# Patent

## Title
Sensor for use in a lithographic apparatus

## Abstract

A sensor for use at substrate level in a high numerical aperature lithographic apparatus, the sensor having a transparent plate that covers a sensing element and includes elements that improve coupling of radiation into the sensing element. The elements include Fresnel lenses, holographic optical elements, inverted Winston Cones, spherical lenses and surface roughening.

## Background
FIELD OF THE INVENTION
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
BACKGROUND
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as enabling the use of a larger effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
Another solution which has been proposed is to provide the liquid supply system with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in FIG. 4. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. Preferably the seal is a contactless seal such as a gas seal. Such as system with a gas seal is disclosed in European Patent Application No. 03252955.4 hereby incorporated in its entirety by reference.
In European Patent Application No. 03257072.3 the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage.
A number of sensors are used at substrate level for evaluating and optimizing imaging performance. These may include transmission image sensors (TIS), spot sensors for measuring exposure radiation dose and integrated lens interferometers at scanner (ILIAS). TIS and ILIAS are described below.
A TIS is a sensor that is used to measure the position at substrate level of a projected aerial image of a mark pattern at the mask (reticle) level. The projected image at substrate level may be a line pattern with a line width comparable to the wavelength of the exposure radiation. The TIS measures these mask patterns using a transmission pattern with a photocell underneath it. The sensor data may be used to measure the position of the mask with respect to the substrate table in six degrees of freedom (three in translation and three in rotation). In addition, the magnification and scaling of the projected mask may be measured. Since the sensor is preferably capable of measuring the pattern positions and influences of all illumination settings (sigma, lens NA, all masks (binary, PSM, etc.)) a small line width is preferable. The TIS may also be used to measure the optical performance of the tool. Different illumination settings are used in combination with different projected images for measuring properties such as pupil shape, coma, spherical aberration, astigmatism and field curvature.
An ILIAS is an interferometric wavefront measurement system that may perform static measurements on lens aberrations up to high order. It may be implemented as an integrated measurement system used for system initialization and calibration. Alternatively, it may be used for monitoring and recalibration “on-demand”.
In systems with high NA and in particular in immersion systems, it has been found that conventional sensors at substrate level suffer poor or no sensitivity to radiation arriving at angles corresponding to an NA of greater than 1. NA is defined as n.sin(θ) where n is the refractive index of the material between the last element of the projection system and the substrate and θ is the angle to the normal of the radiation furthest from the normal.
SUMMARY
It is desirable to provide a sensor at substrate level with high sensitivity and which is suitable for use in a high NA system.
According to an aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector; and
a luminescent layer provided on the back surface of the transmissive plate, the luminescent layer absorbing the radiation and emitting luminescent radiation of a different wavelength, wherein the back surface is rough.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector; and
a Fresnel lens provided on the back surface of the transmissive plate and arranged to couple radiation to the radiation detector.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector; and
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation that is projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector;
wherein a region of the transmissive plate through which the radiation passes has a gradient in its refractive index such that the radiation is refracted towards a normal to the back surface of the transmissive plate.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation that is projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector; and
an inverted Winston cone provided on the back surface of the transmissive plate and arranged to couple radiation to the radiation detector.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector; and
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector;
wherein the radiation detector is mounted directly onto the back surface of the transmissive plate.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector; and
a holographic optical element that is provided on the back surface of the transmissive plate and arranged to couple radiation to the radiation detector.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector;
a convex spherical lens being provided on the back surface of the transmissive plate; and
a cylindrical reflector surrounding the convex spherical lens and arranged to couple radiation exiting the convex spherical lens to the radiation detector.
According to a further aspect of the invention, there is provided a sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector;
a cylindrical body being provided on the back surface of the transmissive plate and being arranged to couple radiation to the radiation detector, the cylindrical body having a reflective coating on its curved side surface and a concave recess in its end surface facing the sensor.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
FIG. 1 depicts a lithographic apparatus according to an embodiment of the invention;
FIGS. 2 and 3 depict a liquid supply system used in a prior art lithographic projection apparatus;
FIG. 4 depicts a liquid supply system according to another prior art lithographic projection apparatus;
FIG. 5 depicts a liquid supply system according to another prior art lithographic projection apparatus;
FIG. 6 depicts an ILIAS sensor module according to the prior art;
FIG. 7 depicts a sensor module according to an embodiment of the present invention;
FIG. 8 depicts radiation coupling into a luminescence layer without surface roughening;
FIG. 9 depicts radiation coupling into a luminescence layer with surface roughening;
FIG. 10 depicts another sensor module according to an embodiment of the present invention;
FIG. 11 depicts another sensor module according to an embodiment of the present invention;
FIG. 12 depicts another sensor module according to an embodiment of the present invention;
FIG. 13 depicts another sensor module according to an embodiment of the present invention;
FIG. 14 depicts another sensor module according to an embodiment of the present invention;
FIG. 15 depicts another sensor module according to an embodiment of the present invention; and
FIG. 16 depicts another sensor module according to an embodiment of the present invention.



