Abstract:
The subject invention relates to a broadband optical metrology system that segregates the broadband radiation into multiple sub-bands to improve overall performance. Each sub-band includes only a fraction of the original bandwidth. The optical path—the light path that connects the illuminator, the sample and the detector—of each sub-band includes a unique sub-band optical system designed to optimize the performance over the spectral range spanned by the sub-band radiation. All of the sub-band optical systems are arranged to provide small-spot illumination at the same measurement position. Optional purging of the individual sub-band optical paths further improves performance.

Description:
PRIORITY CLAIM 
   The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/329,819, filed Oct. 16, 2001, which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The subject invention relates to the field of optical metrology, particularly broadband optical metrology tools that may require a controlled ambient to eliminate the effects of atmospheric optical absorption. Specifically, the invention relates to an optical metrology instrument that includes optimized, spectrally segregated optical systems and gas purged optical paths to improve system performance. 
   BACKGROUND OF THE INVENTION 
   In the prior art there has been considerable effort expended in extending the spectral bandwidth, or wavelength range, of a broad class of optical metrology instruments. Non-contact, optical measurements are heavily utilized in the optics, optical communications and semiconductor industries. The instruments are used in the evaluation and characterization of samples that can include spatially non-uniform distributions of a broad class of materials including, insulators, semiconductors and metals; consequently, the optical properties of these samples can vary markedly with wavelength. Hence, in general, a much greater wealth of information can be extracted from broadband spectroscopic measurements than can be obtained from measurements made over a narrow spectral range. One approach to implementing broadband spectroscopic measurements is set forth in U.S. Pat. No. 6,278,519, which is incorporated herein by reference. 
   At present, leading-edge industrial lithography systems operate in the DUV over a narrow wavelength region of approximately 193 nm. State-of-the art optical metrology systems that operate over the spectral range spanning the DUV-NIR (190 nm–850 nm) characteristically employ two lamps to span this range of measurement wavelengths, a Deuterium lamp for spectroscopic measurements between 190 and 400 nm, and a Xenon lamp for measurements between 400 nm and 800 nm. 
   In the near future systems will operate in the vacuum ultra-violet at an exposure wavelength of 157 nm. Wavelengths in the range between 140 nm–165 nm lie within a region known as the vacuum ultraviolet (VUV), in which the high absorption coefficients of oxygen and water vapor lower the attenuation length in standard air to fractions of a millimeter. (Historically, this light could only be observed under vacuum conditions, hence the designation.) Achieving the transmission and stability necessary for optical metrology in the VUV, in a tool where the optical paths are of order 0.5–2 m requires oxygen and water concentrations in the low parts-per-million (ppm) range averaged over the entire optical path. One approach to achieving this is described in copending U.S. application Ser. No. 10/027385, filed Dec. 21, 2001, now U.S. Pat. No. 6,813,026, and incorporated herein by reference, which discloses a method for inert gas purging the optical path. 
   The technology extension requires that the measurement bandwidth of optical metrology tools be broadened to cover the wavelength range spanning 140–850 nm. This is a daunting challenge. 
   First, optical systems that spectrally segregate the illumination with diffractive elements must account for and suppress unwanted signals produced by harmonic contamination. For example, at wavelength λ a diffraction grating will produce a 1 st  order an intensity maximum at an angular position θ; at a wavelength λ′=λ/2 the same grating, in the 2 nd  order, also produces an intensity maximum at θ. Consequently, when 140 nm light impinges upon a diffractive grating in addition to the 1 st  order diffracted beam, there are 2 nd , 3 rd , 4 th , 5 th  and 6 th  etc. diffractive orders that appear at angular positions corresponding to 1 st  order diffraction at 280 nm, 420 nm, 560 nm, 700 nm and 840 nm light, etc., respectively. Consequently spectrometers that use diffraction gratings to disperse the light must, at a minimum, include order-sorting, optical filters to suppress contributions from the higher orders. 
   Second, most common optical materials undergo solarization, structural and electronic changes to the material that occur upon exposure to VUV and DUV light. VUV and DUV exposure can significantly modify and degrade material optical properties. To avoid the adverse effect of solarization, refractive VUV optical systems must exclusively employ wide bandgap optical materials such as CaF 2 , MgF 2 , LiF, and LaF3. 
