Abstract:
A semiconductor wafer configured for in-process testing of integrated circuitry fabricated thereon. At least two die are separated by a scribe area, and each of the die has at least one complementary metal oxide silicon (CMOS) static random access memory (SRAM) array embedded therein among mixed-signal CMOS circuitry. The mixed-signal CMOS circuitry includes devices with larger feature sizes compared to similar devices of the embedded SRAM array. A first process control monitor (PCM) testline is included, which has a first layout corresponding to the mixed-signal CMOS circuitry. Additionally, a second PCM testline is included, which has a second layout corresponding to the embedded SRAM arrays. The first and second PCM testlines are formed in the scribe area.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to Provisional U.S. Application No. 60/312,352, filed Aug. 16, 2001, which is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to semiconductor devices and, in particular, to in-process electrical testing of integrated circuit (IC) components and process control monitors (PCMs).  
           [0004]    2. Background Art  
           [0005]    Semiconductor integrated circuits (ICs) are formed on substrates, such as silicon wafers, and typically comprise a variety of basic electrical components, such as transistors, resistors, capacitors, and the like. The fabricated components are then interconnected through deposited metal layers, or the like, to form logic or other functions. The various ICs formed on the wafer are typically arranged in a grid pattern and are thus separated by vertical and horizontal scribe areas, also called “streets.” Once the wafer is completely processed, it is cut up (diced) into the individual chips (die). Each die is mounted in a package and the terminals of the die are connected to the package terminals through a wire bonding operation. It is desirable to verify that such integrated basic components are fabricated according to a design specification and have certain properties or values, e.g., a specified gain, resistance, capacitance, etc. An individual component cannot readily be tested, however, once integrated into a circuit.  
           [0006]    In lieu of testing the integrated components (i.e., the components that are part of the ICs on the wafer), “stand-alone” copies of such basic components are tested. The stand-alone copies are fabricated in some location of the surface of the wafer not occupied by the ICs formed on the wafer, e.g., the scribe areas of the wafer. Such stand-alone copies or target components have properties or values of gain, resistance, and the like that are representative of such properties of their IC counterparts since they are fabricated using the same process. As such, it may be assumed that the parameters measured for the target components are similar to those of the non-tested integrated components, and it is therefore appropriate to apply the test results for the target components to the integrated components. Such a quality control methodology is referred to as “in-process electrical testing.” 
           [0007]    During in-process electrical testing, a signal source and measurement device, usually external, are electrically connected to the stand-alone target component to be tested, or DUT (device under test). Electrical connection is typically effected via microprobes between the DUT and the measurement device. Various measurement device systems are commercially available.  
           [0008]    On the wafer, the target component is electrically connected to pads. Microprobes are used to contact the pads, thereby electrically connecting the signal source and the measurement device to the target component. The measurement device is typically used to measure various response or performance parameters of the target component, i.e., parameters that characterize the response or performance of the target component.  
           [0009]    The pads and the target component, collectively, form a “process control monitor” (PCM) or “device monitor,” sometimes referred to as a “tester”. PCMs are commonly used to monitor the quality of wafer processing in IC manufacturing. See U.S. Pat. Nos. 5,523,252 and 6,194,739B1, which are incorporated herein by reference, for descriptions of PCMs and their associated pads. PCMs are incorporated typically in the scribe area between two adjacent dice on a wafer. Modern CMOS (complementary metal oxide semiconductor) IC chips incorporate a significant amount of on-chip SRAMs (static random access memories). The embedded SRAMs typically use smaller design rules than the logic CMOS process used to create the circuits in which the SRAMs are embedded.  
