Patent Publication Number: US-7709794-B2

Title: Defect detection using time delay lock-in thermography (LIT) and dark field LIT

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to defect detection on a sample using time delay lock-in thermography (LIT) to ensure high throughput and improve defect detection sensitivity in production environments. Dark field illumination can be used to minimize background noise in certain LIT embodiments. 
     2. Related Art 
     During the manufacturing process samples may develop localized electrical defects that cause current leakage. Exemplary samples could include photovoltaic materials (e.g. 156 mm×156 mm wafers or 2160 mm×2460 mm panels), semiconductor wafers, or printed circuit boards (PCBs). Electrical defects, such as shunts and localized weak diodes, leak current and therefore can reduce the efficiency of the sample or even jeopardize the functioning of the devices on the sample. Therefore, it is highly desirable to accurately detect the positions of such electrical defects. 
     Defects have high current density passing through them and therefore heat up to a higher temperature than that of the sample. These temperature changes can be detected in the image from a focal plane array (FPA) IR camera. However, the change in temperature at a defect may be 5 orders of magnitude smaller than the background in the image. Thus, separating the defects from background noise may be challenging. 
     Lock-in thermography (LIT) is one known method for locating such defects. In LIT, the sample is modulated, e.g. by direct current injection into the sample or by photocurrent generated from illumination of the sample. When the modulation is by illumination, the method is sometimes called illuminated lock-in thermography (ILIT). Temperature changes caused by heating of the sample from the injected current or photocurrent are modulated at the same frequency. With either form of modulation, multiple frames of IR images are captured while the sample remains stationary. 
     Due to the shot noise of background IR radiation from the sample at room temperature as well as the very small temperature difference between the defects and the rest of the sample, and the limited dynamic range of the IR imaging sensor, a large number of images of the same field of view (FOV) are needed to average out the background noise, thereby improving the signal to noise ratio (SNR). Although the captured images are taken from the identical spatial location, they are a function of time as the temperature of the sample oscillates at the frequency of modulation. In a typical embodiment, the images are filtered by multiplying each image by a weighting factor that varies sinusoidally in time at the same frequency as the modulation or “lock-in” frequency. In general, the improvement of SNR is proportional to the square root of N, wherein N is the total number of frames. 
     Conventional LIT requires that the sample remains stationary while the IR camera acquires the necessary number of images for lock-in averaging. If the size of the sample is greater than the field of view (FOV) of the camera, the sample (or the IR camera) needs to move to a completely different location to capture a new set of IR images after one set of images is captured for one location on the sample. Unfortunately, this stop-go time as well as the settling time (which includes repositioning with its attendant velocity ramp up and ramp down) takes a large portion of the total inspection time, especially for very large samples that can be greater than 2 m×2 m in size, thereby undesirably reducing throughput. This overhead in conventional lock-in thermography becomes a significant limiting factor of inspection throughput. 
     Therefore, a need arises for a technique of detecting defects on a sample that increases inspection throughput compared to conventional LIT while maintaining its accuracy. 
     SUMMARY OF THE INVENTION 
     Conventional lock-in thermography (LIT) techniques require that the sample remains stationary while the IR camera acquires the necessary number of images for lock-in integration. After one set of images is acquired, the sample is replaced or repositioned to capture IR images for a different sample or location. This stationary and repositioning time significantly reduces inspection throughput. 
     To increase inspection throughput, a method of performing time delay LIT on a sample is provided. In this method, the FOV of an IR camera can be moved over the sample at a constant velocity. Throughout this moving, a modulation (e.g. optical or electrical) can be provided to the sample and IR images can be captured using the IR camera. Moving the FOV, providing the modulation, and capturing the IR images can be synchronized. The IR images can be filtered to generate the time delay LIT image, thereby providing defect identification. In one embodiment, this filtering can include sinusoidal weighting at the lock-in frequency that takes into account the number of pixels of the IR camera in a scanning direction. 
     Advantageously, this time delay LIT can be used on various types of samples, e.g. semiconductor wafers, photovoltaic wafers, large panels of photovoltaic material, continuous webs of photovoltaic material, and printed circuit boards. Further, the moving can be done using any efficient moving components, e.g. a scanning stage, bi-directional linear stages in a gantry system, a gantry bridge, a conveyor, and/or at least one roller. 
     In one embodiment, the FOV can be located within a dark field region throughout the moving, thereby providing an improved signal-to-noise ratio (SNR) during filtering. This dark field technique can also be used in what would otherwise be standard ILIT. In this method, the sample is illuminated outside the camera FOV. IR images can be captured using the IR camera, wherein providing the modulation and capturing the IR images are synchronized. The IR images can be filtered to generate the time-averaged image, thereby providing defect identification. Advantageously, the sample can be rotated or moved linearly to reposition the FOV and the dark field region on another section of the sample. At this point, the steps of providing the modulation, capturing the IR images, and filtering the IR images can be repeated. 
     This dark field technique can be used with various types of samples, e.g. semiconductor wafers, photovoltaic wafers, photovoltaic panels, continuous webs of deposited photovoltaic material, and printed circuit boards. Positioning and rotating can include using a scanning stage, bi-directional linear stages in a gantry system, a gantry bridge, a conveyor, a rotating chuck, and/or at least one roller. 