   Third, virtually all optical materials are dispersive; i.e. the material refractive index varies as a function of wavelength, or optical frequency. Therefore, the focal position of an individual lens will be wavelength dependent giving rise to chromatic aberration of the lens. This phenomenon complicates the design of broadband optical systems. Chromatic correction can be achieved, to a limited extent, using lenses with multiple optical elements fabricated from at least two optical materials. The general idea is to select and configure components so that there is partial cancellation of the chromatic effects, thereby increasing the useful wavelength range. The designs may be very complex and involve the use of multiple components arranged in multiple groups. The complexity of the optical design and the cost of system fabrication increase with the optical bandwidth. 
   Note that, to some extent, requirements two and three above are mutually exclusive. It may not be practical to provide a lens design that simultaneously exhibits good transmission in the VUV and chromatic correction over the 140 to 850 nm wavelength range. As proposed herein, one solution to this problem is to spectrally segregate the broadband light and provide multiple lens systems. Each lens operates over a limited wavelength range, or optical sub-band. The sum of the sub-bands constitutes the overall system bandwidth. Here, each lens is designed to operate over a limited wavelength range. This may permit a simpler optical design to be used in each of the sub-band lenses and provide a system architecture that provides superior broadband optical performance at reduced system cost. 
   Accordingly it would be desirable to provide a metrology tool architecture that permits extension of the measurement bandwidth over the spectral region spanning the VUV-NIR 140 nm–850 nm and avoids the problems associate with atmospheric absorption, chromatic aberration and harmonic contamination. 
   The acronyms used in this specification have the following meanings:
         CD=critical dimension   DUV=deep ultraviolet   VUV=vacuum ultraviolet   NIR=near infrared       

   SUMMARY OF THE INVENTION 
   The subject invention relates to a broadband optical metrology system that optimizes performance by dividing the broadband illumination into multiple sub-bands, such that each sub-band spans only a fraction of the frequency width of the original broadband spectrum. The sub-band illumination is transmitted along a unique sub-band optical path that connects the illuminator, the sample and the detector. The sub-band optical path includes a sub-band optical system that is optimized for the range of frequencies contained within the sub-band. In this fashion, a single broadband measurement constitutes multiple narrow-band measurements. 
   Ideally, the individual sub-band optical systems are arranged to produce small-spot illumination of the sample and are configured such that all systems illuminate the sample at the same location. In this way, the broadband information is derived from probing the same spatial region of the sample. The highest system throughput is achieved when the simultaneous measurements are made over all of the sub-bands. To circumvent the adverse effects of atmospheric optical absorption (particularly problematic in the VUV) the sub-band optical path can be purged with an optically transparent gas such as N 2  or He. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 through 4  are schematic diagrams of a generalized optical metrology system comprising illuminator, sample, detector, processor with two light sources and two separate sub-band optical paths. Four representative arrangements of the sub-band optical systems are illustrated. 
       FIG. 1  is a schematic diagram illustrating separate light paths between the source and sample. 
       FIG. 2  is a schematic diagram illustrating separate light paths between the sample and detector. 
       FIG. 3  is a schematic diagram illustrating separate paths between the source and the detector. 
       FIG. 4  is a schematic diagram illustrating a single broadband source and separate light paths between the sample and detector. 
       FIG. 5  is a schematic diagram illustrating common optical paths with separate detectors 
       FIG. 6  is a schematic diagram of a preferred embodiment of the invention including a multiple light source ellipsometer with separate illumination sub-band optical paths and detector sub-band optical paths. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 through 6  are schematic and a highly simplified optical representation has been used to reduce the complexity of the drawings. Herein individual optical components are used to represent functional elements that may be quite complex; for example, a lens in these figures represents an optical system for collecting and focusing light—the physical embodiment of the optical system may employ multiple lenses in multiple groups. 
