Optical metrology tool equipped with modulated illumination sources

Abstract

To provide a system for mitigating effects of coherence artifacts and excessive noise sources in an optical metrology setting.SOLUTION: The optical metrology tool includes a modulatable illumination source 102 configured to illuminate a surface of a sample disposed on a sample stage, a detector 110 configured to detect illumination emanating from the surface of the sample, illumination optics 104 configured to direct the illumination from the modulatable illumination source to the surface of the sample, collection optics 108 configured to direct the illumination from the surface of the sample to the detector, and a modulation control system 112 communicatively coupled to the modulatable illumination source, where the modulation control system 112 is configured to modulate a drive current of the modulatable illumination source at a selected modulation frequency suitable for generating illumination having a selected coherence feature length.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention generally relates to methods and systems for optical measurement, and more particularly, to an optical measurement method and system including a modulated light source.

Background Art

[0002] As the demand for semiconductor device functions that are miniaturized to an unprecedented level continues to increase, the demand for improved optical measurement technology will also continue to increase. Optical measurement technology can include critical dimension (CD) measurement, thin film thickness and composition measurement, and overlay measurement. These optical measurement technologies can be performed using various optical configurations such as reflection measurement optical systems, ellipsometry optical systems, and spectroscopic analysis optical systems.

[0003] Normally, an optical measurement system uses a light source that operates in a constant current or constant light output mode to ensure the optical stability of the system and maintain the noise level within an acceptable limit.

[0004] In an optical measurement configuration provided with a coherent light source, the generation of coherent artifacts such as interference fringes (e.g., "ghost") and speckles resulting from overlapping images is a significant concern in the operation of a given optical measurement tool. Due to the large coherence length of laser-type illumination light, it is difficult to minimize the influence of coherent artifacts. The coherent artifacts that appear in an optical measurement configuration with a coherence length of the illumination used (often 100 m or more) are larger than the distance between the light reflecting surfaces of the measurement tool. Such reflecting surfaces can include lenses, beam splitters, optical fibers, etc. In this case, the primary beam constructively interferes with the illumination from the parasitic beam, generating interference fringes caused by ghosts. This interference condition increases to the extent that the intensity value becomes of the same order of magnitude as the size of the primary beam, and as a result, significantly inhibits the usefulness of a given optical measurement tool.

[0005] Furthermore, some measurement applications require time-series intensity control of multiple light sources that emit different wavelengths of light. The prior art has achieved time-series intensity control using various opto-mechanical and electro-optical devices such as shutters, acousto-optic devices, and Pockels cells. The use of such prior art devices for time-series control of multiple light sources may lead to a decrease in stability and reproducibility.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Therefore, it would be beneficial to provide a system and method for eliminating the deficiencies of the prior art and mitigating the effects of coherent artifacts and excessive noise sources in an optical measurement configuration. Furthermore, it would be beneficial to present a system and method for providing an efficient means for time-series output of a multi-wavelength light source for multi-wavelength optical measurement applications.

Means for Solving the Problems

[0008] An optical measurement tool is disclosed. In one aspect, the optical measurement tool can include, but is not limited to, a tunable light source configured to illuminate a sample surface disposed on a sample stage, a set of illumination optical systems configured to direct illumination from the tunable light source toward the sample surface, a set of condensing optical systems, and a detector configured to detect at least a portion of the illumination emitted from the sample surface, wherein the set of condensing optical systems is configured to direct the illumination from the sample surface toward the detector, and a modulation control system communicatively connected to the tunable light source and configured to modulate the drive current of the tunable light source at a selected modulation frequency suitable for generating illumination having selected coherence characteristics.

[0009] In another aspect, the optical measurement tool can include, but is not limited to, a first light source configured to generate illumination of a first wavelength, at least one additional light source configured to generate illumination of an additional wavelength, the additional wavelength being different from the first wavelength, the first light source and the at least one additional light source configured to illuminate a sample surface disposed on a sample stage, a set of illumination optical systems configured to direct the illumination of the first wavelength and the illumination of the at least one additional wavelength from the first light source and the at least one additional light source toward the sample surface, a set of condensing optical systems, and a detector configured to detect at least a portion of the illumination emitted from the sample surface, wherein the set of condensing optical systems is configured to direct the illumination emitted from the sample surface toward the detector, and a modulation control system communicatively connected to the first light source and the at least one additional light source and configured to modulate the drive current of the first light source to generate a first illumination waveform of the first wavelength and modulate the drive current of the at least one additional light source to generate an additional illumination waveform of the additional wavelength, wherein the pulses of the first illumination waveform are interleaved at least in accordance with the pulses of the additional illumination waveform, and the first illumination waveform and the additional illumination waveform have a selected waveform frequency.

[0010] In other aspects, the optical measurement tool can include, but is not limited to, a first light source configured to generate illumination of a first wavelength, at least one additional light source configured to generate illumination of an additional wavelength, where the additional wavelength is different from the first wavelength, the first light source and the at least one additional light source disposed on a sample stage and configured to illuminate a sample surface, a set of illumination optics configured to direct illumination of the first wavelength and at least one additional wavelength from the first light source and the at least one additional light source toward the sample surface, a set of focusing optics, a detector configured to detect at least a portion of the illumination emitted from the sample surface, where the set of focusing optics is configured to direct the illumination emitted from the sample surface toward the detector, a first illumination switching device communicatively connected to the first light source and configured to control the intensity of the transmitted illumination of the first wavelength, at least one additional illumination switching device communicatively connected to the at least one additional light source and configured to control the intensity of the transmitted illumination of the additional wavelength, and an illumination control system communicatively connected to the first illumination switching device and the at least one additional switching device and configured to modulate the intensity of the transmitted illumination of the first wavelength and the intensity of the transmitted illumination of the additional wavelength by controlling one or more characteristics of the illumination switching devices.

