Abstract:
A method and apparatus implement effective testing of a sense amplifier for an eFuse without having to program or blow the eFuse. After initial processing of the sense amplifier, testing determines whether the sense amplifier can generate a valid “0” and “1” before programming the eFuse. A first precharge device and a second precharge device that normally respectively precharge a true sense node and a complement sense node to a high voltage are driven separately. For testing, one of the precharge devices is conditionally held off to insure the sense amplifier results in a “0” and “1”. This allows the testing of the sense amplifier devices as well as down stream connected devices. Once testing is complete, both precharge devices are controlled in tandem.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for implementing effective testing of a sense amplifier of an eFuse without having to blow the eFuse. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    In known testing arrangements for testing of a sense amplifier of an eFuse, multiple transistors defining the sense amplifier are only tested in the unblown state. If the chip is to be sent to a customer before the eFuses are blown there is no way to know if the sense amplifier will operate properly when the eFuse is blown. 
         [0003]    Typically this lack of effective testing results in a low field-programming yield due to untested faults inside the sense amplifier and surrounding circuits. This yield loss could be avoided if the sense amplifier could be tested without having to blow the eFuse. Typically the transistors that are only tested in one state will have, for example, over half of their faults untested when leaving manufacturing. 
         [0004]    A need exists for a mechanism for effectively testing of a sense amplifier of an eFuse without having to blow the fuse. It is highly desirable to provide such mechanism that does not require additional devices in the sense amplifier. 
         [0005]    As used in the following description and claims, it should be understood that the term eFuse means a non-volatile storage element that includes either an antifuse, which is a programmable element that provides an initial high resistance and when blown provides a selective low resistance or short circuit; or a fuse, which is a programmable element that provides an initial low resistance and when blown provides a selective high resistance or open circuit. 
       SUMMARY OF THE INVENTION 
       [0006]    Principal aspects of the present invention are to provide a method and apparatus for implementing effective testing of a sense amplifier of an eFuse without having to blow the eFuse. Other important aspects of the present invention are to provide such method and apparatus for implementing effective testing of a sense amplifier of an eFuse without having to blow the eFuse substantially without negative effect and that overcome many of the disadvantages of prior art arrangements. 
         [0007]    In brief, a method and apparatus are provided for implementing effective testing of a sense amplifier for an eFuse without having to program or blow the eFuse. After initial processing of the sense amplifier, testing determines whether the sense amplifier can generate both output states (valid “0” and “1” outputs) resulting from an unblown and a blown eFuse before programming the eFuse. A first precharge device and a second precharge device respectively normally precharging a true sense node and a complement sense node of the sense amplifier to a high voltage are driven separately during testing. For testing, the precharge devices are selectively controlled to insure the sense amplifier results in both output states. This enables testing of devices defining the sense amplifier as well as down stream connected devices. Once testing is complete, both precharge devices are controlled in tandem. 
         [0008]    In accordance with features of the invention, test coverage of the sense amplifier is increased by splitting the precharge (PC) signal into two physically different signals. This allows the tester to set the sense amplifier and connected into the same output state (“1” output) that occurs when the eFuse is actually blown without having to blow the eFuse. This testing of the invention significantly improves field-programming yield. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
           [0010]      FIG. 1  is a schematic diagram illustrating an exemplary sense amplifier for implementing eFuse sense amplifier testing in accordance with the preferred embodiment; 
           [0011]      FIGS. 2A and 2B  illustrate normal operation of the eFuse sense amplifier of  FIG. 1  in accordance with the preferred embodiment; and 
           [0012]      FIGS. 3A and 3B  illustrate testing operation of the eFuse sense amplifier of  FIG. 1  in accordance with the preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    In accordance with features of the invention, the sense amplifier of an eFuse is effectively tested without having to blow the eFuse. Electronic fuses or eFuses use a sense amplifier to determine if the eFuse is a logical “0” or logical “1”. The fuse stores information by electrically changing the resistance of a polysilicon resistor. The testing of the present invention effectively tests the states of the sense amplifier that result from both the blown and not blown conditions of the eFuse. 
