Abstract:
A system includes an objective lens, an imaging module, and a measurement module. The objective lens is configured to receive light emitted by a light source, focus the emitted light onto a target object, and receive light reflected by the target object. The imaging module is configured to receive a first portion of the reflected light. The measurement module is configured to receive a second portion of the reflected light and includes a photo detector and an astigmatic lens configured to direct the second beam onto the photo detector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. Pat. No. 7,247,827, filed May 31, 2006, and issued Jul. 24, 2007, and entitled “System for Measurement of the Height, Angle, and Their Variations of the Surface of an Object;” and to Taiwanese patent No. I 264520, issued Oct. 21, 2006, the contents of both of which are incorporated herein by reference. 
         [0002]    This application is also related to co-pending U.S. application Ser. No. __/______ (Attorney Docket No. 70002-216001), filed Oct. 19, 2009, and entitled “Alignment and Anti-Drift Mechanism,” the contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to an optical imaging system. 
       BACKGROUND 
       [0004]    Microfabricated elements, such as micro-mirrors, read/write heads of hard disk drives, acceleration sensors, and electro-acoustic high frequency elements, are widely used in a variety of microelectrical-mechanical systems (MEMS) applications. Measurement of the static or dynamic mechanical motion of these microfabricated elements provides information useful for the design, development, and simulation of such devices. 
         [0005]    Mechanical motion and displacements of MEMS devices are characterized using techniques such as laser interferometry, laser Doppler velocimetry, stroboscopic interferometry, and an astigmatic detection method. An astigmatic detection system (ADS) measures translational displacement of a target object along one axis and angular displacement of the object around two axes. An ADS includes an optical path mechanism in which a detection laser beam passes through a lens assembly and is focused on the surface of the object. The object reflects the light back through the lens assembly and to a position sensitive photosensor. The shape and position of a light spot formed on the photosensor is used to analyze the translational and angular displacement of the object. To position the detection laser beam at a desired position on the surface of the object, the object is observed through an external microscope that includes an eyepiece or other imaging device, such as a CCD sensor. 
       SUMMARY 
       [0006]    In a general aspect, a system includes an objective lens, an imaging module, and a measurement module. The objective lens is configured to receive light emitted by a light source, focus the emitted light onto a target object, and receive light reflected by the target object. The imaging module is configured to receive a first portion of the reflected light. The measurement module is configured to receive a second portion of the reflected light and includes a photo detector and an astigmatic lens configured to direct the second beam onto the photo detector. 
         [0007]    Embodiments may include one or more of the following. The system further includes an objective beam splitter configured to receive the reflected light from the objective lens and to divide the reflected light into the first portion and the second portion. The objective beam splitter is further configured to receive the emitted light and to reflect the emitted light into the objective lens. The system further includes a source beam splitter configured to receive light emitted by the light source and to reflect the emitted light toward the objective lens and to receive the second portion of the reflected light from the objective beam splitter and to transmit the second portion of the reflected light to the astigmatic lens. 
         [0008]    The astigmatic lens is further configured to receive the emitted light and to reflect the emitted light toward the objective lens. The photo detector includes a position sensitive detector, which may be a four quadrant position sensitive detector. The system further includes the light source, which may be a laser. 
         [0009]    The system further includes an illumination light source configured to provide light to the objective lens for illumination of a region of the target object. The light provided by the illumination light source is scattered by the target object. The system further includes an objective beam splitter configured to receive the reflected light and the scattered light from the objective lens and to divide the received light into the first portion and the second portion. 
         [0010]    The imaging module includes an imaging sensor, such as a CCD camera. The imaging module further includes a relay lens configured to receive the first portion of the reflected light and to transmit the first portion of the reflected light to the imaging sensor. The astigmatic lens is a cylindrical lens or an inclined planar light refraction layer. 
         [0011]    An optical imaging system as described herein has a number of advantages. An objective lens shared between the optical imaging system and a measurement system enables simultaneous imaging of a target object and measurement of the translational and/or angular displacements of the object. A detection light spot that is focused on the surface of the object for measurement purposes can be visualized through the optical imaging system and positioned at a desired location on the object surface. Thus, static and dynamic displacements can be measured for specific features of the object. Furthermore, an instrument that integrates optical imaging and measurement is compact and makes efficient use of a minimal number of optical elements. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIGS. 1A and 1B  are diagrams of a first embodiment of an optical path mechanism used with an astigmatic detection system (ADS). 
           [0013]      FIG. 1C  is a diagram of a position sensitive detector. 
           [0014]      FIG. 2  is a diagram of a second embodiment of an optical path mechanism used with an ADS. 
           [0015]      FIG. 3  is a diagram of a measurement system employing the optical path mechanism of  FIG. 2 . 
           [0016]      FIG. 4  is an optical image of a light spot on a microfabricated cantilever obtained using the measurement system shown in  FIG. 3 . 
           [0017]      FIG. 5  is a thermal noise spectrum of the microfabricated cantilever shown in  FIG. 4 . 
           [0018]      FIG. 6  is a diagram of a third embodiment of an optical path mechanism used with an ADS. 
