Abstract:
The soft error rate in a semiconductor memory is improved via the use of a circuit and arrangement adapted to use a mirror bit to recover from a soft error. According to an example embodiment of the present invention, a semiconductor device includes first and mirror memory cells configured and arranged to receive and store a same bit in response to a write operation, with the memory cells more susceptible to a bit error in which the stored bit changes from a first state to a second state than to a change from the second state into the first state. The memory cells are separated by a distance that is sufficient to make the likelihood of both memory cells being upset by a same source very low. For a read operation, the bits stored at the fist and second memory cells are compared. If the bits are the same, the bit from the first and/or mirror bit is read out, and if the bits are different, a bit corresponding to the more susceptible state is read out. In this manner, soft errors can be overcome.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to semiconductor devices and, more specifically, to semiconductor devices relating to memory applications susceptible to errors including soft errors. 
     BACKGROUND 
     In a variety of memory applications, such as SRAM, DRAM, non-volatile memory, thin capacitively coupled thyristor (TCCT) applications, other RAM applications and other memory applications such as CAM, dual-port, FTFO, etc., circuitry used to stare data can experience conditions that result in the stored data being corrupted. Errors common to memory cells include one or more cells sticking at a particular value (e.g. stuck-at-1), a write-zero pulse not being wide enough for a particular bit, a read-one circuit noise margin not being enough, and others. One particular type of error that has presented challenges to memory applications is a soft error. Soft errors occur when data stored in a memory cell is upset in a non-permanent manner, such as when stored data is changed to an incorrect value by a particle interaction. For example, the value of a memory cell may be changed from a “1” to a “0” value or from a “0” to a “1” value. These changes can result in incorrect data and inhibit the performance of memory cells. 
     The rate at which soft errors occur is referred to as the soft error rate, or SER. The SER can be calculated empirically, or else by first simulating the charge created at each node, and then simulating the device behavior to find the charge distribution conditions that lead to a soft error. The spectrum of each node that can result in a charge that causes an error is found, and the fluxes of the relevant parts of each spectrum are integrated to find the total flux and the probability of a bit error. Assuming the errors are probabilistically independent (which is not always the case), the resulting probability of failure can be represented by the equation: 
     
       
         N bit ×P 1-bit   
       
     
     where N bit  is the number of bits in the device and P 1-bit  is the probability related to the failure of a bit. 
     A variety of sources, such as alpha particles, cosmic rays, high-energy, and thermal neutrons, can cause soft errors. These sources can be generated in a variety of manners. For instance, alpha particles may be generated by contamination in metal layers, ceramic portions, and solder contacts of an integrated circuit die, and they generally create a small charge (e.g., 15 fC/micron) and a relatively high flux (e.g., 0.001-0.002/h-cm 2 ). Cosmic background radiation, such as background radiation generated at sea level, generates cosmic rays that typically exhibit a relatively low flux and a relatively high charge. Thermal neutrons, which include very low energy cosmic background neutrons, generally exhibit a relatively high flux and a moderate charge and cause fission in borophosphosilicate gas (BPSG) films. 
     Memory cells typically exhibit a critical charge, or Qcrit, at which a soft error will occur (ie., a charge that will upset the cell). The charge created by these and other sources can be collected in a portion of the cell and when the collected charge (Qcollected) exceeds Qcrit, a soft error occurs. 
     If BPSG is used, the SER is typically dominated by fission. However, in many modem applications, and particularly in DRAM applications, BPSG is reduced or eliminated. In the absence of BPSG and where Qcrit is high the SER is dominated by cosmic rays because they generate a relatively large amount of charge. In applications where BPSG is absent and Qcrit is low, alpha particles dominate the SER. 
     Qcrit for flipping a “0” to a “1” is approximately the same as Qcrit for flipping a “1” to a “0” in symmetric memory cells such as 6-transistor SRAM. However, in asymmetric memory cells, Qcrit01 (Qcrit to flip a bit from “0” to “1”) is very different from Qcrit10 (Qcrit to flip a bit from a “1” to “0”). For example, in TCCT base memory cells the Qcrit01 is significantly less than Qcrit10, and, a soft error detection and correction method which assumes that all soft errors are “0” to “1” flips can be used. 
     In each of the above and in other applications, SER and other error rates continue to present challenges to the continued advancement of the semiconductor industry. 
