Abstract:
A technique for providing high-contrast images of defects in semiconductor devices and arrays of such devices, by illuminating each semiconductor device under inspection with broadband infrared radiation, and then forming an image of radiation that is specularly reflected from the semiconductor device. Many semiconductor devices and arrays of such devices have a metal backing layer that specularly reflects the illumination back into an appropriately positioned and aligned camera, selected to be sensitive to infrared wavelengths at which the semiconductor device materials are relatively transparent.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to optical thermal imaging techniques and, more particularly, to techniques for detecting defects in semiconductor devices or structures. Semiconductor devices are, of course, widely used in a variety of contexts and are typically fabricated as integrated circuit (IC) chips, or as IC wafers containing large arrays of semiconductor devices. Cracks in a semiconductor device have the potential to severely limit its performance. Once a crack has begun, it is highly probable that it will propagate over time to develop into a more significant crack. Therefore, it is important to detect not only large cracks but also small ones.  
         [0002]     Currently, cracks in semiconductor devices are inspected mainly by a thermal method. With an infrared camera, an inspector can see the change in temperature in the device as it is powered up electrically. A “hot” spot may be indicative of the presence of a crack. Typical electrical failures occur when there is temperature rise of over 50° C. By this technique, problems can be detected well in advance of a failure, but such a thermal inspection method can best be used for detection of large cracks, but not for small ones. In addition, a thermal method is an invasive and slow process.  
         [0003]     U.S. Pat. No. 6,806,249 describes an optical method for the inspection of a semiconductor wafer using a brightfield and darkfield arrangement. The method appears to be applied mainly to detect the presence of small particles on the surface of a wafer.  
         [0004]     Most semiconductor devices and arrays have multiple layers of material that render optical inspection difficult. It will be appreciated, therefore, that there is a need for a method for detection of micro-cracks and other defects that is quick, non-invasive and may be used at the array level as well as the device level. The present invention meets and exceeds these requirements.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention resides a method and apparatus for detecting defects in semiconductor devices. In method terms, the invention may be defined as a method for detecting defects in at least one semiconductor device or a portion of the device, the method comprising illuminating at least one semiconductor device or a portion of the device with infrared radiation from a flat-panel illuminator oriented to direct the radiation onto the semiconductor device at a selected incident angle; positioning an infrared camera to receive specularly reflected radiation from the at least one semiconductor device or a portion of the device; and forming an image in the camera, representative of radiation specularly reflected from the at least one semiconductor device or a portion of the device, wherein any defects in the at least one semiconductor device are visible in the image as contrasting features.  
         [0006]     The illuminating step employs a range of wavelengths, including a band at which the semiconductor device materials are relatively transparent. For example, the illuminator radiates light over a broad band of infrared wavelengths and the camera is sensitive in wavelength range of approximately 1-5 μm or 3-5 μm.  
         [0007]     The invention can be used over a wide range of angles of incidence. For example the incident angle of illumination on the semiconductor device under inspection may be in the range of approximately 10° to 30° with respect a normal direction to the semiconductor device.  
         [0008]     The method can be used to inspect a single semiconductor device, or an array of such devices.  
         [0009]     In terms of apparatus, the invention comprises a flat-panel illuminator, for illuminating at least semiconductor device or a portion of the device with infrared radiation, wherein the flat-panel illuminator is oriented to direct the radiation onto the semiconductor device at a selected incident angle; an infrared camera positioned to receive specularly reflected radiation from the at least one semiconductor device or a portion of the device; and means for forming an image in the camera, representative of radiation specularly reflected from the at least one semiconductor device or a portion of the device. Any defects in the at least one semiconductor device or a portion of the device are visible in the image as contrasting features.  
         [0010]     It will be appreciated that the present invention represents a significant advance in the field of semiconductor device inspection techniques. In particular, the invention allows for the inspection of semiconductor devices and arrays in a way that provides high contrast images of any cracks or other defects. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a diagram depicting the apparatus of the present invention, including an infrared illuminator and an infrared camera in relation to a semiconductor device under inspection.  
         [0012]      FIG. 2  is a side view of flat-panel infrared illuminator used in the apparatus of  FIG. 1 .  
         [0013]      FIG. 3  is a diagram depicting an alternative apparatus of the present invention, including an infrared illuminator and an infrared microscope in relation to a semiconductor device under inspection. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     As shown in the drawings for purposes of illustration, the present invention is concerned with the detection of defects in semiconductor devices and arrays having reflective back panels. In the past, thermal imaging techniques have been used to locate defects while power is applied to the semiconductor device under inspection. The conventional thermal inspection technique is slow, invasive, and not suitable for semiconductor structures with multiple layers.  
         [0015]     In accordance with the present invention, a semiconductor device under inspection, indicated by reference numeral  10 , is illuminated with a flat-panel infrared illuminator  12 , and then inspected by means of an infrared camera  14  or other imaging device located to record specular reflections from the an underlying reflective layer in the semiconductor device  10 . Some semiconductor structures, including photodetector arrays and RF detector arrays, include a metal back plane  16 , from which the infrared illumination is specularly reflected. The illuminator  12  is oriented at an angle φ to the semiconductor device  10 , so that radiation from the illuminator has an angle of incidence φ with respect to a line drawn normal (perpendicular) to the semiconductor device. Radiation reaching the back plane  16  is specularly reflected in a direction also having an angle φ with respect to the normal direction. The camera  14  is located on and aligned with this line of reflection from the device  10 . In effect, the camera  13  sees a mirror image (indicated at  12 ′) of the illuminator  12 . The camera  14  produces an image, shown diagrammatically at  20 , in which defects, and even micro-cracks, one of which is indicated at  22 , are clearly visible. Because the camera image  20  is formed from specularly reflected radiation, i.e., radiation that follows essentially straight-line paths from the illuminator  12  to the device  10  and from the device to the camera  14 , any interruption of those straight-line paths, as caused by the presence of a crack, is imaged with diminished brightness in the camera image. Thus the arrangement of the flat-panel illuminator  12  and the camera  14  aligned in the path of specular reflection from the semiconductor device  10 , provides a high-contrast image of any defects or cracks encountered by radiation passing through the multiple layers of the semiconductor device. With appropriate sizing and positioning of the flat-panel illuminator  12  and the camera  14 , the arrangement can be employed to inspect a broad area of a semiconductor device array comprising multiple arrayed devices.  
         [0016]     The camera  14  was selected to provide infrared sensitivity in the range 3-5 microns, although an available 1-5 micron camera also provided good results.  
         [0017]      FIG. 2  shows a little more detail of the illuminator  14 . It consists of a commercially available strip heater  40  on which a copper over-layer  42  approximately 0.0625 inch (1.6 mm) thick is installed to provide a more uniform distribution of heat from the strip heater. The copper over-layer  42  is painted with a high-temperature resistant black paint, as indicated at  44  to enhance the uniform radiation properties of the illuminator. The strip heater may be, for example, a mica strip heater from Watlow Electric Manufacturing Company, St. Louis, Mo. The copper plate is attached to the heater surface, by use of a high-temperature thermal conductive epoxy adhesive, to improve the uniformity of temperature of the source. An example of epoxy adhesive is Duralco  133  aluminum-filled epoxy from Contronics, located in Brooklyn, N.Y. As an alternative to painting the copper surface with a flat black paint, the surface can be coated with black chrome. The black paint or the black-chrome coating increases the emissivity and also diffuseness of the illuminator to improve its performance.  
         [0018]     Based on the geometric ray traces one can estimate roughly a minimum length (L) of the illuminator required for inspection, as given by:
 