           [0019]      FIG. 7  is a diagram of a measurement system employing the optical path mechanism of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Referring to  FIGS. 1A and 1B , an optical path mechanism  100  combines an optical imaging system  1  with an astigmatic detection system (ADS)  2 . A light source  21 , such as a laser or a laser diode, generates a light beam that is formed into a parallel light beam by a collimator lens  22 . The light beam is reflected by an ADS beam splitter  23  and an objective beam splitter  13  toward an objective lens  14 . Objective lens  14  focuses the light beam onto a detection light spot  29  on the surface of an object  3 . Objective lens  14  and objective beam splitter  13  are shared by optical imaging system  1  and ADS  2 . 
         [0021]    Object  3  reflects the incident light back through objective lens  14  and to objective beam splitter  13 , which divides the light into an imaging beam  110  and a measurement light beam  112 . Imaging light beam  110  is transmitted through objective beam splitter  13  and is focused on an image sensor  11  in optical imaging system  1  by a sensor lens  12 . Measurement light beam  112  is reflected by objective beam splitter  13  and is transmitted through ADS beam splitter  23 , a detector lens  24 , and an astigmatic lens  25 , such as a cylindrical lens or an inclined planar light refraction layer, in ADS  2 . 
         [0022]    Referring to  FIGS. 1A-1C , detector lens  24  focuses the measurement beam. The focused beam passes through astigmatic lens  25 , which breaks the rotational symmetry of the beam such that rays propagating in two perpendicular planes have different foci. The beam then strikes a position sensitive detector (PSD) assembly  26  for the measurement of the height, tilt angle, and/or variations thereof of the surface of object  3 . PSD assembly  26  is composed of four quadrants  26   a,    26   b,    26   c,  and  26   d,  each housing a photo sensor that outputs a measurement signal. Translational and angular displacements of object  3  are determined based on the shape and position of the light spot focused on PSD assembly  26 . The algorithms for processing the measurement signals from the four photo sensors of PSD assembly  26  to obtain the displacement of object  3  are described in U.S. Pat. No. 7,247,827, the contents of which are incorporated herein by reference. 
         [0023]    In some embodiments, an external light source is used to illuminate the top surface of object  3  from above or, in the case of a transparent object, from below. Light scattered by the surface of object  3  is collected by objective lens  14  and directed through the optics of optical imaging system  1 , forming an image on image sensor  11 . Image sensor  11  thus receives images of both the surface of object  3  and the detection light spot  29  on the surface of object  3 . With an actuator attached to either object  3  or objective lens  14 , the detection light spot  29  can be positioned at a desired location on the surface of object  3 . The actuator may be any device capable of adjusting the relative position of objective lens  14  and object  3 , such as a piezoelectric material, a voice coil motor, or a translational or rotary stage. 
         [0024]    Referring to  FIG. 2 , a second embodiment of an optical path mechanism  200  combines an optical imaging system  201  and an ADS  202 . In this embodiment, a light source  21  generates a light beam that is reflected by an inclined glass plate  27  through a collimator lens  28 , which forms the beam into a parallel beam. The parallel beam is reflected by objective beam splitter  13  through objective lens  14 . Objective lens  14  focuses the light beam on the surface of object  3 , forming detection light spot  29 . As in the embodiment described above, objective lens  14  and objective beam splitter  13  are shared by optical imaging system  201  and ADS  202 . 
         [0025]    Object  3  reflects the incident light back through objective lens  14  and to objective beam splitter  13 , which divides the reflected light into an imaging light beam  210  and a measurement light beam  212 . Imaging light beam  210  is transmitted through objective beam splitter  13  and is focused on image sensor  11  by sensor lens  12 , forming an image spot on image sensor  11 . Measurement light beam  212  is reflected by objective beam splitter  13  toward collimator lens  28  and glass plate  27 , which behaves as an astigmatic lens. Collimator lens  28  focuses the measurement beam. The focused beam passes through glass plate  27 , which breaks the rotational symmetry of the beam such that rays propagating in two perpendicular planes have different foci. The beam then strikes PSD assembly  26  for measurement of displacements of object  3  at the position of detection light spot  29 . In this embodiment, the dual function of inclined glass plate  27  as a reflector of emitted light toward the objective lens and as an astigmatic lens saves one optical component compared to the embodiment shown in  FIG. 1 . 
         [0026]    As described above, the top surface of sample  3  may be illuminated by an external light source. Light scattered by the surface of object  3  forms an image on image sensor  11 . Image sensor  11  thus receives images of both the surface of object  3  and the detection light spot  29  on the surface of object  3 . With an actuator attached to either object  3  or objective lens  14 , the detection light spot  29  can be positioned at a desired location on the surface of object  3 . The actuator can also move along the axis of the beam in order to adjust the focal plane to a desired position on object  3 . 
         [0027]    Referring to  FIG. 3 , optical path mechanism  200 , including imaging system  201  and ADS  202 , as shown in  FIG. 2 , is used in a measurement system  300  for the simultaneous imaging of a microfabricated cantilever  31  and measurement of translational and angular displacements of the cantilever. In some embodiments, imaging system  201  is implemented as a commercial optical microscope. In this example, object  3 , which includes cantilever  31 , rests on a coarse XYZ linear stage  6 . Light source  21  produces a detection beam that is reflected by astigmatic lens  27 , transmitted through collimator lens  28 , and is reflected toward objective lens  14  by objective beam splitter  13 , as described above. Objective lens  14  focuses the beam into detection light spot  29  on cantilever  31 . An external light source  41  used to illuminate the surface of cantilever  31  generates an illumination beam which is collimated by a convex lens  42  and directed by an illumination beam splitter  43  through objective beam splitter  13  and objective lens  14  and to cantilever  31 . 
         [0028]    In this example, objective lens  14  has a numerical aperture (N.A.) of 0.6 and a focal length of 2.33 mm. The full-width at half-maximum (FWHM) of detection spot  29  is given by 
         [0000]    
       
