WO2003001268A1

WO2003001268A1 - Autofocus system, method and device for optically testing parts in said system

Info

Publication number: WO2003001268A1
Application number: PCT/FR2002/002038
Authority: WO
Inventors: Joseph Cohen-Sabban
Original assignee: Sciences Et Techniques Industrielles De La Lumiere (Sa)
Priority date: 2001-06-25
Filing date: 2002-06-14
Publication date: 2003-01-03
Also published as: FR2826461B1; FR2826461A1

Abstract

The invention relates to a device or system that is used to focus a lens automatically and to the application thereof in relation to methods and devices for optically testing parts. The device for focusing a lens (2) that is used to observe a substrate (4, 104, 204) comprises: a confocal optical system (13) which incorporates said observation lens and which has a longitudinal chromatic aberration in a wavelength interval; a source (12) which is adapted to emit polychromatic incident radiation (AI) having a continuous spectrum in a useful band [λ1, λ2] the wavelength of which is included in said interval; a spectrograph (14) which is adapted to deliver a spectrum of autofocus radiation (AR) reflected by the substrate and transported, in addition to said incident radiation (AI), by the confocal optical system; and movement control means (23, 26) which are used to control a relative movement (25) of the substrate and the observation lens along the optical axis (Z) of the latter, according to the difference between a central wavelength (λMAX) of a line (24) of said spectrum with maximum intensity and a reference wavelength (λREF).

Description

Autofocus system, method and device for optical control of parts incorporating this system

The present invention relates to a device or system for automatic focusing of a lens, and to its application to methods and devices for optical inspection of parts.

The technical field of the invention is that of manufacturing high precision optical control devices.

The present invention is particularly applicable to the automatic focusing of an objective for observing the surface of a substrate, such as a silicon disc ("wafer") or a mask for producing such a disk, said objective forming part of an automated optical device for monitoring the surface of the substrate.

The control of the geometry of such substrates is to date carried out by high precision devices allowing the detection of defects and the presence of impurities on the surface of these substrates.

Such optical monitoring devices usually include a source of illumination of the substrate by monitoring radiation, an objective for observing the surface of the substrate, a photoelectric sensor sensitive to the monitoring radiation transmitted, reflected or backscattered by the surface of the substrate. substrate and transmitted by the observation objective, as well as means for processing the signals delivered by the sensor; these control devices frequently include a source of monochromatic illumination such as an Argon laser, a charge-coupled matrix sensor (matrix CCD), an observation objective of the microscope objective type, and a computer processing the images resulting from the conversion of the signals delivered by the matrix sensor.

In order for the sensor to deliver images of the surface of the substrate of sufficient quality, it is necessary to maintain the focal plane of the objective for observing the surface of the substrate substantially coincident. with the plane along which this surface extends; for this purpose, an automatic focusing device (autofocus) of this lens is used, which generally comprises a sensor sensitive to part of the control radiation reflected by the substrate, and which usually operates by triangulation.

The substrate to be checked is generally placed on a device for positioning the substrate by displacement in translation along three axes (X, Y, Z), one axis of which (called Z) corresponds to the optical axis of the observation objective of the substrate; to control the entire surface of the substrate, it is moved by this device along two axes (X and Y) orthogonal to the optical axis Z; the objective is brought into focus on the plane of the surface to be checked by displacement of the substrate along the Z axis, as a function of a signal delivered by the autofocus device.

An objective of the invention is to propose an improved autofocus device which is adapted to allow precise and rapid focusing of the observation objective of such optical control devices of such substrates.

An objective of the invention is to propose a method and a device for automatic development of this objective which are simple and efficient.

