Abstract:
The present invention provides a dual reference cell sensing scheme for non-volatile memory. A high voltage reference cell and a low voltage reference cell are individually coupled to two sense amplifiers for providing two distinct reference voltages for comparison against the memory cell voltage. The output of the two sense amplifiers is further connected to a second stage sense amplifier to determine the status of the memory. The dual reference cell sensing scheme provides an increased sensing window which increases performance under low voltage application. The dual reference cell sensing scheme can be implemented by either voltage-based, current-based, or ground.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is generally related to non-volatile memory device, and more particularly, to a dual reference cell sensing scheme for non-volatile memory.  
           [0003]    2. Description of Related Art  
           [0004]    Non-volatile memory devices are memory devices that can store data even when voltage is cut off. These non-volatile memory devices are particularly useful in portable devices for storing operating system as well as user data. Recently, high attention and heavy research have been given to non-volatile memory because of its adaptability and flexibility. The speed of the memory is of great importance for non-volatile memory.  
           [0005]    Erasable programmable read only memories (EPROMS) are a fast growing class of non-volatile storage integrated circuits because they have the ability of electrically programming and reading a memory cell in the chip. EPROMs frequently use memory cells that have electrically isolated gates commonly referred to as floating gates. These floating gates are most often completely surrounded by oxide and formed from a polycrystalline silicon (i.e., polysilicon) layer. Information is stored in the memory cells or devices in the form of a charge on the floating gate. Charges are transported to the floating gates by a variety of mechanisms such as avalanche injection, channel injection, tunneling, etc., depending on the construction of the cells. These cells are generally erased by exposing the array to ultraviolet (UV) radiation.  
           [0006]    Electrically erasable and programmable read only memories (EERPOMs) are both electrically erasable and electrically programmable. Charges are placed onto and removed from a floating gate via tunneling of electrons through a thin gate oxide region formed over the substrate. In other instances, charges are removed through an upper control electrode.  
           [0007]    More recently, a new category of electrically erasable devices has emerged, and the devices are frequently referred to as “flash” EPROMs or “flash” EEPROMs. In these memories, memory cells are erased electrically, whereas the cells themselves include only a single device per cell. Also, erasing of the entire array or a block of individual memory cells may be accomplished.  
           [0008]    In accomplishing erase and program verification, a variety of sense amplif sense the state of the memory cells. To accomplish verification by sensing the memory cell being verified by application of a gate potential to its word to a current from a reference cell by the sense amplifier. Typically, EPROM erased cells, identical in structure to the memory cells, which act as these amplifier determines whether the memory cell being verified is drawing mo reference cell which is weighted in some relationship to the memory cell. I amplifier verifies the programmed state of the memory cell. The following in potential for a single cell reference scheme:  
       scheme        :                       1                 Δ                   V   single       =         V     ref        (   H   )         -       V   cell                   or                 Δ                   V   single         =       V   cell     -     V     ref        (   L   )                    
        2           (   1   )                               
 
           [0009]    The reading speed of the non-volatile memory depends on the sensing scheme of the sense amplifiers. Prior art uses a single cell reference scheme for the sense amplifiers and therefore it is not suitable for low voltage application due to the unstable reference voltage and a narrow sensing window of the single cell reference scheme. Furthermore, the dummy cell method and trimming method used in the prior art for setting the reference cell are expensive and inaccurate. Therefore, there is a need for a non-volatile memory with high speed read speed.  
         SUMMARY OF INVENTION  
         [0010]    According to one object of the present invention, a high read speed non-volatile memory is provided.  
           [0011]    According to another object of the present invention, the sensing window of the non-volatile memory is increased.  
           [0012]    According to another object of the present invention, the reference voltage can be easily set.  
           [0013]    According to another object of the present invention, the noise immunity of the non-volatile memory is increased.  
           [0014]    The present invention provides a dual cell reference scheme for non-volat           cells are used in conjunction with two sense amplifiers for providing sensi           reference cell is a high-voltage (HVT) cell and the other one is a (LVT) ce           ground, floating ground, or reference current source and the LVT cell can reference current source. The HVT cell and LVT cell are individually coup           they are both further connected to a common sense amplifier for determin           cell. The equation that shows the change in the potential for a dual referen            
       scheme        :                       1                 Δ                   V   dual       =         (       V     ref        (   H   )         -     V   cell                  )     +     (       V   cell     -     V     ref        (   L   )           )       =     2      Δ                   V   single                
        2           (   2   )                               
 
           [0015]    Equation (2) shows that the change in voltage for the dual reference cell sensing scheme is double of that of the single reference cell sensing scheme. The increase in            dual  expands the sensing window by providing a large input signal difference for sensing. The increase in            dual  significantly increases the read speed of the non-volatile memory. Furthermore due to the expanded sensing window, the accuracy under low voltage application is greatly improved. In a low voltage application, the sensing window is very narrow and therefore the read speed and accuracy is limited. The dual reference cell sensing scheme also has no dead zone which greatly improves performance.  
           [0016]    The reference voltage is easily set because of the increased            dual  and therefore the reference voltage needs not to be very accurately determined being in a midway between the high and the low voltage. The region for determining whether it belongs to high or low voltage is distinctly distinguished because two reference cells are used and therefore the two reference cells can take different values if required. The initial unaltered reference voltage of the manufactured memory can be used as the reference voltage to reduce cost in this dual reference cell sensing scheme.  
           [0017]    The dual reference cell sensing scheme of the present invention is applicable to any non-volatile memory such as EEPROM, EPROM, MASK ROM, FLASH ROM or combination of volatile memory and non-volatile memory.  
