CA1139441A - Optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed - Google Patents
Optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formedInfo
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- CA1139441A CA1139441A CA000353625A CA353625A CA1139441A CA 1139441 A CA1139441 A CA 1139441A CA 000353625 A CA000353625 A CA 000353625A CA 353625 A CA353625 A CA 353625A CA 1139441 A CA1139441 A CA 1139441A
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- plane
- imaging system
- detectors
- radiation
- image
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F9/00—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically
- G03F9/70—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically for microlithography
- G03F9/7003—Alignment type or strategy, e.g. leveling, global alignment
- G03F9/7023—Aligning or positioning in direction perpendicular to substrate surface
- G03F9/7026—Focusing
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
- G03F7/70808—Construction details, e.g. housing, load-lock, seals or windows for passing light in or out of apparatus
- G03F7/70825—Mounting of individual elements, e.g. mounts, holders or supports
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Engineering & Computer Science (AREA)
- Environmental & Geological Engineering (AREA)
- Epidemiology (AREA)
- Public Health (AREA)
- Length Measuring Devices By Optical Means (AREA)
- Automatic Focus Adjustment (AREA)
- Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)
- Exposure Of Semiconductors, Excluding Electron Or Ion Beam Exposure (AREA)
Abstract
15.1.80 1 PHN 9487 ABSTRACT:
Optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed.
An optical imaging system is described which is provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed by the imaging system. After a first reflection on the second plane an auxiliary beam which is obliquely incident on said plane is reflected along itself and mirror-inverted, is sub-sequently reflected a second time on the second plane, and is finally incident on two detectors. The dif-ference signal of the detectors, which is a measure of the said deviation, is independent of a tilted position of the second plane and of local differences in reflection in said plane.
Optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed.
An optical imaging system is described which is provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed by the imaging system. After a first reflection on the second plane an auxiliary beam which is obliquely incident on said plane is reflected along itself and mirror-inverted, is sub-sequently reflected a second time on the second plane, and is finally incident on two detectors. The dif-ference signal of the detectors, which is a measure of the said deviation, is independent of a tilted position of the second plane and of local differences in reflection in said plane.
Description
~394 , PHN 9487 1 10 april 1980 Optical imaging system provided with an opto-electronic detection system for determining a deviation betwecn the image plane of the imaging system and a second plane on which an image is to be formed.
The invention relates to an optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the - imaging system and a second plane on which an image is to S be formed by the imaging system, which detection system comprises a radiation source which produces an auxiliary beam, a reflection element which is disposed in the path of the auxiliary beam which has been reflected a first time by the second plane and which directs the auxiliary beam 10 again towards the second plane, and two radiation-sensitive detectors disposed in the path of the auxiliary beam which has been reflected twice by the second plane, the detectors and the reflection element being connected to the imaging system and the difference between the output 15 signals of the two detectors being a measure of the said deviation.
Lens systems with which small details are to be imaged have a large numerical aperture and thus a small depth of focus. For this type of lens 20 systems, which are for example employed for imaging ~ a mask pattern on a substrate for the manufacture ; of integrated circuits, it is of importance that a deviation between the actual plane of imaging, of for example the mask pattern, and the plane on 25 which the image is to be formed, for example the .,j .
substrate, can be detected so as to enable the lens system to be re-adjusted by means thereof.
It is possible to determine said deviation in a capacitive way. The lens systemshould then be 30 connected to a metal plate and the surface of the ; substrate should be metallized. The variation of the ;` capacitance constituted by the metal plate and the ~ , ' '~
.
`
;. ::. ,. - ;
ll~3~4~
.
PHN 9487 2 10 april 1980 metallized surface i8 a measure of the said deviation.
Said deviation could also be measured with an airpressure sensor. The drawback of these methods is that the distance measurement must be effected outside the field of the - 5 imaging system. Furthermore, the distance between the ; capacitive sensor or the air-pressure sensor and the second plane, for example the surface of the substrate, on which the image is to be formed is very small, so that the occurrence of damage to the sensor or the substrate surface 10 is not unlikely. An optical method of determining the said deviation is more attractive, because in that case measure-' ment is possible in the centre of the field of the imaging `~ system and the detection elements can be arranged at a comparatively large distance from the plane on which 15 the image is to be formed.
United States Patent Specification No. 3,264,935describes how such an optical method may be employed in a slide projector. An auxiliary beam is then sized at the slide to be projected at a large angle of incidence. The angle of 20 incidence is the angle between the auxiliary beam and the optic~al axis of the projection lens sytem. The auxiliary beam which is reflected by the slide is incident on the plane of two reflection-sensitive detectors. When the slide is moved along the optical axis of the projection lens 25 system the auxiliary beam will travel over the detector.
By comparison of the electric output signals of said detectors an indication is obtained of the magnitude and direction of a deviation between the actual and the desired position of the slide.
In order to prevent that the difference signal of the detectors depends on a tilted position of the slide, the detection system in accordance with United States Patent Specification No. 3,264,935 includes a reflection element in the radiation path of the auxiliary bcam which .~ . . .
has been reflected for a first time by the slide, which element again directs the auxiliary beam towards the slide.
