Abstract:
A high density, low voltage, and low-power one time programmable (OTP) memory is based on core cells with a one transistor design. A CLEAN pulse is directed to a single shunt device at the output of the column decoder so spurious charges that may have been stored in the floating nodes can be cleaned up. Such arrangement also allows for the simultaneous initialization of bit lines, data lines, and sensing lines to zero. Core area layout size is substantially reduced, and operational power requirements are exceeding low making these particularly suitable in HF and UHF RFID applications.

Description:
RELATED APPLICATIONS 
     This Application is a continuation in part (CIP) of U.S. patent application Ser. No. 12/796,031, filed Jun. 08, 2010 now U.S. Pat. No. 8,259,518, issued on Sep. 4, 2012, and titled, A NEW LOW VOLTAGE AND LOW POWER XPM CELL, by the present inventors, Jack Zezhong Peng and David Fong. Such Parent Application received a Notice of Allowance that was mailed May 30, 2012. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to non-volatile memory (NVM) devices and memory cell circuits, and more specifically to high density, low voltage, and low-power one time programmable (OTP) core cells and memories important in HF and UHF RFID applications. 
     BACKGROUND 
     The core cells in some memory device types are based on NMOS field effect transistors and bit capacitors. The present inventor, Jack Peng, describes the conventional construction of one time programmable (OTP) memory cells in U.S. Pat. No. 6,667,902, issued Dec. 23, 2003. The circuit impedances are extremely high, and the capacitance values of the bit capacitors are very small. Spurious charges can be induced from nearby unintended sources causing the transistors to weakly turn on and their sense amplifiers to report a wrong data storage result. 
     Various circuits and methods have been developed in the prior art that apply clean up pulses in various ways to nullify the spurious charges on the floating nodes. Some of these add special circuits and more devices, substantially increasing the layout size and cost of the memory. Others apply the clean up pulses broadly and therefore this strategy demands higher power levels to support the cleanup operation that may not be possible in low power applications like passive UHF RFID tags. 
     SUMMARY OF THE INVENTION 
     Briefly, in one embodiment of the present invention, a high density, low voltage, and low-power one time programmable (OTP) memory is based on core cells with a one transistor design. A CLEAN pulse is directed to a single shunt device at the output of the column decoder so spurious charges that may have been stored in the floating nodes can be cleaned up. Such arrangement also allows for the simultaneous initialization of bit lines, data lines, and sensing lines to zero. Core area layout size is substantially reduced, and operational power requirements are exceeding low making these particularly suitable in UHF RFID applications. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments that are illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a low voltage, and low-power one time programmable (OTP) memory and some exemplary core cells with a two transistor design in which a CLEAN pulse is applied to respective WS lines for cleaning spurious charges that may have been stored in the floating nodes; 
         FIG. 2A  is a schematic diagram of another low voltage, and low-power one time programmable (OTP) memory and some exemplary core cells with an extra isolation transistor M added to the basic two transistor design, and in which a CLEAN pulse is applied to respective WS lines for cleaning spurious charges that may have been stored in the floating nodes; 
         FIG. 2B  is a schematic diagram showing an example of read sensing for the memory of  FIG. 2A ; 
         FIGS. 3 and 4  are schematic diagrams of a high density, low voltage, and low-power one time programmable (OTP) memory and some exemplary core cells with a one transistor design, and in which a CLEAN pulse is directed by the column decoder to selected BL column shunt devices so spurious charges that may have been stored in the floating nodes can be cleaned up; and 
         FIGS. 5 and 6  are schematic diagrams of another high density, low voltage, and low-power one time programmable (OTP) memory and some exemplary core cells with a one transistor design, and in which a CLEAN pulse is directed to a single shunt device at the output of the column decoder so spurious charges that may have been stored in the floating nodes can be cleaned up. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  and Voltage Table I represent a memory  100  comprising an array block  102  and core cells. Each core cell is based on two transistors (N) and one floating node comprising a capacitor C and gate g. 
