 50% greater than that of subset SS2.

[0048] Subset SS4 (non-product fields) may be assigned an error threshold value significantly larger than those of subsets SSI, SS2, SS3. By way of example, for all concepts disclosed herein, the allowed dose error for the non-product fields (when expressed in terms of a percentage from a nominal or requested dose value) may be between 2% and 30%, between 2% and 20%, between 2% and 10%, between 2% and 8%, between 3% and 8% or between 3% and 7% for example.

[0049] In an alternative formulation, the allowed dose error for the non-product fields (when expressed in terms of a percentage from a nominal or requested dose value) may be between 2x and 30x an allowed dose error for a product field (again expressed in terms of a percentage from a nominal or requested dose value), between 2x and 20x an allowed dose error for a product field, between 2x and lOx an allowed dose error for a product field, between 2x and 8x an allowed dose error for a product field, between 3x and 8x an allowed dose error for a product field or between 3x and 7x an allowed dose error for a product field for example.

[0050] By way of a specific example, the allowed error for a product field may be between 0.5% and 1.5%, between 0.9% and 1.1% or (e.g., approximately) 1% and the allowed error for a non-product field may be 5x the allowed error for a product field. It is estimated that for a 5% dose error, which results in roughly Inm CD error, the risk of line collapse is likely to be small enough to be acceptable.

[0051] It can be appreciated that, while the dose error may be allowed to be higher for the non-product field, it should still be controlled for the non-product fields, as if this error is too large a resist line collapse and a subsequent cross-contamination effect to neighboring product fields may result. Hence,the non-product fields remain subject to a dose error threshold, rather than being completely uncontrolled. As such, it is proposed that non-product fields still undergo a dose evaluation. Should it be determined that any non-product field is subject to a received dose error higher than the second dose error threshold, it is further proposed that this non-product field will still be re-exposed (e.g., with a top-up dose).

[0052] It can be appreciated that the basic concept of providing a different dose threshold for nonproduct fields may be implemented in isolation to the remainder of the first concept. Therefore, according to a second concept disclosed herein, the exposure for non-product fields may be performed subject to a second (i.e., larger) dose error threshold than the first dose error threshold for the exposures of the (i.e., potentially) product fields.

[0053] To implement the first concept of providing configurable error thresholds per subset of fields, a new interface for the lithographic apparatus is proposed which enables the dose threshold of the first pass exposure to be set for each field (i.e., each subset of fields), e.g., via sub-recipe.

[0054] As such, the interface may be implemented on the lithographic apparatus which enables a user to upload and / or configure a sub-recipe file; e.g., which defines at least the threshold value per field-ID (this may be a simple text file for example). The threshold values may be displayed / represented to the user using a suitable graphical user interface (GUI), e.g., such that they may be visualized on a wafer map or other suitable representation.

[0055] Figure 4(a) is a representation of such a GUI according to an example. Displayed is a wafer map, with each field identified by a Field-ID (numbered sequentially in this example), along with a respective dose error threshold (represented as a percentage). Of course, the GUI may be implemented differently to that represented here.

[0056] Figure 4(b) is a block diagram describing a simplified arrangement suitable for implementing at least the first concept disclosed herein. Layout information 400 and / or computational lithography data 410 (e.g., describing required and / or predicting imaging performance data per field / die) is used to determine a dose threshold sub-recipe 420 which sets the dose threshold for each field / subset. The dose threshold sub-recipe is then fed to the lithographic apparatus 440 via a dose threshold interface 430.

[0057] The dose evaluation sub-recipe may enable the implementation of one or more specific modes. For example, a boost mode may be chosen which maximizes the throughput by increasing the dose threshold for at least some fields (e.g., edge fields), thereby reducing the number of re-exposures and the re-expose time. Alternatively, an imaging mode may be selected which lowers the dose threshold for at least some fields, such as those fields which require more stringent dose performance and / or for those fields which estimated imaging performance is predicted below a threshold (e.g., good / sufficient but not excellent). In another example, a balanced mode may apply a dies-in-spec optimization which optimizes (maximizes) the number of dies within specification. Such a mode may balance the dose threshold per subset with other imaging specifications so as to minimize the number of defective dies.