     A system for performing the time delay LIT can include an IR camera for capturing images of the sample. Scanning components can move the FOV of the IR camera over the sample at a constant velocity. Modulation components can provide a modulation to the sample when moving the FOV. A clock source can synchronize the capturing of images, the moving of the FOV, and the source of the modulation. An image processor can receive the captured images and generate the time delay LIT image to provide defect detection. In one embodiment, a light shield is used to shadow the FOV from the source of illumination for ILIT. 
     A system for performing dark field ILIT can include positioning components for positioning the FOV of the IR camera over the sample. Optical modulation components can provide an optical modulation to the sample after positioning the FOV. A light directing component can provide a dark field region for the FOV. A clock source can synchronize the image acquisition to the modulation. An image processor can receive the captured images and generate the time delay ILIT image to detect defects on the sample. The light directing component can include a light shield or a light pipe. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates an exemplary time delay ILIT system including a dark field illumination. 
         FIG. 2A  illustrates an exemplary acquisition of frames of IR images using conventional LIT. 
         FIG. 2B  illustrates an exemplary acquisition of frames of IR images using time delay LIT. 
         FIG. 2C  illustrates an exemplary sample modulation relative to a plurality of frame triggers. 
         FIG. 3  illustrates an exemplary inspection system including a single IR camera that can move in both x and y directions using a gantry system. 
         FIG. 4  illustrates an exemplary inspection system including multiple IR cameras that can move in one direction using a gantry system. 
         FIG. 5  illustrates an exemplary inspection system including multiple IR cameras that capture images of samples moving on a conveyor. 
         FIG. 6  illustrates an exemplary dark field illumination for the field of view (FOV) that can further minimize background noise. 
         FIG. 7  illustrates an exemplary dark field FOV experimental result, wherein an expanded laser beam modulates current for an illuminated area of the sample. 
         FIG. 8  illustrates an illumination system that can include a light pipe, which ensures that the light generated by a light source is efficiently relayed to a surface of the sample. 
         FIGS. 9A and 9B  illustrate the rotation of a sample to reposition the dark field region for the FOV beneath an exemplary light pipe configuration that can be particularly efficient for smaller samples in an ILIT system. 
         FIG. 10  illustrates an exemplary dark field ILIT system that uses the light pipe configuration of  FIGS. 9A and 9B . 
         FIGS. 11 and 12  illustrate other exemplary dark field ILIT configurations using rotational and linear movements, respectively. 
         FIG. 13  illustrates the dark field ILIT configuration of  FIG. 11  in a system that includes both rotational and linear movements. 
         FIG. 14  illustrates a dark field ILIT in a system including at least one roller for moving a web sample. 
         FIG. 15  illustrates aspects of a solar cell that facilitate forward biasing or reverse biasing of the solar cell during inspection. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     Conventional lock-in thermography (LIT) systems require that the sample remains stationary while the IR camera acquires the necessary number of images for lock-in integration. After one set of images are captured for one location on the sample, the sample is repositioned to capture IR images for a completely different location. This stationary and repositioning time significantly reduces inspection throughput. 
       FIG. 1  illustrates an exemplary time delay LIT system  100  that can significantly increase inspection throughput. In this embodiment, a sample  101  is positioned on an x-y scanning stage  102 . Applying a modulation to the sample can be performed optically (e.g. by using a modulated illuminating light source) or electrically (e.g. by directly applying a current modulation to the sample). In one embodiment, a current driver  106  can be selectively connected to a light source  103  or directly connected to sample  101  using a switch  112 . In other embodiments, system  100  can include the components to provide only one type of modulation, i.e. current driver  106  and light source  103  or only current driver  106 , and eliminate switch  112 . 
     Light source  103  can be constructed using multiple LED modules. However, in other embodiments, light source  103  can be implemented using a standard white light source modulated by a chopper, lasers that are directly modulated, or Q-switch lasers. 
     A clock source  104  can generate a waveform  105 , which is provided to current driver  106 . This waveform is converted to a current that, as described above, can drive light source  103  or is directly connected to sample  101 . Clock source  104  can also generate triggers  107  that activate an IR camera  108  to capture IR images, which in turn are provided to an image processor  110 . Clock source  104  can be connected to a stage controller  109 , which outputs a positioning encoder pulse to scanning stage  102 . In this configuration, as described in further detail below, clock source  104  can advantageously ensure that the speed of sample motion is properly synchronized to the image acquisition frame rate and the modulation rate. In other embodiments, the encoder signal of the stage controller can be used as the clock signal to trigger a function generator for providing modulation to the sample, and also for triggering the IR camera for image acquisition. 
       FIG. 2A  illustrates an exemplary acquisition of frames  201  of IR images using conventional LIT. As described above, to acquire frames  201 , the sample is modulated with a periodic signal, e.g. a sinusoidal function, while the sample remains stationary. Frames  201  are then processed by applying a Fourier filter in time domain at the frequency of modulation. 
     In one embodiment, the discrete sine and cosine transforms are defined as follows. 
     