         
           
             
               
                 D 
                 W 
               
               = 
               
                 
                   k 
                   · 
                   λ 
                 
                 
                   N 
                   . 
                   A 
                   . 
                 
               
             
             , 
           
         
       
     
         [0000]    where λ is the wavelength of the laser beam and k is a constant equal to 0.52. For a laser diode light source  21  with a wavelength of 655 nm, the smallest detection spot  29  has a diameter of 560 nm (FWHM), which is close to the diffraction limit of light. 
         [0029]    The detection light spot  29  is reflected by cantilever  31  and the illumination light from external light source  41  is scattered by the surface of the cantilever. The reflected and scattered light from cantilever  31  passes through objective lens  14  and is transmitted through or reflected by objective beam splitter  13  as described above in conjunction with  FIG. 2 . 
         [0030]    The imaging light beam  210  (i.e., the beam transmitted through objective beam splitter  13 ) is transmitted through illumination beam splitter  43  and arrives at an imaging beam splitter  51 . Imaging beam splitter  51  divides the imaging beam into a first beam and a second beam. The first beam is sent through sensor lens  12  to imaging sensor  11 , (e.g., a CCD sensor). CCD sensor  11  captures optical and/or video images of the surface of cantilever  31  and of the detection light spot  29  and transfers the images to a computer  302  for display on a monitor  304 . The second beam is reflected by imaging beam splitter  51  through a sensor lens  52  and to an eye  53  of a user for viewing the measurement system. The relative position of detection spot  29  and cantilever  31  is visible in the images received at CCD sensor  11  and by an eye  53 . 
         [0031]    Referring to  FIGS. 3 and 4 , an optical image captured by CCD sensor  11  shows microfabricated cantilever  31  with detection light spot  29  focused thereon. The detection light spot  29  can be moved to a desired position on the cantilever by adjusting the position of stage  6 . In some embodiments, stage  6  is fixed and the detection light spot  29  is moved by adjusting the position of objective lens  14 . 
         [0032]    Referring again to  FIG. 3 , the measurement light beam  212  (i.e., the beam reflected through objective beam splitter  13 ) passes through the optical elements of ADS  2  as described above and arrives at PSD assembly  26 . PSD assembly  26  outputs a signal containing information about the translational and angular displacements of the cantilever  31 . Signals generated in PSD assembly  26  pass through a preamplifier  306  and are processed and in computer  302  equipped with a high-speed data acquisition card (DAQ)  308 , such as a NuDAQ PCI-9820 ADLINK card with a bandwidth of 130 MHz and a 14-Bit resolution. The high-speed DAQ  308  is useful for the measurement of thermal noise spectra of micro-cantilever  31 . The output signal of ADS  2  has a linear region of about  6  μm, which is sufficient to detect the movement of microfabricated elements such as cantilever  31 . The maximum measurable angular range of the ADS is ±8° and the noise level of the system is 10 pm (RMS). ADS  2  is capable of measuring the static and dynamic displacement of reflective or transparent objects. 
         [0033]    Referring to  FIGS. 3 and 5 , a thermal noise spectrum  500  of cantilever  31  is measured by ADS  2 . The thickness, width, and length of the cantilever are 2 μm, 50 μm, and 450 μm, respectively. According to the specifications of the cantilever, the fundamental resonance frequency of the first bending mode of the cantilever is between 9-16 kHz. Noise spectrum  500  reveals a first bending mode  502  at 13.0 kHz, in good agreement with the manufactured specifications. A second bending mode  504  at 81 kHz and a third bending mode  506  at 187.9 kHz are also observed. A fundamental torsion mode  508  is detected a frequency of 226.4 kHz, close to the frequency of the third bending mode; and a second torsion mode is detected at a frequency of 631.9 kHz, close to the working bandwidth limit of preamplifier  306 . 
