United States Patent [19]
`Norton et al.
`
`II 1111111111111
`
`US005747813A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,747,813
`May 5, 1998
`
`[54] BROADBAND MICROSPECTRO(cid:173)
`REFLECTOMETER
`
`[75]
`
`Inventors: Adam E. Norton, Palo Alto; Chester
`L. Mallory, Campbell; Hung V. Pham,
`San Jose; Paul Rasmussen, livermore,
`all of Calif.
`
`[73] Assignee: KLA-Tencop. Corporation, San Jose,
`Calif.
`
`[21] Appl. No.: 227,482
`
`[22] Filed:
`
`Apr. 14, 1994
`
`Related U.S. Application Data
`
`[63] Continuation ofSer. No. 899,666, Jun. 16,1992, abandoned.
`Int. CL6
`..................................................... GOlN 21/33
`[51]
`[52] U.S. Cl ....................... 250/372; 250/339.11; 356/448
`[58] Field of Search ............................... 250/372, 339.11;
`3561448, 326, 328
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`Re. 34,783 11/1994 Coates.
`3,214,596 10/1965 Schwerdt, Jr. et al ..
`3,752,559
`8/1973 Fletcher et al ..
`3,975,084
`8/1976 Block .
`3,999,855 1211976 Hirschfeld 0
`4,106,856
`8/19l& Babish.
`4,158,505
`6/1979 Mathisen et al ..
`4,645,349
`211987 Tabata ..................................... 3561382
`4,712,912 1211987 Messerschmidt .
`4,758,088
`7/1988 Doyle .
`4,770,530
`9/1988 VanAken et al. ...................... 3561323
`
`4,795,256
`4,844,617
`4,848,904
`4,899,055
`4,906,844
`4,945,220
`4,983,823
`5,067,805
`
`1/1989 Krause et al. .......................... 356/320
`7/1989 Kelderman et al ..................... 356/372
`7/1989 Sapp et al ..
`211990 Adams .
`3/1990 Hall 0
`7/1990 Mallory et al ..
`111991 Isobe .
`1111991 Code et al ..
`
`FOREIGN PATENT DOCUMENfS
`
`29 05 727 AI 1111979 Germany ............................... 3561448
`57-106846
`7/1982
`Japan ..................................... 3561326
`1-308930 1211989
`Japan ..................................... 3561328
`
`Primary Euzminer-Constantine Hannaher
`Attome)> Agent, or Finn--Majestic, Parsons, Siebert &
`Hsue
`
`[57]
`
`ABSTRACT
`
`An improved method and apparatus for measuring the
`relative reflectance spectra of an observed sample (3) and
`method and apparatus for autofocussing the sample (3). A
`broadband visible and ultraviolet beam ( 42) is split into a
`sample beam (46) and a reference beam (48). The sample
`beam ( 46) is reflected off the surface of tbe sample (3), and
`tbe spectrum of the reflected sample beam ( 46) is compared
`to the spectrum of the reference beam ( 48) to determine the
`relative reflectance spectrum of tbe sample (3). A video
`camera (96) is provided for viewing the sample (3). Tiie
`autofocus system has a course-focus mode and a fine-focus
`mode. In the course-focus mode, the sample (3) is focused
`when the centroid of tbe sample image is centered on a
`position sensitive detector (99). In the fine-focus mode, the
`sample is focused when the intensity of light reaching the
`detector (99) is minimized.
`
`57 Claims, 7 Drawing Sheets
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 1 of 7
`
`5,747,813
`
`I
`
`•
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 2 of 7
`
`5,747,813
`
`~
`I . =;=
`
`I
`I
`
`I
`I
`
`II~ a
`~~ ~
`~---q
`
`I
`I
`
`I
`I
`
`I
`I
`
`CD en
`
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 3 of 7
`
`5,747,813
`
`FIG. 3.
