Apparatus for analyzing multi-layer thin film stacks on semiconductors Number:6,774,997 from the United States Patent and Trademark Office (PTO) owispatent An optical measurement system is disclosed for evaluating samples with multi-layer thin film stacks. The optical measurement system includes a reference ellipsometer and one or more non-contact optical measurement devices. The reference ellipsometer is used to calibrate the other optical measurement devices. Once calibration is completed, the system can be used to analyze multi-layer thin film stacks. In particular, the reference ellipsometer provides a measurement which can be used to determine the total optical thickness of the stack. Using that information coupled with the measurements made by the other optical measurement devices, more accurate information about individual layers can be obtained.<H semiconductors, analyzing, Apparatus, for, a, 6774997.html, Patent, pto, 6774997

Title: Apparatus for analyzing multi-layer thin film stacks on semiconductors

Abstract: An optical measurement system is disclosed for evaluating samples with multi-layer thin film stacks. The optical measurement system includes a reference ellipsometer and one or more non-contact optical measurement devices. The reference ellipsometer is used to calibrate the other optical measurement devices. Once calibration is completed, the system can be used to analyze multi-layer thin film stacks. In particular, the reference ellipsometer provides a measurement which can be used to determine the total optical thickness of the stack. Using that information coupled with the measurements made by the other optical measurement devices, more accurate information about individual layers can be obtained.

Patent Number: 6,774,997 Issued on 08/10/2004 to Rosencwaig, et al.

Inventors: Rosencwaig; Allan (Danville, CA), Opsal; Jon (Livermore, CA)
Assignee: Therma-Wave, Inc. (Fremont, CA)

Appl. No.: 10/395,746

Filed: March 24, 2003

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
141256	May., 2002	6567213
880203	Jun., 2001	6417921
563152	May., 2000	6297880
015839	Jan., 1998	6278519

Current U.S. Class: 356/369

Current International Class: G01B 11/06 (20060101)

Field of Search: 356/364,369

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Primary Examiner: Smith; Zandra V.
Attorney, Agent or Firm: Stallman & Pollock LLP

Parent Case Text

This application is a continuation of prior U.S. application Ser. No. 10/141,256, filed May 8, 2002 now U.S. Pat. No. 6,567,273, which is in turn a continuation of U.S. application Ser. No. 09/880,203, filed Jun. 13, 2001, now U.S. Pat. No. 6,417,921 which is in turn a continuation of U.S. application Ser. No. 09/563,152, filed May 2, 2000, now U.S. Pat. No. 6,297,880, which is in turn is a continuation of U.S. application Ser. No. 09/015,839, filed Jan. 29, 1998, now U.S. Pat. No. 6,278,519.

Claims

What is claimed is:

1. An apparatus for analyzing the characteristics of a semiconductor wafer comprising: a narrowband ellipsometer, said narrowband ellipsometer including a gas discharge laser for generating a narrowband output defining a first probe beam directed to reflect off the sample at a non-normal angle of incidence, said narrowband ellipsometer further including a detector for monitoring the first probe beam after reflection from the wafer and generating first output signals corresponding thereto, said narrowband ellipsometer further including one or more elements selected from the group consisting of polarizers and compensators and wherein a motor is provided for varying the azimuthal angle of at one least of said elements; a broadband spectroscopic ellipsometer, said broadband spectroscopic ellipsometer including a broad band light source for generating a second probe beam, said broadband spectroscopic ellipsometer further including a spectrometer for simultaneously monitoring the intensity of the reflected second probe beam at a plurality of wavelengths after reflection from the wafer, said broadband spectroscopic ellipsometer further including one or more elements selected from the group consisting of polarizers and compensators and wherein a motor is provided for varying the azimuthal angle of at least one of said elements; and a processor for analyzing the characteristics of the wafer based on a combination of the first and second output signals and information regarding the azimuthal positions of the elements in the narrowband and broadband spectroscopic ellipsometers.

2. An apparatus as recited in claim 1, wherein said narrowband ellipsometer includes a rotating compensator.

3. An apparatus as recited in claim 1, wherein the broad band light source of the broadband spectroscopic ellipsometer generates light having a wavelength range between at least 200 and 800 nm.

