Abstract:
A system for analyzing a thin film simultaneously applies a pulsed cleaning beam and a measurement beam to an analysis location on a test sample to enhance measurement accuracy. The pulsed cleaning beam prevents contaminant regrowth on the analysis location during the actual measurement. To minimize the effects of thermal transients from the pulsed cleaning beam on measurement data, cleaning pulses can be timed to fall between data samples. Alternatively, data sampling can be blocked during each cleaning operation (i.e., each cleaning pulse and subsequent cooldown period) or data levels can be clamped at measurement levels from just before the start of the cleaning operation for the duration of the cleaning operation. Alternatively, data samples taken during each cleaning operation can be discarded or replaced with data samples from just before the cleaning operation using post-processing techniques.

Description:
CLAIM OF PRIORITY 
   This application claims priority to U.S. provisional application Ser. No. 60/426,138, filed Nov. 13, 2002 entitled “Film Measurement With Interleaved Laser Cleaning”. 

   FIELD OF THE INVENTION 
   This invention relates generally to measurement systems, and more particularly to a system and method for minimizing contamination effects on metrology operations. 
   BACKGROUND OF THE INVENTION 
   As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Typically, the manufacturing process for modern semiconductor devices includes the formation of a number of layers or “thin films” on a silicon wafer. The thin films can include oxide, nitride, and/or metal layers, among others. To ensure proper performance of the finished semiconductor devices, the thickness and composition of each thin film formed during the manufacturing process must be tightly controlled. 
   Modern thin films have reached the point where the accuracy and reproducibility of thin film metrology (i.e., measurement and/or inspection) can be limited by contamination on the surface of the thin film. For example, airborne molecular contamination (AMC) such as water and other vapors can be absorbed onto the thin film, creating a contaminant layer that adversely affects thin film analysis techniques such as optical ellipsometry, optical reflectometry, grazing-incidence x-ray reflectometry (GXR), x-ray fluorescence (XRF), electron microprobe analysis (EMP), and non-contact electrical analysis—all of which operate by directing a probe beam (optical, x-ray, electron or corona discharge) at the surface of the thin film to be measured. The contaminant layer can also interfere with measurements techniques that physically contact the surface of the thin film, such as contact-based electrical analysis (e.g., spreading resistance analysis). 
   Conventional methods for cleaning thin films include heating the entire wafer in an oven to a temperature of about 300° C. to vaporize any contaminant layer.  FIG. 1   a  shows a conventional oven-based wafer cleaning system  100   a  used to prepare a wafer  110  for thin film analysis, as described in U.S. Pat. No. 6,325,078, issued Dec. 4, 2001 to Kamieniecki. Wafer  110  includes a thin film layer  112  formed on a silicon substrate  111 , and a contaminant layer  113  formed on the surface of thin film layer  112 . Wafer cleaning system  100   a  comprises a wafer stage  120 , and multiple heat lamps  130 . Wafer stage  120  positions wafer  110  under heat lamps  130 , where thermal radiation from heat lamps  130  heats wafer  110  to vaporize contaminant layer  113 . It is believed that this cleaning process is aided by optical photons from heat lamps  130  breaking bonds between contaminant layer  113  and thin film layer  112 . 
     FIG. 1   b  shows another conventional wafer cleaning system  100   b  used to prepare wafer  110  for thin film analysis, as described in U.S. Pat. No. 6,261,853, issued Jul. 17, 2001 to Howell et al. Just as described with respect to  FIG. 1   a , wafer  110  includes a thin film layer  112  formed on a silicon substrate  111  and a contaminant layer  113  formed on the surface of thin film layer  112 . Cleaning system  100   b  incorporates a stage  140  that includes a heating element  141 . Heat generated by heating element  141  is conducted through stage  140  into wafer  110 , thereby providing the heating required to vaporize contaminant layer  113 . A heat exchanger can be coupled to stage  140  to capture excess heat from heating element  141  to minimize undesirable heating of cleaning system  100   b  itself and the surrounding environment. 