 L= 2( D 1+ D 2)* X 1*cos φ/( D 1+ X 1*sin φ),
 
where  X 1= D 1*cos φ*(tan(φ+θ) −tan φ)
 
         [0019]     In these expressions, θ is half of the subtended angle of the camera  14  to the semiconductor device  10  under inspection. X 1  is the portion of the length of the device intercepted by the equal-angle bisector. φ is the angle of incidence of a central-axis ray from the illuminator  12  onto the semiconductor device  10  and is also the angle of reflection, from the semiconductor device, of the same ray. D 1  and D 2  are the distance from the device to the camera and the illuminator, respectively. A minimum width of the cell can also be derived based on these equations using the corresponding dimensions. The actual width and length of the illuminator are preferably larger than these minimum values as determined above.  
         [0020]     Referring to  FIG. 3  for an alternative embodiment of the invention a semiconductor device under inspection, indicated by reference numeral  50 , is illuminated with a flat-panel infrared illuminator  52 , and then inspected by means of an infrared microscope  54  or other imaging device located to record specular reflections from an underlying reflective layer in the semiconductor device  50 . Some semiconductor structures, including photodetector arrays and RF detector arrays, include a metal back plane  56 , from which the infrared illumination is specularly reflected. The illuminator  52  is oriented at an angle φ to the semiconductor device  50 , so that radiation from the illuminator has an angle of incidence φ with respect to a line drawn normal (perpendicular) to the semiconductor device. Radiation reaching the back plane  56  is specularly reflected in a direction also having an angle φ with respect to the normal direction. The infrared microscope  54  is located on and aligned with this line of reflection from the device  50 . In effect, the infrared microscope sees a mirror image (indicated at  52 ′) of the illuminator  52 . The infrared microscope  54  includes an infrared camera  58 , which produces an image, shown diagrammatically at  60 , in which defects, and even micro-cracks, one of which is indicated at  62 , are clearly visible. Because the camera image  60  is formed from specularly reflected radiation, i.e., radiation that follows essentially straight-line paths from the illuminator  52  to the device  50  and from the device to the infrared microscope  54 , any interruption of those straight-line paths, as caused by the presence of a crack, is imaged with diminished brightness in the camera image  60 . Thus the arrangement of the flat-panel illuminator  52  and the infrared microscope  54  aligned in the path of specular reflection from the semiconductor device  50 , provides a high-contrast image of any defects or cracks encountered by radiation passing through the multiple layers of the semiconductor device. With appropriate sizing and positioning of the flat-panel illuminator  52  and the infrared microscope  54 , the arrangement can be employed to inspect a broad area of a semiconductor device array comprising multiple arrayed devices.  
         [0021]     The infrared microscope  54  was selected to provide infrared sensitivity in the range 3-5 microns, although an available 1-5 micron camera also provided good results. An example of an infrared microscope is Infra Scope II Micro-Thermal Imager, manufactured by Quantum Focus Instrument Corporation, Vista, Calif.  
         [0022]     The flat-panel infrared illuminator  52  was selected to provide infrared emission with wavelength 3-5 micron or 1-5 micron. A commercial small-size flat-surface heater, such as a silicone rubber heater or a kapton insulated flexible heater can be conveniently used. The surface temperature is preferably in the range of 50 to 200° C. A thin copper sheet can be attached to the surface of the heater to provide a more uniform temperature distribution across the surface. The surface of the copper sheet (or the bare heater without the copper sheet) can be painted with a high-temperature paint to improve emissivity and diffuseness for source.  
         [0023]     It will be appreciated from the foregoing that the present invention provides a significant improvement in the field of inspection of semiconductor devices and arrays for cracks and defects. In particular, the invention allows for the inspection of multi-layer semiconductor structures and provides a high-contrast image in which any cracks are readily discernable. It will also be appreciated that although a specific embodiment of the invention has been described by way of illustration, various modifications may be made without departing from spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.