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 4 of 7
`
`5,747,813
`
`LIGHT ENTERS
`HERE 41A
`
`LIGHT
`EXITS HERE
`418
`
`I
`1--d
`..,__-~~106
`104
`ITO SAMPLE SPECTROMETER
`I APERTURE 54
`
`d I I
`
`I
`
`45
`
`REFLECTED
`GROSS-SECTION
`OF COMBINED
`BEAM 42
`
`I' 1
`'---+---r---P'I : i \ \
`4~
`: I : 41 \
`I
`I·
`
`C::: 41
`
`40
`
`3
`
`\
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`;
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`I
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`'
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`\
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`1
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`J
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`I
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`I
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`\\\'!,/
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`\
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`I
`
`I I
`
`I
`
`FIG. 4.
`
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 5 of 7
`
`5,747,813
`
`RELATIVE POSITION
`.... OF DETECTOR 99
`
`IMAGE
`DARK
`OF SAMPLE
`SPECTROMETER~~H+ttH~
`PINHOLE 58
`
`IMAGE OF
`LIGHT
`IJJJ:U.I.H+H++f+H.If~APERTURE IN MIRROR
`28
`
`~DARK AREA OUTSIDE
`MEASUREMENT BEAM
`
`WAFER
`
`IN FOCUS
`
`RELATIVE POSITION
`SHADOW., DUE TO VANES OF
`0 BJ ECT IV E 4 0 ORrr.1iPmT~UP""I L,S;,;,;TOn,P ITT'I'4nnl -~rrrrnrrTTn'T'ITTTT'Ii-rrTT10;:,f ,....DETECTOR 99
`CENTROID OF LIGHT FALLING
`ON DETECTOR 99
`OUT-OF- FOCUS IMAGE OF
`APERTURE
`IN MIRROR 28
`
`WAFER OUT OF FOCUS
`
`FIG. 5.
`
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`U.S. Patent
`
`May 5, 1998
`
`Sheet 6 of 7
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`5,747,813
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`IMAGE OF
`DARK
`SAMPLE APERTURE 58-
`
`WAFER
`
`IN FOCUS
`
`_RELATIVE POSITION OF
`DETECTOR 99
`UJ..I+Ifffl"- DARK REGION LYING OUTSIDE
`APERTURE 30
`
`ITI'H'+-H-41-- LIGHT FRINGE REGION
`
`Ll..I.IJ+l-I+Htttttt-LIGHT OUT·OF·FOCUS IMAGE OF
`FOCUS BEAM 63
`
`- DARK REGION
`
`ll'l'H+~DARK IMAGE OF SAMPLE
`APERTURE 58
`
`WAFER OUT OF FOCUS
`
`l POSITION
`
`FIG. 6.
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`U.S. Patent
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`May 5, 1998
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`Sheet 7 of 7
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`5,747,813
`
`--
`
`118
`""90% uv
`..,2% VISIBLE
`
`FIG. 7.
`
`TO CAMERA OPTICS
`
`I
`
`•99.95% uv
`l/V .05 'lo VISIBLE
`
`lfrl22
`
`"'90'/o VISIBLE
`"'2% uv
`f
`y~~4
`
`I
`
`cll2
`
`I
`I
`
`t--uo
`
`TO POSITION
`SENSITIVE DETEC::...;,;TO;o...:..,:R __
`AND VIEWING
`OPTICS
`
`_ _c50 W TUNGSTEN LAMP
`
`75 W XENON LAMP
`
`/'
`
`I
`'
`',,~ 30 WATT DEUTERIUM LAMP
`'
`0.01 +---'L--....,,-----~-~-
`200
`400
`600
`800
`WAVELENGTH (NN)
`FIG. 9.
`
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`5,747,813
`
`1
`BROADBAND MICROSPECTRO(cid:173)
`REFLECTOMETER
`
`This is a continuation of application Ser. No. 07/899,666,
`filed Jun. 16, 1992, now abandoned.
`
`5
`
`BACKGROUND OF THE INVENTION
`
`10
`
`15
`
`2
`The partially reflecting mirror is also difficult to manufacture
`with good efficiency when a very wide range of wavelengths
`are to be used. Even in the best case, the losses due to the
`partially reflecting mirror are squared, as the light must pass
`through the element twice.
