Abstract:
An apparatus for analyzing an integrated circuit to which one or more test signals are applied. An example apparatus includes an objective lens that views reflections from the integrated circuit, a device that houses at least two optical fibers, a component that receives reflections from the objective lens and directs the received reflections to the device, and a photo-diode that receives a reflection received by the device. The apparatus includes a beam splitter that directs reflections from the integrated circuit to a detector. A processing device generates an image signal based on a signal received from the detector and a display outputs an image based on the image signal. The component includes a scan mirror that reflects the collimated reflections to a collimating lens that focuses the reflections from the scan mirror toward the device.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/058,171 filed Jun. 2, 2008, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed towards detection of transient emissions from semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,940,545 by Kash and Tsang describes the use of photon timing detection to determine the switching time of transistors in a semiconductor integrated circuit. Kash et al. describe how typical digital circuits will only emit photons during the switching transient. These photons are detected and the arrival time is recorded. Two techniques are described. Both techniques utilize a time-to-amplitude converter and a multi-channel analyzer to determine the photon arrival time and sort the collected data into a time histogram. The first technique utilizes a single avalanche photodiode with a fiber optic probe placed over the sampling area as the photon detection arrangement. The second utilizes a micro-channel plate array detector with standard microscope optics as the photon detection arrangement. In addition, the second technique utilizes position detection electronics to determine the transverse (XY) location of the detected photon. Kash et al. also describe placing the avalanche photodiode at the position of the micro-channel plate in the microscope arrangement to act as a single point detector. 
     U.S. Pat. No. 6,608,494 by Bruce et al. describes an improvement on Kash et al. with the addition of an ‘aperture element’ 208, and otherwise appears to be identical. The use of apertures in optical systems has been widely known for some time. Bruce et al. indicate the potential use of a scanning laser microscope in their arrangement, but do not explicitly indicate how such a system would be configured (column 3, lines 54-58). Specifically, the issue of how to co-align the laser imaging portion of the microscope with the photon-emission portion of the microscope needed to point at the target is not addressed. 
     Both the above inventions require a separate means of imaging the integrated circuit in order to determine the actual physical location on the circuit that corresponds to the detected emission photons. That is to say an array camera (e.g. a CCD) or some other imaging means is needed to supply navigation to the area of interest and to determine the photon emission location. Although not explicitly indicated, both inventions require some mechanical means to point the detector at the area of interest on the integrated circuit into the detection field of view. For a single point detector and the scale size of current integrated circuits; expensive, high-precision mechanical stages with sub-micron positioning accuracy are required to achieve the required pointing. 
     It is the purpose of the current invention to eliminate the need for: 
     An aperture in the image plane that requires placement over the detection area; 
     An independent imaging system for navigation and pointing; and 
     The requirement for high-precision mechanical translation stages. 
     It is the further purpose of the current invention to allow multiple detection paths that can be conveniently co-aligned and pointed at a target. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for analyzing an integrated circuit to which one or more test signals are applied. An example apparatus includes an objective lens that views reflections from the integrated circuit, a device that houses at least two optical fibers, a component that receives reflections from the objective lens and directs the received reflections to different ones of the at least two optical fibers, and a photo-diode that receives a reflection received by one of at least two optical fibers and adapted to detect a photoemission from the integrated circuit. 
     In one aspect of the present invention, the apparatus includes a beam splitter that directs reflections from the integrated circuit to a detector. Also included is a second component that applies one or more test signals to the integrated circuit, a processing device that generates an image signal based on a signal received from the detector, and a display that outputs an image based on the image signal. 
     In another aspect of the present invention, the component includes a first collimating lens that collimates reflections from the objective lens, a scan mirror that reflects the collimated reflections, a second collimating lens that focuses the reflections from the scan mirror toward the device, and a scan mirror controller that controls position of the scan mirror for progressing through reflections in a predefined pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative examples of the present invention are described in detail below with reference to the following drawings: 
         FIGS. 1 and 2  show single-point transient-emission detection systems formed in accordance with the prior art; 
         FIG. 3  illustrates a system formed in accordance with the present invention; 
         FIGS. 4A , B show image steering performed by the present invention; and 
         FIGS. 5A-C  illustrate end views of exemplary turrets. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The prior art is first described in order to better clarify the differences with the current invention.  FIG. 1  shows the basic components of a single-point transient-emission detection system as described in the first configuration of U.S. Pat. No. 5,940,545 and in U.S. Pat. No. 6,608,494. A microscope objective  10  focuses optical emission  12  from a test circuit  14  onto the end of an optical fiber  16  at the objective focal plane  18 . The optical fiber  16  couples into a single photon counting detector  20  (e.g. an Avalanche Photodiode (APD)). Although the detector  20  could be placed directly into the focal plane  18 , the use of an optical fiber  16  as fiber coupling adds flexibility to the design. Photons arriving at the detector  20  create an electrical pulse which is electrically connected  21  to a time correlated counter  22 . The time correlated counter  22  records the pulse arrival time. The collection of several pulses allows the counter  22  to produce a histogram of the arrival time of the collected photons during the collection sampling period. The counter  22  would typically consist of a time-to-amplitude converter followed by a fast analog-to-digital converter, followed by a multi-channel analyzer as sub-components (not explicitly shown). A clock  24  is used to set the relative timing between the test circuit  14  and the time correlated counter  22  via electrical connections  25 . A computer  26  would typically be used to set system control values and accept data via electrical lines  28 . Typically the computer  26  would also be used to display the histogram. 