The detectors are disposed in the radiation path of the auxiliary beam which has been reflected twice by the slide.
.
PHN 9487 3 10 april 1980 Such an imaging system may also be employed in a device for imaging a mask pattern on a semiconductor substrate for the manufacture of integrated circuits.
Especially for such an application the dctection system exhibits the drawback it is sensitive to local differences in reflection of the substrate. An integrated circuit is formed in a number of process steps, in which consecutively the various mask patterns are to be imaged on the substrate.
When imaging a mask pattern the structures already formed 10 on the substrate in a previous process step may cause scattering. As a result of this scattering the two halves of the radiation spot formed on the two detectors may have different intensities, whilst the distance between the substrate and the imaging lens system is correct.
It is the object of the present invention to provide a detection system which does not have the said drawback. The system in accordance with the invention is ~`^ characteri~ed in that there is provided a first lens system (L2) for focussing a narrow auxiliary beam (b) to a small 20 radiation spot (V) on the second plane (F), that there is provided a second lens system for imaging the said radiation spot on the reflection element, and that the reflection element is a beam-reversing element (retroreflector) (r. L3), by which the beam is reflected along itself and is focussed 25 by the second lens system (L3) in the radiation spot (V) formed when the auxiliary beam is incident on the second plane for a first time.
The auxiliary beam being reflected "along itself"
is to be understood to mean that the chief ray of the reflec-30 ted auxiliary beam coincides with the chief ray of theauxiliary beam which is incident on the beam inverting element, and that the light rays which in said incident auxiliary beam form part of the first and the second beam half-respectively form part of the second and the 35 first beam half in the reflected auxiliary beam. Of the auxiliary beam which is directed towards the detectors both beam halves have been incident on both halves of the 1~3~
P~N 9487 4 10 april 1980 area on the second plane, or substrate, covered by the radiation spot. As a result of this the intensity of both beam halves is influenced to the same extent by any local differences in reflection in said area of the second 5 plane, so that the intensity difference between the two beam halves is independent of said differences in reflection.
In the detection system in accordance with United States Patent Specification No. 3,264,935 it is merely of 10 importance that the auxiliary beam is reflected twice by the sl,ide. It is not necessary that the auxiliary beam impinges twice on the slide at exactly the same location. Neither is the size of the auxiliary beam of importance. For the detection system in accordance with the invention, however, 15 it is essential that a narrow auxiliary beam impinges twice in precisely the same area of the second plane.
The detection system in accordance with the inven-tion may be employed to great advantage in apparatus for the repeated imaging of a mask pattern on a substrate, 20 which apparatus comprises a projection lens system arranged between a mask-pattern holder and a substrate holder, the detectionsystem being employed for determining a deviation between the image plane of the projection lens system and the substrate. In addltion, the lnventicn -----------------------_ . .
~ .
.
PilN 9487 5 10 ap~il 1980 is generally applicable in imaging systems where localdifferences in reflection may occur in the plane in which the image is to be formed, or where said plane may exhibit tilting. Examples of this are microscopes, both reflection microscopes and transmission micros-copes. Because of` the oblique incidence of the auxi-liary beam, a transparent object will also reflect an amount of radiation which is sufficient to be used for the detection of a variation in -the distance between the object to be examined and the microscope objective.
The invention will now be described in more detail, by way of' example, on the basis of a system for irnaging a mask pattern on a substrate. For this reference is made to the drawing7 in which Figure 1, schernatically, represents an imaging system provided with a known opto-electronic detection system, Figure 2 illustrates the principle o* the detection system in accordance with the invention~
and Figure 3 shows an errlbodiTrlellt of such a sys-tem.
In Figure 1 an irnaging system is designat-ed L1. An opto-electronic detection system comprises a radiation source S, which emits an auxiliary beam b, a first lens L2, a second lens L3, and two detec-tors, for example photo-diodes, (hD1 and D2. The detac-tion system is rigidly connected to the lens system L1 via the supports St.
The lens L2 forms a radia-tion spot ~ on the plane p in which an irnage is to be formed by the lens system L1. The lens L3 images said radiation spot on the photo-diodes Dl and D2. Relative to the lens system L1 the detection sys-tem is aligned so that the lens L3 images the intersection of the op-tical axis OO' of the system Ll with the image plane of said system in the plane of the two detectors D1 and D2. If the plane p coincides with the image plane ~3~
PHN 9487 6 10 april 1~80 of the system L1 the radiation spot V is imaged in - the spot V', which is situated symmetrically relative to the detectors D1 and D2. These detectors then re-ceive equal radiation intensities. The output signal Sr of a differential amplifier A which is connected to the detectors D1~and D2 is then for example zero.
In the case of a variation of the distance between the plane p and the lens system L1 the image V' of the ra-diation spot V is moved over the detectors D1 and D2.
If, as is shown in Figure 1, the plane p is moved downwards, the radiation spot V1 formed on said planei8 imaged in V'1 in accordance with the radiation path represented by the dashed lines. The detector D1 then receives a greater radiation intensity than the detector D2 and the signal Sr is for example positive. In the case of an upward movement of the plane p the reverse occurs, and the detector D1 receives a smaller radiation intensity than the detec-tor D2. The signal Sr is then for example negative.