[0058] The proposed method may also enable the setting of a re-exposed field threshold for fields which have undergone a second exposure. This re -exposed field threshold may, for example, be set higher than the original dose threshold for the same field in the first pass. This means that, should the dose performance after repair be (e.g., a small amount) below the first-pass dose threshold, it is still accepted as being in-spec. This will increase the die yield because the imaging performance is an overall metric combining many different aspects (e.g., focus, dose, overlay, lens heating, etc.). As such, a small dose error is not necessarily deterministic for a failed die.

[0059] The first concept has been described in terms of setting the dose error threshold per subset of fields based on a property, a location and / or a predicted imaging performance of the field(s) within that subset. Another factor on which the dose error threshold may be determined (e.g., per subset of fields) is the dose error duration, i.e., the time duration over which the dose error occurs.

[0060] Figure 5 illustrates the reasoning behind this. The current dose evaluation function does not account for the energy profile of an exposure; i.e., whether the dose error occurs over a small number of pulses or many pulses (i.e., its duration). The decision to repair is based only on whether the dose error breaches the (base) dose error threshold, regardless of the length of time over which the breach has occurred. However, the imaging performance (e.g., in terms of local critical dimension uniformity LCDU) is also dependent on this dose error duration.

[0061] Figure 5(a) is a plot of dose error against time (e.g., binned time), showing an example dose error profile 500 with respect to the target dose 510 and the dose error threshold 520. The dose error threshold 520 may be a base dose error threshold, e.g., set as has been described above (e.g., based on a property, a location and / or a predicted imaging performance). It can be seen that the magnitude of the dose error described by the dose error profile 500 does exceed the base dose error threshold 520, but only for a short time (e.g., corresponding to a few, e.g., 10 or fewer, time bins and / or pulses). An example resultant imaged feature 530 corresponding to this dose error profile 500 is also shown. Because the duration of the dose error is very short, the defect 540 (e.g., local critical dimension reduction) is only small and likely to be insignificant; such a defect would not be expected to affect the functioning of this feature. As such, it is likely that re-exposure of this field would not actually be necessary. By contrast, in Figure 5(b), the dose error duration of dose error profile 500’ is significantly longer (e.g., corresponding to 100 or more time bins and / or pulses). The resultant feature 550 has a much larger defect 560 which may affect functioning (e.g., high resistance) and / or might even result in a break in the feature. A field comprising this feature 550 would therefore likely benefit from a further repair exposure.

[0062] Therefore, it is proposed to set the dose error threshold based on the dose error duration, i.e., the duration over which the dose error exceeds a base dose error threshold. The base dose error threshold may comprise any of the dose error thresholds described herein; e.g., the dose error threshold set based on a property, a location and / or a predicted imaging performance of the field(s) within its subset and / or any dose error thresholds set via the interface described above.

[0063] This can be achieved by defining one or more duration thresholds. For example, in its simplest implementation, each field or subset of fields may have a first dose error threshold and a second dose error threshold, such that, if a dose error occurs, its duration is compared to the duration threshold. Should the duration threshold be exceeded (indicative of a long duration), then the (smaller) first dose error threshold is used, otherwise the (larger) second dose error threshold is used. More than one duration threshold may be used, with a corresponding increase in the number of dose error thresholds (e.g., the number of dose error thresholds may equal the number of duration thresholds plus one). In this way, the dose error threshold may be varied between more than two values based on the error duration, such that the dose error threshold has a (stepwise) inversely proportional relationship with the error duration.

[0064] In a further implementation of this concept, a dose error threshold weighting term may be defined, which is calculated based on the error duration. The dose error threshold weighting term may comprise a product of a (e.g., pre-defined) dose evaluation margin and the reciprocal of the error duration (e.g., in terms of the number of time bins over which the error duration occurs). The final dose error threshold for each field may comprise the sum of the base dose error threshold (i.e., the original set dose error threshold for the field) and the dose error threshold weighting term.

[0065] Presented as an equation, the dose error threshold DET may be determined per field (or subset of fields) as:1DET = - — ■ — x DEM -I- DETbase#t binbasewhere #t_bin is the error duration in terms of the number of bins, DEM is the dose evaluation margin and DETbaseis the base dose error threshold.

[0066] By way of an example, the dose evaluation margin may be set to be between 1% and 20%, between 1% and 10%, between 2% and 10%, between 2% and 8% or between 3% and 7%. In a specific example, the dose evaluation margin may be 5%, such that the weighting term may be between 1 (for an error detected in only a single bin) and 0.01 (for an error detected over 100 bins). Therefore, if the base dose error threshold is (purely for example) 1%, the dose error threshold will vary between 6% and 1.01% (or smaller).

