Sensor system

Abstract

A sensor system for measuring the height of a substrate surface, the sensor system comprising: a projection system comprising a radiation source configured to emit a beam of radiation and further comprising a projection grating configured to diffract the beam of radiation to form a first diffraction order and an additional diffraction order, the projection system being configured to direct the first diffraction order and the additional diffraction order onto the substrate to form a grating image; a detection system configured to detect a position or phase of the radiation beam first diffraction order after reflection from the substrate, and configured to detect a position or phase of the radiation beam additional diffraction order after reflection from the substrate; and a processor configured to use the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.

Description

SENSOR SYSTEMCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 24204964.1 which was filed on 7 October, 2024, and of EP application 25157986.8 which was filed on 14 February, 2025, which are incorporated herein in their entirety by reference.FIELD

[0002] The present disclosure relates to a sensor system for measuring the height of a substrate surface, and to an associated method. The system may be located in a lithographic apparatus.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 (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask also referred to as a reticle) onto a layer of radiationsensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. 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 are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 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] Typically, the substrate may include multiple layers comprising one or more materials (each layer having been patterned by a reticle) and each of the layers may comprise multiple repetitions of the same pattern. The patterns of each layer may be arranged as a two-dimensional array. Any and all references to “substrate” are to be understood as comprising a ‘base’ substrate, e.g. a silicon or glass substrate, or any other appropriate base substrate, and any layers provided thereupon, including but not limited to previously exposed and processed patterned layers, and / or photoresist.

[0006] Different points across a surface of the substrate may be at different heights, and so lithographic radiation focused on the surface of the substrate at a first point (at a first height) may not be in focus at a second point (at a second height). To maintain focus on the substrate surface, the heightof the surface of the substrate is measured and corresponding adjustments of the lithographic apparatus are made.

[0007] The height of the surface of the substrate may be measured using a sensor system. However, it is known that light which is used by a sensor system to measure the height of the surface of the substrate may penetrate partially into the substrate surface, instead of being reflected solely from the substrate surface. A partial reflection of the light may occur from layers beneath the surface of the substrate. This partial reflection causes a substrate height measurement error.

[0008] The substrate height measurement error may be referred to as height process dependency (HPD) or apparent surface depression (ASD).

[0009] It may be desirable to provide a system that obviates or mitigates one or more problems associated with the prior art.SUMMARY

[0010] According to a first aspect of the invention, there is provided a sensor system for measuring the height of a substrate surface, the sensor system comprising: a projection system comprising a radiation source configured to emit a beam of radiation and further comprising a projection grating configured to diffract the beam of radiation to form a first diffraction order and an additional diffraction order, the projection system being configured to direct the first diffraction order and the additional diffraction order onto the substrate to form a grating image; a detection system configured to detect a position or phase of at least a portion of the radiation beam first diffraction order after reflection from the substrate, and configured to detect a position or phase of at least a portion of the radiation beam additional diffraction order after reflection from the substrate; and a processor configured to use the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.

[0011] The term “a first diffraction order” does not necessarily require that the diffraction order is the 1st order of the diffracted radiation. Embodiments of the invention may use any combination of diffraction orders. For example, the 2nd diffraction order and the 3rd diffraction order may be used, the 2nd diffraction order and the 4th diffraction order may be used, etc. It may be preferable to use the 1st diffraction order because this may provide a good signal to noise ratio.

[0012] Advantageously, the positions or phases of first and additional diffraction orders may be used to obtain a measurement of the height of the substrate with reduced process dependency.

[0013] The detection system may be configured to detect the positions of the first and additional diffraction orders simultaneously. The detection system may be configured to detect phase of the first and additional diffraction orders simultaneously.

[0014] The detection system may comprise an imaging detector and focusing optics configured to form an image of the grating image at the imaging detector.

[0015] The sensing system may further comprise a processor configured to obtain information relating to frequency components of the grating image, and configured to use the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order.

[0016] The frequency component information may be obtained using a frequency domain representation of the grating image.

[0017] The detection system may comprise two pairs of detectors, a first detector pair configured to detect a position of the radiation beam first diffraction order and a second detector pair configured to detect a position of the radiation beam additional diffraction order.

[0018] The detection system may comprise wedge shaped optics configured to direct the radiation beam additional diffraction order towards the second detector pair, or to direct the radiation beam first diffraction order towards the first detector pair.

[0019] The detection system may comprise a diffraction grating comprising surfaces tilted at at least four different angles, two tilt angles of the tilted surfaces are configured to direct a first order of radiation to a first pair of detectors and two other tilt angles of the tilted surfaces are configured to direct an additional order radiation to a second pair of detectors.

[0020] The projection grating may comprise a first pitch and a second pitch which are both part of the same diffraction structure.

[0021] The projection grating may comprise two adjacent parts, a first part configured to form the first diffraction order and a second part configured to form the additional diffraction order.

[0022] The projection grating may be a 2-dimensional diffraction grating.

[0023] The sensor system may further comprise at least one beam splitter configured to split the radiation beam first diffraction order and the radiation beam additional diffraction order after reflection from the substrate.

[0024] The at least one beam splitter may be located at a pupil plane of the detection system.

[0025] The detection system may comprise a further detector configured to detect a physical quantity of a portion of the radiation beam first diffraction order and a physical quantity of a portion of the radiation beam additional diffraction order. The further detector may comprise a first further detector and a second further detector. The physical quantity may be an intensity.

[0026] According to a second aspect of the invention there is provided a lithographic apparatus or lithographic tool comprising the sensor system of the first aspect.

[0027] According to a third aspect of the disclosure there is provided a method of measuring the height of a substrate surface, the method comprising using a projection grating to diffract a beam of radiation to form a first diffraction order and an additional diffraction order, and directing the first diffraction order and the additional diffraction order onto the substrate to form a grating image; detecting a position or phase of the radiation beam first diffraction order after reflection from the substrate, and detecting a position or phase of the radiation beam additional diffraction order afterreflection from the substrate; and using the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.

[0028] Advantageously, the positions or phases of first and additional diffraction orders may be used to obtain a measurement of the height of the substrate with reduced process dependency.

