Abstract:
The present invention includes a ferro fuse cell comprising a ferroelectric storage capacitor electrically connected to a plate on one side and to a sense amplifier on the other side. A ferroelectric measurement capacitor is electrically connected between the ferroelectric storage capacitor and the sense amplifier.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the implementation of ferro fuse cells. 
   BACKGROUND OF THE INVENTION 
   Electrical fuses are used to store address information for redundancy repair of high-density memories or other set-up information in memory or logic circuit components. In contrast to conventional laser fuses, electrical fuses can be set and reset even in packaged components. Such electrical fuses can be implemented with non-volatile memory cells using ferroelectric capacitors. Read out of such a ferroelectric capacitor memory cell typically requires a certain optimized “measurement” capacitance. In memory circuits, the parasitic bit-line capacitance is used for this purpose. In ferro fuse circuits, typically only one single cell is connected to the sense amplifier, i.e. no bit line or only a very short bit-line exists. Therefore, the bit-line capacitance is replaced by a gate oxide capacitance. 
     FIG. 1  shows a ferro fuse circuit  101  of the prior art. A ferroelectric capacitor (C 1 )  103  is connected to a plate (PL)  105  on one side and to a measurement capacitor (C 2 )  107  on the other side. By pulsing the plate (PL)  105 , a read signal develops on a node F  109  depending on the stored polarization state in the capacitor C 1   103 . If a “1” was stored in the ferroelectric capacitor C 1   103  then a bigger voltage is obtained on the node F  109  (switching). If a “0” was stored then a smaller voltage is obtained on the node F  109  (non-switching). These two voltage levels are compared to a reference voltage VREF  111  by a differential sense amplifier (SA)  113  and the fuse data is available on a node FD  115 . As the read operation is destructive, a write back is required, which is performed automatically by the sense amplifier  113 . 
     FIG. 2  shows another prior art ferro fuse circuit  201 . The circuit  201  uses two cells like the cell  101  in  FIG. 1  are combined to store one single data bit. Two ferroelectric capacitors C 1   203  and C 3   204  are connected to a plate (PL)  205  on one side and to measurement capacitors C 2   207  and C 4   208  on the other. The two ferroelectric capacitors C 1   203  and C 3   204  always contain reversed data (i.e. if a “1” signal is obtained on a node F  209  then a “0” signal is obtained on a node /F  210 , and visa versa). Thus the differential signal between the nodes F  209  and /F  210  is doubled compared to the implementation of FIG.  1 . Fuse data output from a sense amplifier  213  is available on a node FD  215 . Furthermore, no reference voltage is required. However, in the implementation of  FIG. 2 , the area for storage and measurement capacitors is doubled. 
   It would be desirable to produce a ferro fuse having a large signal margin. 
   SUMMARY OF THE INVENTION 
   The ferro fuse of the present invention provides both a large signal margin while reducing chip area and improving the reliability by using ferroelectric capacitors as “measurement” capacitors. 
   In a first example of the present invention, the “measurement” capacitance is implemented using a ferroelectric capacitor which is always in its non-switching state utilizing the high dielectric constant of the ferroelectric film. Alternatively, in a second example of the present invention, the “measurement” capacitance is implemented using a ferroelectric capacitor which is always in the reversed polarization state relative to the ferroelectric storage capacitor. In the first example, the area required for the “measurement” capacitance is reduced by one or two orders of magnitude depending on the chosen ferroelectric material. In the second example, the read signal is significantly increased and the reliability of the fuse circuit is enhanced. 
   In general terms, the present invention includes a ferro fuse cell comprising a ferroelectric storage capacitor electrically connected to a plate on one side and to a sense amplifier on the other side. A ferroelectric measurement capacitor is electrically connected between the ferroelectric storage capacitor and the sense amplifier. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     Further preferred features of the invention will now be described for the sake of example only with reference to the following figures, in which: 
       FIG. 1  shows a ferro fuse circuit of the prior art. 
       FIG. 2  shows a prior art ferro fuse circuit having two storage and two measurement capacitors. 
       FIGS. 3   a  and  3   b  shows the inventive use of ferroelectric capacitors as the measurement capacitors. 
