Abstract:
A radiation thermometer utilizing an off-focus telecentric lens arrangement in chemical vapor deposition reactors. An object assembly of one or more optical components is positioned at a distance equal to its focal length from an aperture stop. The aperture stop is dimensioned so that the chief rays are substantially parallel with the optical axis of the object assembly, and so that the rays that pass through the aperture stop define a narrow solid angle about the chief rays. The off-focus telecentric arrangement thus configured is focused at infinity, but is utilized to capture radiation from a relatively proximate target (e.g., within a couple meters) that is out of focus. The capture of collimated radiation from the target diminishes the contribution of stray radiation, particularly with targets having a highly specular surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The disclosure is directed generally to radiation thermometers or pyrometers, and more specifically to collection optics used therefor. 
       BACKGROUND 
       [0002]    Metalorganic Chemical Vapor Deposition (MOCVD) is a chemical vapor deposition technique for growing crystalline layers in processes such as the production of semiconductors. The MOCVD process is implemented in a reactor chamber with specially designed flow flanges that deliver uniform reactor gas flows to the reactor chamber. 
         [0003]    The temperature of the crystalline layers during the MOCVD process are typically measured using non-contact devices such as radiation thermometers or pyrometers. Such crystalline growth materials include silicon carbide (SiC), zinc selenide (ZnSe), and gallium nitride (GaN) based materials such as GaN and AlGaN. Certain substrates crystalline growth materials have emission characteristics that limit the wavelength of operation for radiation thermometry. For example, GaN, grown on a sapphire substrate, can have a transmittance greater than 50% for wavelengths longer than 450 nanometers (nm) at process temperatures. Thus, at wavelengths longer than 450 nm, a substantial fraction of the radiation leaving the surface of a GaN layer originates from the structure beneath the substrate that is in the line of sight of the radiation thermometer (e.g., a wafer carrier). Radiation that passes through the GaN layer is not indicative of the temperature of the GaN layer. Accordingly, radiation thermometers have been developed that detect radiation at wavelengths lengths shorter than 450 nm (corresponding roughly to the blue, violet and ultraviolet wavelengths). See, e.g., U.S. Patent Application Publication No. 2011/0064114 to Zettler et al. (hereinafter “Zettler”), disclosing a pyrometer adapted to detect radiation in the range of 250 nm to 450 nm. 
         [0004]    An issue with the use radiation thermometers is the detection of unwanted radiation. One source of unwanted radiation is unfiltered radiation that detected from outside the desired band pass of detection. Zettler describes an apparatus and technique that accounts for the contribution of unfiltered radiation. Zettler points out that narrow band pass filters do not totally block infrared radiation. The unblocked infrared radiation can be problematic at the temperatures of operation (about 800° C.) because the blackbody intensity of the radiation in the infrared portion of the electromagnetic spectrum is about 9 orders of magnitude higher than in the primary band pass (i.e., the desired spectral band pass for inferring target temperature) of the narrow band pass filter. The method of Zettler involves the use of a detector that is sensitive over a broad wavelength range (from ultraviolet to the infrared) and filtering the incoming radiation with a narrow band pass filter centered near 410 nm. A longpass filter is then used to effectively block the primary band pass of the narrow band pass filter, but still allow the radiation unfiltered by the narrow band pass filter in the infrared and the near-infrared portions of the electromagnetic spectrum to pass. Zettler infers the radiation that passes through the primary band pass of the narrow band pass filter as the difference between the two measurements, i.e., between the signal attained with only the narrow band pass filter and the signal attained with both the narrow band pass filter and the longpass filter. 
         [0005]    Another source of unwanted radiation is the contribution of “stray radiation.” Stray radiation is reflected radiation that is redirected onto the target by the enclosure or other structures therein via inter-reflection and reflected into the line-of-sight of the radiation thermometer. Consider a wafer carrier with GaN wafers that are being heated to an elevated temperature of 800° C. by, for example, a microwave heating process. The components operating at the elevated temperature, such as the wafer carrier and wafers, will emit radiation in all directions, causing radiation to inter-reflect within the chamber. Some of the inter-reflected radiation will be incident on the surface targeted by the radiation thermometer and contribute to the radiation detected by the radiation thermometer. For GaN crystalline layers at 800° C., the reflectance at 410 nm is approximately 0.2. The stray radiation contribution can significantly bias the temperature value indicated by the radiation thermometer. 
