Abstract:
A method and apparatus for determining the synchronicity of a rotary platen ( 22 ) in a vacuum deposition chamber ( 24 ). A light source ( 64 ) projects a highly collimated light beam ( 66 ) onto the rotating platen ( 22 ), thereby tracing a circular swept path ( 67 ). The swept path ( 67 ) passes alternately through samples ( 20 ) on the platen ( 22 ) and intervening webs ( 58, 60 ). The samples ( 20 ) are significantly more reflective than the webs ( 58, 60 ). The platen ( 22 ) includes an asymmetry feature ( 60 ) along the swept path ( 67 ). A detector ( 62 ) measures light signals reflected from the platen ( 22 ) along the swept path ( 67 ), and generates a unique signal upon encountering the asymmetry feature ( 60 ). A microcontroller generates a trigger pulse synchronized to the unique signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Patent Application No. 61/566,212 filed Dec. 2, 2011, the entire disclosure of which is hereby incorporated by reference and relied upon. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to non-contact, in-situ diagnostics used to monitor various thin film growth parameters during multi-sample deposition on high speed rotation stages. 
         [0004]    2. Related Art 
         [0005]    Essential components for electronic and optoelectronic devices, such as integrated circuits, chips, processors, LEDs, lasers, transistors and solar cells, are made by depositing or growing very thin layers of atoms onto a semiconductor (or other material type) wafer substrate. During the thin film deposition/growth process, a batch of wafer substrates are heated from behind and rotated about a center axis in a vacuum environment. Direct benefits in end component quality and performance can be achieved by precisely controlling growth process properties like temperature and film thickness with high precision and repeatability. 
         [0006]    Numerous methods have been disclosed for monitoring process temperatures and film thicknesses. These include precise and real-time monitoring of the substrate temperature or property. The BandiT™ system from k-Space Associates, Inc., Dexter Mich., USA (kSA), assignee of the subject invention, has emerged as a premier, state-of-the-art method and apparatus for measuring semiconductor substrate temperature. The kSA BandiT system is described in detail in US Publication No. 2005/0106876 and U.S. Publication No. 2009/0177432 the entire disclosures of which are incorporated hereby reference. 
         [0007]    In addition to the use of sophisticated monitoring systems, the production of high-quality semiconductor products can be improved still further with advances in the deposition systems themselves that are used to create the formation of semiconductor nanostructures. In particular, in deposition systems that utilize a multi-wafer rotary platen, opportunities for improvement are manifest. Many such deposition systems lack a positive mechanical lock between the motor driven spindle and the platen, usually as a result of certain structural constraints or methods of sample transfer within those particular systems. Operative connection between the motor drive spindle and platen may be in the form of a magnetic or friction coupling rather than meshing gears or toothed belts. In these cases, the phase angle of the platen may over time drift from the initial spindle arbor phase angle, making the standard spindle arbor “home pulse” signal useless for synchronization. Or, the slippage may be more or less continuous such that the spindle drive system indicates a rotary speed of 1500 RPM for example, but in fact the platen is only spinning at 1480 RPM. 
         [0008]    Such systems lack real-time synchronization between the diagnostic device(s), e.g., a temperature monitoring systems like the kSA BandiT and/or a film thickness measurement system, with the war samples in a multiple sample platen. This lack of synchronization can be more problematic in high speed spindle configurations, where rotation speeds above 1,000 RPM are not uncommon. Typically, in multiple sample platen deposition systems, the diagnostic device must spend many rotations to diagnose the locations of each measurement after they occurred, making real-time monitoring and eventual control impossible. 
         [0009]    There is therefore a need for a system and method to overcome asymmetry issues with respect to platen and drive spindle for the purposes of improving quality and performance during the thin film growth process. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of this invention, an apparatus for determining the synchronicity of a rotary platen in a vacuum deposition chamber is provided. The apparatus includes a vacuum deposition chamber, and a platen supported for rotation about a center axis in the vacuum deposition chamber. The platen is configured to emit light signals about a circular swept path centered about the central axis. The platen includes an asymmetry feature along the swept path. The asymmetry feature has a unique signature with respect to other light signals emitted from the platen along the swept path. The apparatus also includes a rotary spindle drive, and a non-positive coupling operatively connecting the spindle drive to the platen for forcibly rotating the platen about the center axis. A detector is fixed relative to the rotating platen for measuring light signals emitted from the platen along the swept path. The detector is configured to generate a unique: signal in direct response to the unique signature emitted by the asymmetry feature. 