According to a first aspect of the invention, the substrate is illuminated by a source emitting additional polychromatic radiation, called autofocus, which is directed so that this radiation, as well as its part which is reflected by the substrate, pass through said observation objective as well as an additional objective adapted to form, with the observation objective and a substantially punctual or linear spatial filter, a confocal optical system; the chromatic dispersion of the autofocus radiation reflected and transmitted by the confocal optical system is caused by passing through a disperser so as to obtain a spectrum of this radiation; the spectrum thus obtained is analyzed in order to determine the central wavelength of a line of maximum intensity of the spectrum; we then deduce from the value of said central wavelength, the value of a relative displacement (along said Z axis) of the substrate relative to the perfect focusing position of the observation objective, and if necessary causes a relative displacement of said value between the substrate and this goal.

The invention is based on the use of the longitudinal chromatic aberration of the optical system constituted by the additional objective and by the observation objective of the substrate. It follows from this aberration that the position along the Z axis - which is the optical axis of the observation objective - of the confocal image varies, between two extreme positions, as a function of the wavelength considered polychromatic autofocus radiation; the use of this property of these objectives combined with the spatial filtering of the autofocus radiation reflected by the substrate caused by the confocal system allows, when a point on the surface to be checked is located on the Z axis, between said extreme positions delimiting a chromatic range, to observe a line of maximum intensity in the spectrum of the autofocus radiation reflected by said point of the surface of the substrate, the central wavelength of this line characterizing the position on the Z axis of said point of the surface to be checked, this position being able to be determined very precisely and very simply as a function of said central wavelength.

Preferably, the additional polychromatic autofocus radiation has a continuous spectrum in a determined range of wavelength; this range preferably extends in the range (400 nm to 800 nm) of visible light, which in particular facilitates the installation and adjustment of the autofocus system; said additional objective is designed (calculated) so that, for a wavelength of said range which is called reference wavelength and which is preferably the median wavelength of this range, the confocal image of the filter by the autofocus optical system - including the viewing lens - is confused with the focus of the optical control system at the control wavelength; therefore, when the central wavelength of the maximum intensity line of the spectrum of the reflected autofocus radiation coincides with this wavelength of reference, this means that the focus of the optical control system at the control wavelength coincides with the surface to be controlled which caused the monochromatic reflection at said central wavelength; therefore, in this situation, the viewing objective is well focused.

Conversely, when the value of said central wavelength of the maximum intensity line is different from the value of said reference wavelength, this means that the focus of the optical control system is not located in the plane of the surface of the substrate to be checked and that an adjustment is necessary.

This focusing is controlled as a function of the difference between said different values, and preferably achieved by a displacement along the Z axis of the viewing objective of the control system, this displacement being generally proportional to this difference.

The invention makes it possible to control the development of the viewing objective on the surface to be checked with an accuracy which is of the order of a few nanometers or tens of nanometers; the measurement range of the autofocus system is generally of the order of a few microns to a few tens of microns around the position of perfect focus.

The simplicity of the processing to be carried out on the signals (analysis of the spectrum of the reflected autofocus radiation and calculation of a difference in wavelength) and the mechanical action on the viewing lens make it possible to ensure a very short response time. for autofocus, the value of this response time being for example. in the range of 1 to 10 milliseconds.

Preferably, use is made of autofocus radiation extending over a wavelength range outside which the control wavelength is situated; this facilitates the separation of the light beams delivered by the viewing objective; for this purpose, we have on the path of these beams a filter, preferably a dichroic filter; this chromatic separation of the two beams allows their respective simultaneous processing without mutual influence.

Although the device for spatial filtering of autofocus radiation can be materialized by a slot, it is preferably substantially punctual and formed by the heart of one end of a section of optical fiber used to transport this radiation from its physical source (halogen lamp for example) up to said additional objective (incident path), as well as said additional objective to said chromatic disperser - or spectrograph (with prism for example) -

Furthermore, there is preferably the point spatial filter forming the point source of the confocal system, at a distance from the focal point of the additional objective, on the optical axis thereof, and so that the image - called confocal - from this point source, by said additional and observation objectives, is located on the optical axis Z of the latter.

Other characteristics and advantages of the invention appear in the following description which refers to the appended drawings, which illustrate without any limiting character the preferred embodiments of the invention.