     Means are needed to point the image of the optical fiber  16  end point at the desired spot on the test circuit  14 . The prior art suggests use of a separate imaging camera  30  with electrical connection  29  to the computer  30  for display of an image of the test circuit  14 . A fold mirror  32  or similar technique can be used to direct the optical path  12  towards the camera  30 . Some form of illumination (not shown) would also be required. U.S. Pat. No. 6,608,494 suggests that a scanning laser imaging device could be used as the imaging camera  30 . Implicit in the prior art is the need to for a XY translation of the test circuit  14  (or equivalently the entire optical path can be translated) in order to point the image of the fiber  16  end point at the desired spot on the test circuit  14 .  FIG. 1  explicitly includes an XY translation component  34  connected to the computer  26  via an electrical line  31  for performing position control. In addition, some means to optically co-align the camera  30  and the fiber  16  end point is also required. 
     U.S. Pat. No. 6,608,494 is equivalent to U.S. Pat. No. 5,940,545 except for a few additional optical components as shown in  FIG. 2  (non-optical components of  FIG. 1  are suppressed for clarity). The additional components include a variable shutter  40  placed in the focal plane  18  and a relay lens  42 . The relay lens  42  images the variable shutter  40  aperture onto the end of optical fiber  16 . When the image of the variable shutter  40  aperture is smaller than the aperture of the optical fiber  16 , the image becomes the limiting aperture of the optical system. A variable limiting aperture is thereby obtained. 
     As shown in  FIG. 3 , the present invention includes 
     Microscope objective  10   
     Optical emission  12   
     Test circuit  14   
     Objective focal plane  18   
     Single photon counting detector  20   
     Electrical connected  21   
     Time correlated counter  22   
     Clock  24   
     Electrical connections  25   
     Computer  26   
     Electrical connection  28 . 
     These components are similar to those shown in  FIG. 1 . Although (as will be described in more detail below) it is understood that the software control and acquisition of the computer  26  are not identical. 
     The difference between the current invention and the prior art, as illustrated in  FIG. 3 , is the direct integration of the above components into a laser scanning microscope. In the prior art, the scanning laser is a secondary add on for imaging purpose only. That is to say the laser scanning microscope can be removed and replaced with any imaging system. The current invention fully merges these two components. As will be described, direct integration produces several functional improvements. 
     There are many forms of laser scanning microscopes, the one shown in  FIG. 3  being a standard configuration. In this embodiment a laser scanning module  50  is optically coupled to a first optical fiber  52 . The laser scanning module  50  includes a fiber splitter  84  that directs light from a laser  80  to the first optical fiber  52  and from the first optical fiber  52  to a detector  82 . The detector  82  transforms the received light into an electrical signal that is sent to the computer  26  via an electrical connection  90 . The computer  90  digitizes the electrical signal and creates an image for display. 
     The first optical fiber  52  is coupled into a multi-fiber turret  54  (described in more detail later) which places the end of the first optical fiber  52  at the focus of a collimating lens  56 . The collimating lens  56  collimates a divergent light beam  58  from the end of the first optical fiber  52  into a collimated light beam  59  and directs the beam  59  towards a scanning mirror(s)  60 . The scan mirror(s)  60  is rotated in the both the X and Y planes (perpendicular to the optical axis) as controlled by a scan mirror controller  62  through an electrical connection  64  to an actuator (not shown). The scan mirror controller  62  is further connected via an electrical connection  66  to the computer  26 . The computer  26  coordinates the scan motion (typically a raster scan) with data acquisition from the detector  82  so as to form an image. Typically two scan mirrors are used to produce X and Y scanning, however, single mirror arrangements are also utilized. 