       FIGS. 4 and 5  show the major control signals and the obtained voltages for reading “1” and “0” suitable for operating the circuits in  FIGS. 3   a  and  3   b.    
       FIGS. 6 and 7  show other embodiments of the present invention wherein capacitor pairs are always maintained in the reversed polarization states. 
       FIGS. 8 and 9  show the major control signals end the obtained voltages for reading “1” and “0” suitable for operating the circuits in  FIGS. 6 and 7 . 
       FIG. 10  shows how the read signal obtained from the circuits of the present invention are significantly enhanced. 
       FIG. 11  shows an implementation of a differential sense amplifier for use as the sense amplifier of the circuits of  FIGS. 3 ,  6  and  7 . 
       FIG. 12  shows a timing diagram with all the major required control signals for the circuit of FIG.  7 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIGS. 3   a  and  3   b  show embodiments of the present invention. The measurement capacitors C 2   107 ,  207  and C 4   208  of  FIGS. 1 and 2  are replaced by ferroelectric capacitors C 2   307 , C 2   307 ′ and C 4   308  (see  FIGS. 3   a  and  3   b ) similar to the ferroelectric capacitors C 1   103 , C 1   203  and C 3   204  of  FIGS. 1 and 2 . The measurement capacitors C 2   307 , C 2   307 ′ and C 4   308  of  FIG. 3  are always maintained in their non-switching state. Due to the much higher dielectric constant of the ferroelectric material compared to a typical gate oxide, the area required for the measurement capacitors C 2   307 , C 2   307 ′ and C 4   308  can be reduced by one or two orders of magnitude. The read and write back operation is similar to the implementations of  FIGS. 1 and 2 . 
     FIGS. 4 and 5  show the major control signals and the obtained voltages for reading “1” ( FIG. 4 ) and “0” ( FIG. 5 ) suitable for operating the circuits in FIG.  3 . In the figure “SAE” indicates the activation of the sense amplifier. 
     FIGS. 6 and 7  show other embodiments, circuits  601  and  701 , respectively, of the invention. The measurement capacitors C 2   107  of FIG.  1  and C 2   207 ′ and C 4   208  of  FIG. 2  are replaced by ferroelectric measurement capacitors C 2   607  ( FIG. 6 ) and C 2   707  and C 4   708  ( FIG. 7 ) which are similar to the ferroelectric storage capacitors C 1   103  of FIG.  1  and C 1   203  and C 3   204  of FIG.  2 . In these embodiments these measurement capacitors C 2   607 , C 2   707  and C 4   708  are always in the reversed state compared to the corresponding storage capacitor, e.g. if C 1  stores “1” then C 2  stores “0” and vice versa. Additional plate signals (PLO)  617 ,  717  are needed for write back of the data to the measurement capacitors. The signal timing for operating the circuits  601 ,  701  are shown in  FIGS. 8 and 9  for a read “1” and a read “0”, respectively. These timing sequences make sure that each capacitor pair (C 1 /C 2  and C 3 /C 4 ) is always maintained in the reversed polarization state, e.g. C 1 =“1” and C 2 =“0” or C 1 =“0” and C 2 =“1”. 
   The read signal Vsignal obtained from the circuits of the present invention are significantly enhanced as shown in FIG.  10 . The intersection of the characteristics  1005  of the storage capacitor and corresponding characteristics  1003  of the measurement capacitor determines the read signals and the signal differences. 
     FIG. 11  shows an implementation of a differential sense amplifier  1101  for use as the sense amplifier of the circuits of  FIGS. 3 ,  6  and  7 . 
     FIG. 12  shows a timing diagram with all the major required control signals for the circuit  701  of FIG.  7 . During d2 the read signal is developed on the nodes F and /F. During d3 the signal is amplified to full logic levels by the sense amplifier. During d4 the data is transferred from or to an external circuit. During d5-d7 the data write back to the storage and measurement capacitors is preformed. 
   In all of the above embodiments the described components can be formed on the same die. Also, the term “connected” as used in the present disclosure dose not imply that connected components must be in direct physical contact. Rather, the components need only be electrically connected. 
   Thus, although the invention has been described above using particular embodiments, many variations are possible within the scope of the claims, as will be clear to a skilled reader.