`U.S. Pat. No. 4,844,617 (Kelderman et al., 1989)
`describes a confocal focus microscope with an autofocus
`feature. In that confocal microscope, a system of light
`sources, stops, and light intensity detectors is used to auto(cid:173)
`matically detect the focus of the confocal microscope and
`adjust the focus to bring a workpiece into focus. Once the
`workpiece is in focus, light reflected from the surface of the
`workpiece is directed to a spectrometer. However, in that
`device, separate sources are required for field illumination,
`and the direction of focus is not determinable. In addition, a
`separate source for ultraviolet (UV) light is required, and
`only a narrow band of UV light is detected.
`
`SUMMARY OF THE INVENTION
`
`The present invention relates to the field of small spot
`spectral reflectometry. More specifically, in one
`embodiment, the invention provides an improved method
`and apparatus for obtaining an accurate relative reflectance
`spectrum from a sample under a microscope. The samples
`are typically semiconductor wafers which may -contain a
`number of layers over a silicon substrate.
`A relative reflectance spectrum has a variety of uses. The
`thickness of the various films on the wafer can be deter(cid:173)
`mined from a relative re:flectance spectrum. Also, the reflec(cid:173)
`tance at a single wavelength can be extracted. This is useful
`where the re:flectance of photoresist coated wafers at the 20
`In light of these problems, a new apparatus and method
`wavelength of lithographic exposure tools must be found to
`for measuring the relative reflectance spectra of samples has
`determine proper exposure levels for the wafers, or to
`been invented. The present invention provides measurement
`optimize the thickness of the resist to minimize the reflec(cid:173)
`illumination which is split into a sample beam and a refer(cid:173)
`tance of the entire film stack. The refractive index of the film
`ence beam. The sample beam is reflected from the surface of
`can also be determined by analysis of an accurately mea- 25
`sured re:flectance spectrum.
`a sample and is directed onto a diffraction grating and a
`linear photodiode array, or other spectrum detecting means.
`Because of the tight tolerances in the semiconductor arts,
`The first order diffraction beam from the grating is sensed by
`an accurate means for measuring the relative reflectance
`the linear photodiode array. The reference beam is also
`spectrum of a wafer is needed. Typically, monochromatic
`focused onto the diffraction grating and a second linear
`light or broadband light is reflected off the wafer, and the 30
`photodiode array senses the reference spectrum. By dividing
`reflected light is collected and measured. Several difficulties
`the reference spectrum at each wavelength by the spectrum
`arise when trying to collect light reflected from the sample.
`from the sample, the relative reflectance spectrum of the
`First, the sample must be in focus, otherwise the reflected
`sample is found. The reference spectrum is used to eliminate
`light disperses. Second, the sample must not tilt with respect
`the effects of a shift in the spectrum or a variation intensity
`to the optics if the optics are sensitive to shifts in the 35
`of the illumination. The relative reflectance spectrum can be
`reflected light beam or the optics must be designed to be
`used to compare the reflectance of one sample relative to
`insensitive to sample tilt, otherwise the reflected light will
`another sample at any measured wavelength. The absolute
`not be fully captured by the instruments detecting the
`reflectance can be determined by comparing the relative
`reflected beam. Lastly, the light source itself may vary in
`intensity and spectrum, thus distorting the reflectance 40 reflectance spectrum to a known reflectance standard such as
`bare silicon.
`results.
`·
`. The measurement illumination is split into the sample
`U.S. Pat. No. 4,645,349 (Tabata, 1987) describes a film
`beam and the reference beam by a totally reflecting mirror
`thickness measuring device which determines the thickness
`placed across one half of the optical path of the measurement
`of a film from a measured reflectance spectrum. A broadband
`illumination. An autofocus mechanism is provided, with a
`light source (16) illuminates a monochromator (19), which, 45
`coarse focus mode and a fine focus mode. In the coarse focus
`through a partially reflecting mirror (22), illuminates a film
`mode, the autofocus mechanism uses the semicircular or
`(31). The monochromator filters the broadband light by
`semi-annular sample beam to quickly focus the sample
`reflecting it off a diffraction grating (20), and the monochro(cid:173)
`under a microscope objective. The sample beam falls on the
`mator output wavelength is selected by rotating the diffrac(cid:173)
`center of a position sensitive detector if the sample is in
`tion grating with respect to a directional mirror. A reflected 50
`focus, otherwise it falls to one side. The side it falls on
`beam from the film is reflected back along the original
`indicates whether the out-of-focus condition is due to the
`optical path, and is reflected out of the original optical path
`sample being too close to or too far from the microscope
`by the partially re:flecting mirror. The reflected beam then
`objective.