   Although wafer cleaning systems  100   a  and  100   b  use different thermal energy sources (i.e., heat lamps  130  and heating element  141 , respectively), both systems perform a bulk heating operation to remove contaminant layer  113 . The large thermal control components (e.g., lamps, heated stages, heat exchangers, etc.) typically used for bulk wafer heating undesirably increase the cleanroom space required for these conventional cleaning systems. Further exacerbating the problem of excess equipment size, conventional cleaning systems are sometimes stand-alone units used in conjunction with a thin film analysis tool. Therefore, conventional cleaning systems can significantly increase the total footprint required for a complete thin film analysis system. The use of a separate cleaning system also has an adverse effect on throughput, as time must be spent transferring the wafer to and from the cleaning system. In addition, contaminants can redeposit on the cleaned wafer while it is being transferred from the cleaning-system to the film analysis tool. 
   In an attempt to somewhat alleviate these equipment size and recontamination problems, attempts have been made to combine wafer cleaning and measurement capabilities in a single tool. For example, the aforementioned U.S. Pat. No. 6,261,853 describes integrating cleaning system  100   b  with an existing metrology tool (Opti-Probe 5240 from Therma-Wave, Inc.). Also, the Quantox XP tool from KLA-Tencor integrates a wafer cleaning system similar to cleaning system  100   b  with a non-contact electrical film measurement system. However, any bulk wafer heating system must still incorporate the aforementioned (large) thermal control components. Furthermore, even if a combined system is used, the bulk heating operation can significantly degrade overall wafer processing throughput. Several seconds are required to heat the wafer to the temperature required for removal of the contaminant layer, and another several seconds are required to cool down the wafer after cleaning. Any wafer handling operations that must be performed during and after the cleaning operation (e.g., transferring the wafer from the cleaning system to the thin film analysis system) further reduces the throughput. Note also that any delays after cleaning allow contaminant regrowth on the wafer. 
   To improve throughput and reduce system footprint, a laser cleaning system can be incorporated into a metrology system.  FIG. 2   a  shows an integrated laser cleaning metrology system  200 , which is described in detail in co-owned and co-pending U.S. patent application Ser. No. 10/056,271. Metrology system  200  comprises a stage  220 , an energy beam source  230 , and an analysis module  240 . The compact components used in an energy-beam based cleaning system (such as energy beam source  230 ) can be efficiently integrated into metrology system  200  to minimize system footprint. 
   Stage  220  holds a test sample  210  that comprises a thin film layer  212  formed on a substrate  211  and a contaminant layer  213  formed on the surface of thin film layer  212 . Energy beam source  230  directs an energy beam  231  at a spot  214   a  on contaminant layer  213  to expose the underlying portion of thin film layer  212 . Then in  FIG. 2   b , stage  220  positions test sample  210  under analysis module  240  so that a measurement beam  246  can be directed onto thin film layer  212  through an opening  214   b  formed by the laser heating of spot  214   a  during the preceding cleaning operation (as shown in  FIG. 2   a ). Since only a localized portion of contaminant layer  213  is cleaned, the long heating and cooling times associated with conventional cleaning systems can be avoided to improve throughput, and the only delay between cleaning and measurement is the time required to reposition test sample  210  under analysis subsystem  240 —typically 1–2 seconds. 
   However, as metrology parameters become ever more sensitive to AMC, even this 1–2 second delay between cleaning and measurement can allow an excessive amount of AMC recontamination onto the thin film layer. For example, many modern metrology operations require test sample surface stabilities on the order of a tenth of an angstrom. However, AMC regrowth rates can be in the 1 Å/sec range, in which case a repositioning delay of even a second can lead to significant measurement inaccuracies. Furthermore, since the measurement process itself can take a few seconds to complete, significant AMC regrowth can actually take place over the course of the measurement operation. 
   Accordingly, it is desirable to provide a method and system for performing thin film metrology that avoids the aforementioned problems associated with AMC contamination and regrowth. 
   SUMMARY OF THE INVENTION 
   The present invention provides a system and method for concurrent localized cleaning and analysis of a test sample to provide enhanced measurement accuracy. By performing a localized cleaning operation(s) during the actual analysis operation (i.e., “interleaving” cleaning and analysis operations), analysis degradation due to contaminant layer regrowth can be minimized. Furthermore, by eliminating the need for repositioning of the test sample between cleaning and analysis operations, throughput is enhanced while the potential for misalignment (due to the repositioning operation) is reduced. 
