Abstract:
A system and method for detecting defects in semiconductor wafers in a rapid non-destructive manner. Defects in semiconductor wafers can include micropipes and screw dislocations, stress striations, planer defects, polytype inclusions, and others. When a wafer is illuminated by polarized light, the defects induce birefringence of the polarized light that can be visualized by a polariscope to detect defects in wafers. Defects can cause linearly inputted polarized light to emerge as elliptically polarized light after transmission through a wafer having defects. Placing the wafer between a set of polarizers under the cross poles condition allows for a rapid non-destructive system and method for delineating and locating defects within a semiconductor wafer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention is directed to a system and method for determining defect delineation in semiconductor wafers and more specifically a non-destructive system and method for utilizing polarized light microscopy to delineate and map defects in semiconductor wafers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor devices, such as diodes, transistors, and integrated circuits are found everywhere in modern society. These devices are used in automobiles, cell phones, computers, televisions, satellites, and many other products. The semiconductor market has grown at a staggering pace over the years with improvements in performance and reduction in cost measured in orders of magnitude. This drastic improvement in semiconductor technology is due to the skill and technology advances in the ability to miniaturize devices so that more complicated devices can occupy smaller footprints on a wafer. The technology for the manufacture of semiconductors, specifically silicon devices, has matured into a reproducible and reliable manufacturing process. However, the manufacturing process is not perfect and wafers are still created that contain defects. Some of these defects prevent the full area of the wafer from being used as any device located over such a defect causes the device to fail or otherwise become inoperable. When an imperfection exists on a wafer, it is advantageous to know the location and level of imperfection so that the area sufficient for placing a device on a wafer can be known. Further, by performing the automatic mapping of defects during the manufacturing process, the growth parameters can be advantageously modified for improving crystal quality in subsequent wafer growth. Once the wafers are manufactured, the invention can assist with insuring quality control by measuring defects in specific production runs.  
           [0003]    Referring now to a specific example, silicon carbide (SiC) wafers can be used for manufacturing high-frequency, high-powered, or high-temperature operating conditions for devices. Some of the advantages of SiC devices are that they have lower power dissipation, lower current leakage, and higher operational temperatures. In creating SiC devices, a SiC wafer is used as a foundation for the device. Unfortunately, SiC wafers contain defects, including dislocations such as threading edge, screw, and basal plane dislocations, and stacking faults. Specifically, micropipes are small tubular voids in the wafer that are generally oriented normal to the polished surface. The existence of micropipes and screw dislocations in the wafer, especially in high densities, prevents the use of large device areas for more powerful and complex devices. In fact, micropipes and screw dislocations have been identified as the main obstacle for commercializing large-area power devices. Micropipes lead to premature reverse breakdown in the SiC p-n junction and clearly adversely affect the electrical performance of the SiC device. The ability to detect the existence and physical location of the micropipes and screw dislocations on a wafer allows device manufacturers to determine the physical areas of the wafer where devices can be placed so that such defects will not degrade or destroy the functionality of the device.  
           [0004]    Previously, methods for determining the number and location of micropipes in SiC wafers include KOH etching and transmission electron microscope (TEM). However, both are destructive methods resulting in a non-usable SiC wafer and not well suited for production line testing. While other methods exist, they require expensive equipment or special facilities. For example, synchrotron white-beam x-ray topography (SWBXT) requires a synchrotron light source that is available in only a few research facilities in the world. Other methods, such as atomic force microscopy (AFM), scanning electron microscopy SEM, and optical microscopy, while detecting micropipes, cannot detect closed-core screw dislocations in a non-destructive fashion. More specifically, it is advantageous to delineate and locate defects of the epitaxial films or layers. It is known that threading defects, such as micropipes, threading edge and screw dislocations, and grain boundaries, originating in the SiC or other substrate, penetrate the device structure during epitaxial growth and cause device failure or other inoperability. Further, process induced morphological defects can be caused by processes such as cutting, polishing, and preparing a wafer for growth.  
           [0005]    Previously, there has not been an effective method or system to characterize the crystallographic defects and resulting morphological defects in the epilayer. Further, there has not been previously an effective method or system to determining threading defects, their propagation, or their correlation with growth pits of the epilayer, with or without the epilayer present.  
           [0006]    Therefore, the development of a non-destructive, inexpensive, and rapid detection system and method for determining defects in semiconductors is a problem to which much attention should be directed.  
           [0007]    Accordingly, it is an object of this invention to provide for an inexpensive, rapid, non-destructive system and method for determining defects in wafers.  
           [0008]    It is another object of this invention to provide for an inexpensive non-destructive system and method for determining defects in wafers, with or without an epilayer.  
           [0009]    It is another object of this invention to provide for an inexpensive non-destructive system and method for investigating threading defect propagation.  