         [0006]    Stray radiation is enough of an issue when the target is at or near the maximum temperatures within the chamber, which is the case in microwave heating systems. However, when measuring radiation at or near the short wavelengths of the visible spectrum (i.e., in the blue, violet or ultraviolet wavelengths), the problem becomes exacerbated when there are other sources within the chamber that are operating at substantially higher temperatures than the target. Such a heating arrangement transfers heat in accordance with the first law of thermodynamics, which requires that the resistance heating element operate at a temperature that is significantly higher than the crystalline growth layer. An advantage of thermal radiative heating is that the radiation intensity can be tailored to have a profile across the wafer carrier that promotes uniformity of the temperature. 
         [0007]    Consider, for example, the blackbody radiation of a crystalline growth layer at 800° C. According to Planck&#39;s law, the blackbody spectral emissive power at 410 nm and 800° C. is about 2.0×10 −4  watts/m 2 ·μm. Now consider a heating source such as a resistance heating element that transfers heat to the crystalline growth layer via radiation and convection that operates at 1800° C. The blackbody spectral emissive power at 410 nm and 1800° C. is about 1.4×10 3  watts/m 2 ·μm. That is an increase of about 7 orders of magnitude over the blackbody spectral emissive power at 800° C. (a typical operating temperature for crystalline growth layer during CVD operations) at the wavelength of interest ( FIG. 1 ). Accordingly, even if only a fraction of a percent of the radiation at the 410 nm wavelength finds its way onto the detector of the radiation thermometer, the bias to the indicated temperature can be significant. Thus, the stray radiation contribution in chambers that utilize resistance heating elements can be of the same order of magnitude as the unfiltered radiation contribution identified by Zettler. 
         [0008]    Zettler, however, is silent with respect to the contribution of stray radiation, or the effects of having radiation sources within a chamber that can effectively overwhelm the radiation that is emitted from the target. Rather, Zettler treats the target as though it is freely radiating (i.e., has no reflectance contribution). In fact, a target within a CVD chamber at that is operating at the temperatures required for crystalline growth is not freely radiating. 
         [0009]    A radiation thermometer tailored to reduce the effects of unwanted radiation, not only due to unfiltered radiation, but also due to stray radiation, would be welcomed. 
       SUMMARY OF THE INVENTION 
       [0010]    Various embodiments of the invention utilize a so-called “telecentric” optical arrangement, but in an off-focus manner, to limit the contribution of reflected stray radiation in at least three different aspects. First, in a telecentric optical arrangement, the chief rays that are captured from the target are substantially parallel to the optical axis, which substantially limits the stray radiation contribution, particularly if the target has a strong specular reflectivity component. Second, telecentric optical arrangements can also be tailored so that the solid angle subtended by each point on the target is quite small, which also reduces the contribution of stray radiation. Third, the telecentric optical arrangement can be configured to capture a collimated beam of radiation that is emitted from the target, which further reduces the solid angle of radiation captured by the radiation thermometer while increasing the size of the target (and the subsequent signal-to-noise ratio) to the effective diameter of the forward optical elements. In capturing the collimated beam of radiation, the telecentric optical arrangement is utilized in an “off-focus” manner, i.e., is not used for high quality imaging of the surface of the target. Therefore, the components utilized in the telecentric optical arrangement need not be of the superior quality typically associated with commercially available telecentric lens systems. 
         [0011]    Commercially available telecentric lens systems that are utilized, for example, in machine vision systems to provide clear, crisp images of high magnification. These telecentric lens systems provide uniform magnification of all points within an image, regardless of the location of the point within that image. That is, telecentric lens systems used in machine vision systems provide images that are substantially isometric, as opposed to perspective images that are provided with standard imaging systems. One advantage of commercially available telecentric lens systems is that the isometric image can substantially reduce parallax within the image. 
