Abstract:
Systems and methods for measuring an intensity characteristic of a light beam are disclosed. The methods include directing the light beam into a prism assembly that includes a thin prism sandwiched by two transparent plates, and reflecting a portion of the light beam by total-internal-reflection surface to an integrating sphere while transmitting the remaining portion of the light beam through the two transparent plates to a beam dump. The method also includes detecting light captured by the integrating sphere and determining the intensity characteristic from the detected light.

Description:
FIELD 
     The present disclosure relates to measuring the intensity of light beams, and in particular relates to systems and methods for measuring at least one intensity characteristic of a high-intensity light beam. 
     BACKGROUND 
     High-intensity (or high-optical-power) light beams are used in many applications, including thermal processing of semiconductor substrates. Most applications require the high-intensity light beam to have a well-defined intensity profile. For example, in laser annealing applications, the high-intensity beam forms a line image and has a generally Gaussian intensity distribution along the short axis and a generally uniform intensity distribution along the long axis. Typical dimensions for the line image used in thermal processing of semiconductor substrates are hundreds of microns in width (short axis) by a few tens of millimeters in length (long axis). The amount of power in such line images can reach a few kilowatts. 
     It is difficult to accurately measure intensity characteristics, such as an intensity profile, of such high-intensity light beams because the light beam damages the measurement apparatus. One type of measurement apparatus uses image sensors and an attenuator to attenuate the high-intensity beam down to a reasonable (non-damaging) power level and then directs the attenuated light beam to a photodetector such as a CCD or CMOS camera. 
     Unfortunately, this approach suffers from significant inaccuracies because attenuation is always accompanied by aberrations and because measurements at low power do not accurately represent the intensity profile distributions realized at the high power at which the light beam is actually used. 
     Another type of measurement approach is based on scanning the high-intensity light beam past a narrow aperture (e.g., a slit aperture) formed by opposing blades. When high-power densities are involved, however, the measurement needs to be carried out at low-power settings. This reduces the measurement accuracy for essentially the same reason as the image-sensor-based measurement methods. On the other hand, trying to measure the intensity profile at high power to get an accurate measurement typically results in overheating and thereby damaging the blades. Thermal expansion of the blade material can also change the size and/or shape of the slit opening and compromises the measurement. This can occur even at low power. 
     Measurements of the short-axis intensity profile of a line-forming light beam are even more challenging than the long-axis measurements because scanning in the short axis direction requires a very small slit or even a pinhole. Thermal expansion of a small aperture is more pronounced than thermal expansion of a large aperture. For this reason, measurements of the intensity profile along the short axis of a line-forming light beam are usually done using cameras with a large degree of attenuation or even below the laser threshold. But, as noted above, the measurement accuracy suffers. 
     SUMMARY 
     Aspects of the disclosure are directed to measuring at least one intensity characteristic of a high-intensity light beam. Example intensity characteristics include an intensity profile (power/unit area as a function of at least one spatial coordinate), an overall or total intensity (power/unit area), and optical power (intensity x area). In the discussion, the term “power” means “optical power” unless otherwise noted. 
     An aspect of the disclosure is a method of measuring an intensity characteristic of a light beam. The method includes: directing the light beam into a prism assembly that includes a thin prism sandwiched by two transparent plates, wherein the thin prism has a width d and a total-internal-reflecting (TIR) surface having an area; reflecting a portion of the light beam by the TIR surface to an integrating sphere while transmitting the remaining portion of the light beam through the two transparent plates to a beam dump; detecting a portion of the light captured by the integrating sphere; and determining an intensity characteristic of the light beam from the detected light. 
