Abstract:
A non-volatile memory element for storing at least one data item, having a readable memory cell which can be written on with a first part of a data item, the memory cell exhibiting a first characteristic which is electrically irreversibly modifiable according to the first partial data item, at least one readable second memory cell which can be written on by a second partial data item, the second memory cell being electrically irreversibly modifiable according to the second partial data item, and a reader device which is coupled to the first memory cell and second memory cell. The memory element is configured such that the first partial data item and second partial data item are respectively determined according to the data item. The reader device is configured such that it determines the stored data item by comparing the first partial data item with the second partial data item.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of co-pending International Application No. PCT/DE2004/001437 filed Jul. 5, 2004 which designates the United States, and claims priority to German application number DE 103 34 630.9 filed Jul. 29, 2003, which are incorporated herein by reference in their entirety. 

   TECHNICAL FIELD 
   The present invention relates to a memory element for the non-volatile storage of at least one datum. 
   BACKGROUND 
   The non-volatile storage of data or states is often necessary in integrated semiconductor circuits. Non-volatile memories are suitable in particular for holding small to medium volumes of data, such as, by way of example, redundancy data, data keys and self-test results. These can thus be repeatedly made available to the user. 
   A number of non-volatile memory elements are known to the person skilled in the art. As an example thereof, mention shall be made of the textbook “Mikroelektronische Speicher” [“Microelectronic Memories”] by Dietrich Rhein and Heinz Freitag, ISBN 3 211 82354 3, in particular pages 105 to 108 and pages 122 to 127. Non-volatile memories include electrically programmable read-only memories or EPROMs, and flash memories with floating gate technologies. Ferroelectric and magnetic read/write memories are also known. These memories have the disadvantage in common that complicated process steps are needed during their fabrication, which lead to higher product costs. 
   Another form of non-volatile memories uses severable interconnects in the integrated semiconductor circuit, so-called “fuses”. Writing to such memories requires separate devices such as, by way of example, laser cutters or means for generating high currents. It is thus not always possible for the users of such memories themselves to write to the memory. This may be a disadvantage particularly when such memories are used in mobile systems. 
   What is more, severable fuses entail the risk of separated connections at least partly growing together again during operation. As a result, data stored in the memory may be corrupted without this being perceptible to the user. 
   SUMMARY 
   The present invention is based on the problem of providing a cost-effective memory element and a memory element arrangement enabling non-volatile storage of data in a simple and reliable manner. 
   The problem can be solved by means of a non-volatile memory element for storing at least one datum that has at least one first memory cell to which and from which a first partial datum can be written and read out and which has a first characteristic that can be altered electrically irreversibly in a manner dependent on the first partial datum, at least one second memory cell to which and from which a second partial datum can be written and read out and which has a second characteristic that can be altered electrically irreversibly in a manner dependent on the second partial datum, and a read device coupled to the first memory cell and the second memory cell, the memory element being set up in such a way that the first partial datum and the second partial datum are in each case determined in a manner dependent on the datum, and the read device being set up in such a way that it determines the stored datum from a comparison of the first partial datum with the second partial datum. 
   The second partial datum can be the complementary value of the first partial datum, and the read device can be set up in such a way that it determines the datum stored in the memory element from the difference between the first partial datum and the second partial datum. The read device may comprise a differential amplifier for determining the datum from the first partial datum and the second partial datum. The first memory cell and/or the second memory cell each may comprise a resistance element, and the first and/or second characteristic can be the electrical conductivity of the resistance element. The resistance element can be an electrically severable interconnect. The first memory cell and/or the second memory cell each may comprise a transistor and the first and/or second characteristic can be the saturation current of a source-drain path of the transistor. A memory element may further comprise a register coupled to the read device and serving for storing the datum determined by the read device. The first memory cell and/or the second memory cell may comprise a switching element, which prevents or permits an electric current flow through the first memory cell in a manner dependent on an activation signal fed to the switching element. A memory element arrangement may comprise a multiplicity of such non-volatile memory elements, and may comprise a selection device coupled to the multiplicity of non-volatile memory elements, wherein the selection device is set up in such a way that a datum can selectively be stored in a memory element selected from the multiplicity of memory elements or be read out from the said memory element. The multiplicity of memory elements may comprise a common read device. 