[0067] This dose error threshold weighting term may be implemented in combination with a duration threshold, e.g., such that if the duration threshold is exceeded then the dose error threshold weighting term is set to zero. In this way, the weighting term may be variable between zero (duration above the duration threshold) and the dose evaluation margin value (duration equals a single bin). Such a threshold may be between 50 and 200 bins (e.g., 100 bins), purely for example. Typically, the duration of a normal exposure may be in the region of 2000 bins. A dose error duration which exceeds, for example, 100bins may be deemed sufficiently significant in terms of LCDU, such that the base or initial dose error threshold should be used.

[0068] According to a third concept disclosed herein, which may be implemented in combination or separately from the first concept and / or second concept, the non-product fields may be exposed with a greater source power level than the source power level used to expose the product fields.

[0069] However, the requirement that the dose error for the non-product field should still be controlled, if not as tightly as the product fields, means that the source power during exposure of the non-product fields should not be increased too high. Otherwise too many re-exposures will be required.

[0070] When the two concepts disclosed above are combined, the larger dose error allowed for the non-product fields (e.g. in the region of 5% in the example mentioned above) enables the source to be operated at a higher power during exposure of the non-product fields. This reduces the total wafer expose time and will give a productivity improvement.

[0071] By way of a specific example, where the product fields are exposed at a nominal source power (e.g., at a previously optimized source power level), the non-product fields may be exposed, for example, at a level between 1% and 20% greater than nominal, between 1% and 10% greater than nominal, between 2% and 10% greater than nominal, between 3% and 10% greater than nominal, between 3% and 8% greater than nominal or between 4% and 8% greater than nominal.

[0072] In an embodiment, the source power may be automatically adjusted during exposure of the nonproduct fields to optimize the source power for the allowed (e.g., 5%) dose error. As has been described, this may comprise optimizing the source power level to find an optimal balance of the number or percentage of non-product fields needing re-exposure against the time per exposure of the non-product fields so as to maximize productivity. This can be implemented for example in the re-exposure algorithm or re-expose controller described above.

[0073] Figure 6 illustrates the relevant parameters for such an optimization. Figure 6(a) is a plot of reexpose rate RR (% of substrates re-exposed on a logarithmic scale) against relative source power (i.e., relative % to a nominal source power used to expose the product fields). The first plot (black line) 300 corresponds to the exposure of the product fields, for which a dose error threshold is set at a first threshold value e.g., 1%. The second plot (gray line) 310 corresponds to the exposure of the non-product fields, for which a dose error threshold is set at a second threshold value e.g., 5%. The dotted line represents an optimal re-expose rate (e.g., between 0.1% and 1%, here specifically 0.3%). Figure 6(b) is a plot of relative exposure time RET (% relative to a nominal exposure time for the product fields) against relative source power (i.e., relative % to the nominal source power used to expose the product fields). Again, the first plot (black line) 320 corresponds to the exposure of the product fields, for which a dose error threshold is set at a first threshold value e.g., 1% and the second plot (gray line) 330 corresponds to the exposure of the non-product fields, for which a dose error threshold is set at a second threshold value e.g., 5%.

[0074] Arrow 340 represents the power increase for the non-product fields with respect to the source power (100% or nominal) of the product fields. Arrow 350 represents the reduced exposure time of the non-product fields relative to the nominal exposure time for the product fields.

[0075] In the proposed source power optimization method, the fields which are non-yielding and the dose error threshold for the fields may be defined by a user (the former will of course be largely dictated by which fields do not comprise at least one complete die). The proposed optimization uses the allowed dose error per field as input and adjusts the source power separately for the product fields (power 100% in the plots of Figure 5(a) and 5(b)) and the non-product fields (power approximately 106% in the plots of Figure 5(a) and 5(b)) such that for both field types, the average re-expose rate is optimal for productivity.

[0076] A further limit on throughput and productivity is caused by a per-substrate delay after the final field has been exposed, for evaluating the dose level received by the final field (for comparison against the threshold to check if a re-exposure is required), and performing a re-exposure if it is deemed necessary to do so. The final field is often, and can be chosen to be in an exposure routing optimization, a non-product field. Where a re-exposure is required, performing this re-exposure delays the substrate being unloaded and the next substate loaded or the chucks being swapped in a two-stage lithographic apparatus.