     Another aspect of the disclosure is the method as described above, wherein the detection of the portion of the light captured by the integrating sphere measures an amount of optical power, and further includes determining an intensity by dividing a measured amount of optical power by the area of the TIR surface. 
     Another aspect of the disclosure is the method as described above, further comprising repeating the acts therein to measure an intensity for different sections of the light beam to determine an intensity profile for the light beam. 
     Another aspect of the disclosure is the method as described above, further comprising translating the light beam relative to the prism assembly to measure the intensity for the different sections of the light beam. 
     Another aspect of the disclosure is the method as described above, wherein the light beam comprises a line-forming beam that forms a line image. 
     Another aspect of the disclosure is the method as described above, wherein the width d of the TIR prism is in the range from 0.05 mm to 1 mm. 
     Another aspect of the disclosure is the method as described above, wherein the transparent plates have a substantially pentagonal shape and the thin prism has a substantially trapezoidal shape. 
     Another aspect of the disclosure is the method as described above, wherein the thin prism has input and output surfaces configured so that the light beam passes through the input surface at substantially a right angle and the reflected portion of the light beam passes through the output surface at substantially a right angle. 
     Another aspect of the disclosure is the method as described above, wherein the input and output surfaces of the thin prism are coated with an anti-reflection coating. 
     Another aspect of the disclosure is the method as described above, wherein the thin prism and transparent plates have light-transmitting surfaces and wherein the light-transmitting surfaces are coated with an anti-reflection coating. 
     Another aspect of the disclosure is the method as described above, wherein directing the light beam into a prism assembly includes focusing the light beam so that it substantially focuses at the TIR surface. 
     Another aspect of the disclosure is the method as described above, wherein the light beam has an amount of optical power between 10 W and 5 kW. 
     Another aspect of the disclosure is a system for measuring an intensity characteristic of a light beam. The system includes: a prism assembly arranged to receive the light beam at an input side, the prism assembly including a thin prism sandwiched by two transparent plates, wherein the thin prism has a width d and a total-internal-reflecting (TIR) surface, wherein the TIR surface reflects a portion of the light beam, thereby defining an unreflected portion of the light beam; an integrating sphere arranged adjacent a first output side of the prism assembly to receive the reflected portion of the light beam; a beam dump arranged adjacent a second output side of the prism assembly and arranged to receive the unreflected portion of the light beam; a photodetector operably arranged relative to the integrating sphere and adapted to measure an amount of optical power received by the integrating sphere and generate an electrical detector signal representative of the measured amount of optical power; and a processor electrically connected to the photodetector and that includes instructions embodied in a computer-readable medium that cause the processor to determine the intensity characteristic of the reflected portion of the light beam. 
     Another aspect of the disclosure is the system as described above, wherein the TIR surface has an area and wherein the processor determines an intensity by dividing the measured amount of optical power by the TIR surface area. 
     Another aspect of the disclosure is the system as described above, wherein the width d of the TIR prism is in the range from 0.05 mm to 1 mm (i.e., 0.05 mm≦d≦1 mm). 
     Another aspect of the disclosure is the system as described above, further comprising a movable stage that movably supports the prism assembly relative to the light beam. 
     Another aspect of the disclosure is the system as described above, wherein the transparent plates have a substantially pentagonal shape and the thin prism has a substantially trapezoidal shape. 
     Another aspect of the disclosure is the system as described above, wherein the thin prism has input and output surfaces configured so that the light beam passes through the input surface at substantially a right angle and the reflected portion of the light beam passes through the output surface at substantially a right angle. 
     Another aspect of the disclosure is the system as described above, wherein the thin prism and transparent plates have light-transmitting surfaces and wherein the light-transmitting surfaces are coated with an anti-reflection coating. 
     Another aspect of the disclosure is the system as described above, wherein the light beam is substantially focused at the TIR surface. 
     Another aspect of the disclosure is the system as described above, further comprising means for scanning the light beam relative to the prism assembly. 
     Another aspect of the disclosure is the system as described above, wherein the light beam has an amount of optical power between 100 W and 5 kW. 
     Additional features and advantages will be set forth in the Detailed Description that follows and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims thereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary and are intended to provide an overview or framework for understanding the nature and character of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding and are incorporated into and constitute a part of this specification. The drawings illustrate one or more embodiment(s) and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
         FIGS. 1A and 1B  are diagrams of two example embodiments of light-beam-intensity measuring systems according to the disclosure; 
         FIG. 2  is a close-up view of an example line-forming beam and the focused line image it forms at an image plane; 
         FIGS. 3A and 3B  are exploded and assembled views, respectively, of an example prism assembly; 
         FIG. 4A  is a view of the prism assembly that includes a ray trace of an example line-forming beam and the portions of the line-forming beam that are transmitted and reflected by the prism assembly; 
         FIG. 4B  is a front-end view of the prism assembly showing a defocused line image at the front side of the TIR prism assembly; 
         FIG. 4C  is similar to  FIG. 4B  and shows a rotated and defocused line image at the front side of the TIR prism assembly; 
         FIGS. 5A and 5B  are cross-sectional views of the plates and the TIR prism, respectively, of the TIR prism assembly illustrating how one portion of the light beam is transmitted through the plates while another portion is reflected by the TIR surface of the prism; 
         FIG. 6A  is an example intensity profile of a line image formed by a line-forming beam; 
         FIG. 6B  is similar to  FIG. 6A  and shows the slot as defined by the TIR prism assembly superimposed on a portion of the line-image intensity profile; 
         FIG. 7  is a plot of Intensity (relative units) versus x (mm) for different values of the slot width d, along with the exact profile for the line image of  FIG. 6A , with a slot tilt angle α=90°; 
         FIG. 8  is similar to  FIG. 6B  and shows a different slot size and slot tilt angle α; 
         FIGS. 9A and 9B  plot the relative intensity versus Y (mm) for a fixed slot angle α=5 degrees and a different slot width d ( FIG. 9A ) and for a fixed slot width of d=0.25 mm and different slot angles α; 
         FIG. 10A  is similar to  FIG. 8  and shows an example of an alternative scan direction in the x-direction; and 
         FIG. 10B  is a plot of the relative intensity versus x·tan(α) showing the horizontal scan results, the vertical scan results, and the exact intensity distribution. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
     The claims as set forth below are incorporated into and constitute a part of this Detailed Description. 
     The entire disclosure of any publication or patent document mentioned herein is incorporated by reference. 
     Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation. 
       FIGS. 1A and 1B  are diagrams of two example embodiments of a light-beam-intensity measuring system (“system”)  10 . The system  10  is configured to receive and process a relatively high-intensity or high-optical-power light beam (“light beam”)  22 , e.g., a light beam having an optical power of at least 10 W and in a further example of up to 5 kW. 
       FIG. 1A  shows an example of how light beam  22  can be formed. A light source  12  emits an initial light beam  14  of a wavelength λ along an optical axis A1. The light source  12  may include a laser, such as a diode laser, that emits the high-power initial laser beam  14 , where the wavelength λ is nominally between 0.8 and 1 microns. A beam-forming optical system  20  receives the initial light beam  14  and forms light beam  22  to be used for a given application, such as laser annealing or another type of material processing. 
     In an example embodiment, light beam  22  has a location along the length (i.e., direction of travel) at which it is narrowest, i.e., the light beam forms a line image  24  at an image plane IP, as illustrated in the close-up view of  FIG. 2 . In the example shown in  FIG. 2 , light beam  22  is a convergent or focused light beam that forms line image  24  of a length L in the X-direction and a width W in the Y-direction. 
     In the discussion below, image  24  in the form of a line (i.e., a “line image”) is used by way of non-limiting example to facilitate the discussion of the systems and apparatus disclosed herein. When initial light beam  14  forms a line image, it is referred to herein as a “line-forming beam.” Other types of light beams and images, including low-intensity light beams and images, or light beams and images of other shapes, can also be measured in analogous fashion. The systems and methods disclosed herein are advantageous for measuring the intensity of high-intensity light beams because the aforementioned adverse effects are generally avoided. 
     With reference again to  FIG. 1A , system  10  also optionally includes along axis A1 a fold mirror FM that serves to fold system  10  to define a first folded optical axis A1′ and that makes the system more compact. In an example, fold mirror FM is adjustable so that the direction in which folded optical axis A1′ extends is also adjustable. This allows for some flexibility in where system  10  can be located relative to the direction of travel of light beam  22 . In an example, fold mirror FM is configured to provide a known amount of reflectivity for a given wavelength λ or for a given wavelength band Δλ associated with light beam  22 . For convenience, it is assumed that fold mirror FM causes only minimal attenuation of light beam  22 . 
     The system  10  includes along optical axis A1′ a TIR prism assembly  50 , which is shown in an exploded view in  FIG. 3A  and in an assembled view in  FIG. 3B . An example TIR prism assembly  50  has five sides  52 - 56  (i.e., is substantially pentagonal). The TIR prism assembly  50  includes a thin planar TIR prism  51 B that in an example includes four sides  52 B through  55 B (e.g., is substantially trapezoidal). The TIR prism  51 B has a thickness d, which in an example is in the range from 0.05 mm to 1 mm and in another example is 0.25 mm to 1 mm. The TIR prism  51 B is sandwiched between two plates  51 A and  51 C, which in an example are substantially transparent to light beam  22 . 
     In an example, TIR prism  51 B and plates  51 A and  51 C are made of silica. For convenience of polishing and coating, plates  51 A and  51 C can be shaped similarly to TIR prism  51 B and can be optically contacted or glued to the TIR prism to form TIR prism assembly  50 , thereby eliminating the need for adhesives, which may cause damage to the TIR prism assembly by absorbing some of light beam  22 . 
     In an example, transparent plates  51 A and  51 C have five sides  52 A- 56 A and  52 C- 56 C, respectively. In an example, TIR prism assembly  50  is configured so that TIR prism surface  55 B defines a TIR surface having an area A, as described in greater detail below. The TIR surface  55 B also defines a second folded optical axis A1″. 
     Also in an example, TIR prism assembly  50  is configured so that sides  52 A,  52 B and  52 C reside in a common plane at side  52 . The TIR prism assembly  50  is arranged so that side  52  defines an input side, while sides  54  and  55  define first and second output sides. In an example, first and second output sides  54  and  55  are at right angles to axes A1″ and A1′, respectively. 
     With reference again to  FIG. 1A , system  10  includes a beam dump  80  arranged along axis A1′ adjacent second output side  55 . The system  10  also includes a photodetector system  70  arranged along folded optical axis A1″. In an example, photodetector system  70  includes an integrating sphere  71  having an input aperture  72  and an interior  73 . A photodetector  74  is operably arranged to measure diffused light  22 D within interior  73  of integrating sphere  71  and in response generate an electrical detector signal SD that is representative of the detected light. 