[0011] In other aspects, the optical measurement tool can include, but is not limited to, a tunable excitation source configured to generate an illumination beam, a plasma cell having a valve containing a predetermined amount of gas, a set of optical elements configured to shape the illumination beam and focus the illumination beam from the tunable excitation source into the predetermined amount of gas to maintain a plasma therein, a set of illumination optics configured to direct the illumination beam from the plasma cell toward the sample surface, a set of focusing optics, and a detector configured to detect at least a portion of the illumination emitted from the sample surface, where the set of focusing optics is configured to direct the illumination from the sample surface toward the detector, and an excitation control system communicatively connected to the tunable excitation source and configured to modulate the drive current of the tunable excitation source at a selected modulation frequency to impart a time-variation to the plasma contained within the plasma cell.

[0012] It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0013] Many advantages of the present disclosure can be better understood by those skilled in the art by referring to the accompanying drawings.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4A

Figure 4B

Figure 5

Figure 6

Figure 7A

Figure 7B

[0015] Reference is now made in detail to the subject matter of the present disclosure illustrated in the accompanying drawings.

[0016] Generally referring to FIGS. 1 to 7B, an optical measurement tool having the function of a time-modulated light source according to the present invention will be described. The present disclosure relates to systems and methods for performing optical measurements with one or more time-modulated light sources. The time modulation of the illumination emitted from one or more light sources of the measurement system of the present invention provides improved accuracy, precision, and measurement throughput.

[0017] In a sense, the illumination modulation implemented by the present invention, although not limited to, assists in suppressing coherent artifacts such as interference fringes, coherent noise, and speckles in the optical signal to be measured (e.g., angular resolved reflectance or polarization analysis parameters, polarization resolved reflectance or polarization analysis parameters, wavelength resolved reflectance or polarization analysis parameters, etc.). Further, the present invention relates to the time modulation of the illumination output of one or more excitation sources of a light-sustained plasma source. The modulation of the excitation source illumination output achieves a reduction in the noise level in the output illumination of the sustained plasma light source. Further, the present invention relates to the time modulation of the illumination outputs of multiple light sources, thereby enabling time-division interleaving of the illumination outputs of multiple light sources (e.g., lasers and / or lamps). The interleaving of the outputs of the multiple light sources of the present invention achieves improved wavelength stability, noise reduction, and intensity control in measurement applications that require multiple wavelength illumination.

[0018] Generally, the time modulation provided by various embodiments of the present invention provides many benefits. In particular, the present invention realizes a reduction in coherence noise in laser-based measurement applications, time-division interleaving of different types of light sources, and noise reduction in laser-excited plasma sources.

[0019] FIG. 1 is a block diagram showing an optical measurement tool 100 having a time-modulated illumination capability according to an embodiment of the present invention.

[0020] Those skilled in the art will recognize that coherent artifact control is a common challenge in the design of optical measurement tools. In configurations where a given optical measurement tool has one or more coherent light sources (e.g., lasers), the ability to control coherent effects (e.g., speckle and interference fringes) associated with stray light and ghosts becomes increasingly difficult. For example, a given optical measurement tool (see FIGS. 2A and 2B) has multiple optical surfaces. These optical surfaces can include, but are not limited to, beam splitters, lenses, optical fibers, objective lens surfaces, apodizers, and the like. Coherent illumination in optical measurement tools often causes detrimental speckles, fringes, and other coherent artifacts. These can contribute to measurement noise and instability that negatively affect the accuracy and precision of measurements.

[0021] For example, in a given optical system, the beam propagating through the primary path can interfere with a parasitic beam reflected from an optical surface (e.g., a mirror, beam splitter, etc.) of the optical system. To illustrate the detrimental effects of primary beam and parasitic beam interference, the primary beam and parasitic beam are characterized by intensities I1 and I2. The superposition of these two beams provides the following combined beam output.

Equation

[0022] In the above equation, θ represents the relative phase between the primary beam and the plastic beam from the reflecting surface of the optical measurement tool. For illustrative purposes, when I1 = 1 and I2 = 0.0025 (matching the plastic beam that specularly reflects with a reflectivity of 0.25%), the interference term of Equation 1 in the case where the primary and plastic waves interfere constructively will have a magnitude of 10% of the primary beam. This level of interference contribution is not acceptable in most optical measurement tools.

[0023] In contrast, in a configuration where the primary beam and the plastic beam are not coherent with each other, the interference term of Equation 1 will be zero, and the ghost correction of the measurement tool will have a magnitude of 0.25% of the primary beam, which is much easier to handle than the case described above.

[0024] One of ordinary skill in the art will recognize that the standard spectrum of a laser (e.g., a laser based on semiconductor diode technology) includes a single narrow spectral line or multiple narrow spectral lines. Such laser sources commonly have a long coherence length. Due to their wavelength stability and low noise, single-wavelength lasers are ubiquitously used throughout measurement applications. Because of the large coherence length of single-wavelength lasers, which is often greater than 100 m, coherent artifacts during operation in the measurement configuration for the reasons stated earlier in this specification are suppressed.

[0025] In one aspect of the present invention, system 100 includes a modulated light source 102 configured to illuminate the surface of a sample 106 (e.g., a semiconductor wafer) disposed on a sample stage, a detector 110 configured to detect light reflected from the surface of sample 106, and an optical system that serves to optically connect the modulated light source 102 and the detector 110. The optical system can include a set 104 of illumination optics (e.g., lenses, mirrors, filters, etc.) suitable for directing and / or focusing the light from the light source 102 onto the sample 106. The optical system can further include a set 108 of collection optics (e.g., lenses, mirrors, filters, etc.) suitable for directing the light reflected or scattered from the surface of the wafer 106 towards the detector 110. In this method, light can be emitted from the light source 102 and propagated along the illumination arm (via the illumination system 104) to the surface of the sample 106. The light reflected or scattered from the sample 106 can be propagated from the sample 106 to the detector 110 along the collection arm of the system 100 (via the collection system 108). In other aspects, the optical measurement system 100 includes a modulation control system 112 configured to modulate the drive current of a tunable light source 102 (e.g., a laser) at a selected modulation frequency.

[0026] It is noted herein that the optical measurement system 100 of the present invention may be configured to perform any form of optical measurement in the art. For example, the optical measurement system 100 is configured to perform at least one of the following measurement techniques. These techniques are critical dimension (CD) measurement, thin film (TF) thickness and composition measurement, and overlay measurement.

