Patent Publication Number: US-11639959-B2

Title: Defect localization in embedded memory

Description:
RELATED APPLICATION 
     The present application is a Divisional application of application Ser. No. 16/125,162, filed on Sep. 7, 2018, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to localization of defects in semiconductor devices. The present disclosure is particularly applicable to a system and related method for defect localization in embedded memory. 
     BACKGROUND 
     In failure localization of semiconductor devices, bitmapping is an efficient method to isolate embedded memory defects. However, the process of enabling bitmapping is time consuming and resource consuming. Bitmapping is available on high volume products where return on investment is high. Moreover, memory failure defect localization without the use of bitmapping suffers low success rate using conventional static failure analysis approaches Electrically-enhanced laser-assisted device alteration (EeLADA) is a feasible alternative method. However, the current state-of-art in failing bit-cell isolation is inconsistent and limited to a few tens of microns which is not desirable for subsequent physical failure analysis to reveal the defect. 
     A need therefore exists for a system and method for improved defect localization with EeLADA for isolating bit-cell defects. 
     SUMMARY 
     An aspect of the present disclosure is a system for defect localization of embedded memories using enhanced EeLADA at bit-cell resolution. The present system provides improved diagnostic resolution on the failing bit-cell. 
     Another aspect of the present disclosure is a method of defect localization of embedded memories using enhanced EeLADA at bit-cell resolution. 
     According to the present disclosure, some technical effects may be achieved in part by a system including automated testing equipment (ATE) interfaced with a wafer probe including a diagnostic laser for stimulating a device under test (DUT) with the diagnostic laser at a region of interest (ROI). The ATE is configured to simultaneously perform a test run at a test location of the DUT with a test pattern during stimulation of the DUT. Storage includes failing compare vectors of a reference failure log of a defective device. A first profile module is configured to generate a first three-dimensional (3D) profile from each pixel of a reference image of the defective device. A second profile module is configured to generate a second 3D profile from each pixel of the ROI of the DUT. A cross-correlation module is configured to execute a pixel-by-pixel cross-correlation from the first and second 3D profiles and generate a technical signal image corresponding to a level of correlation between the DUT and defective device. 
     Another aspect of the present disclosure is a method including generating a log file comprising fail pins and cycles based upon test vectors on a defective device. A 3D profile of a reference image is generated based upon the log file of the defective device. A DUT is stimulated with a laser from a wafer probe. A log file corresponding to each pixel in a region of interest of the DUT is simultaneously generated. A 3D-profile from each pixel of the region of interest of an image of DUT is generated. A pixel-by-pixel cross-correlation is performed to generate an intensity map that corresponds to a level of correlation between the defective device and DUT. 
     A further aspect of the present disclosure is a method including simultaneously testing a DUT by stimulating the DUT with a laser at a ROI and performing a test run of the DUT with a test pattern. A 3D-profile is generated from each pixel of the ROI. A pixel-by-pixel cross-correlation is performed to generate an intensity map that corresponds to a level of correlation between the DUT and a prior reference failure log. 
     Additional aspects and other features of the present disclosure will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
         FIG.  1    illustrates a block diagram of a test system for defect localization using EeLADA, in accordance with an exemplary embodiment; 
         FIG.  2    is a graphical illustration of a reference failing signature (two-dimensional based on one fail pin), in accordance with an exemplary embodiment; 
         FIG.  3    illustrates an example of a technical signal image, in accordance with an exemplary embodiment; 
         FIG.  4    illustrates an example of the analysis module of the test system of  FIG.  1   ; 
         FIG.  5    illustrates another example of the analysis module of test system of  FIG.  1   ; and 
         FIGS.  6 A,  6 B,  6 C and  6 D  are images demonstrating defect isolation of a programmed defect on a DUT. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring exemplary embodiments. In addition, unless otherwise indicated, all numbers expressing quantities, ratios, and numerical properties of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” 
     The present disclosure addresses and solves the current problem of localization of memory failure at bit-cell resolution using EeLADA. The problem is solved, inter alia, by improved EeLADA techniques to detect memory failures on a bit-cell resolution. The system and method include scanning of a laser and a performing test run of a DUT simultaneously. The test response from the stimulated test response is profiled as a 3D profile including test pins and cycles and occurrence. Each pixel of the stimulated test response is also profiled. A pixel-by pixel cross-correlation is performed to identify defects in the DUT. Additional comparisons to unmatched fail cycles are performed to further enhance the precision defect localization. 