       FIG. 1B  is a top view of system  10 , with TIR prism assembly  50  oriented as in  FIG. 1A , and further illustrates an embodiment that utilizes two integrating spheres  71 , denoted  71 A and  71 B. The integrating sphere  71 A is used to measure the result of a horizontal scan, while integrating sphere  71 B measures light that is totally internally-reflected during a vertical scan. For a vertical scan, fold mirror FM, TIR prism assembly  50 , and beam dump  80  are rotated by 90 degrees. This automatically re-directs useful totally internally reflected light  22 B into integrating sphere  71 B, while integrating sphere  71 A is idle during this measurement. 
     The system  10  also includes a processor  100 , shown in the form of a computer, that receives and processes detector signals SD. In an example, processor  100  includes instructions embodied in a computer-readable medium that cause the processor to perform certain calculations as described below. 
     Method of Operation 
     In the operation of system  10 , light beam  22  is either directed by fold mirror FM to be incident upon TIR prism assembly  50  at input side  52  or is directly incident thereon. In an example where light beam  22  is converging, the light beam focuses down to form line image  24  at TIR surface  55 B. Thus, as best seen in  FIG. 3B  and  FIG. 4A , a defocused line image  24 ′ is formed at input side  52  and at second output side  55  of TIR prism assembly  50 . This situation has the benefit of reducing the energy density at the input and output sides  52  and  55  of TIR prism assembly  50 , which reduces the chance of damaging these sides. 
     The TIR surface  55 B of TIR prism  51 B is angled to reflect by TIR a relatively small portion  22 B of light beam  22  to travel along axis A1″ and through side  54 B at first output side  54  of TIR prism assembly  50 . The amount of light in reflected light beam portion  22 B is defined by width d of TIR prism  51 B. The TIR prism  51 B can be thought of as defining a slot of width d that passes (at side  52 B) and then reflects (at TIR surface  55 B) light beam portion  22 B. 
     To measure an intensity profile of light beam  22 , the light beam is translated relative to TIR prism assembly  50  to make multiple measurements of the light beam until the desired amount of the light beam has been sampled. This can be accomplished by translating light beam  22  (arrow AW1,  FIG. 4B ), by translating TIR prism assembly  50  (arrow AW2), or by a combination of these translations. In addition, beam-forming optical system  20  may be configured to translate light beam  22 , or light source  12  may be configured to translate initial light beam  14  and thus translate light beam  22 . 
     In an example, movable stages  120  can be operably arranged relative to one or more of light source  12 , beam-forming optical system  20  and TIR prism assembly  50  to translate light beam  22  relative to the TIR prism assembly. In other examples, one or more movable stages  120  are used to rotate light beam  22  relative to TIR prism  51 B so that different azimuths (as indicated by azimuth angle α) of the light beam can be sampled, as illustrated in  FIG. 4C . 
       FIG. 5A  illustrates how portions  22 A and  22 C of light beam  22  travel directly through transparent plates  51 A and  51 C, while  FIG. 5B  illustrates how light beam portion  22 B reflected by TIR surface  55 B is directed out of side  54 B and into integrating sphere  71  via aperture  72 . In example embodiments, anti-reflection coatings AR are employed on one or more of sides  52 A,  52 C and  55 A,  55 C of transparent plates  51 A and  51 C and on sides  52 B and  54 B of TIR prism  51 B to optimize optical transmission. 
     Intensity Calculations 
       FIG. 6A  shows an example line image  24  as a two-dimensional intensity distribution (intensity profile) I H =I(x,y) with contours of equal intensity. The length L x  and width W=L y  are shown as corresponding to a rectangular approximation (dark, dashed line RA) based roughly on the third-smallest intensity contour. 
       FIG. 6B  is similar to  FIG. 6A  but also shows a slit aperture denoted as  55 B because it is effectively defined by the TIR surface. Slit aperture  55 B is centered at position (χ, η) in an X-Y coordinate system and tilted by a with respect to X-axis. The light in line image  24  that is transmitted through slit aperture  55 B (or more accurately, that is reflected by TIR surface  55 B) enters integrating sphere  71 , where it forms diffused light  22 D. A portion of diffused light  22 D is measured by photodetector  74 . 
     The measured power P is given by: 
     
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 
                   χ 
                   , 
                   η 
                   , 
                   α 
                 
                 ) 
               
             
             = 
             
               
                 C 
                 · 
                 
                   
                     ∫ 
                     ∫ 
                   
                   
                     S 
                     ⁡ 
                     
                       ( 
                       
                         χ 
                         , 
                         η 
                         , 
                         α 
                       
                       ) 
                     
                   
                 
               
               ⁢ 
               
                 I 
                 ⁡ 
                 
                   ( 
                   
                     x 
                     , 
                     y 
                   
                   ) 
                 
               
               ⁢ 
               
                 ⅆ 
                 x 
               
               ⁢ 
               
                 
                   ⅆ 
                   y 
                 
                 . 
               
             
           
         
       
     
     The power density ρ is thus defined by P/A, or: 
                       ρ   ⁡     (     x   ,   y   ,   α     )       =         C   ·       ∫   ∫       S   ⁡     (     x   ,   y   ,   α     )           ⁢     I   ⁡     (     χ   ,   η     )       ⁢     ⅆ   χ     ⁢     ⅆ   η       A       ,           (   1   )               
where A is the aforementioned area of TIR surface  55 B, and S is the shape function of the TIR surface (e.g., rectangular). The power density ρ(x, y, α) approaches C·I(x, y) when the shape function S is a small pinhole.
 
     For a traditional measurement of an intensity profile along the X-axis (i.e., the long axis) of line image  24 , TIR prism input side  52 B is oriented normal to the X-axis, is centered on Y-axis, is scanned parallel to the y-axis (α=π/2, y=0∀x), and covers all the width (short dimension) of the line image. The measurement result represents an approximation to the distribution in the long axis: 
     
       
         
           
             ρ 
             ⁡ 
             
               ( 
               
                 x 
                 , 
                 0 
                 , 
                 
                   π 
                   2 
                 
               
               ) 
             
           
         
       
     
     In the simplest but most common case, representing almost all useful applications, the intensity distribution is separable, i.e., I(x, y)≈i x (x)·i y (y) so that: 
     
       
         
           
             
               
                 ρ 
                 ⁡ 
                 
                   ( 
                   
                     x 
                     , 
                     0 
                     , 
                     
                       π 
                       2 
                     
                   
                   ) 
                 
               
               ⁢ 
               
                 ⟶ 
                 
                   d 
                   → 
                   0 
                 
               
               ⁢ 
               C 
             
             · 
             
               
                 
                   i 
                   x 
                 
                 ⁡ 
                 
                   ( 
                   x 
                   ) 
                 
               
               . 
             
           
         
       
     
       FIG. 7  plots the long-axis intensity distribution I(x) versus x (mm) for slit widths d of 1 mm, 0.7 mm, 0.4 mm and an ideal (exact) profile. 
       FIG. 8  is similar to  FIG. 6B  and illustrates an example of scanning slit aperture  55 B in the Y-direction. Such a scan presents a signal described by the function ρ(0, y, α) as given by Eq. (1). 
     If again I(x, y)≈i x (x)·i y (y), then it follows that: 
     
       
         
           
             
               
                 
                   
                     
                       ρ 
                       ⁡ 
                       
                         ( 
                         
                           0 
                           , 
                           y 
                           , 
                           α 
                         
                         ) 
                       
                     
                     ⁢ 
                     
                       ⟶ 
                       
                         
                           d 
                           → 
                           0 
                         
                         , 
                         
                           α 
                           → 
                           0 
                         
                       
                     
                     ⁢ 
                     C 
                   
                   · 
                   
                     
                       
                         i 
                         Y 
                       
                       ⁡ 
                       
                         ( 
                         y 
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     This provides a recipe for measuring an intensity distribution in the vertical axis. The maximum permissible sizes of d and α are dictated by the specified accuracy of the measurement. 
       FIG. 9A  plots the relative intensity I(y) versus y (mm) for an angle α=5 degrees and for values of d of 0.7 mm, 0.5 mm and 0.25 mm, as well as for the ideal profile (d→0 mm).  FIG. 9B  is similar to  FIG. 9A  and plots the relative intensity I(y) versus Y (mm) for d=0.25 mm and for values of angle α of 7 degrees, 5 degrees and 2 degrees, as well as for the ideal profile. 
     From  FIGS. 9A and 9B , it is evident that if the light beam width is about 1 mm, then a slit measurement yields a relatively small systematic error, which can be taken into account in the final intensity measurement. 
     The system  10  can be simplified by recognizing that the results of a vertical (y) scan can be computed from a horizontal (x) scan. This means that the measurement setup requires only one translation stage. These scans are completely equivalent; one needs only to substitute the coordinate x with x·tan(α).  FIG. 10A  shows an example of an alternative scan direction in the X-direction, while  FIG. 10B  shows an example plot of the relative intensity versus x·tan(α). 
     It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.