[0027] It is further noted that the optical measurement system 100 of the present invention is not limited to a specific optical configuration or optical measurement function. In some embodiments, the optical measurement system 100 of the present invention can be configured as a reflection measurement system. For example, the optical measurement system 100 can include, but is not limited to, a beam profile reflectometer operating in an angular resolution mode (e.g., a narrow-band beam profile reflectometer), a spectroscopic reflectometer, etc. The generalities of spectral and single-wavelength beam profile reflectometers are described in U.S. Patent No. 6,429,943, filed Mar. 27, 2001, which is hereby incorporated by reference in its entirety.

[0028] In other embodiments, the optical measurement system 100 of the present invention can be configured as a scattering measurement type measurement system. For example, the optical measurement system 100 can include, but is not limited to, a broadband scatterometer (e.g., a broadband spectroscopic scatterometer) or a narrow-band scatterometer.

[0029] In additional embodiments, the optical measurement of the present invention can be configured as an ellipsometry measurement system. For example, the optical measurement system 100 can include, but is not limited to, a beam profile ellipsometer or a spectroscopic ellipsometer. An overview of the principles of ellipsometry is described in Handbook of Elipsometry, 1st ed, by Harland G. Tompkins and Eugene A. Irene, William Andrew, Inc., 2005, which is hereby incorporated by reference in its entirety. Further, Mueller matrix ellipsometry is described in detail in P.S. Hauge, “Mueller Matrix Elipsometry with Imperfect Compensators”, J. of the Optical Soc. of AM. A68(11), 1519 - 1528, 1978; R.M.A Azzam, A Simple Fourier Photopolarimeter with Rotating Polarizer and Analyzer for Measuring Jones and Mueller Matrices, Opt Comm 25(2), 137 - 140, 1978, which are hereby incorporated by reference in their entirety. Further, the concept of “complete” ellipsometry is described in M. L. Aleksandrov et al., “Methods and Apparatus for Complete Elipsometry(review)”, J. Appl. Spectroscopy 44(6), 559 - 578, 1986, which is hereby incorporated by reference in its entirety. An overview of spectroscopic ellipsometry is described in U.S. Patent No. 5,739,909, filed Oct. 10, 1995, which is hereby incorporated by reference in its entirety. An overview of beam profile ellipsometry is described in U.S. Patent No. 6,429,943, filed Mar. 27, 2001, which is hereby incorporated by reference in its entirety as described above.

[0030] Referring now to FIG. 2A, the optical measurement system 100 of the present invention can be implemented as a reflection measurement tool such as tool 200. FIG. 2A is a top schematic view showing a reflection measurement tool suitable for implementation in the present invention. The reflectometer 200 can include a light source 102, an optical system, and a detector 110. The optical system can include a set 104 of illumination optics, a beam splitter 204, and a set 108 of condensing optics. In this regard, light can be emitted from the light source 102 and propagated to the surface of the sample 106 disposed on the sample stage 202 through the illumination optics 104 and the beam splitter 204. The applicant notes that the configuration shown in FIG. 2A is not limiting and is merely for illustrative purposes. As described above, a number of reflectometer-type optical configurations are envisioned to be utilized within the scope of the present invention.

[0031] Referring to FIG. 2B, the optical measurement system 100 of the present invention can be implemented as a scattering measurement / ellipsometry measurement tool such as tool 250. FIG. 2B is a top schematic view showing an ellipsometry measurement tool suitable for implementation in the present invention. The scatterometer / ellipsometer 250 can include a light source 102, an optical system, and a detector 110. The optical system can include a set 104 of illumination optics, a polarizer 206, a set 108 of condensing optics, and an analyzer 208. The illumination and condensing optics can include mirrors, lenses, beam splitters, compensators, and the like. In this regard, light can be emitted from the light source 102 and propagated through the polarizer 206 and the illumination optics 104 to the surface of the sample 106 disposed on the sample stage 202. The light scattered from the sample 106 can then be propagated from the surface of the sample 106 to the detector 110 through the condensing optics 108 and through the analyzer 208. The applicant notes that the configuration shown in FIG. 2B is not limiting and is merely for illustrative purposes. As described above, a number of scattering measurement and ellipsometry optical configurations are envisioned to be utilized within the scope of the present invention.

[0032] In one aspect of the present invention, the modulation control system 112 is configured to modulate the drive current of the tunable light source 102 at a selected modulation frequency. In one aspect, the selected modulation frequency may be suitable for generating illumination having selected coherence characteristics.

[0033] In one embodiment, the selected coherence characteristics can have, but are not limited to, a selected fringe visibility curve (interference fringe sharpness curve). In this regard, the selected modulation frequency may be suitable for generating illumination having a fringe visibility curve (interference fringe sharpness curve). In a further embodiment, the selected modulation frequency may be suitable for generating illumination having a fringe visibility curve (interference fringe sharpness curve) that results in coherent artifacts below a selected level (e.g., a level small enough to permit operation of the measurement tool 100). In other embodiments, the modulation frequency may be suitable for generating a fringe visibility curve (interference fringe sharpness curve) configured to suppress the occurrence of interference fringes having an intensity above a selected level (e.g., an interference fringe intensity small enough to permit operation of the measurement tool 100). In other embodiments, the modulation frequency may be suitable for generating a fringe visibility curve (interference fringe sharpness curve) having a set of intensity peaks located at different distances from the characteristic optical path length of the optical measurement tool 100. The characteristic optical path length of the optical measurement tool 100 can have the distance between a first reflecting surface of the optical measurement tool and a second reflecting surface of the optical measurement tool. In a further embodiment, the modulation frequency may be suitable for generating illumination having a fringe visibility curve (interference fringe sharpness curve) that is substantially different from the fringe visibility curve (interference fringe sharpness curve) of the light source in an unmodulated state. As described above, by sufficiently varying the fringe visibility curve (interference fringe sharpness curve) of the illumination emitted by the light source 102, the effects of coherent artifacts (e.g., speckle and interference fringes) can be removed or at least reduced.