         [0012]    However, the effective range over which a telecentric lens system can provide an isometric image at a given setting is quite limited. This effective range is commonly referred to as the “telecentric depth” (see, e.g., Petrozzo et al., “Telecentric Lenses Simplify Non-Contact Metrology,” Test &amp; Measurement World, Oct. 15, 2001, p. 5). Thus, a paradigm of telecentric lens systems is that they are operable over a narrow range centered about the object plane. The optical components of a machine vision telecentric lens system are of high quality to provide crisp, clear images over the entire image. Furthermore, commercially available telecentric lens systems typically utilize high quality mountings to provide the ability to adjust the focal depth of the object plane. The precise imaging capabilities of commercially available telecentric lens systems drive up the cost. 
         [0013]    For various embodiments of the invention, the telecentric concept is utilized in a way that machine vision systems are not. In one embodiment, the telecentric optical arrangement configured for focusing at infinity while being arranged only a few centimeters from the target. The advantage of this arrangement is that the radiation from each points on the target have substantially the same angle entering the optical system. High quality imaging and the costly optics associated therewith is not needed because the objective is radiation collection and detection rather than object imagery. That is, the telecentric optical arrangement is utilized in an “off-focus” manner to effectively capture a collimated beam of radiation that is emitted from the target surface. Such an arrangement requires neither high quality imaging optics nor sophisticated mountings for fine tuning an image. 
         [0014]    Structurally, in various embodiments of the invention, an off-focus telecentric optical arrangement includes an aperture stop and a first or “object” optical component assembly (herein referred to as the “object assembly”) of one or more optical components. The aperture stop and object assembly can define an optical axis and a first focal length relative to a reference point within the object assembly, the reference point being located on the optical axis. In one embodiment, the aperture stop is located at a distance from the reference point of the object assembly that is substantially equal to the first focal length of the object assembly. By locating the aperture stop at the focal length of the object assembly, the object assembly is focused effectively at infinity for transfer of substantially collimated radiation from an off-focus target through the object assembly and for focusing of the radiation from the off-focus target onto the aperture stop. 
         [0015]    In some embodiments, a second or “image” optical component assembly (herein referred to as the “image assembly”) of one or more optical components can be located opposite said object assembly from said aperture stop and arranged to receive radiation transferred from the object assembly through the aperture stop along the optical axis, the image assembly defining a second focal length relative to a second reference point within the image assembly, the second reference point being located on the optical axis. 
         [0016]    In one embodiment, a “bilateral” telecentric optical arrangement is implemented, in which the chief rays of both the target and the image are parallel to the optical axis. In the bilateral arrangement, the focal length of the object assembly defines approximately the target distance, the aperture stop being located at essentially the back focal plane of the object assembly and at the front local plane of the image assembly. In a bilateral telecentric arrangement, not only is the radiation collected through the object optical component assembly substantially collimated, but the radiation transferred from the image optical component assembly to the detector is also substantially collimated. An advantage of collimating the radiation between the image optical component assembly and the detector is additional rejection of stray light. 
         [0017]    In various embodiments, the off-focus telecentric optical arrangement is provided as a kit for implementation with a new or an existing radiation thermometer in chemical vapor deposition systems. In one embodiment, a telecentric lens arrangement is provided that includes an aperture stop and a forward optical component assembly for collecting radiation from a target, the telecentric lens arrangement being adapted for positioning the aperture stop at the focal length of the forward optical component assembly. Manufacturer-supplied instructions are also provided that instruct the user to orient the forward optical component assembly to intercept radiation emitted from a target within a chemical vapor deposition chamber. In one embodiment, coupling the telecentric lens arrangement with a radiation detector and/or the positioning the aperture stop at the focal length of the forward optical component assembly is performed by the manufacturer; in other embodiments, the step of positioning the aperture stop at the focal length of the forward optical component assembly and/or positioning the aperture is provided on the manufacturer-supplied instructions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a graph of spectral blackbody emissive power according to Planck&#39;s law at various temperatures; 