[0029] The positions of the first and additional diffraction orders may be detected simultaneously. The phase of the first and additional diffraction orders may be detected simultaneously.

[0030] The method may comprise obtaining information relating to frequency components of a detected grating image, and using the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order.

[0031] The frequency component information may be obtained using a frequency domain representation of the grating image.

[0032] An assumption may be made that the substrate has a structure which does not substantially change within a particular area, and that height process dependency errors do not change significantly within that area.

[0033] An assumption may be made that the substrate has a structure is substantially the same within neighboring measurement locations, and that height process dependency errors do not change significantly within the neighboring measurement locations.

[0034] A weight may be assigned to height measured using the radiation beam first diffraction order and a weight is assigned to height measured using the radiation beam additional diffraction order, and the measurements are combined using the weights.

[0035] Features of different aspects of the invention may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 schematically depicts a lithographic apparatus which includes a sensor system according to an embodiment of the invention; andFigure 2 schematically depicts an embodiment of the sensor system in more detail;Figure 3 schematically depicts a projection grating of an embodiment of the sensor system;Figure 4 schematically depicts an alternative embodiment of the sensor system;Figure 5 schematically depicts a further alternative embodiment of the sensor system;Figure 6 schematically depicts an alternative projection grating of an embodiment of the sensor system; andFigure 7 schematically depicts a further alternative embodiment of the sensor system;Figure 8 schematically depicts a further alternative embodiment of the sensor system.DETAILED DESCRIPTION

[0037] 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), a projection system PS and a first substrate table WT1 configured to support a first substrate Wl. In addition, the lithographic apparatus comprises a second substrate table WT2 configured to support a second substrate W2. The lithographic apparatus LA further comprises a sensor system LS according to an embodiment of the invention configured to determine a height map of a substrate surface.

[0038] 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.

[0039] 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 first substrate Wl. For that purpose, the projection system PS may comprise a plurality of mirrors 12,13 which are configured to project the patterned EUV radiation beam B’ onto the first substrate Wl held by the first substrate table WT1. 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 12,13 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0040] The second substrate table WT2 and second substrate W2 are located adjacent to the first substrate table WT1 and the first substrate W 1. The location of the second substrate table WT2 may be referred to as a measurement location, and the location of the first substrate table WT1 may be referred to as an exposure location. Exposure of the first substrate Wl by the patterned EUV radiation beam B’ takes place at the exposure location. Measurement of the second substrate W2 by the sensor system LS and by an alignment sensing system AS take place at the measurement location. The sensor system LS comprises a projection system 15 and a detection system 16 and is configured to provide an output which indicates the height of the surface of the second substrate W2. The alignment sensing system AS measures the positions of alignment marks on the second substrate W2 relative to alignment marks on the second substrate table WT2. After the measurements have been completed, the second substratetable WT2 is moved to the exposure location. Exposure of the second substrate W2 takes into account the height map of the surface of the second substrate .

[0041] 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.

[0042] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0043] The radiation source SO shown in figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0044] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0045] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0046] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that theintermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0047] To clarify the disclosure, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz -rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the disclosure and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the disclosure. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane. In the figures, the height of the substrate W is indicated as the Z -direction.

[0048] The sensor system LS is arranged to measure a topography of a top surface of the substrate W. The sensor system LS may be referred to as a level sensor, or a topography measurement system. A map of the height (z) of the substrate as a function of position (x, y) on the substrate may be generated from measurements obtained using the sensor system LS. This height map may subsequently be used to adjust the vertical position of the substrate W, and / or to adjust the projection system PS, during projection of a pattern from the patterning device MA onto the substrate.

[0049] The sensor system LS may be stationary. The substrate table WT and substrate W may be moved in a scanning movement beneath the sensor system LS. This allows the sensor system to measure height across a surface of the substrate W and thereby generate a height map.

[0050] The sensor system LS comprises a projection system 15, a detection system 16, and a processor 17. The projection system 15 is configured to provide a beam of light which is incident upon the substrate W2 (including any patterned layers thereon) and is then detected by the detection system 16. A position at which the beam of light is incident at the detection system 16 depends upon the height of the substrate W2. This allows the height of the substrate W2 to be measured. The projection system 15 comprises a diffraction grating (not depicted) which may be referred to as a projection grating. The detection system 16 comprises a diffraction grating (not depicted) which may be referred to as a detection grating. An image of the projection grating is formed at the detection grating, and a position of this grating image relative to the detection grating provides the height measurement. More than one detection grating may be provided, as explained further below. More than one projection grating may be provided, as explained further below. An output from the detection system 16 may be processed by the processor 17 to obtain a measured height of the substrate W.

[0051] The sensor system LS may comprise multiple lasers (or other light sources). The sensor system LS may for example comprise a broadband light source, for example a white light source which emits light across the visible spectrum. The sensor system may for example comprise multiple lasers which are configured to emit light at different wavelengths.

[0052] The measured height of the substrate may include a measurement error arising from the reflection of light from multiple layers of the substrate. Embodiments of the invention may provide a measured height which has a reduced measurement error (compared with a height measured using a conventional sensor system).

[0053] The sensor system LS is depicted in more detail in Figure 2. The projection system 15 comprises a laser 18 configured to provide a beam of radiation 19. The beam of radiation 19 is incident upon a diffraction grating 20, which is referred to as the projection grating 20. The projection grating 20 diffracts the radiation beam 19.

[0054] Figure 3 schematically depicts part of the projection grating 20 (on the left hand side of Figure 3). Also depicted in Figure 3 for reference is part of a conventional grating 22 (on the right hand side of Figure 3). Each grating 20, 22 comprises lines (i.e. portions which are opaque to incident radiation) and spaces (i.e. portions which are transparent to incident radiation). In the conventional diffraction grating 22, the lines 24 have the same width as the spaces 26 and have a pitch P. The conventional diffraction grating 22 is a regular grating with a 50% duty cycle. The lines 24 of the grating may have a width of for example a few microns up to tens of microns. In some instances the lines may have a width of hundreds of microns. Similarly, the spaces 26 of the grating may have a width of for example a few microns up to tens of microns. In some instances the lines may have a width of hundreds of microns The conventional diffraction grating 22 maybe referred to as a standard diffraction grating 22. The skilled artisan will recognize that the opacity and transparency of lines and spaces may be reversed relative to the exemplary embodiments described herein.

