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WO1996012981A1 - Dispositif et procede de focalisation automatique destine a un microscope a haute resolution - Google Patents

Dispositif et procede de focalisation automatique destine a un microscope a haute resolution Download PDF

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Publication number
WO1996012981A1
WO1996012981A1 PCT/US1995/013406 US9513406W WO9612981A1 WO 1996012981 A1 WO1996012981 A1 WO 1996012981A1 US 9513406 W US9513406 W US 9513406W WO 9612981 A1 WO9612981 A1 WO 9612981A1
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WIPO (PCT)
Prior art keywords
recited
interest
microscope
light
area
Prior art date
Application number
PCT/US1995/013406
Other languages
English (en)
Inventor
Isaac Katz
Michael Faeyrman
Yehuda Elisha
Shimon Kostianovsky
Yoram Uziel
Joel L. Seligson
Yoram Hanfling
Original Assignee
Kla Instruments Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Kla Instruments Corporation filed Critical Kla Instruments Corporation
Publication of WO1996012981A1 publication Critical patent/WO1996012981A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B21/00—Microscopes
    • G02B21/24—Base structure
    • G02B21/241—Devices for focusing
    • G02B21/244—Devices for focusing using image analysis techniques

Definitions

  • the present invention relates generally to automatic optical focusing methods and more particularly to a method and apparatus for automatically focusing high magnification microscopes on selected areas of interest in a field of view.
  • the autofocus system of the KLA 5000 Coherence Probe uses a single photo-diode covering 1/3 of the linear field of view to detect the coherence of light reflected from an area of a surface to be inspected. Scanning the image in the Z direction, i.e.
  • the disclosed system characterizes an unknown test surface with respect to a known reference surface by using a dual beam interferometer having one wavefront reflected from the unknown test surface of a test object, while the other beam has reference wavefronts reflected from the known reference surface of a reference object.
  • Point coincidences between a return point on the reference surface and a test point on the test surface are indicated by the two reflected beams having a zero path difference.
  • An array of points on the test surface are measured by scanning the interference pattern, point by point, and recording contrast variations by means of a multi-apertured CCD detector, with a CCD aperture corresponding to each test point.
  • the Balasubramanian device is used to determine the surface profile of an object. Consequently, it must utilize the output from every detector in the detector array in order to generate a high resolution comparison between the test and the reference surfaces.
  • the Balasubramanian device considers every portion of a surface as it performs its measurements. In some applications, such as overlay measurements, one may, in succession, focus on the same location but on two different layers of the wafer. Methods and apparatus known in the prior art, for automated high-volume operation, perform this refocusing in an extremely time- consuming manner.
  • an off-set method is used to focus on a suitable area in the same field of view, which is at the same height as the measurement area or at a height having a known difference from that of the measurement area.
  • the manual designation of the optimal focus area during the set-up process can be replaced by having the system automatically select suitable areas for automatic focusing during the measurement phase of the operation.
  • the present invention therefore, provides a process that performs an automatic optimal area selection for automatic focusing in the field of view and describes a method for obtaining optimal focus at the designated location when the measurements are made. Once the designation of the optimal focus area is made, coherence measurements of the measurement areas are performed. In some wafer inspection applications, such as overlay measurements, one may, in succession, focus on the same location but on two different layers of the wafer.
  • one alternative embodiment of the present invention provides for an additional optical channel with its own camera.
  • the system includes two optical channels having different magnifications, so as to speed up operations where a rapid change of magnification is required.
  • High resolution devices particularly those using interferometry, are extremely vulnerable to vibration. Therefore, another alternative embodiment of the present invention provides additional elements to mitigate the effects of vibration.
  • An advantage of the present invention is that it provides a method and apparatus for performing high resolution measurements of certain surface areas of a wafer while automatically focusing on another designated area of the wafer.
  • Another advantage of the present invention is that it provides a method and apparatus for performing high resolution measurements of surface areas of a wafer wherein the effects of vibration are mitigated.
  • Still another advantage of the present invention is that it provides a method and apparatus for performing high resolution measurements of certain areas on a wafer while automatically focusing on the same or a different designated area of the wafer but at different layers of the wafer.
  • Fig. 1 schematically illustrates a coherence microscope system having a pin diode detector array in accordance with the present invention
  • Fig. 2 schematically depicts the light responsive surface areas of the pin diode array shown in Fig. 1
  • Fig. 3 is a block diagram illustrating means providing an interface between the pin diode detector array and the control computer depicted in Fig. 1
  • Fig. 4 schematically depicts an alternative embodiment of the means providing an interface between the pin diode detector array and the control computer depicted in Fig. 1