     Still other aspects, features, and technical effects will be readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated. The disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
       FIG.  1    illustrates a block diagram of a test system for defect localization using EeLADA, in accordance with an exemplary embodiment. The test system facilitates defect isolation in devices. Defect localization is performed in devices such as semiconductor devices or integrated circuits (ICs). Embodiments include testing or analyzing devices or ICs in the manufacturing process to localize defects. Defect localization is facilitated by an EeLADA analysis system. The ICs tested can be any type of IC, such as dynamic or static random access memories, signal processors, microcontrollers or system-on-chip (SoC) devices. Other types of devices may also be useful. The test system of  FIG.  1    includes a scanning microscope module  101 , a test module  103 , and an analysis module  105 . The test system may optionally be provided with other modules. 
     In  FIG.  1   , the scanning microscope module  101  includes a laser source  107 , a photodetector unit  109 , a beam splitter  111 , a scanner unit  113 , a focusing unit  115 , a test stage  117 , an image processor  119  and a display  121 . The scanning microscope, for example, may be a commercially available laser scanning microscope. Such types of scanning microscopes may be from, for example, Thermofisher Scientific, Hamamatsu Photonics, Semicaps and Checkpoint Technologies. 
     The test module  103  in  FIG.  1    includes a reference failure log  123 , a test unit  125 , and a probe stack  127 . The test module includes, for example, commercially available automated testing equipment (ATE) from, for example, Advantest, Teradyne, LTX-Credence, and National Instruments. Other types of ATE may also be useful. 
     The various modules are configured to test and analyze a DUT  129 . For example, a DUT  129  is provided with test signals from the test module  103  and scanned with a laser beam by the scanning microscope module  101 . The laser beam serves to perturb or stimulate the DUT  129  for testing as well as capture the DUT&#39;s image pixel-by-pixel. The image of the DUT  129  may be displayed on the display  121  for user inspection. The DUT  129  is mounted onto the test stage  117 . For example, the test stage  117  supports the DUT  129  for testing. In one embodiment, the DUT  129  is an IC. The DUT  129  may be an individual IC. For example, the DUT  129  may be a die which has been singulated from a wafer with a plurality of ICs by dicing the wafer. Providing unsingulated dies for testing on the test stage may also be useful. The IC includes a plurality of metallization layers formed over the substrate or wafer for interconnecting circuit components, such as transistors, capacitors and resistors. The side of the IC which has the metallization layers is referred to as the “frontside” while the opposite side of the IC is referred to as the “backside”. The IC includes a plurality of pins which allow access to the internal circuitry. For example, the pins may include power and signal pins. The power pins may include various power sources, including ground while signal pins may include input/output (I/O) pins. The signal pins may be bidirectional, unidirectional or a combination thereof. The pins may be in the form of pads for an unpackaged IC. In some cases, the pins may be contact bumps, such as a wafer level packaged IC. The pads or contact bumps are disposed on the frontside of the IC. In other cases, the IC may be a fully packaged IC. In such cases, at least a part of the package is removed for access by the scanning microscope. For example, at least the side of the package which covers the backside of the die is removed to expose the backside of the die for access by the scanning microscope. Decapping to expose the backside of the die may be achieved by laser or chemical techniques. In one embodiment, the backside of the die or IC is disposed on the surface of the test stage  117 . For example, the test stage  117  includes a cavity for accessing by the laser for scanning and defect isolation. 