[0055] The projection grating 20 which forms part of a sensor system LS according to an embodiment of the invention is not a standard diffraction grating. Instead, each space of the projection grating 20 is bisected by a line. Thus, instead of a single space 26 as provided in the conventional grating 22, two spaces 28 are provided, and a line 30 is located between the spaces. Conventional lines 24 having a duty cycle of 50% are also provided on the projection grating 20. As a result, lines 24, 30 having two different widths are present on the projection grating 20. The grating pitch provided by the two spaces 28 and the line 30 which separates the spaces is one third of the grating pitch provided by the conventional lines 24. Thus, the projection grating 20 has two pitches, a first pitch P and a second pitch P / 3. The projection grating is configured to generate a first diffraction order from the first pitch P and to generate an additional diffraction order from the second pitch P / 3. This additional diffraction order has a frequency which is three times the frequency of the first diffraction order. For ease of terminology, the additional diffraction order may be referred to as the third diffraction order. The additional diffraction order may be considered to be a first diffraction order of the P / 3 pitch portions of the projection grating 20. In other embodiments the projection grating 20 may be provided with other combinations of grating pitches.

[0056] The proportion of lines or spaces which have the pitch P / 3 may be selected to provide first order and third order diffraction with generally similar intensities. A projection grating having only thepitch P will generate third order diffraction but the intensity of the third order diffraction will be relatively weak (e.g. around 10%). Using a diffraction grating which includes the pitch P / 3 increases the relative intensity of the third order diffraction.

[0057] The diffraction grating 20 depicted in Figure 3 is merely an example, and other diffraction gratings may be used. In general a diffraction grating which forms part of a sensor system LS according to the embodiment of the invention may include at least two pitches and thus be configured to generate at least two diffraction orders (e.g. first and third diffraction orders). The first and second pitches may be part of the same diffraction structure. A structure of this type may be referred to as a compound diffraction grating.

[0058] Referring again to Figure 2, the first and third diffraction orders of the radiation beam 19 that are generated by the projection grating 20 are depicted. The first diffraction order is labelled +1,-1 and the third diffraction order is labeled +3,-3. Zero order radiation 0 is also depicted. The zero order radiation 0 is blocked by a blocking structure 32. The blocking structure improves signal to noise ratios but in some embodiments may be omitted. The first and third diffraction order radiation is incident at a focusing lens 34. The lens 34 focusses the first and third diffraction order radiation to form an image of the projection grating 20 on the substrate W.

[0059] The detection system 16 comprises a focusing lens 36, wedge shaped optics 38, a first detection grating 40 and associated first detector pair 42a, b, and a second detection grating 44 and associated second detector pair 46a, b. Each detection grating 40, 44 comprises a series of angled surfaces arranged as lines. The angled surfaces deflect incident radiation towards a first detector of a detector pair 42a, 46a or towards a second detector of a detector pair 42b, 46b. The proportion of radiation that is received by each detector of a pair is determined by the position at which radiation is incident upon the detection gratings 40, 44.

[0060] The focusing lens 36 is configured to form an image of the grating image that has been formed on the substrate W. The wedge shaped optics 38 are located at or adjacent to a pupil plane of the detection system 16, which in the depicted embodiment is adjacent to the focusing lens 36. In other embodiments the wedge shaped optics (or other equivalent beam deflection optics) may be located at or adjacent to a different pupil plane of the detection system 16. The wedge shaped optics 38 are positioned to receive and deflect third diffraction order radiation, labelled +3,-3. The wedge shaped optics 38 are positioned such that they do not receive and deflect first diffraction order radiation, labelled +1,-1. The radiation beam 19 may have a coherence and bandwidth such that the 3rd order diffracted radiation is relatively compact in the pupil plane, e.g. sufficiently compact that a substantially all of the 3rdorder diffracted radiation is incident upon the wedge shaped optics 38. Similarly, the radiation beam 19 may have a coherence and bandwidth such that the 1st order diffracted radiation is relatively compact and substantially none of 1storder radiation is incident upon the wedge shaped optics

[0061] The focusing lens 36 forms, using the first diffraction order radiation, a diffraction grating image at the first detection grating 40. In this context the term “image” does not necessarily mean that the image that is formed looks the same as the projection grating 20. Instead, “image” may be interpreted as meaning the result of focusing radiation at an image plane. The radiation that is focused at the image plane (at the first detection grating 40) has undergone frequency filtering. Specifically, the third diffraction frequency and potentially some other diffraction frequencies (depending upon the size and position of the wedge shaped optics 38), has been removed. This is merely an example of frequency filtering, and in other examples different frequency filtering may apply.

[0062] The first detection grating 40 directs some incident radiation onto a first detector 42a and some incident radiation onto a second detector 42b. The ratio of radiation incident at the first and second detectors 42a, b depends upon the position of the grating image at the first detection grating 40. The position of the grating image at the first detection grating 40 depends upon the height of the substrate W. Thus, outputs from the first and second detectors 42a, b of the first detector pair can be used by the processor 17 to obtain a first measurement of the height of the substrate W.

[0063] The focusing lens 36 forms, using the third diffraction order radiation, a diffraction grating image at the second detection grating 44. As explained above, in this context the term “image” does not necessarily mean that the image that is formed looks the same as the projection grating 20. The radiation that is focused at the image plane (at the first detection grating 40) has undergone frequency filtering, with the first diffraction frequency having been removed.

[0064] The second detection grating 44 directs some incident radiation onto a first detector 46a and some incident radiation onto a second detector 46b. The ratio of radiation incident at the first and second detectors 46a, b depends upon the position of the grating image at the second detection grating 44. The position of the grating image at the second detection grating 44 depends upon the height of the substrate W. Thus, outputs from the first and second detectors 46a, b of the second detector pair can be used by the processor 17 to obtain a second measurement of the height of the substrate W. The measurements may be obtained simultaneously. This may also apply for other embodiments.