         [0011]    According to a second aspect of this invention, the apparatus includes a light source for projecting a light beam onto a rotating platen so as to trace a circular swept path of light on the platen as the platen rotates about a center axis. A detector is provided for measuring light signals reflected from the platen along the swept path. The detector is configured to generate a unique signal in direct response to encountering a unique signature in the light signals reflected from the rotating platen, the unique feature corresponding to an asymmetry feature of the platen. A microcontroller includes a non-transitory computer readable medium coded with instructions and executed by a processor to generate a trigger pulse synchronized to the unique signal. The frequency of two successive trigger pulses directly corresponds to the real-time rotational speed of the platen. 
         [0012]    According to a third aspect of this invention, a method is provided for determining the synchronicity of a rotary platen in a vacuum deposition chamber. The method comprises the steps of rotating a platen about a center axis in a vacuum deposition chamber, projecting a light beam onto the rotating platen so as to trace a circular swept path of light on the platen as the platen rotates about the center axis, measuring light signals reflected from the platen along the swept path, identifying a unique signal in direct response to encountering a unique signature in the light signals reflected from the rotating, the unique feature corresponding to an asymmetry feature of the platen, and generating a trigger pulse in response to each identification of the unique signal, the frequency of the trigger pulse being directly proportional to the real-time rotational speed of the platen. 
         [0013]    This invention enables real-time synchronization of the diagnostic system with a multiple sample platen, especially useful in applications where the platen is not mechanically locked to the drive spindle. The invention transmits a trigger pulse signal upon detection of one particular asymmetry on the platen surface, using light either emitted or reflected from the surface features of the platen and the samples. The invention can be constructed so that the physical light source and detection equipment resides outside the vacuum enclosure, with light signals passing through an optical port of the chamber, to provide real-time platen rotational data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features and advantages of the present invention, will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
           [0015]      FIG. 1  is a schematic view of an exemplary thin film deposition process including an apparatus for determining the synchronicity of a rotary platen in a vacuum deposition chamber according to an embodiment of the invention; 
           [0016]      FIG. 2  is a simplified top view of a platen having a sample spacing asymmetry feature; 
           [0017]      FIG. 3  is a perspective view of a reflectance assembly according to an embodiment of the invention; 
           [0018]      FIG. 4  is a graph showing the detector voltage output displaying a typical pulse train from the reflectivity of a spinning asymmetric platen; 
           [0019]      FIG. 5  in an enlarged view of the reflectance detector output indicated at  5  in  FIG. 4 ; 
           [0020]      FIG. 6  is a flow chart describing the steps of the present invention as used to set a V Threshold  value; and 
           [0021]      FIG. 7  is a flow chart describing the steps of the present invention as used to set an A Threshold  value and an Output Pulse. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, an exemplary application of the method, apparatus, and system for achieving real-time synchronization of a diagnostic system to a multiple sample rotating platen is illustrated schematically in  FIG. 1  within the context of a thin film deposition process. A semiconductor film is deposited or otherwise grown on a substrate (collectively a sample  20 ) supported on a rotating carrier or platen  22  within a vacuum deposition chamber  24 , as shown in  FIG. 1 . The substrate may take the form of a disc-like wafer made of any suitable material, such as a silicon or sapphire composition, and the film of any suitable material, for example a semiconductor like Silicon (Si), GaN (Gallium Nitride), Gallium Arsenide (GaAs), and Indium Phosphide (InP), just to name a few. The system typically includes a depositor or a means for depositing the film on the substrate in a highly precise and controlled fashion. The means for depositing the film on the substrate can include any suitable technique including, by way of examples, a chemical vapor deposition process such as metalorganic vapor phase epitaxy (MOVPE), a molecular deposition process such as molecular beam epitaxy (MBE), or other thin-film deposition process including sputtering and the like. 
         [0023]    The platen  22  is rotated about a center axis A inside the chamber  24  by a spindle drive  25 . A coupling  27  operatively connects the spindle drive  25  to the platen  22 . The invention is particularly adapted for deposition systems in which the platen  22  is not positively mechanically locked to its drive spindle  25  through the coupling  27 . That is, the coupling  27  may be in the form of a magnetic or friction or fluid coupling, or some other construction, where the rotation of the platen  22  is not positively linked to the rotation of the spindle chive  25  (as compared to geared and toothed coupling arrangements that do provide a positive link). In non-positively coupled  27  drive systems  25 , there exists opportunity for slippage between the platen  22  and drive spindle  25 . In these instances, the standard spindle arbor “home pulse” signal captured by a spindle detector  29  will be useless for synchronizing the true angular position of the platen  22 . For the instantaneous angular position of the platen  22 , the lack of synchronicity makes the standard spindle arbor “home pulse” signal unreliable. When slippage occurs in operation, the lack of synchronicity means that the real-time platen  22  rotation speed slower that that indicated by measurements based on the spindle detector  29 . 
