US 20050105090 A1
A method and system for spectroscopic ellipsometry employing reflective optics to measure a small region of a sample by reflecting radiation (preferably broadband UV, visible, and near infrared radiation) from the region. The system preferably has an autofocus assembly and a processor programmed to determine from the measurements the thickness and/or complex refractive index of a thin film on the sample. Preferably, only reflective optics are employed along the optical path between the polarizer and analyzer, a sample beam reflects with low incidence angle from each component of the reflective optics, the beam is reflectively focused to a small, compact spot on the sample at a range of high incidence angles, and an incidence angle selection element is provided for selecting for measurement only radiation reflected from the sample at a single, selected angle (or narrow range of angles). The focusing mirror preferably has an elliptical shape to reduce off-axis aberrations in the focused beam. Some embodiments include both a spectrophotometer and an ellipsometer integrated together as a single instrument. In such instrument, the spectrophotometer and ellipsometer share a radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Preferred embodiments of the ellipsometer employ a rotating, minimal-length Rochon prism as a polarizer, and include a spectrometer with an intensified photodiode array to measure reflected radiation from the sample, and a reference channel (in addition to a sample channel which detects radiation reflected from the sample).
1. A spectroscopic ellipsometer for measuring a sample, including:
a source which emits broadband radiation;
a polarizer for polarizing the broadband radiation, thereby producing a sample beam;
an analyzer positioned for receiving radiation of the sample beam that has reflected from the sample, wherein the analyzer produces an output beam in response to said radiation;
detector means for detecting the output beam; and
all-reflective optics between the polarizer and the analyzer, wherein the sample beam reflects with low incidence angle from each component of the all-reflective optics, and wherein the all-reflective optics reflectively focuses the sample beam to a small spot on the sample.
The invention relates to methods and systems for obtaining ellipsometric and reflectance measurements of a small region of a sample over a range of UV (and preferably also visible) wavelengths, and optionally also for determining, from the measurements, the thickness and refractive index of a very thin film on the sample. The sample can be a semiconductor wafer having at least one thin layer over a silicon substrate. Preferred embodiments of the invention include both a spectrophotometer and an improved spectroscopic ellipsometer which share a common focal point on the sample and preferably a common radiation source.
Among the well known nondestructive testing techniques are the techniques of spectroreflectometry and spectroscopic ellipsometry, which measure reflectance data by reflecting electromagnetic radiation from a sample. In spectroscopic ellipsometry, an incident radiation beam having a known polarization state reflects from a sample (generally at high incidence angle), and the polarization of the reflected radiation is analyzed to determine properties of the sample. Since the incident radiation includes multiple frequency components, a spectrum of measured data (including data for incident radiation of each of at least two frequencies) can be measured. Typically, the polarization of the incident beam has a time-varying characteristic (produced, for example, by passing the incident beam through a mechanically rotating polarizer), and/or the means for analyzing the reflected radiation has a time-varying characteristic (for example, it may include a mechanically rotating analyzer). Examples of spectroscopic ellipsometry systems are described in U.S. Pat. No. 5,329,357, issued Jul. 12, 1994 to Bernoux, et al., and U.S. Pat. No. 5,166,752, issued Nov. 24, 1992 to Spanier, et al.
In the technique of spectroreflectometry an incident radiation beam reflects from a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation includes multiple frequency components (or is monochromatic with a time-varying frequency), so that a spectrum of measured data (known as a reflectance spectrum or relative reflectance spectrum) including data regarding reflected intensity of incident radiation having each of at least two frequencies is measured. Systems for spectroreflectometry are described in U.S. Pat. No. 5,241,366 issued Aug. 31, 1993 to Bevis et al., and U.S. Pat. No. 4,645,349, issued Feb. 24, 1987 to Tabata, and the following U.S. patent applications assigned to the assignee of the present invention: U.S. Ser. No. 07/899,666, filed Jun. 16, 1992 (abstract published on Apr. 26, 1994 as the abstract of U.S. Pat. No. 5,306,916), and pending U.S. Ser. No. 08/218,975, filed Mar. 28, 1994.
Reflectance data (measured by spectroscopic ellipsometry, spectroreflectometry, or other reflection techniques) are useful for a variety of purposes. The thickness of various coatings (either single layer or multiple layer) on a wafer can be determined from spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample in response to incident radiation having known polarization state), or a reflectance spectrum or relative reflectance spectrum.