       
         
           
             
               
                 
                   
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     Where I m,n   i  is the pixel value of the (m,n)th pixel of the ith frame, m=1, 2, . . . N x , n=1, 2, . . . N y , i=1, 2, 3 . . . , f 1  is the frequency of modulation, f 2  is the frame rate (preferably an even integer multiple of f 1 ), P is the pixel size on the sample, N x  and N y  are the number of pixels in one frame in the x and y directions, and N F  is the total number of frames (e.g. an integer multiple of the number of modulation cycles). 
     Note that certain samples may respond differently to different phases of modulation. However, notably, the sine and cosine transforms can be combined to generate an amplitude independent of phase. Specifically, using the values for S m,n  and C m,n  as computed by Equations 1 and 2, the amplitude A and phase image φ are given by:
 
 A =√{square root over ( S   2   +C   2 )}  Equation 3
 
     
       
         
           
             
               
                 
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     In contrast,  FIG. 2B  illustrates an exemplary acquisition of frames  202  of IR images using time delay LIT. As described above in reference to  FIG. 1 , unlike conventional LIT, multiple image frames are acquired in time delay LIT while the sample moves at a constant speed (thus, the imaged locations as measured in a y direction change over time). Advantageously, the speed of motion (dy/dt) can be synchronized to the frame rate of the image acquisition. 
     In one embodiment, the sample can move by a distance of one pixel within the time duration of one frame. Thus, in one embodiment, the total number of frames for time delay LIT is the same as the number of pixels of the FOV of the IR camera in the scan direction. Note that image capture can begin with the FOV only slightly overlapping the sample (e.g. by one pixel or less) to ensure that even the edges of the sample are in fact imaged multiple times. 
     In other embodiments, the distance that a sample moves between two consecutive frames can be integer multiples, e.g. 1, 2, 3 . . . pixels, which allows higher inspection speed at a fixed frame rate. The integer multiple approach provides lower sensitivity because the total number of frames for LIT is reduced by a factor equal to the number of pixels moved. In yet another embodiment, the distance that the sample moves between two consecutive frames can be less than 1 pixel (e.g. generically 1/N pixel: ⅕ pixel, ¼ pixel, ⅓ pixel, ½ pixel, etc.), which allows higher inspection accuracy, but results in slower inspection speed. In one embodiment, a predetermined number of frames can be designated for capture during each modulation cycle (e.g. at least 4), thereby determining inspection accuracy as well as the allowed inspection speed. 
     In accordance with any embodiment of time delay LIT, as the sample is modulated at a fixed frequency, each imaging pixel of the sample is imaged multiple times as the sample continuously moves across the field of view (FOV) of the IR camera. Therefore, an image for each imaging pixel is read out multiple times by a line of the pixels of the IR imaging sensor, which can form part of the IR camera. The captured images in a time delay LIT image are given by the following sine and cosine transforms, which together provide Fourier filtering. 
     