  • Fig. 5 illustrates an alternative dual camera embodiment of a coherence microscope of the present invention having a second optical channel
  • Fig. 6 shows typical waveforms of outputs from the pin diode detector array
  • Fig. 7 is a flow chart depicting the sequence of operation for designating which diodes of the pin diode detector array are to be utilized in the focusing operation.
  • Fig. 1 is a schematic representation of an improved coherence microscope system 103 having an autofocusing feature in accordance with the present invention. Some, but not all portions of the coherence microscope system 103 of the present invention are described in U.S. Patent 4,818,110 which is assigned to the assignee of the present invention, and which is incorporated by reference herein. As depicted in the drawing, a specimen 100 is disposed on a computer-controlled, motorized and metered x-y stage 101 and is mounted so as to lie perpendicular to a main optical axis 102 of the microscope system.
  • a light source 104 provides a beam of broadband illumination which impinges on the specimen 100 via a beam splitter 106 and an objective lens 108.
  • the light reflected by the specimen 100 travels to a camera 110 via objective lens 108, beam splitter 106, an intermediate lens 112, a beam splitter 114, and a lens 116 which forms a magnified image of the specimen 100 on the focal plane of the camera 110.
  • the camera 110 generates video data, corresponding to the inspected surface area of the wafer 100, which is transmitted, via a line 111, to a video A/D unit 144. Digitized video data generated by the unit 144 is transmitted, via a line 143, to a computer 136.
  • a reference light path 99 which is collinear with the source beam path 107 and which by means of beam splitters 106 and 114 provides a reference wavefront to both the camera 110 and the pin diode array 118.
  • the reference path 99 includes a lens 122 and a planar mirror 124.
  • the path 99 may be blocked by a shutter 125. When the shutter 125 is disposed so as to block the path 99, the system 103 functions as a conventional microscope.
  • Focusing of the microscope system 103 is performed by moving an assembly 126 of the previously mentioned vertically along the Z direction, i.e. along the direction indicated by the arrow 105.
  • the assembly 126 Under control of a computer 136, the assembly 126 is moved vertically by a motor and lead assembly (or other suitable means) 127 for macro- movements, and by a piezoelectric flexure, or the like, 129 for micro-movements.
  • a sensor 128 is attached to the assembly 126 and measures the vertical movement of the assembly 126 with respect to a linear scale 130 which is stationary with respect to the specimen 100. The output of sensor 128 is fed to computer 136.
  • An analog output signal from the array 118 is transmitted, via a line 119, to an interface unit 121 which converts the analog signal to a digital signal.
  • the digital signal is transmitted, via a line 123, to the computer 136.
  • the digitized video or other computer generated images are output, via a line 137, to a display monitor 146.
  • Fig. 2 illustrates one embodiment of the pin diode array 118 which may be obtained, for example, from Centronic Limited, Electro Optics Division, Centronic House, New Addington, Croydon CR90BG, United Kingdom.
  • the detector array 118 includes a plurality of square shaped detectors 132 each having a separate electrical output that produces a current proportional to the energy impinging on the corresponding square.
  • Each square is indicated by a numeral 132 ⁇ where i ranges from 1 to N, with N being the number of squares in the detector array 118.
  • the square in the center of the array (and perhaps in other parts of the array as well) includes a subarray of detectors 133 having a much smaller "footprint" than that of the other detectors. This is to provide increased sensitivity in the corresponding portion of the area of interest.
  • Fig. 3 schematically depicts the functional components of the unit 121 that provides the interface between the detection array 118 and the computer 136. As partially shown in Fig.
  • an electrical signal is output from each square 132 of the array 118, with each square 132 representing a single photo-sensor or photo-diode that provides a signal which is communicated, via lines 119 x - 119 N and pre-amplifiers 132 x to 132 N , to an analog selector 134.
  • the computer 136 selects, via a control interface 138, and lines 123a and 137, the particular detectors (i.e. squares 132 A of the array 118) that are to contribute to an input signal to a sealer and output driver unit 14*0.
  • the input to the unit 140 is transmitted via lines 135 ! - 135 M , where M is the number of detectors selected.
  • any combination of photo-sensors or photo-diodes can be selected for processing by the computer 136.
  • the unit 140 is used to maintain the dynamic range of electronic output and to prevent device saturation. Because several detector outputs may be summed together, the total voltage may exceed the saturation voltage of the subsequent electronics.
  • the unit 140 linearly reduces the summation weight of each photo-sensor selected for processing. Thus, if two photo-sensors are selected, their respective weights will be 0.5. If three photo- sensors are used, their respective weighting will be 0.333, etc.
  • the total weighted sum of the selected detectors is kept at 1.
  • the electronic circuit is not limited to this scheme of weighting the sums, and other weighting schemes may be used.
  • Unit 140 includes an output driver component (not shown) in the form of a low impedance amplifier through which an analog signal is transmitted, via a line 141, to the A/D converter 142.
  • the digital signal from the converter 142 is input into the computer 136, via a line 123b.
  • the video A/D 144 receives its input from the camera 100, and transmits the digitized video to the computer 136.
  • the digitized video or computer generated images may then be displayed on the monitor 146.
  • the computer can also superimpose on the image the pattern of detector array 118.