           [0010]    Therefore, the PCM testline used to monitor the logic CMOS process is not ideally suited for logic-embedded SRAM ICs. What is needed is a PCM testline designed exclusively for logic-embedded SRAMs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed to a method and apparatus comprising a semiconductor wafer configured for in-process testing of integrated circuitry fabricated thereon. At least two die are separated by a scribe area, and each of the die has at least one complementary metal oxide silicon (CMOS) static random access memory (SRAM) array embedded therein among mixed-signal CMOS circuitry. The mixed-signal CMOS circuitry includes devices with larger feature sizes compared to similar devices of the embedded SRAM array. A first process control monitor (PCM) testline is included, which has a first layout corresponding to the mixed-signal CMOS circuitry. Additionally, a second PCM testline is included, which has a second layout corresponding to the embedded SRAM arrays. The first and second PCM testlines are formed in the scribe area.  
           [0012]    In a preferred embodiments, the second PCM testline comprises at least one 6-transistor memory cell.  
           [0013]    In other embodiments, the second PCM testline comprises at least one decoder, and/or at least one sense amplifier, and/or at least one data output circuit.  
           [0014]    In another embodiment, the second PCM testline comprises facsimiles of embedded SRAM array devices located at different locations within the embedded SRAM array, thereby permitting in-process testing to evaluate process variations that are location specific.  
           [0015]    In another embodiment, the second PCM testline comprises facsimiles of embedded SRAM array devices located at different locations within the embedded SRAM array, thereby permitting in-process testing to track across-die variations of critical dimensions.  
           [0016]    In another embodiment, the second PCM testline comprises facsimiles of embedded SRAM array devices located at different locations within the embedded SRAM array, thereby permitting in-process testing to track across-die alignment variations.  
           [0017]    In another embodiment, the second PCM testline comprises facsimiles of embedded SRAM array devices located at different locations within the embedded SRAM array, thereby permitting in-process testing to track across-wafer variations of critical dimensions.  
           [0018]    In yet another embodiment, the second PCM testline comprises facsimiles of embedded SRAM array devices located at different locations within the embedded SRAM array, thereby permitting in-process testing to track across-wafer alignment variations.  
           [0019]    These and other objects, advantages and features will become readily apparent in view of the following detailed description of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]    The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears.  
         [0021]    [0021]FIG. 1 shows a conventional prior art wafer having a plurality of dies arranged thereon;  
         [0022]    [0022]FIG. 2 is an enlarged view of a region of the wafer of FIG. 1, showing horizontal and vertical streets located between dies;  
         [0023]    [0023]FIG. 3 shows an array of like devices that may experience different optical effects during photolithograph; and  
         [0024]    [0024]FIG. 4 shows SRAM PCMs, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The preferred embodiment of the present invention will now be discussed in detail. While specific features, configurations and arrangements are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other steps, configurations and arrangements may be used without departing from the spirit and scope of the invention. Indeed, for the sake of brevity, conventional electronics, manufacturing of CMOS logic and CMOS SRAMs, and other functional aspects of the method/apparatus (and components of the individual operating components of the apparatus) may not be described in detail herein. Furthermore, for purposes of brevity, the invention is frequently described herein as pertaining to SRAMs for use in electrical or electronic systems. It should be appreciated that many other manufacturing techniques could be used to create the SRAMs described herein, and that the techniques described herein could be used in connection with other devices. Moreover, it should be understood that the spatial descriptions (e.g., “next to”, “above”, “below”, “up”, “down”, etc.) made herein are for purposes of illustration only.  
         [0026]    Moreover, the terms chip, integrated circuit, monolithic device, semiconductor device and microelectronic device are often used interchangeably in this field. The present invention is applicable to all of the above as they are generally understood in the field.  
         [0027]    The terms pins, pads and leads refer to input and/or output terminals of a connector, device, chip, printed circuit, or the like, which are used to provide electrical connection to one or more connectors, devices, chips, printed circuits, or the like.  