       
         
           
             
               
                 
                   
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     Where I m,n   (i+n−1)  is the pixel value of the (m,n)th pixel of the (i+n−1)th frame of the IR images, i=1, 2 . . . , m=1, 2, . . . N x , n=1, 2, . . . N y , f 1  is the frequency of modulation, and f 2  is the frame rate. Preferably f 2  is an even integer (≧4) multiple of f 1 . N x  and N y  are the number of pixels in one frame in the x and y directions. Note that the index n appears in both the subscripts of pixel index and the superscript of frame index of I m,n   (i+n−1) , which defines the tracking each pixel of a specific spatial position as it moves across the FOV of the IR camera. The speed V of the moving sample is given by:
 
V=Pf 2   Equation 7
 
     Where P is the pixel size on sample. As described above, the speed V of the moving sample, the sample modulation, and the frame triggers can be synchronized to ensure a desired frame capture.  FIG. 2C  illustrates an exemplary sample modulation  203  relative to a plurality of frame triggers  204 . In other embodiments, the speed of the moving sample can be generalized to be greater than or less than 1 pixel per frame interval (time duration between two consecutive frames); equation 7 is then written as:
 
V=kPf 2 .  Equation 8
 
     In one embodiment, k can be an integer of greater than 1, for example, k=2, 3, 4, . . . . In this case, the pixels of each frame can be binned in the scan (y) direction by the number of pixels equal to k. The effective number of pixels in the y direction is reduced by a factor of k, and equations 5 and 6 still apply as long as the image is down-sampled to the effective number of pixels. In another embodiment, k can be less than 1. For example, the sample may move half a pixel per frame interval when k=½, or one third of a pixel when k=⅓. In this case, the effective number of pixels per frame in the scan direction is increased by a factor of 1/k. The effective image may be reconstructed to larger size by re-sampling of the image through interpolation methods such as nearest neighborhood, linear, spline, or cubic interpolations. Equations 5 and 6 still apply as long as the image size in the scan direction is re-sampled to the effective number of pixels increased by the factor of 1/k. Note that the phase and amplitude can then be computed using equations 3 and 4. 
     Note that the sensor of the IR camera can have a rectangular format, with rectangular sensor elements (wherein a square is considered as a special case of a rectangle). In one embodiment, the sample moves at a constant speed in a direction parallel to one of the edges of the rectangular sensor. Note that P, i.e. the imaging pixel size on the sample, can be computed by the size of the sensor element along the scan direction divided by the magnification of the imaging lens. 
     In one embodiment of image processor  110 , a technique called time delayed integration (TDI) can synchronize pixel shifting with movement of the sample. TDI is described in detail in Reissue U.S. Pat. RE 37,740, entitled “Method and apparatus for optical inspection of substrates”, which issued on Jun. 11, 2002. However, in this reference, TDI captures only one instance of each imaging pixel (i.e. a line scan imaging mode). Notably, TDI can be modified to keep track of multiple captured images for each imaging pixel as the FOV moves across the sample, thereby allowing TDI to be used in the context of time delay LIT. This tracking can be performed by a computer-implemented software program installed in image processor  110 . 
     Moreover, also in image processor  110 , a single frequency Fourier filter (or matched filter, at the same frequency of modulation) in the time domain can be applied to the captured image, over a window of the multiple frames. As described above, each frame can be shifted by a predetermined number of pixels (1, 2, 3 . . . ) in the scan direction when applying the Fourier filter. 
     In Equations 5 and 6, each x-column i in the final image is a weighted sum from multiple frames of images, where image n contributes to this sum the column i+n−1. 
     By using a continuous scan of a sample, time delay LIT can advantageously eliminate the undesirable stop-go action of conventional LIT inspection systems, thereby significantly reducing inspection overhead time. Therefore, high throughput inspection in a production environment can be implemented. Notably, by varying the number of pixels moved, time delay LIT can advantageously optimize a desired speed/sensitivity balance. 
     Note that when the images of the sample are captured, the sample could be moving with respect to the IR camera (e.g. using scanning stage  102  of  FIG. 1 ) or the IR camera could be moving with respect to the sample. For example,  FIG. 3  illustrates an exemplary inspection system  300  including a single IR camera  301  that can move in both x and y directions by a gantry system, which includes linear stages  302  that allow camera movement in an x direction and a linear stage  303  that allows camera movement in a y direction. As shown in  FIG. 3 , alternating horizontal and vertical movements result in a serpentine scan of a sample  304 . 
