Abstract:
Methods and systems that excite a test structure with an excitation beam and then sense the response of the test structure after some prescribed time interval with respect to the excitation. One or more detectors detect an emission from the test structure at a location of the test structure that is offset from a position on the test structure that is coincident with the excitation beam as the beam is scanned across the test structure.

Description:
PRIORITY CLAIM 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/554,496 filed Mar. 18, 2004, the contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to scanning imaging systems. 
   BACKGROUND OF THE INVENTION 
   There exist several techniques in which a radiation source is used to rapidly (pulse) excite a test structure, then after a time delay the test structure is examined using a detector. Examples include:
         A pulse of light from a flash lamp is used to rapidly heat a test structure. An infrared camera rapidly records multiple images of the test structure at multiple time delays after the heating. Heat flow in the test structure can be derived from these images.   Time resolved spectroscopy typically utilizes a pulsed laser source to excite atoms or molecules of a test structure. A fast optical detector is used to detect fluorescent decay of the excitation. Alternatively, a second fast laser pulse is used to “probe” the test structure.       

   It is noted in these examples that the optical detector can be both passive (e.g. a camera), or active (e.g. a laser probe). It is also noted that a multiplicity of signals collected at a multiplicity of time delays is often recorded. 
   In these techniques, the excitation source must be rapidly pulsed and the detection must be time delayed via electronic or other means with respect to the excitation pulse. 
   Therefore, there exists a need to more effectively and efficiently analyze an excited test structure. 
   SUMMARY OF THE INVENTION 
   The current invention uses the properties of a scanning system to supply an impulse of radiation to a position on a test structure and to supply a built in time delay for the detection. 
   Specifically, an embodiment of the present invention describes methods and systems that excite a test structure with an excitation beam and then sense the response of the test structure after some prescribed time interval with respect to the excitation. 
   One or more detectors detect an emission from the test structure at a location of the test structure that is offset from a position on the test structure that is coincident with the excitation beam as the beam is scanned across the test structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a diagram illustrating an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows the basic components of an embodiment of the present invention as well as a specific implementation as a laser scanning or confocal microscope. The present invention senses properties of a test structure  2 . The test structure  2  can be any physical object, such as an integrated circuit, a package in which the integrated circuit is housed, a printed circuit board, composite materials, etc. In one embodiment, the present invention excites and then detects a time delay response of the test structure  2 . 
   An excitation source  4  generates an excitation beam  6  that is used to induce an excitation in the test structure  2 . Examples of the excitation source  4  including but are not limited to:
         Optical, e.g. a laser   Electron beam   Ion beam   Acoustic.       

   Each excitation source  4  can induce a range of possible excitations, including but not limited to:
         Thermal heating   Ionization   Photo excitation of atoms and molecules   Photo carrier generation (e.g. in semiconductors)   Material ablation.       

   The excitation beam  6  is directed towards a scanning device  8 . In the case where the excitation beam  6  is an optical beam, the scanning device  8  may include a scanning mirror assembly  102  coupled with a lens  104  that focuses the excitation beam  6  onto the test structure  2 . In the case where the excitation beam  6  includes an electron or ion beams, the scanning device  8  may include equivalent electromagnetic beam deflectors and focusing elements (not shown). The purpose of the scanning device  8  is to direct the excitation beam  6  on to the test structure  2  and to scan the excitation beam  6  across the test structure  2 . Any assembly that produces this result is acceptable. As shown in  FIG. 1 , an arrow  12  indicates a possible upwards scanning motion of the excitation beam  6 . In another embodiment, the test structure  2  is moved to supply the scanning function. 
   The excitation beam  6  produces an excitation in the test structure  2  that is moved with a velocity, V, across the surface of the test structure  2 . Thus, for a specific position on the test structure  2 , the excitation beam  6  appears as a pulse. A duration of the pulse is the width of the excitation beam  6  at the surface of test structure  2  divided by the velocity V. The excitation produced by this pulse can in turn produce a re-emission from the test structure  2 . The re-emission can take on a variety of forms, including but not limited to:
         Thermal radiation   Fluorescent decay   Ablation fragments   Electrons or ions.       

   The re-emission can be passive as in the above list or involve an active secondary probe as would be the case for an active laser probe for physical distortion of the test structure  2 . 
   The re-emission is collected by the scanning device  8 . The scanning device  8  sends the collected re-emission back to a detection device  10 . The detection device  10  detects the re-emission of a location on the test structure  2  at some point in time after the excitation of that location on the test structure  2  occurs. Also, the detection device  10  transforms the magnitude of the re-emission into a signal that can be displayed as an image. 
   As the excitation beam  6  is being scanned across the test structure  2 , the location on the test structure  2  that the detection device  10  detects is behind the location on the test structure  2  that is presently being excited by the excitation beam  6 . As shown in  FIG. 1 , the detection device  10  includes one or more detectors  206  or  208 . In this example, the detector  208  is positioned to collect the delayed re-emissions of the test structure  2  from a beam splitter  202 . Because the detector  208  is not positioned to receive direct reflections off of the test structure  2 , re-emissions are sensed by the detector  208 . 
   The excitation beam  6  and the re-emissions detection point are scanned across the test structure  2  at a velocity V. A distance D is a distance that the re-emissions detection point is behind the position of the excitation beam, thereby causing the detection of the re-emission to take place at a time delay or time interval after the excitation takes place. The time delay or interval is the ratio of the distance over the velocity, D/V. 
   Both distance D and velocity V can be adjusted to allow a range of time delays. Velocity V can be adjusted using scanning controls of the scanning device  8 . Distance D can be adjusted by changing position of the detector. 
   In another embodiment, multiple detectors may be used at various positions for simultaneously detecting re-emissions at a plurality of distances (or time delays) after excitation. 
   The details of  FIG. 1  allow description of a specific example that may clarify the more general description. In one example, the excitation source  4  is a laser and the excitation beam  6  is a laser beam. The laser beam passes through a scanning mirror assembly (the scanning device  8 ), which deflects the laser beam at an angle versus time (the angle changes over time). The lens  104  transforms the angular scan into a position scan on the test structure  2 . 
   In this example, excitation caused by the laser beam heats the surface of the test structure  2 . The increased temperature that occurs will increase the amount of thermal emission that occurs. The thermal emission is the re-emission. Thermal conduction from the heated area into the cooler parts or the test structure  2  causes the thermal emission to decrease with time. A measurement of the rate of decrease in the thermal emission is a direct measurement of the thermal conduction properties of the test structure  2 . 
   Also in this example, the beam splitter  202  allows passage of the laser beam (the excitation beam  6 ) while redirecting the re-emission towards a second lens  204 . The second lens  204  focuses the thermal emission towards the detectors  206  or  208  that are optical detectors. The optical detectors convert the thermal emission (the re-emission) into electrical signals  306  or  308 . The electrical signals  306  or  308  are converted into images that are presented on a display  302 . Thus, the output of one of the optical detectors will cause a single thermal image to be displayed on the display  302 . 
   The first detector  206  collects thermal radiation from a position coincident with the excitation beam  6 . The second detector  208  collects thermal radiation from a position at a distance D behind the excitation beam  6 , which is to say at a time of D/V where V is the scan velocity described above. There can be a multiplicity of detectors with a multiplicity of positions, allowing signals from a multiplicity of time delays to be collected in parallel. An equivalent apparatus could also be used to detect fluorescent decay or other types of re-emissions for the test structure  2 . 
   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. For example, because the excitation beam and emission may be any of the type described above, the lenses  104  and  204  is selected from any of number of different types of focusing devices according to the excitation beam and emission. 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.