         [0028]    The terms metal line, trace, wire, conductor, signal path and signalling medium are all related. These related terms are generally interchangeable and appear in order from most specific to most general. In this field, metal lines are sometimes referred to as traces, wires, lines, interconnect or simply metal. Metal lines, generally aluminum (Al) or an alloy of Al and copper (Cu), are conductors which provide signal paths for coupling, or interconnecting, electrical circuitry. Conductors other than metal are available in microelectronic devices. Materials such as doped polysilicon, doped single-crystal silicon (often referred to simply as diffusion, regardless of whether such doping is achieved by thermal diffusion or ion implantation), titanium (Ti), molybdenum (Mo), or refractory metal silicides are examples of other conductors. Signalling medium is the most general term and encompasses the others.  
         [0029]    As discussed below, the following sections more fully describe the present invention.  
         [0030]    Before describing the present invention, it is useful to discuss prior art wafers and the manner in which PCMs are arranged thereon for in-process electrical testing. Referring now to FIG. 1, there is shown a conventional wafer  100  and a plurality of dies  102  formed thereon. The dies  102  are discrete regions of the wafer at which circuitry (the IC) is patterned. All dies resident on a wafer may contain the same circuitry, such as is typical for mass production of a particular IC, or each die in a plurality of such dies disposed on a single wafer may have a different circuit configuration from other dies on the wafer. Typically, the wafer is diced to separate the dies  102  for use. When separated, the individual dies are typically referred to as an IC or chip.  
         [0031]    Referring now to FIG. 2, there is shown an enlarged view of region  200  of wafer  100  of FIG. 1. Adjacent dies, such as adjacent dies  102   a  and  102   b,  and adjacent dies  102   a  and  102   c,  are separated by respective vertical and horizontal scribe areas (also called streets)  202   a,    202   b.  PCMs or testers  204  operable to test various stand-alone electrical components, counterparts of which stand-alone components are incorporated within the circuits comprising any of the dies  102  on wafer  100 , are located in the scribe areas between such dies, such as the scribe areas  202   a,    202   b.    
         [0032]    As illustrated in FIG. 3, the inventors have observed that devices at different locations in an array of like devices  300  may experience different optical effects during photolithograph. For example, an edge device  302  is not surrounded on all four sides by similar devices, but may be adjacent to a different device  304  on one side, and a like device  306  on the other side. Similarly, a corner device  308  may be adjacent to one or more different device types  310  and  312 . Thus, edge and comer devices can be exposed to optical effects during photolithography that devices in the center of an array of common devices do not experience during processing. Such process variations can impact operational characteristics of the variously positioned devices (i.e., how the circuits behave).  
         [0033]    This problem is exasperated in circuits comprising embedded memory. For example, many contemporary circuit designs, such as mixed (analog and digital) signal circuits operating at radio frequencies of 2 GHz (gigahertz) and above, require CMOS SRAM memory arrays. Such SRAM memory arrays are often laid-out (or “embedded”) at various, non-symmetric locations of the circuit. Thus, edge and corner memory cells of each embedded SRAM memory array are often located adjacent different devices or components.  
         [0034]    The inventors have observed that embedding of SRAM memory arrays can cause operating characteristics of the center, edge, and corner memory cells to vary. Additionally, the inventors have observed that memory array edge and corner cells fail more often than middle cells. This variation of operating characteristics between cells located at different regions of an array and increased failure rates of edge and corner cells may be attributable to optical effects during photolithography. Based on these observations, the inventors have developed a new in-process electrical testing technique to measure and compensate for such variations.  
         [0035]    Because the SRAM memory arrays are embedded, they not provided with separate pinouts. Thus, direct in-process testing is of the embedded SRAM memory arrays is not possible. Adding pinouts for testing embedded SRAM memory arrays is not practical.  
         [0036]    For the case of a 6-transistor SRAM, for example, it is desirable to monitor the two pull-up PMOS transistors, two pull-down transistors, and the two pass-gates. In addition, it is desirable to monitor junction leakage and gate oxide leakage in the SRAM array. Several conventional an SRAM cell circuits would be apparent to a person skilled in the art.  
         [0037]    [0037]FIG. 4 illustrates the scribe areas  202   a  and  202   b  discussed above. A exemplary die  402  comprises standard CMOS logic  404  in which SRAM memory arrays  406  are embedded. Typically, the standard (e.g., mixed-signal) CMOS logic circuitry  404  comprises devices with larger feature sizes compared to similar devices of the embedded SRAM memory arrays  406 .  