     FIG. 1  illustrates an embodiment of the invention  10 , wherein two spectrally segregated light sources  12  and  14  are used to illuminate the sample. Light source  12  emits illumination over a first spectrum. The optical path  28  traversed by light of the first spectrum includes light source  12 , optical system  16 , sample  20 , optical system  22  and detector  24 . Optical system  16  focuses a portion of the light emitted by source  12  onto sample  20  where the light interacts with and reflects from the sample. A portion of the reflected radiation is collected by optical system  22  and focused onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   Light source  14  emits illumination over a second spectrum substantially different from the first. The optical path  30  traversed by light of the second spectrum includes light source  14 , optical system  18 , sample  20 , optical system  22  and detector  24 . Optical system  18  focuses a portion of the light emitted by source  14  onto sample  20  where the light interacts with and reflects from the sample. A portion of the reflected radiation is collected by optical system  22  and focused onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   Note that in this arrangement the optical paths  28  and  30  in the region between sample  20  and detector  24  are substantially identical. However, those portions of optical paths  28  and  30  between source  12  and sample  20 , and source  14  and sample  20  are different. The spatial separation of the light contained within the first and second spectra permits optical systems  16  and  22  to be designed to optimize performance over a limited spectral range: optical system  16  is optimized for wavelengths included within the first spectrum, optical system  18  is optimized for wavelengths included within the second spectrum. 
   Optical systems  16  and  18  are configured to illuminate the sample at the same location with substantially identical spot-sizes. In this way, the aberrations of both optical systems may be reduced which permits superior focusing performance and a smaller illuminated region on the sample, which, in turn, permits measurement of smaller regions of the sample and the measurement of smaller features. 
   The arrangement admits several variations. For example in one arrangement both light sources are broadband, e.g. light source  12  is a Xe lamp and light source  14  is a deuterium lamp. In this case optical system  16  is optimized for performance over a wavelength range spanning approximately 400–850 nm. Optical system  18  is optimized for performance over a wavelength range spanning approximately 190–400 nm. 
   Different optimization criteria may be employed in designing optical systems  16  and  18 . For example, it may be important to insure that an identical illumination spot is used to probe the sample at all illumination wavelengths, i.e. the optical designs must minimize chromatic aberration. Generally, chromatic aberrations of optical systems increase with the range of operating wavelengths; therefore, chromatic correction is more readily accomplished over a limited wavelength range. Optical system  18  is designed to operate over a sub-band of optical frequencies spanning the wavelength range between 190 and 400 nm. Similarly, optical system  16  is designed to operate over a sub-band of optical frequencies spanning the wavelength range between 400 and 850 nm. Superior correction can be achieved over the entire range of operating wavelengths since optical systems  16  and  18  may be individually chromatically corrected over their respective sub-band frequencies. 
   In the present case the optimum design for optical systems  16  and  18  may be very different, with each system employing a unique set of optical elements fabricated from unique materials. For example, optical system  18  will require the use of higher quality and therefore more expensive UV compatible optical materials such as UV grade fused silica and CaF 2 . Optical system  16  may employ a wider variety of lower cost optical materials. 
   Alternatively, cost may be the most important factor governing the optical design and a given degree of chromatic aberration may be acceptable. As discussed above, chromatic correction of two individual sub-band optical systems is simpler and requires fewer optical elements than chromatic correction of a single broadband optical system that spans the entire wavelength range. Since fewer elements are required the cost is reduced. 
   Additionally, it may be desirable to provide superior environmental control for one of the sub-band optical systems, e.g. sub-band optical system  18  which operates over the wavelength region spanning 190–400 nm. In this case, and as illustrated in the preferred embodiments of  FIGS. 3–6 , it is desirable to house the optical system in an enclosure purged with inert gas. Gas purging can prevent unwanted optical absorption and mitigate against degradation of the sub-band optics due to chemical adsorption of reactive species on the optical surfaces. The concept of optimizing a sub-band optical path can include selection or design of lenses and other optical elements and/or environmental controls which can improve the performance of the system. 
   Lastly, although the preceding discussion has been undertaken with regard to the use of broadband sources, one or both of light sources  12  and  14  may be a laser. In this case optical systems  16  and/or  18  are optimized at the laser wavelength(s). 