   One fundamental idea of the invention consists in redundant storage of the datum. In the memory element, the first partial datum is stored in the first memory cell and the second partial datum is stored in the second memory cell by irreversibly altering the electrically alterable characteristic of the first memory cell and of the second memory cell, respectively. Such a characteristic may be a physically measurable parameter whose value is changed irreversibly in a manner dependent on the respective partial datum. This may take place by means of a degradation of a physical quantity such as a conductivity, for example. To express it clearly, the invention is thus also based on the utilization of irreversible degradation operations in the memory circuit, more precisely in the first memory cell and/or in the second memory cell. 
   The first partial datum and also the second partial datum are defined in a manner dependent on the datum to be stored. By way of example, the first partial datum and the second partial datum may correspond in terms of their value to the datum to be stored. However, the first partial datum and/or the second partial datum may also correspond to the complementary value of the datum to be stored. 
   When the datum is read out from the memory element, this is determined by a read device from a comparison between the first partial datum and the second partial datum. 
   This arrangement advantageously ensures a secure read-out of the datum stored in the memory element. If the first partial datum is no longer able to be reliably determined, for example as a result of uncontrollable physical processes, then the comparison with the second partial datum provides for a secure indication of the stored datum. 
   The memory element arrangement has a multiplicity of memory elements according to the invention having a selection device coupled to the multiplicity of memory elements, which selection device is set up in such a way that a datum can selectively be stored in a memory element selected from the multiplicity of memory elements or be read out from the said memory element. 
   Thus, a further basic concept of the present invention consists in a memory element arrangement with a multiplicity of the non-volatile memory elements already mentioned. By means of the selection device, it is possible for a plurality of data to be stored in a respective memory element. In the example of a binary system, a bit is thus in each case stored in a memory element. Greater volumes of data can thus advantageously be stored. 
   It is possible to realize the memory element arrangement in an arrangement comprising a plurality of components for example by each memory element and the driving device being embodied in a dedicated component. However, the memory element arrangement may also be embodied in a single semiconductor circuit which can advantageously be fabricated with the aid of a submicron CMOS process. 
   In a preferred development, the second partial datum is the complementary value of the first partial datum. In this case, the read device is set up in such a way that the stored datum is determined from the difference between the first partial datum and the second partial datum. 
   What is advantageous here, in particular, is that the first partial datum and the second partial datum are stored by means of different states of the alterable characteristic. Interference or undesirable external influences thus influence the stored partial data to different extents. 
   Preferably, in this development, the read device for determining the datum from the first partial datum and the second partial datum is embodied in the form of a differential amplifier. This embodiment permits a cost-effective and simple realization. 
   In one embodiment of the invention, the first characteristic and/or the second characteristic is the conductivity of a resistance element. This may take place by virtue of the fact that the conductivity can be irreversibly altered by influencing the charge carrier zone in a manner dependent on a partial datum to be stored. Such influencing of the charge carrier zone may be caused for example by hot carrier effects. One advantage of this arrangement is that it can readily be fabricated in a customary semiconductor process, such as in CMOS, for example. 
   In a preferred development, the resistance element comprises an electrically severable interconnect, a so-called fuse track, which can be destroyed in a manner dependent on a partial datum to be stored. This destruction quite generally takes place as a result of electromigration. A further cause may be melting of the fuse track as a result of thermal effects of an electric current. The distinctly measurable difference between the resistance values before and after storage of a partial datum is advantageous here. If a fuse track is severed, then its conductivity falls to the value zero. 