     The laser  107  generates a radiation or light beam which is directed to the backside of the die. For example, the beam is focused on the backside of the die through the test stage  117 . The wavelength of the laser beam may be from about 1000-1400 nm. Other wavelengths may also be useful. The wavelength used may depend on the type or material of the substrate of the die as well as application method. For example, the wavelength should be below the bandgap of the substrate material of the die. The laser may be configured to operate as a continuous-wave laser or a pulsed laser. 
     The laser can be configured to operate as a pulsed laser (e.g., pulse mode). Various techniques may be employed to configure the laser to operate in the pulse mode. For example, an electro-optical modulator (EOM), a mode-locker, or a laser chopper may be employed. The frequency of the pulsed beam may be from about 1 kHz to about 500 kHz or greater. Other pulse frequencies may also be useful, for example, a pulse width of a laser beam is preferably less than 200 μs. The pulse width may be about 50 μs. Other pulse widths may also be useful. In one embodiment, the duty cycle of the pulse width is about 50%. Other duty cycles may also be useful. 
     The test system may be employed to identify soft or hard IC failures. In the case of identifying soft failures, the laser may be operated in either a continuous or a pulse mode. To identify hard failures, the laser may be operated in either a continuous or a pulse mode but the pulse mode is more effective. Other configurations of the laser for defect analysis may also be useful. 
     The incident laser beam is used to perturb the electrical characteristics of the transistors during testing. For example, the incident laser beam may serve as a heating source to heat the backside of the die to perturb the IC. The laser may be in continuous mode at a wavelength of around 1340 nm. To generate carriers, the laser may be in the continuous or pulse mode and the wavelength may be about 1064 nm. For example, the carrier generation can be caused by optical beam induced current (OBIC) effects. 
     The scanner  113  is employed to scan the backside of the DUT  129  with the laser beam. For example, the scanner  113  is disposed in the path of the laser beam from the laser source  107  and directs the beam to the backside of the DUT  129 . The scanner  113 , for example, may be controlled to scan the laser or laser beam in an x-y direction in the plane of the back of the DUT  129 . Various types of scanners  113  for scanning the laser may be used. For example, the scanner  113  may be a step (non-continuous) or raster (continuous) scanner. The scanner  113 , for example, scans the complete backside of the IC pixel-by-pixel. The scanner may include an output position signal which enables determination of the position of the laser beam on the backside of the die or DUT  129 . 
     In one embodiment, the focusing unit  115 , which is disposed in the beam path between the scanner  113  and test stage  117 , focuses the laser beam from the scanner  113  to the backside of the IC. The focusing unit  115 , for example, may be an optical column. For example, the focusing unit may include an objective lens for focusing the beam onto the backside of the IC. The lens may be an air gap or immersion lens. Other type of lenses or focusing units may also be useful. For example, the focusing unit may include curved mirrors. The focusing unit focuses the beam having a predetermined spot size. The spot size, for example, may be about 150-200 nm. Other spot sizes may also be useful. The spot size, for example, depends on the focal length of the lens of the focusing unit. The focal length of the lens can be selected depending on a desired resolution limit for the measurements. 
     The laser is also used to obtain a light image of the portion of the DUT or IC  129  on which the beam is focused. The light image is obtained from the reflected laser beam. For example, the reflected laser beam from the backside of the DUT  129  is sampled to obtain the image. The reflected laser beam is directed to the photodetector  109  via the beam splitter  111 , which is located between the laser  107  and scanner  113 . The photodetector  109  detects the reflected beam and generates a detector output signal of the reflected image. For example, the photodetector  109  detects the intensity of the reflected beam and generates a detector output signal. 
     The image processor  119  processes the detector output signal and generates an image of the portion of the DUT  129  sampled. The image, for example, is a reflected laser image of a pixel of the DUT  129  sampled. The location of the pixel may be determined by the location output signal from the scanner  113 . The image may be displayed on the display  121 . For example, as each pixel of the DUT  129  is scanned, the image may be displayed on the display  121  in real time. The image may be stored in memory. For example, the reflected laser image may be stored in memory of the image processor  119 . The image may be stored in other storage locations. For example, the image may be stored in a server. 