                                                                                                                                                                                             VOLTAGE TABLE I                       Cell   V(WP)   V(WS)   V(BL)   V(BR)                                    Programming   A SW/SB   5.5   V   2.5   V   0   V   floating           B SW/UB   5.5   V   2.5   V   2.5   V   floating           C UW/SB   2.5   V   0   V   0   V   floating           D UW/UB   2.5   V   0   V   2.5   V   floating       Read   A SW/SB   1.0   V   0   V   0   V   V sensing +                B SW/UB   1.0   V   0   V   floating   floating                C UW/SB   0   V   0   V   0   V   V sensing −                D UW/UB   0   V   0   V   floating   floating                                   sensing line                        
Where the voltage values can be +/− by 10%˜50% for a 0.13 μm CMOS process, depends on the process and detailed implementation of the decoding circuits. In an 0.18 μm process, these voltages will be correspondingly higher.
 
     In memory array block  102  the core cells are arranged in rows (m) and columns (s) with internal bit line (BLs) and bit read (BRs) interconnections to corresponding column decoders and sense amplifiers, and write pulse line (WP) and write sense line (WS) row (m) interconnections. Each core cell includes an NMOS transistor (Nms) with a floating gate connected through a node (Gms) to a bit capacitor (Cms). When a core cell has been previously programmed with data, its node (Gms) will typically express a voltage of about 0.4˜0.9 v, and if it had not been previously programmed with data, node (Gms) will ideally express a voltage of about zero. But, when a write pulse line (WPm) is turned on from zero to about 1.0 v, node (Gms) may be induced to about 0.4 v which can weakly turn on a transistor (Nms) and cause a sense amplifier  106  to falsely report such core has been previously programmed. So, a mechanism to correct this situation can be used to apply a short, “clean pulse” CLEAN  108  on a corresponding write sense line (WSm) during a write program line (WPm) action to remove any residual charges that may be left on node (Gms). 
     Spurious voltages from nearby unintentional sources may also be induced into nodes (Gms) which can weakly turn on corresponding transistors (Nms) and cause false reports such have been previously programmed. A second mechanism can be used to apply a clean pulse on all the write sense lines (WSm) in parallel to remove any residual charges that may be left on nodes (Gms). But that comes at a cost in power consumption that may not be tolerable in particular mobile applications with limited power resources. 
     In one embodiment, an extra isolation device is added to limit the power consumed during clean pulses.  FIGS. 2A-2B , and Voltage Table II represent a memory  200  comprising an array block  202  and exemplary core cells. Each core cell includes an extra isolation transistor (M) controlled by an extra set of write row (WR) control lines. 
                                             VOLTAGE TABLE II                   Cell   V(WP)   V(WS)   V(WR)   V(BL)   V(BR)                   Pro-   A SW/SB   5.5 V   2.5 V     0 V   0 V   float       gram-   B SW/UB   5.5 V   2.5 V     0 V   2.5 V     float       ming   C UW/SB   2.5 V   0 V   0 V   0 V   float           D UW/UB   2.5 V   0 V   0 V   2.5 V     float       Read   A SW/SB   1.0 V   pulse, 0 V   1.0 V     0 V   V sensing           B SW/UB   1.0 V   0 V   1.0 V     0 V, float   0 V, float           C UW/SB     0 V   0 V   0 V   0 V   V sensing           D UW/UB     0 V   0 V   0 V   0 V, float   0 V, float                               sensing                               line                    
Where the voltage values can be +/− by 10%˜50% for 0.13 μm CMOS process, depends on the process and detailed implementation of the decoding circuits. For a 0.18 μm process, the exemplary voltages will be correspondingly higher. When reading a cell core A, all the other WP, WS and WR lines for the non-selected rows are set to zero. For the selected row, WRm should be held at 1.0 v, and WPm is switched from zero to 1.0 v. The corresponding WSm will require a short, clean positive pulse. The charges in all the other floating nodes g 1 s, . . . g(m−1)s, . . . g(m+1)s will not affect the core A reading result because all the isolation devices M 1 s, . . . M(m−1)s, . . . M(m+1)s, . . . Mns, are turned off. So only one pulse on the selected WS line is needed, instead of being applied in parallel on all the WS lines, and saves a lot of power. In a 64×64 array, a 90% power savings can be realized for the WS clean pulse alone.
 
     Unfortunately, while the clean pulse current is now limited, the extra isolation transistors (M) can add 20% to the core area layout size. 