         [0024]    The deposition system preferably includes one or more features for determining meaningful properties of the sample  20 , and which depend on precise synchronicity between drive  25  and platen  22 . For purposes of example, the features may include devices for real-time monitoring process temperatures and film thicknesses. Taking only the example of temperature assessment, real-time monitoring may be accomplished with a unit like the BandiT™ system from k-Space Associates, Inc., Dexter Mich., USA (kSA). The system of  FIG. 1  includes a light source  26  for interacting light with the sample  20  to produce diffusely scattered light. The light source  26  is typically a quartz halogen lamp mounted outside the deposition chamber  24  that directs light toward the sample  20 . The light provided by the light source  26  is both visible and not visible to the naked eye. A control unit  28  containing a lamp controller unit  30  is connected to the light source  26  by a light source power cable. A computer  32 , such as a laptop or standard central processing unit, employing a suitably configured software program, simultaneously monitors and operates the lamp controller unit  30  and other components of the system. The computer  32  is connected to the control unit  28  by a USB cable  34 . 
         [0025]    In the exemplary application of  FIG. 1 , the deposition system includes a heat source  36 , which heats the samples  20  from behind. In an alternative configuration, the light source  26  and the heat source  36  may be integrated into the same component. The temperature of the sample  20  is monitored and controlled as variations in temperature ultimately affect quality and composition of the film deposited on the substrate. The system includes a temperature control  38 , such as a PID temperature control  38 , which is connected to the computer  32  and can be manually operated by a user of the system. 
         [0026]    Light diffusely scattered from the sample  20  is analyzed to determine the optical absorption edge wavelength of the sample  20 , which is used to calculate or determine by look-up the temperature or other properties of the sample  20 . The optical absorption edge can also be referred to as the band edge or band gap. The system includes a detector  40  for collecting diffusely scattered light from the film  20 . The detector  40  is typically a Si-based detector  40 . The detector  40  includes a housing  42 , which is also mounted outside the deposition chamber  24  proximate to a transparent view port at an angle that is non-specular to the light source  26 . The detector  40  includes an adjustable tilt mount  44  comprising a micrometer-actuated, single-axis tilt mechanism built into the front of the detector  40  to assist in pointing the detector  40  at the sample within the chamber  24 . The detector  40  also includes focusing optics  46  assisting in the collection of the diffusely scattered light. 
         [0027]    The exemplary system includes a spectrometer  48 , such as a solid state spectrometer  48  or an array spectrometer  48 , for producing a spectra from or based on the diffusely scattered light from the film and collected by the detector  40 . The optical absorption edge wavelength of the film is determined based on the spectra. The step of determining the optical absorption edge wavelength of the film based on the spectra includes accounting for the semiconductor material and the thickness of the film. 
         [0028]    The exemplary system further includes an optical fiber unit  50 , including a first optical fiber  52  coupled to the spectrometer  48  and a second optical fiber  54  running co-linear to first optical fiber  52  and coupled to a visible alignment laser  56  for aid in alignment of the detector  40 . The optical components are optimized, using appropriate optical coatings, for either infrared or visible operation depending on the characteristics of the sample  20  being measured. The computer  32  is connected to the alignment laser  56  and the spectrometer  48  by the USB cable  34 . The software program is employed to control the alignment laser  56  and spectrometer  48 . 
         [0029]    Less than optimal measurement/monitoring data may be experienced if the rotating platen  22  falls out of synchronicity with the chive spindle  25 . This type of problem usually does not occur when the platen  22  and drive spindle  25  are mechanically linked through gears or belts. However, when they are not mechanically linked as in some drive system configurations, a loss of synchronicity can appear and even grow over time. When this happens, devices that measure and monitor relevant film growth characteristics can report less than accurate data, and result in less than ideal real-time thin film deposition information. 
         [0030]    The present invention overcomes this deficiency by calibrating the angular position of the platen  22  at regular intervals, such as once each revolution. The invention accomplishes this goal by identifying a particular asymmetry on the platen  22  surface as a reference point, and then transmitting a “home pulse” signal at each encounter of the reference point. A particularly novel aspect of this invention is that light is used to generate the home pulse signal. The light used may either be emitted or reflected from the platen  22  and/or the samples  20 . 
