GB2096316A

GB2096316A - Optical position detector

Info

Publication number: GB2096316A
Application number: GB8209505A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1981-04-03
Filing date: 1982-03-31
Publication date: 1982-10-13
Also published as: CA1194176A; GB2096316B; DE3211928C2; JPS6355002B2; FR2503416B1; NL8101668A; DE3211928A1; JPS57178103A; FR2503416A1

Abstract

Optical position detectors, eg for the accurate registration of a mask pattern on a substrate in the manufacture of integrated circuits, are provided with a wide lock-in range by means of a prism P of large apex angle beta , typically 165 DEG , attached to one object V, eg the table. A light source B, a lens L and a pair of radiation detectors D1, D2 are attached as an assembly to the other body, eg the substrate support. The image of B provided by L is focussed on the prism apex when the objects are in registration, the split beams b1 and b2 illuminating the two detectors equally. When the objects are out of registration, the detectors are unequally illuminated, providing a difference signal for servo control of registration. The difference signal is maintained for a displacement of prism P over the whole width of the prism base, the deflection of beams b1 and b2 depending only on the base angle alpha . The prism may have reflective surfaces eg four mutually inclined surfaces so-operating with four detectors to measure movement in two dimensions, and light guides may be utilised. Rotational registration of a turntable is achieved by a detector system at the periphery thereof. <IMAGE>

Description

SPECIFICATION Optical position detector The invention relates to a device for detecting the position of an object, which device comprises a a radiation source and a radiation-sensitive detection system, which system comprises at least two detectors which are arranged in line with each other in a direction of movement of the object, the radiation distribution on the detection system being a measure of a deviation between the actual and the desired position of the object.

United States Patent Specification 3,207,904 describes a system for positioning the contact stripes of a transistor. The reflecting stripes are then illuminated by a radiation beam and imaged onto a radiation-sensitive detection system by means of a microscope objective. This detection system comprises four quadrants, each quadrant comprising a mask behind which a rndiatonsenstive element such as a photo-conductor or a radiation sensitive semiconductor element is arranged. A comparison of the output signals of said radiation-sensitive elements enables the position of the contact stripes to be measured in two orthogonal directions and the angular position of the transistor to be detected.

Optical position detection equipment is employed in many fields, inter alia in apparatus for projecting a mask pattern onto a substrate, which apparatus is used in the fabrication of integrated circuits. This apparatus comprises a rotatable mask table on which different masks can be arranged which are to be imaged during the consecutive process steps. Furthermore, the apparatus comprises a so-called substrate changer, by means of which an exposed substrate is removed from the apparatus and a substrate to be exposed can be placed into the apparatus. For positioning both the substrate changer and the mask table an optical position-detection device may be employed.

For these and other uses of an optical position detection device for positioning rapidly moving objects with great accuracy, an early indication is required that the object to be positioned is approaching its desired position or angular position, so that the speed with which the object is moved can be reduced and the desired linear or angular position reached with the desired accuracy.

It is an object of the present invention to provide such a device. The invention provides a device for detecting the position of an object, which device comprises a radiation source, an optical prism, and a radiation-sensitive detection system, which system comprises at least two detectors which are arranged in line with each other in the direction of movement of the object, the radiation distribution on the detection system being determined by said prism as a measure of a deviation between the actual and the desired position of the object, and which prism comprises at least one refracting edge which is disposed transversely opposite the direction of movement of the object, the angle between the prism surfaces enclosing said edge being substantially greater than 900.

Owing to the large apex angle, which is for example of the order of 1650, a given displacement of the prism and the radiation beam relative to each other results in a substantially smaller displacement of the beam over the radiation-sensitive detection system, so that for larger displacements of the prism relative to the radiation beam a sufficient amount of radiation still reaches the detection system. It can be demonstrated that when a radiation-transmitting or a reflecting prism is used under certain circumstances the lock-in range can be extended by a factor of 2 1 ~our~ a2 2a2 respectively, a being the base angle of the prism expressed in radians.