  • An alternative embodiment of the interface unit 121 is depicted as unit 121' in Fig. 5.
  • the unit 121' uses as many A/D converters 142' as there are detectors 132. In this embodiment each of the detector outputs are sampled via a multiplexer 143 forming the output stage of converter assembly 142' .
  • Two alternative embodiments of the present invention 103 include elements that improve the immunity of the system to vibration.
  • the light source 104 ordinarily an incandescent bulb, is replaced with a Xenon flash lamp having a flash duration of less than 0.01 seconds. If the flash is synchronized with the vertical retrace of camera 110, there will not be any apparent tearing of the image due to vibrations. However, vibrations may cause successive frames to be slightly displaced with respect to each other, but this can be corrected by cross-correlating these frames with each other and determining and correcting for the shift in the memory of computer 136.
  • a typical overlay target on wafers has two levels which can be up to 2 microns apart, vertically. This separation is more than the depth of focus of the microscope objectives normally used, even when the microscope is operated in the conventional or non- interferometric mode.
  • the stage slews to a measurement point and focuses at a first Z level, the system takes a measurement, and then slews to a second Z level. This slewing movement in the z-direction is relatively slow because of the settling time.
  • the speed of the operation may be increased by adding another optical path having another camera.
  • the second optical channel shown at 151 in Fig.
  • a beamsplitter 150 is comprised of a beamsplitter 150, a mirror 152, a lens 154 and a camera 156.
  • the camera 156 is moved along the Z-axis by means of a servo-driven motor 157 and a suitable drive linkage 158.
  • a servo-driven motor 157 is moved along the Z-axis by means of a servo-driven motor 157 and a suitable drive linkage 158.
  • the difference in the focusing level of the two cameras will remain the same for all measurements.
  • camera 156 need not move with respect to camera 110 as the stage moves from one measurement area to another. It is unlikely that cameras 156 and 110 can be adjusted mechanically so that their fields of view correspond to each other exactly, pixel-for-pixel.
  • an automatic measurement operation can start.
  • the shutter 125 is opened so as to unblock the optical path 99.
  • the first wafer to be measured is loaded on the stage 101.
  • the stage is driven to the pre-stored x/y scale position such that the desired measurement point is disposed coincident with the optical axis.
  • the system attempts to acquire focus by using the outputs from each of the designated photo- sensors of the array 118.
  • the focus area is "in focus” if the coherence measurement computed from the output of the designated diodes is maximized. That is, the coherence values computed from the designated photo- sensor outputs should be substantially identical to the coherence values obtained during the set-up procedure. If the focus area is "in focus", then the area of measurement, i.e. target of measurement, should be located on the optical axis, i.e. in the center of the image.
  • the measurements of the target can be acquired by the computer and processed as taught by U.S. Patents 4,818,110 and 5,112,129 and co-pending U.S. patent application serial number 08/025,435 assigned to the assignee of the present invention. In most cases focus is easily achieved.
  • the stage due to errors in the x/y measurements, may not be at the proper location. Therefore, the field of view designation, as referred to by the structures on the wafer, will be incorrect. Such an error makes it impossible to achieve focus.
  • the stage is driven in a spiral search pattern, and at designated points the system automatically attempts to achieve focus. Once this is accomplished, the system tries to locate the targets of measurement. In the great majority of cases, the target will be close to the optical axis and to position it on the axis requires only a determination of the x/y off-sets by cross-correlating the reference image, acquired during set-up, with image seen by the camera. The stage is then driven to the right location.
  • a suitable flat site is selected for achieving focus.
  • the relative heights of the flat site and the measurement location are determined.
  • the system is focused on the flat site and the stage is moved to the measurement site and the microscope height is adjusted by the amount determined during set-up.
  • An alternate implementation of the invention is to use camera 110 both to acquire the image for focusing and to perform the measurements.
  • the operator can view the image as stored in the computer and designate the area via a mouse where focus is to be achieved.
  • the coherence function is then computed but only for the pixels of the designated area.
  • beamsplitter 114, lens 120 and diode array 118 may be omitted.
  • a “fast” camera is a camera which can operate at a frame rate significantly greater than 30 Hz. If such a camera is not used, the amount of time required to detect focus will be considerable.
  • the above description illustrates the focusing method used for coherence microscopes.
  • the present invention primarily relates to microscopes using Linnik interferometry, and is particularly useful in conjunction with all types of interference microscopes and techniques including those described by U. S. Patents 4,818,110 and 4,885,583. However, the concepts of the present invention can also be used, with some modifications, in conventional microscopes, i.e. microscopes not using interferometric techniques.
  • the same basic approach may be used with conventional microscopes, i.e., those equivalent to the system 103 having the shutter 125 closed.
  • the contrast instead of coherence, is maximized.
  • the operator designates a group of diodes, which "see” a bright (high reflectance) area of the target, and another group, which "see” a dark (low reflectance) area of the target.