           [0019]      FIG. 2  is a sectional view of an off-focus telecentric radiation thermometer operatively coupled to a MOCVD chamber in an embodiment of the invention; 
           [0020]      FIG. 3  is a sectional view of an off-focus telecentric radiation thermometer and light trap operatively coupled to a MOCVD chamber in an embodiment of the invention; 
           [0021]      FIG. 4  is an off-focus telecentric optical arrangement in an embodiment of the invention; 
           [0022]      FIG. 5  is a sectional view of an off-focus telecentric radiation thermometer operatively coupled to a MOCVD chamber utilizing a flow extender in an embodiment of the invention; and 
           [0023]      FIG. 5A  is an enlarged partial sectional view of the MOCVD chamber and flow extender of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring to  FIG. 1 , a family of curves  10  depicting the spectral blackbody emissive power in accordance with Planck&#39;s law at various temperatures is presented. The visible spectral region  12 , coinciding approximately with the 400 nm to 700 nm wavelength band, is also identified in  FIG. 1 . In relation to the previous discussion of the effect of temperature on the blackbody emissive power at 410 nm, first and second reference points  14  and  16  are identified in  FIG. 1  at 1073 K and 2073 K, respectively (corresponding to 800° C. and 1800° C., respectively). 
         [0025]    Referring to  FIGS. 2 and 3 , a MOCVD reactor system  20  utilizing a radiation thermometer  22  having an off-focus telecentric optical arrangement  24  is depicted in embodiments of the invention. The MOCVD reactor system  20  includes a reactor chamber  26  operatively coupled with a flow flange  28  to define an enclosure  30 . The flow flange  28  includes laminar flow plates  31  through which the gases for the MOCVD process are introduced into the reactor chamber  26 . Disposed within the reactor chamber  26  is a wafer carrier  32  having a top surface  34  defining wafer pockets  35  and a bottom surface  36  and operatively coupled with a spindle  38  that defines a rotation axis  40 . A body shutter  42  can be removably inserted adjacent the interior wall of the reactor chamber  26  and surrounds the wafer carrier  32 . 
         [0026]    A resistance heating array  44  is disposed beneath the wafer carrier  32  for radiative coupling with the bottom surface  36  of the wafer carrier  32 . The resistance heating array  44  can include a peripheral heating element  45  and can be surrounded by a cylinder  46  and also bounded beneath with a reflector plate  48  to enhance radiative coupling between the resistance heating array  44  and the wafer carrier  32 . 
         [0027]    The radiation thermometer  22  is mounted atop the flow flange  28  and oriented to view the top surface  34  of the wafer carrier  32  through a view port window  52 . In one embodiment, the view port window  52  is disposed in a recess  54 , which can be actively cooled. 
         [0028]    The off-focus telecentric optical arrangement  24  includes a first or forward optical component assembly  62  (herein referred to as the “object assembly”  62 ) and a second or rearward optical component assembly  64  (herein referred to as the “image assembly”  64 ). The object assembly  62  is characterized as having an effective radial dimension  65  ( FIG. 4 ), i.e., the maximum radial dimension over which the object assembly  62  effectively transfers radiation onto an aperture stop  66 . 
         [0029]    The aperture stop  66  is disposed between the object and image assemblies  62  and  64 . In one embodiment, the object and image assemblies  62  and  64  and the aperture stop  66  are arranged concentrically along an optical axis  68 . The optical axis  68  is the axis about which radiation detected by the radiation thermometer  22  propagates. The optical axis  68  can be straight, such as depicted herein, or can be tortuous, for example when planar or focusing mirrors are implemented for the transfer of radiation. The optical axis  68  can be centered about a target  72 , characterized as having a target area  74 . The radiation thermometer  22  also includes a detector  76  for detecting electromagnetic radiation. 
         [0030]    It is noted that, for purposes of this disclosure, an “optical component assembly” can comprise a plurality of optical components (as depicted), or can comprise a single optical component such as a single lens. While the optical components depicted herein comprise lenses, it is understood that other optical components, such as focusing mirrors and fiber optic bundles can also be utilized to achieve the radiation transfer. 