[0034] In other embodiments, the selected modulation frequency can be suitable for generating illumination having a coherence length less than the selected length (i.e., a coherence length shorter than the distance between the optical elements of system 100). For example, the selected modulation frequency can be suitable for generating illumination having a coherence length less than the coherence length of the light source 102 in an unmodulated state (e.g., the coherence length of the light source before modulation). In other examples, the selected modulation frequency can be suitable for generating illumination having a coherence length less than the characteristic optical path length of the optical measurement tool 100. For example, the selected modulation frequency can be suitable for generating illumination having a coherence length smaller than the distance between the first reflecting surface and the second reflecting surface of the optical measurement tool 100. As described above herein, by reducing the coherence length of the illumination emitted by the light source 102 to be less than the distance between the reflecting surfaces within the measurement tool 100, the influence of coherent artifacts (e.g., speckles and interference fringes) can be removed or at least reduced.

[0035] In one embodiment of the present invention, the modulation control system 112 can serve to drive the current of one or more laser light sources at a selected frequency. For example, the modulation control system 112 can serve to modulate the drive current of a laser light source (e.g., a multi-longitudinal mode laser light source) in order to achieve a modulated fringe visibility curve (interference fringe sharpness curve) in the laser light output, so that the modulated fringe visibility curve (interference fringe sharpness curve) of the laser light source is suitable for reducing coherent artifacts within the optical measurement tool 100 to be less than a selected tolerance level. In other examples, the modulation control system 112 can serve to modulate the drive current of the laser light source in order to generate illumination having a coherence length less than a selected level.

[0036] FIG. 3 is a conceptual diagram showing the intensity spectra from a laser source without drive current modulation 302 and from a laser source with drive current modulation 304. As shown in FIG. 3, in the case of direct current drive, the spectrum 302 associated with the laser source includes the multiple longitudinal modes of the laser cavity. The spectrum 304 shown in FIG. 3 shows a broad envelope of each spectral peak of curve 302. In this regard, high-speed modulation of the drive current of the laser source provides broadening and smoothing of the intensity spectrum 304. The change in the fringe visibility curve (the clarity curve of the interference fringes) can serve to suppress the coherent artifacts (e.g., interference fringes) described above in this specification. Further, it is noted herein that the optical surfaces of a given optical measurement tool (e.g., 100) can be relatively easily configured to be separated by a distance sufficient to render the effects of parasitic interference negligible when the light source 102 is in a modulated state such that it matches the intensity spectrum 304. The applicant notes that the above descriptions regarding the fringe visibility curve (the clarity curve of the interference fringes), the coherence length, and the distance between optical elements are for illustrative purposes only and should not be construed as limiting.

[0037] In a further embodiment, the modulation control system 112 can modulate the drive current of the tunable light source 102 at a frequency in the radio frequency (RF) range. It is further noted herein that the specific frequency at which the control system 112 drives the tunable light source 102 can be selected by trial and error. For example, the modulation frequency implemented can be a frequency that serves to reduce the coherence length of the illumination from the tunable light source 102 to be below the characteristic optical path length of the optical measurement system 100. For example, the characteristic optical path length of the optical measurement system 100 can have the distance between two or more reflecting surfaces of the optical measurement tool 100. As another example, it is recognized that neither the coherence length nor the fringe visibility curve (the sharpness curve of the interference fringes, as shown above) needs to be measured to perform the modulation of the light source 102. In this sense, the control system 112 can sweep the modulation frequency of the control system 112 until a satisfactory detector 110 output is achieved.

[0038] In a further aspect of the present invention, the modulation control system 112 of the optical measurement tool 100 comprises one or more processors (not shown) communicatively connected to the tunable light source 102 and can be configured to control the modulation of the light source 102. The modulation control system 112 is configured to execute a modulation control algorithm 118 stored as a set of program instructions 116 on a carrier medium 114 (e.g., a non-transitory storage medium). The program instructions 116 are configured such that one or more processors of the control system 112 execute one or more of the various steps described in this disclosure.

[0039] It will be appreciated that the various control steps related to modulation control described throughout this disclosure may be performed by a single computer system or alternatively by a plurality of computer systems. Further, the different subsystems of system 100 can comprise a computer system suitable for performing at least some of the steps described above. Further, one or more computer systems can be configured to perform any other steps of the methods of the embodiments described herein.

[0040] Modulation control system 112 can comprise, but is not limited to, a personal computer system, a mainframe computer system, a workstation, an image computer, a parallel processor, or other devices known in the art. Generally, the terms “computer system,” “computing system,” or “computer control system” can be defined broadly to include any device having one or more processors that execute instructions from a storage medium.

[0041] The methods executed by program instructions 116 as described herein can be transmitted or stored via carrier medium 114. The carrier medium can be a transmission medium such as a wire, cable, or wireless transmission link. The carrier medium can include non-transitory storage media such as read-only memory, random access memory, magnetic or optical disks, or magnetic tape.

[0042] In other embodiments, control system 112 can be communicatively coupled to light source 102 or any other subsystem of system 100 in any manner known in the art. For example, modulation control system 112 can be communicatively coupled to the various subsystems of system 100 via a wired line or a wireless connection.

[0043] In other embodiments of the present invention, the tunable light source 102 can include narrowband light sources known in the art. In one embodiment, the light source 102 can include, but is not limited to, one or more lasers. For example, the laser light source can include, but is not limited to, one or more semiconductor lasers. In another example, the laser source can include, but is not limited to, a diode-pumped solid-state laser. In another example, the laser source can include, but is not limited to, a supercontinuum laser. Further, a first light source that emits illumination in a first spectral region can be combined with a second light source that emits illumination in a second spectral region.

[0044] In other aspects, the detector 110 can comprise an optical detection system known in the art that is optimized for implementation in a reflectometer, a scatterometer, a spectrometer, or an ellipsometer configuration. For example, the detector 110 can include, but is not limited to, at least one of a CCD array, a CMOS array, a one-dimensional photodiode array, a two-dimensional photodiode array, and the like.

[0045] FIG. 4 is a diagram showing a multi-source light source 102 according to an alternative embodiment of the present invention. In one aspect, the multi-source light source 102 of the optical measurement tool 100 includes two or more single light sources, and each single source has a different output wavelength. In one aspect, the present invention achieves stable intensity balance and control of multiple light sources. Those skilled in the art will generally recognize that turning on and off light sources, such as lasers and LEDs, results in a decrease in stability and an increase in noise. The applicants understand that instability and noise generation are limited in configurations that satisfy a periodic waveform. Thus, periodic waveform operation serves to maintain the average and stable temperature, electrical, and optical characteristics of the light source, resulting in improved wavelength stability and noise reduction.

