Abstract:
Apparatus and methods for measuring characteristics of a metallic target as well as other interior surfaces of a sputtering chamber. The apparatus includes a sensor configured to emit an energy beam toward a surface of interest and to detect an energy beam therefrom, the detected energy beam being indicative of parameters of a characteristic of interest of the surface of interest. Quantitative and qualitative characteristics of interest may be determined. A sputtering system including the apparatus and operable according to the methods of the invention is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/352,699, entitled “Device for Measuring the Profile of a Metal Film Sputter Deposition Target, and System and Method Employing Same,” filed Jan. 27, 2003, now U.S. Pat. No. 6,811,657 issued on Nov. 2, 2004, the disclosure of which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to sputter deposition of materials on substrate surfaces. More specifically, the present invention relates to methods and apparatus for measuring characteristics of a sputtering target and other surfaces within a sputtering vacuum chamber. 
     2. State of the Art 
     A thin film of metallic material may be deposited on a substrate using a sputter deposition process wherein a metallic target is attacked with ions, causing atoms or small particles of the target to be ejected from the target and deposited on the substrate surface.  FIG. 1  illustrates a cross-sectional schematic of a conventional sputtering apparatus  10  comprising a vacuum chamber  12  having inner chamber walls  13 , a gas inlet  14  and a gas outlet  16 . The vacuum chamber  12  may further include a window  15  comprising a material that is transparent to predetermined wavelengths of electromagnetic radiation. The sputtering apparatus  10  further comprises a substrate support pedestal  24  and a metallic target  22  attached to a sputtering cathode assembly  18 , each located within the vacuum chamber  12 . The pedestal  24  may be configured to secure a substrate  26  thereto with a biasable electrostatic chuck, a vacuum chuck, a clamping structure, or a combination of methods. The substrate  26  may be transported to and from the pedestal  24  manually or with a robotic arm or blade (not shown). 
     During the sputtering process, the vacuum chamber  12  is filled with an inert gas, such as argon, through the gas inlet  14  and then reduced to a near vacuum through the gas outlet  16 . The target  22  is negatively charged to cause electrons to be emitted from an exposed surface  23  of the target  22  and move toward an anode (not shown). A portion of the moving electrons strike atoms of the inert gas, causing the atoms to become positively ionized and move towards the negatively charged target  22 . The electrons, inert gas atoms, and ions form a plasma which is typically intensified and confined over the target surface  23  by a magnetic field generated by a magnet assembly  20  located proximate the target  22 . The magnet assembly  20  may comprise one or more permanent magnets or electromagnets located behind and/or to the side of the target  22 . A portion of the ions discharging from the plasma strikes the target surface  23  at a high velocity, causing atoms or small particles of the target  22  material to be ejected from the target surface  23 . The ejected atoms or small particles then travel through the vacuum chamber  12  until they strike a surface, such as the surface of the substrate  26 , forming a thin metallic film thereon. 
     Residue deposits comprising the ejected atoms or small particles and byproducts are also deposited on the inner chamber walls  13  and other surfaces within the sealed vacuum chamber  12  during the deposition process. The accumulation of the residue deposits on the inner chamber walls  13  may be a source of contamination as a plurality of substrates  26  is successively processed in the vacuum chamber  12 . Thus, the vacuum chamber  12  must be opened to atmosphere and cleaned after a predetermined amount of operation time has elapsed under vacuum or when contamination is detected on a substrate  26  that has undergone the deposition process. Opening and cleaning the vacuum chamber  12  is costly and time consuming. Therefore, it would be advantageous to clean the vacuum chamber  12  only when a predetermined amount of residue deposits have accumulated on the inner chamber walls  13  and other surfaces within the vacuum chamber  12 . 
     The magnetic field formed over the target surface  23  by the magnet assembly  20  confines the electrons emitted from the target  22  to an area near the target surface  23 . This greatly increases the electron density and the likelihood of collisions between the electrons and the atoms of the inert gas in the space near the target surface  23 . Therefore, there is a higher rate of ion production in plasma regions near the target surface  23  where the magnetic field intensity is stronger. Varying rates of ion production in different plasma regions causes the target surface  23  to erode unevenly. Typically, the configuration of the magnet assembly  20  produces a radial variation of thick and thin areas, or grooves, within a diameter of the target surface  23 .  FIG. 2  illustrates a cross-sectional perspective view of a typical erosion profile of a cylindrical metallic target  22 , such as the metallic target  22  shown in  FIG. 1 , which has been used in a sputtering process.  FIG. 2  illustrates a target surface  23  before erosion has occurred as well as an eroded target surface  32  that has eroded unevenly across the length of a diameter of the target  22 . Due to the geometry of a magnetic field surrounding the target  22 , the target surface  32  has eroded nearly symmetrically about a center line  30  dividing the length of the diameter. 
     Referring now to  FIGS. 1 and 2 , the target  22  may comprise a rare metal, such as gold, platinum, palladium or silver, or may comprise, for example, aluminum, titanium, tungsten or any other target material conventionally employed in the semiconductor industry. Therefore, it is advantageous to consume as much of the target  22  material during sputter deposition processes as possible before replacing an eroded target  22 . Further, replacing an eroded target  22  before the end of its useful life may be a difficult and time-consuming task. However, it is important to replace the target  22  before a groove “punches through” the target  22  material and exposes portions of the cathode assembly  18  to erosion, causing damage to the cathode assembly  18  and contaminating the sputtering apparatus  10 . For example, the target  22  material in the area of grooves  28  shown in  FIG. 2  may erode before the remainder of the target  22  material and expose the cathode assembly  18  to ionic bombardment from the surrounding plasma. 
     It may also be advantageous to replace or condition the sputtering target  22  when certain characteristics of the target surface  23  become degraded during the sputtering process. For example, the smoothness of the target surface  23  may degrade over time. The roughened target surface  23  may affect the consistency of the deposition formation on the substrate  26  and may also be an indication of the amount of target  22  consumption. Therefore, it may be advantageous to replace the target  22  when the target surface  23  reaches a predetermined roughness level. 
     As another example of degraded target surface  23  characteristics, certain targets  22 , such as targets  22  comprising Ag 2 Se (hereinafter “silver selenide”), may exhibit hair-like growths or asperities (not shown) during the sputtering process. A portion of the asperities may be ejected from the target surface  23  during the plasma ion bombardment and land on substrate  26 , forming defects therein. Typically, by the time the asperities have grown on the target surface  23  so as to create noticeable defects on the substrate  26 , the target  22  is no longer useful and must be replaced. Therefore, to avoid forming defects on the substrate  26  and to prolong the useful life of the target  22 , it may be advantageous to detect the asperities while the vacuum chamber  12  is under vacuum. 
     The useful life of a metallic sputtering target  22  is typically estimated by determining the cumulative deposition time for the target  22 . A deposition time is chosen in an attempt to guarantee that the target  22  material will never be completely removed at any given location and may take into account the thickness of the target  22 , the material used for the target  22 , and the effect of intensifying and confining the plasma over the target surface  23  by a magnetic field generated by the magnet assembly  20  in a predetermined configuration. However, if the characteristics of the plasma distribution change due, for example, to reconfiguring the magnet assembly  20  to produce a magnetic field with a different geometry, the erosion of the target surface  23  may be changed and could result in localized enhanced metal removal and the possible punching through of target  22  to the cathode assembly  18  before the expiration of the estimated deposition time. 
     Directly measuring the characteristics of the target surface  23  or the vacuum chamber  12  is difficult and time consuming. Opening the vacuum chamber  12  to inspect the target surface  23  or inner chamber walls  13  requires several hours of idle time while the vacuum chamber  12  is baked out under post-vacuum inspection. Accurate measurement of the target surface  23  while the sputtering apparatus  10  is under vacuum is difficult because the gap distance d between the target  22  and the pedestal  24  may be as small as 25 millimeters. Typical measurement devices are too large to be inserted into the gap between the target  22  and the pedestal  24  to profile the target surface  23  while the vacuum chamber  12  is under vacuum. Further, measurement devices placed near the target  22  during a sputtering process may be damaged by exposure to metal deposition. 
     In view of the above-noted shortcomings in the art, it would be advantageous to prevent contamination from residue deposits on the inner chamber walls  13  and other surfaces and to prevent premature replacement, over-consumption or degradation of the target  22  by providing a technique and device to measure the inner chamber walls  13  and the target surface  23  while the vacuum chamber  12  is under vacuum. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention, in a number of embodiments, relates to methods and apparatus for measuring the characteristics of a metallic sputtering target and other surfaces within a sputtering chamber. 
     An apparatus according to one embodiment of the present invention may comprise a sensor configured to emit a first energy beam toward a target surface and to detect a second energy beam emitted from the target surface. The sensor may be coupled to a thin profile arm configured to move or transport the sensor over the target surface between the target and a substrate support pedestal to a plurality of measurement locations. The arm may be configured to attach to a robotic device. The sensor and the arm are configured, positioned and sized to be inserted into a narrow gap existing between the target surface and the pedestal. The arm may also be configured to remove the sensor from the gap and to shield the sensor during a sputtering process. 
     In another embodiment of the present invention, the sensor may comprise a source element configured to emit a collimated light beam and at least one detector. According to one aspect of the invention, the at least one detector is arranged as a linear array of detection elements and the source element is positioned so as to emit the collimated light beam at an acute angle with respect to the linear array. The linear array is positioned relative to the source element so as to be illuminated by a reflection of the collimated light beam. The distance from the sensor to the target surface or the percentage of target erosion may be calculated by determining the location in the array of the detection element or elements illuminated by the reflection of the collimated light beam. According to another aspect of the invention, the at least one detector may be configured, positioned and sized to collect a coherent reflection of the collimated light beam and a substantial portion of scattered light beams from the target surface. The roughness of the target surface may be calculated by comparing the coherent reflection and scattered light beams. According to a further aspect of the invention, the sensor may comprise a source configured to emit an energy beam substantially parallel to the target surface toward the at least one detector. The presence of asperities on the target surface may be detected by analyzing the energy beam after passing proximate to the target surface. 
     An apparatus according to yet another embodiment of the present invention may comprise a sensor configured to emit a first energy beam toward a surface in a chamber and to detect a second energy beam emitted from the surface to analyze residue deposits thereon. The sensor may be coupled to a thin profile arm configured to move or transport the sensor proximate to the surface. Alternatively, the sensor may be positioned outside the chamber and configured to emit the energy beam through a window in the chamber. The sensor may be configured to perform a spectral analysis on the second energy beam. 
     In yet another embodiment of the present invention, a sensor may comprise a transmitter optically coupled to a source collimator configured to collimate a light beam as it exits an optical fiber. The sensor may further comprise a receiver optically coupled to one or more collection collimators, each collection collimator being configured to collect a light beam incident thereon into a corresponding optical fiber. 
     The present invention, in additional embodiments, also encompasses a sputter deposition system incorporating the sensors of the present invention and methods of measuring surface characteristics. 