[0077] According to a fourth concept disclosed herein, it is proposed that the delay due to waiting for the dose evaluation result and performing any necessary re-exposure of the last field can be eliminated by never re-exposing this final field. Such an approach may be taken when the final field is a nonproduct field (and as such the exposure routing may ensure this). This means that the substrate unloading, chuck swap or any required re-exposure(s) (i.e., of other fields) can be performed immediately after exposing the final field. For the avoidance of doubt, the final field describes the last field to be exposed (temporally) on each substrate.

[0078] It can be appreciated that this fourth concept may be implemented individually, or in combination with one or more of the aforementioned first concept, second concept and third concept.

[0079] In an embodiment where this fourth concept is combined with at least the third concept, it is proposed that this final (non-yielding) field is exposed at the nominal source level used for the yielding dies rather than the increased source level used for the other non-product fields. This prevents, or at least greatly reduces the likelihood of, the final field exceeding the allowed dose error threshold. This is likely as the allowed dose error threshold for this final field may be the second dose error threshold, being greater than the threshold for the yielding dies, as this final die is non-yielding. Alternatively, this final field may be exposed at a source level below nominal (e.g., between 1% and 10% or between 1% and 5% below nominal source level).

[0080] The benefit of allowing a larger dose error for the non-product fields and exposing these fields at a higher power will depend on the amount of non-product fields. By way of an example, for an expected range of 5 to 20 non-product fields per substrate, the productivity benefit accrued may bebetween an 0.1 % and 1 % improvement in throughput. In addition, the time benefit of not performing a dose evaluation and never re-exposing the last non-product field may be in the region of 70 ms per substrate. This translates to a similar benefit in terms of throughput.

[0081] According to a fifth concept disclosed herein, the routing of the second exposure over the substrate may be dependent upon the dose error threshold. By way of example, for a typical dose error threshold of 1%, it may be deemed acceptable to move the stages to directly expose only the fields requiring re-exposure, without necessarily following the meander of the first exposure (i.e., the first exposure route). This is because, with such a small dose top-up, the effect of heating history on overlay is likely to be negligible.

[0082] Therefore, it is proposed that the first exposure route (i.e., the exposure route taken during the first exposure) is only followed (at least to the final field requiring re-exposure) if one or more of the fields requiring re -exposure had a dose exposure threshold greater than a route determination threshold. This route determination threshold may be a percentage below 3%, below 2.5%, below 2%, below 1.8% or below 1.6% for example. Otherwise, an alternative exposure route is used; e.g., a more direct exposure route to those fields requiring re-exposure compared to following the first exposure route.

[0083] Where this fifth concept is used in combination with the first concept, and specifically where the first concept using a dose error threshold which is dependent on error duration, this fifth concept may be implemented by making the routing decision specifically dependent upon the dose error threshold weighting term (and therefore specifically dependent upon the error duration).

[0084] 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. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0085] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic apparatuses or lithographic tools. Such a lithographic apparatus may use vacuum conditions or ambient (non-vacuum) conditions.