`illuminates a photo-multiplier tube (26), and the output of
`the photo-multiplier tube is connected to a graphics device 55
`In the fine focus mode, apertures of similar size at each of
`(30), which is also connected to a wavelength output of the
`two focal points of the microscope objective are used. A
`focussing beam. which is from the same light source as the
`monochromator, allowing the graphics device to display a
`graph of reflectance versus wavelength. However, since a
`measurement illumination, passes through one aperture,
`scanning monochromator is used, obtaining the reflectance
`passes through the objective, is reflected off the sample, and
`is focused again onto the second aperture. If the sample is in
`spectrum is time consuming, and no means is provided to 60
`ensure that the intensity of the incident light is uniform over
`focus, the light from the first aperture will pass through the
`the time period of measurement. Furthermore, the system of
`second aperture. Consequently, the light falling on the stop
`Tabata assumes the sample is in focus. If the sample is not
`which forms the second aperture indicates whether the
`in focus, the reflected light may not be sufficiently focused
`sample is in focus by the amount of light falling on the stop.
`by the objective to provide a useful spectrum. The optics 65
`This light falling on the stop is reflected to an intensity
`also present special problems, because the diffraction grat(cid:173)
`detector. The autofocus mechanism adjusts the focus of the
`ing must be precisely aligned with the directional mirror.
`sample so as to minimize the light falling on the detector.
`
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`3
`The present invention has many applications, such as
`measuring reflectance spectra, measuring refractive indices,
`measuring fibn thicknesses, and determining lithographic
`exposure times, to name a few.
`A further understanding of the nature and advantages of 5
`the inventions herein may be realized by reference to the
`remaining portions of the specification and the attached
`drawings.
`
`BRIEF DESCRWTION OF TilE DRAWINGS
`
`FIG. 1 is a top view of an embodiment of a broadband
`small spot spectral reflectometer, camera and autofocus
`apparatus according to the present invention;
`FIG. 2 is a front view of the apparatus of FIG. 1;
`FIG. 3 is an isometric view of the apparatus of FIG. 1;
`FIG. 4 is a simplified illustration of a beamsplitter mirror
`and an objective, showing their relative placement;
`FIG. 5 is an illustration of the light patterns created on a
`position sensitive focus detector in the autofocus system in
`a coarse focus mode when a sample is out of focus;
`FIG. 6 is an illustration of the light patterns created on a
`position sensitive focus detector in the autofocus system in
`a fine-focus mode and the fine focus response as a function
`of the distance between a sample and an objective;
`FIG. 7 is a partial illustration of an alternative embodi(cid:173)
`ment of a small spot spectral reflectometer using flip-in
`dichroic mirrors;
`FIG. 8 shows a beamsplitter cube with black glass applied 30
`to unused faces; and
`FIG. 9 is a graph of the output of various lamp types.
`
`4
`shown in the graph of FIG. 9.JYpically, a tungsten lamp and
`a deuterium lamp are used in combination to cover the same
`spectrum covered by a xenon lamp, however this combina-
`tion still leaves a gap in brightness in the mid-UV wave(cid:173)
`lengths. Brightness of the spectrum is important, because
`with less intensity, light must be collected for longer periods,
`thus lower intensities slow the measurement process.
`Normally, xenon lamps are disfavored due to the instability
`of their intensity and the fact that the arc may shift positions.
`10 However, in the present invention both of these drawbacks
`have been overcome.