[0065] In an alternative arrangement, the wedge shaped optics may be configured to receive and deflect the 1storder diffracted radiation.

[0066] If the substrate W provided a perfect reflection of the incident radiation, and if the optics of the projection system 15 and detection system 16 were perfect, then the substrate height as measured using the third order radiation would be the same as the substrate height as measured using the first order radiation. However, as a result of height process dependency the substrate height measured using the third order radiation may differ from the substrate height measured using the first order radiation. The difference between the measured heights may be used to obtain a correction (which may be a partial correction for height process dependency).

[0067] The reason for the height process dependency difference as seen for third order radiation and first order radiation is now explained. The chief ray of the radiation beam 19 incident upon the substrateW subtends an angle 0 with respect to a normal from the substrate. The first and third diffraction orders will subtend different angles with respect to the normal when incident upon the substrate W. Height process dependency (HPD) or apparent surface depression (ASD) may occur due to penetration of some of the incident radiation into the substrate W and reflection of that radiation from structures within the substrate. The penetration of incident radiation into the substrate W, and reflection from structures within the substrate, will be different for different angles of incident radiation. Thus, each diffraction order will experience a different height process dependency. The height process dependency experienced by each diffraction order will vary depending upon the structure of an area of the substrate upon which the radiation beam 19 is incident. The structure of the substrate may be known, and may repeat for an array of exposure areas (which may be dies) distributed across the substrate.

[0068] The focusing lens 34 which forms part of the projection system 15 is not a perfect lens but instead includes some imperfections which will cause aberration of radiation that passes through the focusing lens. The aberration experienced by the radiation when passing through the focusing lens 34 depends upon the spatial position of the radiation in the focusing lens. The spatial position of the radiation in the focusing lens is determined by the diffraction order of the radiation, as schematically depicted. Thus, the first and third diffraction orders will experience different aberrations due to the focusing lens 34 of the projection system 15. These aberrations are generally constant and do not contribute to height process dependency.

[0069] In the detection system 16 of the sensor system LS however, imperfections in the focusing lens 36 do contribute to height process dependency. This is because variations of layer thicknesses, and of structure, within the substrate W will modify an angle at which radiation is reflected from the substrate. The variation of layer thickness and of structure may also modify a wavelength dependent reflectivity of the substrate W, and thus modify the amplitude of radiation reflected from the substrate. As a result of these effects, the spatial positions at which radiation passes through the focusing lens 36 of the detection system 16 will vary. The radiation will therefore experience varying aberrations in the focusing lens 36 depending upon the layer thicknesses and structure of the substrate W. These varying aberrations will contribute to height process dependency.

[0070] The first and third radiation diffraction orders each interact with the substrate W at different angles, and each pass through the detection system focusing lens 36 at different spatial positions. The measurements provided by the first and second detector pairs 42a, b, 46a, b can therefore be used to obtain a corrected measured height. This may be done for example using a linear model or a non-linear model. One example of a linear model is: tfl(x) = tfO(x) * (1 + al) + 01 Equation 1H3 x) = HO(x) * (1 + a3) + 03 Equation 2where HO is the actual height of the substrate, Hl is the height measured using the first diffraction order, H3 is the height measured using the third diffraction order, al and a 3 are scaling errors for the first and third diffraction orders respectively, and 01 and 03 are offset errors for the first and third diffraction orders respectively.

[0071] It may be assumed that the structure of the substrate W remains substantially the same for a particular area of the substrate. Where this is the case, the scaling errors al, a3 and offset errors 01, 03 may be assumed to remain constant within that particular substrate area. A set of measured height values Hl, H3 are measured for different positions within the particular substrate area. The scaling errors al, a3 and offset errors 01, 03 are caused by the structure of the substrate W, and because the structure remains substantially constant these errors remain substantially constant and may be assumed to be constant. Thus, multiple sets of measured heights have been obtained for different substrate heights, with the scaling and offset errors remaining constant. This provides sufficient information to solve Equations 1 and 2 thereby determine the scaling errors al, a3 and offset errors 01, 03. These determined scaling errors al, a3 and offset errors 01, 03 are used to correct the measured height values Hl, H3 to obtain the actual height HO of the substrate. In some cases the correction may be partial, that is the height process dependency of the measured height value is reduced but not completely eliminated.

[0072] The assumption that the structure of the substrate W remains substantially the same for a particular area of the substrate may be based upon for example knowledge of the substrate structure. The knowledge of the substrate structure may be obtained from a prior measurement of a different area of the same substrate. An array of dies may be formed in the substrate. Each die will have the same structure. Thus, a measurement of the structure of one die may be obtained and used to identify areas of that die within which the structure remains the same. Once this identification has been performed, the assumption may be applied for any substrate which comprises that die. The die may be partially formed, i.e. with some layers of the eventually completed die not yet having been formed. The measurement of the die structure may be performed outside of a lithographic apparatus.

[0073] In an alternative approach, an assumption may be made that the structure of the substrate is substantially the same within multiple neighbouring measurement locations. Multiple height measurements made at those locations will provide sufficient information to determine the scaling and offset errors, thereby allowing the actual height HO of the substrate to be determined. In some cases the correction may be partial, that is the height process dependency of the measured height value is reduced but not completely eliminated. This assumption may not always be correct. Locations where the assumption does not apply may be identified for example by the presence of steep measured height gradients. At locations where the assumption does not apply, the method may be omitted.

[0074] In general, the processor 17 may use the height as measured by the first diffraction order and by the additional diffraction order to obtain an improved height measurement (compared with measuring using a conventional measurement). The measured heights may be used to obtain estimates of process dependency errors, allowing correction for those errors (including partial correction).

[0075] Modelling may use the measured heights to determine weightings to be applied to measurements obtained using the first diffraction order and measurements obtained using the additional diffraction order. The measurements may then be combined using those weightings. The weightings may be different for different substrate areas which have different substrate structures.