         [0031]    In one embodiment, the radiation thermometer  22  is oriented so that the optical axis  68  is substantially normal to the top surface  34  of the wafer carrier  32  ( FIG. 2 ). In another embodiment, the radiation thermometer  22  is oriented so that the optical axis  68  is at an acute angle  78  relative to a direction normal to the top surface  34  of the wafer carrier  32  ( FIG. 3 ). In one embodiment, a light trap  82  is arranged at a mirrored angle of the optical axis  68  ( FIG. 3 ) in three-dimensional space. That is, the light trap  82  is arranged to subtend a reflection of the optical axis  68  from a hypothetical mirrored surface at the top surface  34  of the wafer carrier  32 . 
         [0032]    Referring to  FIG. 4 , the off-focus telecentric optical arrangement  24  of the radiation thermometer  22  is described in greater detail. The object assembly  62  is characterized as having a focal length F 1  that is measured from a reference point  84  on the optical axis that is on or within the object assembly  62 . A “focal length” is the distance from the reference point  84  at which rays parallel to the optical axis  68  passing through by the object assembly  62  are focused. For the off-focus telecentric optical arrangement  24 , the aperture stop  66  is positioned at this convergence point, i.e., at the focal length F 1  of the object assembly. 
         [0033]    The off-focus telecentric optical arrangement  24  is further depicted as having a distance L 1  between the image assembly and the aperture stop and a distance L 2  between the image assembly  64  and the detector  76 . The aperture stop  66  is also characterized as having a major dimension  86 . Herein, the “major dimension”  86  is the diameter of a circular aperture or the largest dimension of a non-circular aperture (e.g., the diagonal of a rectangular aperture). 
         [0034]    In one embodiment, the distance L 1  is substantially equal to the focal length of the image assembly  64 , such that the radiation transferred by the image assembly  64  to the detector  76  is substantially collimated. This arrangement is herein referred to as a “bilateral” telecentric optical arrangement. In a bilateral telecentric arrangement, not only is the radiation collected through the object assembly  62  substantially collimated, but the radiation transferred from the image assembly  64  to the detector  76  is also substantially collimated (as depicted in the figures). An advantage of collimating the radiation transferred from the image assembly  64  to the detector  76  is that additional stray light is rejected. Such scattered radiation can originate on the surfaces of the various optical components in the system, as well as off-axis radiation that enters the radiation thermometer  22 . The collimation of the radiation between the image assembly  64  and the detector  76  rejects more of the radiation entering the image assembly  64  at angles that are not parallel to the optical axis  68 . 
         [0035]    In one embodiment, the distance L 2  can also be substantially equal to the focal length of the image assembly  64 . However, L 2  is not constrained to any particular dimension in a bilateral telecentric optical arrangement. 
         [0036]    A ray bundle  88  is characterized as a cluster of rays including a central or “chief” ray  92 , all originating from an infinitesimal point  94  on the target  72 . The ray bundle  88  comprises all the rays originating from the infinitesimal point  94  that are within a solid angle  96  centered about the chief ray  92 . The chief ray  92  is parallel to but offset from the optical axis  68 . Each infinitesimal point  94  within the target area  74  emits a similar bundle of rays that are collected by the object assembly  62 . 
         [0037]    The solid angle  96  is a function of the major dimension  86  and a target distance L 3 , which is the distance from a forward-most surface  98  of the object assembly  62  to the target  72 . The smaller the solid angle  96  of the ray bundle  88 , the closer the rays in the ray bundle  88  are to being parallel with the optical axis  68 , and the greater the rejection of stray light. For a given target distance L 3 , the smaller the major dimension  86  the smaller the solid angle  96 . Also, for a given major dimension  86  of the aperture stop  66 , a longer target distance L 3  will provide a smaller solid angle  96  for enhanced rejection of stray light. Generally, the target distance L 3  is not of specified dimension because of the off-focus, parallel ray collection. Non-limiting examples of the target distance L 3  for MOCVD chambers is less than two meters. In one embodiment, the target distance L 3  is substantially the focal length of the object assembly  62 . In one embodiment, the target distance L 3  is on the order of 200 mm to 300 mm (for example, 250 mm). 