In the Figures, corresponding reference symbols indicate corresponding parts.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes:
an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation).
a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;
a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and
a projection system (e.g. a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.
The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.
The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.
The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.
The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
As shown in FIG. 5, a liquid supply system is used to supply liquid to the space between the final element of the projection system and the substrate. The reservoir 10 forms a contactless seal to the substrate around the image field of the projection system so that liquid is confined to fill a space between the substrate surface and the final element of the projection system. The reservoir is formed by a seal member 12 positioned below and surrounding the final element of the projection system PL. Liquid is brought into the space below the projection system and within the seal member 12. The seal member 12 extends a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The seal member 12 has an inner periphery that at the upper end preferably closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case.
The liquid is confined in the reservoir by a gas seal 16 between the bottom of the seal member 12 and the surface of the substrate W. The gas seal is formed by gas, e.g. air or synthetic air but preferably N2 or another inert gas, provided under pressure via inlet 15 to the gap between seal member 12 and substrate and extracted via first outlet 14. The overpressure on the gas inlet 15, vacuum level on the first outlet 14 and geometry of the gap are arranged so that there is a high-velocity air flow inwards that confines the liquid.
FIG. 6 shows an ILIAS sensor module 20 according to the prior art. This module has a shearing grating structure 21 as a radiation-receiving element, supported by a transmissive plate 22, which may be made of glass or quartz. A quantum conversion layer 23 is positioned immediately above a camera chip 25 (a radiation-detecting element), which is in turn mounted on a substrate 28. The substrate 28 is connected to the transmissive plate 22 via spacers 26 and bonding wires 27 that connect the radiation-detecting element to external instrumentation. An air gap is located between the quantum conversion layer 23 and the transmissive plate 22. In a setup such as this designed to be sensitive to 157 nm radiation, for example, the air gap within the sensor cannot easily be purged so that it will contain significant proportions of oxygen and water, which absorb radiation. As a result, signals may be lost. Due to the large difference in diffractive index between the quartz or glass sealing plate 22 and the air in the gap, the critical angle is small and radiation at larger angles to the normal, corresponding to NA>1, may be lost. In addition to signal loss, the sensitivity of the sensor may not uniform with angle of incidence.
FIGS. 7 to 15 depict improved substrate-level sensors according to embodiments of the invention. In the following embodiments, parts that are equivalent to parts of the sensor of FIG. 6 are identified by like reference numerals and a detailed description is omitted for conciseness.
In the embodiment of FIG. 7, the quantum conversion (luminescent) layer 23 is positioned on the back surface of the transparent plate 22, rather than on the front of the camera chip (radiation detecting element) 25. As the quantum conversion layer has a higher refractive index than air, the critical angle is larger and less radiation is internally reflected in the transparent plate 22. However, as shown in FIG. 8, the material of the quantum conversion layer, which may be a phosphor, is porous, so there may be incomplete coverage of the back surface of the transparent plate 22. Thus, more radiation is internally reflected in the transparent plate 22 than would be expected. The sensitivity of the sensor may be improved by making the back surface 22a of the transparent plate 22 rough.
The roughness of surface 22a has the effect that radiation propagating through the transparent plate encounters areas of the surface 22a at a variety of angles. There is therefore a loss of transmission at angles close to the (global) normal but an increase in transmission at angles further from the global normal, as shown in FIG. 9. The net effect is an increase in the uniformity of the sensor responsivity with incident angle. The diffuseness of the surface may cause some blurring of the image on the camera 25, but this is acceptable especially if it is less than the pixel size, e.g. 25 μm, and can therefore be neglected. The camera 25 may therefore need to be close to or directly against the conversion layer 23. Alternatively, a lens or fiber-optic bundle may be used to couple the radiation emitted by the conversion layer 23 to the camera without loss of spatial information.
The roughness of the surface 22a can be created by any known method, including omitting final polishing steps in the manufacture of the plate.
The surface roughness should be such that the variation in slope across the surface is at least as large as the amount by which the NA is greater than 1, i.e. Δθ>sin−1 (NA−1). This ensures that a ray at the maximum NA will always have an angle of incidence less than the critical angle at some point on the surface.
The roughness RDq determined by a surface roughness tester should be larger than tan (sin−1 (NA−1)), e.g. in the range of from 0.1 to 0.5.
A further embodiment of the sensor 30, shown in FIG. 10, has a Fresnel lens 31 formed in the back surface 22a of the transparent plate 22. The Fresnel lens is designed such that all radiation passing through the aperture (e.g. pin hole or grating) in chrome layer 21 is incident at the quartz or glass/air interface at less than the critical angle. The Fresnel lens can be formed by many known techniques, e.g. lithographic patterning and etching.