           [0010]    It is another object of this invention to provide for an inexpensive non-destructive system and method for investigating the relationship between wafer defects and device performance.  
         SUMMARY OF THE INVENTION  
         [0011]    The above objectives are achieved according to this invention by utilizing a polarized light microscope for performing a rapid, inexpensive, and non-destructive defect delineation of defects including micropipes, stressed striations, inclusions, dislocations, and grain boundaries. This invention can easily determine the location and therefore map defects of wafers including expitoxial films such as SiC, GaN, AlN and AlGaN. The invention provides for A system for delineating defects in a semiconductor wafer comprising a computer readable medium in communications with a CCD and a display monitor, an analyzer disposed adjacent to the CCD for analyzing polarized light received by the CCD and transmitted through the semiconductor wafer, a polarized light source for transmitting light through the semiconductor wafer so that the CCD can receive image information representing defects illuminated by the polarized light; and, a set of computer readable instructions included within the computer readable medium for allocating a plurality of scan regions associated with the semiconductor wafer, receiving scan information representing at least one scan region from the CCD, and displaying the scan information on the display monitor so that defects of the semiconductor wafer illuminated by the polarized light may be rapidly determined in a non-destructive manner. The system can also contain computer readable instructions for storing the scan information within the computer readable medium, cumulating the scan information to create a digital map representing the entire area of the semiconductor wafer and displaying the digital map. A movable scanning plate can be included, having an actuator, in communications with the computer readable medium; and, the set of computer readable instructions include instructions for receiving wafer placement information representing the location of the wafer on the scanning plate, actuating the scanning plate according to the wafer placement information so that scan information is received by the CCD for at least one of the plurality of scan regions associated with the semiconductor wafer. The invention can also contain an actuator for actuating the CCD in communications with the computer readable medium; and, the set of computer readable instructions include instructions for receiving wafer placement information representing the location of the wafer on the scanning plate, actuating the CCD according to the wafer placement information so that scan information is received by the CCD for at least one of the plurality of scan region associated with the semiconductor wafer. The invention can also contain a reflective light source disposed adjacent to the scanning plate for reflecting light off the wafer to be received by the CCD and, the computer readable instructions include instructions for receiving reflective light information reflected off the wafer and displaying the reflective light information on the display monitor so that distortions in the reflective light, representing defects in the wafer, can be viewed. A set of defect characteristics contained in the computer readable medium and the set of computer readable instructions include instructions for reviewing the scan information, determining the existence of defects by identifying defects according to the defect characteristics in the scan information so that a system to automatically locate defects in the semiconductor wafer is provided. The invention can contain computer readable instructions for determining areas of contrast differentiation representing the wafer edge according to the scan information, creating boundary information according to the areas of contrast differentiation, and calculating the area of the wafer according to the boundary information.  
           [0012]    The invention is also a method of delineating defects in a semiconductor wafer comprising the steps of providing a semiconductor, placing the semiconductor between a polarized light source and an analyzer; segmenting the semiconductor into a plurality of scan regions; and, viewing at least one scan region according to the analyzer to provide scan information for at least one scan region so that a non-destructive method for determining defects in the semiconductor wafer using polarized light is provided. The steps of displaying the scan information so that any defects in the semiconductor wafer can be viewed, storing the scan information for subsequent review, viewing a plurality of scan regions for providing a plurality of scan information; and, cumulating the plurality of scan information from the scan region so that a map having a plurality of scan information for a plurality of scan regions is provided can also be included. The steps of displaying the map having the plurality of scan information, providing a set of defect characteristics and, reviewing the scan information for determining the existence of defects by identifying defects according to the set of defect characteristics shown in the scan information can also be included. The steps of reflecting reflective light off the semiconductor wafer; and, viewing the reflective light so that distortions created by defects within the semiconductor wafer can be seen representing defects within the semiconductor wafer, determining areas of contrast differentiation representing the wafer edge according to the scan information, creating boundary information according to the areas of contrast differentiation representing the boundaries of the semiconductor wafer, and calculating the area of the wafer according to the boundary information can also be included. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic of the invention;  
         [0014]    [0014]FIG. 2 is a flowchart of the invention; and  
         [0015]    [0015]FIG. 3 is a schematic of the results of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The detailed description that follows may be presented in terms of program procedures executed on a computer or network of computers. These procedural descriptions are representations used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. These procedures herein described are generally a self-consistent sequence of steps leading to a desired result. These steps require physical manipulations of physical quantities such as electrical or magnetic signals capable of being stored, transferred, combined, compared, or otherwise manipulated computer readable medium that is designed to perform a specific task or tasks. Actual computer or executable code or computer readable code may not be contained within one file or one storage medium but may span several computers or storage mediums. The term “host” and “server” may be hardware, software, or combination of hardware and software that provides the functionality described herein.  