     SUMMARY 
     The present invention is directed to overcoming the above-mentioned challenges and others related to the types of devices and applications discussed above and in other memory cells. The present invention is exemplified in a number of implementations and applications, some of which are summarized below. 
     According to an example embodiment of the present invention, a memory arrangement is adapted for error recovery using first and second (mirror) bits. The memory arrangement includes a first asymmetric memory cell and a mirror asymmetric memory cell, each asymmetric memory cell being adapted to receive and store a same bit in response to a write operation. An output circuit detects and compares the bits stored at the first and mirror asymmetric memory cells. An error is detected when the bits stored at the first and mirror cells are different (e.g., when one of the cells has been upset by an error, such as excess charge near the cell). In response to detecting an error on either the first or the mirror cell, the output circuit is adapted to provide an output corresponding to the bit received and stored. In one implementation, this detection and correction scheme involves performing a logical AND operation between the first cell and the mirror cell in the case that errors are presumed to flip a “0” into a “1” only, and in the case that errors are presumed to flip a “1” into a “0” only, a logical OR operation is performed between the first cell and the mirror cell. This scheme is able to detect and correct an error to either one of the first cell or the mirror cell. 
     In another example embodiment, the present invention is directed to a method for recovering from errors in a memory arrangement having first and mirror asymmetric memory words, each word comprising one or more cells. During a write cycle, bits provided (written) to the first asymmetric memory word are also provided to the mirror asymmetric memory word. Between consecutive write cycles, memory cells from each word can become upset. The bits stored at each memory cell are compared for a read operation, and when the values stored at the first and mirror memory cells are different, a bit corresponding to the received and stored bit at the first asymmetric memory word is read out. In one implementation, this detection and correction scheme involves performing a logical AND operation between the first word and the mirror word in the case that errors are presumed to flip a “0” into a “1” only, and in the case that errors are presumed to flip a “1” into a “0” only, a logical OR operation is performed between the first word and the mirror word. This scheme is able to detect and correct all errors in which either one or none of any two corresponding memory cells in the first word and mirror word have an error. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments. 
    
    
     BRIRF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 is a flow diagram for error recovery in a memory cell, according to an example embodiment of the present invention; 
     FIG. 2 shows a series of original and mirror bits, used in connection with another example embodiment of the present invention; 
     FIG. 3 is a circuit configured and arranged for error recovery, according to another example embodiment of the present invention; 
     FIG. 4 is another circuit configured and arranged for error recovery, according to another example embodiment of the present invention; and 
     FIG. 5 is a thyristor-based memory cell arrangement adapted for error recovery, according to another example embodiment of the present invention. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not necessarily to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is believed to be applicable to a variety of different types of memory applications, and has been found to be particularly useful for devices susceptible to errors including soft errors. While the present invention is not necessarily limited to such devices, various aspects of the invention may be appreciated through a discussion of various examples using this context. 
     According to an example embodiment of the present invention, a memory arrangement is adapted for error recovery using a comparison of bits stored at first and second memory cells (e.g., DRAM, SRAM, non-volatile memory or TCCT memory cells). The bits are sufficiently separated such that the likelihood of both bits becoming upset in response to a single error source (e.g., static charge) is relatively low. Each of the first and second memory cells receives and stores a bit received from a same source during a write cycle, and the bit value at each cell is detected and compared for a subsequent read cycle. If the bits are the same, the value of the first (or second) bit is read out. If the bits are different, an output corresponding to the bit originally received and stored at the first (and second) memory cell is provided. In one particular implementation, the memory arrangement is adapted to output a “0” whenever the bits are different. In this manner, errors causing a memory cells to be upset (e.g., switched from a “0” to a “1”) can be recovered from by detecting whether or not the values of the cells are different. This method works for a variety of memory cells, including those that can easily be switched from a “0” to a “1” (small Qcrit for “0” to “1”) but cannot be easily be switched from a “1” to a “0” (large Qcrit for “1” to “0”) by an error. For example, TCCT based memory cells have an asymmetric sensitivity (different Qcrit) for flipping between “0” and “1” states. 