For determining the position of an object in one direction a prism with two inclined surfaces and two radiation-sensitive detectors is employed. If the position of an object is to be determined in two orthogonal directions a prism with four inclined surfaces and four detectors should be used.

It is to be noted that it is known per se from United States Patent Specification 2,703,505 to employ a reflecting prism in a device for positioning an object, which prism serves for splitting a radiation beam into two sub-beams which are reflected to a detector, the radiation distribution on the detectors being a measure of the deviation between the actual and the desired position of the object. However, the purpose of this prism is not to extend the lockin range. The detectors, which are for example photo-tubes, have a comparatively large radiation-sensitive surface.

The apex angle of the prism in the known device is approximately 900 and the base angle of said prism is approximately 450.

A device for detecting the angular position of a rotatable object may comprise, between the radiation source and the prism, a lens whose focal length is equal to the distance between the axis of rotation and the lens. The lens ensures that the chief ray of the beam is always incident on the prism at the same angle. Said lens may be a cylindrical lens whose cylinder generating line is parallel to the axis of rotation of the object.

Preferably, the prism is connected to the object whose position is to be detected and the other elements of the detection device are arranged to be stationary.

In order to obtain a compact construction of the detection device the prism is preferably a reflecting prism.

A maximum lock-in range and a compact construction are obtained in the case of a reflecting prism if the chief ray of the beam which is emitted by the radiation source makes an angle which differs from 900 with the refracting edge of the prism.

In order to concentrate the beams which are reflected by the prism use can be made of separate optical elements such as optical fibres, lenses and the like. Preferably, in a device in accordance with the invention mirrors are arranged between an incident beam lens system, which forms a radiation spot on the prism, and the prism which mirrors reflect the beams reflected by the prism to the pupil of the lens system. The incident beam lens system is then also used for focusing the reflected beams on the detectors.

If, in addition, mirrors are arranged between the radiation source and the incident beam lens system images of the radiation source are formed near said source, permitting the detectors and the source, which may be light-emitting diode (LED), to be arranged on one support.

The device in accordance with the invention may be used with advantage in an apparatus for imaging a mask pattern onto a substrate, which apparatus comprises a mask table and a substrate changer, which should be positioned rapidly and accurately. In such apparatus the detector elements of a first position-detection device are connected to position the mask table and the detector elements of a second position-detection device are connected to position the substrate changer.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a known device for detecting the position of an object, Figure 2 shows the radiation-sensitive detection system employed in said device, Figure 3 represents the variation of the position signal as a function of the displacement of the object in said device, Figures 4 and 5 show devices in accordance with the invention, comprising a radiationtransmitting and a radiation-reflecting prism respectively, Figure 6 shows a reflecting prism with inclined surfaces for use in a device in accordance with the invention, Figures 7 and 8 represent the beam deflection as a function of the displacement of the object for a radiation-transmitting and a reflecting prism respectively, Figure 9 represents the variation of the position signal as a function of the displacement in a device in accordance with the invention, Figures 1 ova, 1 Ob and 1 Oc show different optical elements for concentrating the radiation reflected by a prism onto the detectors, Figures 1 1 and 12 show embodiments of a device for detecting the position of a rotatable object, Figure 13 shows an apparatus for projecting a mask pattern on a substrate, Figures 1 4a and 1 4b represent the radiation path in the case of a reflecting prism, if the chief ray of the exposure beam is incident on the refracting edge at a right angle and at an angle which differs from 900 respectively, Figure 15 shows an embodiment of a positiondetection device comprising additional mirrors for the reflected beams, Figure 16 shows the arrangement of the detectors relative to the radiation source in an embodiment of the position-detection device and Figure 17 shows an embodiment of the position detection device in which one element performs the functions of the exposure lens system and the mirrors.