The reflectance of a sample (or sample layer) at a single wavelength can be extracted from a reflectance or relative reflectance spectrum. This is useful where the reflectance of photoresist coated wafers at the wavelength of a lithographic exposure tool must be found to determine proper exposure levels for the wafers, or to optimize the thickness of the resist to minimize reflectance of the entire coating stack.
The refractive index of a coating on a sample (or layer thereof) can also be determined by analysis of spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample, in response to incident radiation having known polarization state) or an accurately measured reflectance spectrum.
It would be useful for a variety of industrial applications to determine the thickness of a very small region of a very thin film (less than 30 angstroms in thickness) on a substrate from reflectance measurements (with sub-angstrom measurement repeatability) of the sample (e.g., where the sample is a semiconductor wafer and the very thin film is coated on a silicon substrate of the wafer). It would also be useful for a variety of industrial applications to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of a layer of a sample, where the layer has unknown thickness in a broad range from more than 10 microns to less than 10 angstroms.
It would also be useful to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of any selected layer of a multiple layer stack (where each layer has unknown thickness in a range from more than 10 microns to less than 10 angstroms). Such multiple layer stacks are often produced during the manufacture of semiconductor integrated circuits, with the stacks including various combinations of material such as SiO2, Si3N4, TiN, Poly-Si, and a-Si.
Because of the tight tolerance requirements typically required in the semiconductor arts, an extremely accurate method and apparatus (e.g., having sub-angstrom repeatability) is needed for determining film thickness and refractive index measurements from reflectance data from a very small, and preferably compact region (e.g., a microscopically small region of size less than 40 micron×40 micron) of a wafer. However, it had not been known how to accomplish this using an ellipsometer with all-reflective optics (for use with broadband UV radiation). Conventional ellipsometers had employed transmissive optics to direct a beam at a sample, either with relatively high incidence angles (angles substantially greater than the zero degree incidence angle of “normally” incident radiation at a sample) as in above-cited U.S. Pat. No. 5,166,752, or with low incidence angle (normal or nearly normal incidence at the sample). The inventors have recognized that such transmissive optics are unsuitable for use with broadband radiation of ultraviolet (or UV to near infrared) wavelengths, and have also recognized that beams of such radiation incident on reflective ellipsometer components with high incidence angles undesirably undergo a large change in polarization upon reflection from each such reflective component. The inventors have also recognized that the change in beam polarization upon reflection from each optical component of an ellipsometer should be small relative to the polarization change (due to specific properties of the sample itself) occurring on reflection from the sample, and that such small polarization changes can be achieved by reflecting an ellipsometer beam from optical components of an ellipsometer only at small incidence angles (where the ellipsometer reflectively focuses the beam to a small, compact spot on the sample, with rays of the beam incident at the sample at a substantial range of high incidence angles).
Until the present invention, it had not been known how to meet the needs set forth in all three preceding paragraphs, and avoid the described limitations of the prior art set forth in these three preceding paragraphs.
The spectroscopic ellipsometry method and apparatus of the invention employs reflective optics to measure a small (and preferably compact) region of a sample (e.g., a microscopically small, square-shaped spot on the sample) by reflecting broadband radiation having a range of UV (and preferably also visible and near infrared) wavelengths from the region. The method and apparatus of the invention optionally also determines from the measurements the thickness and/or complex refractive index of a thin film on the sample (such as a layer of a multiple layer stack over a silicon substrate of a semiconductor wafer). Preferred embodiments of the inventive ellipsometer employ only reflective optics (along the optical path between the polarizer and analyzer) to avoid aberration and other undesirable effects that would otherwise result from transmission of broadband ultraviolet (UV) radiation through transmissive optics, and direct the beam so that it reflects with low incidence angle from each such reflective optical component. Preferred embodiments of the inventive ellipsometer focus a beam having elongated cross-section from an elliptical focusing mirror to a small, compact spot on the sample at a range of high incidence angles. The elliptical shape of the mirror surface reduces off-axis aberrations such as “coma” in the focused beam. Use of a reflective focusing element (rather than a transmissive lens) eliminates chromatic aberration in the focused beam.
Preferred embodiments of the invention include a spectrophotometer and an improved spectroscopic ellipsometer integrated together as a single instrument. In such integrated instrument, the spectrophotometer and ellipsometer share a broadband radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Some of these embodiments include means for operating a selected one of the spectrophotometer and the ellipsometer. Others of the embodiments include means for supplying a portion of the radiation from the source to each of the spectrophotometer and ellipsometer subsystems, thus enabling simultaneous operation of both subsystems to measure the same small sample region.
Preferred embodiments of the inventive ellipsometer reflect a beam from a focusing mirror (where the beam has low incidence angle at the mirror) to focus a beam onto a small, square-shaped spot on a sample with high incidence angle. Preferably, the beam focused onto the spot has a substantial range of high incidence angles (e.g., the beam is a converging beam whose rays are incident at the sample with incidence angles in the range from about 63.5 degrees to 80.5 degrees), and a means is provided for selectively measuring only a portion of the radiation reflected from the sample after being incident at a single, selected high incidence angle (or narrow range of high incidence angles). In preferred implementations of these embodiments, a beam having elongated cross-section is focused from an elliptical focusing mirror to a compact spot on the sample, and the numerical aperture of the focusing mirror is sufficiently large to focus the reflected beam with a desired (sufficiently large) range of high incidence angles.
Preferred embodiments of the inventive ellipsometer also employ a rotating, minimal-length Rochon prism to polarize the broadband radiation beam incident on the sample (and also employ a fixedly mounted analyzer). The prism preferably has only the minimum length needed to enable the beam to pass through its clear aperture, because the prism's length is proportional to the amount of chromatic aberrations introduced by the prism. Alternatively, a phase modulator can be substituted for a rotating polarizer, or a fixedly mounted, minimal-length polarizing element can be employed with a rotating analyzer.
Other preferred embodiments of the inventive ellipsometer include a spectrometer which employs an intensified photodiode array to measure reflected radiation from the sample. Each photodiode in the array measures radiation, having wavelength in a different range, reflected from the sample. The intensified photodiode array may include an intensifier means, which preferably includes a top photocathode surface which emits electrons in response to incident photons, means for accelerating the electrons to a bottom phosphor surface, and a fiber optic coupler for directing photons emitted from the bottom phosphor to the photodiode array.
In some embodiments, the inventive ellipsometer includes a reference channel (in addition to a sample channel which detects radiation reflected from the sample). Illuminating radiation from the source is split into a sample beam and a reference beam, preferably by a bifurcated optical fiber. The sample beam reflects from the surface of a sample and is directed to the sample channel detector. The reference beam does not reflect from the sample, but is directed to the reference channel detector. By processing reference signals from the reference channel detector, as well as signals from the sample channel detector, the thickness of a very thin film on the sample (or the sample's refractive index) can be more accurately determined.
The invention has many applications, such as measuring refractive indices, measuring film thicknesses, and determining lithographic exposure times, and (in embodiments including a spectrophotometer) measuring reflectance spectra.
Throughout the specification, including in the claims, the phrase “incidence angle” of radiation at a surface denotes the angle between the normal to the surface and the propagation direction of the radiation. Thus, radiation with normal incidence at a sample surface has an incidence angle of zero degrees, and radiation with grazing incidence at such surface has an incidence angle substantially equal to 90°. Throughout the specification, including in the claims, the phrase “high incidence angle” denotes an incidence angle greater than 30°. Throughout the specification, including in the claims, the phrase “broadband radiation” denotes radiation whose frequency-amplitude spectrum includes two or more different frequency components. For example, broadband radiation may comprise a plurality of frequency components in the range from 230 nm to 850 nm, or a plurality of frequency components in the range from 400 nm to 700 nm.
A preferred embodiment of the focused beam spectroscopic ellipsometer of the invention will be described with reference to
Beam 9 (radiation emitted from lamp 10 and then polarized in polarizer 5) is reflected from sample 3 through a slit in aperture plate 6A to collection mirror 6, is then reflected from mirror 6 to mirror 7, and is then directed by mirror 7 through analyzer 8 into a spectrometer. The spectrometer (to be described in detail below) comprises entrance slit member 69, folding mirror 170, Ebert spherical mirror 171, prism 172, and detector 173. Alternatively, an Ebert-Fastie or Czerny-Turner spectrometer can be employed.