   The arrangement of  FIG. 1  may be generalized to include multiple light sources and multiple optical systems. Furthermore, it may be employed in systems that include one or more optical systems used separately or in combination including reflectometers, ellipsometers, scatterometers, and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
     FIG. 2  illustrates an embodiment of the invention  40 , wherein two spectrally segregated light sources  12  and  14  are used to illuminate the sample. Light source  12  emits illumination over a first spectrum and light source  14  emits illumination over a second spectrum. The optical path  28  traversed by light of the first spectrum includes light source  12 , beam combiner  25 , optical system  32 , sample  20 , optical system  34  and detector  24 . The optical path  30  traversed by light of the second spectrum includes light source  14 , beam combiner  25 , optical system  32 , sample  20 , optical system  36  and detector  24 . 
   Note that optical paths  28  and  30  are substantially coincident in the region between beam combiner  25  and sample  20  and that optical system  32  is used to provide small-spot illumination of sample  20  with light originating from both of the light sources  12  and  14 . Beam combiner  25  may be a prism, a beam-splitter, a dichroic mirror, or other suitable element which functions to combine a portion of the light emitted by sources  12  and  14 . 
   Following reflection from sample  20  light paths  28  and  30  diverge. A portion of the reflected illumination with wavelengths within the first spectrum is collected by optical system  34 , and a portion of the reflected illumination with wavelengths within the second spectrum is collected by optical system  36 . Optical systems  34  and  36  are separately optimized for the spectral regions spanning the first and second spectra respectively. In this way, the aberrations of both optical systems may be reduced which permits superior detection performance. 
   Optical systems  34  and  36  focus the collected ration onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   The arrangement admits several variations. For example in one arrangement both light sources are broadband, e.g. light source  12  is a Xe lamp and light source  14  is a deuterium lamp. Here optical system  34  is optimized for performance over a wavelength range spanning approximately 400–850 nm. Optical system  36  is optimized for performance over a wavelength range spanning approximately 190–400 nm. In another arrangement one or both of the sources  12  and  14  may be a laser. In this case optical systems  34  and/or  36  are optimized at the laser wavelength(s). 
   The arrangement of  FIG. 2  may be generalized to include multiple light sources and multiple optical systems. Furthermore, it may be employed in systems that include one or more optical systems used separately or in combination including reflectometers, ellipsometers, scatterometers, and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
     FIG. 3  illustrates an arrangement of the invention  50  that employs separate optical paths for each of the spectrally separated light sources  12  and  14 . Light source  12  emits illumination over a first spectrum. The optical path  28  traversed by light of the first spectrum includes light source  12 , optical system  16 , sample  20 , optical system  34  and detector  24 . Optical system  16  focuses a portion of the light emitted by source  12  onto sample  20  where the light interacts with and reflects from the sample. A portion of the reflected radiation is collected by optical system  34  and focused onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   Light source  14  emits illumination over a second spectrum substantially different from the first. The optical path  30  traversed by light of the second spectrum includes light source  14 , optical system  18 , sample  20 , optical system  36  and detector  24 . Optical system  18  focuses a portion of the light emitted by source  14  onto sample  20  where the light interacts with and reflects from the sample. A portion of the reflected radiation is collected by optical system  22  and focused onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   The spatial separation of the light contained within the first and second spectra permits optical systems  16  and  22  to be designed to optimize performance over a limited spectral range: optical system  16  is optimized for wavelengths included within the first spectrum, optical system  18  is optimized for wavelengths included within the second spectrum. Optical systems  16  and  18  are configured to illuminate the sample at the same location with substantially identical spot—sizes. 
   In this way, the aberrations of both optical systems may be reduced which permits superior focusing performance and a smaller illuminated region on the sample, which, in turn, permits measurement of smaller regions of the sample and the measurement of smaller features. Further, optical systems  34  and  36  are separately optimized for the spectral regions spanning the first and second spectra respectively. In this way, the aberrations of both optical systems may be reduced which permits superior detection performance. 
   The arrangement admits several variations. For example in one arrangement both light sources are broadband, e.g. light source  12  is a Xe lamp and light source  14  is a deuterium lamp. Here optical systems  16  and  34  are optimized for performance over a wavelength range spanning approximately 400–850 nm. Optical systems  18  and  36  are optimized for performance over a wavelength range spanning approximately 190–400 nm. In another arrangement one or both of the sources  12  and  14  may be a laser. In this case optical systems  16  and/or  18  are optimized at the laser wavelength(s). 