Figure 1 represents the principle of a known device for detecting the position of an object relative to a reference position. Said device comprises a source B, which emits a beam b. A lens system L concentrates the radiation from the source in the plane of a detection system D. The lens system L may form a sharp image B' of the source B, but this is not necessary. If the position of the object is to be determined in only one direction, for example the direction x in Figure 1, the detection system D comprises two detectors D, and D,, for example photo diodes, whose separating line is normal to the direction x. The output signals of these detectors are applied to the inputs of a differential amplifier A, whose output signal is a measure of the deviation between the desired and the actual position of the object, which bears the reference V in Figure 1.Said object is connected to one or more elements of the position-detection device, for example, as is shown in Figure 1, to the lens system L. The output signal of the differential amplifier A is applied to an object control device, which is schematically represented by the block C in Flgure 1.

If the object is in the correct position, the image B' of the source B is situated symmetrically relative to the detectors D, and D2 and the output signals of these detectors are equal. If the object has shifted to the left or the right relative to the desired position, the output signals of the detectors D1 and D2 are unequal. As a result of this, the control device will move the object to the right or to the left respectively until the detector signals are equal.

If the position of the object is to be determined in two orthogonal directions, the detection system D should comprise four detectors D1, D2, D3 and D4, as is shown in Figure 2. The detectors D3 and D4 are connected to a second differential amplifier, not shown, whose output signal is applied to a second control device, not shown, for moving the object transversely of the direction x.

Figure 3 represents the variation of the output signal Es of the differential amplifier A as a function of the position S. The position SO is the desired position of the object. If the object is in the position S, or S2, the image B' is situated entirely above one of the detectors. The magnitude of the positioning range p is approximately equal to M times e, M being the magnification of the lens system L and e the diameter of the radiation source B. If the object is in position S3 or S4, the chief ray of the beam b is incident on the outer edge of a detector. If the object moves further outwards, the beam b falls entirely beyond the detectors and position detection is no longer possible. The range between S3 and S4 is the lock-in range.

For a number of uses, in general those where rapidly moving objects or objects having a large mass are to be positioned with great accuracy, it is necessary to have an early indication that the object is approaching the positioning range in order to achieve stable control and to prevent overshooting of the object. Steps may then be taken to control the speed of the object in due time, in such a way that the positioning range is reached with the desired, comparatively low, speed. For this purpose, means are provided to detect whether one of the limits of the lock-in range, that is one of the outer edges at the points S3 and S4 in Figure 3 of the curve for Es, has been reached on the way to SO. In the said cases it is therefore desirable to have a maximum lock-in range I.

The lock-in range of the known device is limited by the width d of the detectors or by the vignetting which may occur in the case of large displacements. In cases in which the source B or detection system D are connected to the movable object, the lock-in range I is 2d. If the detectors Dr and D2 are photo-diodes, d will be comparatively small, for example 2 mm, and the beam b will soon fall outside the range of the detection system D. If the lens system L is connected to the object, the lock-in range I (assuming that no vignetting occurs) is given by: I=2d/M, M being the magnification of the lens system. The lock-in range may then be extended by reducing M.

In accordance with the present invention the lock-in range, for a specific positioning range, can be extended by arranging a prism P having a small base angle cg and a large apex angle ss in the radiation path. Figures 4 and 5 schematically represent embodiments of a device in accordance with the invention comprising a radiationtransmitting and a radiation-reflecting prism respectively. In comparison with a radiationtransmitting prism a reflecting prism has the advantage that the detection device can be more compact and that the object configuration is less constrained by optical requirements.

In the devices in accordance with Figures 4 and 5 the radiation spot formed by the lens system is divided by the prism P. The sub-beams b, and b2 thus formed are received by the photo-diodes D, and D2. The locations where the sub-beams are incident on the photo-diodes are now irrelevant, provided that the chief rays of said beams are incident on the photo-diodes.

If the position of an object is to be determined in two orthogonal directions, a prism in accordance with Figure 6, that is a prism having four inclined surfaces, should be used. Each of said surfaces reflects a sub-beam (b,, b2, b3 and b4) to an associated detector (D1, D2, D3 and D4).