   In a further embodiment, the first memory cell and/or the second memory cell in each case have at least one transistor. A saturation current of the transistor can be degraded in a manner dependent on a partial datum to be stored. This degradation of the saturation current takes place as a result of hot carrier effects at the gate oxide of the transistor. The charge carrier channel of the transistor is thus influenced in such a way that the charge carrier flow is restricted. Preferably, during a writing operation, a current flows through the resistance element in a different direction than during a reading operation. Due to the asymmetry of the damage, the distribution of an electric field in the transistor will likewise turn out to be asymmetrical. This amplifies the degradation of the saturation current which, in the event of a read-out of the stored value, flows in the opposite direction to the current when writing the datum. The cause of this effect is based on the effective screening of the charge carrier channel in the transistor by the damage of the gate oxide and the lack of support of the channel flow in the region of the damage by the additional field of the drain terminal. The degradation is thus particularly pronounced if the current which damages the gate oxide flows in a different direction than the current during a measurement of the saturation current. 
   In a further preferred development, the memory element according to the invention has a register coupled to the read device and serving for securing the datum determined by the read device. In this case, a data output of the read device is connected to the input of a register. This advantageously reduces the number of accesses to the memory cell to a minimum. The data have to be transferred from the memory cell into the register only when a corresponding value is not present there. The small number of accesses to the memory cell enables a further degradation thereof to be prevented as far as possible. 
   In an alternative development, the first memory cell and/or the second memory cell have a switching element, which prevents or permits a current flow through the respective element in a manner dependent on an activation signal fed to the switching element. Consequently, an unnecessary influencing of degradation of the alterable characteristic may likewise be prevented. A current advantageously flows only if a partial datum is written to the respective memory cell, or if the datum is determined by the read device. 
   In a development of the control element arrangement, the multiplicity of memory elements have a common read device, so that unnecessary chip area that causes costs can be saved in an embodiment as an integrated semiconductor circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below using a plurality of exemplary embodiments with reference to the drawing, in which: 
       FIG. 1  shows a memory element in accordance with a first exemplary embodiment of the invention, in which the degradation of the saturation current of a transistor is utilized for storing data, 
       FIG. 2  shows a memory element in accordance with a second exemplary embodiment of the invention, in which the degradation of the saturation current of a transistor is utilized for storing data, 
       FIG. 3  shows the performance of a writing and reading operation in a memory element in accordance with  FIG. 2 , and 
       FIG. 4  shows a memory element in accordance with a third exemplary embodiment of the invention, which has severable fuses. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows the circuit construction of a memory element in which the degradation of the saturation current of a transistor is utilized for storing data. The memory element has a first memory cell group  101  (illustrated in dotted fashion), which in each case has a first memory cell  103  (illustrated in dashed fashion) with a first signal connection  105  and a second memory cell  104  (illustrated in dashed fashion) with a second signal connection  107 . A second memory cell group  102  is illustrated as a circuit block with a first signal connection  106  and a second signal connection  108 . Its internal construction is equivalent to that of the memory cell group  101 . It is possible to provide as many additional further memory cell groups as desired in each case having any desired number of transistors in the memory cells. 
   The first memory cell  103  and the second memory cell  104  respectively comprise a plurality of transistors  110 ,  111 ,  112  and  113 ,  114 ,  115 , the source-drain paths of which are connected in series. A respective signal path via the series circuit of this source-drain path in the first memory cell  103  or in the second memory cell  104  connects the first signal connection  105  to a control connection  125 . 
   The gate terminals of the transistors  110 ,  111 ,  112  and  113 ,  114 ,  115  receive an access control signal from an access control output of a decoder  109  via a cell selection line. In this case, the memory cell group  101 ,  102  is assigned a respective access control output of the decoder  109 . The decoder  109  additionally has a first input for an activation signal  116  and a parallel input for address signal  117 . 
   A data input  118  for a data signal to be stored is connected via an inverter  119  to the input of a first tristate driver  120  and directly to the input of a second tristate driver  121 . The first tristate driver  120  and the second tristate driver  121  are inhibited or opened by means of a state control signal provided at a respective inhibiting input. The state control signal is introduced into the switching element via a state control input  122  and likewise passed to the gate terminal of a first PMOS transistor  123  and to the gate terminal of a second PMOS transistor  124  and also to the control connection  125 . A respective source terminal of the PMOS transistors  123 ,  124  is fed with an applied voltage V DD  from a constant-voltage source  126 . 