4. An apparatus as recited in claim 1, wherein the second probe beam is directed to reflect off the wafer at a non-normal angle of incidence.

Description

FIELD OF THE INVENTION

The present invention relates to optical analyzers, and more particularly to an optical measurement system having a stable single wavelength ellipsometer and a broadband spectroscopic measurement module to accurately characterize multi-layer thin film stacks.

BACKGROUND OF THE INVENTION

There is considerable interest in developing systems for accurately measuring the thickness and/or composition of multi-layer thin films. The need is particularly acute in the semiconductor manufacturing industry where the thickness of these thin film oxide layers on semiconductor substrates is measured. To be useful, the measurement system must be able to determine the thickness and/or composition of films with a high degree of accuracy. The preferred measurement systems rely on non-contact, optical measurement techniques, which can be performed during the semiconductor manufacturing process without damaging the wafer sample. Such optical measurement techniques include directing a probe beam to the sample, and measuring one or more optical parameters of the reflected probe beam.

In order to increase measurement accuracy and to gain additional information about the target sample, multiple optical measuring devices are often incorporated into a single composite optical measurement system. For example, the present assignee has marketed a product called OPTI-PROBE, which incorporates several optical measurement devices, including a Beam Profile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and a Broadband Reflective Spectrometer (BRS). Each of these devices measures parameters of optical beams reflected by the target sample. The BPR and BPE devices utilize technology described in U.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which are incorporated herein by reference.

The composite measurement system mentioned above combines the measured results of each of the measurement devices to precisely derive the thickness and composition of the thin film and substrate of the target sample. However, the accuracy of the measured results depends upon precise initial and periodic calibration of the measurement devices in the optical measurement system. Further, recently developed measurement devices have increased sensitivity to more accurately measure thinner films and provide additional information about film and substrate composition. These newer systems require very accurate initial calibration. Further, heat, contamination, optical damage, alignment, etc., that can occur over time in optical measurement devices, affect the accuracy of the measured results. Therefore, periodic calibration is necessary to maintain the accuracy of the composite optical measurement system.

It is known to calibrate optical measurement devices by providing a reference sample having a known substrate, with a thin film thereon having a known composition and thickness. The reference sample is placed in the measurement system, and each optical measurement device measures the optical parameters of the reference sample, and is calibrated using the results from the reference sample and comparing them to the known film thickness and composition. A common reference sample is a "native oxide" reference sample, which is a silicon substrate with an oxide layer formed thereon having a known thickness (about 20 angstroms). After fabrication, the reference sample is kept in a non-oxygen environment to minimize any further oxidation and contamination that changes the thickness of the reference sample film away from the known thickness, and thus reduces the effectiveness of the reference sample for accurate calibration. The same reference sample can be reused to periodically calibrate the measurement system. However, if and when the amount of oxidation or contamination of the reference sample changes the film thickness significantly from the known thickness, the reference sample must be discarded.

For many optical measurement devices, reference samples with known thicknesses have been effective for system calibration. Oxidation and contamination that routinely occurs over time with reference samples is tolerable because the film thickness change resulting from the oxidation/contamination is relatively insignificant compared to the overall thickness of the film (around 100 angstroms). However, new ultra-sensitive optical measurement systems have been recently developed that can measure film layers with thicknesses less than 10 angstroms. These systems require reference samples having film thicknesses on the order of 20 angstroms for accurate calibration. For such thin film reference samples, however, the changes in film layer thickness resulting from even minimal oxidation or contamination are significant compared to the overall "known" film layer thickness, and result in significant calibration error. Therefore, it is extremely difficult, if not impossible, to provide a native oxide reference sample with a known thickness that is stable enough over time to be used for periodic calibration of ultra-sensitive optical measurement systems.

There is a need for a calibration method for ultra-sensitive optical measurement devices that can utilize a reference sample that does not have a stable or known film thickness.

There is also a need in the industry to improve the accuracy of these type of measuring systems to permit characterization of samples having multiple thin film layers formed thereon. More particularly, in the semiconductor industry, semiconductor material substrates are now being fabricated with multiple thin film layers. Each film layer can be formed from a different material. Common layer materials include oxides, nitrides, polysilicon, titanium and titanium-nitride.