         [0038]    The artisan will recognize that there is a tradeoff between the magnitude of the solid angle  96  required and the magnitude of the target area  74  required to achieve a given signal-to-noise ratio. That is, for a given target distance L 3 , smaller solid angles  96  (e.g., smaller major dimensions  86 ) can be utilized for larger target areas  74 , generally enhancing the rejection of stray radiation, whereas larger solid angles  96  (e.g., larger major dimensions  86 ) are required for smaller target areas  74 . Target size is limited by other factors, including the dimension of the view port window  52 , the effective radial dimension of the image assembly  64 , and the desired field of view of the target  72  on the wafer carrier  32 . Thus, for smaller target areas  74  that require a larger major dimension  86  of the aperture stop  66 , and at shorter target distances L 3 , the stray light rejection of the off-focus telecentric optical arrangement  24  can be vitiated. 
         [0039]    In certain non-limiting embodiments, the major dimension  86  of the aperture stop  66  is approximately ⅓ or less of the effective radial dimension  65  of the object assembly  62 . In one embodiment, the major dimension  86  of the aperture stop  66  is in the range of 1 mm to 20 mm. 
         [0040]    For a typical target of crystalline growth material, inter-reflected radiation that is reflected from the target  72  has a strong specular component. That is, a large fraction of the radiation incident on the surface of the crystalline growth structure will be reflected at the same angle as the angle of incidence. Accordingly, a disproportionate amount of the stray radiation that enters standard radiation thermometers (i.e., one without at telecentric optical arrangement) is reflected off the target  72  at angles that are not parallel to the optical axis  68 . Accordingly, by reducing the solid angle  96  of the ray bundle  88 , the contribution of stray radiation is also substantially reduced. 
         [0041]    Consider the orientation of the radiation thermometer  22  in  FIG. 2 . Radiation that is specularly reflected from the target  72  and into the radiation thermometer  22  must have first been inter-reflected or emitted from the view port window  52 . The viewport can be configured to reduce the amount of reflected radiation therefrom, for example by the use of antireflective coatings and/or by disposing the view port window  52  within the recess  54 , which can be actively cooled, to limit the amount of radiation incident upon the view port window  52 . 
         [0042]    Consider the orientation of the radiation thermometer  22  in  FIG. 3 . The light trap  82 , arranged as generally described and as depicted in  FIG. 3 , functions to trap radiation that would otherwise be incident on the target  72  at the angle of specular reflection for the optical axis  68 . The light trap  82  can also be configured as discussed above—e.g., with an antireflective window within a recess  54 —to limit transfer of inter-reflected radiation onto the target  72 . 
         [0043]    To verify the theory of operation of the off-focus telecentric optical arrangement  24 , the geometry and operating conditions of the enclosure  30  as substantially depicted in  FIG. 2  and described herein was modeled using the Advanced System Analysis Program (ASAP), a three-dimensional ray tracing program provided by the Breault Research Organization, Inc. of Tucson, Ariz., U.S.A. The ASAP model was executed to identify stray radiation paths and to analyze stray radiation entering the view port window  52 . The peripheral heating element  45  was set as a radiation source operating at a temperature of 1800° C. The wafer carrier  32  (modeled as including wafers in the wafer pockets  35 ) was modeled as both a radiation source at 800° C. and as a scattering media. The wafer pockets  35  were assumed to be carrying wafers having an emittance of 0.8 at the wavelengths of interest. The blackbody emissive power for the radiation sources was established based on Planck&#39;s law at a wavelength of 405 nm. The interior walls of the enclosure  30  (including the body shutter  42 , laminar flow plates  31  and view port window  52  were also modeled as scattering media. 
         [0044]    The radiation thermometer  22  was modeled for two different collection optics: a “standard” optical arrangement having a target diameter of 10 mm at a 1:1 magnification; and the off-focus telecentric optical arrangement as depicted and described herein, with a target diameter of approximately 30 mm. The amount of 405 nm radiation emitted from the target  24  and directly entering the radiation thermometer  22  (“signal radiation”) was compared with the amount of 405 nm radiation inter-reflected within the enclosure  30  and entering the radiation thermometer  22  (“stray radiation”) for each optical arrangement. The results are presented in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 ASAP model results predicting the fraction of radiation wavelength 
               