         [0031]    This invention takes advantage of the naturally large differences in reflectivity between the samples  20  and the areas of platen  22  adjacent to the samples  20 . These areas resemble radial spokes hereafter referred to as webs  58 ,  60 . The sides of each web  58 ,  60  have opposing concave profiles formed by opposing circular segments of two adjacent samples  20 . The narrowest portion of the webs  58 , i.e., the narrowest spacing between adjacent samples  20 , is labeled “X” in one representative web  58  location shown in  FIG. 2 . The narrowest spacing X occurs only at narrow webs  58 , which comprise all of the webs  58  except the one web  60 . Platens  22  used in many commercially available deposition systems are designed to receive samples  20  arranged in a predetermined pattern such that one of the webs  60  is wider than the other narrow webs  58 . The single wide web  60  may be used by the present invention as a distinctive point of reference on the platen  22 . The width of the wide web  60  is labeled “Y” in  FIG. 2 . The incident light intensity changes dramatically at these web  58 ,  60  locations, relative to the high reflectivity of the samples  20 , as the platen  22  spins. 
         [0032]    A microcontroller, which may be incorporated into the control unit  28  or as a stand-alone component, digitizes the voltage generated from a silicon detector  62 , and then runs an analysis to determine the location of the asymmetry. The microcontroller includes a non-transitory computer readable medium coded with instructions and executed by a processor to perform the steps described below. At each encounter of the asymmetry, the microcontroller transmits a 5 micro-second trigger pulse at the trailing edge of the asymmetry. Suitable programming within the microcontroller can compensate for varying rotation rates and changing reflectivity conditions. Details of one exemplary algorithm to determine the position of the asymmetry will be described in detail below, however those of skill in the art may envision other techniques to achieve the same end effects and based upon the same core concepts of this invention. 
         [0033]    Referring now to  FIG. 3 , one embodiment of the invention utilizes a highly collimated light source, generally indicated at  64 , such as a low power CW laser directed at normal incidence to the surface of the platen  22 . The laser beam produced by the light source  64  is indicated by broken line  66  in  FIG. 1 . The transitory spot at which the laser beam  66  strikes the platen  22 , as well as its circular swept path  67  traced around the platen  22 , is shown in  FIG. 2 . The position of the light source  64  is arranged so that the swept path  67  passes through the samples  20  and the intervening webs  58 ,  60 . The portion of the swept path  67  that crosses any web  58 , 60  is identified as a web segment. Each web segment has a length, with the length of the web segments for the narrow webs  58  being generally equal and shorter than the length of the web segment for the one wide web  60 . Preferably, the web segments all coincide with the narrowest spacings X, Y of the respective webs  58 ,  60 . However, in an alternative arrangement the web segments could be offset from the actual narrowest spacings X, Y, provided they all pass through some portion of the webs  58 ,  60  and samples  20  at an equal radial measure from the central axis A. In other words, the swept path  67  must pass alternately through webs  58 ,  60  and samples  20  so that the large differences in reflectivity can be used to indicate the periodic occurrence of the asymmetric wide web  60 . 
         [0034]    A beam splitter  68  redirects the reflected light through a narrow band pass optical filter  70  to focus on the solid state silicon detector  62 . The optical filter  70  ensures that stray light from other sources as well as emitted light (i.e., black body radiation) from a hot platen  22  does not interfere with the reflected signal. The entire assembly may be integrated into a single housing, allowing for fine angle adjustments to compensate for any tilt between platen  22  and exterior support fixtures. 
         [0035]    The light source  64  may be a simple 660 nm diode laser with integrated collimation/focusing lens housed within the cylinder block at the top. The laser wavelength &amp; filter are chosen to yield best sensitivity depending on the sample  20  and platen  22  materials. The beam splitter cube  68  is fixed within a central mounting block  74  which also acts as beam stop as a safety to prevent stray laser light from escaping. The silicon detector  62  is mounted to a cylindrical lens holder  72  which houses the focusing lens and optical filter specific to the particular laser wavelength. The mounting block  74  for the assembly is preferably on a spring loaded fine adjustment mounting plate  76  to allow correction for any tilt (i.e., deviation from parallel) between the platen  2  and the mounting block  74  exterior to the chamber  24 . 