Attempts to characterize samples having multiple thin layers with conventional techniques is difficult since each layer has a different thickness and different optical characteristics. The best approaches found to date to characterize such complex stacks is to utilize multiple measurement techniques which generate independent data that can be analyzed by a processor. Devices now exist which are capable of making both ellipsometric (phase) and spectrophotometric (magnitude) measurements and integrating the results in a microprocessor. The ellipsometers in these devices can include multiple wavelength and multiple angle of incidence measurements. Similarly, the spectrophotometers in some of these devices can be arranged to make measurements at multiple angles of incidence.

While these systems have had reasonable success, further accuracy in analyzing the characteristics of individual layers in a multi-layer stack is always desirable. The subject system, which includes a wavelength stable calibration ellipsometer can be modified to improve the characterization of individual layers of multi-layer thin film stack.

SUMMARY OF THE INVENTION

The present invention is a thin film optical measurement system with a wavelength stable ellipsometer that can be used for calibration and to enhance the characterization of multi-layer thin film stacks. When used for calibration purposes, the stable wavelength ellipsometer functions to precisely determine the thickness of a film on a reference sample. The measured results from the calibration ellipsometer are used to calibrate other optical measurement devices in the thin film optical measurement system. By not having to supply a reference sample with a predetermined known film thickness, a reference sample having a film with a known composition can be repeatedly used to calibrate ultra-sensitive optical measurement devices, even if oxidation or contamination of the reference sample changes the thickness of the film over time.

The calibration reference ellipsometer uses a reference sample that has at least a partially known composition to calibrate at least one other non-contact optical measurement device. The reference ellipsometer includes a light generator that generates a quasi-monochromatic beam of light having a known wavelength and a known polarization for interacting with the reference sample. The beam is directed at a non-normal angle of incidence relative to the reference sample to interact with the reference sample. An analyzer creates interference between S and P polarized components in the light beam after the light beam has interacted with reference sample. A detector measures the intensity of the light after the beam has passed through the analyzer. A processor determines the polarization state of the light beam entering the analyzer from the intensity measured by the detector. The processor then determines optical properties of the reference sample based upon the determined polarization state, the known wavelength of light from the light generator and the at least partially known composition of the reference sample. The processor operates at least one other non-contact optical measurement device that measures an optical parameter of the reference sample. The processor calibrates the other optical measurement device by comparing the measured optical parameter from the other optical measurement device to the determined optical property from the reference ellipsometer.

The reference ellipsometer has the further benefit in that it can be used to very accurately measure the overall optical thickness of an unknown multi-layer stack on a substrate. In this context, the term total optical thickness refers to the effective thickness of the stack which corresponds to a single uniform layer with uniform optical parameters (i.e. n and k). A stable wavelength ellipsometer is an excellent tool for determining the total optical thickness of a layer or a stack having a thicknesses less than 500 angstroms and is the best tool for stacks having a thickness of 200 angstroms or less.

The reference ellipsometer, which provides only a single wavelength, single angle of incidence output, is not suitable for analyzing the individual layers in a stack. Such analysis requires additional measurements typically from spectroscopic tools such as spectrophotometers and spectroscopic ellipsometers. However, the latter tools alone have difficulty producing sufficient information to accurately characterize the stack.

In accordance with the subject invention, the output from the wavelength stable ellipsometer is used by the processor to determine the overall optical thickness of the multi-layer stack. This information is used by the processor to reduce the uncertainty of the analysis based on the spectroscopic measurements. By taking a number of measurements at different wavelengths with one or more different techniques, very accurate information about layer composition and thickness can be determined.

Other aspects and features of the present invention will become apparent by a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a composite optical measurement system with the calibration ellipsometer of the present invention.

FIG. 2 is a side cross-sectional view of the reflective lens used with the present invention.

FIG. 3 is a plan view of an alternate embodiment of the light source for the calibration ellipsometer of the present invention.

FIG. 4 is a plan view of the composite optical measurement system with multiple compensators in the calibration ellipsometer of the present invention.

FIG. 5 is an illustration of a multi-layer stack on a sample.

FIG. 6 is a flow chart illustrating the steps which can be carried out to characterize individual layers of a multi-layer stack using measurements from both a stable wavelength ellipsometer and a multi-wavelength measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a composite thin film optical measurement system 1 having a wavelength stable reference ellipsometer 2 that is used, in conjunction with a reference sample 4 having a substrate 6 and thin film 8 with known compositions, to calibrate non-contact optical measurement devices contained in the composite thin film optical measurement system 1.

FIG. 1 illustrates the composite optical measurement system 1 that has been developed by the present assignees, which includes five different non-contact optical measurement devices and the reference ellipsometer 2 of the present invention.

Composite optical measurement system 1 includes a Beam Profile Ellipsometer (BPE) 10, a Beam Profile Reflectometer (BPR) 12, a Broadband Reflective Spectrometer (BRS) 14, a Deep Ultra Violet Reflective Spectrometer (DUV) 16, and a Broadband Spectroscopic Ellipsometer (BSE) 18. These five optical measurement devices utilize as few as two optical sources: laser 20 and white light source 22. Laser 20 generates a probe beam 24, and white light source 22 generates probe beam 26 (which is collimated by lens 28 and directed along the same path as probe beam 24 by mirror 29). Laser 20 ideally is a solid state laser diode from Toshiba Corp. which emits a linearly polarized 3 mW beam at 673 nm. White light source 22 is ideally a deuterium-tungsten lamp that produces a 200 mW polychromatic beam that covers a spectrum of 200 nm to 800 nm. The probe beams 24/26 are reflected by mirror 30, and pass through mirror 42 to sample 4.

The probe beams 24/26 are focused onto the surface of the sample with a lens 32 or lens 33. In the preferred embodiment, two lenses 32/33 are mounted in a turret (not shown) and are alternatively movable into the path of probe beams 24/26. Lens 32 is a spherical, microscope objective lens with a high numerical aperture (on the order of 0.90 NA) to create a large spread of angles of incidence with respect to the sample surface, and to create a spot size of about one micron in diameter. Lens 33 is illustrated in FIG. 2, and is a reflective lens having a lower numerical aperture (on the order of 0.4 NA) and capable of focusing deep UV light to a spot size of about 10-15 microns.

Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No. 5,181,080, issued Jan. 19, 1993, which is commonly owned by the present assignee and is incorporated herein by reference. BPE 10 includes a quarter wave plate 34, polarizer 36, lens 38 and a quad detector 40. In operation, linearly polarized probe beam 24 is focused onto sample 4 by lens 32. Light reflected from the sample surface passes up through lens 32, through mirrors 42, 30 and 44, and directed into BPE 10 by mirror 46. The position of the rays within the reflected probe beam correspond to specific angles of incidence with respect to the sample's surface. Quarter-wave plate 34 retards the phase of one of the polarization states of the beam by 90 degrees. Linear polarizer 36 causes the two polarization states of the beam to interfere with each other. For maximum signal, the axis of the polarizer 36 should be oriented at an angle of 45 degrees with respect to the fast and slow axis of the quarter-wave plate 34. Detector 40 is a quad-cell detector with four radially disposed quadrants that each intercept one quarter of the probe beam and generate a separate output signal proportional to the power of the portion of the probe beam striking that quadrant. The output signals from each quadrant are sent to a processor 48. As discussed in the U.S. Pat. No. 5,181,080 patent, by monitoring the change in the polarization state of the beam, ellipsometric information, such as .psi. and .DELTA., can be determined. To determine this information, the processor 48 takes the difference between the sums of the output signals of diametrically opposed quadrants, a value which varies linearly with film thickness for very thin films.

Beam profile reflectometry (BPR) is discussed in U.S. Pat. No. 4,999,014, issued on Mar. 12, 1991, which is commonly owned by the present assignee and is incorporated herein by reference. BPR 12 includes a lens 50, beam splitter 52 and two linear detector arrays 54 and 56 to measure the reflectance of the sample. In operation, linearly polarized probe beam 24 is focused onto sample 4 by lens 32, with various rays within the beam striking the sample surface at a range of angles of incidence. Light reflected from the sample surface passes up through lens 32, through mirrors 42 and 30, and directed into BPR 12 by mirror 44. The position of the rays within the reflected probe beam correspond to specific angles of incidence with respect to the sample's surface. Lens 50 spatially spreads the beam two-dimensionally. Beam splitter 52 separates the S and P components of the beam, and detector arrays 54 and 56 are oriented orthogonal to each other to isolate information about S and P polarized light. The higher angles of incidence rays will fall closer to the opposed ends of the arrays. The output from each element in the diode arrays will correspond to different angles of incidence. Detector arrays 54/56 measure the intensity across the reflected probe beam as a function of the angle of incidence with respect to the sample surface. The processor 48 receives the output of the detector arrays 54/56, and derives the thickness and refractive index of the thin film layer 8 based on these angular dependent intensity measurements by utilizing various types of modeling algorithms. Optimization routines which use iterative processes such as least square fitting routines are typically employed. One example of this type of optimization routine is described in "Multiparameter Measurements of Thin Films Using Beam-Profile Reflectivity," Fanton, et. al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993. Another example appears in "Simultaneous Measurement of Six Layers in a Silicon on Insulator Film Stack Using Spectrophotometry and Beam Profile Reflectometry," Leng, et. al., Journal of Applied Physics, Vol. 81, No. 8, page 3570, 1997.

Broadband reflective spectrometer (BRS) 14 simultaneously probes the sample 4 with multiple wavelengths of light. BRS 14 uses lens 32 and includes a broadband spectrometer 58 which can be of any type commonly known and used in the prior art. The spectrometer 58 shown in FIG. 1 includes a lens 60, aperture 62, dispersive element 64 and detector array 66. During operation, probe beam 26 from, white light source 22 is focused onto sample 4 by lens 32. Light reflected from the surface of the sample passes up through lens 32, and is directed by mirror 42 (through mirror 84) to spectrometer 58. The lens 60 focuses the probe beam through aperture 62, which defines a spot in the field of view on the sample surface to analyze. Dispersive element 64, such as a diffraction grating, prism or holographic plate, angularly disperses the beam as a function of wavelength to individual detector elements contained in the detector array 66. The different detector elements measure the optical intensities of the different wavelengths of light contained in the probe beam, preferably simultaneously. Alternately, detector 66 can be a CCD camera, or a photomultiplier with suitably dispersive or otherwise wavelength selective optics. It should be noted that a monochrometer could be used to measure the different wavelengths serially (one wavelength at a time) using a single detector element. Further, dispersive element 64 can also be configured to disperse the light as a function of wavelength in one direction, and as a function of the angle of incidence with respect to the sample surface in an orthogonal direction, so that simultaneous measurements as a function of both wavelength and angle of incidence are possible. Processor 48 processes the intensity information measured by the detector array 66.

Deep ultra violet reflective spectrometry (DUV) simultaneously probes the sample with multiple wavelengths of ultra-violet light. DUV 16 uses the same spectrometer 58 to analyze probe beam 26 as BRS 14, except that DUV 16 uses the reflective lens 33 (FIG. 2) instead of focusing lens 32. To operate DUV 16, the turret containing lenses 32/33 is rotated so that reflective lens 33 is aligned in probe beam 26. The reflective lens 33 is necessary because solid objective lenses cannot sufficiently focus the UV light onto the sample.