     One method according to the present invention comprises emitting an energy beam, illuminating a first location on a target surface, detecting a reflection of the energy beam from the first location, and analyzing the detected reflection of the energy beam to determine a distance from the point of emission to the first location. Another method according to the present invention comprises detecting a coherently reflected portion of an energy beam from a target surface, detecting a scattered portion of the energy beam, and relating the coherently reflected portion and the scattered portion to a surface roughness. Yet another method according to the present invention comprises emitting an energy beam substantially parallel to a target surface, measuring a change to the energy beam, and relating the change to a presence of asperities on the target surface. A further method according to the present invention comprises performing a spectral analysis on an energy beam received from a surface. 
     Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, which illustrate what are currently considered to be best modes for carrying out the invention: 
         FIG. 1  is a cross-sectional side view schematic of a sputtering apparatus; 
         FIG. 2  is a cross-sectional perspective side view of an erosion profile of a cylindrical, metallic target; 
         FIGS. 3A–3C  are cross-sectional side view schematics according to the present invention of a portion of a sputtering apparatus comprising a sensor configured, sized and positioned to be inserted between a target surface and a pedestal or near a vacuum chamber wall; 
         FIG. 4  is a top view schematic of a sensor configured to measure the erosion of a sputtering target surface according to one embodiment of the present invention; 
         FIG. 5  is a side view schematic of the sensor of  FIG. 4  and a portion of a sputtering apparatus; 
         FIG. 6  is a top view schematic of a sensor comprising a transceiver and detectors, the sensor configured to the roughness of a sputtering target surface according to another embodiment of the present invention; 
         FIG. 7  is a side view schematic of the transceiver of  FIG. 6  and a roughened target surface; 
         FIG. 8  is a partial side view schematic of the sensor of  FIG. 6  and the roughened target surface shown in  FIG. 7 ; 
         FIG. 9  is a top view schematic of a sensor configured to detect asperities on a sputtering target surface according to yet another embodiment of the present invention; 
         FIG. 10  is a side view schematic of the sensor of  FIG. 9  and a portion of a target surface having asperities; 
         FIG. 11  is a block diagram of a sputter deposition system comprising a sensor assembly according to one embodiment of the present invention; 
         FIGS. 12A–12C  are block diagrams of sensor assemblies according to one embodiment of the present invention; and 
         FIG. 13  is a block diagram of a receiver suitable for use in the sensor assembly of  FIG. 12C . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 3A–3C  each illustrate a cross-sectional schematic according to the present invention of a portion of a sputtering apparatus, such as the sputtering apparatus  10  shown in  FIG. 1 , wherein a sensor  50  is positioned relative to a surface of an inner chamber wall  13 , a surface of pedestal  24  or surface  23  of target  22  to be analyzed. As shown in  FIG. 3A , a sensor  50  coupled to a thin profile arm  44  is configured and sized to be inserted into a gap between a target  22  and a pedestal  24 . The arm  44  may be configured to detachably attach to a chamber robot  40  configured to translate the sensor  50  over the target surface  23 , or at least a portion thereof. The chamber robot  40  may further be configured to protect the sensor  50  during the sputtering process by removing the sensor  50  from the sputtering area or by shielding the sensor  50 . The arm  44  may be interconnected to the chamber robot  40  through an articulating arm  42  configured to provide movement in at least one plane. In another embodiment of the present invention, the sensor  50  may detachably attach to a substrate pickup arm (not shown) connected to the chamber robot  40  and configured to transport a substrate (not shown) to and from the pedestal  24  using a pickup device (not shown), such as a clamp, vacuum chuck or electrostatic chuck, to attach the substrate thereto. In yet another embodiment, the sensor  50  may be configured to attach directly to the pickup device. 
     As shown in  FIG. 3A , the sensor  50  is sized, positioned and configured to measure the characteristics of the target surface  23  by transmitting a signal  46  toward the target  22  and receiving a reflected or emitted signal  48  from the target surface  23 . The transmitted signal  46  may be an energy beam selected from the group comprising a visible light beam, an ultraviolet light beam, an infrared (hereinafter “IR”) light beam, a radio frequency (hereinafter “RF”) beam, a microwave beam and an ultrasound beam. To profile the target surface  23 , the chamber robot  40  may be configured to position the sensor  50  at a plurality of locations relative to the target surface  23 . Further, the sensor  50  may be configured, such as by using a multiplexor, to scan a portion (as opposed to a single point) on the target surface  23  while positioned at one location relative to the target surface  23 . 
     As shown in  FIG. 3B , the sensor  50  may be sized, positioned and configured to measure the characteristics of the pedestal  24  by transmitting the signal  46  toward the pedestal  24  and receiving the reflected or emitted signal  48  from the pedestal  24 . Alternatively, although not shown in  FIG. 3B , the sensor  50  may be positioned and configured to measure the characteristics of the substrate  26  shown in  FIG. 1  or deposits thereon. For example the sensor  50  may be configured to detect deposition defects on the substrate  26  or to detect when the deposition process is complete. 
     As shown in  FIG. 3C , the sensor  50  is sized, positioned and configured to measure the characteristics of an inner chamber wall  13  by transmitting the signal  46  toward the inner chamber wall  13  and receiving the reflected or emitted signal  48  from the inner chamber wall  13 . Similarly, the sensor  50  may be sized, positioned and configured to measure the characteristics of any surface in the vacuum chamber  12 . Alternatively, although not shown in  FIGS. 3A–3C , the sensor  50  may be positioned outside the vacuum chamber  12  and configured to pass the transmitted signal  46  through the window  15  shown in  FIG. 1  such that the transmitted signal  46  may reflect off one or more surfaces within the vacuum chamber  12  and exit the vacuum chamber  12  as reflected or emitted signal  48  through the same window  15 , or a different window (not shown). 