     In this embodiment, sample  304  is a single sample (e.g. a thin film, large-scale solar panel formed on a glass substrate). Note that in other embodiments using this gantry system, sample  304  could be replaced with multiple samples. 
     Multiple parallel IR cameras can further improve inspection speed. For example,  FIG. 4  illustrates an exemplary inspection system  400  including 3 IR cameras  401 , although other embodiments can include fewer or more IR cameras (note that other system components, such as those components shown in  FIG. 1 , are not shown for simplicity). In this embodiment, IR cameras  401  can provide a single pass scan in a direction  402  using a gantry bridge  403 . 
       FIG. 5  illustrates an exemplary inspection system  500  including 4 IR cameras  501 , although other embodiments can include fewer or more IR cameras. In this embodiment, IR cameras  501  can be positioned on a stationary beam  502 , whereas samples  503  can move in a direction  504  using tracks  505 , which form part of a conveyor  506 . 
     In one embodiment, an IR camera can be implemented using a medium wave infrared (MWIR) camera having a sensor resolution of 320×256 pixels. The inspection system including this IR camera can include the following operating characteristics: a frame rate of 433 frames per second (fps), an imaging resolution of 0.5 mm, a sample speed of 216 mm/s, and an inspection speed of 276 cm 2 /s. 
     Referring back to the time delay LIT system  100 , the use of light source  103  to provide current modulation can result in some heat generation. Specifically in the case of solar cells, some portion of the illumination light is converted to heat due to the limited efficiency of solar cells to convert light power to electric power. The heat generated by the illumination can increase the background IR emission, which results in greater background noise and thus lower detection sensitivity. Notably, because the excessive heat due to illumination is generated at the same frequency as the defect signal modulation, the emissivity difference between different materials (such as metal grid lines vs. silicon) shows in the LIT image as a non-uniform background noise that may not be easily removed, thereby further reducing the defect sensitivity. 
     Therefore, in one embodiment, system  100  can use a light shield  111  to create a dark field region for the FOV of the IR camera. In one embodiment, light shield  111  can be positioned above sample  101  by 2-4 mm, or any other distance that limits illumination of the sample. For example,  FIG. 6  illustrates a dark field region  602  that could be provided by light shield  111  for protecting an FOV  603  on a sample  601 . In this case, an illuminated area  604  occurs outside dark field region  602 . Notably, although illuminated area  604  is limited to be outside of FOV  603 , the photocurrent generated by such illumination can quickly flow into the area of FOV  603 . 
     Therefore, the sample heating due to excessive photon energy is constrained to be outside of FOV  603 . As a result, this indirect illumination advantageously minimizes the background noise inside FOV  603 . However, of interest, despite using dark field region  602  for FOV  603 , defects are still visible to the IR camera. 
     For example,  FIG. 7  illustrates an exemplary experimental result, wherein an expanded laser beam modulates current for an illuminated area  702  of the sample. Defects that leak current appear as hot spots  701 . As shown in  FIG. 7 , (1) the background heating is higher where the light directly illuminates the sample, i.e. inside illuminated area  702 , (2) the background heating is much lower outside illuminated area  702 , and (3) the defects still appear as hot spots  701  even though they are outside illumination area  702  because current flows freely across the sample. 
     Referring back to  FIG. 1 , a predetermined area outside the FOV of IR camera  108  (e.g. a band of illumination substantially parallel to the border of the FOV) can be illuminated by light source  103  (e.g. an array of LEDs) as defined by light shield  111 . Notably, light shield  111  can advantageously reduce the background heating of the FOV, thereby increasing the signal to noise ratio (SNR) of the defect in the captured images. Better SNR results in higher throughput (i.e. shorter integration times at a given sensitivity) and/or higher sensitivity. 