       FIGS. 3-4 , and Voltage Table III represent a memory  300  in an embodiment of the present invention. Memory  300  includes an array block  302 , a column decoder  304 , and a sense amplifier  306 . The array block  302  is shown here with representative cell cores (A)  310 , (B)  320 , (C)  330 , and (D)  340 .  FIG. 4  provides circuit details for an exemplary way to implement column decoder  304  and sense amplifier  306 . 
                                     VOLTAGE TABLE III                   Cell   V(WP)   V(WS)   V(BL)                   Programming   A SW/SB   5.5 V   2.5 V     0 V           B SW/UB   5.5 V   2.5 V     2.5 V             C UW/SB   2.5 V   0 V   0 V           D UW/UB   2.5 V   0 V   2.5 V         Read   A SW/SB   1.0 V   0 V   0 V           B SW/UB   1.0 V   0 V   0 V, float           C UW/SB     0 V   0 V   0 V           D UW/UB     0 V   0 V   0 V, float                    
Where the voltage values can be +/− by 10%˜50% for a 0.13 μm CMOS process, depends on the process and detailed implementation of the decoding circuits. In an 0.18 μm process, the voltages will be correspondingly higher.
 
     In contrast with the cell cores of memory  200  in  FIG. 2 , devices M 1 s, Mms, Mns, M 1 t, Mmt, Mnt, . . . . have all been eliminated and their functions essentially replaced, in  FIG. 3 , by devices NMs  350  and NMt  352  on the columns. Such reduces each core cell to one device NA, NB, NC, or ND with one corresponding floating node gms, gmt, gns, or gnt, e.g., implemented with a half transistor. The present inventor, Jack Peng, describes the construction in greater detail in U.S. Pat. No. 7,269,047, issued Sep. 11, 2007. The size of core area  302  of memory  300  can be reduced by about 20% over that of memory  200  ( FIG. 2 ). 
     However, floating node gms still needs to be cleaned by a short pulse CLEAN  360  after Wpm  361  ( FIG. 4 ) goes on, and before SENSE-ENABLE  362  goes on. 
     Assume core A has been selected for reading. After selected WPm and WSm have been turned on, a short pulse of signal clean will remove all stored charges in node gms. All the other floating nodes in selected column BLs will not be affected, because their WS lines are turned off. See Voltage Table III. When CLEAN  360  goes off, the voltage level on the input node BL of the voltage comparator will be determined by the status of core A  310 . If it has been previously programmed, then its BL will charge up to a high voltage level. If it has not, its BL will stay at a low level. When a SENSE-ENABLE  362  arrives, the status of core A  310  is sensed and thereafter latched. 
     In contrast to some conventional sensing schemes that need to separately initialize the voltage level of bit lines, data lines, and sense lines, embodiments of the present invention use the clean pulse to both clean any erroneously stored charges in the floating node, and to initialize the bit lines, data lines, and sensing lines to zero. 
     Power is conserved when reading core A because no DC current flows in non-selected bit line BLs. Memory  300  therefore retains most of the benefits provided by memory  200 . 
       FIGS. 5 and 6  represent a memory  500  in an embodiment of the present invention alternative to memory  300 . Voltage Table III applies here as well. Memory  500  includes a core array block  502 , column decoders  504 , and a sense amplifier  506 . Instead of using individual transistors NM  350  and  352  ( FIGS. 3 and 4 ) on each of the columns, memory  500  uses a single transistor NM  508  for spurious charge cleaning that shunts a connection between column decoder  504  and sense amplifier  506 . 
       FIG. 6  shows some exemplary circuits in greater detail. Column selection between BLs and BLt to BL, for example, is highly simplified over the more complex way needed when individual transistors NM are needed in every column. CLEAN no longer needs to be steered to the selected column by the column decoder as it was in  FIGS. 3 and 4 . In  FIGS. 5 and 6 , CLEAN  510  is directed to a single transistor NM  508 . Memory  500  therefore accomplishes its objectives in reduced operating power and minimal layout area. 
     Although the present invention has been described above with reference to preferred embodiments, those skilled in the art will readily appreciate that various modifications and improvements of the described embodiment s can be implemented without departing from the spirit and scope of the invention as defined by the claims of this application.