     The surface characteristics measured by the sensor  50  shown in  FIGS. 3A–3C  may be obtained, for example, through spectroscopy techniques utilizing the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules on the surface being analyzed to qualitatively or quantitatively study the atoms or molecules, or to analyze physical processes occurring on the surface. Referring to  FIG. 3C , spectroscopy may be used to measure the amount and composition of residue deposits on the inner chamber wall  13 . In one embodiment of the present invention, the signal  46  transmitted toward the inner chamber wall  13  is an IR light beam and the absorption spectrum of the residue deposits on the inner chamber wall  13  is measured using IR absorption spectroscopy. IR absorption spectroscopy is the measurement of the wavelength and intensity of the absorption of the IR light by the inner chamber wall  13  and the residue deposits thereon. As discussed in relation to  FIG. 12C  below, Fourier-transform infrared (hereinafter “FTIR”) spectroscopy may be used, for example, to measure the absorption spectrum using Fourier-transform techniques and a Michelson interferometer. 
     In another embodiment of the present invention, Raman spectroscopy is used to measure the amount and composition of residue deposits on the inner chamber wall  13 . When the transmitted signal  46  illuminates the surface of the inner chamber wall  13 , a portion of the transmitted signal  46  is scattered in various directions. Light scattered due to vibrations in molecules or optical phonons in solids is Raman scattered light. When the transmitted signal  46  strikes the inner chamber wall  13  or the residue deposits thereon, the light is scattered elastically (i.e., Rayleigh scattering) and inelastically (i.e., Raman scattering), generating Stokes and anti-Stokes lines. In the present embodiment, the reflected or emitted signal  48  represents a Raman scattered beam. Raman spectroscopy is the measurement of the wavelength and intensity of the inelastically scattered light of reflected or emitted signal  48  from the inner chamber wall  13  or the residue deposits thereon. The Raman scattered light of reflected or emitted signal  48  occurs at wavelengths that are shifted from the transmitted signal  46  by the energies of molecular vibrations. Raman spectroscopy may provide structure determination, multicomponent qualitative analysis, and quantitative analysis of the residue deposits on the inner chamber wall  13 . The mechanism of Raman scattering is different from that of IR absorption. Therefore, Raman spectroscopy and IR absorption spectroscopy may each be used to provide complementary information about the residue deposits on the inner chamber wall  13 . 
     Returning to  FIG. 3A , to determine the amount of erosion at any location on the target surface  23 , the reflected or emitted signal  48  may be analyzed to determine a relative distance between the sensor  50  and the target surface  23 . It may not be necessary to measure the relative distance between the sensor  50  and the target surface  23  at every point on the target surface  23 . Due to the radial symmetry of the erosion of the target surface  23 , it is only necessary to determine the relative distance between the sensor  50  and the target surface  23  at points located linearly between the center line  30  of the target surface  23  and an outside edge  25  of the target surface  23 , as shown in  FIG. 2 . Thus, measuring the relative distance between the sensor  50  and the target surface  23  approximately every ten millimeters linearly between the center line  30  and an outside edge  25  may provide sufficient resolution to prevent punching through a target  22  having a diameter of approximately thirty centimeters. 
     In one embodiment of the present invention, the relative distance between the sensor  50  and the target surface  23  is measured by measuring the time delay between the emission of the transmitted signal  46  and detection of the reflected or emitted signal  48 , multiplying the measured time delay by the speed of the transmitted signal  46  and dividing by two. In another embodiment, the distance between the sensor  50  and the target surface  23  may be determined by indirectly establishing the time delay by measuring a phase difference between the transmitted signal  46  and the reflected or emitted signal  48 . In a phase measurement sensor  50 , the transmitted signal  46  may comprise a modulated signal. In yet another embodiment, the transmitted signal  46  may be a pulsed signal and the reflected or emitted pulse signal  48  may be detected only during a predetermined time window such that increased time delay between transmission and detection causes less of the pulse to be detected. Thus, the detected power level of the reflected or emitted pulse signal  48  is inversely proportional to the distance traveled. Other embodiments for measuring the distance between the sensor  50  and the target surface  23 , as presently known in the art, may also be employed. 
       FIG. 4  illustrates a top view schematic of a sensor  52  according to one embodiment of the present invention. The sensor  52  is attached to a thin profile arm  44 , such as the arm  44  shown in  FIG. 3A . Sensor  52  comprises a source element  54  and a detector array  55 . The source element  54  has a thin profile so as to fit between the target  22  and the pedestal  24 , as shown in  FIG. 3A . The source element  54  is configured to generate a collimated light beam. By way of example only, and not by limitation, the source element  54  may comprise a laser diode. Alternatively, the source element  54  may comprise a collimator, such as a lens, configured to collimate or focus light exiting an optical fiber to a desired beam diameter or spot size. As will be seen below, the collimated light emitted from the source element  54  minimizes extraneous reflections and enhances signal detection. Use of a collimated light beam as an energy beam is currently preferred, although the invention is not so limited. 
     The detector array  55  comprises a plurality of detectors or detector elements  56  (ten shown) disposed side by side in a linear array, each detector  56  having a thin profile so as to fit between the target  22  and the pedestal  24 , as shown in  FIG. 3A . Each detector  56  in the detector array  55  is configured to produce an electronic sensory signal related to the magnitude of the radiation received thereon. By way of example only, and not by limitation, each detector may comprise a photodiode or a charge coupled device (hereinafter “CCD”). Alternatively, each detector  56  in the detector array  55  may comprise a collimator, such as a lens, configured to collect light into an optical fiber. 
       FIG. 5  illustrates a side view schematic of the sensor  52  and arm  44  shown in  FIG. 4 . As shown in  FIG. 5 , the source element  54  is positioned so as to emit a transmitted beam  60  at a predetermined transmission angle α in relation to the arm  44 . Although not shown, it may also be advantageous to position each detector  56  of the detector array  55  at an angle in relation to the arm  44  so as to align with a corresponding reflected beam, such as reflected beams  62 ,  66  and  72 . 