         [0034]    Referring to  FIG. 6 , a third embodiment of an optical path mechanism  600  combines an optical imaging system  601  and an ADS  602 . In this embodiment, light source  21  generates a light beam that is reflected by parallel astigmatic lens  27  through a collimator lens  28  and to objective beam splitter  13 . Objective beam splitter  13  reflects the light beam through objective lens  14 , which focuses the light beam into detection light spot  29  on the surface of object  3 . As in the embodiment described above, objective lens  14  and objective beam splitter  13  are shared by optical imaging system  601  and ADS  602 . 
         [0035]    Object  3  reflects the incident light back through objective lens  14  and to objective beam splitter  13 , which divides the reflected light beam into an imaging light beam  610  and a measurement light beam  612 . Imaging light beam  610  is transmitted through objective beam splitter  13 , a lens  16 , and a relay convex lens  17  mounted on a fine adjustment device  18 . Relay lens  17  focuses the imaging light beam into an image spot on image sensor  11 . Measurement light beam  612  is reflected by objective beam splitter toward collimator lens  28  and astigmatic lens  27 . Astigmatic lens  27  focuses the beam onto PSD assembly  26  for measurement of the displacement of object  3 . 
         [0036]    As described above, the top surface of sample  3  may be illuminated by an external light source. Light scattered by the surface of object  3  forms an image on image sensor  11 . Image sensor  11  thus receives images of both the surface of object  3  and the detection light spot  29  on the surface of object  3 . With an actuator attached to either object  3  or objective lens  14 , the detection light spot  29  can be positioned at a desired location on the surface of object  3 . 
         [0037]    Referring to  FIG. 7 , a second embodiment of a measurement system  700  employs optical path mechanism  600  as shown in  FIG. 6  for the simultaneous imaging of cantilever  31  and measurement of translational and angular displacements of the cantilever. Detection light spot  29  is focused on cantilever  31 , which is also illuminated by external light source  41 . The detection light spot  29  is reflected by cantilever  31 ; the illumination light from external light source  41  is scattered by the surface of cantilever  31 . The reflected and scattered light from cantilever  31  passes through objective lens  14  and is transmitted through or reflected by objective beam splitter as described above in conjunction with  FIG. 6 . 
         [0038]    The imaging light beam  610  (i.e., the beam transmitted through objective beam splitter  13 ) is focused by lens  15  into relay convex  17 , from which the beam is directed by imaging beam splitter  51  to CCD imaging sensor  11  and to an eye  53 . The position of relay convex  17  is adjustable along the direction of the light beam by fine adjustment device  18 . The measurement light beam  612  (i.e., the beam reflected through objective beam splitter  13 ) passes through the optics of ADS  2  and impinges on PSD  26 , from which translational and angular displacements of cantilever  31  are detected and measured. 
         [0039]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention. For instance, the optical path mechanisms used in conjunction with an ADS for simultaneous imaging and measurement are not limited to those paths described herein. Rather, any configuration of an optical path mechanism that achieves optical imaging and measurement of translational and angular displacement using a single objective lens may be used. Other embodiments are within the scope of the following claims, which claims define the scope of the invention.