   A first data line  127  connects an output of the first tristate driver  120  to a drain terminal of the first PMOS transistor  123 , to the first signal connection  105 ,  106  and to a first voltage input of a differential amplifier  129 . 
   A second data line  128  connects an output of the second tristate driver  121  to a drain terminal of the second PMOS transistor  124 , to the second signal connection  107 ,  108  and to a second voltage input of the differential amplifier  129 . 
   An output data signal is provided at a voltage output  130  of the differential amplifier  129 , and is passed to a signal input of a multiplexer  131 . The multiplexer  131  has two signal outputs that are respectively connected to a data input of two registers  132 ,  133  set up as D-type flip-flops. 
   The output data signal provided at the signal input of the multiplexer  131  is switched in a manner dependent on activation signals input at the control input of the multiplexer  131 . The control inputs are respectively connected to an access control signal output of the decoder  109 . 
   The method of operation of the memory element illustrated in  FIG. 1  is based on particular properties of MOSFET transistors. The latter are subject to a degradation of the saturation current in the course of their operation. The degradation is particularly pronounced if the electric current when writing to the memory element has a different direction or a different sign than the electric current which flows through the memory cell in reading out a stored datum. This phenomenon is connected with the asymmetrical damage of the transistor in the drain region thereof. The effect is additionally amplified by means of the series circuit of the transistors  110 ,  111 ,  112  and  113 ,  114 ,  115  in the first memory cell  103  and in the second memory cell  104 , respectively, as illustrated in  FIG. 1 . In the present embodiment, each memory cell  103 ,  104  comprises three transistors  110 ,  111 ,  112  and  113 ,  114 ,  115 , respectively. It is equally possible to use more or fewer transistors per memory cell  103 ,  104 . The first memory cell  103  and the second memory cell  104  increase the sensitivity of the system. The data stored in the memory cells  103 ,  104  are evaluated differentially by means of the differential amplifier  129 . 
   The cell selection lines are driven by the decoder  109 , which, on the basis of an address information item of the address signal  117  sets an access control signal on one of the cell selection lines to the value logic “1”, provided that the activation signal  116  likewise has a value logic “1”. For a better understanding it is assumed below that the value logic “1” corresponds to a potential V DD  and the value logic “0” corresponds to a zero potential. 
   A voltage pulse shall additionally be defined below for a simpler representation. In this case, a 0-1-0 pulse is a temporal pulse in which, on an electrical connection, firstly a zero potential is present, which is switched over to the voltage V DD  during a specific time duration. Correspondingly, a 1-0-1 pulse on an electrical connection is an applied potential V DD  which is switched over to a zero potential during a specific time duration. The time duration and thus the length of the pulse result from the effect respectively desired and may have a different length. 
   By virtue of the potential present on the cell selection line, there are present in a memory cell group  101 ,  102 , at the gate terminals of the transistors  110 ,  111 ,  112  and  113 ,  114 ,  115 , respectively, in each case such large electrical potentials that the charge carrier channels of the respective source-drain paths are open. A current can thus flow through them. The respective memory cell group  101 ,  102  is thus activated. At the same time, on account of the access control signal, the multiplexer  131  switches the signal path from the voltage output  130  to a register  132 ,  133  assigned to the memory cell group  101 ,  102 . 
   In order to write to a memory cell group  101 ,  102 , the latter is selected by means of the address signal  117 . The data signal to be stored is provided at the data input  118 . A switching state is established by the state control signal being set to the value logic “1”. As a result, the first tristate driver  120  and second tristate driver  121  are activated and the first PMOS transistor  123  and the second PMOS transistor  123  are turned off. The data input  118  is thus connected to the first signal connection  105 ,  106  and the second signal connection  107 ,  108 . The complement of the data signal to be stored is present at a first signal connection  105 ,  106 , while the value of the data signal to be stored is present at the second signal connection  107 ,  108 . The value logic “1” is present in the channel connection  125 . In order to impress the datum to be stored into the memory cell group  101 ,  102  a 0-1-0 pulse is provided as activation signal  116 . As a result, the memory cell group  101 ,  102  is activated and a current flows through the first memory cell  103  or through the second memory cell  104 , which current degrades the saturation current of the transistors  110 ,  111 ,  112  or  113 ,  114 ,  115 , respectively. The length of the 0-1-0 pulse of the activation signal  116  is chosen correspondingly in order to achieve a measurable degradation. 