       FIG. 5  also illustrates the sensor  52  positioned in relation to a portion of a target  22 , such as the target  22  shown in  FIG. 2 . The number of detectors  56  in the detector array  55  and the position of each detector  56  relative to the source element  54  are dependent upon the distance between the sensor  52  and the target  22 . For illustration purposes, three surfaces  23 ,  32 ,  70  are referenced in  FIG. 5  corresponding to different target  22  erosion states. The first target surface  23  corresponds to a new or unused target  22  that has not yet been exposed to a sputtering process. The transmitted beam  60  illuminates the new target surface  23  and reflects back toward the detector array  55  as reflected beam  62 . To configure the dimensions of the detector array  55 , the vertical distance z between the new target surface  23  and the sensor  52  may be predetermined. Thus, assuming the incident angle β of the transmitted beam  60  and the reflected angle β′ of the reflected beam  62  are equal, the distance x between the source element  54  and the nearest detector  56  in the detector array  55  (i.e., the detector  56  illuminated by the reflected beam  62 ) is given by: 
             x   =     2   ⁢     (     z     tan   ⁢           ⁢   α       )               (   1   )               
     The next target surface  32  shown in  FIG. 5  corresponds to a target  22  that has been used in a sputtering process wherein approximately one-third of the target  22  material has been eroded. As discussed above in relation to  FIG. 2 , the target surface  32  has eroded unevenly. The transmitted beam  60 , now represented by dashed line  64 , illuminates the eroded target surface  32  and reflects back toward the detector array  55  as reflected beam  66 . The reflected beam  66  illuminates a detector  56  in the detector array  55  located approximately one-third of the distance between the detector  56  located nearest the source element  54  and the detector  56  located farthest from the source element  54 . Therefore, it may be determined that approximately one-third of the target  22  material has been eroded at the measured location along the target surface  32 . 
     The next target surface  70  shown in  FIG. 5  corresponds to the interface between the target  22  and the cathode assembly  18 , as shown in  FIG. 1 . The transmitted beam  60 , now represented by dashed line  68 , illuminates the target interface surface  70  and reflects back toward the detector array  55  as reflected beam  72 . The reflected beam  72  illuminates a detector  56  in the detector array  55  located farthest from the source element  54 . Thus, it may be determined that substantially all of the target  22  material has been eroded at the measured location along the target interface surface  70 . As discussed above, use of the present invention to detect target consumption prevents the target interface surface  70  from being punched through and exposing portions of the cathode assembly  18  to erosion from the sputtering process. Therefore, it may be advantageous to replace the target  22  before the target interface surface  70  is detected. 
       FIG. 6  illustrates a top view schematic of a sensor  80  according to another embodiment of the present invention. The sensor  80  is attached to a thin profile arm  44 , such as the arm  44  shown in  FIG. 3A . The sensor  80  comprises a transceiver  82  and a two-dimensional detector matrix  86  comprising a plurality of detectors  84  ( 24  shown). The transceiver  82  and the detectors  84  each have a thin profile so as to fit between the target  22  and the pedestal  24 , as shown in  FIG. 3A . As shown in  FIG. 6 , the transceiver  82  is positioned in row  87  of the detector matrix  86 . Each detector  84  in the detector matrix  86  is configured to produce an electronic sensory signal related to the magnitude of the radiation received thereon. By way of example only, and not by limitation, each detector  84  may comprise a photodiode or a CCD. Alternatively, each detector  84  may comprise a collimator, such as a lens, configured to collect light into an optical fiber. 
       FIG. 7  illustrates a side view schematic of the transceiver  82  shown in  FIG. 6  positioned in relation to a portion of a target surface  88 . As shown in  FIGS. 7 and 8 , the target surface  88  has roughened during a deposition process. The transceiver  82  comprises a source element  92  and a detector  94 . The transceiver  82  may also comprise a light-directing element  96 , such as a mirror. The source element  92  is configured to transmit a coherent light beam  97  of wavelength λ toward the roughened target surface  88 . Use of a collimated coherent light beam as an energy beam is presently preferred, although the invention is not so limited. By way of example only, and not by limitation, the source element  92  may comprise a laser diode. Alternatively, the source element may comprise a collimator, such as a lens, configured to collimate or focus coherent light exiting an optical fiber to a desired beam diameter or spot size. 
     A first portion of the transmitted coherent light beam  97  is coherently reflected by the roughened target surface  88  in the specular direction back toward the transceiver  82  as reflected coherent beam  98  (offset for illustration only). The reflected coherent beam  98  is directed to the detector  94  by the light-directing element  96  where the power of the reflected coherent beam  98  is measured. By way of example only, and not by limitation, the detector  94  may comprise a photodiode or a CCD. Alternatively, the detector  94  may comprise a collimator, such as a lens, configured to collect the coherent light into an optical fiber. 
       FIG. 8  illustrates a side view schematic of the sensor  80  and arm  44  shown in  FIG. 6 .  FIG. 8  also illustrates the sensor  80  positioned in relation to a portion of the roughened target surface  88 . For illustrative purposes,  FIG. 8  shows a cross-sectional view of the sensor  80  along row  87  of the detector matrix  86 . As discussed above in relation to  FIG. 7 , the transceiver  82  is positioned and configured to illuminate a portion of the roughened target surface  88  with the transmitted coherent light beam  97  and to detect the reflected coherent beam  98 . A second portion of the transmitted coherent light beam  97  is reflected and scattered by the roughened target surface  88  in a three-dimensional cone-like direction back toward the detectors  84  in the detector matrix  86  as scattered light beams  90  (four beams shown). The dimensions of the detector matrix  86  are configured and positioned to detect a substantial portion of the scattered light beams  90 . 