   A metrology system in accordance with an embodiment of the present invention comprises a cleaning subsystem, an analysis subsystem, a focusing subsystem, and a stage. The stage holds a test sample (such as a wafer) to be analyzed by the analysis subsystem. The analysis subsystem can comprise any metrology system or systems, including an ellipsometry system, such as a single-wavelength ellipsometry system (SWE) or a spectroscopic ellipsometry system (SE), a reflectometry system, a contact-based electrical measurement system, a non-contact electrical measurement system, a GXR system, an XRF system, an EMP system, and/or a scanning electron microscope (SEM) inspection or review system. More generally, the cleaning subsystem can be integrated with any sort of measurement system, such as metrology systems used in the production of semiconductor devices. 
   The focusing subsystem positions the analysis subsystem and the cleaning subsystem relative to the stage such that a measurement beam (or probe) from the analysis subsystem and a pulsed cleaning beam generated by the cleaning subsystem are simultaneously focused on the test sample. According to an embodiment of the invention, the analysis subsystem includes a measurement emitter for generating and directing the measurement beam at a analysis location, and also includes a measurement receiver for measuring the output beam(s) generated from (i.e., reflected by, emitted from, scattered by, etc.) the analysis location in response to the measurement beam so that the test sample can be analyzed. The cleaning subsystem includes a cleaning beam emitter for generating and directing the cleaning beam at the same analysis location. By performing a cleaning operation during the analysis operation, the cleaning system minimizes contaminant regrowth and provides a stable analysis environment for the analysis subsystem. 
   According to an embodiment of the invention, the cleaning beam can comprise a series of cleaning pulses, i.e., a series of on/off states. This in turn helps to minimize any effect the cleaning operation might have on the measurement operation. Depending on the characteristics of the pulsed cleaning beam, the pulse (on) portions may introduce local effects that could affect the measurements being taken by the analysis subsystem (of course, the non-pulse (off) portions of the pulsed cleaning beam will have no effect on the measurements). For example, each pulse of a laser cleaning beam could cause localized heating of the test sample that could in turn affect measurements taken at this elevated temperature. 
   Depending on the specific cleaning effects and the sensitivity of the analysis subsystem to those effects, various approaches can be taken to minimize their impact. According to an embodiment of the invention, if the cleaning effects are small enough, they can simply be ignored. According to another embodiment of the invention, the width (i.e., the duration of the pulse) and period (i.e., the time between the start of one pulse and the next) of the cleaning pulses in the pulsed cleaning beam could be timed to fall between measurement samples taken by the measurement subsystem. According to another embodiment of the invention, the analysis subsystem can include a clamp circuit that clamps measurement samples taken during each cleaning pulse (and during the cooldown period after each cleaning pulse) at the level of a measurement sample just before the cleaning pulse. According to another embodiment of the invention, post-processing can be performed on the sampled data to delete or replace measurement samples taken during each cleaning pulse and associated cooling period. 
   The present invention will be more fully understood in view of the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
       FIGS. 1   a  and  1   b  are schematic diagrams of conventional wafer cleaning systems. 
       FIGS. 2   a  and  2   b  are schematic diagrams of another wafer cleaning system. 
       FIGS. 3   a ,  3   b ,  3   c  and  3   d  are schematic diagrams of a metrology system including interleaved cleaning capability in accordance with an embodiment of the invention. 
       FIG. 4  is a beam diagram showing localized cleaning and measurement in accordance with an embodiment of the invention. 
       FIG. 5  is a plan view of the metrology system of  FIGS. 3   a – 3   c  in accordance with an embodiment of the invention. 
       FIGS. 6   a ,  6   b , and  6   c  are graphs comparing sampling rate with cleaning pulses and cleaning effects according to an embodiment of the invention. 
       FIG. 7  is a schematic diagram of a clamping circuit according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3   a  shows a metrology system  300  that includes interleaved cleaning capability in accordance with an embodiment of the invention. Metrology system  300  includes a stage  320 , an analysis subsystem  330 , a cleaning subsystem  360 , and a focusing subsystem  370 . Stage  320  holds a test sample  310  that comprises a thin film layer  312  formed on a substrate  311 . Substrate  311  can comprise any structure on which thin film layer  312  can be formed, including a single-layer structure (such as a silicon wafer) or a multi-layer structure (such as an additional thin film layer or layers formed on a silicon wafer). Test sample  310  also includes a contaminant layer  313  formed on the surface of thin film layer  312 . Contaminant layer  313  can comprise any unwanted material, such as AMC, on the surface of thin film layer  312 . Note that while contaminant layer  313  is shown covering the entire surface of thin film layer  312  for explanatory purposes, contaminant layer  313  can also only partially cover thin film layer  312 . 