[0076] In general, the measurements obtained using the first diffraction order and the measurements obtained using the additional diffraction order may be processed to obtain an improved height measurement (compared with measuring using a conventional measurement). Statistical analysis may be used. For example principal component analysis may be used.

[0077] Measurements obtained using the first diffraction order and measurements obtained using the additional diffraction order may be combined with other measurements. This may provide a further height measurement accuracy improvement. The other measurements may comprise one or more of measurements made using additional wavelengths, and measurements made using multiple polarisations.

[0078] An alternative embodiment of the invention is schematically depicted in Figure 4. The embodiment in Figure 4 corresponds with the embodiment in Figure 2, except that the wedge shaped optics are not present, and there is a single detection grating 50. The diffraction grating 50 comprises surfaces tilted at four different angles. Two tilt angles of the tilted surfaces are configured to direct a first order of radiation, which may for example be first order radiation (+1,-1) to a first pair of detectors 42a, b. Two other tilt angles of the tilted surfaces are configured to direct an additional order of radiation, which may for example be third order radiation (+3,-3) to a second pair of detectors 46a, b. Thus, the detection grating 50 has surfaces tilted at at least four angles.

[0079] The detection grating may for example be made as follows: divide the area of the to-be- designed detection grating into an array pixels (i.e. an array of square or rectangular areas). Assign half of the pixels to first order radiation and half of the pixels to third order radiation, for example in a random way. Then assign either a positive or a negative tilt angle to each pixel (a generally equal number of each being provided). Then manufacture the detection grating with the assigned slopes.

[0080] A further alternative embodiment of the invention is schematically depicted in Figure 5. The embodiment in Figure 5 corresponds with the embodiment in Figure 2, except that the projection grating 60 has a different form. The projection grating 60 comprises two adjacent parts. A first part 62 is configured to generate 1storder diffraction. A second part 64 has a grating pitch which is 1 / 3 of the pitch of the first grating part. The second part 64 of the projection grating 60 therefore provides diffraction which has a diffraction angle which corresponds with the 3rdorder diffraction angle for the first part 62 of the projection grating. For ease of terminology this diffraction is referred to as 3rdorder diffraction, and in Figure 5 is labelled +1(3), -1(3). The first and second parts 62, 64 are separated in a direction which corresponds with the Y-direction (scanning direction of the lithographic apparatus) when the projection grating image is formed on the substrate. Cartesian coordinates for the grating are shown in Figure 5. The orientation of the projection grating 60 will in practice include a Z -directioncomponent due to the angle 0 subtended by the beam 19 emitted by the projection system 15 relative to the substrate W. Although this embodiment is described a second part with a grating pitch 1 / 3 of the pitch of the first part, in other embodiments the grating of the second part may have a different pitch. For example the pitch of the second part may be or 1 / 2 or 1 / 4 of the pitch of the first part (or some other value). The grating pitch should be sufficiently large that the diffracted radiation falls within a numerical aperture of the lens 36 (or other focussing optics). The grating shown in Figure 5 is configured to generate 1stand 3rdorder diffraction, but may alternatively be configured to generate diffraction orders, or a different combination of diffraction orders.

[0081] The projection system 15 forms an image of the projection grating 60 on the substrate W. The detection system 16 forms an image of this projection grating image at the detection grating 66. The form of the detection grating 66 corresponds with the form of the projection grating 60. That is, the detection grating 66 comprises a first part with a pitch that corresponds with the pitch of the first part 62 of the projection grating 60, and has a second part with a pitch that corresponds with the second part 64 of the projection grating. A first pair of detectors 42a, b is used to detect radiation (+1,-1) which has passed through the first part of the detection grating 66, and a second pair of detectors 46a, b is used to detect radiation (+1(3), -1(3)) which has passed through the second part of the detection grating. In embodiments where the projection grating is configured to generate radiation with diffraction orders different to the first and third orders, the detection grating will correspondingly be configured to detect the orders of radiation corresponding to the orders of the projection grating.

[0082] Although the projection grating 60 is depicted as being a single element provided with first and second parts, in other embodiments the projection grating may comprise two separate elements. Providing the projection grating as a single element is preferred because this provides better controlled relative positioning of the first and second parts (compared with providing them as separate elements). When two separate elements are used for the first and second parts there may be a greater risk of drift of the relative positions of the first and second parts.

[0083] In the depicted and described embodiments of the invention, the projection grating is a 1- dimensional grating. That is, the grating has a pitch in one direction but has no pitch in the orthogonal direction. Embodiments of the invention may comprise a 2 -dimensional projection grating. An example of a 2-dimensional projection grating that may be used by an embodiment of the invention is schematically depicted in Figure 6. The 2-dimensional grating of Figure 6 is a 2-dimensional version of the grating depicted in Figure 3. The projection grating 70 comprises a plurality of square spaces 72 each of which is bisected by orthogonal lines 74, 76. The square spaces 72 are provided as a two- dimensional array.

[0084] The projection grating 70 is configured to generate 1stand 3rdorder diffracted radiation in two orthogonal directions. The orthogonal directions may be referred to herein as X and Y directions, although due to the angle 0 subtended with respect to the substrate W the Y -direction may include a Z- direction component. The depicted projection grating 70 is merely an example. Other 2-dimensionaldiffraction gratings configured to generate 1stand 3rdorder diffraction (or other diffraction orders) may be used.

[0085] The projection system 15 used in connection with a 2 -dimensional grating may correspond with the projection system as described further above. The projection system 15 will form an image of the 2-dimensional grating at the substrate W. The detection system is modified compared with the above described detection systems. The modification allows for additional detection in an orthogonal direction. For example, in the embodiment depicted in Figure 2 a pair of wedge shaped optics 38 are used to separate 1stand 3rdorder diffracted radiation so that these can be detected by separate detector pairs 42a, b, 46a, b. When a 2-dimensional projection grating 70 is used, another pair of wedge shaped optics are provided, the wedge shaped optics being separated in X-direction. These wedge shaped optics are used to separate 1stand 3rdorder diffracted radiation and direct the 1stand 3rdorder diffracted radiation to separate detector pairs.