FIG. 1 schematically illustrates the main elements constituting an optical control device according to the invention and their arrangement for the automatic focusing of the observation objective of the substrate to be checked.

FIG. 2 schematically illustrates on an enlarged scale the paths traveled between the objective of observation of the substrate and the substrate, by the control radiation on the one hand, and by the chromatic components of the autofocus radiation on the other hand; in this figure, there is illustrated in a shifted manner on the left a substrate in a position of perfect focusing of the control radiation, and there is illustrated in a shifted position on the right a substrate in a position requiring a lens development; the offset illustration is only intended to improve the clarity of the figure and to avoid providing two separate figures; of course the actual position of the substrate is that illustrated in Figure t.

FIGS. 3 and 4 are two spectrograms of the reflected and dispersed autofocus radiation, corresponding respectively to the situations of perfect focus and faulty focus which are illustrated in the left and right part of FIG. 2; in these figures we have also identified the central wavelength of the control radiation which is identified λ _C τRL-

With reference to FIGS. 1 and 2, the control device 1 comprises, in a known manner, an objective 2 for observing the surface 3, 103, 203 of a substrate 4, 104, 204 to be checked.

The substrate is placed on a table 5, 105, 205, provided with guide means (such as slides) and actuation means (such as jacks) for moving the substrate, under the control of a computer (6 Figure 1), along two orthogonal axes X and Y, which are perpendicular to the optical axis Z of the lens 2; these displacements of the substrate along the axes X and Y make it possible to successively observe the elementary (punctual) regions forming the surface 3, 103, 203 of the substrate, by successively centering these said regions on the optical axis of the objective.

The control device further comprises a sighting objective 7 whose optical axis coincides with that of objective 2 and which makes it possible, with the latter, to form on an array sensor 8 an image of the elementary region of the surface. of the substrate which is observed by objective 2; the signals delivered by the sensor 8 are transmitted to the computer 6 by a link 9 for conversion into images and processing of these images in order to determine whether the geometry of the patterns inscribed or engraved on the surface of the substrate conforms to a predefined model; this optical control system can also make it possible to detect the presence of foreign bodies on the surface of the substrate. In order to carry out this optical control, the surface of the substrate is illuminated by incident control radiation C1 emitted by a first light source 1 0; this radiation is generally monochromatic, for example at a wavelength λc _TR L of 365 nm in the case of a source of the ARGON laser type.

The surface 3, 1 03, 203 reflects, backscatter or transmits the CI radiation to form a control CR radiation called reflected which is transmitted to the sensor 8 by the objectives 2 and 7 of the control device to form on this sensor the image of the substrate.

The source emitting the control radiation can, as illustrated in FIG. 1, be placed on the same side of the substrate as the objective 2 d ^J observation; for this purpose - a semi-reflecting plate 1 1 reflects the radiation Cl emitted by the source 1 0 towards the objective 2 and transmits the radiation CR reflected by the surface of the substrate and transmitted by the objective 2, towards the objective 7 and sensor 8.

Alternatively, in the case where the substrate is transparent for the control wavelength λ _C τ _RL , the source 1 0 and the objective 2 can be arranged on either side of the substrate, for example aligned along the Z axis; in this case, the blade 1 1 can be deleted.

According to the invention, the control device comprises an autofocus device comprising a source 12 of white light - called autofocus radiation - and / or of continuous polychromatic spectrum, means for illuminating the surface of the substrate by this radiation which comprise a confocal optical system 1 3 incorporating said observation objective 2, and a spectrograph 14 adapted to the chromatism of the confocal optical system.

The optical system 1 3 comprises, in addition to the observation objective 2, an additional objective 1 5, and a point spatial filter constituted by an end 16 of the heart of a section 1 7 of an optical fiber which is disposed at a distance from the focus of objective 15 so as to form a point source for the confocal system; Al radiation emitted by the source 1 2 is transported by the fiber 17 to this confocal point source; this radiation Al transmitted by this objective is reflected by a dichroic blade 1 8 towards objective 2; the combined chromatism of objectives 15 and 2 causes, at the output of objective 2, a dispersion along the Z axis of the position of the image of this point source, according to the wavelength considered of the monochromatic components of the polychromatic radiation incident Al from source 12, as illustrated in FIG. 2.