     A reflected laser image of the DUT  129  may be obtained by scanning the whole DUT  129  with the laser beam. For example, scanning the DUT  129  pixel-by-pixel with the laser beam may be employed to generate a complete image of the DUT  129 . In one embodiment, prior to commencing the testing, a complete image of the DUT  129  is obtained. 
     The test module  103  includes a reference failure log unit  123 . The reference failure log unit  123  contains prior failure data of interest. The prior failure data of interest, for example, are obtained from a sort test of failed ICs and logging the failing compare vectors. Other techniques for obtaining prior failures or failing compare vectors of interest may also be useful. In one embodiment, the failing compare vectors are test vectors of interest from the sort test. As shown in  FIG.  1   , the tester unit  125  receives a test pattern  131  for testing the DUT  129 . The test pattern  131 , for example, is (3D matrix of test vectors corresponding to specific pin names of the IC and cycle numbers. For example, the test vectors may be pointers to bias or a waveform table. The test vectors may be input or outputs. In the case of inputs, they serve as driving signals. In the case of outputs, also called compare test vectors, they serve as expected signals which are compared with actual outputs from the DUT  129  corresponding to the cycle. The rising edge of the tester&#39;s clock signal may serve as a reference when input vectors are applied or when output vectors are compared. Typically, one vector or pin is tested per clock cycle. Testing more than one vector per clock cycle may also be useful. Typically, there is more than one pin under test in a clock cycle. Testing, for example, is performed while an image of the IC is being obtained from an initial scan. The synchronous or simultaneous scanning of the laser and test run is illustrated in  FIG.  1    by way of directional line  130  connecting tester  125  and scanner  113 . 
     The probe stack  127  is mounted onto the DUT  129 . For example, the probe stack  127  includes electrical connections connected to pads or contacts of the IC. This enables the tester unit  125  to communicate with the IC. For example, the tester unit  125  provides a test pattern to the IC for testing as well as reads the outputs from the IC for comparison with expected values via the probe stack  127 . Within a test cycle, the laser beam scans the backside of the DUT  129  to perturb the IC one test location (pixel location) at a time until the whole DUT  129  is tested. Testing at each location may be referred to as a complete test run of all the test cycles or test sequence. At each location or test run, the tester tests the IC with the test pattern. The result of the test pattern (e.g., measured or output test vector) is compared with the expected values of the test pattern to determine whether the output test vector is a failed test vector. For example, failed test vectors are output test vectors which do not match the expected values. The analysis module  105  includes a first profiler unit  133 , second profiler unit  135 , cross-correlation function module  137  and image processing unit  139 . The first profiler  133  can be implemented either using hardware or software methods. The first profiler is configured to generate a 3D profile from the reference failure log  123  of the defective or bad device. The failing details on a bad or defective device represent the reference failing signature. 
     As shown in  FIG.  2   , a graphical representation of a number of occurrences (Y-axis) at different failing cycles (X-axis) of a reference fail signature is illustrated. In this embodiment, there is 1 failing pin with 20 failing cycles (X-axis) and the occurrences per cycle are registered and normalized. This forms the reference profile. During EeLADA evaluation on a DUT  129  (i.e., good die), as the laser rasters the region of interest on the DUT, a laser-stimulated profile (not shown for illustrative convenience) is generated for each pixel. The cross-correlation function module  137  then performs pixel-by-pixel comparisons of failing signatures between reference and different pixels as stimulated on the DUT. The cross-correlation function module  137  generates a signal image  141  based on the pixel-by-pixel comparison. The signal image  141  is created by the cross-correlation function module  137  based on the level of correlation. A threshold on the correlation level determines the final signal image that suggests the localized signals. 