         [0038]    SRAM PCM testlines  408  according to the present invention comprise transistors that are laid-out in a manner that is identical to the layout of the on-chip SRAM memory arrays  406 . However, independent access is provided to the terminals of the individual transistors (not shown). PCMs  410  corresponding to the standard CMOS logic  404  are also formed as part of the testlines. The PCMs  408  and  410  can be arranged in the scribe areas in many different patterns. For example, PCMs  408  can be formed in the vertical scribe areas and PCMs  410  can be formed in the horizontal scribe areas, or vices versa. PCMs  408  and  410  can be formed in alternating patterns in both the vertical and horizontal scribe areas. Alternatively, PCMs  408  and  410  can be formed in any application specific manner in accordance with the teaching herein, as would be apparent to a person skilled in the relevant art.  
         [0039]    To obtain individual connection to the SRAM memory transistors, it is important not to alter the layout of the SRAM memory cell up to and including the contacts to the transistors. To mimic the layout of an SRAM array  406 , a smaller array (called a mini-array) of SRAM cells are used in the testline  408 . In a preferred embodiment, Metal-1 connections are made to independently access the individual NMOS and PMOS transistors, respectively. Thus, basic parameters such as cell current can be monitored in production. Typically, the memory cell at the edge or the corner of the memory array sees a different layout environment compared to a memory cell in the middle of the array. The SRAM testline of the present invention includes transistors from memory a memory cell, edge, and at the corner of the memory array to monitor process variations. Based on such testing using the techniques of the present invention, layout modifications, process modifications, or combinations thereof can be made on subsequent design and tape-out iterations to compensate for such process variations.  
         [0040]    In advanced CMOS technology, non-negligible across-die and across-wafer variation of critical dimensions and alignment occurs. To monitor the impact of such variation, the testline of the present invention is designed to include variations to dimensions of the (a) active region—the channel of the transistor, (b) gate length of the transistor, and (c) size of the contacts. The magnitude of the variations is determined by the extent of across-die uniformity that the process technology can provide, as would be apparent to a person skilled in the relevant art.  
         [0041]    In another testline embodiment, SRAM PCM testlines  408  can include a speed path having address loading, address decoding, data sensing, and data output circuits, and the like to facilitate monitoring of the speed path in the embedded SRAM. More specifically, to monitor the junction leakage of P+ to Nwell, or N+ to Psubstrate, a test memory array is generated with all P+ nodes connected in parallel using metal interconnect to a terminal, and similarly all N+ nodes to another terminal. Such a test device can be used to monitor the extent of junction leakage in a SRAM array in production. Also, across-wafer variation of all transistor and junction parameters can be monitored.  
         [0042]    In yet another testline embodiment to monitor the cycle time of a memory, which is the speed at which data can be accessed from the memory, a full access path is placed in the SRAM testline. This consists of loading of the address on the latches, subsequent decoding and data access and data output. The data access path can be monitored as a delay chain or as a ring oscillator. Such a test circuit provides a vehicle to monitor the memory performance. Other test circuits will become apparent to those persons skilled in the art in view of the teachings of the present invention.  
         [0043]    In addition, the testline can include a sensitive sense amplifier so that the actual sensing circuit of the SRAM  406  can be monitored for manufacturing variations.  
         [0044]    Prior art typically uses a standard CMOS logic testline to monitor process quality. Such testlines do not provide accurate information about how process variations affect SRAM performance. In addition, with prior art critical dimension and overlay variations are not included.  
         [0045]    The advantages of the proposed testline is that it enables regular accurate monitoring of across-die variation of device parameters associated with a SRAM array and thus reduce yield improvement cycle.  
         [0046]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention as defined in the claim(s). Among other reasons, this is true in light of (later) developing technology and terms within the relevant art(s). Thus the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.