               
                 attributed to signal and stray radiation at 405 nm for a standard and 
               
               
                 a telecentric optical arrangement (target at 800° C.; peripheral 
               
               
                 heating element at 1800° C.) 
               
             
          
           
               
                   
                   
                 Off-Focus 
               
               
                   
                 Standard Optics 
                 Telecentric Optics 
               
               
                   
                   
               
             
          
           
               
                   
                 Signal Radiation 
                 30% 
                 61% 
               
               
                   
                 Stray Radiation 
                 70% 
                 39% 
               
               
                   
                 Temperature Error at 
                 41° C. 
                 16° C. 
               
               
                   
                 405 nm wavelength 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    The ASAP model predicted that, for a radiation thermometer utilizing standard optics, about 70% of the radiation flux at the 405 nm wavelength on the detector is attributed to stray radiation. Utilization of the off-focus telecentric optical arrangement  24 , however, reduced the stray radiation contribution to 39%. These stray light contributions cause temperature bias errors of approximately 41° C. and 16° C., respectively. That is, the bias error of the temperature measurement for the off-focus telecentric optical arrangement  24  is almost ⅔ less for the off-axis telecentric optical arrangement than for a standard lens system. 
         [0046]    Referring to  FIGS. 5 and 5A , the performance of the off-focus telecentric optical arrangement  24  was also verified experimentally. For this experiment, a MOCVD reactor system utilizing a flow extender  120 . The flow extender  120  includes an upper end  122  that extends above the top surface  34  of the wafer carrier  32  and is attached to the body shutter  42  using connectors  124 . Flow extenders are utilized to improve the flow and thermal characteristics crystalline growth environment, but also tend to dramatically increase the stray radiation signal received by radiation thermometers viewing the wafer carrier and wafers. The reactor system was operated at approximately 800° C. for an extended period of time with the wafer carrier (including GaN crystalline growth material in the wafer pockets) so that the thermal environment within the enclosure was at quasi-steady state (i.e., the components of the MOCVD reactor system were thermally saturated). A measurement with a radiation thermometer was made with the resistance heating array energized. Then, the power to the resistance heating array was cut, and a second measurement made with the radiation thermometer within a ten second time period. At the 405 nm wavelength, the stray radiation from the resistance heating array ceases almost immediately upon the power being cut, whereas the target continues to emit radiation at essentially the same emissive power as immediately prior to the power termination because of the thermal capacitance of the target. Accordingly, it was assumed that the first measurement included a stray radiation component from the resistance heating array at the 405 nm wavelength, whereas the second measurement did not. The experiment was performed for both a standard optical pyrometer utilizing a standard in-focus optical arrangement, as well as for a radiation thermometer utilizing the off-focus telecentric optical arrangement  24 . Both radiation thermometers were operated nominally at the 405 nm wavelength. The results are presented in Table 2. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Measurement results estimating the fraction of radiation wavelength 
               
               
                 attributed to signal and stray radiation at 405 nm for a standard and 
               
               
                 a telecentric optical arrangement (target at 800° C.; 
               
               
                 chamber with flow extender) 
               
             
          
           
               
                   
                   
                 Off-Focus 
               
               
                   
                 Standard Optics 
                 Telecentric Optics 
               
               
                   
                   
               
             
          
           
               
                   
                 Signal Radiation 
                 36% 
                 69% 
               
               
                   
                 Stray Radiation 
                 64% 
                 31% 
               
               
                   
                 Temperature Error at 
                 34° C. 
                 12° C. 
               
               
                   