     FIG. 1 is a flow diagram for error recovery in an asymmetric memory cell that can easily be switched by an error from a “0” to a “1” , but not vice versa, according to another example embodiment of the present invention. At block  110 , bits stored at original and mirror TCCT memory cells are detected, and the detected bits are compared at block  120 . If the comparison indicates that the detected bits have the same value at block  130 , an output corresponding to the bit stored at the original memory cell is read out at block  140 . If the comparison indicates that the detected bits do not have the same value at block  130 , a “0” bit is read out at block  150 . In this instance, if the bit originally received and stored at the first memory cell is a “0” and has not been upset, the output being “0” corresponds correctly to the bit. Accordingly, if the bit originally received and stored at the first memory cell has been upset and is a “1,” the output being “0” corresponds correctly to the bit received and stored at the first memory cell, prior to being upset. This is particularly useful in TCCT applications where errors (bit upsets) cause a bit to change from a “0” to a “1” value, and not from a “1” to a “0” value. 
     The present invention is applicable to a variety of semiconductor circuit arrangements and memory applications. FIG. 2 is a schematic of one such circuit arrangement  200  having a series of original words  210  and mirror words  220  (e.g., respective memory arrays), each word including original and mirror bits that are a part of a series of bits  214  and  224 , respectively. Using word  212  and mirror word  223  in an example, bit  213  is compared to bit  223  using comparison circuitry. If the comparison shows that the bits are equal, no error has been detected and an output (a “1” or “0”) corresponding to bit  213  is read out. However, if the comparison shows that the bits are not equal, an error has been detected and a “0” is output. 
     In a more particular example embodiment, the mirror words  220  are adapted to store values that are different than the values stored in the original words  210 . This is particularly useful in applications where error recovery is not necessary at all times and/or for all bits. For example, it may be useful to use all bits for memory storage in a particular application. It may also be useful to selectively apply an error recovery scheme to some of the bits in a memory array, but not for others. In this instance, mirrored data is stored in the mirror bits only for those bits to which error recovery is applied, and the remaining mirror bits are used to store information. This is particularly useful for increasing the memory storage capability of the circuit arrangement  200  while maintaining error recovery for selected bits. 
     FIG. 3 is an arrangement of original and mirror words  310  and  320 , respectively, and circuitry adapted to detect and compare original and mirror bits, according to another example embodiment of the present invention. Each arrangement of original and mirror words includes a series of original and mirror bits  314  and  324 , respectively. Referring to original and mirror words  312  and  322  as an example, original bit  313  is electrically coupled to a first sense amplifier  340 , and mirror bit  323  is electrically coupled to a second sense amplifier  330 . The sense amplifiers are electrically coupled to an AND gate  350  adapted for comparing the signals from the sense amplifiers. If the original bit and the mirror bit have the same value, the AND gate reads out the value of the original bit at node  360 . However, when the original bit and the mirror bit are different, a “0” is read as the bit out. In one implementation, the bit out value at node  360  is defined as 
     BIT OUT=(bit) AND (mirror bit), 
     where the value “bit” is the output from sense amplifier  340  and the value “mirror bit” is the output from sense amplifier  330 . 
     The original bits ( 313 ,  314 ) are separated from their corresponding mirror bits ( 323 ,  324 ) by a distance that is sufficient to inhibit the possibility of an original bit and its mirror bit being upset by a single source, such as an alpha particle or other excess charge. In one implementation, original bit  313  is separated from mirror bit  323  by more than about one micron, and in another implementation the original and corresponding mirror bits are separated by more than about ten microns. Moreover, in applications where sources of error exhibit a relatively strong charge and/or flux, the separation of the original and mirror bits is selectively increased to achieve an acceptable probability that a single source would upset both the original and mirror bits. 
     In another example embodiment of the present invention, the circuitry including the AND gate  350  and the sense amplifiers  330  and  340  is replaced by a different comparison circuit adapted for comparing the values of the original and mirror bits. The comparison circuit is electrically coupled to the bits  313  and  323  and adapted to compare their values. As with the AND gate in FIG. 3, the comparison circuit is adapted to read out the value of the original bit when the original and mirror bits are the same, and to read out a “0” when the original and mirror bits are different. In other implementations, logic operator circuits including circuitry such as an OR gate, an inverter and/or other commonly available circuitry are used to effect the comparison. 