     The roughness of the target surface  88  may be expressed as a root-mean-square surface roughness (hereinafter “RMS — Roughness”) and may be determined as a function of the wavelength λ of the transmitted coherent light beam  97 , the detected power of the reflected coherent beam  98 , and the detected power of the scattered light beams  90 . From the detected coherent reflected beam  98  power (hereinafter “P Coherent ”) and the detected scattered light  90  power (hereinafter “P Scattered ”), a scattering ratio is given by: 
               Scattering   ⁢           ⁢   Ratio     =       P   Scattered         P   Scattered     +     P   Coherent                 (   2   )             
 
     The ratio of the RMS — Roughness divided by the wavelength λ of the transmitted coherent light beam  97 , or RMS — Roughness/λ, is related to the scattering ratio in equation (2). If the target surface  88  is relatively smooth, P Coherent  will be large compared to P Scattered . Thus, the scattering ratio will be relatively small and the ratio RMS — Roughness/λ will also be relatively small. As the target surface  88  becomes increasingly rough, P Scattered  increases and P Coherent  approaches zero. Thus, the scattering ratio becomes increasingly large and the ratio RMS — Roughness/λ will also become increasingly large. Thus, for a given wavelength λ of the transmitted coherent light beam  97 , the RMS — Roughness may be characterized. 
       FIG. 9  illustrates a top view schematic of a sensor  100  according to another embodiment of the present invention. The sensor  100  is attached to a thin profile arm  44 , such as the arm  44  shown in  FIG. 3A . The sensor  100  comprises a source element  102  and a detector  104 . The source element  102  and the detector  104  each have a thin profile so as to fit between the target  22  and the pedestal  24 , as shown in  FIG. 3A . The source element  102  is configured to generate an energy beam. By way of example only, and not by limitation, the source element  102  may comprise a laser diode. Alternatively, the source element  102  may comprise a collimator configured to collimate or focus light exiting an optical fiber to a desired beam diameter or spot size. The detector  104  is configured to produce an electronic sensory signal related to the magnitude of the energy beam received thereon. By way of example only, and not by limitation, the detector  104  may comprise a photodiode or a CCD. Alternatively, the detector  104  may comprise a collimator, such as a lens, configured to collect light into an optical fiber. 
       FIG. 10  illustrates a side view schematic of the sensor  100  shown in  FIG. 9  positioned in relation to a portion of a target surface  110 . As shown in  FIG. 10 , the target surface  110  comprises a plurality of asperities  112  that have grown thereon during a deposition process. As discussed above, the target surface  110  may comprise silver selenide or any target material which manifests protrusion defects. As shown in  FIG. 10 , the source element  102  is positioned and configured to emit an energy beam  114  substantially parallel to the target surface  110  toward the detector  104 . The sensor  100  is configured and positioned such that the energy beam  114  illuminates or otherwise interacts with a portion of the asperities  112 . Thus, it may be advantageous to move the sensor  100  in a plane perpendicular to the target surface  110  as well as in a plane parallel to the target surface  110 . The presence of the asperities  112  on the target surface  110  is detected by an interruption of the energy beam  114  by a portion of the asperities  112  between the source element  102  and the detector  104 . Similarly, the presence of the asperities  112  may be detected by a reduction in the intensity or power of the detected energy beam  114  caused by interactions with a portion of the asperities  112 . Alternatively, the presence of the asperities  112  may be detected by measuring the roughness of the target surface  110  as described above in relation to  FIGS. 6–8 . 
       FIG. 11  is a block diagram of a sputter deposition system  180  according to the present invention. The sputter deposition system  180  comprises a controller  182  electrically coupled to chamber circuitry  190 , a sensor assembly  200 , an input device  184 , an output device  186  and a data storage device  188 . The controller  182  is configured to communicate an electronic transmit signal to the sensor assembly  200 . Upon receipt of the transmit signal from the controller  182 , the sensor assembly  200  is configured to transmit a beam of energy. The sensor assembly  200  is configured to generate electronic sensory signals related to the magnitude of an emitted, reflected or scattered energy beam received thereon. The controller  182  is configured to receive and analyze the sensory signals. 
     Referring to  FIGS. 1 and 11 , the sensor assembly  200  may be located substantially within the vacuum chamber  12 . Alternatively, the sensor assembly  200  may be located substantially outside of the vacuum chamber  12  or partially outside the vacuum chamber  12 . For example, the sensor assembly  200 , or a portion thereof, may be located outside of the vacuum chamber  12  and configured to transmit the energy beam through the window  15 . Similarly, the sensor assembly  200  may be configured to receive the emitted, reflected or scattered energy beam as it exits the same window  15  or a different window (not shown). 
     The controller  182  may be configured to interface with the chamber circuitry  190 , including chamber robot circuitry  192 , to control the position of the sensor assembly  200  or a portion thereof relative to a surface in the vacuum chamber  12 , the placement and removal of a substrate  26  on the pedestal  24 , sputter processing times, and other sputtering process and vacuum chamber  12  operations. The controller  182  may further be configured to perform computer functions such as executing software to perform desired calculations and tasks. 
     The input device  184  may include, by way of example only, an Internet or other network connection, a mouse, a keypad or any device that allows an operator to enter data into the controller  182 . The output device  186  may include, by way of example only, a printer or a video display device. The data storage device  188  may include, by way of example only, drives that accept hard and floppy discs, tape cassettes, CD-ROM or DVD-ROM. 