   The arrangement of  FIG. 3  may be generalized to include multiple light sources and multiple optical systems. Furthermore, it may be employed in systems that include one or more optical systems used separately or in combination including reflectometers, ellipsometers, scatterometers and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
   The separation of the light paths illustrated in  FIG. 3  has further advantages. For example, one or more of the measurement systems may be maintained in a purged environment. In  FIG. 3  the metrology system comprising source  14 , light path  30 , optical system  18 , sample  20 , optical system  36  and a portion of detector  24  is located within a purged region  38 . In practice, region  38  may include several isolated regions and enclosures that surround the system components  14 ,  18 ,  36 , and portions of  24 , and beam transport tubes which surround the optical paths that connect the system components and the sample. Region  38  may be purged of atmospheric constituents using inert gas such as N 2  or He permitting improvement of the system performance and extension of the operating wavelengths into the VUV spectral region, including the 157 nm wavelength (F 2  laser transition) used in leading-edge optical lithography. (See the above cited copending U.S. application Ser. No. 10/027385, now U.S. Pat. No. 6,813,026,for discussions of purging technology.) 
   For VUV applications, optical systems  18  and  36  are optimized for performance over a spectral range spanning a portion of the spectral region between 140 and 190 nm and employ optical components made from VUV compatible materials selected from the group consisting of UV grade fused silica, CaF 2 , BaF 2 , LaF 3 , LiF, SrF 2 , MgF 2  and fluorine doped fused silica. 
   The arrangement admits several variations. For example in a preferred embodiment including two broadband light sources light source  12  is a Xe lamp and light source  14  is a deuterium lamp that includes a VUV window. A separate, gas purged metrology system is used for measurement over the spectral range spanning the spectral region between 140 and 190 nm. In the spectral region between 190 and 850 nm, the remainder of the system is configured as illustrated in  FIG. 1 , e.g. both sources illuminate the sample, and a common path optical system  22  and detector  24  are employed. 
   The arrangement of  FIG. 3  may be further generalized to include multiple light sources, multiple optical systems and multiple purging systems. Furthermore, it may be employed in systems that include one or more optical instruments used separately or in combination including reflectometers, ellipsometers, scatterometers, and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
     FIG. 4  illustrates an arrangement of the invention that employs a single ultra-broadband light source  13 . By ultra-broadband we mean to indicate that the light source emits illumination over a very broad spectral range including, for example, the DUV and VUV, or the DUV, visible and NIR, or the VUV, DUV, visible, and NIR. 
   A portion of the illumination  29  emitted by ultra-broadband light source  13  is collected by broadband optical system  32  and focused to provide small-spot illumination of sample  20 . The incident illumination interacts with and reflects from sample  20 . A portion of the reflected illumination is incident onto beamsplitter  31  that divides the reflected portion into two spectrally segregated optical beams  28  and  30 . 
   The spectral segregation may be accomplished with a one or more elements selected from the group consisting of, for example, dichroic mirrors, grating, prisms or their equivalent. Beam  28  comprises a first sub-band spectrum that includes illumination with wavelengths or frequencies spanning a first portion of the original ultra-broadband spectrum. Similarly, beam  30  comprises a second sub-band spectrum that includes illumination with wavelengths or frequencies spanning a second portion of the original ultra-broadband spectrum. 
   In the preferred embodiment illustrated in  FIG. 4 , beamsplitter  31  is a dichroic mirror, which is designed to reflect the first sub-band spectrum  28  and transmit the second sub-band spectrum  30 . Illumination within the first sub-band spectrum reflects from fold-mirror  33  and is collected and focused by sub-band optical system  34  onto detector  24 . Illumination within the second sub-band spectrum is transmitted through dichroic mirror  31 , and collected and focused by sub-band optical system  36  onto detector  24 . Detector  24  generates output signals in response to the detected illumination and processor  26  records and analyzes those signals. 
