Abstract:
A method of testing semiconductor devices on a wafer, including a tasting circuit formed on the wafer for providing an output signal indicative of at least one operational characteristic of the devices. The output signal provided by the testing circuit is compatible for monitoring using an integrated circuit tester. The testing circuit includes an oscillator, an N-bit counter, and an N-bit shift register, all formed on the semiconductor wafer. The tester resets the counter and enables the oscillator, at which time the oscillator produces oscillator pulses at an oscillator frequency. During a predetermined time period, the counter receives and counts the oscillator pulses from the oscillator, and produces a pulse count corresponding to the number of oscillator pulses received. The shift register receives the count from the counter as an N-bit digital data word. The tester shifts the N number of bits of the digital data word out of the shift register, and manipulates the bits to determine a count value. The tester then determines an oscillator frequency value by dividing the count value by a time value corresponding to the predetermined time period.

Description:
FIELD 
     This invention relates to the field of testing of mass-produced semiconductor device. More particularly the invention relates to measuring a characteristic of a test device on a semiconductor chip to determine the performance of other devices on the chip. 
     BACKGROUND 
     Process monitor circuits are used in semiconductor manufacturing processes to monitor, at various points during production, whether the processes are producing semiconductor devices on a very large scale integrated (VLSI) circuit chip that perform according to specification. Monitoring is typically done by measuring the characteristics of the monitor circuit that are related to the process conditions used to fabricate the integrated circuits on a monolithic wafer. The process monitor circuits may be placed within every one of the production circuits, or dice, produced. In this manner, the process monitor circuit in each packaged device can be used as a data point to determine process characteristics. 
     One type of process monitor circuit is an oscillator circuit manufactured as part of the production integrated circuit wafer. When the oscillator circuit is excited, its oscillation frequency gives an indication of the performance of transistor devices in the oscillator circuit, and of the performance of other devices on the wafer. 
     Most VLSI circuit testers are not capable of monitoring an oscillator frequency directly. Thus, to measure oscillator frequency, integrated circuit manufacturers have had to resort to building expensive mixed-signal testers implemented in a “rack-and-stack” test equipment format. 
     What is needed, therefore, is a testing circuit on the production chip that provides an oscillator frequency output signal that may be monitored by a standard VLSI circuit tester. 
     SUMMARY 
     The above and other needs are met by a testing circuit formed on a semiconductor wafer that provides an output signal indicative of at least one operational characteristic of semiconductor devices formed on the wafer. The output signal provided by the testing circuit is compatible for monitoring using an integrated circuit tester. The testing circuit includes an oscillator, a counter, and a shift register, all formed on the semiconductor wafer. The oscillator produces oscillator pulses at an oscillator frequency when the oscillator is energized, where the oscillator frequency is indicative of at least one operational characteristic of the semiconductor device. The counter receives and counts the oscillator pulses from the oscillator, and produces a count corresponding to the number of oscillator pulses received by the counter during a predetermined length of time. The shift register receives the count from the counter as a digital data word comprising multiple bits. 
     In another aspect, the invention provides a system for determining at least one operational characteristic of semiconductor devices formed on a semiconductor wafer. The system includes an oscillator formed on the semiconductor wafer for producing oscillator pulses at an oscillator frequency when the oscillator is enabled, where the oscillator frequency is indicative of at least one operational characteristic of the semiconductor devices. The system also includes a counter formed on the semiconductor wafer for receiving and counting the oscillator pulses from the oscillator, and for producing a count corresponding to the number of oscillator pulses received by the counter during a predetermined length of time. A shift register formed on the semiconductor wafer receives the count from the counter as a digital data word comprising multiple bits. The system further includes a tester for receiving the count from the shift register, for determining the oscillator frequency based on the count and the predetermined time period, and for determining the at least one operational characteristic of the semiconductor devices based on the oscillator frequency. 
     In a most preferred embodiment, the tester is operable to reset the counter, enable the oscillator to begin producing the oscillator pulses, disable the oscillator after a predetermined time period, shift out the multiple bits of the count from the shift register, and determine the oscillator frequency based upon the multiple bits of the count and the predetermined time period. 
     In yet another aspect, the invention provides a method for testing a semiconductor device to determine at least one operational characteristic of the semiconductor device. The method includes resetting an N-bit counter, enabling an oscillator on the semiconductor device to produce oscillator pulses at an oscillator frequency during a predetermined time period, and counting the oscillator pulses with the counter during the predetermined time period to determine a pulse count. The method also includes producing a digital data word comprising N number of bits corresponding to the pulse count, and shifting the N number of bits of the digital data word into a shift register having a most significant bit and a least significant bit. The N number of bits of the digital data word are shifted out of the shift register, and are manipulated to determine a count value. The method further includes determining an oscillator frequency value by dividing the count value by a time value corresponding to the predetermined time period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