     The sensor assembly  200  may comprise a sensor (not shown) such as the sensors discussed above or the embodiments disclosed below in  FIGS. 12A–13 . For example, the sensor assembly  200  may comprise the sensor  52  attached to the arm  44  shown in  FIGS. 4 and 5 . Referring to  FIGS. 4 ,  5  and  11 , the controller  182  is configured to communicate an electronic transmit signal to the source element  54 . Upon receipt of the transmit signal from the controller  182 , the source element  54  is configured to transmit a beam of collimated light. The beam of collimated light may be a pulsed beam of collimated light. Each detector  56  is configured to generate an electronic sensory signal related to the magnitude of the radiation received thereon. The controller  182  is configured to receive and compare each of the sensory signals to determine which one of the detectors  56  was illuminated with the greatest magnitude of radiation. The controller  182  may be configured to receive the sensory signals during a predefined time window in relation to the communication of the transmit signal to the source element  54 . 
     The controller  182  is further configured to determine the relative distance from the sensor  52  to a target surface  23 ,  32 ,  70 . As described above in relation to  FIG. 5 , the controller  182  may be configured to estimate the relative amount of erosion at a location along the target surface  23 ,  32 ,  70  according to the relative position of the detector  56  in the detector array  55  illuminated with the greatest amount of radiation. For example, if a detector  56  located at the center of the detector array  55  is determined by the controller  182  to be illuminated by a reflected beam, then the controller  182  may be configured to estimate that half of the target  22  material has been eroded at the position along the target surface  23 ,  32 ,  70  being measured. Alternatively, the distance from the sensor  52  to the target surface  23 ,  32 ,  70  may be determined as a function of the transmission angle α and the distance between the source element  54  and the detector  56  being illuminated. For example, if the transmission angle α and the distance x between the source element  54  and the nearest detector  56  in  FIG. 5  are known, then equation (1) above may be used (assuming the incident angle β of the transmitted beam  60  and the reflected angle β′ of the reflected beam  62  are equal) to determine the distance z between the sensor  52  and the target surface  23  as: 
             z   =     x   ⁡     (       tan   ⁢           ⁢   α     2     )               (   3   )             
 
       FIGS. 12A–12C  illustrate block diagrams of sensor assemblies  202 ,  230 , and  260 , suitable for use as the sensor assembly  200  shown in  FIG. 11 . The sensor assemblies  202 ,  230 ,  260  shown in  FIGS. 12A–12C  employ fiber optics to reduce the size of a portion of the sensor assemblies  202 ,  230 ,  260  to be positioned within a sealed chamber (not shown), such as the vacuum chamber  12  shown in  FIG. 1 .  FIG. 12A  illustrates a block diagram of sensor assembly  202  according to one embodiment of the present invention. The sensor assembly  202  comprises a source element  210  and a plurality of reception elements  212  (four shown) attached to a thin profile arm  214 , such as the arm  44  shown in  FIGS. 4 and 5 . The source element  210  comprises a collimator, such as a lens, configured to collimate or focus light exiting an optical fiber  216  to a desired beam diameter or spot size. Each reception element  212  comprises a collimator, such as a lens, configured to collect light incident thereon into an optical fiber assembly  218 . The sensor assembly  202  further comprises a transmitter  204  coupled to the source element  210  through the optical fiber  216  and a receiver  206  coupled to each of the plurality of reception elements  212  through the optical fiber assembly  218 . The optical fiber assembly  218  comprises a plurality of optical fibers, each optical fiber configured to couple to one reception element  212 . 
     Referring to  FIGS. 11 and 12A , the transmitter  204  is configured to receive a transmit signal from the controller  182  and to transmit a beam of collimated light to the source element  210  through the optical fiber  216 . The beam of collimated light may be a pulsed beam of collimated light. For each reception element  212 , the receiver  206  is configured to receive a light beam through the optical fiber assembly  218  and to generate an electronic sensory signal related to the magnitude of the radiation collected at the respective reception element  212 . The receiver  206  is further configured to transmit each of the sensory signals to the controller  182 . The controller  182  is configured to receive and compare each of the sensory signals to determine which one of the reception elements  212  was illuminated with the greatest magnitude of radiation. The controller  182  may be configured to receive the sensory signals during a predefined time window in relation to the communication of the transmit signal to the source element  210  from the transmitter  204 . The controller  182  is further configured to determine the relative distance from the source element  210  to an object (not shown), as described above. 
       FIG. 12B  illustrates a block diagram of sensor assembly  230  according to another embodiment of the present invention. The sensor assembly  230  comprises an imaging device  236 , a transceiver  238  and a scattered light reception element  240  attached to a thin profile arm  242 . The imaging device  236  may comprise a lens. The transceiver  238  is configured to emit a coherent light beam  250  and to receive a reflected coherent light beam  251  (offset for illustration only). The transceiver  238  comprises a source collimator (not shown), such as a lens, configured to collimate or focus the coherent light beam  250  exiting an optical fiber  244  to a desired beam diameter or spot size. The transceiver  238  also comprises a coherent light reception element (not shown). The transceiver  238  may also comprise a light-directing element (not shown), such as a mirror, configured to direct the coherent light beam  250  from the source collimator out of the transceiver  238  and/or to direct the reflected coherent light beam  251  into the transceiver  238  to the coherent light reception element. The coherent light reception element in the transceiver  238  and the scattered light reception element  240  each comprise a collimator, such as a lens, configured to collect light incident thereon into an optical fiber assembly  246 . The sensor assembly  230  further comprises a transmitter  232  coupled to the transceiver  238  through the optical fiber  244  and a receiver  234  coupled to the transceiver  238  and to the scattered light reception element  240  through the optical fiber assembly  246 . 