[0046] In one aspect of the present invention, the tunable light source 102 of the system 100 includes a first light source 402a configured to generate illumination of a first wavelength (λ1), a second light source 402b configured to generate illumination of a second wavelength (λ2), …, and an Nth light source 402c configured to generate illumination of an Nth wavelength (λ N ).

[0047] In an additional aspect of the present invention, the modulation control system 112 is communicatively connected to the first light source 402a, the second light source 402b, …, the Nth light source 402c by means known in the art (e.g., a wireline or wireless connection). In a further aspect, the modulation control system 112 is configured to execute a multi-source control algorithm 120 suitable for controlling the waveform of each illumination output of the light sources 402a - 402c. The modulation control system 112 (via the control algorithm 120) is configured to modulate the drive current of the first light source 402a to generate a first illumination waveform of the first wavelength (e.g., a stepped waveform of a selected frequency). Further, the modulation control system 112 is configured to modulate the drive current of the second light source 402b to generate a second illumination waveform of the second wavelength. In this way, the pulses of the first illumination waveform are interleaved with the pulses of the second illumination waveform, and the first illumination waveform and the second illumination waveform have a selected waveform frequency. It is further noted herein that the combined waveform can include any number of component waveforms. Thus, the pulses of the first illumination waveform are interleaved with the pulses of the second illumination waveform, …, the Nth waveform. The interleaving of the various waveforms from the light sources 402a - 402c enables time-series measurement at multiple wavelengths. Further, since the modulation of the illumination from the light sources 402a - 402c is achieved by drive current modulation, the present invention eliminates the need for various optomechanical components such as optical shutters, chopper wheels. As such, the embodiment shown in FIG. 4A provides a simplified approach to multi-wavelength intensity control in the optical measurement tool 100.

[0048] In other embodiments, the multi-source light source 102 includes a plurality of wavelength combiners 404a, 404b, 404c configured to combine the beams 403a, 403b, 403c emitted from the light sources 402a, 402b, 402c, respectively. In this regard, the wavelength combiners 404a - 404c serve to spatially combine the beams and enable a temporary interleaving of the source waveforms implemented by the algorithm 120 executed by the modulation control system 112. Following the temporary interleaving and spatial combination of the beams of wavelengths, the combined waveform output 408 can be directed to the illumination system 104 of the optical measurement tool 100. It is further noted that the light source 102 can include additional optical elements such as the steering mirror 406. The Applicant notes that the optical configuration shown in FIG. 4A and above is not limiting and is to be construed as exemplary. It is recognized herein that a plurality of equivalent optical configurations are implemented to spatially combine and temporarily interleave the waveforms of the light sources 402a, 402b, …, 402c. The spatial combination of multiple laser beams into a single combined beam is generally described in U.S. Patent Application No. 13 / 108,892 to Hill et al., filed May 16, 2011, which is incorporated herein by reference in its entirety.

[0049] In one embodiment, the modulation of the first, second, …, Nth light sources performed by the modulation control system 112 includes switching the drive current of a laser or LED light source. Switching the light source drive current in this method can generate a stepped (e.g., ON / OFF) or substantially stepped waveform pattern for the respective illumination outputs of the light sources 402a - 402c. In this regard, as shown in FIG. 4A, the multi-source used enables channel selection and relative intensity control in a “color” sequential manner. The purpose of the present disclosure is that the term “color” is used to describe the primary wavelength (e.g., peak wavelength) of each light source. Further, the term “color” should not be construed as applying to a specific portion of the electromagnetic spectrum. It is envisioned that the wavelength of a given light source may exist outside the visible spectrum. For example, the spectral regions of the outputs of the light sources 402a - 402c can include the visible, UV, and IR spectral regions.

[0050] FIG. 4B is a conceptual diagram of a graph 450 of a set of interleaved waveforms from three light sources of different wavelengths λ1, λ2, λ3. The pulse train 451 shown in FIG. 4B represents either the input drive current of the light sources or the output intensity of the light sources at their respective wavelengths (e.g., λ1, λ2, λ N )). In this regard, the pulse train 451 includes a set of pulses 452 at wavelength λ1, a set of pulses 454 at wavelength λ2, …, at wavelength λ NIt consists of a set of pulses. The input drive current (not shown in FIG. 4B), the duty cycle (i.e., the width of each pulse for a given wavelength), and the output power (i.e., the height of each pulse for a given wavelength in FIG. 4B) generally vary for each wavelength waveform and are noted herein to be selected based on the requirements of a given optical measurement system. Further, it is recognized herein that the drive current can be switched between zero and the nominal peak current, or alternatively, can follow a more complex periodic pattern (e.g., the lower limit can be chosen to be a non-zero current). The frequency, duty cycle, and peak current and power levels of the waveform can be selected for the optimal performance of the light source (e.g., a laser) and other elements of the measurement tool 100, which can include a beam monitor, detectors (e.g., one or more CCDs), and an autofocus subsystem. It is further noted that changing the duty cycle and output power can also assist in achieving the desired intensity levels and balance of the multiple light sources 402a - 402c. It is further recognized that the repetition frequency of the waveform of the pulse train 451 can be about 100 Hz. As such, the repetition frequency of the multi-source of the present invention is much slower than the modulation frequency (e.g., the RF frequency) of the single source of the light source 102 described above herein. Thus, the control method for the interleaved color sequential operation (e.g., in the 100 Hz frequency range) and the control method for reducing noise / coherence effects (e.g., at the RF frequency) can be implemented simultaneously. In this regard, the control system 112 can drive a given light source (e.g., 402a - 402c) having multiple periodic waveforms operating on significantly different time scales. For example, in addition to the interleaving of the waveforms of the light sources 402a, 402b, 402c, one or more of 402a, 402b, 402c can undergo a high-speed modulation operation (following the RF frequency) to reduce coherent artifacts for a given single source.