[0086] 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, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0087] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also beimplemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0088] Aspects of the invention are described in the following numbered clauses.1. A method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; and deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field; wherein a respective dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location, a dose error duration over which the dose error occurs and / or a predicted imaging performance of the field(s) within that subset.2. A method according to clause 1 , comprising an initial dose error threshold setting step, comprising: setting the dose error threshold value for each subset based on said one or more of: a property, location and / or predicted imaging performance of the field(s) within the subset.3. A method according to clause 1 or 2, wherein said dose error threshold value is set, for each said subset, based on a location on the substrate, of the field(s) within that subset.4. A method according to clause 3, wherein one or more subsets further from a center of the substrate are set a larger dose threshold than one or more subsets closer to the center.5. A method according to any preceding clause, wherein said dose error threshold value is set, for each said subset, based on a predicted and / or estimated imaging performance of the exposure of the field(s) within that subset.6. A method according to clause 5, wherein a smaller dose threshold is set for subsets associated with a lower estimated imaging performance.7. A method according to any preceding clause, wherein said dose error threshold value is set, for each said subset, based on a required imaging performance of the exposure of the field(s) within that subset and / or a die type exposed on the field(s) within that subset.8. A method according to clause 7, wherein a smaller dose threshold is set for subsets associated with a higher required imaging performance.9. A method according to any preceding clause, wherein said dose error threshold value is set, for each said subset, based on a desired speed for exposing the field(s) within that subset.10. A method according to any preceding clause, wherein said dose error threshold values are set to maximize dies within specification.11. A method according to any preceding clause, comprising setting a re-exposed field threshold for fields which have undergone a second exposure.12. A method according to any preceding clause, wherein said re-exposed field threshold is set higher than the original dose threshold for that field.13. A method according to any preceding clause, comprising performing for each field for which the dose error exceeds a base dose error threshold: determining the dose error duration, said dose error duration comprising a duration over which the dose error exceeds the base dose error threshold; and determining said dose error threshold based on the dose error duration.14. A method according to clause 13, wherein said base dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location and / or a predicted imaging performance of the field(s) within that subset.15. A method according to clause 13 or 14, comprising performing for each said field for which the dose error exceeds the base dose error threshold: comparing said dose error duration to one or more duration thresholds; and determining said dose error threshold based on this comparison.16. A method according to clause 13, 14 or 15, comprising performing for each said field for which the dose error exceeds the base dose error threshold: determining a dose error threshold weighting term based on the error duration; and determining said dose error threshold based on the dose error threshold weighting term and the base dose error threshold.17. A method according to clause 16, wherein said dose error weighting term is determined as a product of a dose evaluation margin and the reciprocal of the error duration.18. A method according to clause 16 or 17, wherein said dose error threshold is determined as the sum of the base dose error threshold and the dose error threshold weighting term.19. A method according to clause 16, 17 or 18, comprising determining an exposure routing for said second exposure based on said dose error threshold weighting term.20. A method according to any preceding clause, comprising determining an exposure routing for a second exposure, where required, based on said dose error threshold of at least one field requiring the second exposure.21. A method according to clause 20, wherein a first exposure route is followed if said dose error threshold of at least one field requiring the second exposure is greater than a route determination threshold, said first exposure route comprising the exposure route taken during the first exposure, at least to the final field requiring second exposure; otherwise a second exposure route is followed wherein the second exposure route is more direct to the at least one field requiring the second exposure than the first exposure route.22. A method according to clause 21, wherein said route determination threshold comprises a percentage below 3%.23. A method according to clause 21, wherein said route determination threshold comprises a percentage below 2%.24. A method according to any preceding clause, wherein said plurality of fields comprise product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; and wherein at least one said subset comprises a non-product field subset comprising said non-product fields.25. A method according to clause 24, wherein the dose error threshold for said non-product field subset, when expressed in terms of a percentage with respect to the target dose value, is greater than 2%.26. A method according to clause 24, wherein the dose error threshold for said non-product field subset, when expressed in terms of a percentage with respect to the target dose value, is greater than 3%.27. A method according to clause 24, wherein the dose error threshold for said non-product field subset, when expressed in terms of a percentage with respect to the target dose value, is greater than 4%.28. A method according to any of clauses 24 to 27, wherein the dose error threshold for said nonproduct field subset, when expressed in terms of a percentage with respect to the target dose value, is less than 30%.29. A method according to any of clauses 24 to 27, wherein the dose error threshold for said nonproduct field subset, when expressed in terms of a percentage with respect to the target dose value, is less than 10%.30. A method according to any of clauses 24 to 27, wherein the dose error threshold for said nonproduct field subset, when expressed in terms of a percentage with respect to the target dose value, is less than 8%.31. A method according to any of clauses 24 to 30, wherein the dose error threshold for said nonproduct field subset is between 2x and 30x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.32. A method according to any of clauses 24 to 30, wherein the dose error threshold for said nonproduct field subset is between 2x and lOx the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.33. A method according to any of clauses 24 to 30, wherein the dose error threshold for said nonproduct field subset is between 3x and 8x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.34. A method according to any of clauses 24 to 33, wherein a final field exposed in said plurality of first exposures is a non-product field, and the method comprises not performing the step