Radiation (e.g., from lamp 10) is reflected from sample 3 back to objective 40, and is focused by objective 40 onto optical elements or sensors within subsystem 80 (for use in performing pattern recognition, controlling the focusing of beam 9 onto sample 3, and optionally displaying an image of all or part of the sample). The
Sample 3 is typically a semiconductor wafer with at least one thin layer 3 a (shown in
The illumination subsystem of
Lamp 10 emits beam 12 through heatsink window 10A and then through lamp housing window 14, to mirror 16. Windows 10A and 14 are unnecessary for optical reasons, but function to keep lamp cooling air from being drawn through the optical path, thereby avoiding noise due to shimmering of the arc image. A xenon arc lamp is preferred over other lamps such as tungsten or deuterium lamps, because a xenon lamp will produce radiation having a flatter spectrum in the wavelength-range from UV to near infrared. Alternatively, a tungsten lamp and a deuterium lamp can be used in combination to cover the substantially the same spectrum covered by a xenon lamp, but this lamp combination typically has a gap in brightness in the mid-UV wavelengths. Brightness of the spectrum is important, because with less intensity, reflected radiation must be collected for longer periods. The lower intensities slow the measurement process. In alternative embodiments, a lamp is chosen which emits broadband UV radiation without emitting significant visible or near infrared radiation.
Preferably, optical fiber 1 is made of fused silica, a UV transmitting material, and has a core diameter of 365 microns.
The illumination subsystem optionally includes actuator 17A connected to mirror 17. Actuator 17A operates to move mirror 17 between a first position (shown in
Also described below (with reference to
With reference again to
Entrance slit member 2 is a substrate (preferably made of stainless steel) through which an elongated, rectangular entrance slit (60 microns×500 microns) has been etched. Because of the elongated shape of the entrance slit, elliptical focusing mirror 4 images the entrance slit as a small (25 micron×25 micron), compact (square-shaped) spot on sample 3, by reflectively focusing the beam 9 onto sample 3 at high incidence angle. Polarized beam 9 is incident at mirror 4 with a low incidence angle. Due to its orientation and the shape of its elliptical focusing surface, mirror 4 images the entrance slit Mirror 4 has a numerical aperture (0.15 or greater, in preferred-implementations of
The preferred shape of focusing mirror 4's reflective surface is elliptical. As is well known, an elliptical mirror has two foci. In embodiments in which mirror 4 is an elliptical mirror, sample 3 should be positioned at one focus of the mirror and the entrance slit (through member 2) should be positioned at the other focus of the mirror.
The elongated shape of the entrance slit in member 2, with the described design and orientation of mirror 4, results in focusing of beam 9 onto a small, compact (preferably square-shaped) spot on sample 3 with high incidence angle.
In alternative embodiments of the invention, other combinations of an entrance slit and a focusing mirror are employed (in place of elements 2 and 4 of
Designing the reflective surface of mirror 4 to have its preferred elliptical shape (rather than a spherical shape, for example) reduces off-axis aberrations (such as the aberration known as “coma”) in the focused beam incident on the sample. Use of a reflective elements (mirrors 4, 6, and 7) between the polarizer and analyzer, rather than transmissive lenses, minimizes chromatic aberration in the analyzed beam which reaches spectrometer entrance slit member 69.
Collection mirror 6 receives that portion of the diverging beam reflected from sample 3 which passes through an aperture in apertured plate 6A. Mirror 6 preferably has a focal length of 70 mm and a diameter of 20 mm. Mirror 6, because it is spherical, introduces coma into the beam. However, the aberration spreads the beam in a direction parallel to the long axis of the spectrometer entrance slit so it does not affect the light transmission properties of the instrument. In addition the spectrometer entrance slit is preferably rotated by approximately 5 degrees in a plane perpendicular to the surface normal in order to better pass the aberrated beam.