     In an alternative embodiment, the 3D profile can be generated by plotting out a current profile of an entire test run. A DUT can have at least one power supply. When the laser rasters pixel-to-pixel, at each pixel, there is a complete test run on the test pattern, as discussed above. Therefore, at each pixel, the current values of the power supplies can be extracted at each cycle. When a device is powered up without a test pattern run, the current is stable at a DC value. When a test pattern is run, however, the current will fluctuate. The current fluctuation provides a profile comparison. 
       FIG.  3    represents an example of a signal image  141  with correlation represented by way of the brightness of the pixels in grey scale. Pixel blocks  143  represents a correlation near 0 and pixel blocks  145  represents a correlation near 1. The remaining pixel blocks represent varying correlations between 0 and 1. The bright pixel blocks  145  on technical image  141  represent a match between the reference failing profile to each stimulated event. 
       FIG.  4    further illustrates the analysis module  105  of test system of  FIG.  1   . As discussed previously, the first profiler  133  receive reference failure log  123  from the bad or defective device. The first profiler creates a 3D profile  147  of the reference failure log  123 . The x-axis represents fail cycles, the y-axis represents fail pins and the z-axis represents occurrence frequency or cycle repeats. The second profiler  135  creates a 3D profile  149  from the DUT  129 . 
     The 3D profile  149  is a profile of each pixel of a ROI of the DUT stimulated EeLADA evaluation on a DUT  129 . The number of profiles is equivalent to the total number of pixels in the ROI frame. 
     A pixel-by-pixel cross-correlation is performed by the cross-correlation module  137  to generate the signal image  141 . As discussed previously, signal image  141  depicts a correlation by way of the brightness of the pixels. Pixel blocks  143  represents a correlation near 0 and pixel blocks  145  represents a correlation near 1. The bright pixel blocks  145  on technical image  141  represent a match between the reference failing profile to each stimulated event. 
     A final signal technical image or defect isolation signal image  151  is generated by the image processing unit  139 . Pixel blocks  153  in the final signal technical image or defect isolation signal image  151  represent bit cell level failure in the DUT  129 . Additional comparisons to unmatched fail cycles  155  can be performed to further enhance the precision of the defect isolation. 
       FIG.  5    further illustrates another example of the analysis module  105  of test system of  FIG.  1   . The difference between the embodiments of  FIG.  4    and  FIG.  5    concerns the processing of the un-matched fail cycles  155  to the reference. For example, the laser-stimulated profile  149  may comprise fail cycles that are not present in the reference profile  147 . The present embodiment includes these un-matched cycles as the profile for cross-correlation. The presence of these un-matched cycles in the cross-correlation module affects the level of correlation significantly and improves the quality signal images  141  and  151  in some cases. It should be highlighted the un-matched events may refer to un-matched fail pin information in addition to fail cycles. 
       FIGS.  6 A through  6 D  represent experimental test images following a programmed defect in a DUT. In the experimental test, one or more programmed defects is created on a good die using 1340 nm continuous wave (cw) laser and solid immersion lens (SIL). In this example, the black spot  157  in the reflected laser optical image, as shown in  FIG.  6 A , indicates the location of a single defect. EeLADA is performed on the DUT and the results are presented in a signal technical image or defect isolation signal image of  FIG.  6 B . By overlaying the optical image and signal technical image, the known defect location  157  is compared to the EeLADA signal. As shown in the enlarged image of  FIG.  6 D , the accuracy of the defect localization is about 3 μm. 
     The embodiments of the present disclosure can achieve several technical effects, such as improved diagnostic resolution of embedded memory on the failing bit-cell. Devices formed in accordance with embodiments of the present disclosure enjoy utility in various industrial applications, e.g., microprocessors, smart phones, mobile phones, cellular handsets, set-top boxes, DVD recorders and players, automotive navigation, printers and peripherals, networking and telecom equipment, gaming systems, and digital cameras. The present disclosure enjoys industrial applicability in any of various types of semiconductor devices including embedded memory and application-specific integrated circuits. 
     In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.