[0051] In other aspects of the present invention, one or more of light sources 402a - 402c can have a broadband light source known in the art. In one embodiment, one or more of light sources 402a - 402c can have an HLS as described above, but is not limited thereto. In another example, one or more of light sources 402a - 402c can have a xenon arc lamp. According to another example, one or more of light sources 402a - 402c can have a deuterium arc lamp. In other embodiments, one or more of light sources 402a - 402c can have any discharge plasma source known in the art, but is not limited thereto. In other embodiments, one or more of light sources 402a - 402c can have a laser-driven plasma source, but is not limited thereto. In a further embodiment, one or more spectral filters (not shown) can be arranged between the output of one or more broadband filters and wavelength couplers 404a - 404c to spectrally filter the spectral output of one or more broadband light sources.

[0052] In other aspects of the present invention, one or more of light sources 402a - 402c can have a narrowband light source known in the art. In one embodiment, one or more of light sources 402a - 402c can include one or more lasers, but is not limited thereto. For example, one or more of light sources 402a - 402c can include one or more semiconductor lasers, but is not limited thereto. In another example, one or more of light sources 402a - 402c can include a diode-pumped solid-state laser, but is not limited thereto. In another example, one or more of light sources 402a - 402c can include a supercontinuum laser, but is not limited thereto. In other embodiments, one or more of light sources 402a - 402c can include one or more light-emitting diodes, but is not limited thereto. It will be appreciated by those skilled in the art that the above-described light sources are not representative of limitations and are merely to be construed as illustrative. In a general sense, a light source capable of generating illumination in the ranges of the visible, infrared, and ultraviolet spectra is suitable for implementation in one or more of light sources 402a - 402c.

[0053] It is further recognized herein that the plurality of light source sets 402a - 402c can include a combination of narrowband and broadband. For example, one or more of the light sources 402a - 402c can include a laser source, while one or more of the remaining light sources can consist of a broadband lamp (e.g., a laser - produced plasma source) with a fixed or wavelength - switchable spectral filter.

[0054] FIG. 5 shows a multi - source light source 102 having intensity - switching capabilities according to an alternative embodiment of the present invention. In one aspect, the multi - source light source 102 of the optical measurement tool 100 can include two or more single light sources, and each single source has a different output wavelength. In an additional aspect, the multi - source light source 102 of FIG. 5 has a set of illumination switching devices 502a, 502b, 502c. In this regard, the intensity contribution to the combined output beam 408 of each light source 402a, 402b, 402c can be controlled by using the illumination switching devices 502a, 502b, 502c respectively. Further, the modulation control system 112 can be configured to control the illumination switching devices 502a - 502c via an illumination switching algorithm, thereby controlling the intensity of each wavelength component of the combined beam 408. In this way, the modulation control system 112 can control the waveforms associated with each wavelength λ1, λ2, …, λ N such that the combined waveform, duty cycle, and intensity of each wavelength component of the selected frequency are transmitted.

[0055] In one embodiment, one or more of the illumination switching devices 502a, 502b, 502c can include, but are not limited to, a Pockels cell disposed between a first polarizer and a second polarizer. In this regard, each wavelength channel λ1, λ2, and λ NThe Pockels cells associated therewith can serve as digital ON / OFF intensity switching in response to the signals transmitted from the modulation control signals. In a further embodiment, the switching period of each Pockels cell may be much shorter than the integration time of the detector 110, eliminating the need for phase synchronization between the Pockels cell and a given light source 402a-402c and / or the detector 110.

[0056] In other embodiments, one or more of the illumination switching devices 502a, 502b, 502c can include, but are not limited to, acousto-optic switching devices. In a general sense, any high-speed optical switching device known in the art may be used.

[0057] FIG. 6 shows a spectral monitoring system 602 configured to monitor one or more spectral characteristics of a tunable light source 102 according to an embodiment of the present invention. It is recognized herein that accurate knowledge of the spectral characteristics of illumination is desirable in a configuration where noise and coherent artifacts are reduced (e.g., by reducing the coherence of illumination). In one embodiment, the spectral monitoring system 602 can be utilized to monitor the peak or center wavelength of each light source. Accurate monitoring of the spectral output of the illumination beam can reliably reduce the coherence length of a given illumination beam below an acceptable level, so the spectral monitoring system 602 is particularly useful in a drive current modulated diode laser type light source (previously described herein). In this regard, one or more portions of the spectral monitoring system 602 can be disposed along the illumination path 604 of the optical measurement tool 100. In this sense, the spectral monitoring system 602 can measure one or more spectral characteristics of the illumination emitted from the tunable light source 102. In one embodiment, the one or more spectral characteristics can include, but are not limited to, the intensity spectrum beyond a selected wavelength range, the position of one or more spectral peaks of interest (e.g., the position of the center wavelength), the full width at half maximum (FWHM) of the spectral peak of interest, and the like.

[0058] In a further embodiment, the spectrum monitoring system can be communicatively connected to the modulation control system 112. In this regard, the results of the spectrum measurement of the illumination of the illumination path 604 can be transmitted to the control system 112. In a further embodiment, the modulation control system 112 can store the results of the spectrum monitoring process in a storage medium for future use.

[0059] In one embodiment, the spectrum monitoring system 602 can monitor one or more spectral characteristics of the illumination from the light source 102 in real time or near real time. For example, the spectrum monitoring system 602 can have a spectrometer suitable for real-time measurement of one or more spectral characteristics of the illumination from the light source 102. For example, the spectrum monitoring system 602 can have a diffraction grating spectrometer, but is not limited thereto. The applicants note that the diffraction grating spectrometer is particularly useful in measuring the spectral characteristics (e.g., the center wavelength) of the light source used in the optical measurement tool of the present invention.