Preferably, the prism P is connected to the movable object and the source B, the lens system L and the detection system D are stationary.

However, it is alternatively possible to arrange the prism 2 to be stationary and to connect the other elements B, L and D, to the movable object.

The device in accordance with the invention employs the fact that in the case of a specific linear displacement of the prism P relative to the beam b and where the angle at which the subbeams are deflected remains constant, the subbeams are subject to a substantially smaller lateral displacement than with prior art methods.

Thus, even in the case of larger deviations between the actual and the desired position of the object the sub-beams will be incident on the detectors. Since the displacement of the subbeams is smaller than the displacement of the prism relative to the incident beam b, it is less likely that problems will occur with respect to vignetting, with reception of the beams by the detectors, or with re-imaging on said detectors.

Figure 7, in the case of a radiation transmitting prism, shows how far a sub-beam is moved in the case of a displacement of the prism over a distance s. The deviation S, or the angular difference between the beam which is incident on the prism P and a beam which is deviated by the prism, is given by the well-known law of refraction: nsin a=n, sin a, where a is the angle of incidence of the beam on an inclined surface of the prism and a, the angle between the deviated beam and the normal to said plane, whilst n and n, are the refractive index of the prism material and air respectively.For small angles cr and ar the following equation is then valid: na=a, (for n,=1 ) The deviation 6=(a,-a) is then #=(n-1 )a.

For a glass prism for which n==1 .5, which yields (5=0.5a. Figure 7 clearly shows that for the beam shift A=h. sin S in which h=x. tan a so that for small angles a and S: A=S.a.#.=0.5a2s, so that 2h s= a2 In the case of a l-to-l image of the source the shift A should not exceed the width d of the photodiode, so that the maximum displacement sax If the object to the left or to the right that can still be detected is: d sax=2 a2 in a position detection device without prism the maximum shift that can be still measured is equal to d.The extension of the lock-in range is therefore approximately 2/a2 where a is expressed in radians. Since for a prism of small base angle, i.e. a small angle wedge, the deviation S is substantially constant independently of the position of the prism, the foregoing also applies to the situation in which the beam b is first incident on the inclined surfaces of the prism.

Figure 8 shows how much, in the case of a reflecting prism, a sub-beam is shifted for a displacement of the prism relative to the exposure beam over a distance s. The following equation is now valid for the shift A: A=h' sin 2~, where h'=s.tan a, so that for a small angle : A A=s.2a2 and s= 2a2 The extension of the pull-in range is approximately 2a2 in the case of a 1 -to-1 image of the source.

Figure 9 schematically represents the extension of the lock-in range Ii is the lock-in range of the device without prism and 12 that of the device comprising a prism. In an embodiment using a reflecting prism whose base angle a is approximately 6.50, 12 may in principle be approximately 37xi,.

Figures 7 and 8 only show the chief rays of the beams. The incident beam is of a converging beam which forms a radiation spot at the location of the prism. The prism derives two or four diverging beams from said incident beam. In order to avoid an excessive beam diameter at the location of the detectors, which would necessitate the use of large detector surfaces, various steps may be taken. Preferably, as is indicated in Figure 1 ova, two additional lenses L, and L2 are arranged between the prism and the photo-diodes, which lenses image the radiation spot onto the photo-diodes.

It is alternatively possible to catch the subbeams b1 and b2 with two optical fibre guides F, and F2, which are arranged near the prism and which for example have a large opening at the location of the prism and a small opening at the location of the photo-diodes. Figure 1 Ob shows an embodiment using such optical fibre guides.