   The spectral segregation of the light contained within the first and second spectra permits optical systems  34  and  36  to be designed to optimize performance over a limited spectral range: optical system  34  is optimized for wavelengths included within the first spectrum, optical system  36  is optimized for wavelengths included within the second spectrum. In this way, the aberrations of both optical systems may be reduced which permits superior detection performance. 
   In this arrangement, the spectrally segregated light reaching detector  24  is also spatially separated. Consequently, as illustrated in  FIG. 4 , detector  24  could consist of two detectors: a first detector optimized for the wavelength region comprising the first sub-band spectrum and a second detector optimized for the wavelength region comprising the second sub-band spectrum. 
   Further, the metrology system comprising source  13 , light path  29 , optical system  32 , sample  20 , beamsplitter  31 , light paths  28  and  30 , fold mirror  33 , optical systems  34  and  36  and a detector  24  may be located within a purged region  38 . In practice, region  38  may include several isolated regions and enclosures that surround the system components  14 ,  18 ,  36 , and portions of  24 , and beam transport tubes which surround the optical paths that connect the system components and the sample. Region  38  may be purged of atmospheric constituents using inert gas such as N 2  or He permitting improvement of the system performance and extension of the operating wavelengths into the VUV spectral region. 
   The arrangement of  FIG. 4  may be generalized to include multiple light sources and multiple optical systems. Furthermore, it may be employed in systems that include one or more optical systems used separately or in combination including reflectometers, ellipsometers, scatterometers and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
     FIG. 5  illustrates an embodiment of the invention  60 , wherein two spectrally segregated light sources  12  and  14  are used to illuminate the sample, and separate detection systems  70  and  72  are employed to detect illumination after reflection from and interaction with sample  20 . 
   Light source  12  emits illumination over a first spectrum and light source  14  emits illumination over a second spectrum. The optical path  28  traversed by light of the first spectrum includes light source  12 , beam combiner  25 , optical system  32 , sample  20 , optical system  22 , beam-splitter  31  and one, or both, of the detectors  70  and  72 . The optical path  30  traversed by light of the second spectrum includes light source  14 , beam combiner  25 , optical system  32 , sample  20 , optical system  22 , beam-splitter  31  and one or both, of the detectors  70  and  72 . 
   Note that optical paths  28  and  30  are substantially coincident in the region between beam combiner  25  and beam splitter  31  and that optical system  32  is used to provide small-spot illumination of sample  20  with light originating from both of the light sources  12  and  14 . Beam combiner  25  may be a prism, a beam-splitter, a dichroic mirror, or other suitable element which functions to combine a portion of the light emitted by sources  12  and  14 . 
   Following reflection from sample  20  a portion of the reflected illumination is collected by optical system  22 ; the optical system is optimized over the spectral region spanning both sources  12  and  14 . Optical system  22  focuses the collected illumination onto the detectors  70  and  72 . Beam-splitter  31  divides the focused illumination so that each detector receives a portion of the focused illumination, and generates separate output signals in response thereto. 
   In  FIG. 5 , beam-splitter  31  is shown as a spectrally selective element. Illumination originating from light source  12  is directed along optical path  28  toward detector  70 . Illumination originating from light source  14  is directed along optical  30  toward detector  72 . This arrangement permits optimization of the response of each detector over a limited spectral region, thereby improving detection system performance. 
   One or both of detector systems  70  and  72  may be located within a N 2  purged environment  38  to limit the adverse effects of temperature variation, atmospheric optical absorption, etc., and improve the temporal stability of the detector response. Processor  26  records and analyzes the detector output signals. 
   The arrangement admits several variations. For example in one arrangement both light sources are broadband, e.g. light source  12  is a Xe lamp and light source  14  is a deuterium lamp. Here detector  72  is optimized for performance over a wavelength range spanning approximately 400–850 nm. Detector  70  is optimized for performance over a wavelength range spanning approximately 190–400 nm. 