         [0036]    The silicon detector  62  preferably has an integrated amplifier with adjustable gain so that the reflection signal can be set to saturate at the higher reflectivity sample  20  surfaces and there is sufficient voltage range between the samples  20  and the platen webs  58 ,  60 . The output is sent directly to an analog input to the microcontroller for analysis of the output pulses. A sample output is graphically depicted in  FIG. 4 , and an enlarged view of the relevant region of the output is shown in  FIG. 5 . 
         [0037]      FIGS. 6 and 7  are flow charts describing the basic sequence of microcontroller operations. In particular,  FIG. 6  lays out basic steps to set the V Threshold  value, whereas  FIG. 7  lays out basic steps to set the A Threshold  value and the Output Pulse, as described more fully below. 
         [0038]    In a preferred implementation, the microprocessor runs at an internal clock speed that is fast enough to poll the detector  62  signal with enough resolution to divide the reflectance from a full platen  22  rotation into at least 1600 points at rotation speeds of 1500 rpm. This is not intended as a limitation of present microcontrollers, but is deemed generally sufficient for the rotation speeds and platen  22  constructions predominant in the current population. Naturally, these exemplary specifications can be increased for higher speeds and other platen  22  designs. Also, as microprocessor performance increases with advances in technology, the ability to operate at even higher resolution, higher speeds and smaller platen  22  features will be possible. Other limitations would be the silicon detector  62  response time, which if needed could be overcome with an increase in laser  64  power. 
         [0039]    In the initial voltage measurements, the microcontroller establishes a value for the maximum and minimum voltages, labeled V max  and V min  in  FIG. 5 . This is made over an average of 65,000 samples, or approximately 40 rotations at  1500  rpm (˜1.6 seconds in real time). These values are updated on that time frame in order to track changes in reflectivity that will occur during film growth on both samples  20  and on the platen  22 . This allows an automatic correction for real time changes during film growth. Once these values are obtained, the microcontroller calculates an average value (V Ave. ) and then in order to establish a threshold value to begin counting the width of the reflection minima, the following equation may be used: 
         [0000]        V   Threshold =( V   Ave.   −V   Min. )× S   VT   +V   Min.  
 
         [0040]    The variable S VT  is a sensitivity factor, similar to a “tooling factor” in other instruments, with values ranging between 0 and 1, and allows the user to compensate for different sample/platen reflectivity, distance to platen  22 , and detector  62  gain and noise levels. A typical value is ≈0.3. Once V Threshold  is determined, a count is made of the number of successive measurements that satisfy V&lt;V Threshold . The count corresponds to the length of the web segment. Both the average count of the web width. Web Ave. , and the longest count of the web width. Web Max , are determined over 50 webs. In  FIG. 5 , the web width, i.e., length of web segment, is identified by the variable τ web  at or proximate to the V Threshold  value. 
         [0041]    An “asymmetry threshold”. A Threshold , is then calculated from: 
         [0000]        A   Threshold =Web Max. −(Web Max. −Web Ave. )× S   WT  
 
         [0000]    The variable S WT  is a count width threshold sensitivity factor, again allowing the user to compensate for conditions as in the above case, e.g., to account for film growth on samples  20  and platen  22  over time. A typical value for S WT ≈0.5. When it is determined that the above threshold is satisfied, a 5 μS trigger pulse is sent out by the microcontroller. Of course, if necessary the exact duration of trigger pulse can be longer or shorter than 5 μS to suit the application. In this manner, a pulse synchronized to the rotation of the platen  22  is continually generated. In other words, the trigger pulse is generated once per evolution of the platen  22 , and can be used to calculate the real-time angular position of the platen  22  or the real-time angular speed of the platen  22  or both. Additional determinations can be made as well, such as a coupling  27  slippage assessment, and other useful metrics. 
         [0042]    In an alternative implementation, the user can input the number of samples  20  on the platen  22  (or the number of webs  58 ,  60 ). This input can then be entered into the algorithm and act as a check on the asymmetric trigger position. The check is made by setting the trigger pulse to automatically send every X successive measurements detected, where X is the number of samples  20  input (or the number of webs  58 ,  60 ). In the case of very weak reflectance signal from the samples  20  (as a result of either a strong deconstructive light interference or a very rough sample  20  surface), the algorithm may be programmed to automatically revert to outputting every X successive measurements, and thus not rely on detecting the asymmetry in wide web  60  width relative to the other narrow webs  58 . In this implementation, the system will trigger properly with a symmetric platen (not shown), except that while the trigger pulse will occur in the same position for every rotation, the absolute trigger position with respect to a specific sample  20  on the platen) will not be known. 
         [0043]    The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention.