Patent Publication Number: US-2003222209-A1

Title: Compact, large angle beam stabilization module

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The subject matter disclosed generally relates to the field of laser beam stabilization modules.  
       [0003] 2. Background Information  
       [0004] Manufacturing process equipment may contain a laser to perform work on a piece part. For example, semiconductor fabrication equipment utilize lasers to perform photolithographic processes. Fabrication equipment may be subjected to vibration loads that vary the location of the laser beam and reduce the accuracy of the process. Additionally, commercial lasers are subject to drift which also changes the position of the laser beam. It is desirable to integrate a stabilization system into the equipment that will maintain a desired location of the laser beam.  
       [0005]FIG. 1 shows a laser beam stabilization system  1  of the prior art. The stabilization system  1  maintains a position of a laser beam  2  emitted by a laser  3  and reflected by bending mirrors  4 . The system  1  includes a first fast steering mirror (FSM)  5  and a second fast steering mirror  6  that can adjust the position of the laser beam  2 . Each FSM  5  and  6  includes a reflective mirror  7  that is pivoted by a plurality of actuators  8 .  
       [0006] The actuators  8  receive input signals from a controller  9 . The controller  9  receives error signals from a pair of photodetectors  10  and  11 . A portion of the laser beam  2  is reflected onto the photodetectors  10  and  11  by mirrors  12 . Photodetector  10  is used to determine a displacement error in the location of the laser beam  2 . Photodetector  11  is used to determine a tilt error in the location of the laser beam  2 .  
       [0007] The first FSM  5  is pivoted to correct for any undesired lateral displacement of the laser beam  2 . The second FSM  6  is then pivoted to correct for an undesired angle or tilt in the laser beam  2 . Unfortunately, the second FSM  6  must compensate for not only the tilt error in the laser beam but the additional tilt angle created by the pivoting first FSM  5 . This reduces the dynamic range for correcting tilt error. The tilt of the first FSM  5  can be minimized by placing the second FSM  6  far away from FSM  5 , but this increases the size of the stabilization system  1 . Additionally, the residual error in the system is also increased by the larger input error to the second FSM  6 .  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] A stabilization module for a light beam that travels along an optical path. The module includes a tilt compensation device and a displacement compensation device located along the optical path. A tilt feedback assembly is coupled to the tilt compensation device. A displacement feedback assembly is coupled to the displacement compensation device.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a schematic of a stabilization module of the prior art;  
     [0010]FIG. 2 is a schematic of a stabilization module;  
     [0011]FIG. 3 is a schematic of an embodiment of the stabilization module; and,  
     [0012]FIG. 4 is a schematic of an alternate embodiment of the stabilization module;  
     [0013]FIG. 5 is a schematic of an alternate embodiment of the stabilization module.  
    