   In order to read out a datum from a memory cell group  101 ,  102 , the latter is likewise selected by means of the address signal  117 . A read state is established by the state control signal being set to the value logic “0”. As a result, the two tristate drivers  120 ,  121  are inhibited, while the first PMOS transistor  123  and second PMOS transistor  124  are open. The value logic “1” is present at the first signal connection  105 ,  106  and also at the second signal connection  107 ,  108 , while the value logic “0” is present at the channel connection  125 . If the activation signal then obtains a value logic “1”, the gate terminals of the transistors  110 ,  111 ,  112  and  113 ,  114 ,  115  of the first memory cell  103  and of the second memory cell  104 , respectively, in the selected memory cell group  101 ,  102  are open. A current can flow, limited by the saturation current. In accordance with the respective saturation current and the thereby afforded conductivity of the first memory cell  103  and of the second memory cell  104 , the voltage is dropped across the first signal connection  105 ,  106  and the second signal connection  107 ,  108 . A voltage difference between the first signal connection  105 ,  106  and the second signal connection  107 ,  108  is detected by means of the differential amplifier  129  and stored as a value in one of the registers  132 ,  133  via the multiplexer  131 . As long as the registers  132 ,  133  are supplied with a supply voltage, the datum stored from the memory cell group  101 ,  102  can be provided by the register  132 ,  133 . 
     FIG. 2  illustrates a second embodiment of the memory element, which differs from  FIG. 1  by the use of a read amplifier  201  (illustrated in dotted fashion). The memory cell groups  101 ,  102  are constructed as in  FIG. 1  and are likewise activated via an access control output of a decoder  109 .  FIG. 2  likewise differs from  FIG. 1  in that the tristate drivers  120 ,  121  are activated by a tristate control signal  202 . By contrast, the control connection  125  is connected to a control input  200 . 
   The first signal connection  105 ,  106  is connected to a line  127  of the read amplifier  201  and the second signal connection  107 ,  108  is connected to the second line  128  of the read amplifier  201 . The potentials on the first line  127  and the second line  128  can be equalized by means of a transistor  203 . For this purpose, the transistor  203  is switched or turned off via an equalization signal input  204 . 
   Furthermore, the first line  127  and the second line  128  are coupled to one another via an NMOS latch  210  (illustrated in dashed fashion) and a PMOS latch  220  (illustrated in dashed fashion). The read amplifier  201  provides the stored datum and the complement with respect thereto at a first output  205  and at the second output  206 , respectively. 
   The NMOS latch  210  has a feedback transistor stage comprising two NMOS transistors  211 ,  212 . In this case, the gate terminal of a respective one of the NMOS transistors  211 ,  212  is connected to the source terminals of the respective other NMOS transistor  211 ,  212 . The source terminal of one NMOS transistor  211  is connected to the first line  127 , while the source terminal of the other NMOS transistor  212  is connected to the second line  128 . The drain terminals of the two NMOS transistors  211 ,  212  are coupled to a switching transistor  213 , which connects them through to a zero potential in a manner dependent on a voltage signal at a first supply input  214 . 
   The PMOS latch  220  is constructed analogously from a feedback transistor stage comprising two PMOS transistors  221 ,  222 . The source terminals thereof are coupled to a supply voltage V DD  via the source-drain path of a further switching transistor  223 . The further switching transistor  223  switches in a manner dependent on a voltage signal provided to it at a second supply input  224  by means of an inverter  225 . 
     FIG. 3  shows the performance of a writing and reading operation in a memory element in accordance with the embodiment in  FIG. 2 . During the writing operation, firstly the signal at the control input  200  is set to the value logic “1”. The value logic “0” is present at the first supply input  214  and the second supply input  224  and also at the equalization signal input  204 . The read amplifier  201  is thus not activated. 
