Abstract:
Herein are described layouts of test structures and scanning methodologies that allow large probe currents to be used so as to allow the detection of resistive defects with a resistance lower than 1 MΩ while at the same time allowing a sufficient degree of localization to be obtained for root cause failure analysis. The detection of resistances lower than 1 MΩ nominally requires a probe current greater than 1 micro ampere for detection on an electron beam inspection system.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/755,942, filed Jan. 3, 2006, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to apparatus and methods for inspecting and testing semiconductor wafers during circuit fabrication. 
     BACKGROUND OF THE INVENTION 
     Test structures are often utilized in the semiconductor device industry as a method for rapidly developing a process flow. Many existing methodologies currently exist such as KLA Tencor&#39;s uLoop™, Applied Material&#39;s, Inc. ADL™ and the Real Vision™ jointly developed by Applied Materials Inc. and PDF Solutions Inc. These techniques allow a large number of devices to be rapidly tested allowing determination of root cause failure analysis in a relatively short period of time. 
     There are two basic requirements of any such technique to be useful for rapid diagnostics and root cause failure analysis. These are 1) detection of the electrical error and 2) localization of the electrical error for failure analysis. For example, electrical test by probe techniques can rapidly measure a very large number of devices. However, when an electrical error is detected it is only localized to a somewhat large area on the test chip. This means that a failure technique such as focused ion beam cross sectioning can only be applied if the defect is further localized by a following technique such as ADL™. 
     Common to most techniques such as those mentioned above is a relation between the sensitivity to resistance failures, and the amount of current that can be driven into the device. This is in general true because the voltage that can be generated across a resistive error is determined by the current as dictated by Ohm&#39;s law. This is particularly important for electron microscopy techniques where the current density determines the amount of current that can be injected into a small feature. The total current in the illuminating beam can be made arbitrarily large, but only by allowing the spot size to also become large. 
     The invention herein is specifically directed towards electron microscopy techniques and as such the following background material, which pertains specifically to electron microscopy, is presented. As the device scale shrinks to 45 nm technology and beyond, the amount of current that can be injected into an individual device feature becomes limited. For example, using conventional retarding field electron optics, it is possible to provide 200 nanoamperes into a Gaussian shaped probe with a FWHM of 100 nm. A typical feature such as a contact will have a nominal dimension of 1.5 times the device node so that a contact of the 45 nm nose would have a diameter of approximately 70 nm. In this case, only about 30% of the primary beam current or 60 nA would strike the contact. The other 70% would only contribute to the noise of the measurement. 
     When testing electrically contiguous structures of large spatial extent, the capacitance of the structure will grow proportionally with the area subtended. The total dose required to saturate the capacitance and reach the resistive regime of the electrical response of the structure is therefore also proportional to the area subtended by the structure. The total dose is the product of the current injected by the probe and the time that the probe impinges upon the structure. Typically, as in the case of via chain and metal comb test structures, the defect of interest is a faulty contact or metal short respectively. In both cases, these are resistive defects and the capacitance of the structure is not of interest. In these cases, if the exposure time is short relative to the RC response of the structure with capacitance C containing a defect with capacitance R, the sensitivity will be compromised. To maximize sensitivity, it is necessary to expose the structure under test for a sufficient amount of time to saturate the capacitance of the structure. Accordingly, improved test structures are desirable. 
     SUMMARY 
     Herein are described layouts of test structures and scanning methodologies that allow large probe currents to be used so as to allow the detection of resistive defects with a resistance lower than 1 MΩ while at the same time allowing a sufficient degree of localization to be obtained for root cause failure analysis. The detection of resistances lower than 1 MΩ nominally requires a probe current greater than 1 micro ampere for detection on an electron beam inspection system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, several preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a system, according to an embodiment of the invention, wherein a pad is used to allow high current to be injected into the device and said pad integrated into an individual structure; 
         FIG. 2  illustrates a system, according to an embodiment of the invention, wherein a pad is used to allow high current to be injected into the device and said pad is integrated into a structure with a specific geometrical relationship to a second structure; 
         FIG. 3  illustrates a system, according to an embodiment of the invention, wherein a double serpentine structure is used to allow high current to be injected into the device during both a detection and localization step; 
         FIG. 4  illustrates a system, according to an embodiment of the invention, wherein structures are laid out sequentially in the stage scan direction; 
         FIG. 5  illustrates a system, according to an embodiment of the invention, wherein structures are laid out both sequentially and mirrored; and 
         FIG. 6  illustrates a system, according to an embodiment of the invention, providing an example of a non-linear scan used to both detect and localize a defect within a test structure with a pad integrated to allow high current to be injected into the device. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to an implementation consistent with the present invention as illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same or like parts. Although discussed with reference to these illustrations, the present invention is not limited to the implementations illustrated therein. Hence, the reader should regard these illustrations merely as examples of embodiments of the present invention, the full scope of which is measured only in terms of the claims following this description. 