Furthermore, as shown in Figure 1 Oc, two light diffuser H, and H2 may be arranged behind the prisms, which diffusers ensure that, if a sub-beam is incident on them, some radiation is incident on the associated detector. In order to increase the amount of radiation on the detectors D, and D2, lenses L3 and L4 may be arranged between the radiation diffusers H, and H2 and the detectors D1 and D2. Steps may be taken to ensure that in the arrangements in accordance with Figures 1 ova, 1 Ob and I Oc the sub-beams do not fill the pupils completely, so that, in the case of a displacement of the prism no vignetting occurs, because the displacement of the subbeams is then only a fraction of the pupil diameter.

So far, it has been assumed that the object to be positioned moves along a line. However, the device may also be used for detecting the angular position of a rotary object. In that case an additional step should be taken in order to ensure that the beam is always incident on the prism at the same angle. For this purpose, a lens is arranged near the prism, which lens ensures that the chief ray of incident beam is always parallel to the imaginary connecting line between the axis of rotation of the prism and the refracting edge of the prism. The lens may be a cylindrical lens, because the lens action is required in one direction only.

Figures 1 1 and 12 show two embodiments of a device comprising a reflecting prism for detecting the angular position of a rotary object.

In these figures the axis of rotation of the object V is designated RA. R is the connecting line between the axis of rotation and the apex of the prism P and Cl is a cylindrical lens. The cylinder generating line CA of said lens should be parallel to RA. The focal length fcL of this lens should be equal to the distance r between the centre of the cylindrical lens and the axis of RA.

If, as is shown in Figure 11, the prism is arranged on the object V and the remainder of the optical system, that is the radiation source, the lens system and the detectors, are arranged stationary in the housing H, the cylindrical lens is a negative lens for which fCL=~r. If the prism P is stationary and the remainder of the optical system moves with the object, as shown in Figure 12, the cylindrical lens is a positive lens whose focal length is fc#=+r.

The embodiment shown in Figure 1 1 may be used in an apparatus for the repeated imaging of a mask pattern on a substrate in the fabrication of integrated circuits. The substrate is then exposed a large number of times via the mask pattern, the substrate being shifted relative to the projection system over a desired distance between two consecutive exposures. After the entire substrate has been exposed via the mask pattern, the substrate is removed from the apparatus, to be subjected to further process steps. Subsequently, a new substrated may be placed in the apparatus, which substrate may be exposed via the same or a different mask pattern. In order to remove such an exposed substrate and to place an unexposed substrate into the apparatus. a so-called substrate change is employed.

An integrated circuit is fabricated in a number of process steps in which, consecutively, different mask patterns are imaged onto a substrate. In order to place the various mask patterns in the apparatus a so-called mask table is employed, in which a plurality, for example two, of mask patterns can be placed. Both the substrate changer and the mask table can be positioned with the aid of a device in accordance with the invention.

Figure 13 shows an embodiment of an apparatus for the repeated imaging of a mask pattern on a substrate. A projection system, for example comprising a mercury lamp LA, an elliptical mirror EM, an element In which provides a homogeneous radiation distribution within the projection beam, and a condenser lens CO, illuminates a mask pattern M" which is arranged on a mask table MT. On said table a second mask pattern M2 may be placed. By rotating the table about the axis MA the second mask pattern can be moved into the projection beam. The beam which passes through the mask pattern M, traverses a projection lens system PL, which is represented schematically and which forms an image of the mask pattern on the substrate. The substrate W rests on an air-cushioned substrate table WT.The projection lens system PL and the substrate table are mounted in a housing HO, which is closed at the bottom by a, for example granite, base plate BP and at the top by the mask table. The apparatus further comprises a substrate change WC, which is rotatable about an axis WA and which is moreover adjustable in height. For further details as regards the construction and operation of the projection apparatus, reference is made to the article "Stepand-Repeat Wafer Imaging", in: "Solid State Technology" June 1980, pages 80-84.