  • the signal from the first group is denoted by S ⁇ , and from the second group by S m ⁇ n . (These signals are measured for each diode relative to the dark-level output of that diode.)
  • Assembly 126 is driven vertically, and the best focus is deemed to occur where the contrast is at a maximum.
  • the system of the present invention is substantially distinguishable from the device disclosed in the referenced Balasubramanian patent which uses a CCD array to provide a surface profile. Since a profile of the entire surface of the test object is required, all the apertures (i.e. sensors) of the array must be utilized in order to assure that no portions of the surface will be omitted from measurement. It is important to note that the device in Balasubramanian attempts to identify and measure all local irregularities in height that are present in the test object. In fact, it is these irregularities that are measured by the diode array in order to obtain a true indication of the accuracy of replication of the test object with respect to the reference object.
  • the system of the present invention uses the diode array in a secondary optical system to perform an autofocusing function in order that a primary optical system may be used to accurately inspect characteristics of the wafer.
  • a primary optical system may be used to accurately inspect characteristics of the wafer.
  • only those areas of the wafer that facilitate focusing are selected, e.g. perhaps only flat sites of a certain height on the wafer will be selected. Consequently, while the array could perhaps cover an entire field of view, only a portion of the photo-sensors of the array will be designated.
  • the system of the present invention essentially ignores areas of the wafer surface that can detract from the focusing operation, and is thus immune to detracting wafer features such as local irregularities in height.
  • the set-up operation can be speeded up by automatically selecting the diodes corresponding to the area of interest.
  • the method of automatically identifying the area of interest is described below.
  • the object is to designate all diodes that cover the area, so as to make the system more immune to local irregularities in height.
  • Fig. 6 illustrates typical waveforms or modulation envelopes of the diode outputs from array 118 while assembly 126 is moved vertically i.e., in the Z direction.
  • Waveform A is a typical waveform for an area suitable for focusing because it exhibits a good signal-to-noise (S/N) ratio, i.e.
  • waveform E is clearly unsuitable because this envelope of the waveform has no sharp peaks.
  • Waveforms A, B, C, and D have good signal-to-noise ratios and the diodes providing the outputs forming these waveforms are suitable candidates for selection.
  • the modulation envelope C peaks at a different location in Z from the waveforms A, B and D. This disparity indicates that the regions covered by detectors producing waveforms A, B and D, are at different Z levels from the region covered by the detector producing waveform C.
  • the diodes producing waveform C are not suitable candidates for selection.
  • Envelopes A and B peak at the same Z value but the waveforms are 180 degrees out of phase. This indicates that the detector producing one of the waveforms covers a surface region that is overlaid by a transparent substance, such as a photoresist. Adding waveforms A and B together would result in phase cancellation and therefore no optimum focal indication.
  • the envelopes of B and D have simultaneous peaks and are in- phase. Therefore, these waveforms can be added together.
  • a z-scan waveform output from each detector is collected. Specifically, the output from each diode is measured as the system scans or translates in the z direction away from the stage. Typical waveforms are illustrated in Fig. 6.
  • the signal- o- noise (S/N) ratio of each waveform is determined using methods known in the art.
  • waveforms having multiple peaks are eliminated in the step indicated at 212.
  • a set of single peak waveforms remain for consideration after the completion of this step.
  • the step indicated at 214 of the remaining single peak waveforms, those that peak within 4 micrometers of each other are identified and grouped into a plurality of waveform groups.
  • step indicated at 216 within each group of waveforms, eliminate waveforms from the same group which have phase differences greater than ⁇ /4 from the average of the remainder of the group. If the grouping by phase results in more than one subgroup of the original group, choose the subgroup with the highest S/N ratio.
  • the result of step 216 is several groups of waveforms, each within a phase difference of 7r/4.
  • for every combination of waveforms within a group compute the sum of the waveforms to generate a summed waveform for each group. Then, for each summed waveform, compute a summed waveform S/N ratio. Choose the diode combination that provides the highest signal/noise ratio.
  • primary camera 100 the camera whose focus coincides with that of the diode array, is focused at the height of one target during set-up by moving assembly 126 appropriately. Then, while assembly 126 is held fixed, secondary camera 156 is focused at the level of the second target by moving the secondary camera along the Z-axis relative to the primary camera.
  • These operations during the set-up adjust the relative focus levels of the two cameras to correspond to the levels needed in the subsequent automatic measurements.
  • all targets have identical height differences. Therefore, if the primary camera is focused by driving assembly 126 vertically, this automatically ensures that the secondary camera will also be in focus on its target.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Vision & Pattern Recognition (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Microscoopes, Condenser (AREA)
  • Automatic Focus Adjustment (AREA)
  • Focusing (AREA)