   To perform a metrology operation, an alignment operation is first performed using focusing subsystem  370  to properly align analysis subsystem  330  and cleaning subsystem  360  with test sample  310 . According to an embodiment of the invention, focusing subsystem  370  includes a focusing emitter  380  and a focusing receiver  390 . For explanatory purposes, focusing emitter  380  is shown comprising an alignment beam source  381 , a protective filter  382 , directional optics  383 , and a lens  384 , while focusing receiver  390  is shown comprising a position-sensitive detector  391 , a protective filter  392 , directional optics  393 , and a lens  394 . However, focusing emitter  380  and focusing receiver  390  can comprise any components that are capable of providing the desired focusing performance. 
   To perform a focusing operation, alignment beam source  381  generates an alignment beam  375 . According to an embodiment of the invention, alignment beam source  381  can comprise various light sources, including white-light lamps, such as xenon discharge lamps or tungsten halogen lamps, light-emitting diodes (LEDs), or near-infrared (NIR) lasers. Alignment beam  375  passes through protective filter  382  and is guided by directional optics  383  through a lens  384  that focuses alignment beam  375  onto an analysis location  314 . Alignment beam  375  penetrates contaminant layer  313 , which is very thin and therefore has a negligible effect on the focusing operation. A resulting reflected alignment-beam  376  is then focused by lens  394  and directed by directional optics  393  through protective filter  392  onto position sensitive detector  391 . Position sensitive detector  391  measures the positional characteristics of reflected alignment beam  376 , from which an accurate position for test sample  310  can be trigonometrically determined. Protective filters  382  and  392  prevent stray light (e.g., ambient light, measurement or cleaning light, etc.) from damaging or interfering with the measurements of focusing subsystem  370 . For example, if alignment beam source  381  is a white-light lamp or a NIR laser, protective filters  382  and  392  can comprise NIR filters (i.e., filters that only pass NIR-light). 
   Once the position of test sample  310  is known, the position of analysis-subsystem  330  can be adjusted relative to stage  320  via an optional adjustment mechanism  331  in analysis subsystem  330  and/or an optional adjustment mechanism  321  in stage  320 . This positioning operation ensures that a measurement beam generated by analysis subsystem  330  (discussed below with respect to  FIG. 3   d ) is properly focused on test sample  310 . An adjustment mechanism  361  in cleaning subsystem  360  is likewise used to position cleaning subsystem  360  relative to stage  320  to ensure that a cleaning beam generated by cleaning subsystem  360  (discussed below with respect to  FIG. 3   b ) is properly focused on test sample  310 . 
   Once the alignment operation is completed, an initial cleaning operation can be performed by cleaning subsystem  360  to place analysis location  314  in the desired condition for metrology by cleaning away the overlying portion of contaminant layer  313 . As shown in  FIG. 3   b , according to an embodiment of the invention, cleaning subsystem  360  includes a cleaning beam emitter  362  that directs a cleaning beam  366  at analysis location  314  on test sample  310 . For explanatory purposes, cleaning beam emitter  362  includes a cleaning beam source  363 , directional optics  364 , and a lens  365 . However, cleaning beam emitter can comprise any components capable of providing the desired cleaning functionality. 
   Cleaning beam source  363  generates cleaning beam  366 , which is then guided by directional optics  364  through lens  365 , which focuses cleaning beam  366  onto analysis location  314 . Cleaning beam  366  is configured to remove enough of contaminant layer  313  to reveal analysis location  314  of thin film layer  312 . This removal process can comprise either an interaction with contaminant layer  313  and/or an interaction with the underlying portion(s) of thin film layer  312  and/or substrate  311 . For example, cleaning beam  366  could comprise a laser tuned to heat contaminant layer  313  directly or heat the underlying portion of thin film layer  312  or substrate  311 . Note that the specific contaminant layer removal mechanism will depend on the type of cleaning beam used. 
   Cleaning subsystem  360  and focusing subsystem  370  can share a common optical path to optimize layout efficiency and permit the sharing of focusing optics to reduce system cost.  FIG. 3   c  shows a metrology system  300 - 1  that includes interleaved cleaning capabilities in accordance with another embodiment of the invention. Metrology system  300 - 1  is substantially similar to metrology system  300  shown in  FIGS. 3   a  and  3   b , except that focusing receiver  390  and cleaning beam emitter  362  share a common lens  395  for focusing reflected alignment beam  376  onto position sensitive detector  391  and for focusing cleaning beam  366  onto analysis location  314 , respectively. Because they use lens  395 , reflected alignment beam  376  and cleaning beam  366  share a common optical path (i.e., are aligned) between lens  395  and analysis location  314  of test sample  310 . A dichroic mirror  394  reflects cleaning beam  366  and transmits reflected alignment beam  376 , thereby placing the two beams in alignment. As noted above, position sensitive detector  391  and alignment beam source  381  are protected from scattered cleaning light by protective filters  392  and  382 , respectively. 
   During cleaning operations, the potential for damage to the underlying thin film layer  312  and/or substrate  311  during the cleaning process is minimal because of the localized action of cleaning beam  366 . Furthermore, the fact that cleaning and measurement are interleaved in time allows a lower cleaning beam power to be used than is the case where cleaning and measurement are separated in space and time (i.e., where cleaning intervals are much greater). This lower cleaning beam power also serves to reduce the possibility of damage to the sample. The risk of damage can be further reduced by performing the cleaning operation on non-functional regions of test sample  310  (e.g., regions such as scribe lines that will not be part of the functional portion(s) of the final devices to be made from test sample  310 ). 
   To minimize the effect of the interleaved cleaning process on the beam characteristic measurements being taken by metrology system  300 , cleaning beam emitter  362  shown in  FIG. 3   b  can provide cleaning beam  366  as a pulsed beam. According to an embodiment of the present invention, cleaning beam source  363  could comprise a pulsed laser. For example, contaminant layer  313  could comprise a 5 angstrom thick layer of water and organic materials (which is similar to contamination layers often formed on modern thin film layers during production). A number of pulses or even a single pulse from a 5–100 μJoule laser having a 1–100 ns pulse duration could then heat the desired portion of contaminant layer  313  to between roughly 300° C. to 1000° C., which is a temperature range sufficient to vaporize that portion of contaminant layer  313 . 
   According to another embodiment of the invention, cleaning beam source  363  could comprise a Q-switched laser delivering a relatively, high peak power, such as a frequency-doubled or tripled YAG (yttrium aluminum garnet) laser operating at wavelengths of 532 nm or 355 nm, respectively. According to another embodiment of the invention, other types of pulsed lasers operating at different wavelengths might be used including pulsed diode or alexandrite lasers. 
   According to another embodiment of the invention, a continuous laser, such as an argon-ion laser, could be externally modulated (such as with an acousto-optic or electro-optic modulator) to produce pulses. According to another embodiment of the invention, cleaning beam source  363  could comprise a flashlamp coupled to focusing optics to direct the high intensity light to the desired area on contaminant layer  313 . According to other embodiments of the invention, cleaning beam emitter  362  can comprise a pulsed microwave source, a pulsed gas jet source, a pulsed acoustic source, a pulsed dry ice jet, or a pulsed ion beam source. 
   To provide the desired amount of initial cleaning, cleaning subsystem  360  can apply cleaning beam  366  to analysis location  314  until a predefined number of pulses have been applied to the region. Note that the duration and/or period of the cleaning pulses in cleaning beam  366  during this initial cleaning process do not necessarily have to be the same as the duration and/or period of the cleaning pulses used for the interleaved cleaning operation (i.e., the cleaning operation performed concurrently with the measurement operation). The cleaning pulses of cleaning beam  366  form an opening  315  in contaminant layer  313 , as shown in  FIG. 3   d , through which the actual measurement operation can them be performed. 