     In one embodiment of the invention, a conducting pad is electrically connected to the structure under test as shown in  FIG. 1 . In one instance, the pad end of the structure is floating and the opposite end is electrically connected to the wafer. In another instance, both ends are floating and the end opposite the pad is connected to a large capacitance. The dimension of the pad is chosen to allow the entirety of the primary probe current of the inspection system to enter the structure. Since the pad can have a larger contiguous area than that of the devices under test, very large primary beam currents may be used while still allowing the entirety of the current to enter the device, e.g. 1 uA. Furthermore, the size and shape is also chosen to allow a sufficient total dose to enter the structure to saturate its capacitance to the underlying wafer. The dose is the product of the time the probe spends on the pad and the primary probe current. The structure under test could be, as a specific non-limiting example, a contact or via chain system. 
     As illustrated in  FIG. 2 , in another embodiment of the invention a conducting pad is placed in series with a structure that has a geometrical relationship to a second structure in a way that allows the testing of a desired process step. The pad geometry is chosen based on the same criteria as in the previously described embodiment. Here, the structure to which the pad is connected is electrically isolated, and the second structure is electrically connected to the underlying wafer or to a large capacitance. The structures could be, as a specific non-limiting example, a comb structure for testing shorts between adjacent metal lines. 
     In the two particular embodiments described above in connection with  FIGS. 1 and 2 , an electrical defect is localized to the structure or structures associated with a specific pad. A preferred layout is for the structure to have a short height and large width. This allows the defect to be localized quite tightly in one dimension while still allowing a large number of devices to be tested. A second technique such as that used in ADL™ implemented either on the inspection system or later on a dedicated review system can be used to complete the localization of the defect. 
     As illustrated in  FIG. 3 , in another embodiment of the invention a double serpentine pattern is used. In this case, a very large spot size can be used while still allowing a large portion of the current to enter the device under test. The specific amount of current entering the device will be proportional to the density of the pattern of the structure. The structure under test could be, as a specific non-limiting example, a contact or via chain system. This structure allows for both the detection and a localization steps to be carried out with high current. 
     In a further embodiment of the invention suitable for electron beam inspection systems with a moving stage, structures such as those in the above embodiments are arranged sequentially in the direction of the stage axis, as illustrated in  FIG. 4 . This allows the pads or structures to be moved under the beam in rapid precession. Additionally, the primary beam can be scanned in the direction perpendicular to the stage motion. 
     In a further embodiment of the invention suitable for electron beam inspection systems with a moving stage, in addition to arranging the structures such as those in the above embodiments sequentially in the direction of the stage axis, they are also arranged in a mirror format (e.g., in opposing pairs) as illustrated in  FIG. 5 . By both moving the stage and scanning the beam perpendicular to the stage direction, two sets of structures can be tested with one stage motion. The structures on the left and right do not have to be the same. 
     In a further embodiment of the invention, a non-linear scan line is used by the electron beam inspection system to both detect and localize a defect in one step as illustrated in  FIG. 6 . For a portion of the scan line where the beam directed over a pad region the beam is scanned slowly to allow the injection of a high dose so as to sufficiently charge the structure to allow the defect to be detected. For a second portion of the scan line in which the beam is scanned over the structure under test the beam is scanned quickly. For this embodiment, the width of the structure must be limited to the scan field of the electron microscope. 
     In another embodiment of the invention, a dedicated extremely high current microscope (&gt;1 uA) with detection capabilities is added to an electron beam inspection system. This allows the electron microscope of the inspection system to be optimized for currents associated with other applications. One non-limiting example of how this might be implemented is as follows. In a first step, the structures are scanned under the high current microscope using the stage. No advanced scanning capability is required for the high current microscope so the microscope can be small and inexpensive. The detected signal during this step is monitored to detect the presence of an electrical error. When an electrical error is sensed, the position of the structure under test is stored. In a following step, the stage is used to bring this structure under the scan field of the electron microscope of the inspection system. In a final step, this electron microscope is used to localize the electrical error. 
     The foregoing description of various implementations of the invention has been presented for purposes of illustration and description. It is not exhaustive and does not limit the invention to the precise form or forms disclosed. The scope of the invention is defined by the claims and their equivalents.