[0060] In other embodiments, the spectral monitoring system 602 can monitor one or more spectral characteristics of the illumination from the light source 102 for calibration purposes. For example, the spectral monitoring system 602 can monitor one or more spectral characteristics of the illumination from the light source during the tool setting calibration process. For example, since the spectral monitoring system 602 can measure one or more spectral characteristics of the illumination from the light source 102 in the tool setting calibration process, optical metrology measurements are performed on a calibration object (i.e., an object having known parameters such as a known CD, known thin film thickness and / or composition, known overlay, etc.). By using the results of the metrology measurements (e.g., thickness measurement) and the results of the measured spectral characteristics of the illumination, the control system 112 can execute the spectral monitoring calibration algorithm 119 stored in the carrier medium 114. The modulation control system 112 can periodically calibrate or "recalculate" one or more spectral characteristics of the illumination from the light source 102 based on the measurement of the calibration sample and the measured spectral characteristics. Further, the frequency of spectral calibration may depend on the spectral stability of a given light source.

[0061] In one embodiment, the calibration sample can consist of a sample having a known thin film thickness. For example, the calibration sample can include, but is not limited to, a sample having a known oxide layer thickness (e.g., a silicon W chip having a known oxide film thickness). In this regard, the thickness of the calibration sample can be calibrated during the calibration process executed by the control system 112. The spectral characteristics of the calibration sample can be monitored periodically using each data channel of the system 100 (e.g., all wavelengths of the illumination, polarization state, etc.). Based on the monitoring by the spectral monitoring system 602, the control system 112 can calculate the spectral characteristics (e.g., each wavelength value of the spectrum) of the light source 102.

[0062] In an additional aspect, the modulation control system 112 can input the results from the measurement of one or more spectral characteristics of a given sample into the sample modeling software of the control system 112. In this regard, the sample modeling software executed by the control system 112 serves to associate the measured data from the sample with a given optical model. The optical model to be implemented can utilize, as inputs, one or more spectral characteristics of a given test sample acquired by the spectral monitoring system 602.

[0063] It is noted herein that the spectral monitoring system 602 can have any spectral monitoring / measurement device known in the art. For example, the spectral monitoring device 602 can include, but is not limited to, any spectrometer known in the art (e.g., a diffraction grating spectrometer).

[0064] FIG. 7A is a block diagram showing an optically driven plasma illumination subsystem 700 with a modulated excitation source suitable for implementation in the optical measurement tool 100 of the present invention. It is noted herein that the operation of a plasma source with an excitation source (e.g., an excitation laser) driven in constant current mode causes noise greater than the desired level of noise for optical measurement applications. The present invention relates to modulating the drive current of the excitation laser of the plasma source to reduce the noise level in the output illumination of the plasma source. In particular, the excitation control system 701 of the optically driven (e.g., laser-driven) illumination subsystem 700 can serve to reduce the noise level within a specific frequency bandwidth by modulating the excitation source 702 at a frequency greater than the bandwidth of the detector 110. In this regard, the modulation frequency is selected such that the laser modulation does not cause aliasing for the detected target frequency range.

[0065] In one aspect, a plasma-based illumination subsystem 700 of an optical measurement tool 100 can have an adjustable excitation source 702 configured to generate illumination (e.g., illumination of a selected wavelength), and a plasma cell 706 suitable for containing a selected gas (e.g., argon, xenon, mercury, etc.). Further, the subsystem 700 can include an optical system assembly 704 (e.g., a focusing optical system, a shaping optical system, an adjustment optical system, etc.) configured to condition and shape the beam emitted from the excitation source 702 and further configured to focus the beam within the volume of gas contained within the bulb of the plasma cell 706. It is noted herein that the beam shaping and conditioning elements of the subsystem 700 can be utilized for the purpose of optimizing or at least improving the shaping of the beam emitted from the excitation source 702 in order to maximize the excitation effect in the plasma cell 706 (or at least to achieve a selected level of the excitation effect). Further, the beam shaping optics can be used to optimize the shaping of the plasma within the plasma cell 706. By focusing light from the excitation source 702 within the volume of gas contained within the plasma cell 706, energy is absorbed by the gas or plasma within the bulb of the plasma cell 706, exciting the gas species to generate or maintain a plasma.

[0066] In a further aspect, the broadband illumination emitted by the plasma cell 706 is directed through the illumination optics 104 of the optical measurement tool 100 towards the sample 106. The collection optics 108 of the optical measurement tool 100 then direct the illumination reflected or scattered from the sample 106 towards the detector 110.

[0067] The generation of a plasma within an inert gas species is generally described in U.S. Patent Application No. 11 / 695,348, filed Apr. 2, 2007; and U.S. Patent No. 7,435,982, issued Oct. 14, 2008, which are hereby incorporated by reference in their entirety. In a general sense, the subsystem 700 can be construed to extend to any plasma-based light source known in the art.

[0068] FIG. 7B is a schematic diagram showing a laser-driven illumination subsystem 700 according to an embodiment of the present invention. In one embodiment, the optical system 704 of the subsystem 700 can include, but is not limited to, a beam adjustment / shaping optical system 717 configured to adjust / shape the beam from the modulated excitation source 702. Further, the optical system 704 can include a set of focusing optical systems 716 suitable for focusing the illumination from the excitation source 702 within the volume of the gas 707 contained within the bulb of the plasma cell 706.

[0069] In additional embodiments, the subsystem 700 can include various additional optical elements. For example, the subsystem 700 can include, but is not limited to, a steering mirror 718 suitable for directing the illumination 721 from the modulated excitation source 702 towards the plasma cell 706. In a further example, the subsystem 700 can include a splitter / dichroic mirror 722 suitable for transmitting the illumination from the excitation source 702 to the plasma cell 706 and further reflecting the broadband illumination emitted by the plasma cell 706 (and directed by the ellipse 720) along the output path 724 towards the set of illumination optics 104 of the optical measurement tool 100 (described above herein), but is not limited thereto.

[0070] The applicant notes that the above description of the laser-driven illumination subsystem 700 should be construed as being in no way limiting and merely exemplary. It is noted herein that the laser-driven plasma illumination subsystem is suitable for implementation in the present invention.

[0071] For example, the ellipse 720 may be configured to act as a light collecting element for illumination emitted from the excitation source 702, and the ellipse 720 can serve to collect the illumination 721 within the volume of the gas 707 of the plasma cell 706. In this regard, the ellipse 720 can be configured to collect laser illumination from the excitation source 702 into the plasma cell 706 and direct broadband radiation from the plasma cell 706 toward the downstream illumination optical system 104 of the measurement tool 100. In this embodiment, the subsystem 700 can include a collimator (not shown) configured to collimate the illumination emitted from the excitation source 702.