The signal Sr may be applied to a servo-system~ not shown, by means of which the position of the lens : system L1 can be corrected.
` The angle of incidence C~ at which the ~; auxiliary beam b is incident on the plane p is select-ed to be as great as possible, for example of the or-der of 80, in order to obtainaa maximum sensitivity to positional errors of the plane p relative to the lens system L1 and in order to obtain a maximum re-flection of the auxiliary beam by the plane p.
In accordance with the invention the path of the beam b which has been reflected by the plane p includes a beam-invert ng element or retro-reflec-tor, Such a retro-reflector may be constituled by a so-termed l!cats-eyel', which, as is shown in Figure 2, comprises a lens L3 and a mirror r, the mirror being arranged in the focal plane of the lens L3. Such a "cats-eye" reflects a beam along i~self. Figure 2 shows only the part of the path of the beam b in the ~3~
PHN g487 7 10 april 1980 vicinity of the plate p. The beam b forms the radia-tion spot V on said plane. The beam that has been re-flected by the plane p is directed to the mirror r by the reflection prism P4. After reflection at this S mirror the beam b traverses the same path in the re-verse direction.
A small portion b' of the beam b is shown dark in order to indicate how this part traverses the radiation path. Upon the first arrival at the plane p this part is a portion of the beam half b1. The beam portion b' is then situated in the right-hand part Vr of the radiation spot V. After reflection at the plane p and at the reflection prism P4 the beam portion b' passes through the left-hand part of the lens L3. Upon reflection at the mirror r the beam portion b' passes through the right-hand part of the lens L3 and i8 then situated in the left-hand part V
in the radiation spot V. After double reflection at the plane p the beam portion b' has become part of the beam half b2. This applies to all beam portions form-ing the bea~half b1. Thus, after double reflection at the plane p the beam half bl has been in contact both with the right-hand part and with the left-hand part ~of the area on the plane p underneath the ra-; 25 diation spot V. Obviously, the same applies to the , beam half b2. A~ a result of this neither local dif-ferences in reflection as a result of scattering structures in the plane p nor irregularities in this plane or an oblique position of this plane relative to the lens ~ystem Ll can affect the intensity dis-tribution over the beam halves b1 and b2.
The detection system in accordance with the invention may alternatively employ a concave mirroP-or a combination of a plane mirror and a cylindrical lens as inverting element; with a magni-fication of -1.
Figure 3 shows an embodiment of a detection ; system in accordance with the invention for use in an ~13~4~
apparatus for imaging mask patterns on a substrate. The surface of the substrate is represented by the plane ~ and the projection lens system by Ll. This system is movable along its optical axis 00' by drive means, known per se and not shown, which are controlled by the signal Sr, derived from the signals, from the two detectors. In directions perpendicular to the optical axis 00' the projection lens system is immobile in its housing H. As is indicated in Figure 3, the projection lens system may be secured in the housing A by means of springs Sp. Instead of springs it is alter-natively possible to employ diaphragms as fixing means.
In an apparatus for the repeated imaging of a mask pattern on a substrate the projection column and the substrate are movable relative to each other in two mutually perpendicular directions. For more details about such an apparatus reference is made to Canadian Patent No. 1,078,240 of Bouwhuis, et al, issued May 22, 1980. Furthermore, it is to be noted that in such a device the image plane of the projection lens system sub-stantially coincides with the focal plane of this system.
In order to ensure that also for small displacements of the plane ~ relative to the lens system Ll a sufficiently large signal Sr, ; i.e. a sufficiently large difference between the radiation intensities on the two detectors, is obtained, the radiation spot V should have a high brightness. Therefore, a laser is preferably used as radiation source S.
The radiation beam b produced by the laser should be stable. Preferably, a semiconductor-diode laser, DL in Figure 3, for example, an AlGaAs diode laser is used, which can be arranged close to the substrate. It is alter-natively possible to employ a gas laser, which is arranged at a larger distance from the substrate, the radiation of said ~ -8-,4~.
PHN 9487 9 10 april 1980 laser being passed to the substrate via a light-conducting fibre.
The laser beam b is converted into a parallel beam by the lens L4 and is subsequently directed to the substrate surface p via reflection at the beam splitter 8S and the reflection prisms P2 and P3. The lens L2 forms the radiation spot V on the substrate. Subsequently the beam b traverses the radiation path described with reference to Figure 2. A part of the beam b which has been reflected twice by the substrate surface is transmitted to the two detectors Dl and D2 by the beam splitter BS. The lens L5 forms an image of the radiation spot V on these detectors.
The beam splitter BS may comprise the semi-transparent mirror of a semitransparent prism. Instead, it is alternatively possible to employ a polarization splitting prism, the radiation path between said prism and the substrate including a ~ plate, where ~ represents the wavelength of the beam b. The radiation from the source DL is then polarized in such a way that it is reflected by the polari-zation splitting prism. The beam b then traverses the ~
plate twice, as a result of which the plane of polarization of the radiation, that is reflected twice by the substrate 3 i5 rotated through 90 in total, so that the beam is then transmitted by the prism.