`Off-axis paraboloid mirror 16 collimates light beam 12,
`which can be optionally filtered by flip-in UV cutoff filter 18
`and color filter wheel 20. Flip-in UV cutoff filter 18 is used
`15 in part to limit the spectrum of light beam 12, so that when
`light beam 12 is dispersed by a diffraction grating, the first
`and second order diffraction beams do not overlap. Part of
`light beam 12 is reflected by fiat mirror 22 onto concave
`mirror 24 to form measurement beam 25. Concave mirror 24
`20 focuses measurement beam 25 onto the end of large-core
`silica fiber 26, which acts as a radial uniformer. Thus,
`regardless of the intensity pattern in a cross section of
`measurement beam 25 at the input of large-core silica fiber
`26, the output of large-core silica fiber 26 is radially sym-
`25 metric. Without this radial uniformer, it is possible that the
`arc in lamp 10 might shift and cause the intensity of light
`across a cross section of measurement beam 25 to shift
`causing apparent fluctuations in the relative reflectance
`spectrum.
`Another part of light beam 12, field illumination beam 34,
`is focused by large achromat 32 near fold mirror 36, causing
`fold mirror 36 to reflect an image of lamp 10 toward small
`achromat 38. Small achromat 38 collects the light in field
`illumination beam 34 before the light reflects off aperture
`35 mirror 28. Aperture mirror 28 is a fused silica plate with a
`reflective coating on one side, with a 150 J.UU square etched
`from the reflective coating to provide an aperture for mea(cid:173)
`surement beam 25. The aperture is placed at one conjugate
`of an objective 40 (see FIG. 2). The field illumination can be
`40 tnmed off by placing field illumination shutter 31 in the
`optical path of field illumination beam 34.
`The narrow measurement beam 25 and wide field illumi(cid:173)
`nation beam 34 are rejoined at aperture mirror 28, with field
`illumination beam 34 reflecting off the front of aperture
`mirror 28, measurement beam 25 passing through the aper(cid:173)
`ture. The use of flip-in fine focus aperture 30 is explained in
`detail below.
`FIGS. 2 and 3 show more clearly the reflectometer,
`viewing and autofocus subsystems of optical system 8,
`including objective 40, a beamsplitter mirror 45, a sample
`beam 46, a reference beam 48, a concave mirror 50, a fiat
`mirror 43, a reference plate 52 with a reference spectrometer
`pinhole 56, a sample plate 54 with a sample spectrometer
`pinhole 58, a second fold mirror 68, a diffraction grating 70,
`a sample linear photodiode array 72, a reference linear
`photodiode array 74, a flip-in reference photodiode 95, a
`flip-in sample photodiode 93, an achromat 80 with a short
`focal length, a right angle prism 82, a beamsplitter cube 84,
`60 a penta prism 86, achromats 88, 90 with long focal lengths,
`a third fold mirror 89, a focus detector 98, a neutral density
`filter wheel 97 (not shown in FIG. 3, for clarity), a fourth
`fold mirror 91, and a video camera 96.
`Several magnifications are possible for objective 40. In
`one embodiment, a 15X Schwarzchild design all-reflective
`objective, a 4X Nikon CFN Plan Apochromat, color cor(cid:173)
`rected at three wavelengths, and a lX UV transmissive
`
`DESCRWTION OF Tiffi PREFERRED
`EMBODIMENfS
`
`FIGS. 1, 2 and 3 each show the same embodiment of an
`optical system according to the present invention, but from
`different angles. The elements are explained below in con(cid:173)
`junction with the figure which most clearly shows their
`placement with respect to other elements, and for clarity,
`some elements have been omitted from some figures.
`Referring to FIG. 1, an optical system 8 for measuring the
`relative reflectance spectrum of a wafer 3 and focussing
`wafer 3 with respect to the optical system includes an 45
`illumination subsystem, a reflectometer subsystem, a view(cid:173)
`ing subsystem, and an autofocus subsystem, wherein any
`given optical element may be part of more than one sub(cid:173)
`system.
`The illumination subsystem includes a lamp 10, typically 50
`a xenon arc lamp, which emits a light beam 12 of visible
`and/or UV light, a lamp housing window 14, an off-axis
`paraboloid mirror 16, a flip-in UV cutoff filter 18, a color
`filter wheel 20, a flat mirror 22, a concave mirror 24, a
`large-core silica fiber 26, an aperture mirror 28 with a flip-in 55
`40 J.UU fine focus aperture 30, a large achromat 32, a field
`illumination shutter 31, a fold mirror 36, and a small
`achromat 38.