Broadband spectroscopic ellipsometry (BSE) is discussed in pending U.S. patent application Ser. No. 08/685,606, filed on Jul. 24, 1996, which is commonly owned by the present assignee and is incorporated herein by reference. BSE (18) includes a polarizer 70, focusing mirror 72, collimating mirror 74, rotating compensator 76, and analyzer 80. In operation, mirror 82 directs at least part of probe beam 26 to polarizer 70, which creates a known polarization state for the probe beam, preferably a linear polarization. Mirror 72 focuses the beam onto the sample surface at an oblique angle, ideally on the order of 70 degrees to the normal of the sample surface. Based upon well known ellipsometric principles, the reflected beam will generally have a mixed linear and circular polarization state after interacting with the sample, based upon the composition and thickness of the sample's film 8 and substrate 6. The reflected beam is collimated by mirror 74, which directs the beam to the rotating compensator 76. Compensator 76 introduces a relative phase delay .delta. (phase retardation) between a pair of mutually orthogonal polarized optical beam components. Compensator 76 is rotated at an angular velocity .omega. about an axis substantially parallel to the propagation direction of the beam, preferably by an electric motor 78. Analyzer 80, preferably another linear polarizer, mixes the polarization states incident on it. By measuring the light transmitted by analyzer 80, the polarization state of the reflected probe beam can be determined. Mirror 84 directs the beam to spectrometer 58, which simultaneously measures the intensities of the different wavelengths of light in the reflected probe beam that pass through the compensator/analyzer combination. Processor 48 receives the output of the detector 66, and processes the intensity information measured by the detector 66 as a function of wavelength and as a function of the azimuth (rotational) angle of the compensator 76 about its axis of rotation, to solve the ellipsometric values .psi. and .DELTA. as described in U.S. patent application Ser. No. 08/685,606.

Detector/camera 86 is positioned above mirror 46, and can be used to view reflected beams off of the sample 4 for alignment and focus purposes.

In order to calibrate BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18, the composite optical measurement system 1 includes the wavelength stable calibration reference ellipsometer 2 used in conjunction with a reference sample 4. Ellipsometer 2 includes a light source 90, polarizer 92, lenses 94 and 96, rotating compensator 98, analyzer 102 and detector 104.

Light source 90 produces a quasi-monochromatic probe beam 106 having a known stable wavelength and stable intensity. This can be done passively, where light source 90 generates a very stable output wavelength which does not vary over time (i.e. varies less than 1%). Examples of passively stable light sources are a helium-neon laser, or other gas discharge laser systems. Alternately, a non-passive system can be used as illustrated in FIG. 3 where the light source 90 includes a light generator 91 that produces light having a wavelength that is not precisely known or stable over time, and a monochrometer 93 that precisely measures the wavelength of light produced by light generator 91. Examples of such light generators include solid state lasers, laser diodes, or polychromatic light sources used in conjunction with a color filter such as a grating. In either case, the wavelength of beam 106, which is a known constant or measured by monochrometer 93, is provided to processor 48 so that ellipsometer 2 can accurately calibrate the optical measurement devices in system 1.

The beam 106 interacts with polarizer 92 to create a known polarization state. In the preferred embodiment, polarizer 92 is a linear polarizer made from a quartz Rochon prism, but in general the polarization does not necessarily have to be linear, nor even complete. Polarizer 92 can also be made from calcite. The azimuth angle of polarizer 92 is oriented so that the plane of the electric vector associated with the linearly polarized beam exiting from the polarizer 92 is at a known angle with respect to the plane of incidence (defined by the propagation direction of the beam 106 and the normal to the surface of sample 4). The azimuth angle is preferably selected to be on the order of 30 degrees because the sensitivity is optimized when the reflected intensities of the P and S polarized components are approximately balanced. It should be noted that polarizer 92 can be omitted if the light source 90 emits light with the desired known polarization state.

The beam 106 is focused onto the sample 4 by lens 94 at an oblique angle. For calibration purposes, reference sample 4 ideally consists of a thin oxide layer 8 having a thickness d, formed on a silicon substrate 6. However, in general, the sample 4 can be any appropriate substrate of known composition, including a bare silicon wafer, and silicon wafer substrates having one or more thin films thereon. The thickness d of the layer 8 need not be known, or be consistent between periodic calibrations. The useful light from probe beam 106 is the light reflected by the sample 4 symmetrically to the incident beam about the normal to the sample surface. It is noted however that the polarization state of nonspecularly scattered radiation can be determined by the method of the present invention as well. The beam 106 is ideally incident on sample 4 at an angle on the order of 70 degrees to the normal of the sample surface because sensitivity to sample properties is maximized in the vicinity of the Brewster or pseudo-Brewster angle of a material. Based upon well known ellipsometric principles, the reflected beam will generally have a mixed linear and circular polarization state after interacting with the sample, as compared to the linear polarization state of the incoming beam. Lens 96 collimates beam 106 after its reflection off of the sample 4.

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