When a point on a reflecting or backscattering surface such as the surface 3, 1 03, 203 of the substrate, is located on the Z axis between the two point confocal images F1 and F2 corresponding respectively to the minimum wavelength λ ₁ of the useful wavelength band of radiation Al, and at the maximum wavelength λ ₂ of said useful wavelength band, this results in reflected autofocus radiation AR which is transmitted back by the objectives 2 and 15 and the spatial filter 1 6; this radiation is deflected at the outlet of the section 17 by a semi-reflecting plate 1 9, towards the entry of the spectrograph 1 4.

The spectrograph comprises a spatial dispersing element 20 - such as a prism - which selectively deflects the monochromatic components ARD of the incoming radiation AR as an output; a plurality of photoelectric sensors, for example the sensors of a CCD strip

21, delivers a plurality of signals respectively representative of the light intensity of the AR radiation in consecutive narrow spectral bands of said useful wavelength band; these signals are delivered to the input of a computer 23 connected to the sensor 21 by a link 22; this computer reconstructs from these signals a spectrum of the reflected autofocus radiation AR, then determines the central wavelength λ _A x of the line (24, FIGS. 3 and 4) of maximum intensity of this spectrum.

As a function of the position of this central wavelength λMAX with respect to a reference wavelength λ _REF , the computer 23 controls the displacement in translation 25 along the Z axis of a support 26 of the objective 2 which is movable along this axis, by means of conventional means of guidance and actuation not shown and by means of a link 27 connecting the support 26 to the computer 23.

The method and the device according to the invention require, during the installation of the autofocus device, to determine the value of the reference wavelength λ _REF forming part of the useful band [λ λ ₂ ] of wavelength , for which the image of the point source 1 6 by the autofocus system - including the objective 2 of observation - is confused with the focal point F of the optical system 2, 7 of control.

With reference to FIG. 2, the confocal image point of the objective 2 for the minimum wavelength λi d the useful band [λ-i, λ ₂ ] of the incident autofocus radiation Al, is marked F1; similarly, the confocal image point for the maximum wavelength λ ₂ of this useful band, is marked F 2; these two image points surround the focal point F of the objective 2 for the control wavelength λc _TR which is located outside this useful band (see FIGS. 3 and 4); the distance measured along the Z axis and separating the focal points F1 and F2 constitutes the "measuring range" of the focusing system; if the point of intersection 28 of the Z axis and of the surface 1 03, 203 was outside this "measurement range", this surface would be "lost to view" by the autofocus device; this possibility can be easily avoided (after initial adjustment) thanks to the very short response time of the device according to the invention.

When the point 28 is located in the interval [F1, F2] without being confused with the focal point F, which corresponds to the position of the surface 203 of the substrate 204, FIG. 2, the confocal system does not effectively return a luminous flux AR in return that in a narrow spectral band (or line of maximum intensity) centered on the wavelength λ _MA X _> which corresponds to a spectrum as represented in FIG. 4, which results from the analysis of the AR flux by the spectrograph 14, and which allows the computer 23 to determine the difference between this wavelength λ _MA χ and the reference wavelength λ _REF which is generally the median value of the useful interval [λ-i, λ ₂ ], then to control the displacement along Z of objective 2.

Conversely, when point 28 is confused with focus F, the spectrum obtained at the output of the AR flow analysis spectrograph returned by the confocal system is as shown in FIG. 3, where the central wavelength λ Ax of the line 24 of this spectrum coincides with the reference wavelength, which corresponds to good focusing.