     FIG. 1 is a functional block diagram of a preferred embodiment of a testing circuit according to the present invention, 
     FIG. 2 is a flow chart depicting a preferred embodiment of a method according to the present invention, and 
     FIG. 3 is a flow chart depicting further detail of a preferred embodiment of a method according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a functional block diagram of a testing circuit  10  according to a preferred embodiment of the invention. The major elements of the circuit  10  include an oscillator  20 , a counter  30 , and a shift register  40 . The function of each of these elements will be explained in greater detail below, as well as their construction and how they interact with one another. In the following discussion, an electrically conductive path from one element to another is referred to as a line. However, it will be appreciated that in the various embodiments, one or more electrical paths may be utilized where only a single line is depicted in the drawings. 
     The testing circuit  10  is preferably formed on a semiconductor wafer  12 , along with many production integrated circuits  14 , preferably using the same methods and processes of formation as those used to create the integrated circuits  14 . In this manner, when the testing circuit  10  is analyzed, the results provide an indication of how the production integrated circuits  14  will function under similar circumstances. Thus, the invention provides an effective digital process monitor circuit. 
     The testing circuit  10  receives input from, and provides output to a test apparatus, such as a tester  50 . The tester  50  may be a dedicated tester having only the capability to perform the functions described hereafter, or more preferably is a programmable tester capable of performing the described functions in conjunction with many other functions. Testers such as models Vista Logic, Logic 100, or SC212 manufactured by Credence, Inc., or model J-921 manufactured by Teradyne, Inc., and other similar testers known in the art, are acceptable for the functions described herein. 
     The input and output lines mentioned above will be briefly described here, and then their functions will be more fully explained hereafter. The oscillator  20  receives an enable signal from the tester  50  on line  64 , the counter  30  receives a reset signal on the line  62 , and the shift register  40  receives a shift signal on line  60 . The output of the oscillator  20  is preferably provided to the counter  30  on the line  22 , the output of the counter  30  is preferably provided to the shift register  40  on the line  32 , and the output of the shift register  40  is preferably provided to the tester  50  on the line  42 . 
     In the preferred embodiment, the oscillator  20  is a ring oscillator consisting of strings of CMOS devices. When enabled, such as by being energized, the oscillator  20  generates pulses at an oscillator frequency which is dependent on and proportional to the switching speed of the gates of the CMOS devices that comprise the oscillator  20 . An example of such a ring oscillator is described in U.S. Pat. No. 5,867,033, the contents of which are incorporated herein by reference. 
     The measurement of the oscillator frequency is preferably commenced by resetting the counter  30  to zero by providing an input on line  62 . By thus initializing the counter  30 , the circuit  10  is prepared to start the digital counting process that forms the basis of the method. Next, the oscillator  20  preferably receives the enable signal from the tester  50  on the line  64 , where the enable signal persists for a predetermined period of time. Upon receipt of the enable signal, the oscillator  20  begins generating alternating high and low logic signals, or pulses, at the oscillator frequency on the line  22 . The oscillator continues generating the pulses during the predetermined period of time. 
     The counter  30  receives and counts the pulses on the line  22 . In the preferred embodiment, the counter  30  is a ripple counter having ten bits. However, it will be appreciated that the counter  30  could be any other type of digital counter having any number of bits, where the number of bits define the resolution of the counter. 
     As mentioned previously, the enable signal on the line  64  provided to the circuit  10  only lasts for a predetermined length of time, during which the pulses produced by the oscillator  20  are counted by the counter  30 . At the end of this predetermined length of time, the enable signal is curtailed, at which point the counter  30  has preferably not been overrun. If the counter  30  has been overrun, because, for example, the advance of process technology has made the CMOS gates of oscillator  20  switch faster than when the circuit was originally designed, then an enable signal having a shorter predetermined duration can be supplied on line  64 , thereby allowing the oscillator  20  less time in which to cycle, and producing fewer pulses on line  22 . Alternately, the counter  30  could be redesigned to have a higher capacity. Adding one bit to counter  30  would double its capacity. The ability to double the capacity of the counter  30  by merely adding one bit provides the circuit  10  with an enormous ability to adapt to faster gate speeds without a commensurately enormous addition of circuit elements. 
     As described in more detail hereinafter, the output from the counter  30 , that is, the number of pulses counted during the predetermined period, is provided in the form of a digital data word on the line  32  to the shift register  40 . The shift register  40 , which preferably has a resolution of the same number of bits as the counter  30 , receives the pulse count word, and shifts the bits of the word out to the tester  50  on the line  42 . The shift register  40  shifts out the bits based on the shift signal received on the line  60 . 