                 405 nm wavelength 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    The measurement results show that, for a radiation thermometer utilizing standard optics, about 64% of the radiation flux at the 405 nm wavelength on the detector is attributed to stray radiation. Utilization of the off-focus telecentric optical arrangement  24 , on the other hand, reduced the stray radiation contribution to about 31%. These stray light contributions cause temperature bias errors of approximately 34° C. and 12° C., respectively. Again, the bias error of the temperature measurement for the off-focus telecentric optical arrangement  24  is about ⅔ less for the off-axis telecentric optical arrangement than for a standard lens system. 
         [0048]    In one embodiment, the detector  76  comprises a photon counter (i.e., photomultiplier tube, or PMT) having a cut off wavelength of 700 nm, and thus is insensitive to infrared radiation. Therefore, the use of a PMT as the detector largely eliminates the concerns of inadequate filtering in the infrared portion of the spectrum identified in Zettler. A filtering device  102  can be used to filter the PMT so that only wavelengths primarily in the region of blue, violet or ultraviolet light are detected. 
         [0049]    Another advantage of the PMT is the fast time response that it provides, which is a factor for CVD chambers that utilize high rotational speeds of the wafer carrier, such as the TURBODISC systems manufactured by Veeco Instruments of Somerset, N.J., U.S.A. The TURBODISC system is described generally at Mitrovic, et. al., “Reactor Design Optimization Based on 3D CFD Modeling of Nitrides Deposition in MOCVD Vertical Rotating Disc Reactors,” June 2005 (available at http://www.wpi.edu/academics/che/HMTL/CFD_in_CRE_IV/Mitrovic.pdf, last visited 16 Jun. 2012). The high rotation rates of such systems can require data acquisition rates from the radiation detector  76  on the order of 10 kHz, which PMTs can provide. 
         [0050]    Non limiting examples of the spectra transmitted by the filtering device includes a center wavelength in the range of 380 nm to 420 nm and a band width (full width at half maximum) in the range of 10 nm to 70 nm. In one embodiment, the filtering device  102  further comprises a band pass filter in combination with a colored glass filter. A non-limiting example of a filter combination is the 10BPF25-400 band pass filter from Newport (center wavelength of 400±3.5 nm; full width at half maximum of 25±3.5 nm) with a FGB25 colored glass filter from Thorlabs (local cut-off wavelength of 400 nm), which combine to define a primary band pass that passes radiation nominally in the 390 nm to 420 nm band pass. 
         [0051]    In one embodiment, a non-limiting example of the component sizing and layout for the off-focus telecentric optical arrangement  24  includes: the object assembly  62  comprising a plano-convex lens having a 50.8 mm diameter and a focal length of 249.2 mm (e.g., LA1301-A from Thorlabs, Inc.) located a distance F 1  of 249.2 mm from the aperture stop  66 ; the image assembly  64  comprising a plano-convex lens having a 25.4 mm diameter and a focal length of 75.0 mm (e.g., LA1608-A from Thorlabs, Inc.) located at a distance L 1  of 75 mm from the aperture stop  66  and a distance L 3  of 75 mm from the detector. In another embodiment, the object assembly  62  further comprises an achromatic doublet having a 50.8 mm diameter and a focal length of 100 mm (e.g., AC508-100-A from Thorlabs) in combination with the plano-convex lens above to shorten the focal length F 1  of the object assembly to approximately 87 mm, as well as to shorten the overall length of the assembly. In this latter arrangement, an achromatic doublet having a shorter focal length (e.g., 30 mm) can, for example, be utilized as the image assembly  64  for closer proximity to the aperture (e.g., AC254-030-A from Thorlabs). 
         [0052]    The lenses in the above-referenced examples can comprise any material suitable for transmitting radiation in the visible and/or ultraviolet portions of the electromagnetic spectrum, such as borosilicate glass, barium fluoride and fused silica. They can also be coated with antireflective coatings. 
         [0053]    Alternatively, other filtering devices and techniques can be implemented in combination with the off-focus telecentric optical arrangement  24  presented herein. For example, the detector and filtering arrangement Zettler can be implemented. In some embodiments, a water cooled CCD or a solid state detector such as an avalanche photodiode can be utilized. 
         [0054]    In operation, the wafer carrier is rotated about the rotation axis  40  while being radiantly heated by the heating array  44 . The rotation rate of the wafer carrier  32  about the rotation axis  40  can vary substantially, depending on the operating parameters and design criteria of the MOCVD reactor system  20 . PMT 
         [0055]    The radiation thermometer  22  and off-focus telecentric optical arrangement  24  of the invention is not limited to systems that implement heating sources other than resistance-type heaters. The various embodiments of the invention can be utilized, for example, some CVD reactor systems utilize microwave heating sources. 
         [0056]    The following references are hereby incorporated by reference in their entirety except for express definitions and patent claims contained therein: U.S. Patent Application Publication No. 2011/0064114 to Zettler et al.; Petrozzo et al., “Telecentric Lenses Simplify Non-Contact Metrology,” Test &amp; Measurement World, Oct. 15, 2001; Mitrovic, et. al., “Reactor Design Optimization Based on 3D CFD Modeling of Nitrides Deposition in MOCVD Vertical Rotating Disc Reactors,” June 2005 (available at http://www.wpi.edu/academics/che/HMTL/CFD_in_CRE_IV/Mitrovic.pdf, last visited 16 Jun. 2012); co-pending U.S. patent application Ser. No. ______ entitled “Control of Stray Radiation in a CVD Chamber,” naming Guray Tas and Jing Zhou inventors and filed on even date herewith. 
         [0057]    References to relative terms such as upper and lower, front and back, left and right, or the like, are intended for convenience of description and are not contemplated to limit the invention, or its components, to any specific orientation. All dimensions depicted in the figures may vary with a potential design and the intended use of a specific embodiment of this invention without departing from the scope thereof. 
         [0058]    Each of the additional figures and methods disclosed herein may be used separately, or in conjunction with other features and methods, to provide improved devices, systems and methods for making and using the same. Therefore, combinations of features and methods disclosed herein may not be necessary to practice the invention in its broadest sense and are instead disclosed merely to particularly describe representative embodiments of the invention. 
         [0059]    For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in the subject claim.