Claims

1. Method for developing an objective (2) for observing a substrate (4, 104, 204), characterized in that it comprises the following operations:

- having an additional objective (15) and a quasi-point spatial filter (1 6) to form with the observation objective a confocal optical system (13) having a longitudinal chromatic aberration in a wavelength interval,

- illuminate the substrate with polychromatic (Al) incident autofocus radiation whose spectrum is continuous in a useful band

[λi, λ ₂ ] of wavelength included in said wavelength interval, by placing a point of the substrate in a chromatic range [F1, F2] of the optical axis (Z) of the objective of observation;

- cause said confocal optical system to transport said incident autofocus radiation (A1) as well as an essentially monochromatic reflected autofocus radiation (AR) corresponding to the reflection or backscattering of said incident autofocus radiation by said point on the substrate ;

- causing a chromatic dispersion of said reflected autofocus radiation to obtain a spectrum thereof;

determining a central wavelength (AX) of a maximum intensity line (24) of said spectrum and determining a difference between said central wavelength and a reference wavelength (λ _REF ), and causing a relative displacement (25), along the axis (Z), of the substrate and the observation objective, as a function of said difference, in order to obtain the focusing.

2. Method according to claim 1, in which the substrate is further illuminated by a control radiation (Cl) whose wavelength (s) (λ _C τRi_) is (are) located (s) outside said useful band [λi, λ ₂ ].

3. Method according to claim 1 or 2, wherein said useful band is located in the visible light range.

4. Method according to any one of claims 1 to 3, wherein said displacement (25) is a displacement of said objective (2) of observation.

5. Method according to any one of claims 1 to 4, in which one transports said radiation (Al) of incident autofocus and said radiation (AR) of autofocus reflected by a section (17) of optical fiber of which one end ( 1 6) forms said quasi-point spatial filter of the confocal optical system.

6. Device for developing an objective (2) for observing a substrate (4, 1 04, 204), characterized in that it comprises:

- a confocal optical system (1 3) including said observation objective and having a longitudinal chromatic aberration in a wavelength interval;

- A source (1 2) adapted to emit polychromatic incident radiation (Al) and of continuous spectrum in a useful band [λi, λ ₂ ] of wavelength included in said interval;

- a spectrograph (1 4) adapted to deliver a spectrum of autofocus radiation (AR) reflected by the substrate and transported - as well as said incident radiation (Al) - by said confocal optical system,

- displacement control means (23, 26) for controlling a relative displacement (25) of the substrate and the observation objective along the optical axis (Z) thereof, as a function of a difference between a central wavelength (λ _MA x) of a line (24) of maximum intensity of said spectrum and a reference wavelength

(λ F).

7. Device according to claim 6, which further comprises a source (1 0) adapted to emit a radiation (Cl) control whose wavelength (s) (λ _C τ _R i.) is (are) located outside of said useful band [λi, λ].

8. Device according to claim 6 or 7, which comprises a movable support (26) for the observation objective which is connected to a computer (23) controlling its displacement in translation along the optical axis (Z).

9. Device according to any one of claims 6 to 8, which comprises at least one section (7) of optical fiber allowing the transport of said incident (Al) and reflected (AR) autofocus radiation between said source (12) and said confocal system (13) on the one hand, and between said confocal system (13) and said spectrograph (14) on the other hand.

10. Device according to claim 9, wherein one end (1 6) of a fiber section forms a quasi-point spatial filter for said confocal system (13).

1 1. Device according to any one of Claims 6 to 10, which further comprises means (1 8) for separating the control radiation (Cl, CR) and the autofocus rays (Al, AR), these means comprising preferably a dichroic filter.

12. Device (1) for optical control of a substrate (4, 104, 204) comprising:

- an objective (2) for observing the substrate,

- a source (10) adapted to emit a control radiation (Cl) to illuminate the substrate,

- a sensor (8) sensitive to the control radiation (CR) emitted by the substrate under the effect of illumination by the source;

- a computer (6) connected to the sensor for processing images of the substrate resulting from the conversion of signals delivered by the sensor, - a device according to any one of claims 6 to 11.