     Referring to  FIGS. 11 and 12B , the transmitter  232  is configured to receive a transmit signal from the controller  182  and to transmit the coherent light beam  250  to the transceiver  238  through the optical fiber  244  where the source collimator emits the coherent light beam  250  through the imaging device  236 . The imaging device  236  is configured to direct the reflected coherent light beam  251  to the transceiver  238  where it is passed to the receiver  234  through the optical fiber assembly  246 . The imaging device  236  is further configured to direct scattered light beams  252  (two shown) to the scattered light reception element  240  where they are passed to the receiver  234  through the optical fiber assembly  246 . The receiver  234  is configured to generate an electronic sensory signal in response to each of the received reflected coherent light beam  251  and scattered light beams  252 . The receiver  234  is further configured to transmit each of the sensory signals to the controller  182 . The controller  182  is configured to receive and analyze the sensory signals as described above in relation to  FIGS. 6–8  to determine the roughness of a surface (not shown) illuminated by the coherent light beam  250 . 
       FIG. 12C  illustrates a block diagram of sensor assembly  260  according to yet another embodiment of the present invention. The sensor assembly  260  comprises a source element  266  and a reception element  268  attached to a thin profile arm  270 . The source element  266  comprises a collimator, such as a lens, configured to collimate or focus light exiting an optical fiber  272  to a desired beam diameter or spot size. The reception element  268  also comprises a collimator, such as a lens, configured to collect emitted, reflected or scattered light incident thereon into an optical fiber  274 . The sensor assembly  260  further comprises a transmitter  262  coupled to the source element  266  through the optical fiber  272  and a spectrometer  264  coupled to the reception element  268  through the optical fiber  274 . 
     Referring to  FIGS. 11 and 12C , the transmitter  262  is configured to receive a transmit signal from the controller  182  and to transmit a beam of collimated light to the source element  266  through the optical fiber  272 . The beam of collimated light may comprise multiple wavelengths. The spectrometer  264  is configured to receive the collected light incident upon the reception element  268  through the optical fiber  274  and to analyze the collected light using spectroscopy techniques. The spectrometer  264  is further configured to generate electronic sensory signals related to the spectroscopic analysis and to transmit the sensory signals to the controller  182 . The controller  182  is configured to receive the sensory signals and to correlate the sensory signals to spectra previously stored in a database in the data storage device  188 . Thus, the sensory signals may be correlated to compositional data to determine elemental, isotropic and structural characteristics of a surface (not shown) illuminated by the transmitted beam of collimated light. For example, as discussed above, the sensory signals may be correlated to determine the amount and composition of residue deposits on the inner chamber wall  13  shown in  FIG. 3C . 
     The spectrometer  264  shown in  FIG. 12C  employs a Michelson interferometer. However, the scope of the present invention includes all spectrometers and spectroscopy techniques presently known in the art. The spectrometer  264  comprises a beam splitter  280 , a moving mirror  282 , a fixed mirror  284  and a receiver  286 . As an example of one spectroscopy technique suitable for use with the present invention, the spectrometer  264  may be used with Fourier-transform techniques to perform FTIR spectroscopy on the collected light incident upon the reception element  268 . In this example, it is assumed that the collimated light beam transmitted by source element  266  is an IR light beam and that the collected light incident upon the reception element  268  is a reflection of the IR light beam from a surface, such as the inner chamber wall  13  shown in  FIG. 3C . The reflected IR light beam exiting the optical fiber  274  is directed onto the beam splitter  280 . The beam splitter  280  directs approximately half of the reflected IR light beam to the moving mirror  282  and approximately half of the reflected IR light beam to the fixed mirror  284 . After reflecting off the moving mirror  282  and the fixed mirror  284 , the components of the reflected IR light beam are recombined by the beam splitter  280  and directed to the receiver  286 . The moving mirror  282  and the fixed mirror  284  produce constructive and destructive interference in the recombined IR light beam which is detected by the receiver  286 . The receiver  286  is configured to convert the detected interference into sensory signals, which are then analyzed by the controller  182  in  FIG. 11  to determine the concentration and composition of the surface being analyzed. 
     As another example of a spectroscopy technique suitable for use with the present invention, the spectrometer  264  may be used to perform Raman spectroscopy on the collected light incident upon the reception element  268 . In this example, it is assumed that the collimated light beam transmitted by source element  266  comprises multiple wavelengths and that the collected light incident upon the reception element  268  is Raman scattered light from a surface illuminated with the collimated light beam, such as the inner chamber wall  13  shown in  FIG. 3C . In this example, the Raman scattered light is processed by the spectrometer  264  as described above. However, the Raman scattered light undergoes additional Raman spectroscopy once it reaches the receiver  286 .  FIG. 13  illustrates a receiver  300 , such as the receiver  286  shown in  FIG. 12C , configured to perform Raman spectroscopy. 
     The receiver  300  comprises a first lens  306 , a grating  308 , a second lens  310 , and a detector  312 . The Raman scattered light is directed onto the grating  308  by the first lens  306 . The grating  308  disperses the Raman scattered light through the second lens  310  where it is focused onto the detector  312 . The detector  312  may be selected from the group comprising a CCD camera, an intensified CCD detector, a charge injection device, a photomultiplier tube detector array, a photodiode array (hereinafter “PDA”), an intensified PDA, or an avalanche photodiode array. The detector  312  is configured to generate sensory signals representative of the Raman spectra received thereon. The sensory signals are then analyzed by the controller  182  in  FIG. 11  and compared to Raman spectra previously stored in a database in the data storage device  188 . Thus, structural analysis, multicomponent qualitative analysis, and quantitative analysis may be performed to determine the characteristics of the surface being analyzed. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.