         [0014]    Having reference now to the drawings, in  FIG. 1 , there is shown an exemplary sense amplifier generally designated by the reference character  100  for implementing eFuse sense amplifier testing in accordance with the preferred embodiment. 
         [0015]    Sense amplifier  100  is used for an electronic fuse, or eFuse  102  to determine if the fuse  102  is a logical “0” or logical “1”. The fuse  102  stores information by electrically changing the resistance of a polysilicon resistor. Sense amplifier  100  includes true and complement sensing nodes respectively labeled S_T and S_C. A first precharge P-channel field effect transistor (PFET)  104  is connected between a positive voltage supply rail VDD and the true sensing node S_T that is connected via a pair of series connected N-channel field effect transistor (NFETs)  106 ,  108  to the eFuse  102 . A second precharge P-channel field effect transistor (PFET)  110  is connected between the positive voltage supply rail VDD and the complement sensing node S_C that is connected via a pair of series connected N-channel field effect transistor (NFETs)  112 ,  114  to a reference resistor  116 . 
         [0016]    Sense amplifier  100  includes a pair of cross-coupled inverters connected to the true and complement sensing nodes S_T and S_C, as shown. A PFET  120  and an NFET  122 , and a PFET  124  and an NFET  126  respectively form the cross-coupled inverters. A pull-up PFET  128  connects PFETs  120 ,  124  to the positive voltage supply rail VDD and a pull-down NFET  130  connects NFETs  122 ,  126  to ground. 
         [0017]    The eFuse  102  and reference resistor  116  are connected to a common node labeled FSOURCE and a connected via a pair of series connected N-channel field effect transistor (NFETs)  140 ,  142  to ground. A fuse programming circuit coupled to the eFuse  102  includes a NAND gate  150  receiving two inputs, BLOW_FUSE, FUSE_SOLUTION and providing an output applied to an inverter  152 , and a pair of series connected N-channel field effect transistor (NFETs)  154 ,  156  connected between the eFuse  102  to ground. 
         [0018]    The reference resistor  116  is, for example, about ½ the difference between a “0” and “1” resistance of fuse  102 . The fuse  102  and the reference resistor  116  are used to generate signal for the sense amplifier, that converts them to a digital “0” or “1” value. 
         [0019]    A sense amplifier signal control  160  generates signals SIGDEV, FSET, and PRECHARGE that are applied to the sense amplifier  100  in normal operation as illustrated in  FIGS. 2A and 2B . The sense amplifier signal control  160  generates signals SIGDEV, FSET, and two separate precharge control signals PC_TRU, PC_CMP that are applied to the sense amplifier  100  during testing operation as illustrated in  FIGS. 3A and 3B  in accordance with features of the invention. 
         [0020]    Referring to  FIGS. 2A and 2B , the sense amplifier  100  initializes by precharging both sides of the sense nodes S_C, S_T to a high voltage with a low PRECHARGE signal applied to both PFETs  104 ,  110 . The FSET signal is inverted by an inverter  210  and applied to PFET  128  and the FSET signal is directly applied to NFET  130 . The sensing signals SIGDEV are applied to NFETs  106 ,  112  on the two sides of the amplifier  100  and the amplification process commences. However, after initial processing of the silicon, it is desirable to test whether the sense amplifier  100  can generate a valid “0” and “1” before blowing or programming the fuse  102 . With the eFuse  102  not blown the sense amplifier  100  will result in an output “0” at the output TRUE of inverter  134  of  FIG. 1 , with S_C high and S_T low, as shown in  FIG. 2B . 
