WO1997040422A1 - Amelioration d'image par interferometrie - Google Patents

Amelioration d'image par interferometrie Download PDF

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Publication number
WO1997040422A1
WO1997040422A1 PCT/GB1997/001006 GB9701006W WO9740422A1 WO 1997040422 A1 WO1997040422 A1 WO 1997040422A1 GB 9701006 W GB9701006 W GB 9701006W WO 9740422 A1 WO9740422 A1 WO 9740422A1
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WO
WIPO (PCT)
Prior art keywords
sample
image data
interference
interferometer
intensity
Prior art date
Application number
PCT/GB1997/001006
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English (en)
Inventor
Shimon Dov Yanowitz
Nigel Peter Smith
Michael John Hammond
Andrew Michael Christian Smout
Andrew Peter Keens
Original Assignee
Bio-Rad Micromeasurements Limited
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Publication date
Application filed by Bio-Rad Micromeasurements Limited filed Critical Bio-Rad Micromeasurements Limited
Priority to AU25163/97A priority Critical patent/AU2516397A/en
Publication of WO1997040422A1 publication Critical patent/WO1997040422A1/fr

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Classifications

    • 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
    • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
    • G03F7/70605—Workpiece metrology
    • G03F7/70616—Monitoring the printed patterns
    • G03F7/70633—Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching

Definitions

  • the invention relates to a method and apparatus for the inspection and ideally measurement of surface patterns particularly, but not exclusively, in instances where surface topography may be low or the surface is planarised.
  • the method and apparatus of the invention is particularly suited to the field of computerised optical tools such as metrology tools and so to the automated inspection and measurement of patterns on layers in a semiconductor multilayer wafer fabrication process.
  • An overlay metrology tool operates on an overlay target which often comprises two rectilinear marks. Typically, the bigger mark which is called the Outer, is printed as part of the selected reference layer. The smaller mark, called the Inner, is then printed inside the outer as part of the new layer that is being superimposed.
  • a typical overlay mark is illustrated in Figure 1. The tool determines the overlay error by first determining the centre point of each mark, and then computing the distance between these two centre points.
  • Optical inspection of small overlay marks or patterns has been performed using imaging apparatus of a conventional nature such as a microscope in combination with electronic image processing. In this way images have been viewed in order to determine the nature of their alignment.
  • imaging apparatus of a conventional nature such as a microscope in combination with electronic image processing.
  • images have been viewed in order to determine the nature of their alignment.
  • chemical and mechanical processes are now being used to polish and so planarize wafer surfaces and these techniques undesirably degrade the optical appearance of the mark on the wafer surface.
  • the polishing process itself may introduce asymmetries into the appearance of the mark that undesirably distort any measurements of overlay error that are made.
  • Figure 1 shows two camera images of overlay marks, each consisting of a plurality of pixels or image points with various intensities arranged in rows and columns.
  • the right hand side of the Figure shows an image in which the contrast between the two overlay marks and the wafer substrate is relatively high and which is therefore suitable for conventional analysis.
  • the left hand side of the Figure shows an image where the contrast between the outer mark and the wafer substrate has been degraded as a consequence of chemical and/or mechanical processing. It is of note that instances may occur where more than one mark is degraded and in these instances an image would be presented where both marks were difficult to distinguish.
  • the first step in measuring the overlay error represented by an image such as those in Figure 1 is to take one or more lines passing through the selected mark.
  • such lines are selected having regard to the geometry of the mark. For example, where a rectangular mark is used such lines are typically vertical and/or horizontal and lie along rows or columns in the image.
  • the line gives rise to an image cross-section profile where the displacement at each point in the profile represents the image signal level in each corresponding pixel along said line.
  • edge detection techniques are applied to locate the centres of the marks in the profiles.
  • the edges are separately determined for each mark in each axis.
  • the centre of each mark is then determined as the midpoint between opposite edges in the profile.
  • the overlay measurement result is determined as the distance between the centre points of the two marks.
  • a problem in the measurement process may occur as a result of poor contrast in the cross sections of either mark, for example where wafer fabrication processes, such as the chemical and/or mechanical polishing (CMP) process or other planarization processes, have degraded the topography and hence the contrast and of the optical image of overlay marks.
  • CMP chemical and/or mechanical polishing
  • Linnik interferometer for measuring an overlay misregistration during semiconductor wafer fabrication is described in US patent 5,438,413 and therefore will not be discussed in great detail herein.
  • the use of the Linnik interferometer is characterised by the use of an interference optical system that includes a sample channel and, distinct therefrom, a reference channel.
  • the two channels are typically positioned perpendicular with respect to each other and a beam splitter is used to direct wave energy such as light along each channel.
  • a camera is provided to detect the magnitude of mutual coherence between wave energy reflected from the sample and wave energy reflected from the reference mirror. The magnitude of this mutual coherence data is then used to generate a synthetic image of the sample. This synthetic image effectively enhances differences between parts of the sample surface and so enhances the contrast of the image.
  • Figure 3 shows the intensity response of the interference signal along the Z axis, which is dependent on the phase of the light reflected from the sample.
  • a further disadvantage associated with the Linnik interferometer concerns the requirement for long-path lengths of the image and referenced paths. This results in large fluctuations in these paths as a result of vibrations, thermal changes and other factors all of which contribute to degrade the quality of the obtained interference signals. It can therefore be seen that there is a need to provide a means for improving the visualisation of relatively flattened sample surfaces, but based on interferometry in order to obtain the advantages associated with the sensitivity of an interference signal to small height variations in a sample.
  • the Mirau interferometer consists of a special objective fitted to an otherwise standard bright-field metallurgical microscope. This objective has a plate beam splitter after its final element that allows the camera to simultaneously focus on the surface of the wafer and a small plane mirror spot deposited on a transparent surface placed equidistantly above the beam splitter.
  • the Mirau interferometer is illustrated in Figure 4. Constructive and destructive interference between these two images occurs, depending on the exact phase of the signal and reference rays.
  • the Mirau interferometer requires that a plane beam splitter be placed after the finite element of the objective, and thus that the objective needs to be positioned relatively farther from the sample to allow room for this beam splitter. It follows that the Numerical Aperture (NA) of the Mirau objective, affected by this distance, cannot be as large a figure as that achievable with the Linnik. Since the resolution of the objective is proportional to its Numerical Aperture, it follows that the resolution of the Mirau will be somewhat smaller. Given the need in bright field imaging to maximise resolution in order to visualise relatively small features, including low topography features such as those produced by chemical and or mechanical polishing of semiconductor waters, the Mirau has therefore been disregarded by those skilled in the art.
  • TIS Tool-Induced-Shift
  • a Mirau interferometer can be used to advantage for the measurement of low contrast overlay targets. It is therefore an object of the invention to provide an apparatus for the inspection and/or measurement of a pattern or mark on a relatively planarised surface which makes use of a Mirau interferometer.
  • a method of inspecting and measuring the degree of alignment between a first pattern or mark provided on a first surface and a second pattern or mark provided on a second surface which second surface is aligned with respect to said first surface so as to determine the amount of displacement error in said alignment comprising;
  • an interference optical system characterised in that it includes a single channel through which wave energy from a sample and wave energy from a reference passes prior to interference between same so as to provide interference image data of said sample as a result of mutual coherence between wave energy reflected from at least a part of said sample surface and wave energy reflected from at least a part of said reference surface;
  • said first and second patterns or marks are ideally aligned so as to be concentric such that the synthetic image is examined in order to determine the concentric nature of the said first and second marks and more specifically to determine whether the centre points of said first and second marks are perfectly aligned.