Abstract

Un procédé et un appareil permettent de focaliser automatiquement un microscope à haute résolution (103). Pendant la préparation, l'utilisateur désigne des zones de chaque champ observé où une mesure doit être effectuée, et pour chaque zone intéressante, il déplace une partie mobile (126) du microscope le long de son axe optique (axe Z, 102), tout en mesurant des intensités d'image dans des sous-zones distinctes propres à cette zone. Ces intensités d'image sont alors évaluées, et celles qui présentent le meilleur rapport signal sur bruit et se rapportent à un point commun, situé sur l'axe Z, sont choisies et les sous-zones correspondantes identifiées. Pendant des inspections ultérieures de cette zone intéressante, seule la lumière reflétée depuis les sous-zones identifiées servira à focaliser le microscope. Cette invention sert aussi bien en microscopie conventionnelle qu'en interférométrie.
PCT/US1995/013406 1994-10-21 1995-10-20 Dispositif et procede de focalisation automatique destine a un microscope a haute resolution WO1996012981A1 (fr)

Applications Claiming Priority (2)

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US32767994A 1994-10-21 1994-10-21
US327,679 1994-10-21

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US6341180B1 (en) 1997-12-18 2002-01-22 Cellavision Ab Image content autofocus for microscopy using a noise-insensitive focus filter
EP1353211A3 (fr) * 2002-03-28 2004-01-21 National Institute of Advanced Industrial Science and Technology Système de microscope de transmission tridimensionnel, méthode d'affichage d'image et dispositif de traitement d'image
EP1494059A1 (fr) * 2003-07-03 2005-01-05 Carl Zeiss SMS GmbH Procédé de focalisation automatique lors de l'imagerie optique d'un objet
EP1441215A4 (fr) * 2001-10-31 2007-10-03 Olympus Corp Dispositif d'observation de type lecteur optique
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