         [0021]    As shown in  FIG. 2B , reading the eFuse  102  includes normal control signals as follows:
   1) Initially, PRECHARGE ON, (PFETs  104 ,  110  turned on) SIGDEV OFF (NFETs  106 ,  112  turned off), FSET OFF (PFET  128  off, NFET  130  off)   2) SIGDEV ON (NFETs  106 ,  112  turned on)   3) FSET ON (PFET  128  turned on, NFET  130  turned on)   4) PRECHARGE OFF (PFETs  104 ,  110  turned off)   5) SIGDEV OFF (data can be read) (NFETs  106 ,  112  turned off)   6) FSET OFF (PFET  128  off, NFET  130  off)   7) PRECHARGE ON (PFETs  104 ,  110  turned on)   
 
         [0029]    In accordance with features of the invention, after initial processing of the silicon defining sense amplifier  100 , the sense amplifier  100  is tested to determine whether the sense amplifier  100  can generate a valid “0” and “1” outputs before programming or blowing eFuse  102 . When the eFuse  102  is not blown the sense amplifier  100  will result in an output “0”. When the fuse is blown the sense amplifier  100  will result in an output “1”. Testing of the sense amplifier  100  includes both states of the sense amplifier  100  that result from both the blown and not blown conditions of the eFuse  102  without requiring that the eFuse be programmed or blown. 
         [0030]    Referring to  FIGS. 3A and 3B  in accordance with features of the invention testing of the sense amplifier  100  is provided without requiring any additional devices to be added to the sense amplifier. The FSET signal is inverted by an inverter  210  and applied to PFET  128  and the FSET signal is directly applied to NFET  130 . The sensing signals SIGDEV are applied to sensing node NFETs  106 ,  112  on the two sides of the amplifier  100 . 
         [0031]    As shown in  FIG. 3B , the method to read  0  with an unblown fuse  102  is illustrated near the bottom of  FIG. 3B , with signal PC_CMP is held low keeping PFET  110  on, and PC_TRU switched off early turning PFET  104  off. Since the eFuse  102  is unblown, the fuse can be read normally as shown in  FIG. 2B , however, PC_TRU can be switched off early as shown in  FIG. 3B , while it should be understood that this is unnecessary. As shown in  FIG. 3B , reading  0  with the unblown eFuse  102  includes testing control signal as follows:
   1) Initially, PC_TRU ON, and PC_CMP ON, (PFETs  104 ,  110  turned on) SIGDEV OFF (NFETs  106 ,  112  turned off), FSET off (PFET  128  off, NFET  130  off)   2) SIGDEV ON (NFETs  106 ,  112  turned on)   3) FSET ON (PFET  128  turned on, NFET  130  turned on)   4) PC_TRU OFF, PC_CMP ON (PFET  104  turned on, PFET  110  turned off)   5) SIGDEV OFF (data can be read) (NFETs  106 ,  112  turned off)   6) FSET OFF (PFET  128  off, NFET  130  off)   7) PC_TRU ON, PC_CMP ON (PFET  104  turned on, PFET  110  turned on)   
 
         [0039]    The method to read  1  with an unblown eFuse  102  is illustrated near the bottom of  FIG. 3B  starting with signal PC_TRU that is held low keeping PFET  104  on, and PC_CMP switched off early turning PFET  110  off. As shown in  FIG. 3B , reading  1  with the unblown eFuse  102  includes testing control signal as follows:
   1) Initially, PC_TRU ON, and PC_CMP ON, (both low with PFETs  104 ,  110  turned on) SIGDEV OFF (NFETs  106 ,  112  turned off), FSET off (PFET  128  off, NFET  130  off)   2) SIGDEV ON (NFETs  106 ,  112  turned on)   3) FSET ON (PFET  128  turned on, NFET  130  turned on)   4) PC_TRU ON, PC_CMP OFF (PFET  104  turned on, PFET  110  turned off)   5) SIGDEV OFF (data can be read) (NFETs  106 ,  112  turned off)   6) FSET OFF (PFET  128  off, NFET  130  off)   7) PC_TRU ON, PC_CMP ON (PFET  104  turned on, PFET  110  turned on)   
 
         [0047]    While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.