The difference signal Sr of the two detectors Dl and D2 is primarily determined by the distance between the image plane of the projection lens sytem Ll and the plane p. However, this signal is also dependent on the total intensity of the auxiliary beam b which has been reflected twice by the plane p. This intensity may vary as a result of a variation in the radiation-source intensity or as a result of variations in the reflection or transmission coefficients of the optical elements in the radiation path.
In order to eliminate the effect of variations in the total beam i~tensity which is incident on the detectors Dl and D2, the output signals of the detectors may be added, yielding a sum signal St. In an analog divider circuit the signal Slr = Sr/St can then be derived, which signal is a .
.
1~13S~
PHN 9487 10 10 apri] 1980 measure of the positional error of the plane p of the substrate relative to the projection lens system and which is independent of intensity variations of the radiation source and of variations in the reflection or transmission coefficients in the radiation path. In order to ensure that also in the case of poor reflection or transmission coeffi-cients an amount of radiation is obtained on the detectors which is sufficient for the required accuracy, the radiation source should be set to a maximum radiation power. This may lead to a reduction of the life of the radiation source, especially if this source is a semiconductor diode laser.
Furthermore, a drift-free analog divider is a comparatively expensive eIement.
The afore-mentioned drawbacks can be avoided, in 15 accordance with a preferred embodiment of the detection system in accordance with the invention, by using the sum signal St for controlling the intensity of the radiation source so that the total radiation intensity which is incident on the detectors Dl and D2 remains constant. If the 20 radiation source is a radiation-emitting diode the magnitude of the electric current with which said radiation source is driven may then be corrected for this purpose.
A semiconductor diode laser, for example an AlGaAs laser, is preferably operated with current pulses, 25 because this is most favourable for the life of such a laser.
However, the angle at which such a laser emits its radiation can vary when the magnitude of the electric current through the laser is varied. The intensity of the radiation which is emitted by a diode laser is suitably corrected by 30 varying the pulse repetition rate of the electric current pulses at constant pulse width.
As is shown in Figure 3, the signals from the detectors Dl and D2 are applied to a subtractor circuit Al, on whose output the signal Sr appears, and to an adder circuit A2, on whose output a signal St is available.
The signal St may for example be applied to an input of a differential amplifier A3, whose other input is connected to a reference source Ref. The output of the differential ~Y'. ~ t`.~
PHN 9487 11 10 april 1980 amplifier is connected to an oscillator VC0, which supplies a train of pulses whose frequency is determined by the voltage on its input. The output of the oscillator is connected to a pulse generator PG. The electric current required for the operation of the diode laser DL is supplied by the pulse generator in the form of pulses of constant duration and with a repetition rate which is equal to that oF the pulses from the oscillator VCOO The average intensity of the diode laser could also be controlled by adapting the lO duration of the elec-tric current pulses instead of adapting the pulse repetiti.on rate.
The setting of the diode laser is such that for the maximum radiation loss to be anticipated in the radiation path the total amount of radiation incident on the 15 detectors just suffices to attain the required accuracy, which is inter alia determined by the leakage current and the noise of the detectors. In the case of smaller radiation losses the radiation source need emit a smaller radiation intensity, which is Favourable for the life of the source.
The displacement ~ of the radiation spot over the photo-diodes Dl and D2 is given by:
~ 4.M.I-I. sin ~ .
In this formu:La H is the displacernent of the plane p, CX the angle of incidence of the focussing beam on 25 said plane, and M the magnification of the lens sytem L5 L2. This magnification is equal to the ratio of the focal length of the lens L5 to that of the lens L2. As the focussing beam has been reflected twice by the plane ~, the sensitivity of the detection system has also increased by a 30 factor 2. As a result of this a factor 4 instead of a factor
The invention relates to an optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the - imaging system and a second plane on which an image is to S be formed by the imaging system, which detection system comprises a radiation source which produces an auxiliary beam, a reflection element which is disposed in the path of the auxiliary beam which has been reflected a first time by the second plane and which directs the auxiliary beam 10 again towards the second plane, and two radiation-sensitive detectors disposed in the path of the auxiliary beam which has been reflected twice by the second plane, the detectors and the reflection element being connected to the imaging system and the difference between the output 15 signals of the two detectors being a measure of the said deviation.
Lens systems with which small details are to be imaged have a large numerical aperture and thus a small depth of focus. For this type of lens 20 systems, which are for example employed for imaging ~ a mask pattern on a substrate for the manufacture ; of integrated circuits, it is of importance that a deviation between the actual plane of imaging, of for example the mask pattern, and the plane on 25 which the image is to be formed, for example the .,j .
substrate, can be detected so as to enable the lens system to be re-adjusted by means thereof.
It is possible to determine said deviation in a capacitive way. The lens systemshould then be 30 connected to a metal plate and the surface of the ; substrate should be metallized. The variation of the ;` capacitance constituted by the metal plate and the ~ , ' '~
.
`
;. ::. ,. - ;
ll~3~4~
.
PHN 9487 2 10 april 1980 metallized surface i8 a measure of the said deviation.