In order to enable the mask table to be positioned when a mask is placed in the projection beam, a reflecting prism Pa and a negative lens L5 are arranged in accordance with the invention, on the mask table. The prism reflects a beam emitted by a radiationsource/detection unit H, to said unit, in which the beam is received by the detection, system comprising two detectors, in the manner~ described in the foregoing. The difference signal of the two detectors is used to position the mask table by means which are known per se and which are not described in further detail. The use of the position detection system (P,, L5 and H,) provides a signal, at an early instant, that the mask table is approaching the desired position, so that the speed of rotation of the table can be reduced and the desired position is reached at a sufficiently low speed.Diametrically opposite to the first prism-lens pair (P1. L5 a second prism-lens pair (P2, L6) may be provided for positioning the second mask pattern M2.

For positioning the substrate changer WC there is provided an analog position-detection system, comprising a prism P3, a lens L7 and a radiationsource/detention unit H2. The prism P3 and the lens L, are stationary, whilst the radiationsource/detection unit is arranged on the moving changer, in order to obtain a compact construction. The lens is a positive lens.

In the position-detection device comprising a reflecting prism, in which the chief ray of the incident beam is perpendicularly incident on the refracting edge of the prism, the chief rays of the 2' hown in Figures reflected beams b1 and b as is shownin 1 4a, are situated one on each side of the incident beam b, whilst moreover the chief rays of all beams are situated in one plane. Care must be taken that the reflected beams do not overlap the incident beam. For a correct separation of the beams the base angle a of the reflecting prism should comply with the condition; sinx > x" where a1 is the aperture angle of the incident beam system on the prism side. For a large lock-in range a should be minimal, so that in practice a will be substantially equal to (x,.

In order to obtain a more compact construction and a larger lock-in range of the positiondetection device, the prism p is preferably slightly inclined relative to the chief ray of the incident beam b. As is shown in figure 1 4b, the chief rays of the reflected beams are then situated in a different plane than the chief ray of the incident beam b. The reflected beams b1 and b2 may then be situated nearer to each other and the base angle a' may be smaller than the base angle a in Figure 10a.

The reflected beams b1 and b2 can be focused onto the photo-diodes by lenses, in which case one lens and the associated photo-diode may constitute one unit.

In one embodiment of the device in accordance with Figure 1 4b the lock-in range was approximately +10 mm, which range is determined by the dimensions of the prism P.

In a position-detection device having a reflecting prism the incident beam lens system L may also be used for focusing the beams which are reflected by the prism P onto the detectors.

For this purpose an additional mirror AM for each reflected beam should be arranged between the prism P and the lens system L, as is shown in Figure 1 5. For the sake of clarity said Figure only shows the radiation path of the beam b1, which is reflected by the upper part of the prism P and is subsequently directed to the detector D1 by the upper mirror AM. The chief rays of the beams b and b, are represented by broken lines.

The radiation spots formed by means of a prism P and the mirrors AM are separated from the source B, so that the detectors may be arranged around said source. In the case of a prism having four inclined surfaces and four auxiliary mirrors AM the four detectors D1, D2, D3 and D4 are positioned in the plane of the radiation source B as is shown in Figure 16.

The separation between the source and the images of said source is given by the angle O in Figure 1 5. The angle P at which the mirrors AM should be arranged relative to the optic axis is determined by the aperture angle y of the lens system L on the image side in accordance with the relation fl (p=y 2 in the case that the beams do not overlap each other immediately after reflection. The base angle of the reflecting prism should then be equal to At the location of the lens system L the distance between the mirrors AM and the optical axis is then equal to 1, 1 being the maximum pupil radius of the system L.

The mirrors Am should extend so far that the reflected beams can be received, when said beams are shifted parallel to themselves when the prism is displaced. The sub-beams b, and b2 should then also be capable of moving over the pupil of the incident beam system L. Since said pupil is filled completely by the sub-beams when the prism occupies the centre position, said displacement may give rise to loss of radiation. In order to mitigate such a loss a field lens FL, represented by the broken lines in Figure 15, may be arranged between mirrors Am and the prism P.

The focal length of the lens FL is equal to the distance of said lens to the pupil of the incident beam system L. The reflected beams then always pass through the centre of the pupil of L.

