Abstract:
A system for analyzing a thin film uses an energy beam, such as a laser beam, to remove a portion of a contaminant layer formed on the thin film surface. This cleaning operation removes only enough of the contaminant layer to allow analysis of the underlying thin film, thereby enhancing analysis throughput while minimizing the chances of recontamination and/or damage to the thin film. An energy beam source can be readily incorporated into a conventional thin film analysis tool, thereby minimizing total analysis system footprint. Throughput can be maximized by focusing the probe beam (or probe structure) for the analysis operation at the same location as the energy beam so that repositioning is not required after the cleaning operation. Alternatively, the probe beam (structure) and the energy beam can be directed at different locations to reduce the chances of contamination of the analysis optics.

Description:
RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/056,271, entitled “Laser-Based Cleaning Device for Film Analysis Tool” filed Jan. 23, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to field of thin film measurement, and in particular to a method and apparatus for cleaning the surface of a thin film to improve measurement accuracy. 
   2. 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 measurements can be limited by contamination on the surface of the thin film. For example, the absorption of water and other vapors onto the thin film can create a contaminant layer that adversely affects thin film analysis technique 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 heat 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  10   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 the optical photons from heat lamps  130  effectively 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  150  coupled to stage  140  captures excess heat from heating element  141 , thereby minimizing 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 when it is 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 reduce the throughput. 
   Accordingly, it is desirable to provide an efficient wafer cleaning system for thin film measurement systems that does not require lengthy heating and cooling times and does not require dedicated wafer handling steps. 
   SUMMARY 
   The present invention provides localized contaminant layer removal from a thin film surface, thereby enabling accurate and repeatable analysis of the thin film by a measurement tool. By using a concentrated energy beam to clean only the portion of the thin film to be measured by the measurement tool, the thin film analysis can be performed without the long heating and cooling times associated with conventional cleaning systems. Furthermore, the compact components used in an energy beam-based cleaning system can be incorporated into the thin film measurement tool itself, thereby eliminating any delays related to transferring the wafer to and from a stand-alone cleaning system. This integration also minimizes the total footprint required for a thin film analysis system, and since the wafer can be cleaned and analyzed in the same process chamber, redeposition of contaminants on the cleaned portion of the wafer can be prevented. 
   A thin film analysis system in accordance with an embodiment of the present invention comprises an energy beam source, an analysis module, and a stage. The stage holds a test sample (such as a wafer) that includes a thin film layer to be measured by the analysis module. The analysis module can comprise any thin film analysis system or systems, including a single-wavelength ellipsometer (SWE, such as described in co-owned, co-pending U.S. patent application Ser. No. 09/298,007), a spectroscopic ellipsometer (SE, such as described in co-owned U.S. Pat. No. 5,608,526), a reflectometer (such as described in co-owned U.S. Pat. No. 5,747,813), a non-contact electrical measurement system (such as described in co-owned U.S. Pat. No. 5,485,091), a GXR system (such as described in co-owned, co-pending U.S. patent application Ser. No. 10/005,610), a contact-based electrical measurement system, an XRF system, and/or an EMP system. More generally, this cleaning system can be used with any sort of inspection or metrology system used in the production of semiconductor devices. According to an embodiment of the present invention, the energy beam source is incorporated into a conventional thin film analysis tool, thereby minimizing the total footprint of the thin film analysis system. 
   The energy beam source is configured to direct an energy beam at a contaminant layer on the surface of the thin film layer. The energy beam heats a portion of the contaminant layer until that portion of the contaminant layer is vaporized. This process can be aided by direct photon excitation of the bonds between the contaminant layer and the thin film layer. The area of the thin film layer exposed by this cleaning operation can then be analyzed by the analysis module. The size of this analysis area required by the analysis module for performance of the thin film analysis can be used to determine the minimum required power and size of the energy beam. By minimizing the power and size of the energy beam, the risk of damage to the test sample is small. This risk of damage can be further reduced by performing the cleaning and measuring operations at non-functional regions of the test sample. According to an embodiment of the present invention, the energy beam source can comprise a laser, such as a Q-switched pulsed laser. According to another embodiment of the present invention, the energy beam source can comprise a flashlamp with appropriate focusing optics. 
   According to an embodiment of the present invention, a probe beam generated by the analysis module (e.g., a low-power laser beam, a white light beam, a corona discharge, an x-ray beam, etc.) is directed at the same location on the test sample as the energy beam produced by the energy beam source. Alternatively, a physical probe structure (e.g., a four-point probe in a spreading resistance tool) (can be aimed at the same location on the test sample as the energy beam produced by the energy beam source. Consequently, the test sample does not need to be moved between the cleaning and measurement operations, thereby maximizing analysis throughput. Furthermore, because the measurement operation can be performed immediately after the cleaning operation, the chances of the cleaned portion of the thin film layer (i.e., the analysis area) being recontaminated before the measurement operation are minimized. 
   According to another embodiment of the present invention, the probe beam (or probe structure) from the analysis module and the energy beam are directed at different locations on the test sample. The test sample (and/or the analysis module) is then repositioned after the cleaning operation to align the probe beam (or probe structure) with the analysis area of the thin film layer. This allows the focusing optics or probe structure of the analysis module to be kept out of the vicinity of the portion of the contaminant layer being vaporized, thereby minimizing the risk of any contaminant redeposition on the measurement focusing optics or probe structure. 
   The present invention will be more fully understood in view of the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  and  1   b  show conventional wafer cleaning systems. 
       FIGS. 2   a  and  2   b  show a thin film analysis system in accordance with an embodiment of the present invention. 
       FIGS. 3   a  and  3   b  show a thin film analysis system in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2   a  shows a thin film analysis system  200  in accordance with an embodiment of the present invention. Analysis system  200  comprises a stage  220 , an energy beam source  230 , and an analysis module  240 . Stage  220  holds a test sample  210  that comprises a thin film layer  212  formed on a substrate  211 . Substrate  211  can comprise any structure on which thin film layer  212  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  210  also includes a contaminant layer  213  formed on the surface of thin film layer  212 . Contaminant layer  213  can comprise any unwanted material on the surface of thin film layer  212 . 
   An analysis operation performed using analysis system  200  actually comprises two steps—a cleaning operation and a measurement operation. During the cleaning operation, a small portion of contaminant layer  213  is removed. The exposed portion (i.e., analysis area) of thin film layer  212  is then analyzed during the measurement operation. According to an embodiment of the present invention, the position of stage  220  can be shifted relative to energy beam source  230  and analysis module  240  to enable thin film analysis at multiple locations on test sample  210 . According to an embodiment of the present invention, stage  220  can include a positioning mechanism  221  to enable this positional shifting. 
   To perform a cleaning operation, energy beam source  230  directs an energy beam  231  at a spot  214   a  on contaminant layer  213 . Energy beam  231  is configured to remove a portion of contaminant layer  213  by heating contaminant layer  213  directly or by heating the underlying portion of thin film layer  212  or substrate  211 . The portion of contaminant layer  213  heated in this manner is eventually vaporized, thereby exposing the underlying portion of thin film layer  212 . As noted previously, this removal process can be aided by other mechanisms besides heating, including the direct stimulation of the bonds between contaminant layer  213  and thin film layer  212  by photons from energy beam  231 . 
   Because the heating from energy beam source  230  is confined to a localized area, the cleaning operation can be performed very rapidly, which minimizes any impact on analysis throughput. The potential for damage to the underlying thin film layer  212  and/or substrate  211  is minimal because only a small portion of test sample  210  is heated. This risk of damage can be further reduced by performing the cleaning operation on non-functional regions of test sample  210  (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  210 ). 
   The amount of contaminant layer  213  to be removed depends on the measurement requirements of analysis module  240 . Modern thin film analysis tools generally require an analysis area of at least 20 μcm×20 μcm. Therefore, at least a 20 μcm×20 μcm portion of contaminant layer  213  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  213  could be removed. 
   According to an embodiment of the present invention, energy beam source  230  could comprise a pulsed laser. For example, contaminant layer  213  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 1–1000 ns pulse duration could then heat the desired portion of contaminant layer  213  to between roughly 300° C. to 1000° C., which is a temperature range sufficient to vaporize that portion of contaminant layer  213 . According to another embodiment of the present invention, energy beam source  230  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 present 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 present 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 a pulse. According to another embodiment of the present invention, energy beam source  230  could include focusing optics such as an optical fiber  232  (shown using dotted lines) and a lens system to deliver a beam of the desired size and energy to spot  214   a  from a remote location, i.e., the optional optical fiber  232  could transmit energy beam  231  from a remote beam generator to spot  214   a . According to another embodiment of the present invention, energy beam source  230  could comprise a flashlamp coupled to focusing optics to direct the high intensity light to the desired area on contaminant layer  213 . 
   Once the cleaning operation is completed, the measurement operation can be performed. Because test sample  210  does not need to be transferred to a different tool or process chamber, the measurement operation can be performed immediately following the cleaning operation, so that the chances of recontamination of the exposed portion (analysis area) of thin film layer  212  are minimized. For explanatory purposes,  FIG. 2   b  depicts analysis module  240  as including a xenon lamp  241 , a rotating polarizer  242 , a focusing mirror  243 , a fixed polarizer  244 , a spectrometer  245 , and a CCD detector  246  for performing a spectroscopic ellipsometry analysis. However, analysis module  240  can comprise a system or systems for any type of analysis that would benefit from removal of contaminant layer  213 , including SWE, SE, reflectometry (optical or x-ray), GXR, XRF, EMP, and non-contact or contact-based electrical analysis, among others. Note that analysis system  200  can comprise a conventional thin film analysis system to which energy beam source  230  is added, thereby minimizing the footprint of analysis system  200 . 
   As indicated in  FIG. 2   b , contaminant layer  213  includes an opening  214   b  formed by the laser heating of spot  214   a  during the preceding cleaning operation (as shown in  FIG. 2   a ). The measurement operation therefore can be performed directly on thin film layer  212  through opening  214   b . Xenon lamp  241  directs a diverging light beam  247  through rotating polarizer  242  at focusing mirror  243 , which reflects and focuses beam  247  through opening  214   b  in contaminant layer  213  onto an analysis area  215  on thin film layer  212 . Light beam  247  is reflected by thin film layer  212  as a diverging beam, which passes through fixed polarizer  244  and spectrometer  245  before being measured by CCD detector  246  to determine the thickness of thin film layer  212 . 
   In this manner, a localized cleaning operation can be efficiently combined with a measurement operation to ensure accurate and repeatable thin film analyses. Because both energy beam  231  and the probe beam from analysis module  240  (here represented by light beam  247 ) are simultaneously directed at substantially the same location on test sample  210 , the position of test sample  210  does not have to be adjusted between cleaning and measurement operations. Therefore, the measurement operation can be performed immediately after the cleaning operation to ensure that a new contaminant layer is not reformed over analysis area  215 . 
     FIG. 3   a  shows a thin film analysis system  300  in accordance with an embodiment of the present invention. Analysis system  300  comprises a stage  320 , an energy beam source  330 , and an analysis module  340 . Stage  320  includes a positioning mechanism  321  and a platform  322 . Positioning mechanism  321  allows the position of platform  322  to be shifted relative to energy beam source  330  and analysis module  340 . Platform  322  holds a test sample  310  that comprises a thin film layer  312  formed on a substrate  311 . Substrate  311  can comprise any material on which thin film layer  312  can be formed, including a single material (such as a silicon wafer) or a plurality of materials (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 on the surface of thin film layer  312 . 
   Unlike in analysis system  200  shown in  FIGS. 2   a  and  2   b , energy beam source  330  and analysis module  340  are not simultaneously focused at the same location on test sample  310 . Consequently, an analysis operation performed using analysis system  300  actually comprises three steps—a cleaning operation, a positioning operation, and a measurement operation. During the cleaning operation, a small portion of contaminant layer  313  is removed by the energy beam from energy beam source  330 . Then during the positioning operation, test sample  310  is positioned such that the probe beam of analysis module  340  is aligned with the portion of thin film layer  312  exposed during the cleaning operation. The exposed portion of thin film layer  312  can then be analyzed by analysis module  340  during the measurement operation. 
   To perform a cleaning operation, energy beam source  330  directs an energy beam  331  at a point  314   a  on contaminant layer  313 . Energy beam  331  is configured to remove a portion of contaminant layer  313  by heating contaminant layer  313  directly or by heating the underlying portion of thin film layer  312  or substrate  311 . The portion of contaminant layer  313  heated in this manner is vaporized, thereby exposing the underlying portion of thin film layer  312 . 
   Because the heating from energy beam source  330  is confined to a localized area, the cleaning operation can be performed very rapidly, which minimizes any impact on analysis throughput. The potential for damage to the underlying thin film layer  312  and/or substrate  311  is minimal because only a small portion of test sample  310  is heated. This risk of damage can be further reduced by performing the cleaning operation on non-functional regions of test sample  310 . 
   The amount of contaminant layer  313  to be removed depends on the measurement requirements of analysis module  340 . As described previously, modern thin film analysis tools generally take measurements within a roughly 20 μcm×20 μcm spot. Accordingly, energy beam source  330  could comprise a 5–100 μJoule pulsed laser with a pulse duration of 1–100 ns, which would be capable of vaporizing a 20 μcm×20 μm (or slightly larger) portion of a 5 angstrom thick contaminant layer (contaminant layer  313 ) of adsorbed water vapor. According to an embodiment of the present invention, energy beam source  330  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 present invention, energy beam source  330  could include focusing optics such as an optical fiber and a lens system to deliver a beam of the desired size and energy to spot  314   a  from a remote location, i.e., the optical fiber could transmit energy beam  331  from a remote beam generator to spot  314   a . According to another embodiment of the present invention, energy beam source  330  could comprise a flashlamp coupled to focusing optics to direct the high intensity light to the desired area on contaminant layer  213 . 
   As indicated in  FIG. 3   b , the cleaning operation creates an opening  314   b  in contaminant layer  313  (at spot  314   a  shown in  FIG. 3   a ), thereby exposing an analysis area  315  of thin film layer  312 . A positioning operation then aligns analysis area  315  with the probe beam from analysis module  340 , in this case an electron beam (e-beam)  346 . This positioning operation is performed by positioning mechanism  321 , which shifts platform  322  relative to analysis module  340  (as indicated by the phantom lines). While a lateral shift is indicated in  FIG. 3   b , any other type of positioning motion could be used, including a rotational or vertical shift. In this manner, the probe beam focusing optics in analysis module  340  can be maintained at a distance from the portion of contaminant layer  313  being removed during the cleaning operation (point  314   a  shown in  FIG. 3   a ). This in turn minimizes the risk of any of vaporized contaminant layer  313  redepositing on the probe beam focusing optics or probe structure of analysis module  340 . 
   After the positioning operation is completed, the measurement operation can be performed by analysis module  340 . Because test sample  310  does not have to be transferred to a different tool or process chamber, there is little chance of recontamination of analysis area  315 . For explanatory purposes,  FIG. 3   b  depicts analysis module  340  as including a corona discharge gun  344 , a charge mask  345 , and a vibrating probe  346  for performing a non-contact electrical analysis, as described in co-owned U.S. Pat. No. 5,485,091. However, analysis module  340  can comprise a system or systems for any type of analysis that would benefit from removal of contaminant layer  313 , including SWE, SE, reflectometry, GXR, XRF, EMP, and non-contact or contact-based electrical analysis, among others. Note that analysis system  300  can comprise a conventional thin film analysis system to which energy beam source  330  is added, thereby minimizing the footprint of analysis system  300 . 
   As indicated in  FIG. 3   b , the measurement operation is performed through opening  314   b  formed in contaminant layer  313  during the preceding cleaning operation. Corona discharge gun  344  produces a corona discharge  347  that is shaped into a negative charge beam  348  by charge mask  345 . Negative charge beam  348  deposits a negative charge onto analysis area  315  through opening  314   b  in contaminant layer  313 . The resulting change in surface potential can then be measured by vibrating probe  346  to determine the thickness and electrical properties of thin film layer  312 . 
   In this manner, a localized cleaning operation can be efficiently combined with a measurement operation to ensure accurate and repeatable thin film analyses. By allowing the position of test sample  310  to be shifted between the cleaning and measurement operations, energy beam  331  and the probe beam of analysis module  340  (here represented by negative charge beam  348 ) do not need to be focused at the same location on test sample  310 . Therefore, the focusing optics of analysis module  340  can be distanced from any contamination released during the cleaning operation. 
   Although the present invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to one of ordinary skill in the art. Thus, the invention is limited only by the following claims.