     In one embodiment shown in  FIG. 8 , an illumination system  800  can include a light pipe  802  that can ensure that the light generated by a light source  801  is efficiently relayed to a surface of sample  804  without a light shield. Note that light pipes can be particularly effective for analyzing smaller samples, e.g. small-scale solar cells (for example, 6″×6″) and semiconductor wafers, to limit light dispersion to only the samples for which images are being collected. In one embodiment, to further limit light dispersion, an optional Fresnel lens  803  can be used to focus the light from light pipe  802  onto sample  804 . 
     Light pipe  802  can be implemented using a solid block of glass that guides the light by total internal reflection of the sidewalls of light pipe  802 . In another embodiment, light pipe  802  can be implemented using a hollow tube with mirror surfaces inside. In any implementation of light pipe  802 , a clearly defined illumination area (e.g. rectangular) is projected into sample  804 . 
     Advantageously, a light pipe can be configured to cover large or small areas of a sample. In any configuration, a light pipe can provide a relatively sharply defined border for the dark field region as well as the illuminated area. For example, a light pipe could sharply define the borders of illuminated area  604  of  FIG. 6  (and thus also the border of dark field region  602 ). In contrast, the outside border of illuminated area  604 , if created by a light shield, would typically be diffused, whereas the inside border would be relatively sharply defined (assuming that the light shield is close enough to the sample). 
       FIGS. 9A and 9B  illustrate an exemplary configuration for a light pipe configuration that can be particularly efficient for smaller samples, e.g. semiconductor wafers or solar cells, in what would otherwise be a conventional LIT system. In this configuration, a sample  910  can be divided into (i.e. characterized as having)  4  quadrants, e.g.  901 ,  902 ,  903 , and  904 , and the shape of a light pipe  900  is substantially matched to three quadrants of sample  910 . In  FIG. 9A , quadrants  902 ,  903 , and  904  are illuminated by light pipe  900 , whereas quadrant  901 , which is in a dark field region, can be imaged by an IR camera (not shown for simplicity). Another quadrant can be imaged by rotating sample  910  relative to light pipe  900 . For example, from  FIG. 9A  to  FIG. 9B , sample  910  is rotated counter clockwise by 90 degrees relative to light pipe  900 . Thus, quadrants  901 ,  903 , and  904  are illuminated by light pipe  900 , whereas quadrant  902 , which is in dark field region, can be imaged by the IR camera. Therefore, all quadrants  901 ,  902 ,  903 , and  904  can be inspected by rotating sample  910  three times. 
       FIG. 10  illustrates an exemplary dark field LIT system  1000  including light pipe  900  and sample  910 . In system  1000 , sample  910  is positioned on a rotating chuck  1001  that can perform the desired rotations (e.g. 90 degree rotations). Light pipe  900  can direct the light from LED module  1002  onto sample  910 . An IR camera  1003  can capture images from the dark field quadrant of sample  910 . In this embodiment, IR camera  1003  can capture multiple shots of the dark field quadrant over time as sample  910  is current modulated by the light directed by light pipe  900 . After a desired number of images have been captured by IR camera  1003 , rotating chuck  1001  can be rotated to expose another quadrant of sample  910 . 
     In other embodiments, a multi-sample dark field LIT system can be implemented. For example,  FIG. 11  illustrates an exemplary configuration including four samples  1101 . Block  1102  delineates the border of a dark field region. In this case, after an IR camera (not shown for simplicity) simultaneously captures the desired number of dark field images from samples  1101 , then each of samples  1101  can be rotated (e.g. clockwise by 90 degrees as shown by the arrows using four chucks, not shown for simplicity) to begin capturing images from different quadrants of samples  1101 . 
     Note that other embodiments can include different divisions of the sample. For example,  FIG. 12  illustrates an exemplary configuration including a dark field region  1200  and three samples  1201 ,  1202 , and  1203  on a conveyor belt  1204 . In this case, the camera first images the left side of sample  1201  and the right side of sample  1202  within dark field region  1200 . The conveyor belt  1204  next moves one sample width to the right (i.e. in a linear motion, as indicated by the arrow), and the camera images the left side of sample  1202  and the right side of sample  1203  within dark field region  1200 . In another embodiment, the conveyor belt moves continuously and time delayed lock-in thermography is used to process the image as described earlier. In this embodiment, the width of the FOV must be less than the width of the sample so that part of the sample is always illuminated as the sample passes beneath the dark field region. For example, for a rectangular FPA with 320×256 pixels, the IR camera would be oriented so that the width of the cell normal to the direction of motion is covered by 320 pixels, and the width of the cell parallel to the direction of motion is covered by 256 pixels. 