[0072] According to another example, the system 700 is configured to separate the illumination 721 emitted by the excitation source 702 from the broadband radiation 724 emitted by the plasma cell 706 without the need for a beam splitter 722. In this regard, the illumination optical system 104 of the optical measurement tool 100 can be configured to receive the broadband radiation 724 directly from the plasma cell 706. For example, since the excitation source NA721 can be separated from the plasma irradiation NA724, the excitation source NA721 is vertically oriented while the plasma radiation is collected along a horizontal path.

[0073] In an additional aspect, the illumination subsystem 700 includes an excitation control system 701 communicatively coupled to the tunable excitation source 702, and the modulation control system 701 is configured to modulate the drive current from the tunable excitation source 702 at a selected modulation frequency to impart a time-variation within the plasma / gas contained within the plasma cell 706. For example, the time-variation can include, but is not limited to, a time-varying thermal distribution within the plasma / gas within the plasma cell 706. In a further aspect, the excitation control system 701 can control the tunable excitation source 702 via an excitation control algorithm 720 stored as a set of program instructions 116 within the carrier medium 114.

[0074] In one embodiment, the tunable excitation source 702 of the illumination subsystem 700 can include, but is not limited to, one or more lasers. The Applicant further states for clarity that various components of the optical metrology tool 100 that exist downstream from the illumination optics 104 are not shown in FIG. 7B. However, the Applicant notes that the various components and subsystems of the optical metrology tool 100 described above in this specification are to be construed broadly to the optically driven plasma source shown in FIGS. 7A and 7B. Further, the optically sustained plasma source shown in FIGS. 7A and 7B can be implemented in the reflectometer, scatterometer, ellipsometer, or spectrometer configurations described above in this specification.

[0075] It is noted herein that the frequency of modulation of the excitation source 702 should be well above the Nyquist frequency of the detection equipment of the optical metrology tool 100 in order to minimize aliasing that occurs within the detector 110.

[0076] Furthermore, the modulation depth must be selected such that large characteristic variations are achieved within the plasma of the plasma cell 706 without reducing the power density in the plasma to a level where the plasma can no longer be sustained. In a further aspect, the excitation control system 701 can serve to modulate the drive current of the laser excitation source 702, and as a result, modulate the excitation laser intensity and wavelength. Modulation of the intensity and wavelength in the optical output of the excitation source 702 can serve to create vibrational characteristics (e.g., temperature distribution) within the plasma of the plasma cell 706. Since the plasma emission from the plasma cell 706 generally passes through a number of optical components including one or more apertures that limit the spatial extent of the imaged plasma via the optical system, modulation of the spatial distribution of the plasma source can contribute to the same order of magnitude as the modulation of the spatially integrated power collected from the light source. The applicants have found a significant reduction in the noise level over a wide range of amplitude modulation at modulation frequencies from about 20 kHz to 40 kHz. The applicants have also shown that rectangular and sinusoidal modulation of the excitation source 702 is effective in reducing the noise level. It is noted that the above frequency range and waveform types are in no way limiting and are presented for illustrative purposes only. Various modulation waveforms and frequency ranges are envisioned to be within the scope of the present invention.

[0077] It is further noted herein that by controlling the above-described plasma characteristics and integrating multiple modulation periods of each detector sample, the illumination subsystem 700 can serve to reduce the random effects on the overall noise level of the overall optical measurement tool 100.

[0078] It is further contemplated herein that each of the above-described method embodiments can include any other steps of any other method described herein. Further, each of the above-described methods can be performed by any system described herein.

[0079] While specific embodiments of the subject matter of the invention described in this specification are illustrated and described, those skilled in the art can make changes and modifications based on the teachings herein without departing from the subject matter of the invention described herein and its broad aspects. Thus, it will be apparent that the appended claims encompass those within their scope and all such changes and modifications are encompassed within the true spirit and scope of the subject matter of the invention described herein.

[0080] Furthermore, it is understood that the invention is defined by the appended claims. Although specific embodiments of the invention are shown, it is clear that various modifications and embodiments of the invention can be made by those skilled in the art without departing from the scope and spirit of the above disclosure. Therefore, the scope of the invention should be limited only by the claims appended hereto. Many of the present disclosure and its attendant advantages will be understood by the foregoing description, and it is contemplated that various changes in the form, construction, and arrangement of the components can be made without departing from the disclosed subject matter of the invention or sacrificing all of the advantages of its elements. The described form is merely exemplary, and the appended claims hereinafter are intended to encompass and have those variations.

Claims

1. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with An optical metrology tool, wherein the excitation control system reduces the noise level within a particular frequency band by modulating the excitation source at a frequency greater than a bandwidth of the detector.

2. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool, wherein the plasma cell is configured to contain at least one of argon, xenon, or mercury.

3. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool, wherein the specimen comprises a semiconductor wafer.

4. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool further comprising a dichroic mirror configured to transmit illumination from the excitation source to the plasma cell and to reflect broadband illumination emitted by the plasma cell.

5. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool is configured as a reflectometer, a scatterometer, an ellipsometer, or a spectrometer.

6. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool, wherein the modulatable excitation source is modulated at a frequency between 20 kHz and 40 kHz.

7. a modulatable excitation source configured to generate an illumination beam; a plasma cell having a valve for receiving a quantity of gas; a set of optical elements configured to shape the illumination beam and focus the illumination beam from the modulatable excitation source into the volume of gas to sustain a plasma within the volume of gas; a set of illumination optics for directing the illumination beam from the plasma cell onto the surface of a specimen; A set of focusing optics; a detector configured to detect at least a portion of illumination emanating from a surface of the sample, the collection optics set configured to direct illumination from the surface of the sample to the detector; an excitation control system communicatively connected to the modulatable excitation source, the excitation control system configured to modulate a drive current of the modulatable excitation source at a selected modulation frequency to impart a time variation within the plasma contained within the plasma cell; Equipped with The optical metrology tool, wherein the modulatable excitation source is configured to modulate at least one of an intensity or a wavelength of an illumination beam.

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