[0086] In another example, instead of the detection diffraction grating 50 comprising surfaces tilted at four different angles, the detection diffraction grating may comprise surface tilted at eight different angles. Four pairs of detectors may be used to detect the orthogonally separated 1stand 3rdorder diffracted radiation.

[0087] In an embodiment, the projection grating may comprise a 2 -dimensional grating having two parts. A first part may be configured to generate 1storder diffraction in 2-dimensions. A second part may have a grating pitch which is 1 / 3 of the pitch of the first grating part (the second part also being configured to generate diffraction in 2-dimensions). The detection grating may have two corresponding parts. Four detector pairs may be provided, two pairs to detect radiation that was diffracted by the 1stpart of the projection grating and two pairs to detect radiation that was diffracted by the 2ndpart of the projection grating. The projection grating having two parts may be provided as a single element or may be provided as two elements.

[0088] The detectors of embodiments of the invention may be photodiodes. In described embodiments two detectors are provided per diffraction order (for a given grating direction). However, more than two detectors may be provided per diffraction order (for a given grating direction).

[0089] A further alternative embodiment of the invention is schematically depicted in Figure 7. In this embodiment the projection system 10 corresponds with the projection system depicted in Figures 2 and 3 and described further above. The projection system 10 may have the form of any of the above described projection systems.

[0090] The detection system 16 comprises a focussing lens 36 and an imaging detector 82 (which may be referred to as a camera). The focussing lens 36 forms an image at the imaging detector 82. The image formed at the imaging detector 82 is an image of the projection grating 20 as formed on the substrate W. In general, any form of focussing optics may be used to form the image at the imaging detector. The processor 17 receives a signal output from the imaging detector 82.

[0091] The phases in the height-sensitive direction of frequency orders (which correspond with diffraction orders) can each be related to a measured height of the substrate. Detection of the diffraction orders using an imaging detector may be advantageous in that it does not require additional components and is therefore simpler to implement.

[0092] The processor 17 may apply a Fourier transform (e.g. a Fast Fourier Transform) to the signal output from the imaging detector 82 in order to obtain a frequency domain representation of the grating image. The phase of the first diffraction order and the phase of the additional diffraction order (e.g. 3rddiffraction order) may be determined by the processor 17 using this frequency domain representation. In general, frequency component information relating to the image detected by the imaging detector may be obtained using any suitable method. A correlation method which uses a representation of the projection grating may for example be used to obtain frequency component information.

[0093] For any embodiment of the invention, the processor 17 may calculate a weighted sum of the heights as measured by each diffraction order. This may provide a height measurement which is less affected by height process dependency than a conventional measurement. The measured heights for different orders can be compared to measured heights at different wavelengths or polarizations (which may also be obtained using the sensor system LS). The measured heights for different orders may be combined with measured heights for different wavelengths and / or polarizations to provide a height measurement which is less affected by height process dependency than a conventional measurement.

[0094] When a 2-dimensional projection grating is used, the phases in the non -height-sensitive direction of frequency orders (which correspond with diffraction orders) can be measured. These are phase changes in the X-direction. The phase changes are sensitive to variations in the layers which make up the substrate W but are not sensitive to the height of the substrate. The phase changes in the non-height sensitive direction can be incorporated in a weighted sum to improve the process robustness.

[0095] In some sensor systems LS a series of radiation beams which are distributed along the X- direction are provided simultaneously. This allows simultaneous illumination of a portion of the substrate using those beams of radiation, thereby allowing a height of the substrate W to be mapped more quickly (compared with using a single beam of radiation). Different beams of radiation will pass through different positions in the focussing lens 36 of the detection system (the positions being distributed in the X-direction), and thus will experience different aberrations. This may be expressed by saying that different level sensor spots experience different height process dependency.

[0096] A further alternative embodiment of the invention is schematically depicted in Figure 8. In this embodiment the projection system 10 corresponds with the projection system depicted in Figures 2 and 3 and described further above. The projection system 10 may have the form of any of the above described projection systems. Merely for the purposes of illustration, the detection system 16 comprises a focussing lens 36 and an imaging detector 82 (which may be referred to as a camera) as in the embodiment of Figure 7. Alternatively, the detection system 16 may comprise a pair of detectors 42, 46 as shown in the embodiments of Figures 2 and 4. The embodiment of Figure 8 comprises a beam splitter84 configured to split the radiation beam incident thereupon. Thus, in the exemplary embodiment of Figure 8, the first and third diffraction order radiation is incident on beam splitter 84 after reflection from the substrate and is in turn split into a first portion 86 and a second portion 88. In the embodiment of Figure 8, the first portion 86 is transmitted through the beam splitter 84 and the second portion 88 is reflected at the beam splitter 84. The first portion 86 comprises a first first diffraction order portion and a first third diffraction order portion. The second portion 88 comprises a second first diffraction order portion and a second third diffraction order portion. The first portion 86 is detected at a detector, which in the embodiment of Figure 8 is imaging detector 82 and detection is as described for the embodiment of Figure 7. Alternatively, the first portion 86 may be detected at a plurality of detectors as per previously described embodiments. The second portion 88 is detected at a further detector 90. Further detector 90 may be a camera. Alternatively, further detector 90 may comprise a plurality of further detectors, for example a first further detector and a second further detector.

[0097] The first further detector and the second further detector may be for example photodiodes. Each of the first further detector and the second further detector is configured to detect a physical quantity of the radiation detected thereupon. The physical quantity may for example be an intensity. In an embodiment, the first further detector may be configured to detect the intensity of the positive diffraction orders and the second further detector may be configured to detect the intensity of the negative diffraction orders.

[0098] The intensity of the individual diffraction orders of the radiation beam reflected from the substrate surface have a dependency on the interaction with the substrate. The intensities of the individual diffraction orders of the radiation beam can be used as a metric for optical changes due to interactions with the substrate, for example to correct for height process dependencies due to the interaction with underlying layers in the stack. Wedge shaped optics and / or lenses (not shown) configured to direct a part or whole of the first and / or second portions towards one or more detectors may be employed. Such wedges and / or lenses may for example be used to spatially separate diffraction orders.