`The illumination system provides a combined beam 42
`comprising a measurement beam 25 and a field illumination
`beam 34. Lamp 10 emits light beam 12 through a lamp
`housing window 14. The lamp housing window is not
`necessary for optical reasons, however it is provided to
`contain lamp 10 should the lamp crack and explode. A xenon
`lamp is preferred over other lamps such as tungsten or 65
`deuterium lamps, because a xenon lamp will give a flatter
`output covering a spectrum from UV to near infrared as
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`5
`The relative reflectance spectrum can then be simply
`objective are mounted on a rotatable turret which allows for
`one of the three objectives to be placed in the optical path of
`obtained by dividing the sample light intensity at each
`sample beam 46.
`wavelength by the relative reference intensity at each wave(cid:173)
`The measurement of the relative re:tlectance spectra of
`length. Typically, this might involve 512 division
`wafer 3 will now be described. A typical relative re:tlectance s
`computations, where 512-diode linear photodiode arrays are
`spectrum 102 is shown in FIG. 2. When field illumination
`used to record the sample and reference spectra. A typical
`shutter 31 is placed in the path of field illumination beam 34,
`spectrum ranges from 220 nm to 830 nm.
`combined beam 42 comprises only measurement beam 25.
`In one embodiment of the present invention, diffraction
`Combined beam 42 is split by beamsplitter mirror 45, a
`grating 70 is a concave holographic grating and the spec(cid:173)
`totally re:tlecting mirror placed so as to de:tlect half of 10
`trometer pinholes are 15 mm apart. The diffraction grating is
`combined beam 42 towards objective 40, thus forming
`holographically corrected to image multiple spectra, since
`sample beam 46, the unde:tlected half of combined beam 42
`the 15 mm spacing does not allow for both beams to be
`fonning reference beam 48. Importantly, because sample
`centered on the grating. One such grating is a multiple
`beam 46 and reference beam 48 are derived from the same
`spectra imaging grating supplied by Instruments S.A. Also,
`source, lamp 10, and because combined beam 42 is radially
`uniform, reference beam 48 and sample beam 46 have 15
`the grating is designed so that the angle of the detector
`causes re:tlections off the detector to fall away from the
`proportionally dependent spectral intensities. Furthermore,
`grating.
`since beamsplitter mirror 45 is a totally re:tlecting mirror in
`half of an optical path rather than a partially re:tlecting mirror
`FIG. 2 shows more clearly the positioning of beamsplitter
`in the entire optical path, a continuous broadband spectrum
`mirror 45 with respect to objective 40. Combined beam 42,
`is re:tlected with good brightness.
`20 which may include field illumination, is re:tlected off beam(cid:173)
`Reference beam 48 does not initially interact with beam(cid:173)
`splitter mirror 45 toward wafer 3. When re:tlectance spectra
`splitter mirror 45, but instead illuminates concave mirror 50.
`measurements and autofocussing are being performed, the
`Concave mirror 50 is slightly off-axis, so reference beam 48
`field illumination is off to minimize scattered light. An
`is re:tlected onto the reverse face of beamsplitter mirror 45,
`objective pupil stop 41 is placed in the optical path as shown
`where fiat mirror 43 re-re:tlects reference beam 48 into 25
`in FIG. 4.
`alignment with reference spectrometer pinhole 56. Hat
`Since light travels both directions through objective 40,
`mirror 43 is provided to realign reference beam 48 with
`objective pupil stop 41 is both an entrance pupil stop and an
`sample beam 46 so that both beams pass through their
`exit pupil stop. Furthermore, since the light passing through
`respective spectrometer pinholes substantially parallel. This
`allows for simpler alignment of the spectrometer element for 30 objective 40 has a semicircular cross section, one half (41a)
`of objective pupil stop 41 is the entrance pupil stop, and the
`both channels, since the reference beam enters the spec-
`trometer parallel to the sample beam.
`other half (41b) of objective pupil stop 41 is the exit pupil
`Since reference beam 48 does not interact with the surface
`stop. The exit half 41b is slightly larger than the entrance
`of beamsplitter mirror 45 which re:tlects beam 46, there is no
`half 41a to allow for deviation of the re:tlected light due to
`loss in reference intensity as reference beam 48 passed 35 wafer tilt.