Said deviation could also be measured with an airpressure sensor. The drawback of these methods is that the distance measurement must be effected outside the field of the - 5 imaging system. Furthermore, the distance between the ; capacitive sensor or the air-pressure sensor and the second plane, for example the surface of the substrate, on which the image is to be formed is very small, so that the occurrence of damage to the sensor or the substrate surface 10 is not unlikely. An optical method of determining the said deviation is more attractive, because in that case measure-' ment is possible in the centre of the field of the imaging `~ system and the detection elements can be arranged at a comparatively large distance from the plane on which 15 the image is to be formed.
United States Patent Specification No. 3,264,935describes how such an optical method may be employed in a slide projector. An auxiliary beam is then sized at the slide to be projected at a large angle of incidence. The angle of 20 incidence is the angle between the auxiliary beam and the optic~al axis of the projection lens sytem. The auxiliary beam which is reflected by the slide is incident on the plane of two reflection-sensitive detectors. When the slide is moved along the optical axis of the projection lens 25 system the auxiliary beam will travel over the detector.
By comparison of the electric output signals of said detectors an indication is obtained of the magnitude and direction of a deviation between the actual and the desired position of the slide.
In order to prevent that the difference signal of the detectors depends on a tilted position of the slide, the detection system in accordance with United States Patent Specification No. 3,264,935 includes a reflection element in the radiation path of the auxiliary bcam which .~ . . .
has been reflected for a first time by the slide, which element again directs the auxiliary beam towards the slide.
The detectors are disposed in the radiation path of the auxiliary beam which has been reflected twice by the slide.
.
PHN 9487 3 10 april 1980 Such an imaging system may also be employed in a device for imaging a mask pattern on a semiconductor substrate for the manufacture of integrated circuits.
Especially for such an application the dctection system exhibits the drawback it is sensitive to local differences in reflection of the substrate. An integrated circuit is formed in a number of process steps, in which consecutively the various mask patterns are to be imaged on the substrate.
When imaging a mask pattern the structures already formed 10 on the substrate in a previous process step may cause scattering. As a result of this scattering the two halves of the radiation spot formed on the two detectors may have different intensities, whilst the distance between the substrate and the imaging lens system is correct.
It is the object of the present invention to provide a detection system which does not have the said drawback. The system in accordance with the invention is ~`^ characteri~ed in that there is provided a first lens system (L2) for focussing a narrow auxiliary beam (b) to a small 20 radiation spot (V) on the second plane (F), that there is provided a second lens system for imaging the said radiation spot on the reflection element, and that the reflection element is a beam-reversing element (retroreflector) (r. L3), by which the beam is reflected along itself and is focussed 25 by the second lens system (L3) in the radiation spot (V) formed when the auxiliary beam is incident on the second plane for a first time.
The auxiliary beam being reflected "along itself"
is to be understood to mean that the chief ray of the reflec-30 ted auxiliary beam coincides with the chief ray of theauxiliary beam which is incident on the beam inverting element, and that the light rays which in said incident auxiliary beam form part of the first and the second beam half-respectively form part of the second and the 35 first beam half in the reflected auxiliary beam. Of the auxiliary beam which is directed towards the detectors both beam halves have been incident on both halves of the 1~3~
P~N 9487 4 10 april 1980 area on the second plane, or substrate, covered by the radiation spot. As a result of this the intensity of both beam halves is influenced to the same extent by any local differences in reflection in said area of the second 5 plane, so that the intensity difference between the two beam halves is independent of said differences in reflection.
In the detection system in accordance with United States Patent Specification No. 3,264,935 it is merely of 10 importance that the auxiliary beam is reflected twice by the sl,ide. It is not necessary that the auxiliary beam impinges twice on the slide at exactly the same location. Neither is the size of the auxiliary beam of importance. For the detection system in accordance with the invention, however, 15 it is essential that a narrow auxiliary beam impinges twice in precisely the same area of the second plane.
The detection system in accordance with the inven-tion may be employed to great advantage in apparatus for the repeated imaging of a mask pattern on a substrate, 20 which apparatus comprises a projection lens system arranged between a mask-pattern holder and a substrate holder, the detectionsystem being employed for determining a deviation between the image plane of the projection lens system and the substrate. In addltion, the lnventicn -----------------------_ . .
~ .
.
PilN 9487 5 10 ap~il 1980 is generally applicable in imaging systems where localdifferences in reflection may occur in the plane in which the image is to be formed, or where said plane may exhibit tilting. Examples of this are microscopes, both reflection microscopes and transmission micros-copes. Because of` the oblique incidence of the auxi-liary beam, a transparent object will also reflect an amount of radiation which is sufficient to be used for the detection of a variation in -the distance between the object to be examined and the microscope objective.
The invention will now be described in more detail, by way of' example, on the basis of a system for irnaging a mask pattern on a substrate. For this reference is made to the drawing7 in which Figure 1, schernatically, represents an imaging system provided with a known opto-electronic detection system, Figure 2 illustrates the principle o* the detection system in accordance with the invention~
and Figure 3 shows an errlbodiTrlellt of such a sys-tem.
In Figure 1 an irnaging system is designat-ed L1. An opto-electronic detection system comprises a radiation source S, which emits an auxiliary beam b, a first lens L2, a second lens L3, and two detec-tors, for example photo-diodes, (hD1 and D2. The detac-tion system is rigidly connected to the lens system L1 via the supports St.