   For explanatory purposes, measurement subsystem  330  is shown in  FIG. 3   d  as comprising a measurement emitter  340  and a measurement receiver  350 , and an optional computer  359 . Measurement emitter includes a measurement beam source  341 , an optional acousto-optical modulator  342 , directional optics  343 , a polarizer  344 , a focusing lens  345  and a rotating waveplate  346 . Measurement receiver  350  includes a detector circuit  351 , an interference filter  352 , a polarizer  353 , a focusing lens  354 , and a rotating waveplate  355 . Therefore, measurement subsystem  330  as shown in  FIG. 3   d  includes components for performing single wavelength ellipsometry (SWE), such as described in co-owned, co-pending U.S. patent application Ser. No. 09/298,007, herein incorporated by reference. Note, however, that as mentioned above, analysis subsystem  330  can comprise any type of analytical assembly, including spectroscopic ellipsometry (SE, such as described in co-owned U.S. Pat. No. 5,608,526, herein incorporated by reference), reflectometry (optical or x-ray, such as described in co-owned U.S. Pat. No. 5,747,813, herein incorporated by reference, or GXR, such as described in co-owned, co-pending U.S. Patent Application, herein incorporated by reference), non-contact electrical analysis (such as described in co-owned U.S. Pat. No. 5,485,091, herein incorporated by reference), XRF, EMP, SEM inspection/review, or contact-based electrical analysis (e.g., spreading resistance analysis), among others. 
   To perform an SWE measurement process, measurement beam source  341  generates a measurement beam  335 . According to an embodiment of the invention, measurement beam source  341  can comprise a helium-neon (HeNe) laser with a wavelength of 633 nm. Optional acousto-optical modulator  342  can then be used to pulse measurement beam  335  if desired. Directional optics  343  direct measurement beam  335  through polarizer  344  and then through focusing lens  345 . Passing through rotating waveplate  346 , measurement beam  335  has its polarization continuously modulated from circular to linear and back again and directed onto the portion of thin film layer  312  exposed through opening  315  in contaminant layer  313  (i.e., analysis location  314 ). In response to measurement beam  335 , an output beam  336  is generated from (in this case reflected by) analysis location  314 . Note that depending on the specific measurement process being used, output beam  336  can comprise a single beam (e.g., if the measurement process comprises ellipsometry) or multiple beams (e.g., if the measurement process comprises XRF). Output beam  336  passes through rotating waveplate  355 , focusing lens  354 , polarizer  353 , and interference filter  352  before striking detector circuit  351  (typically a photodiode circuit). Detector circuit  351  measures the resulting intensity profile as a function of time to allow calculation of the desired thin film characteristics—for example, by optional computer  359 . 
   To ensure that accurate measurements are taken by measurement subsystem  330 , measurement beam  335  must have clear access to analysis location  314 . Accordingly, the specific amount of contaminant layer  313  to be removed by cleaning beam  366  depends on the beam characteristics of measurement beam  335  (and the measurement characteristics of measurement subsystem  330 ). Modern thin film analysis tools generally require an analysis area of at least 20 μm×20 μm. Accordingly, at least a 20 μm×20 μm portion of contaminant layer  313  would need to be removed for such systems. However, to ensure that the entire analysis area is uniformly cleaned, a larger portion of contaminant layer  313  could be removed.  FIG. 4  shows a detail view of the portion of test sample  310  around analysis location  314 , showing relative sizes of a cleaning beam spot  467  (through contaminant layer  313 ) produced by cleaning beam  366  and a measurement beam spot  437  (on thin film layer  312 ) produced by measurement beam  335 , according to an embodiment of the invention. By focusing measurement beam  335  down to a smaller spot size (at analysis location  314 ) than the spot size of cleaning beam  366 , greater tolerance for x-y alignment between alignment subsystem  330  and cleaning subsystem  360  (shown in  FIG. 3   b ) is provided. 
   Note that while  FIGS. 3   a – 3   d  show measurement subsystem  330 , cleaning subsystem  360 , and focusing subsystem  370  in a “stacked” configuration for explanatory purposes, the subsystems can take any desired arrangement. For example,  FIG. 5  shows a plan view of metrology system  300  in accordance with an embodiment of the invention. While cleaning subsystem  360  remains aligned with focusing subsystem  370 , measurement subsystem  330  is oriented perpendicular to focusing subsystem  370 , such that the plan view directional component (i.e., parallel to the surface of test sample  310 ) of measurement beam  335  is perpendicular to the plan view directional components of cleaning beam  366  and alignment beam  375  (note that other non-parallel arrangements could also be implemented). This perpendicular arrangement can allow measurement subsystem  330  and focusing subsystem  340  to be more efficiently packed into metrology system  330 . 