of deciding whether to perform a second exposure for the final field, and / or never performing a second exposure for the final field.35. A method according to clause 34, comprising exposing the final field in the first exposure at or below a nominal power level used to expose said product fields, the power level describing a power level of an illumination source used in said exposures.36. A method according to clause 34 or 35, comprising, immediately after exposure of the final field, performing one of: swapping a chuck supporting the substrate with a second chuck comprising an unexposed substrate having been measured; unloading the substrate and loading an unexposed substrate; or performing any of said second exposures on said substrate.37. A method according to any of clauses 24 to 36, wherein said method comprises: performing said plurality of first exposures; performing the second exposures on each product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the first dose error threshold value; and performing the second exposures on each non-product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the dose error threshold for said non-product field subset.38. A method according to clause 37, comprising performing said first exposures and / or said second exposures for said non-product fields with a second power level, said second power level being greater than a nominal power level used to expose said product fields, wherein the power levels describe a power level of an illumination source used in said exposures.39. A method according to clause 38, comprising performing an optimization to individually optimize the nominal power level and second power level according to said first dose error threshold value and said dose error threshold for said non-product field subset value, so as to maximize productivity.40. A method according to clause 39, comprising performing said optimization and automatically adjusting the illumination source power in real time during said first exposures.41. A method according to any of clauses 38 to 40, wherein said second power level is between 1 % and 20% greater than the nominal power level.42. A method according to any of clauses 38 to 40, wherein said second power level is between 1% and 10% greater than the nominal power level.43. A method according to any of clauses 38 to 40, wherein said second power level is between 2% and 10% greater than the nominal power level.44. A method according to any of clauses 38 to 40, wherein said second power level is between 3% and 10% greater than the nominal power level.45. A method according to any of clauses 24 to 44, wherein a target dose to be received in said first exposures is the same for said product fields and said non-product fields.46. A method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures, the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; deciding whether to perform a second exposure on each product field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a first dose error threshold value; and deciding whether to perform a second exposure on each non-product field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a second dose error threshold value, the second dose error threshold value being greater than the first dose error threshold value.47. A method according to clause 46, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 2%.48. A method according to clause 46 or 47, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 3%.49. A method according to any of clauses 46 to 48, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 4%.50. A method according to any of clauses 46 to 49, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 30%.51. A method according to any of clauses 46 to 49, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 10%.52. A method according to any of clauses 46 to 49, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 8%.53. A method according to any of clauses 46 to 52, wherein the second dose error threshold is between 2x and 30x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.54. A method according to any of clauses 46 to 52, wherein the second dose error threshold is between 2x and 20x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.55. A method according to any of clauses 46 to 52, wherein the second dose error threshold is between 2x and lOx the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.56. A method according to any of clauses 46 to 52, wherein the second dose error threshold is between 3x and 8x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.57. A method according to any of clauses 46 to 56, wherein a final field exposed in said plurality of first exposures is a non-product field, and the method comprises not performing the step of deciding whether to perform a second exposure for the final field, and / or never performing a second exposure for the final field.58. A method according to clause 57, comprising exposing the final field in the first exposure at or below a nominal power level used to expose said product fields, the power level describing a power level of an illumination source used in said exposures.59. A method according to clause 57 or 58, comprising, immediately after exposure of the final field, performing one of: swapping a chuck supporting the substrate with a second chuck comprising an unexposed substrate having been measured; unloading the substrate and loading an unexposed substrate; or performing any of said second exposures on said substrate.60. A method according to any of clauses 46 to 59, wherein said method comprises: performing said plurality of first exposures; performing the second exposures on each product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the first dose error threshold value; and performing the second exposures on each non-product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the second dose error threshold.61. A method according to clause 60, comprising performing said first exposures and / or said second exposures for said non-product fields with a second power level, said second power level being greater than a nominal power level used to expose said product fields, wherein the power levels describe a power level of an illumination source used in said exposures.62. A method according to clause 61, comprising performing an optimization to individually optimize the nominal power level and second power level according to said first dose error threshold value and said second dose error threshold value, so as to maximize productivity.63. A method according to clause 62, comprising performing said optimization and automatically adjusting the illumination source power in real time during said first exposures.64. A method according to any of clauses 61 to 63, wherein said second power level is between 1% and 20% greater than the nominal power level.65. A method according to any of clauses 61 to 63, wherein said second power level is between 1% and 10% greater than the nominal power level.66. A method according to any of clauses 61 to 63, wherein said second power level is between 2% and 10% greater than the nominal power level.67. A method according to any of clauses 61 to 63, wherein said second power level is between 3% and 10% greater than the nominal power level.68. A method according to any of clauses 46 to 67, wherein a target dose to be received in said first exposures is the same for said product fields and said non-product fields.69. A method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate, the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; wherein: said first exposures for said product fields are