`beamsplitter mirror 45. While reference beam 48 does
`Even with a wide field of view, objective pupil stop 41
`interact with a mirror on the reverse side of beamsplitter
`does not cause vignetting of the light re:tlected from wafer 3,
`mirror 45, these two mirrors are independent, since no light
`since objective pupil stop 41 is in the plane of the back focal
`passed through beamsplitter mirror 45. Indeed, in an alter-
`point of objective 40. However, since objective pupil stop 41
`native embodiment where the two re:tlecting surfaces of 40 is in the back focal plane, beamsplitter mirror 45 cannot also
`be in the back focal plane. To prevent the beamsplitter mirror
`beamsplitter mirror 45 cannot easily be placed together on
`45 from vignetting the re:tlected sample beam 46 when a
`one optical element, the re:tlecting surfaces exist on separate
`wide field of view is used, beamsplitter mirror 45 is placed
`mirror elements.
`The focal length of concave mirror 50 is such that
`with its edge 104 a slight distance, d, which is about 0.4 to
`reference beam 48 is in focus at reference spectrometer 45 1.2 mm, from an optical axis 106 of objective 40. Edge 104
`pinhole 56. The light passing through reference spectrom-
`is also an acutely angled edge, to avoid interference between
`the side of beamsplitter mirror 45 and the re:tlected sample
`eter pinhole 56 and re:tlecting off fold mirror 68 is dispersed
`beam 46. The acute angle, a., is about 10° from normal to the
`by diffraction grating 70. The resulting first order diffraction
`beam is collected by reference linear photodiode array 74,
`mirror surface.
`thereby measuring a relative reference spectrum.
`50 With a concave grating, the local efficiency varies sub-
`Sample beam 46 is re:tlected off beamsplitter mirror. 45
`stantially across its face. When wafer 3 tilts, the beam falling
`towards objective 40, where sample beam 46 is focused onto
`on the grating shifts slightly, and changes the signal level.
`wafer 3, and the re:tlected sample beam 46 is focused by While this effect causes less than a half percent change in
`objective 40 onto sample spectrometer pinhole 58. The
`re:tlectance measurements, a map of a wafer's re:tlectance
`re:tlected sample beam 46 does not interact with beamsplitter 55 would be strongly in:tluenced by the :tlatness of the wafer. To
`mirror 45 on the re:tlected path, because sample beam 46
`avoid this sensitivity, a different spectrum measuring means
`is used, namely flip-in photodiodes 93, 95.
`passed through the space behind beamsplitter mirror 45,
`where reference beam 48 also passes. The light passing
`While :tlip-in photodiodes only measure a single intensity
`through sample spectrometer pinhole 58 and re:tlecting off
`value, often the value is a broadband average over a spec-
`fold mirror 68 is dispersed by diffraction grating 70. As with 60 trum of interest and provides sufficient information for
`the reference beam, the resulting first order diffraction beam
`calculating the full relative re:tlectance spectrum. Flip-in
`of the sample beam is collected by sample linear photodiode
`photodiodes 93, 95 are selected such that they have a broad
`array 72, thereby measuring the sample spectrum. Because
`sensitivity to wavelengths in the lN band, with both pho-
`the two beams cross at diffraction grating 70, the photodiode
`todiodes having substantially similar peak sensitivity wave-
`array apparently aligned with sample beam 46 in FIG. 3 is 65 lengths. The sensitivity of each photodiode drops off gradu-
`in fact the photodiode array for reference beam 48, and vice
`ally for higher and lower wavelengths. Consequently, when
`versa.
`the response of sample photodiode 93 is divided by the
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`7
`response of reference photodiode 95, the result is an indi(cid:173)
`cation of the reflectance of wafer 3 over wavelengths in the
`UV band, with the peak sensitivity wavelength having more
`weight in the measure of reflectance than other wavelengths.