     In one embodiment, both rotational and linear movements can be included in a dark field LIT system. For example,  FIG. 13  illustrates a dark field LIT system configuration  1300  including a plurality of samples  1301  that can be positioned on rotating chucks  1304  (one shown for simplicity), which in turn can be secured to a conveyor  1303 . In the configuration shown in  FIG. 13 , four samples  1301  can be simultaneously imaged as described in reference to  FIG. 11 . After the desired images are captured from all quadrants (using rotating chucks  1304 ), then the next four samples  1301  can be moved into position (using conveyor  1303 ) relative to dark field region  1302  for the next round of image capture. 
     Notably, as shown above, providing the dark field region for the FOV can be included in both time delay LIT and conventional LIT systems to advantageously reduce background noise when optical modulation is used. Moreover, this dark field LIT can be used for numerous types of samples, e.g. semiconductor wafers, solar cells, solar panels, PCBs, and continuous webs. 
     For example,  FIG. 14  illustrates an exemplary dark field LIT system  1400  in which a web sample  1401  can be advanced using rollers  1403 . An exemplary web sample is a stainless steel ribbon (e.g. approximately 14 inches wide) on which photovoltaic material can be deposited. After the desired images are captured in a dark field region  1402 , another portion of web sample  1401  can be positioned under dark field region  1402  using rollers  1403  and then imaged. In one embodiment, dark field LIT system  1400  could include other rollers for positioning web sample  1401  for subsequent processing (e.g. physical cutting of web sample  1401 ). In another embodiment, dark field LIT system  1400  can be easily converted into a time delay, dark field LIT system. That is, rollers  1403  can be used to provide the constant velocity used in a time delay LIT system. Note that other embodiments can include fewer or more rollers to provide the advancement of the web sample. Typically, a system implementation using a web sample includes at least one roller. 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying figures, it is to be understood that the invention is not limited to those precise embodiments. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. As such, many modifications and variations will be apparent to practitioners skilled in this art. 
     For example, as described above for time delay LIT, when the images of the sample are captured, the sample could be moving with respect to the IR camera or the IR camera could be moving with respect to the sample. As used herein, moving an FOV of the IR camera over the sample is meant to describe either movement. Notably, either movement can provide the same captured images. 
     Further, note that when time delay LIT is combined with a dark field region for the inspection of multiple samples (e.g. see samples  503  of  FIG. 5 ), then the modulation of any one sample will vary over time (because the percentage of the sample exposed to the light field (versus dark field) varies over time). However, this modulation variation can be compensated for by the appropriate programming of the image processor (e.g. see image processor  110  of  FIG. 1 ). 
     Yet further, referring back to  FIG. 15 , two different electrical modulations can be performed on samples: forward bias electrical modulation and reverse bias electrical modulation. For example, in the case solar cell  1500 , a reverse bias could be applied by connecting the positive terminal to N-layer  1501  (e.g. using metallic fingers  1504  on the top surface of solar cell  1500 ) and the negative terminal to P-layer  1502  (e.g. using a metallic layer  1503  on the back surface of solar cell  1500 ). In contrast, a forward bias could be applied by connecting the negative terminal to N-layer  1501  and the positive terminal to P-layer  1502 . Each electrical modulation could be used to detect a different type of defect. For example, in one embodiment, the forward bias current modulation can be used to detect defects that behave more like a diode but have a low open circuit voltage. 
     Note that although the directed illumination configurations described herein provide a border of illumination around the FOV, other embodiments could provide different illumination shapes. That is, because current flows freely through the sample, another illumination configuration could include a plurality (≧2) of illuminated blocks distributed around the FOV that still allow modulation of the FOV. 
     Accordingly, it is intended that the scope of the invention be defined by the following claims and their equivalents.