   A memory cell group  101 ,  102  is written to by being selected by means of an address signal  117  and being activated by the activation signal  116  by means of a 0-1-0 pulse. At the same time, the tristate drivers  120 ,  121  are momentarily opened by a 1-0-1 pulse of the tristate control signal  202 , so that a potential corresponding to the value of the datum present at the data input  118  and to its complement, respectively, is present at the first signal connection  105 ,  106  and at the second signal connection  107 ,  108 . 
   During the writing operation, the datum and the complement thereof are impressed into the selected memory cell group  101 ,  102  by means of an irreversible change in the saturation currents of the transistors  110 ,  111 ,  112  and  113 ,  114 ,  115 , respectively. 
   In order to read out the datum contained in a memory cell group  101 ,  102 , the said datum is selected by means of the address signal  117 . The potential at the control input  200  is set to the value logic “0”. A potential having the value logic “0” is initially present at the first supply input  214  and the second supply input  224  as well as at the equalization signal input  204 . An arbitrary signal having a valid value logic “0” or logic “1” is provided at the data input. 
   By means of a 1-0-1 pulse of the tristate control signal  202 , the first line  127  and the second line  128  are occupied by a value 0 and 1, respectively, by a conductive connection to the data input being momentarily produced. The potentials of the first line  127  and the second line  128  are then equalized by means of a 0-1-0 pulse at the equalization signal input  204 , so that a potential V DD /2 is present on both. 
   Afterwards, the datum stored in the selected memory cell group  101 ,  102  is read out by a 0-1-0 pulse simultaneously being provided at the activation input  116  and also at the first supply input  214  and at the second supply input  224 . 
     FIG. 4  shows the circuit construction of a memory element having severable fuses. The circuit differs from  FIG. 1  in a different embodiment of the first memory cells  103  and of the second memory cell  104  and also in that the control connection  125  is set to a zero potential. 
   The first memory cell  103  and the second memory cell  104  respectively comprise an activation transistor  403 ,  404 , the source-drain path of which couples the first and second signal connection  105 ,  106 ,  107 ,  108 , respectively, to a thin interconnect (fuse)  401 ,  402 . The gate terminals of the activation transistors  403 ,  404  are connected to the access control output of the decoder  109 . 
   A writing and reading operation is effected in accordance with the sequence described in  FIG. 1 . A current flowing through the thin interconnect  401 ,  402  destroys the latter and thus its conductivity. A possible healing of the severing of the thin interconnect  401 ,  402  is insignificant due to the differential construction of the memory element, since the datum stored in the memory element is determined from the comparison of the first partial-datum with the second partial datum. 
   LIST OF NUMERALS 
   
       
         101 , 102  Memory cell group 
         103  First memory cell 
         104  Second memory cell 
         105 , 106  First signal connection 
         107 , 108  Second signal connection 
         110 , 111 , 112  Transistor 
         113 , 114 , 115  Transistor 
         109  Decoder 
         116  Activation signal 
         117  Address signal 
         118  Data input 
         119  Inverter 
         120  First tristate driver 
         121  Second tristate driver 
         122  State control input 
         123  First PMOS transistor 
         124  Second PMOS transistor 
         125  Control connection 
         126  Constant-voltage source 
         127  First data line 
         128  Second data line 
         129  Differential amplifier 
         130  Voltage output 
         131  Multiplexer 
         132 , 133  Register 
         200  Control input 
         201  Sense amplifier 
         202  Tristate control signal 
         203  Transistor 
         204  Equalization control input 
         205  First output 
         206  Second output 
         210  NMOS latch 
         211 , 212  NMOS transistor 
         213  Switching transistor 
         214  First supply input 
         220  PMOS latch 
         221 , 222  PMOS transistor 
         223  Switching transistor 
         224  Second supply input 
         225  Inverter 
         401 , 402  Thin interconnect 
         403 , 404  Activation transistor