[0099] The intensities of individual diffraction orders (i.e. positive diffraction order intensity I+and negative diffraction order intensity I ) can be used to determine a normalized asymmetry A:The normalized asymmetry A can in turn be used to correct the measured height signal Hmeas for height process dependency, providing a height correction HCorr:Hcorr= Hmeas+ cI+■ / ++ c'~ ■ I~ + CA■ Awhere cI+, c1’ and cAare scaling factors of the positive order intensity, negative order intensity, and asymmetry, respectively. The scaling factors may be determined via training measurements on previous substrates, or may alternatively be selected based on a similar response at multiple areas on a substrate (e.g. for a plurality of fields).

[0100] The beam splitter 84 may provide for an uneven splitting of the radiation beam incident thereupon. For example, the beam splitter 84 may provide that 90% of light incident on the beamsplitter is transmitted, such that the first portion 86 comprises 90% of the first and third diffraction order radiation. It follows that the same beam splitter 84 would provide that 10% of the light incident on it is reflected, such that the second portion 88 comprises 10% of the first and third diffraction order radiation. Advantageously, by selecting a ratio of splitting at the beam splitter such that a limited percentage of the radiation is directed to the second portion 88 for detection of order intensities (e.g. 10%), the intensity of the radiation in the first portion 86 used for the height measurement is not reduced significantly and measurement accuracy is maintained.

[0101] The beam splitter 84 may be integrated into the detection system, or alternatively the beam splitter may be a separate component.

[0102] The beam splitter 84 may be located at a pupil plane of the detection system. The second portion 88 thereby comprises a radiation signal comprising two half-pupils, namely a first half-pupil and a second half-pupil. Advantageously, when detecting two half-pupils, the projection grating may comprise a simple spot configured to fill the majority of the pupil with light, and no detection grating is necessary.

[0103] Embodiments of the invention have been described in connection with the 1stand 3rddiffraction orders. These diffraction orders may advantageously provide relatively strong signals compared with other diffraction orders. However, other diffraction orders including higher orders may be used.

[0104] It is not essential that the 1stdiffraction order is used. Embodiments of the invention may use any combination of diffraction orders. For example, the 2nd diffraction order and the 3rd diffraction order may be used, the 2nd diffraction order and the 4th diffraction order may be used, etc. It may be preferable to use the 1st diffraction order because this may provide a good signal to noise ratio.

[0105] In embodiments which use to adjacent grating parts to form the first and additional diffraction orders, the first grating part may have a pitch which is not an integer multiple of the pitch of the second grating part.

[0106] While the methods described herein have been described in relation to a substrate W that has, or will be, exposed to lithographic radiation (i.e. the radiation beam B), as will be clear to the skilled person, the methods (and corresponding apparatus) may beneficially be adapted for use with substrates that are not exposed to lithographic radiation.

[0107] Embodiments of the invention may form part of a lithographic apparatus (e.g. as depicted). Embodiments of the invention may form part of a lithographic tool. Examples of lithographic tools are mentioned further below.

[0108] Likewise, while the methods described herein have been described in relation to an EUV lithographic apparatus, as will be clear to the skilled person, the methods may be beneficially adapted for use with a DUV lithographic apparatus.

[0109] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin -film magnetic heads, etc.

[0110] 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 apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0111] 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.

[0112] 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 be implemented 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.

[0113] 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 maybe made to the invention as described without departing from the scope of the claims set out below. Aspects of the invention are set out in the clauses below.1. A sensor system for measuring the height of a substrate surface, the sensor system comprising: a projection system comprising a radiation source configured to emit a beam of radiation and further comprising a projection grating configured to diffract the beam of radiation to form a first diffraction order and an additional diffraction order, the projection system being configured to direct the first diffraction order and the additional diffraction order onto the substrate to form a grating image; a detection system configured to detect a position or phase of at least a portion of the radiation beam first diffraction order after reflection from the substrate, and configured to detect a position or phase of at least a portion of the radiation beam additional diffraction order after reflection from the substrate; and a processor configured to use the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.2. The sensor system of clause 1, wherein the detection system comprises an imaging detector and focusing optics configured to form an image of the grating image at the imaging detector.3. The sensor system of clause 2, wherein the sensing system further comprises a processor configured to obtain information relating to frequency components of the grating image, and configured to use the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order.4. The sensor system of clause 3, wherein the frequency component information is obtained using a frequency domain representation of the grating image.5. The sensor system of clause 1, wherein the detection system comprises two pairs of detectors, a first detector pair configured to detect a position of the radiation beam first diffraction order and a second detector pair configured to detect a position of the radiation beam additional diffraction order.6. The sensor system of clause 5, wherein the detection system comprises wedge shaped optics configured to direct the radiation beam additional diffraction order towards the second detector pair, or to direct the radiation beam first diffraction order towards the first detector pair.7. The sensor system of clause 5, wherein the detection system comprises a diffraction grating comprising surfaces tilted at at least four different angles, two tilt angles of the tilted surfaces are configured to direct first order radiation to a first pair of detectors and two other tilt angles of the tilted surfaces are configured to direct third order radiation to a second pair of detectors.8. The sensor system of any preceding clause, wherein the projection grating comprises a first pitch and a second pitch which are both part of the same diffraction structure.9. The sensor system of any of clauses 1 to 5, wherein the projection grating comprises two adjacent parts, a first part configured to form the first diffraction order and a second part configured to form the additional diffraction order.10. The sensor system of any of any preceding clause, wherein the projection grating is a 2 -dimensional diffraction grating.11. The sensor system of any preceding clause, further comprising at least one beam splitter configured to split the radiation beam first diffraction order after reflection from the substrate into a first radiation beam first diffraction order portion and a second radiation beam first diffraction order portion, and to split the radiation beam additional diffraction order after reflection from the substrate into a first radiation beam additional diffraction order portion and a second radiation beam additional diffraction order portion.12. The sensor system of clause 11, wherein the at least one beam splitter is located at a pupil plane of the detection system.13. The sensor system of clause 11 or 12, wherein the detection system comprises a further detector configured to detect a physical quantity of one of the first radiation beam first diffraction order portion and second radiation beam first diffraction order portion and one of the first radiation beam additional diffraction order portion and second radiation beam additional diffraction order portion.14. The sensor system of clause 13, wherein the further detector comprises a first further detector configured to detect a physical quantity of one of the first radiation beam first diffraction order portion and second radiation beam first diffraction order portion and a second further detector configured to detect a physical quantity of one of the first radiation beam additional diffraction order portion and second radiation beam additional diffraction order portion.15. The sensor system of clause 14 wherein the physical quantity is an intensity.16. A lithographic apparatus or lithographic tool comprising the sensor system of any of clauses I to 15.17. A method of measuring the height of a substrate surface, the method comprising: using a projection grating to diffract a beam of radiation to form a first diffraction order and an additional diffraction order, and directing the first diffraction order and the additional diffraction order onto the substrate to form a grating image; detecting a position or phase of at least a portion of the radiation beam first diffraction order after reflection from the substrate, and detecting a position or phase of at least a portion of the radiation beam additional diffraction order after reflection from the substrate; and using the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.18. The method of clause 17, wherein the method comprises obtaining information relating to frequency components of a detected grating image, and using the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order.19. The method of clause 17, wherein the frequency component information is obtained using a frequency domain representation of the grating image.20. The method of any of clauses 17 to 19, wherein an assumption is made that the substrate has a structure which does not substantially change within a particular area, and that height process dependency errors do not change significantly within that area.21. The method of any of clauses 17 to 19, wherein an assumption is made that the substrate has a structure is substantially the same within neighboring measurement locations, and that height process dependency errors do not change significantly within the neighboring measurement locations.22. The method of any of clauses 17 to 21, wherein a weight is assigned to height measured using the radiation beam first diffraction order and a weight is assigned to height measured using the radiation beam additional diffraction order, and the measurements are combined using the weights.