A sensor 40 according to a further embodiment is shown in FIG. 11 and has a region 41 in the transparent plate behind the aperture (e.g. pin hole or grating) in chrome layer 21 that has a gradient in its refractive index. This can be created by locally selective doping of the quartz or glass material forming transparent plate 22 and makes it possible to arrange that all rays passing through the aperture are incident on the quartz/air interface at near normal angles.
FIG. 12 shows a further embodiment of a sensor 50 which uses an inverted Winston cone 51 to reflect all the rays so that they are incident on the bottom surface 52 at less than the critical angle, so there is no internal reflection and maximum transmission into the sensor 25. A Winston cone is an off-axis parabola of revolution designed to maximize collection of incoming rays within some field of view and is described further in Winston, R. “Light Collection within the Framework of Geometric Optics.” J. Opt. Soc. Amer. 60, 245-247, 1970, which document is hereby incorporated by reference in its entirety. The Winston cone 51 in this embodiment is a solid piece of quartz or glass, preferably formed integrally with the transparent plate 22, and has a reflective coating on its side surfaces 53.
In sensor 60, shown in FIG. 13, the sensor 25 is mounted directly onto the back surface 22a of the transparent plate 22. For this purpose a glue that is stable under the radiation to be detected and has a refractive index close to that of the quartz or glass of the transparent plate 22 can be used.
A further sensor 70, show in FIG. 14, uses a holographic element 71 located on the back surface of transparent plate 22, to direct radiation onto the sensor 25. The holographic pattern required can readily be made by known techniques. A diffractive optical element may be used instead.
As shown in FIG. 15, a sensor 80 according to a further embodiment has a convex spherical lens 81 formed in the back surface of transparent plate 22 and a reflective cylinder 82 to direct radiation to the sensor 25. The spherical lens 81 has its center close to the aperture in the chrome layer 21 so that the angles of incidence at the quartz/air interface are near normal for all rays.
The spherical lens 81 is preferably formed integrally with the transparent plate but may also be formed as a separate body and attached using a suitable glue, i.e. one that is stable under the exposure radiation and has a refractive index close to that of the lens.
However, the cylindrical reflector 82 is preferably made as a separate body and attached to the transparent substrate or sensor thereafter. This is because the requirements on the manner and accuracy of its attachment are considerably less strict than those on its shape.
In the embodiment of FIG. 16, the sensor 90 has a cylindrical projection 91 provided on the rear side of the transparent plate 22. The distal end of the projection has a concave cut-away shape, forming a lens. The outer surface 93 of the projection 91 is polished and coated to increase its reflectivity. The concave surface may also be coated to increase its transmissivity. As shown in the figure, radiation can reach the sensor 25 by one of three routes. At small angles to the normal, the radiation will pass directly through the concave surface 92 to reach the sensor 25. At larger angles to the normal, the radiation will be internally reflected at the concave surface 92 and be reflected by the side surface 93 back on to the concave surface 92, which it will pass through to reach the sensor. At still larger angles to the normal, the radiation will be reflected by the side surface 93 to pass through the concave surface 92 and thence to the sensor.
Although a luminescence, or quantum conversion, layer has not been shown in the embodiments of FIGS. 11 to 16, one may be provided as convenient on the sensor or elsewhere. It will also be appreciated that features from the different embodiments of the invention may be combined.
The radiation-receiving element may comprise a grating and or an element having a pinhole, depending on the function of the sensor.
The sensors may be located at the level of the substrate and in particular such that the radiation-receiving element 21 is at substantially the same distance from the final element of the projection system as the substrate W.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (LV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm).
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.
The present invention can be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above.
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

1. A sensor for use at a substrate level in a lithographic projection apparatus having a projection system with a numeric aperture that is greater than 1 that is configured to project a patterned radiation beam onto a target portion of a substrate, the sensor comprising:
a radiation-detector;
a transmissive plate having a front surface and a back surface, the transmissive plate being positioned such that radiation projected by the projection system passes into the front surface of the transmissive plate and out of the back surface thereof to the radiation detector; and
a luminescent layer provided on the back surface of the transmissive plate, the luminescent layer absorbing the radiation and emitting luminescent radiation of a different wavelength, wherein the back surface is rough.

2. The sensor according to claim 1, wherein the back surface of the transmissive plate includes a surface roughness in a range of 0.1 to 0.5.

3. The sensor according to claim 1, wherein the radiation detector includes a plurality of pixels and the roughness of the back surface of the transmissive plate causes a blurring of an image that is projected onto the radiation detector, wherein the blurring is less than a size of one of the pixels.