The lens L2 forms a radia-tion spot ~ on the plane p in which an irnage is to be formed by the lens system L1. The lens L3 images said radiation spot on the photo-diodes Dl and D2. Relative to the lens system L1 the detection sys-tem is aligned so that the lens L3 images the intersection of the op-tical axis OO' of the system Ll with the image plane of said system in the plane of the two detectors D1 and D2. If the plane p coincides with the image plane ~3~
PHN 9487 6 10 april 1~80 of the system L1 the radiation spot V is imaged in - the spot V', which is situated symmetrically relative to the detectors D1 and D2. These detectors then re-ceive equal radiation intensities. The output signal Sr of a differential amplifier A which is connected to the detectors D1~and D2 is then for example zero.
In the case of a variation of the distance between the plane p and the lens system L1 the image V' of the ra-diation spot V is moved over the detectors D1 and D2.
If, as is shown in Figure 1, the plane p is moved downwards, the radiation spot V1 formed on said planei8 imaged in V'1 in accordance with the radiation path represented by the dashed lines. The detector D1 then receives a greater radiation intensity than the detector D2 and the signal Sr is for example positive. In the case of an upward movement of the plane p the reverse occurs, and the detector D1 receives a smaller radiation intensity than the detec-tor D2. The signal Sr is then for example negative.
The signal Sr may be applied to a servo-system~ not shown, by means of which the position of the lens : system L1 can be corrected.
` The angle of incidence C~ at which the ~; auxiliary beam b is incident on the plane p is select-ed to be as great as possible, for example of the or-der of 80, in order to obtainaa maximum sensitivity to positional errors of the plane p relative to the lens system L1 and in order to obtain a maximum re-flection of the auxiliary beam by the plane p.
In accordance with the invention the path of the beam b which has been reflected by the plane p includes a beam-invert ng element or retro-reflec-tor, Such a retro-reflector may be constituled by a so-termed l!cats-eyel', which, as is shown in Figure 2, comprises a lens L3 and a mirror r, the mirror being arranged in the focal plane of the lens L3. Such a "cats-eye" reflects a beam along i~self. Figure 2 shows only the part of the path of the beam b in the ~3~
PHN g487 7 10 april 1980 vicinity of the plate p. The beam b forms the radia-tion spot V on said plane. The beam that has been re-flected by the plane p is directed to the mirror r by the reflection prism P4. After reflection at this S mirror the beam b traverses the same path in the re-verse direction.
A small portion b' of the beam b is shown dark in order to indicate how this part traverses the radiation path. Upon the first arrival at the plane p this part is a portion of the beam half b1. The beam portion b' is then situated in the right-hand part Vr of the radiation spot V. After reflection at the plane p and at the reflection prism P4 the beam portion b' passes through the left-hand part of the lens L3. Upon reflection at the mirror r the beam portion b' passes through the right-hand part of the lens L3 and i8 then situated in the left-hand part V
in the radiation spot V. After double reflection at the plane p the beam portion b' has become part of the beam half b2. This applies to all beam portions form-ing the bea~half b1. Thus, after double reflection at the plane p the beam half bl has been in contact both with the right-hand part and with the left-hand part ~of the area on the plane p underneath the ra-; 25 diation spot V. Obviously, the same applies to the , beam half b2. A~ a result of this neither local dif-ferences in reflection as a result of scattering structures in the plane p nor irregularities in this plane or an oblique position of this plane relative to the lens ~ystem Ll can affect the intensity dis-tribution over the beam halves b1 and b2.
The detection system in accordance with the invention may alternatively employ a concave mirroP-or a combination of a plane mirror and a cylindrical lens as inverting element; with a magni-fication of -1.
Figure 3 shows an embodiment of a detection ; system in accordance with the invention for use in an ~13~4~
apparatus for imaging mask patterns on a substrate. The surface of the substrate is represented by the plane ~ and the projection lens system by Ll. This system is movable along its optical axis 00' by drive means, known per se and not shown, which are controlled by the signal Sr, derived from the signals, from the two detectors. In directions perpendicular to the optical axis 00' the projection lens system is immobile in its housing H. As is indicated in Figure 3, the projection lens system may be secured in the housing A by means of springs Sp. Instead of springs it is alter-natively possible to employ diaphragms as fixing means.
In an apparatus for the repeated imaging of a mask pattern on a substrate the projection column and the substrate are movable relative to each other in two mutually perpendicular directions. For more details about such an apparatus reference is made to Canadian Patent No. 1,078,240 of Bouwhuis, et al, issued May 22, 1980. Furthermore, it is to be noted that in such a device the image plane of the projection lens system sub-stantially coincides with the focal plane of this system.
In order to ensure that also for small displacements of the plane ~ relative to the lens system Ll a sufficiently large signal Sr, ; i.e. a sufficiently large difference between the radiation intensities on the two detectors, is obtained, the radiation spot V should have a high brightness. Therefore, a laser is preferably used as radiation source S.