     Under the control of a software algorithm described below, the tester  50  determines a pulse count value based on the bits shifted out of the shift register  40 , and divides the pulse count value by the predetermined time period to determine the oscillator frequency. The oscillator frequency can then be compared to tolerance limits related to the design of the specific production integrated circuits  14  located on the wafer  12  with the process monitor circuit  10 . For example, if the oscillator frequency is below a first specified value, or above a second specified value, the wafer  12  could be scrapped. However, if the oscillator frequency is between the first and second specified values, then passing the semiconductor devices  14  on the wafer  12 , such as by further processing, could be accomplished. 
     In this manner, the tester  50  may be used to determine the oscillator frequency, and thus the performance of the devices  14  on the wafer  12 , without directly measuring the oscillator frequency. Therefore, there is no need for a rack of specialized test equipment dedicated to directly measuring the oscillator frequency. 
     According to a preferred embodiment of the present invention, the testing circuit  10  preferably uses only a single output line  42  and three input lines  60 ,  62 , and  64  for determining the exact frequency of operation of the oscillator  20 . Because of the small number of connection lines used in implementing the testing circuit  10 , the testing circuit  10  does not unduly compete with the production circuits  14  for pin-outs in the finished package, and thus may be readily integrated within a semiconductor device  14 , such as a CMOS device. 
     With reference to FIGS. 2 and 3, a preferred method of operation of the tester  50  will be described. The tester  50  sets the reset signal high on the reset line  62  to reset (zero) the counter  30  (step  100 ). The oscillator  20  is energized and begins producing pulses on the line  22 , when the enable signal from the tester  50  is provided on the enable line  64  (step  102 ). During the predetermined time period of duration t g , the counter  30  counts the oscillator pulses (step  104 ). In the preferred embodiment, the tester  50  de-energizes the oscillator  20  at the completion of the predetermined period of time, preferably by removing the enable signal from the enable line  64  (step  106 ). 
     In a most preferred embodiment, the oscillator  20  is energized and allowed to stabilize before the tester  50  enables the counter  30  to count the oscillator pulses for the predetermined period of time. Then, at the end of the predetermined period of time, the tester  50  disables the counter  30  from counting any more of the oscillator pulses. Thus, in this most preferred embodiment, the oscillator  20  is able to stabilize at a given rate prior to commencement of the time period during which the counter  30  counts the oscillator pulses. 
     The counter  30  generates an N-bit word representing the number of pulses counted during the predetermined time period (step  108 ). This N-bit pulse count word is loaded into the shift register  40 , preferably serially (step  110 ). However, it should be appreciated that the bits of the pulse count word could also be loaded into the register  40  in parallel. The tester  50  shifts the N number of bits of the pulse count word out of the shift register  40 , determines the pulse count value based thereon (step  112 ), and determines the oscillator frequency based on the pulse count value and the length of the predetermined period of time (step  114 ). 
     The preferred process performed by the tester  50  to shift the bits out of the shift register  40  and to determine the count value is depicted in FIG.  3 . The pulse count value, CV, and an index value, I, are first set to zero (steps  112   a  and  112   b ). An N-bit test pattern word is accessed from memory in the tester  50  (step  112   c ). In the preferred embodiment, and in the example provided below, all of the bits b tpN  of the test pattern word are set high; that is, bits b tp1 , through b tpN , are all set to one. Alternately bits b tp1  through b tpN  are all set to zero, with the method as described below changed appropriately to account for the different initial value of the bits. 
     Preferably, the tester  50  can unload the shift register  40  from the most significant bit (MSB) or from the least significant bit (LSB), depending at least in part upon the configuration of the testing circuit  10 . A value indicating the unloading mode may be stored as a configuration parameter in the tester memory, or may be selected by the operator of the tester  50  when so prompted during execution of the tester software (step  112   d ). 
     If the shift register  40  is to be unloaded from the MSB, the tester  50  shifts out the bit value b cN  (step  112   e ), and accesses the bit value b tp(N−1)  from the bit position N−I in the test pattern word (step  112   f ). If b tp(N−1)  equals b cN  (step  112   g ), such as may be determined by an AND operation, the count value is determined at step  112   h  according to: 
       CV=CV+ 2 N−1−I .  (1) 
     If b tp(N−1)  does not equal b cN  (step  112   g ), the count value remains unchanged (step  112   i ). 
     If I is less than N-1 (step  112   j ), I is incremented by one (step  112   k ), and the tester  50  provides the shift signal on the shift line  60  to shift the bits of the pulse count word by one bit position to the left in the register  40  (step  112   l ). The process then loops back to step  112   e  to shift out the next bit from the MSB position, and the process continues until I is not less than N−1. 
     If I is not less than N−1 at step  112   j , the tester  50  then determines the oscillator frequency, f o , at step  114  according to:                f   o     =       CV     t   g       .             (   2   )                                
     If the shift register  40  is to be unloaded from the LSB (as depicted in FIG.  1 ), the tester  50  shifts out the bit value b c1  from the pulse count word (step  112   m ), and accesses the bit value b tp(I+1)  from the bit position I+1 in the test pattern word (step  112   n ). If b tp(I+1)  equals b c1  (step  112   o ), the count value is determined at step  112   p  according to: 
     