     A tube lens  68  focuses the scanned light onto the microscope focal plane  18 . The tube lens  68  also transforms the angular motion of the scan mirror(s) into a positional motion in the focal plane  18 . The light from the tube lens  68  then couples through the microscope objective  10  in the usual manner. Light reflected from the test circuit  14  is coupled back into the end of first optical fiber  52  via optical reciprocity, where it is directed towards the detector  82  by the fiber splitter  84 . The detected intensity versus scan position is used (typically by the computer  26 ) to produce a laser scanning microscope image of the test circuit  14 . 
     A second optical fiber  70  is attached to the multi-fiber turret  54  such that the end of the second optical fiber  70  is also at a focus (i.e. in the focal plane) of the collimating lens  56 . The second optical fiber  70  serves the same purpose as the optical fiber  16  in  FIG. 1 , that is to say it couples the optical emission  12  from the test circuit  14  into the single photon counting detector  20 . This coupling integrates the photo timing components directly into the laser scanning microscope. 
     This arrangement of the first optical fiber  52  and the second optical fiber  70  within the multi-fiber turret  54  is shown in more detail in  FIG. 4 . Displacement in the XY plane (perpendicular to the optical axis) of the end of either the first or second optical fibers  52 ,  70  produces an angular shift in the collimated light beam  59  formed by the collimator  56 . This shift is shown in exaggerated form (typical shifts are less than 1 degree) in  FIGS. 4A  and B. As shown in  FIG. 4A , an upward displacement (along Y-axis) of the first optical fiber  52  from the optical axis causes a downward angle in the collimated light beam  59 .  FIG. 4B  shows that a downward displacement (along Y-axis) of the second optical fiber  68  from the optical axis causes the collimator  64  to make the collimated light beam  59  have an upward angle. 
     These angular shifts can be compensated via a rotation of the scan mirror(s)  60  as shown in  FIGS. 4A  and B so that the optical beams return to parallel with the optical axis. In both cases a focus spot  72  in the focal plane  18  is returned to the on-axis (central) position via rotation of the scan mirrors(s)  60 . There is no other affect to the optical system. The current invention takes advantage of this optical invariance to place multiple fibers into the multi-fiber turret  54  with their end points located in the focal plane of the collimating lens. The end faces of the multi-fiber turrets  54 - 1  thru  54 - 3  are shown in  FIGS. 5A-C . The end face includes a bundle of multiple fibers  72 . Cases of three  FIG. 5A , four  FIG. 5B  and seven  FIG. 5C  fibers  72  being shown in a close packing arrangement. An offset  74 - 1  thru  74 - 3  from the center of each bundle for the fibers is fixed by the fiber diameter. Typical fibers are  125  micron in diameter, so this offset is small, less than or equal to the fiber diameter for the configurations in  FIG. 3 . The angular shift produced by the offset is related to the fiber offset via the collimator focal length, f, is as follows:
 
Angular shift= f *fiber offset
 
     Since the fiber offset is fixed via the fiber diameter and the collimator focal length is known, the angular shift between fibers  72  within the multi-fiber turret  54  is known and readily compensated by fixed offsets in the scanning mirror(s)  60 . If needed, further calibration can be made by coupling the laser scanner module  50  into each of the fibers in the turret and recording the offset in the resultant image. 
     In operation, the laser scanning module  50  attached to the first optical fiber  52  in the multi-fiber turret  54  is turned on and used to produce an image of the test circuit  14 . The desired position on the test circuit  14  is then located on the image. The scan mirror(s)  50  is then rotated using an actuator (not shown) to point the end of the second optical fiber  70  at the desired position on the test circuit  14  using the known offset between the two fibers  52  and  70 . The laser scanning module  50  is then turned off and the emissions  12  are collected into the second optical fiber  70  and detected as described earlier. Note that an XY translation stage (item  34  in  FIG. 1 ) is no longer required for pointing a fiber. 
     Note that additional fibers in the multi-fiber turret  54  can be used to couple in other detection technologies or allow different types of fibers (single and multi-mode) to be used for a single detection. Specifically, use of fibers with differing core diameters produces a different limiting apertures, eliminating the need for the variable shutter  40 . 
     Note that the described optical arrangement would typically also include the use of field lenses between the ends of the fibers  72  and the collimating lens  56  to reduce vignetting effects. These lenses are not shown as they should be obvious to any optics designer, have no direct impact on the present invention and would tend to obscure the description. That is to say these additional lenses are desirable, but not required. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.