         [0017]    The present invention is described below with reference to flowchart illustrations of methods, apparatus (“systems”) and computer program products according to the invention. It will be understood that each block of a flowchart illustration can be implemented by a set of computer readable instructions or code. These computer readable instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine such that the instructions will execute on a computer or other data processing apparatus to create a means for implementing the functions specified in the flowchart block or blocks.  
         [0018]    These computer readable instructions may also be stored in a computer readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in a computer readable medium produce an article of manufacture including instruction means that implement the functions specified in the flowchart block or blocks. Computer program instructions may also be loaded onto a computer or other programmable apparatus to produce a computer executed process such that the instructions are executed on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Accordingly, elements of the flowchart and schematics support combinations of means for performing the special functions, combination of steps for performing the specified functions and program instruction means for performing the specified functions. The present invention is now described more fully herein with reference to the drawings in which the preferred embodiment of the invention is shown. This invention may, however, be embodied any many different forms and should not be construed as limited to the embodiment set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.  
         [0019]    Referring now to FIG. 1, the invention is described in further detail. Computer  10  is shown connected to a digital imaging apparatus such as a charged-coupled device (CCD)  12 , which can include a microscope or other magnification means, to provide for digital images derived from scanning plate  16 . The scanning plate can be operatively associated with computer  10  so that the scanning plate is articulated to provide movement of wafer  14  placed on plate  16  allowing CCD  12  to scan the entire surface of wafer  14 . The computer can be informed of the physical location of the wafer on the plate and scan the wafer according to the wafer location information to insure the entire surface of the wafer is scanned. Further, the wafer can be segmented into scan regions so that the CCD captures information for each scan region and sends the captured information to the computer for storage and cummulation. By cumulating the digital information for each scan region, defects for the entire wafer can be displayed.  
         [0020]    In order to detect the defect of a wafer, this invention uses photoelasticity since the defects of the wafer induce a change in the stress distribution around the defects location and such change can be detected to determine the location of the defect. To detect defects, light source  20 , polarized through polarizing plate  18 , is transmitted through wafer  14  and the optical stress is detected by CCD  12 , using analyzer  13 . The scan information from CCD is transmitted to computer  10 . Therefore, the wafer is placed between the polarizers in the crossed poles condition. The defects of the wafer induces a stress around the defect location that is captured by the CCD. Further, the brightness of the defects as shown on the monitor corresponds to the defect distribution across the wafer. The CCD can scan a specific region or the entire wafer and collect and store the various scan regions in a database  22 . Computer readable medium  24  can contain instructions for controlling the scanning plate  16 , CCD  12 , and for detecting defects from the information gathered from the CCD. In its operation, the wafer can be scanned, in regions or otherwise, an example of which shown as  34 , then displayed on monitor  28  as image  30 . Within image  30 , an image of the wafer  32  is shown with various regions such as  34 . The computer readable instructions allow for the magnification of the wafer, and any defects, for ease of viewing.  
         [0021]    In one embodiment, reflective light can also be utilized in this invention. In lower quality wafers, the density of the micropipes may be too great to allow for the delineation of each micropipe as the results of the illumination from polarized light may overlap. Therefore, the intensity of the polarized light can be reduced or eliminated so as to reduce or prevent such overlap. Reflective light is then reflected off the wafer and used to delineate the micropipe, and screw defects, when used in conjunction with the polarized light. Reflective light causes a darkened area to exist with the butterfly shape so as to assist in further delineating the micropipe. Reflective light source  11  is disposed adjacent to scanning plate  16  so that light is reflected from reflective light source  11  onto wafer  14  and received by CCD  12 . Distortions of the reflective light are caused by defects in the wafer, especially the epitaxial layer, and recorded by the CCD. Therefore, the defects can be identified through distortions of the reflective light.  
         [0022]    A set of defect characteristics  62  can be contained with the computer readable medium so that the scan information can be compared to the set of defects characteristics so that the defects of the wafer can be identified. The set of defect characteristics can include information representing the butterfly shape  64 , the size of the butterfly shape, or other defect  66 , and the brightness  68 . Further, a wave shape  70  can be included representing screw dislocations within the wafer. The wave shape is further illustrating as generally  61  of FIG. 3.  
         [0023]    In providing polarized light, two types of polarizers could be used. First, a plane polariscope can be used for linear polarization of the light source transmitted through the wafer and received by the analyzer which results are recorded by the CCD. Second, a circular polariscope can be used with the addition of quarter wave plates on either side of the wafer and inside the space defined within the polarizer and analyzer. Using either polariscope, the defects are detected as the stressed areas (defects) of the wafer, an optically isotropic material, become anisotropic thereby showing double refraction which can be detected and captured through a CCD. The defects induce stress around the defect that is thereby detected.  