The radiation beam b produced by the laser should be stable. Preferably, a semiconductor-diode laser, DL in Figure 3, for example, an AlGaAs diode laser is used, which can be arranged close to the substrate. It is alter-natively possible to employ a gas laser, which is arranged at a larger distance from the substrate, the radiation of said ~ -8-,4~.
PHN 9487 9 10 april 1980 laser being passed to the substrate via a light-conducting fibre.
The laser beam b is converted into a parallel beam by the lens L4 and is subsequently directed to the substrate surface p via reflection at the beam splitter 8S and the reflection prisms P2 and P3. The lens L2 forms the radiation spot V on the substrate. Subsequently the beam b traverses the radiation path described with reference to Figure 2. A part of the beam b which has been reflected twice by the substrate surface is transmitted to the two detectors Dl and D2 by the beam splitter BS. The lens L5 forms an image of the radiation spot V on these detectors.
The beam splitter BS may comprise the semi-transparent mirror of a semitransparent prism. Instead, it is alternatively possible to employ a polarization splitting prism, the radiation path between said prism and the substrate including a ~ plate, where ~ represents the wavelength of the beam b. The radiation from the source DL is then polarized in such a way that it is reflected by the polari-zation splitting prism. The beam b then traverses the ~
plate twice, as a result of which the plane of polarization of the radiation, that is reflected twice by the substrate 3 i5 rotated through 90 in total, so that the beam is then transmitted by the prism.
The difference signal Sr of the two detectors Dl and D2 is primarily determined by the distance between the image plane of the projection lens sytem Ll and the plane p. However, this signal is also dependent on the total intensity of the auxiliary beam b which has been reflected twice by the plane p. This intensity may vary as a result of a variation in the radiation-source intensity or as a result of variations in the reflection or transmission coefficients of the optical elements in the radiation path.
In order to eliminate the effect of variations in the total beam i~tensity which is incident on the detectors Dl and D2, the output signals of the detectors may be added, yielding a sum signal St. In an analog divider circuit the signal Slr = Sr/St can then be derived, which signal is a .
.
1~13S~
PHN 9487 10 10 apri] 1980 measure of the positional error of the plane p of the substrate relative to the projection lens system and which is independent of intensity variations of the radiation source and of variations in the reflection or transmission coefficients in the radiation path. In order to ensure that also in the case of poor reflection or transmission coeffi-cients an amount of radiation is obtained on the detectors which is sufficient for the required accuracy, the radiation source should be set to a maximum radiation power. This may lead to a reduction of the life of the radiation source, especially if this source is a semiconductor diode laser.
Furthermore, a drift-free analog divider is a comparatively expensive eIement.
The afore-mentioned drawbacks can be avoided, in 15 accordance with a preferred embodiment of the detection system in accordance with the invention, by using the sum signal St for controlling the intensity of the radiation source so that the total radiation intensity which is incident on the detectors Dl and D2 remains constant. If the 20 radiation source is a radiation-emitting diode the magnitude of the electric current with which said radiation source is driven may then be corrected for this purpose.
A semiconductor diode laser, for example an AlGaAs laser, is preferably operated with current pulses, 25 because this is most favourable for the life of such a laser.
However, the angle at which such a laser emits its radiation can vary when the magnitude of the electric current through the laser is varied. The intensity of the radiation which is emitted by a diode laser is suitably corrected by 30 varying the pulse repetition rate of the electric current pulses at constant pulse width.
As is shown in Figure 3, the signals from the detectors Dl and D2 are applied to a subtractor circuit Al, on whose output the signal Sr appears, and to an adder circuit A2, on whose output a signal St is available.
The signal St may for example be applied to an input of a differential amplifier A3, whose other input is connected to a reference source Ref. The output of the differential ~Y'. ~ t`.~
PHN 9487 11 10 april 1980 amplifier is connected to an oscillator VC0, which supplies a train of pulses whose frequency is determined by the voltage on its input. The output of the oscillator is connected to a pulse generator PG. The electric current required for the operation of the diode laser DL is supplied by the pulse generator in the form of pulses of constant duration and with a repetition rate which is equal to that oF the pulses from the oscillator VCOO The average intensity of the diode laser could also be controlled by adapting the lO duration of the elec-tric current pulses instead of adapting the pulse repetiti.on rate.
The setting of the diode laser is such that for the maximum radiation loss to be anticipated in the radiation path the total amount of radiation incident on the 15 detectors just suffices to attain the required accuracy, which is inter alia determined by the leakage current and the noise of the detectors. In the case of smaller radiation losses the radiation source need emit a smaller radiation intensity, which is Favourable for the life of the source.
The displacement ~ of the radiation spot over the photo-diodes Dl and D2 is given by:
~ 4.M.I-I. sin ~ .
In this formu:La H is the displacernent of the plane p, CX the angle of incidence of the focussing beam on 25 said plane, and M the magnification of the lens sytem L5 L2. This magnification is equal to the ratio of the focal length of the lens L5 to that of the lens L2. As the focussing beam has been reflected twice by the plane ~, the sensitivity of the detection system has also increased by a 30 factor 2. As a result of this a factor 4 instead of a factor
2 occurs in the above expression for the displacement ~ .
In a realized embodirrlent of a detection system in accordance with the invention a displacement of the plane p over only 0.1/um could still be detected in a 35 satisfactory manner.