  • said interference optical system comprises a Mirau interferometer and thus said sample and reference wave energy travels in a single channel typically perpendicular to the plane of the sample surface.
  • More preferably still said apparatus consists of a metallurgical microscope and a Mirau interferometer and more specifically a standard bright-field metallurgical microscope and a Mirau interferometer.
  • an apparatus for inspecting and measuring the degree of alignment between a first pattern or mark provided on a first surface and a second pattern or mark provided on a second surface, which second surface is overlain and aligned with respect to said first surface, so as to determine the amount of displacement error in said alignment comprising: an overlay metrology tool including an interference optical system characterised in that said system includes a single channel through which wave energy from a sample and wave energy from a reference passes prior to interference between same so as to provide interference image data of said sample as a result of mutual coherence between wave energy reflected from at least a part of said sample surface and wave energy reflected from at least a part of said reference surface and imaging means for converting said interference image data into a synthetic image of said sample.
  • said interference optical system comprises a Mirau interferometer.
  • a Mirau interferometer in an overlay metrology tool.
  • a method as herein described for digital phase extraction and/or enhancement of interference image data provided by the use of a Mirau objective in a metrology tool.
  • each of the pixels in the image will undergo a cyclical change in intensity as constructive and destructive interference with the reference beam occurs. If the intensity of a given pixel is plotted against focus (Z-axis), the resulting graph will look like that of Figure 3.
  • / is the intensity
  • A is the amplitude function of the signal
  • ⁇ 0 is the initial value of the phase at location zero.
  • the Phase of the interference intensity is the argument of the cosine function ⁇ z+ ⁇ 0 . It is linear in Z. This means that when focusing on a sample, the phase of the intensity signal at each point of the sample image, is proportional to the topography of the sample at that point. Creating a synthetic image each pixel of which reflecting the phase value at that pixel, will therefore produce a topography map of the sample. Such a map image can be used for metrology purposes.
  • the intensity level at each point must be sampled at constant intervals along the Z axis. Extracting the phase and amplitude at every point can ideally be performed in the Fourier domain, by taking the Fourier Transform of the Z samples at each image point.
  • This process also allows to determine the best focus position for the image. It is simply the location in Z where the amplitude is the highest.
  • a method of digital phase extraction of interference image data provided by an interferometer metrology tool which method comprises measuring the intensity of each interference image data point having regard to the following equation;
  • Aa A ( z > cos( ⁇ z+ ⁇ 0 )
  • a method for creating a synthetic image of a sample using interference image data of said sample which method involves providing interference image data relating to said sample so as to provide a synthetic interference image of said sample made up of a plurality of pixels and then measuring selected pixel intensities of said image using the following equation;
  • phase extraction in order to recover data the phase can only be recovered in the range 0 to ⁇ , or - ⁇ to ⁇ as shown in figure 5. That means that unlike the original phase which is linear in Z, the extracted phase will only be linear in segments of Z. This implies that in general, an accurate topography image of the sample cannot be created, unless the topography changes are very small and the Z position is determined to a great degree of accuracy, so that the entire topography can be fitted in one segment.
  • a further advantage of this new method relative to the elaborate phase extraction is that it is much simpler in terms of numerical computations and therefore much faster to perform. Better measurement precision can be obtained by using this method since the straightforward difference operation used to derive the synthetic image is much less sensitive to the noisy and fluctuating signal which is characteristic of low contrast images, than the more elaborate phase extraction operation.
  • a method of digital phase enhancement of interference image data provided by an interferometer metrology tool which method comprises measuring the intensity of interference image data at selected pixel locations; modifying the intensity difference between said pixels; and then averaging said modified difference over at least one wave cycle to produce an intensity difference that is independent of Z position but representative of the phase difference between adjacent pixels.
  • said modification involves rectifying said intensity difference by taking an absolute value.
  • a first pixel is compared with a second pixel positioned at least one pixel remote therefrom, thus neighbouring pixels are typically not compared rather a pixel designated 1 would be compared with a pixel designated 3 or greater, regardless of direction.
  • a method for creating a synthetic image of a sample using interference image data of said sample which method involves providing interference image data relating to said sample and then enhancing said data by measuring the intensity of interference image data at selected pixel locations; modifying the intensity difference between said pixels; and then averaging said modified difference over at least one wave cycle to produce an intensity difference that is independent of Z position but representative of the phase difference between adjacent pixels.
  • said modification involves rectifying said intensity difference by taking an absolute value.
  • Figure 1 is an illustration of typical Bar-in-Bar type overlay marks and respective horizontal intensity profiles. Right: good contrast inner and outer marks. Left: case of low contrast outer mark;
  • Figure 2 shows an optical layout of the Linnik interferometer
  • Figure 3 shows characteristic intensity response of white light interference along the Z axis. The intensity varies much more rapidly than its envelope which, in turn, varies more rapidly than the bright-field intensity would.
  • Figure 4 shows an optical lay out of the Mirau interferometer
  • Figure 5 shows the phase of the signal of Figure 3, as extracted by means of numerical computations. Unlike the original phase, the extracted phase is triangular and varies between 0 to ⁇ (it can also be extracted to the range - ⁇ to ⁇ , in which case it will have a saw-tooth form with discontinuities at the boundaries).
  • Figure 6 shows the intensity variation of two adjacent pixels differing slightly in topography (and hence phase), along two periods in Z.
  • the third (smaller) signal shows the difference in intensities. It also has the same period.
  • Figure 7 shows the rectified difference signal and its average (the horizontal line) over two periods.
  • Figure 8 is a flow chart showing the method of the invention.
  • the invention therefore essentially concerns the use of interference techniques to undertake overlay measurements in the semiconductor industry. More specifically, the invention concerns the use of an Mirau interferometer to undertake such measurements and thus, as shown in Figure 8, the invention concems the provision of interference image data relating to a sample, by way of using a Mirau interferometer, and the subsequent analysis of said data either by digital phase extraction and/or digital phase enhancement so as to provide a synthetic image of said sample. Once the synthetic image has been provided one can then use this to determine the degree of displacement error between a first pattern or mark provided on a first surface and a second pattern or mark provided on a second surface when undertaking metrology measurements.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