Claims

CLAIMS1. A sensor system for measuring the height of a substrate surface, the sensor system comprising: a projection system comprising a radiation source configured to emit a beam of radiation and further comprising a projection grating configured to diffract the beam of radiation to form a first diffraction order and an additional diffraction order, the projection system being configured to direct the first diffraction order and the additional diffraction order onto the substrate to form a grating image; a detection system configured to detect a position or phase of at least a portion of the radiation beam first diffraction order after reflection from the substrate, and configured to detect a position or phase of at least a portion of the radiation beam additional diffraction order after reflection from the substrate; and a processor configured to use the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.

2. The sensor system of claim 1, wherein the detection system comprises an imaging detector and focusing optics configured to form an image of the grating image at the imaging detector.

3. The sensor system of claim 2, wherein the sensing system further comprises a processor configured to obtain information relating to frequency components of the grating image, and configured to use the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order, and wherein the frequency component information is obtained using a frequency domain representation of the grating image .

4. The sensor system of claim 1, wherein the detection system comprises two pairs of detectors, a first detector pair configured to detect a position of the radiation beam first diffraction order and a second detector pair configured to detect a position of the radiation beam additional diffraction order.

5. The sensor system of claim 4, wherein the detection system comprises wedge shaped optics configured to direct the radiation beam additional diffraction order towards the second detector pair, or to direct the radiation beam first diffraction order towards the first detector pair.

6. The sensor system of claim 4, wherein the detection system comprises a diffraction grating comprising surfaces tilted at at least four different angles, two tilt angles of the tilted surfaces are configured to direct first order radiation to a first pair of detectors and two other tilt angles of the tilted surfaces are configured to direct third order radiation to a second pair of detectors.

7. The sensor system of any of claims 1 to 4, wherein the projection grating comprises two adjacent parts, a first part configured to form the first diffraction order and a second part configured to form the additional diffraction order.

8. The sensor system of any preceding claim, further comprising at least one beam splitter configured to split the radiation beam first diffraction order after reflection from the substrate into a first radiation beam first diffraction order portion and a second radiation beam first diffraction order portion, and to split the radiation beam additional diffraction order after reflection from the substrate into a first radiation beam additional diffraction order portion and a second radiation beam additional diffraction order portion; and a further detector configured to detect a physical quantity of one of the first radiation beam first diffraction order portion and second radiation beam first diffraction order portion and one of the first radiation beam additional diffraction order portion and second radiation beam additional diffraction order portion.

9. The sensor system of claim 8, wherein the at least one beam splitter is located at a pupil plane of the detection system, and wherein the physical quantity is an intensity.

10. A lithographic apparatus or lithographic tool comprising the sensor system of any of claims 1 to 9.

11. A method of measuring the height of a substrate surface, the method comprising: using a projection grating to diffract a beam of radiation to form a first diffraction order and an additional diffraction order, and directing the first diffraction order and the additional diffraction order onto the substrate to form a grating image; detecting a position or phase of at least a portion of the radiation beam first diffraction order after reflection from the substrate, and detecting a position or phase of at least a portion of the radiation beam additional diffraction order after reflection from the substrate; and using the detected positions or phases of first and additional diffraction orders to determine a measurement of the height of the substrate.

12. The method of claim 11, wherein the method comprises obtaining information relating to frequency components of a detected grating image, and using the frequency component information to determine a phase of the radiation beam first diffraction order and a phase of the radiation beam additional diffraction order, and wherein the frequency component information is obtained using a frequency domain representation of the grating image.

13. The method of any of claims 11 to 12, wherein an assumption is made that the substrate has a structure which does not substantially change within a particular area, and that height process dependency errors do not change significantly within that area.

14. The method of any of claims 11 to 12, wherein an assumption is made that the substrate has a structure is substantially the same within neighboring measurement locations, and that height process dependency errors do not change significantly within the neighboring measurement locations.

15. The method of any of claims 11 to 14, wherein a weight is assigned to height measured using the radiation beam first diffraction order and a weight is assigned to height measured using the radiation beam additional diffraction order, and the measurements are combined using the weights.

Images