performed at a nominal power level; said first exposures for said non-product fields are performed at a second power level being greater than the nominal power level; said power levels describing a power level of an illumination source used in said first exposures; and a target dose to be received in said first exposures is the same for said product fields and said nonproduct fields.70. A method according to clause 69, comprising performing an optimization to individually optimize the nominal power level and second power level according to a first dose error threshold value and a second dose error threshold value, so as to maximize productivity.71. A method according to clause 70, comprising performing said optimization and automatically adjusting the illumination source power in real time during said first exposures.72. A method according to any of clauses 69 to 71 , wherein said second power level is between 1 % and 20% greater than the nominal power level.73. A method according to any of clauses 69 to 71 , wherein said second power level is between 1 % and 10% greater than the nominal power level.74. A method according to any of clauses 69 to 71 , wherein said second power level is between 2% and 10% greater than the nominal power level.75. A method according to any of clauses 69 to 71 , wherein said second power level is between 3% and 10% greater than the nominal power level.76. A method according to any of clauses 69 to 75, wherein a final field exposed in said plurality of first exposures is a non-product field, and the method comprises, immediately after exposure of the final field such that the step of deciding whether to perform a second exposure is not performed for the final field and / or a second exposure is never performed for the final field, performing one of: swapping a chuck supporting the substrate with a second chuck comprising an unexposed substrate having been measured;unloading the substrate and loading an unexposed substrate; or performing any of said second exposures on said substrate.77. A method according to clause 76, comprising exposing the final field at or below said nominal power level.78. A method according to any of clause 69 to 77, comprising: determining a received dose value per field of the plurality of fields in the plurality of first exposures; deciding whether to perform a second exposure on each product field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to the target dose which is above a first dose error threshold value; and deciding whether to perform a second exposure on each non-product field, or all non-product fields except a final field, only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to the target dose which is above a second dose error threshold value, the second dose error threshold value being greater than the first dose error threshold value.79. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 2%.80. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 3%.81. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is greater than 4%.82. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 30%.83. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 10%.84. A method according to clause 78, wherein the second dose error threshold, when expressed in terms of a percentage with respect to the target dose value, is less than 8%.85. A method according to any of clauses 78 to 84, wherein the second dose error threshold is between 2x and 30x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.86. A method according to any of clauses 78 to 84, wherein the second dose error threshold is between 2x and lOx the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.87. A method according to any of clauses 78 to 84, wherein the second dose error threshold is between 3x and 8x the first dose error threshold, when said dose error thresholds are expressed in terms of a percentage with respect to the target dose value.88. A method according to any of clauses 78 to 87, comprising:performing the second exposures on each product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the first dose error threshold; and performing the second exposures on each non-product field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above the second dose error threshold.89. A method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; determining, for each of said plurality of fields except a final field exposed in said plurality of first exposures, whether each of these fields received a sufficient dose with respect to a target dose in the first exposure; and deciding whether to perform a second exposure on each field except the final field only if it is evaluated to have received an insufficient dose; wherein said deciding step is not performed on said final field and / or a second exposure is never performed for the final field regardless of the dose it received, said final field comprising a non-product field.90. A method according to clause 89, comprising exposing the final field at or below a nominal power level used to expose said product fields, the power level describing a power level of an illumination source used in said exposures.91. A method according to clause 89 or 90, comprising, immediately after exposure of the final field, performing one of: swapping a chuck supporting the substrate with a second chuck comprising an unexposed substrate having been measured; unloading the substrate and loading an unexposed substrate; or performing a second exposure on any of said plurality of fields evaluated to have received an insufficient dose.92. A method according to any of clauses 24 to 91, wherein said non-product fields are comprised at a periphery of the substrate.93. A method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field; and determining an exposure routing for the second exposure, where required, based on said dose error threshold of at least one field requiring the second exposure.94. A method according to clause 93, wherein a first exposure route is followed if said dose error threshold of at least one field requiring the second exposure is greater than a route determination threshold, said first exposure route comprising the exposure route taken during the first exposure, at least to the final field requiring second exposure; otherwise a second exposure route is followed wherein the second exposure route is more direct to the at least one field requiring the second exposure than the first exposure route.95. A method according to any preceding clause, wherein said method comprises: performing said plurality of first exposures; performing the second exposures on each field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above its respective dose error threshold value.96. A computer program comprising program instructions operable to perform the method of any preceding clause, when run on a suitable apparatus.97. A non-transient computer program carrier comprising the computer program of clause 96.98. A processing system comprising a processor and a storage device comprising the computer program of clause 96.99. A lithographic apparatus comprising the processing system of clause 98.100. A lithographic apparatus of clause 99, comprising an illumination system configured to condition a radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate.101. A lithographic apparatus of clause 100, comprising an illumination source for providing said radiation beam.102. A lithographic apparatus of clause 99, 100 or 101, further comprising a dose threshold interface for receiving dose threshold data describing respective thresholds for one or more subsets of fields of a substrate being exposed.