L'invention se rapporte à un procédé de contrôle et de mesure du degré d'alignement entre un premier motif ou marque sur une première surface et un second motif ou marque réalisé sur une seconde surface, cette seconde surface étant alignée par rapport à la première. Le procédé consiste à utiliser des données d'image d'interférence qui sont générées par un interféromètre Mireau.
PCT/GB1997/001006 1996-04-19 1997-04-11 Amelioration d'image par interferometrie WO1997040422A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU25163/97A AU2516397A (en) 1996-04-19 1997-04-11 Image enhancement using interferometry

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB9608178.1A GB9608178D0 (en) 1996-04-19 1996-04-19 Image enhancement using interferometry
GB9608178.1 1996-04-19

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GB (1) GB9608178D0 (fr)
WO (1) WO1997040422A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0999475A2 (fr) * 1998-10-30 2000-05-10 Canon Kabushiki Kaisha Système pour détection de position et appareil d' exposition l' utilisant
US7054071B2 (en) * 2004-07-08 2006-05-30 Spectel Research Corporation Mireau interference objective lens

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5073018A (en) * 1989-10-04 1991-12-17 The Board Of Trustees Of The Leland Stanford Junior University Correlation microscope
EP0562133A1 (fr) * 1992-03-23 1993-09-29 Erland Torbjörn Sandström Méthode et appareillage de soudage
US5375175A (en) * 1992-03-06 1994-12-20 The Board Of Trustees Of The Leland Stanford Junior University Method and apparatus of measuring line structures with an optical microscope by data clustering and classification
US5438413A (en) * 1993-03-03 1995-08-01 Kla Instruments Corporation Process for measuring overlay misregistration during semiconductor wafer fabrication
US5633714A (en) * 1994-12-19 1997-05-27 International Business Machines Corporation Preprocessing of image amplitude and phase data for CD and OL measurement

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5073018A (en) * 1989-10-04 1991-12-17 The Board Of Trustees Of The Leland Stanford Junior University Correlation microscope
US5375175A (en) * 1992-03-06 1994-12-20 The Board Of Trustees Of The Leland Stanford Junior University Method and apparatus of measuring line structures with an optical microscope by data clustering and classification
EP0562133A1 (fr) * 1992-03-23 1993-09-29 Erland Torbjörn Sandström Méthode et appareillage de soudage
US5438413A (en) * 1993-03-03 1995-08-01 Kla Instruments Corporation Process for measuring overlay misregistration during semiconductor wafer fabrication
US5633714A (en) * 1994-12-19 1997-05-27 International Business Machines Corporation Preprocessing of image amplitude and phase data for CD and OL measurement

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
KINO G S: "SCANNING OPTICAL MICROSCOPY", JOURNAL OF VACUUM SCIENCE AND TECHNOLOGY: PART B, vol. 8, no. 6, 1 November 1990 (1990-11-01), pages 1652 - 1656, XP000169227 *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0999475A2 (fr) * 1998-10-30 2000-05-10 Canon Kabushiki Kaisha Système pour détection de position et appareil d' exposition l' utilisant
EP0999475A3 (fr) * 1998-10-30 2002-05-08 Canon Kabushiki Kaisha Système pour détection de position et appareil d' exposition l' utilisant
US6906805B1 (en) 1998-10-30 2005-06-14 Canon Kabushiki Kaisha Position detecting system and exposure apparatus using the same
US6972847B2 (en) 1998-10-30 2005-12-06 Canon Kabushiki Kaisha Position detecting system and exposure apparatus using the same
US7054071B2 (en) * 2004-07-08 2006-05-30 Spectel Research Corporation Mireau interference objective lens

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AU2516397A (en) 1997-11-12
GB9608178D0 (en) 1996-06-26

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