The aperture in plate 6A is preferably elongated, and oriented to pass only the radiation which has reflected from sample 3 after reaching the sample at a single incidence angle (or narrow range of incidence angles). The aperture is preferably about 2 mm tall (in the Z-direction shown in
In accordance with the invention, actuator 62 can position plate 6A at any selected one of a range of positions in the optical path of reflected beam 9, so that the slit (aperture) through plate 6A will pass only those rays of the reflected beam which have reflected from sample 3 at incidence angles in a selected narrow range. For example, actuator 62 can be operated to move plate 6A (downward along the Z-axis in
To measure a complicated film stack, it is necessary to perform multiple independent measurements at different settings of one or more measurement parameters (such as wavelength or incidence angle). Spectroscopic ellipsometric measurement (at a fixed incidence angle) simultaneously provides data for multiple wavelengths of radiation reflected from the sample. Varying incidence angle in a sequence of spectroscopic ellipsometric measurements provides data about the sample which usefully supplements the data obtained at one fixed incidence angle.
The width of the slit through apertured plate 6A determines the spreading of the incidence angles associated with the measured portion of the radiation reflected from sample 3, and the location of the slit's center determines the average incidence angle associated with the measured portion of such reflected radiation. Preferably, actuator 62 includes means for controlling both the slit width and the location of the slit's center. However, in some embodiments of the invention, the slit width and/or the location of the slit center are fixed. In embodiments in which the location of the slit center can be controlled, such location will typically be chosen to be close to Brewster's angle for the sample being measured. For example, when the sample is a flat panel display comprising films deposited on a glass substrate, it is useful to locate the slit center so that plate 6A passes only rays reflected from the flat panel display after being incident at angles in a narrow range centered at 57° (since Brewster's angle for glass is about 57° at visible wavelengths). The latter embodiment would require substitution of a differently shaped focusing mirror for above-described elliptical focusing mirror 4 (since above-described mirror 4 could not focus radiation to sample 3 at incidence angles close to 57 degrees).
Apertured plate 6A functions as an incidence angle selection element. An alternative position for the incidence angle selection element of the invention is shown in
It should be understood that in each of
With reference again to
With reference to
The area within polarizer 5 bounded by rectangular perimeter 5D in
Rochon prism 5 of
To measure a sample, analyzer 8 typically remains fixed while polarizer 5 rotates about the optical axis. Analyzer 8 is mounted so as to be free to rotate into a different angular orientation when a new sample is placed in the instrument (or when a new measurement is to be conducted on the same sample). This technique of “analyzer tracking” is well known in the field of ellipsometry.
Alternative embodiments of the invention employ an alternative type of polarizer (and analyzer), such as a Glan-Taylor polarizer (which is a polarizer well known in the art). Other embodiments employ a phase modulator (such as a photoelastic modulator) in place of a rotating polarizer. Other alternative embodiments employ an analyzer that rotates during measurement of a sample, with a fixedly mounted, minimal-length polarizer (or another fixedly mounted polarizer).
With reference again to
Preferably, detector 173 is an intensified photodiode array of the type shown schematically in
We next describe two embodiments of an autofocus assembly for the inventive ellipsometer. One such assembly is shown in
The autofocus assembly of
The reason for use of split photodiode detector 94 can be appreciated by considering the following explanation, which contrasts a conventional autofocus system with the autofocus assembly of
In a conventional autofocus system, the sample stage scans in one direction (typically the z-direction as shown in
However, this conventional autofocus technique is not useful with the spectroscopic ellipsometer of
To find the best focus position for the ellipsometer of the invention, it is thus necessary to separate f(z) from I(t). It is mathematically possible to do so, but an undesirably complicated algorithm must be implemented (so that focus speed will almost certainly be compromised).
However, by supplying two measured signals to processor 100 (one signal FA(z,t) from photodiode 94A and another signal FB(z,t) from and photodiode 94B), processor 100 can be programmed to quickly (and efficiently) determine all the useful information of the conventional focus signal f(z). This is accomplished by programming processor 100 to determine the following new focus signal:
The shape of F(z) in
As the sample position z increases from “e” to “g”, spot 94C sweeps across diode 94A (but not diode 94B), so that the ratio F(z) remains constant or decreases. Finally, as the sample position z increases from “g” to “h”, spot 94C is projected onto neither diode 94A nor 94B.
The auto focus system of
In designing the autofocus assembly of the invention, it is important to consider that the image intensity seen by the camera is time-varying, and that the speed at which the video image can be digitized and processed should be sufficiently high to enable autofocus.