   The concurrent application of cleaning beam  366  to analysis location  314  during the measurement process clears away any regrowth of contaminant layer  313  in opening  315  that would otherwise compromise the accuracy of the measurement data. However, depending on the characteristics of cleaning beam  366 , the individual cleaning pulses of the beam may or may not introduce some inaccuracy into the measurements taken by measurement subsystem  330  by disturbing the test sample away from an analysis baseline condition. For example, if cleaning beam  366  comprises laser pulses for vaporizing AMC on the surface of test sample  310 , localized heating produced by those laser pulses may affect measurement accuracy. Also, the cleaning beam may cause an excess of charge carriers in and around analysis location  314  that can affect the metrology operation and produce erroneous results. Other cleaning effects may induce similar disturbances. According to an embodiment of the invention, because the recovery period from disturbances caused by such “cleaning effects” is typically much shorter than the time required for significant AMC regrowth, cleaning effects can simply be ignored; i.e., any measurement inaccuracy due to the cleaning beam will simply be accepted. 
   According to other embodiments of the invention, the cleaning effects can be compensated for in various ways, such as properly setting cleaning pulse timing, adjusting the measurement sampling characteristics, or selectively processing the raw data measurements. According to an embodiment of the invention, implementation of any of these compensation techniques can be simplified by setting the cleaning pulse rate (i.e., the number of cleaning pulses per unit time) of cleaning beam  366  equal to a submultiple of the sampling rate of measurement system  330 , in which case cleaning beam  366  would introduce a substantially constant cleaning effect at constant intervals. 
     FIG. 6   a  shows an example sampling rate graph for detector circuit  351  of measurement subsystem  330  shown in  FIG. 3   d . The data sampling begins at a time to, and has a sampling period Ps (i.e., the time between the start of one sampling pulse and the start of the next sampling pulse; equal to 1/sampling rate) and sample width Ws (i.e., the duration of a sampling pulse). Nine samples are shown, taken at times t 0 –t 8  (although any number of samples can be taken).  FIG. 6   b  shows an example cleaning pulse graph for cleaning beam  366  of cleaning subsystem  360  (shown in  FIG. 3   d ) that could be used in conjunction with the sampling rate profile shown in  FIG. 6   a . Prior to time to (i.e., prior to the start of data sampling), a quantity of cleaning pulses having an intensity IN′, widths Wc′ and a period Pc′ are applied to the test sample beginning at time t 0 ′ to perform the initial cleaning operation described with respect to  FIG. 3   b . Once data sampling has begun (i.e., after time T 0 ), cleaning pulses are applied having an intensity IN, widths Wc and period Pc. Note that cleaning pulse intensity IN, width Wc, and period Pc can be different from cleaning pulse intensity IN′, width Wc′ and period Pc′, respectively. For example, to reduce the time required for the initial cleaning operation, cleaning pulse width Wc′ can be set larger than cleaning pulse width Wc and/or period Pc′ can be set shorter than period Pc. 
     FIG. 6   c  shows a possible temperature profile for test sample  310  shown in  FIG. 3   d  when subjected to cleaning beam  366  having the cleaning pulse profile shown in  FIG. 6   b . Each cleaning pulse shown results in a corresponding temperature spike—i.e., cleaning pulses at times tc 1 , tc 2 , and tc 3  shown in  FIG. 6   b , produce corresponding temperature spikes at the same times in  FIG. 6   c . As indicated, the local temperature of the test sample rises from a steady-state temperature Tss to an elevated temperature Tel. Because the analysis location is generally a small portion of a much larger test sample, any heating from a cleaning pulse is rapidly dissipated, hence the narrow widths of the temperature spikes shown in  FIG. 3   c . For example, in a silicon wafer, the temperature spike from a 60 ns cleaning pulse from a 532 nm laser will have a width on the order of 1 μs. 
   According to an embodiment of the invention, by timing the cleaning pulses to fall between data samples, the temperature disturbances caused by the cleaning pulses have time to dissipate and therefore not affect the actual measurements. By setting the cleaning pulses to occur immediately after the completion of a data sample, the allowable recovery period (cooling time) for that cleaning pulse can be maximized. For example, time tc 1  of  FIG. 6   b  could be set equal to time t 2  of  FIG. 6   a  added to sample width Ws. Similarly, time tc 2  could be set equal to time t 5  added to sample width Ws and time tc 3  could be set equal to time t 8  added to sample width Ws. 
   Note that even if some portion of cleaning-induced temperature effects “bleeds” into some of the data samples, by averaging multiple samples to obtain final measurements, such transient heating effects will be “smoothed out”. This smoothing effect can be enhanced by increasing the sampling rate of the detection circuit (e.g., photodiode detection circuit  351  shown in  FIG. 3   d ). However, the higher the sampling rate of the detector, the more susceptible the detector becomes to high frequency noise. 