    
     DETAILED DESCRIPTION  
     [0014] Disclosed is a laser beam stabilization module that has a tilt compensation device and a separate displacement compensation device. Each compensation device is coupled to a feedback loop to correct for tilt and displacement errors in a laser beam stabilized by the module. Providing a pure displacement compensation device eliminates the additional tilt error found in two mirror stabilization modules of the prior art.  
     [0015] Referring to the drawings more particularly by reference numbers, FIG. 2 shows a stabilization module  50 . The module  50  can stabilize and maintain a light beam  52  that is emitted from a light source  54 . The beam  52  travels along an optical path. The light source  54  may be a laser that emits a laser beam. The beam  52  can be reflected by bending mirrors  56 . The module  50  may be a separate assembly that is attached to the light source  54  and mirrors  56 . For example, the module  50  may be attached to a laser machine or a fiber Bragg forming apparatus. The module stabilizes the laser beam of the equipment.  
     [0016] The module  50  includes a tilt compensation device  58  and a displacement compensation device  60 . The tilt compensation device  58  can vary the tilt angle of the beam  52 . The displacement compensation device  60  can laterally displace the beam  52 . The beam  52  may enter the module  50  through an input aperture  62 . The beam  52  may exit the module  50  through a semi-reflective mirror  64  and exit aperture  66 .  
     [0017] A portion of the light beam  52  may be directed onto photodetectors  68  and  70  by semi-reflective mirror  64  and bending mirrors  72  and  74 . An imaging lens  76  images the surface of the mirror  64  onto photodetector  68 . A focusing lens  78  focuses the beam onto photodetector  70 . The photodetectors  68  and  70  may be quadrature devices that can provide output signals that are processed to determine beam movement.  
     [0018] Photodetector  68  is used to determine lateral displacement of the beam  52 . Photodetector  70  is used to determine an undesirable tilt of the beam  52 . Photodetector  70  is preferably located at the focal point of lens  78  so that the detector  70  only detects a change in the beam tilt independent of any lateral displacement of the beam  52 .  
     [0019] The photodetectors  68  and  70  are connected to a controller  80 . The controller  80  includes amplifiers  82  and  84  to amplify the output signals of the detectors  68  and  70 . The controller  80  also contains error control and driver circuits  86  and  88  that provide output signals to the compensation devices  60  and  58 , respectively. The error control circuits  86  and  88  may include proportional-integral-derivative (PID) feedback control for eliminating tilt and displacement errors detected by photodetectors  68  and  70 , respectively.  
     [0020] In operation, the light beam  52  is directed through the module  10 . A deviation in the tilt angle of the beam  52  will be detected by photodetector  70 , processed by control circuit  88  and corrected by actuating the tilt compensation device  58 . Likewise, a lateral deviation of the beam  52  will be detected by photodetector  68 , processed by control circuit  86  and corrected by actuating the displacement compensation device  60 . Providing separate tilt and displacement compensation and detection systems de-couples the tilt error from the displacement error. For example, if the systems were not de-coupled, the tilt compensation device would interpret pure lateral displacement as a tilt error and actually increase the error. With the arrangement shown in FIG. 2, the tilt compensation device  58  corrects the tilt so that the beam is travelling parallel to the desired optical path. The displacement device  60  can then move the beam over to the desired position.  
     [0021] It is preferable to place the displacement compensation device  60  after the tilt compensation device  58  on the optical path of the beam  52 . Placing the displacement device  60  before the tilt device  58  would increase the amount of displacement that the device  60  would have to compensate for by the tangent of the tilt error multiplied by the propagation between devices  58  and  60 . This would reduce the dynamic range of the module  50 .  
     [0022]FIG. 3 shows an embodiment of the module  50  wherein the tilt compensation device  58  is a fast steering mirror (FSM) and the displacement compensation device  60  is a fast steering plate (FSP). The FSM includes a mirror  90  that can be pivoted by a plurality of actuators  92  driven by control circuit  88 . The FSP includes a transmissive plate  94  that is pivoted by actuators  96  driven by control circuit  86 . The plate  94  uses refraction and varying impingement angles to vary the lateral position of the beam. This embodiment is preferable for monochromatic light beams. A light beam with multiple wavelengths may produce chromatic feedback errors.  
     [0023]FIG. 4 shows another embodiment wherein the displacement compensation device  60  has a pair of reflective mirrors  96  that are each moved by a linear translator  98  (only one mirror is shown). One mirror  96  may move the beam  52  along an x axis, the other mirror  96  may move the beam  52  along an orthogonal y axis. Each mirror  96  may reflect the beam  52  in an orthogonal direction resulting in 90 degree turn from the input beam  52 . The translators  98  may include voice coil motors.  
     [0024]FIG. 5 shows another embodiment that has a scan lens  100  which focuses the light beam to a point on a work piece  102 . Focusing the beam to a point eliminates the need for the displacement compensation device and accompanying feedback system.  
     [0025] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.  
     [0026] For example, the tilt compensation device  58  and displacement compensation device  60  may be mounted to the same mechanical platform, or mounted to different mechanical platforms.