`FIG. 7 shows an alternative embodiment of optical system
`8, where flip-in photodiodes 93, 95 are fixed outside the
`reference and sample spectrometer paths, and mirrors are
`used to optionally deflect the beams onto the fixed photo(cid:173)
`diodes. FIG. 7 only shows one such system, however two
`identical systems are used, one for the sample channel and
`one for the reference channel. Combined visible and UV
`light in beam 110 passes through entrance pinhole 112,
`which is equivalent to reference spectrometer pinhole 56 or
`sample spectrometer pinhole 58. Beam 110 is then partially
`reflected by flip-in dichroic mirror 114. Most of the visible
`light passes through flip-in dichroic mirror 112 and most of
`the UV light is reflected towards a fused silica lens 116.
`Fused silica lens 116 is provided to increase the optical path
`for greater flexibility in mounting optical elements, but is not
`required. Beam 118, containing about 90% of the UV light
`from beam 110 and only about 2% of the visible light from
`beam 110, is reflected off stationary dichroic mirror 120,
`which further filters out visible light Beam 118 is reflected
`onto photodiode 122, which measures the intensity of light
`in beam 124.
`In another embodiment, mirror 114 is large enough to
`reflect both the sample beam and the reflected beam, and
`mirror 120 is large enough to reflect both the sample beam
`and the reference beam onto respective photodiodes. Pho(cid:173)
`todiode 122 serves the same purpose as photodiodes 93, 95,
`however, because of the filtering provided by the dichroic
`mirrors, the sensitivity of photodiode 122 to visible light is
`less of a concern. Mirror 114 need not be a flip-in niirror.
`Mirror 114 can be a fixed mirror, where the attenuation of
`UV light in the beams directed to diffraction grating 70 is not
`a !X"Oblem.
`In yet another embodiment, photodiode 122 is fixed to
`intercept beam 118 without reflection from mirror 120, thus
`requiring only one dichroic mirror. This arrangement is
`useful where one mirror sufficiently filters out enough
`unwanted visible light from beam 110 to avoid intensity
`detecting errocs by photodiode 122.
`Because of the innovative arrangement of optical ele(cid:173)
`ments used to measure relative reflectance spectra, many of 45
`the elements of in the spectra measurement subsystem are
`also used in the autofocus subsystem. For example, since
`relative reflectance spectrum measurement in the embodi(cid:173)
`ment shown in the figures uses beamsplitter mirror 45 as
`opposed to a partially reflecting mirror, the resulting beam
`reflected from the wafer 3 has a circularly asymmetric cross
`section. This allows for detecting the direction of focus as
`well as the relative distance to move objective 40 or wafer
`3 to achieve an in-focus condition, whereas with a circularly
`symmetric cross section, the direction of focus cannot be
`detected.
`The autofocus subsystem of optical system 8 uses the
`image reflected from sample plate 54. Sample plate 54 is a
`reflective fused silica plate with an aperture. For simplicity,
`an identical reflective fused silica plate with an aperture is
`used as reference plate 52, however reference plate 52 need
`not be reflecting.
`The image reflected from sample plate 54 is also used for
`viewing wafer 3. As shown in FIGS. 2 and 3, sample beam
`46 is partially reflected off sample plate 54, through short
`focal length achromat 80, right angle prism 82, into beam(cid:173)
`splitter cube 84. Beamsplitter cube 84 splits the incoming
`
`8
`beam into a camera beam 65 and a focus beam 63. Camera
`beam 65 is then reflected in penta prism 86, focused by long
`focal length achromat 90, and reflected into video camera 96
`by fold mirror 91. A penta prism is used instead of a mirror,
`5 so that the image received by video camera 96 is a non(cid:173)
`inverted image of wafer 3.
`In the embodiment shown in FIGS. 2 and 3, long focal
`length achromat 88 and turning mirror 89 direct beam 63
`onto detector 99. In an alternative embodiment (not shown),
`10 where less space is available, long focal length achromat 88
`is replaced by a medium focal length achromat and a
`negative lens such as a barlow lens. Turning mirrors are used
`if detector 99 is not mounted in the path of beam 63.
`Beamsplitter cube 84 is positioned slightly off-axis so that
`unwanted reflections from the faces of beamsplitter cube 84
`are skewed out of the optical path of the entering beam. This
`is accomplished by rotating the beamsplitter cube 1° to 10°,
`preferably 3° to 5°, about an axis normal to the reflection
`surface within the cube.