In a realized embodirrlent of a detection system in accordance with the invention a displacement of the plane p over only 0.1/um could still be detected in a 35 satisfactory manner.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed by the imaging system, which detection system comprises a radiation source which produces an auxiliary beam, a reflection element which is disposed in the path of the auxiliary beam which has been reflected a first time by the second plane and which directs the auxiliary beam again twoards the second plane, and two radiation-sensitive detectors disposed in the path of the auxiliary beam which has been reflected twice by the second plane, the detectors and the reflection element being connected to the imaging system and the difference between the output signals of the two detectors being a measure of the said deviation, characterized in that there is provided a first lens system (L2) for focussing a narrow auxiliary beam (b) to a small radiation spot (V) on the second plane (P), that there is provided a second lens system for imaging the said radiation spot on the reflection element, and that the reflection element is a beam-reversing element (retro-reflector) (r, L3), by which the beam is reflected along itself and is focussed by the second lens system (L3) in the radiation spot (V) formed when the auxiliary beam is incident on the second plane for a first time.
2. An optical imaging system as claimed in Claim 1, characterized in that the outputs of of the two detectors are connected to an adder circuit whose output is connected to a control circuit for correcting the intensity of the radiation source, in such a way that the sum of the detector signals remains-constant.
3. An optical imaging system as claimed in Claim 2, in which the radiation source is a semiconductor diode laser which emits radiation pulses, characterized in that the PHN 9487 13 10 april 1980 control circuit comprises an oscillator which is controlled by the sum voltage from the detectors, the output of said oscillator being connected to an electric current source which controls the diode laser.
4. An apparatus for the repeated imaging of a mask pattern on a substrate, which apparatus comprises a projec-tion lens system arranged between a mask-pattern holder and a substrate holder, provided with a detection system as claimed in Claim 1, the difference between the output signals of the detectors being a measure of a deviation between the image plane of the projection-lens system and the substrate.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NLAANVRAGE7904579,A NL186353C (en) | 1979-06-12 | 1979-06-12 | DEVICE FOR IMAGING A MASK PATTERN ON A SUBSTRATE EQUIPPED WITH AN OPTO-ELECTRONIC DETECTION SYSTEM FOR DETERMINING A DEROGATION BETWEEN THE IMAGE OF A PROJECT SYSTEM AND THE SUBSTRATE PLATE. |
| NL7904579 | 1979-06-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1139441A true CA1139441A (en) | 1983-01-11 |
Family
ID=19833337
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000353625A Expired CA1139441A (en) | 1979-06-12 | 1980-06-09 | Optical imaging system provided with an opto-electronic detection system for determining a deviation between the image plane of the imaging system and a second plane on which an image is to be formed |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4356392A (en) |
| JP (1) | JPS5632114A (en) |
| CA (1) | CA1139441A (en) |
| DE (1) | DE3021622C2 (en) |
| FR (1) | FR2458830B1 (en) |
| GB (1) | GB2052090B (en) |
| NL (1) | NL186353C (en) |
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| DE3834117A1 (en) * | 1988-10-07 | 1990-04-12 | Zeiss Carl Fa | COORDINATE MEASURING DEVICE WITH AN OPTICAL PROBE |
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| NL9001611A (en) * | 1990-07-16 | 1992-02-17 | Asm Lithography Bv | DEVICE FOR IMAGING A MASK PATTERN ON A SUBSTRATE. |
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| JPS5422845A (en) * | 1977-07-22 | 1979-02-21 | Toshiba Corp | Automatic focusing device |
| DE2744130A1 (en) * | 1977-09-30 | 1979-04-12 | Siemens Ag | DEVICE FOR CONTACT-FREE MEASUREMENT OF THE DISTANCE OF A SURFACE OF AN OBJECT FROM A REFERENCE PLANE |
-
1979
- 1979-06-12 NL NLAANVRAGE7904579,A patent/NL186353C/en not_active IP Right Cessation
-
1980
- 1980-06-09 DE DE3021622A patent/DE3021622C2/en not_active Expired
- 1980-06-09 GB GB8018792A patent/GB2052090B/en not_active Expired
- 1980-06-09 CA CA000353625A patent/CA1139441A/en not_active Expired
- 1980-06-11 FR FR8012988A patent/FR2458830B1/en not_active Expired
- 1980-06-11 US US06/158,653 patent/US4356392A/en not_active Expired - Lifetime
- 1980-06-12 JP JP7846280A patent/JPS5632114A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| DE3021622A1 (en) | 1980-12-18 |
| US4356392A (en) | 1982-10-26 |
| JPS5632114A (en) | 1981-04-01 |
| JPS6337364B2 (en) | 1988-07-25 |
| GB2052090B (en) | 1983-04-13 |
| NL186353B (en) | 1990-06-01 |
| NL186353C (en) | 1990-11-01 |
| DE3021622C2 (en) | 1984-01-12 |
| FR2458830A1 (en) | 1981-01-02 |
| NL7904579A (en) | 1980-12-16 |
| GB2052090A (en) | 1981-01-21 |
| FR2458830B1 (en) | 1988-02-19 |
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