     FIG. 4 is an error recovery arrangement  400  having original and mirror bits  413  and  423  located in isolated active regions  415  and  425 , respectively. The read current from the bits when one bit is on and the other is off (e.g., one bit is “1” and the other bit is “0”) is referred to as I cell , and the read current when both bits are on is thus about 2×I cell . Therefore, when the bits are different, the read current is equal to about I cell , and when both bits are “1” the read current is about 2×I cell . In this regard, the bits are electrically coupled to a sense amplifier  435  that is adapted to read a “0” bit out at node  460  in response read current being less than about 2×I cell . This effects a proper read out because either there has been an error, in which a “0” should be read out, or there has been no error and the value of both of the bits is “0” (and thus I cell  is about zero). Accordingly, a “1” bit is read out in response to the read current being about 2×I cell . 
     The error recovery scheme of the present invention is adaptable for implementation with a variety of memory cell arrangements. FIG. 5 shows one such memory arrangement  500  including thyristor-based memory cells  510  and  540  in a substrate  505 , each having a thyristors  520  and  550  electrically coupled in series with pass gates  530  and  560 , respectively. The thyristor  520  includes emitters  522  and  528  separated by base regions  524  and  526 , and a gate  530  capacitively coupled to base region  526 . The gate  529  is electrically coupled to a second word line (WL 2 ) and adapted to form a conductive channel between the emitters  522  and  528  via the base regions  524  and  526  in response to a voltage being applied to WL 2 . Emitter region  528  is also a source/drain region for the pass gate  530  and is adapted to store a bit. The pass gate  530  also includes source/drain region  534  and a gate  532  electrically coupled to a first word line (WL 1 ). The pass gate is adapted to form a conductive channel between the source/drain regions  528  and  534  in response to a voltage being applied to WL 1 . A bitline  570  is coupled to the source/drain region  534  and adapted for reading out the bit from the emitter region  528 . 
     Memory cell  540  has similar features to memory cell  510  and is similarly numbered, and the discussion above regarding memory cell  510  also applies to memory cell  540 . In addition, gate  562  is also electrically coupled to WL 1 , and gate  559  is also electrically coupled to WL 2 . The bitline  570  is electrically coupled to a comparison circuit  580 , such as the one shown in FIG. 4, that is adapted to detect that the bits stored in each memory cell are different. The comparison circuit is adapted to read out a “0” value when the bits are different and to read out the value stored at the bits when they are the same. 
     The memory arrangement  500  in FIG. 5 shows both memory cells sharing bitline  570  and source/drain region  534 , as would be applicable to the circuit arrangement shown in FIG.  4 . However, in another example embodiment, separate bitlines are used (e.g. as would be applied to FIG. 3) with similar cells, except wherein the separate bitlines are electrically coupled to separate source/drain regions of the respective pass gates and to the comparison circuit  580 . 
     The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Based on the above discussion and illustrations, those skilled in the art will readily recognize that various modifications and changes may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein. Such changes may include, but are not necessarily limited to: altering the shapes, locations, and sizes of the illustrated words, bits and the thyristors and connections to which they may be applied; adding structures to the semiconductor devices; using asymmetric memory cells other than TCCT memory cells (e.g., SRAM, DRAM, non-volatile memories); using asymmetric memory cells that are easily disturbed and flipped from a “1” to a “0” , but not vice versa, with the corresponding logic (e.g., a OR gate or other circuitry that achieves the same logic, such as sensing circuitry that is wired to both the first and mirror cells and outputs a “1” when the read current is equal to or greater than approximately I cell , and outputs a “0” when the read current is less than approximately I cell ); using an inverter that changes a signal from a “1” to a “0” and vice versa, wherein the error detection schemes described herein are correspondingly reversed to correspond to the inverted signals (e.g., susceptibility of a flip from a “0” to a “1,” after inversion, corresponds to the susceptibility of a flip from a “1” to a “0”); increasing the number of PN sections in thyristors of the memory cells; interchanging P and N regions in the device structures and/or using PMOSFETS interchangeably with NMOSFETS. In addition, for more information regarding TCCT implementations to which the present invention is applicable, reference may be made to U.S. Pat. No. 6,229,161, dated May 8, 2001 and entitled “Semiconductor Capacitively-Coupled NDR Device And Its Applications In High-Density High-Speed Memories And In Power Switches,” which is fully incorporated herein by reference. Such modifications and changes do not depart from the true spirit and scope of the present invention that is set forth in the following claims.