         [0024]    Referring now to FIG. 2, the operation of this invention is shown in more detail. The process for detecting defects of a wafer begins with step  34 . The wafer is placed upon the scanning bed in step  36 . Providing the location of wafer  14  on scanning bed  16  begins by scanning the wafer at step  38 . The image resulting from the scan is displayed on monitor  28 . The boundaries of the wafer are then indicated at step  40  so that the computer readable instructions can associate the indicated onscreen boundaries of the wafer with the physical location of the CCD so that the scanning plate boundaries are known for scanning. This allows the computer readable instructions to “learn” the boundaries of the wafer so that the entire wafer can be scanned by manipulating the scanning plate. The computer readable instructions include instructions for being able to determine the wafer size from the scan information rather than relying upon the measurements of the wafer manufacturer or manual input. The boundaries of the wafer can also be determined by detecting the contrast differentiation in the light received by the CCD representing the scanning plate and wafer. Where there is a sufficient contract differentiation, there is a boundary of the wafer. Determining the boundary information allows for the wafer area to be calculated. Alternatively, the wafer could be static with the CCD itself manipulated to scan the entire surface of the wafer or the CCD of such high resolution that the entire wafer can be scanned without manipulation of the scanning plate or CCD. The scanning begins by scanning the present scan region at step  42  and the results from the scan stored in a computer readable medium, with location information, at step  44 . By storing the results from each scan, the scanning process can be halted mid-process and resumed without the need to restart the scanning process. The determination of whether there are more regions to scan is made at step  46  and if so, the process returns to step  42  to scan the next region. If the determination is made that all regions have been scanned, then the scanning information can be retrieved from the computer readable medium at step  48 , cumulated to form a map representing the image of the wafer at step  50 , and display the map at step  52  so that the defects discovered can be seen. Through these steps, wafer defects can be magnified to tens of hundreds of micrometers in dimensions thereby allowing the location and mapping of defects to be more easily performed. Further, using color imaging technics for corresponding optical stress behavior derived from wafer defects, the nonuniformity of the wafer can be illustrated in an inexpensive, rapid, non-destructive manner. Traditional defect detection techniques simply did not allow for such location determination and mapping.  
         [0025]    Further, the computer readable instructions can also automatically detect defects in the wafer at step  51  for display. When automatically detecting defects, the computer readable instructions can scan the digital image information cumulated from the scan information step  54  and compare the information with defect information at step  56  to provide a defect map of the defect locations on the wafer at step  58 . Defect information can include the appearance of micropipes in the commonly known butterfly shape that can be detected in the digital image by comparing the digital image information with defect characteristics information to see if the pattern in the digital image corresponds to a micropipe on the wafer. More specifically, the defect information concerning micropipes can represent the butterfly shape, the size of the butterfly shape, and brightness of the illuminated defect. Based upon predetermined values for size and brightness, the computer readable instructions can compare the scan information with the existence of the butterfly shape, a predetermined size and a predetermined brightness to determine if a micropipe is present. Further, the defect information can contain a wave shape pattern that can be compared to the scan information. When the wave shaped pattern is determined to exist in the scan information, a closed core screw dislocation is known to be present.  
         [0026]    Referring now to FIG. 3, the results from this invention can be seen in further detail. The results of the invention are illustrated by showing, through the photoelasticity utilization of the invention described herein, that micropipes are located at  60   a  through  60   c  of wafer  14  and any device placed over this region would experience integrity problems. By providing such a map, the defect type, density, and location can be discovered so that the quality of the wafer and potential locations for devices can be known.  
         [0027]    It should be understood that the detection of micropipes and screw dislocations are not the only defects that can be detected by this invention. Detection can also be made for domain boundaries, stress striation, dislocation walls, polytype inclusions, planar defects and other such defects. Further the results obtained, available in under 30 minutes from this invention, have been compared with results obtained by SWBXT and an almost one-to-one micropipe and screw dislocation matching between the results of the two analyses was shown.  
         [0028]    It should also be known that this invention need not be limited to SiC wafers, but can also be used for other semiconductor materials and compound semiconductors including GaAS, InP, GaN, AlN, and AlGaN. Further, this invention can specifically delineate defects of wafers of substrates, even when the substrate is already covered with the epitaxial film, since a unique feature of this invention is that it can characterize a SiC wafer with or without an epilayer. Therefore, this invention provides the opportunity to determine threading defect propagation and the correlation with growth pits in the epilayer. This invention allows for the relationship between the threading defects and growth pits in the epilayer to be established. Since this invention allows for viewing micropipes, screw dislocations, stress striations, and dislocation walls that propagate through the substrate to the epilayer and open various growth pits in the epilayer surface.  
         [0029]    While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.