       
           CV=CV+ 2 I .  (3) 
       
     
     If b tp(I+1)  doe not equal b c1  (step  112   o ), the count value remains unchanged (step  112   q ). 
     If I is less than N−1 (step  112   r ), I is incremented by one (step  112   s ), and the tester  50  provides the shift signal on the shift line  60  to shift the bits of the pulse count word by one bit position to the right in the register  40  (step  112   t ). The process then loops back to step  112   m  to shift out the next bit from the LSB position, and the process continues until I is not less than N−1. 
     For an example of the operation of the tester  50  according to the process depicted in FIG. 3, consider a situation where N is four, the pulse count word in the shift register  40  is 1101 (b c4 =1, b c3 =1, b c2 =0, b c1 =1), and the test pattern word is 1111 (b tp4 =1, b tp3 1, b tp2 =1, b tp1 =1). Table I indicates the values of CV for each pass through the loop as the shift register  40  is unloaded from the MSB, and Table II indicates the values of CV for each pass through the loop as the shift register  40  is unloaded from the LSB. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 I 
                 b cN   
                 b tp(N−I)   
                 CV 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 1 
                 1 
                 8 
               
               
                   
                 1 
                 1 
                 1 
                 12 
               
               
                   
                 2 
                 0 
                 1 
                 12 
               
               
                   
                 3 
                 1 
                 1 
                 13 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 I 
                 b cI   
                 b tp(I+I)   
                 CV 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 1 
                 1 
                 1 
               
               
                   
                 1 
                 0 
                 1 
                 1 
               
               
                   
                 2 
                 1 
                 1 
                 5 
               
               
                   
                 3 
                 1 
                 1 
                 13 
               
               
                   
                   
               
             
          
         
       
     
     To continue with this example, if the predetermined time period, t g , during which the thirteen pulses are counted is one millisecond, the oscillator frequency is:          f   o     =       CV     t   g       =       13   .001     =     13   ,   00                 0                   Hz   .                                  
     As described in the foregoing, the counter  30  and shift register  40  of the preferred embodiment of the invention are located on the same semiconductor wafer  12  as the oscillator  20  and the semiconductor devices  14 . However, one skilled in the art will appreciate that the counter  30  and register  40  could be located on a test board that is separate from the wafer  12 . Alternatively, the counter  30  and register  40  could be integrated into the tester  50 . Thus, as indicated by the appended claims, the invention is not limited to any particular location of the counter  30  or the register  40 . 
     The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.