The alternative autofocus assembly of
Signals indicative of the position and size of the spot are supplied from camera 91 to processor 100. In response to these signals, processor 100 generates focus control signals that are used for focusing the sample (e.g., the focus control signals are used for controlling the position of sample stage 63). Where camera 91 is part of focusing and pattern recognition subsystem 80 of
Apertured mirror 93 has an aperture therethrough which allows polarized beam 9 from polarizer 5 to pass unimpeded to mirror 4. Apertured mirror 93 also reflects off-axis illuminating radiation from source 92 toward mirror 4. This off-axis illuminating radiation is reflected to camera 91, where it enables camera 91 to “see” the position of the spot to which beam 9 is focused on the sample (and to enable pattern recognition and auto focus operations).
Next, with reference to
Reference beam 109 does not reflect from sample 3, but is directed directly to the spectrometer. Specifically, beam 109 reflects from mirror 171 (i.e., from a slightly different spot on mirror 171 than the spot from which beam 9 reflects) to prism 172. The components of beam 109 having different wavelengths are refracted in different directions from prism 172 to mirror 171, and from mirror 171 to reference channel detector 273. Detectors 173 and 273 are identical, but have slightly offset positions, so that the former receives only radiation of beam 9 reflected from mirror 171, and the latter receives only radiation of beam 109 reflected from mirror 171.
Alternatively, a plate with a double entrance slit is substituted for plate 69 of
By processing reference signals from reference channel detector 273 with signals from sample channel detector 173, the thickness (or refractive index) of a thin film on sample 3 can (under some conditions) be more accurately determined than with the
An alternative technique for obtaining a reference beam is to modify the
Variations on the
We have described many embodiments of the inventive spectroscopic ellipsometer. In alternative embodiments of the inventive ellipsometer, polarized radiation having only one wavelength (rather than broadband radiation) is reflected from the sample. These embodiments can include a spectrometer as in
Other embodiments of the invention are not an ellipsometer alone, but a spectrophotometer integrated together with an ellipsometer (preferably any of the above-described spectroscopic ellipsometers) as a single instrument.
In one such preferred embodiment, the
In another such embodiment, the
The illumination system provides both measurement beam 25 and field illumination beam 34 to beam divider 45. Off-axis paraboloid mirror 16 collimates beam 12 from lamp 10, and the beam is then optionally filtered by flip-in UV cutoff filter 18 and color filter wheel 20. UV cutoff filter 18 is used in part to limit the spectrum of beam 12 so that when beam 12 is dispersed by a diffraction grating, the first and second order diffraction beams do not overlap. Part of beam 12 is reflected by flat mirror 22 onto concave mirror 24 to form measurement beam 25.
Field illumination beam 34, another part of beam 12, is focused by large achromat 32, so that fold mirror 36 reflects an image of lamp 10 toward small achromat 38. Small achromat 38 collects the radiation in beam 34 before it reflects from aperture mirror 28. Aperture mirror 28 is preferably a fused silica plate with a reflective coating on one side, with a 150 micron square etched from the reflective coating to provide an aperture for beam 25. The aperture is placed at one conjugate of objective 40. The field illumination can be turned off by placing field illumination shutter 31 in the optical path of field illumination beam 34.
Narrow measurement beam 25 and wide field illumination beam 34 are rejoined at aperture mirror 28, with field illumination beam 34 reflecting off the front of aperture mirror 28, and measurement beam 25 passing through the aperture.
The reflectometer, viewing, and autofocus subsystems of
Objective 40 is preferably a reflective objective (as shown in
Reference beam 48 does not initially interact with beamsplitter mirror 45, but instead illuminates concave mirror 50. Concave mirror 50 is slightly off-axis, so reference beam 48 is reflected onto the reverse face of beamsplitter mirror 45, and flat mirror 43 re-reflects reference beam 48 into alignment with the reference spectrometer pinhole through plate 52. Flat mirror 43 realigns reference beam 48 with sample beam 46 so that both beams pass through their respective spectrometer pinholes substantially parallel.
The focal length of concave mirror 50 is such that reference beam 48 is in focus at the reference spectrometer pinhole (which extends through plate 52). The radiation passing through the reference spectrometer pinhole and reflecting from fold mirror 68 is dispersed by diffraction grating 70. The resulting first order diffraction beam is collected by reference linear photodiode array 74, thereby measuring a reference reflectance spectrum.