[0089] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method for exposing a substrate in an exposure process, comprising: determining a received dose value per field of a plurality of fields having been exposed on the substrate in a plurality of first exposures; and deciding whether to perform a second exposure on each said field only if its respective received dose value in the first exposure is evaluated to comprise a dose error with respect to a target dose which is above a respective dose error threshold value for that field; wherein a respective dose error threshold is set for each of one or more subsets of said plurality of fields based on one or more of: a property, a location, a dose error duration over which the dose error occurs and / or a predicted imaging performance of the field(s) within that subset.

2. A method as claimed in claim 1, comprising an initial dose error threshold setting step, comprising: setting the dose error threshold value for each subset based on said one or more of: a property, location and / or predicted imaging performance of the field(s) within the subset.

3. A method as claimed in claim 1 or 2, wherein said dose error threshold value is set, for each said subset, based on one or more of: a location on the substrate, of the field(s) within that subset; a predicted and / or estimated imaging performance of the exposure of the field(s) within that subset; and / or a required imaging performance of the exposure of the field(s) within that subset and / or a die type exposed on the field(s) within that subset.

4. A method as claimed in any preceding claim, wherein said dose error threshold value is set, for each said subset, based on a desired speed for exposing the field(s) within that subset.

5. A method as claimed in any preceding claim, wherein said method comprises: performing said plurality of first exposures; performing the second exposures on each field for which its respective received dose value in the first exposure is evaluated to comprise a dose error which is above its respective dose error threshold value.

6. A method as claimed in any preceding claim, comprising setting a re-exposed field threshold for fields which have undergone a second exposure.

7. A method as claimed in any preceding claim, wherein said plurality of fields comprise product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; and wherein at least one said subset comprises a non-product field subset comprising said non-product fields8. A method as claimed in claim 7, wherein a final field exposed in said plurality of first exposures is a non-product field, and the method comprises one or both of: not performing the step of deciding whether to perform a second exposure for the final field, and / or never performing a second exposure for the final field; and / or exposing the final field in the first exposure at or below a nominal power level used to expose said product fields, the power level describing a power level of an illumination source used in said exposures.

9. A method as claimed in claim 7 or 8, comprising performing said first exposures and / or said second exposures for said non-product fields with a second power level, said second power level being greater than a nominal power level used to expose said product fields, wherein the power levels describe a power level of an illumination source used in said exposures.

10. A method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate, the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; wherein: said first exposures for said product fields are performed at a nominal power level; said first exposures for said non-product fields are performed at a second power level being greater than the nominal power level; said power levels describing a power level of an illumination source used in said first exposures; and a target dose to be received in said first exposures is the same for said product fields and said nonproduct fields.

11. A method for exposing a substrate in an exposure process, comprising: performing a plurality of first exposures over a plurality of fields on the substrate the plurality of fields comprising product fields which are at least potentially yielding and one or more non-product fields which have no potential to yield; determining, for each of said plurality of fields except a final field exposed in said plurality of first exposures, whether each of these fields received a sufficient dose with respect to a target dose in the first exposure; and deciding whether to perform a second exposure on each field except the final field only if it is evaluated to have received an insufficient dose;wherein said deciding step is not performed on said final field and / or a second exposure is never performed for the final field regardless of the dose it received, said final field comprising a non-product field.

12. A method as claimed in claim 11, comprising exposing the final field at or below a nominal power level used to expose said product fields, the power level describing a power level of an illumination source used in said exposures.

13. A computer program comprising program instructions operable to perform the method of any preceding claim, when run on a suitable apparatus.

14. A processing system comprising a processor and a storage device comprising the computer program of claim 13.

15. A lithographic apparatus comprising the processing system of claim 14.

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