   For example, in certain applications it is advantageous to employ array detectors. In this case, detector  70  is a UV enhanced photodiode or UV enhanced CCD array. Detector  72  is a photodiode or CCD array selected to provide high sensitivity and good quantum efficiency throughout the visible and NIR. In applications requiring narrow band detectors, detectors  70  and  72  are, respectively, photomultiplier tubes with photocathode materials optimized for the UV and visible-NIR regions of the spectrum. In another arrangement one or both of the sources  12  and  14  may be a laser. In this case detectors  70  and  72  may be photodiode detectors optimized at one or more laser wavelength(s). 
   The arrangement of  FIG. 4  may be generalized to include multiple light sources and multiple optical systems. Furthermore, it may be employed in systems that include one or more optical systems used separately or in combination including reflectometers, ellipsometers, scatterometers, and optical CD metrology tools operating as single wavelength metrology systems and broadband spectroscopic instruments. 
     FIG. 6  illustrates a preferred embodiment of a broadband spectroscopic ellipsometry system  80 . Ellipsometry system  80  includes two separate measurement systems; one optimized for VUV operation, the other for operation in the UV-NIR spectral range. Detector system  24  may include separate VUV  72  and UV-VIS  70  detectors. 
   The VUV ellipsometer is comprised of a VUV light source  14 , optical system  18 , sample  20 , optics system  36  and detector  72 . The VUV ellipsometer is isolated from the ambient environment and maintained in a N 2  purged environment  38 , in order to remove optically absorbing atmospheric constituent from the optical path  30 . 
   Optical system  18  includes condenser  56 , polarizer  58 , and focusing system  62  and is configured to provide small-spot illumination of the sample with polarized broadband VUV light at a measurement location. Optical system  36  includes collection optics  64 , wave-plate  66  and analyzer  68  configured to collect a portion of the illumination reflected from the sample and measure the change in the illumination produced by interaction with the sample. 
   Detector  72  generates output signals in response to the detected illumination at a plurality of wavelengths spanning the spectral region between 140 and 190 nm. Processor  26  records and analyzes those output signals. Optical systems  18  and  36  are fabricated from VUV transparent materials and components selected from the group consisting of UV grade fused silica, CaF 2 , BaF 2 , LaF 3 , LiF, SrF 2 , MgF 2  and fluorine-doped fused silica. Ideally, the VUV light source  14  is a deuterium lamp with a VUV window. In the preferred embodiment polarizer  58  and analyzer  68  are VUV grade Rochon prisms and wave-plate  66  is manufactured form MgF 2 . 
   The UV-NIR ellipsometer is comprised of a UV-NIR light source  12 , optical system  16 , sample  20 , optics system  34  and detector  70 . The UV-NIR ellipsometer is illustrated with optical path  28  maintained in the ambient environment; however, if so desired, this system may also be maintained in a N 2  purged environment in order to improve the performance of the UV-NIR measurement system. 
   Optical system  16  includes condenser  42 , polarizer  44 , and focusing system  46  and is configured to provide small-spot illumination of the sample with polarized broadband UV-NIR light at a measurement location. Optical system  34  includes collection optics  48 , wave-plate  52  and analyzer  54  configured to collect a portion of the illumination reflected from the sample and measure the change in the illumination produced by interaction with the sample. 
   Detector  70  generates output signals in response to the detected illumination at a plurality of wavelengths spanning the spectral region between 190 and 850 nm. Processor  26  records and analyzes those output signals. 
   Optical systems  16  and  34  are optimized for the wavelength region spanning 190–850 nm. In the preferred embodiment, the UV-NIR light source  12  includes both deuterium and Xe lamps arranged to provide broadband illumination of the sample over the wavelength range spanning 190–850 nm. In the preferred embodiment polarizer  44  and analyzer  54  are Rochon prisms. 
   The VUV and UV-NIR optical systems are arranged so that the measurement regions of the VUV and UV-NIR ellipsometers are substantially identical, e.g. the size and shape of the illuminated area at the measurement location and the numerical aperture of the collection systems are virtually identical in the VUV and UV-NIR. 
   The arrangement of  FIG. 5  may be further generalized to include additional light sources and additional measurement systems that employ gas purged optical paths. Possible generalizations include systems that use one or more measurement technologies employed separately or in combination including instruments commonly known as reflectometers, ellipsometers, scatterometers, and optical CD metrology tools operating as single wave-length metrology systems and broad-band spectroscopic instruments.