By also arranging auxiliary mirrors AM' between the source B and the lens system L in Figure 15, the images of the source can be placed very close to the source itself. The radiation source B, in the form of a light-emitting diode, and the photo-diodes may then be arranged on one support. The mirrors AM' are arranged at an angle with the optical axis, which is equal to or substantially equal to the angle y.

The mirrors AM may be formed by a block of a material, for example glass, which may have a square cross-section. The two, or four, reflected sub-beams are reflected by the side faces of said block, in which case total internal reflection may be utilized if the sub-beams are incident on said faces at sufficiently large angles. The side faces may be internally or externally reflecting. On the one end of the block lens system L is located and on the other end of the field lens FL.

Preferably, as is shown in figure 17, the end faces LS, and LS2 of the block ML are curved, so that said surfaces have a lens action. The surface LS, functions as an incident beam lens L and the surface LS2 as a field lens (FL). The lens element LS2 has a positive power and its focal length is equal to the length of the block.

Claims

1. A device for detecting the position of an object, which device comprises a radiation source, an optical prism, and a radiation-sensitive detection system which system comprises at least two detectors which are arranged in line with each other in the direction of movement of the object, the radiation distribution on the detection system being determined by said prism as a measure of deviation between the actual and the desired position of the object, and which prism comprises at least one refracting edge which is disposed tranversely opposite the direction of movement of the object, the angle between the prism surfaces enclosing said edge being substantially greater than 900.

2. A device as claimed in Claim 1, for determining the angular position of a rotatable object, wherein between the radiation source and the prism a lens is arranged, whose focal length is equal to the distance between the axis of rotation and the lens.

3. A device as claimed in Claim 1 or Claim 2, wherein the prism is connected to the object.

4. A device as claimed in any one of Claims 1, 2 or 3, wherein the prism is a reflecting prism.

5. A device as claimed in Claim 4, wherein the chief ray of the beam emitted by the radiation source makes an angle which differs from 900 with the refracting edge of the prism.

6. A device as claimed in Claim 4 or Claim 5, wherein mirrors are arranged between an incident beam lens system, which forms a radiation spot on the prism, and the prism which mirrors reflect the beams reflected by the prism into the pupil of the lens system.

7. A device as claimed in Claim 6, wherein mirrors are arranged between the radiation source and the incident beam lens system, which mirrors reflect the reflected beams which pass through the lens system to the vicinity of the radiation source, the radiation-sensitive detectors being arranged around said source.

8. A device as claimed in Claim 6, wherein between the radiation source and the prism a hollow reflecting body of square cross-section is arranged, the end faces of said body acting as lenses.

9. Apparatus for imaging a mask pattern onto a substrate, which apparatus comprises a mask table, an exposure system, a projection lens system, a substrate table, and a substrate changer and which is furthermore provided with positiondetection devices as claimed in any one of the preceding Claims, the detector elements of a first position-detection device being connected to position the mask table and the detector elements of a second position-detection device being connected to position the substrate changer.

10. A device for detecting the position of an object substantially as described with reference to Figure 4 of the accompanying drawings.

1 1. A device for detecting the position of an object substantially as described with reference to Figure 5 or Figure 6 of the accompanying drawings.

12. A device for detecting the position of an object substantially as described with reference to Figure 1 Oa or Figure 1 Ob or Figure 1 Oc of the accompanying drawings.

13. A device for detecting the angular position of an object substantially as described with reference to Figure 1 1 or Figure 12 of the accompanying drawings.

14. A device for detecting the position of an object substantially as described with reference to Figures 14 and

1 5 of the accompanying drawings.

1 5. A device for detecting the position of an object substantially as described with reference to Figures 6 and 16 of the accompanying drawings.

16. A device for detecting the position of an object substantially as described with reference to Figure 17 of the accompanying drawings.

17. Apparatus for imaging a mask pattern onto a substrate substantially as described with reference to Figure 17 of the accompanying drawings.