   To avoid the problem of high frequency noise, a metrology system in accordance with another embodiment of the invention includes a low-bandwidth measurement receiver coupled with a measurement emitter that includes a modulator for the measurement beam (e.g., acousto-optical modulator  342  shown in  FIG. 3   d ). Then, a high-frequency modulated measurement beam can be used to provide the desired high sampling rate, while the low bandwidth detector minimizes the problem of high-frequency noise. The modulator blocks the measurement beam during the cleaning pulse and any subsequent cooling period so that no information from the sample is received during cleaning (and cooling) operations. The modulator can also block the measurement beam between sampling pulses to avoid introducing artifacts at the cleaning laser pulse rate. Blocking the measurement laser between each sampling pulse would introduce a known, fixed artifact at the sampling rate that can be accounted for in downstream processing, thereby effectively making each inter-sampling interval identical, whether or not a cleaning pulse occurs. 
   According to another embodiment of the invention, the detection circuit includes a clamp circuit to blank out any information received from the sample during cleaning pulses (and any subsequent recovery-period).  FIG. 7  shows a photodiode detector circuit  700  in accordance with an embodiment of the invention. Photodiode detector circuit  700  includes a photodiode PD, an amplifier circuit  702 , and a clamp circuit  710 . Amplifier circuit  702  comprises an op-amp  701 , a resistor R, and a capacitor C. Photodiode PD is connected to the negative input terminal of op-amp  701 , and resistor R and capacitor C are connected in parallel across the negative and output terminals of op-amp  701  to control the time constant (and therefore the bandwidth) of the amplifier circuit. Clamp circuit  710  includes a sample/hold circuit  720 , a low-pass filter  730 , an analog-to-digital (A/D) converter  740 , and a control circuit  750 . Note that the bandwidth of amplifier circuit  702  should be greater than the sampling rate of A/D converter  740  to ensure that raw data Draw always provides a valid data signal to sample/hold circuit  720 . 
   Sample/hold circuit  720  is coupled to receive raw data Draw from op-amp  701  and a control signal CLAMP from control circuit  750 . During non-cleaning periods of operation, sample/hold circuit passes raw data Draw as processed data Dpro to low-pass filter  730 , which in turn filters out any high frequency noise and passes filtered data Dfil to A/D converter  740 , which samples filtered data Dfil to generate final output data Dout. However, during cleaning operations (and any subsequent recovery periods), control circuit  750  asserts control signal CLAMP, which places sample/hold circuit  720  into hold mode. This causes sample/hold circuit  720  to set processed data Dpro to the level of raw sample data Draw just prior to the start of the cleaning operation and hold processed data Dpro at that level until after the cleaning and recovery period has passed. 
   According to another embodiment of the invention, data processing software in optional-computer  359  shown in  FIG. 3   d  can be used to delete data samples taken during cleaning operations and any subsequent recovery periods. This data deletion will generally not be problematic since the data is typically highly oversampled. According to another embodiment of the invention, the data processing software could replace the deleted data samples with data equal to the last data sample taken before the start of the cleaning operation. 
   According to another embodiment of the invention, measurement subsystem  330  shown in  FIG. 3   b  comprises a spectroscopic ellipsometry (SE) measurement system, in which rotating waveplates  346  and  355  are not present, and polarizer  353  comprises a rotating analyzer. In an SE measurement system, the intensity of output beam  336  is low, so that any measurements of this beam must be integrated over many milliseconds by detector circuit  351  before being read out. At least eight such integrations (“readout cycles”) must be performed during one rotation of polarizer  353 . According to an embodiment of the invention, the cleaning pulses in cleaning beam  366  can be synchronized with the readout timing of detector circuit  351  so that there are a constant integer number of cleaning pulses per readout cycle. The transient perturbations caused by the cleaning pulses are then accepted and taken to be constant. According to another embodiment of the invention, measurement beam source  341  can be a broadband flashlamp or pulsed plasma source that fires at a predetermined rate, and the cleaning pulses of cleaning beam  366  are timed to fall between firings such that any disturbances such as temperature spikes are allowed to dissipate before the next measurement is taken. 
   The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. Thus, the invention is limited only by the following claims and their equivalents.