Sample beam 46 is reflected from beamsplitter mirror 45 towards objective 40, which focuses sample beam 46 onto wafer 3, and the reflected sample beam 46 is focused by objective 40 onto the sample spectrometer pinhole (which extends through plate 54). The reflected sample beam 46 does not interact with beamsplitter mirror 45 on the reflected path, because sample beam 46 passed through the space behind beamsplitter mirror 45, through which reference beam 48 also passes. The radiation passing through the sample spectrometer pinhole and reflecting from fold mirror 68 is dispersed by diffraction grating 70. As with the reference beam, the resulting first order diffraction beam of the sample beam is collected by sample linear photodiode array 72, thereby measuring the sample spectrum.
The relative reflectance spectrum can be simply obtained by processing the outputs of arrays 72 and 74 in processor 100, by dividing the sample light intensity at each wavelength (the output of array 72) by the reference intensity at each wavelength (the output of array 74). Typically, this involves 512 division computations, in cases in which each of arrays 72 and 74 is a 512-diode linear photodiode array. A typical relative reflectance spectrum will include components ranging from 220 nm to 830 nm.
In some embodiments, diffraction grating 70 is a concave holographic grating and the spectrometer pinholes (through plates 52 and 54) are 15 mm apart. This embodiment of diffraction grating 70 is holographically corrected to image multiple spectra, since the 15 mm spacing does not allow for both beams to be centered on the grating. One such grating is a multiple spectra imaging grating supplied by Instruments S.A. It is also desirable that grating 70 be designed so that the angle of detectors 72 and 74 causes reflections from the detectors to propagate away from the grating.
Detector 98 has a position output, which is dependent on the position of the centroid of the radiation falling on detector 98, and an intensity output, which is dependent on the incident intensity at detector 98. Detector 98 is positioned to avoid dark regions of the out-of-focus image. In the coarse-focus mode, the centroid of the image falling on detector 98 indicates not only the direction in which focus lies, but also how far out of focus wafer 3 is. The Z position of wafer 3 (the separation between wafer 3 and objective 40) is then adjusted until the centroid of the light falling on detector 98 is centered near the center of detector 98. With the appropriate positioning and feedback mechanism, wafer 3 can be kept in coarse focus while the wafer is being moved in the X and Y directions.
For fine focus, flip-in aperture member 30 is flipped into the optical path of measurement beam 25, resulting in a smaller square image reaching detector 98. The smaller square image has a size of about 40 microns with an 1× objective. Since aperture member 30 has an aperture the same size as the aperture through plate 54, and since the two apertures are at conjugates of objective 40, when wafer 3 is in focus, very little radiation strikes plate 54 (away from the aperture through plate 54) to be reflected onto detector 98. Thus in the fine-focus mode, the intensity output of detector 98 is used to bring wafer 3 into focus, with the Z position of wafer 3 being adjusted until the intensity output of detector 98 is minimized.
There are several other hardware features important to a preferred implementation of the
Another feature is that lamp housing window 14 should be very thin to reduce chromatic aberration in the measurement illumination path. This chromatic aberration causes the UV and visible images of the arc of lamp 10 projected onto aperture mirror 28 to separate, creating problems with the 15× focus curve.
Another feature is that means for adjusting the lamp housing's position along the z-axis shown in
The autofocus subsystem of
The image reflected from sample plate 54 is also used for viewing wafer 3. As shown in
As shown in
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. Similarly, penta prism 86 is rotated in the plane of reflection to remove unwanted reflections from the field of view. Additionally, to capture stray radiation from unwanted internal reflections within beamsplitter cube 84, black glass is glued to the unused surfaces of beamsplitter cube 84. In this way, only the desired internal reflection of beam 65 and beam 63′ exit beamsplitter cube 84.
Several embodiments of optical systems according to the present invention have been described. The description is illustrative and not restrictive. Many other variations on the invention will become apparent to those of skill in the art upon review of this disclosure. Merely by way of example, the sample measured by the invention need not be a wafer, but can be any other reflective object; and fold mirrors can be removed where space allows, and additional fold mirrors can be provided where space is limited. The scope of the invention should be determined not merely with reference to the above description, but should be determined with reference to the appended claims along with their full scope of equivalents.