           [0018]    The present invention also provides a method for sensing a non-volatile memory with dual reference cells, comprising first providing first sense amplifier, a second sense amplifier, and a third sense amplifier, wherein each of the sense amplifiers has a first polarity input terminal and a second polarity input terminal. Then, a first voltage reference signal is aplied to the first polarity input terminal of the first sense amplifier, and a second voltage reference signal is applied to the second polarity input terminal of the second sense amplifier. A cell reference signal, which is to be sensed, is applied to the second polarity input terminal of the first sense amplifier and the first polarity input terminal of the second sense amplifier. And, outputs of the first sense amplifier and the second sense amplifier are input to the third sense amplifier.  
           [0019]    In the foregoing method, the first polarity input terminal is a negative input terminal and the second polarity input terminal is a positive input terminal. The first polarity input terminal is a positive input terminal and the second polarity input terminal is a negative input terminal. The first voltage reference signal is from a high voltage reference cell. The high voltage reference cell can be a ground voltage, a floating ground, or a reference current source. The second voltage reference signal is from a low voltage reference cell.  
           [0020]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0022]    [0022]FIG. 1 is a schematic diagram of the current-based dual reference cell sensing scheme according to a preferred embodiment of the present invention.  
         [0023]    [0023]FIG. 2 is a voltage diagram of the high voltage cell of the dual reference cell sensing scheme according to a preferred embodiment of the present invention.  
         [0024]    [0024]FIG. 3 is a voltage diagram of the low voltage cell of the dual reference cell sensing scheme according to a preferred embodiment of the present invention.  
         [0025]    [0025]FIG. 4 is a schematic diagram of the voltage-based dual reference cell sensing scheme according to a preferred embodiment of the present invention.  
         [0026]    [0026]FIG. 5 is a schematic diagram of the voltage-based dual reference cell sensing scheme according to another preferred embodiment of the present invention.  
         [0027]    [0027]FIG. 6 is a circuit implementation of the voltage-based dual reference cell sensing scheme according to a preferred embodiment of the present invention.  
         [0028]    [0028]FIG. 7 is a circuit implementation of the voltage-based dual reference cell sensing scheme according to another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    [0029]FIG. 1 shows a dual reference cell sensing scheme of a preferred embodiment of the present invention. Reference numerals  102 ,  106 , and  110  represent current sources from a memory cell (CELL in the FIG. 1), a high voltage (HVT) reference cell, and a low voltage (LVT) reference cell respectively. The current sources  102 ,  106 , and  110  are connected to capacitors  104 ,  108 , and  112  respectively. The capacitors  104 ,  108 , and  112  are used for holding the charges from the current sources  102 ,  106 , and  108  until it reaches a required level. The current source  102  of the memory cell sends out a signal CMI through a common node to a positive input of a sense amplifier  114  and to a negative input of a sense amplifier  116 . A signal TREF HVT from the high voltage reference cell is sent to a negative input of the sense amplifier  114 . A signal TREF LVT from the low voltage reference cell is sent to a positive input of the sense amplifier  116 . The signal CMI is individually compared to the signal TREF HVT from the high voltage reference cell by the sense amplifier  114  and to the signal TREF LVT from the low voltage reference cell by the sense amplifier  116 . An output          A1 from the sense amplifier  114  is further connected to the sense amplifier  118  and an output          A2 from the sense amplifier  116  is also further connected to the sense amplifier  118  for determining the state of the memory cell.  
         [0030]    [0030]FIGS. 2 and 3 show a voltage diagram of the sense amplifiers  114  and  116  the non-volatile memory. The sense amplifier  114  compares the input sign outputs a          A1 according to the equation: equation: 
         Δ SA 1 =Av ×( CMI−TREF HVT )  (3) 
         [0031]    wherein Av represents an adjustable bias according to the sense amplifie           change in voltage of the sense amplifier  114 . Similarly, the sense amplifie          signals CMI and TREF LVT and outputs a          A2 according to the equation equation: 
         Δ SA 2 =Av ×( TREF LVT−CMI)   (4) 
         [0032]    wherein Av represents an adjustable bias according to the sense amplifier and          A2 represents the change in voltage of the sense amplifier  116 . The signals          A1 and          A2 are sent to the sense amplifier  118  for comparison to determine whether the voltage lies in the high or low voltage region. As shown in FIG. 2, while reading a HVT reference cell, an up branch senses a small difference in the          A1 and a down branch senses a significant difference in the          A2 and therefore the voltage will be pulled down by the sense amplifier  118  and the data is determined to be 1. Similarly shown in FIG. 3, the up branch senses a small difference in the          A2 and the down branch senses a significant difference in the          A1 and therefore the voltage will be pulled up by the sense amplifier  118  and the data is determined to be 1. The state of the memory cell is thereby determined in this dual reference cell sensing scheme of the present invention.  
         [0033]    The read speed of the non-volatile memory is directly related to the           of Although the output           of the sense amplifier  118  can be adjusted by a b (4), the original sensing window cannot be expanded merely by adjusting memory devices in prior art has limited usefulness in low voltage applicati Av is overlarge, noise problem occurs and performance decreases in the scheme. The dual reference cell sensing scheme of the present invention window without the need of a large amplification. The bias Av can be adju that natural noise immunity is high. Furthermore the dual reference cell se suitable for low voltage application because of the increased sensing wind          . The following equation satisfies the increase in speed due to the incre           voltage: 
         Δ V=ΔI×t   (5) 
         [0034]    wherein,           is the change in voltage,           is the change in current, and t is the time. Therefore if the           is increased, or doubled in the dual reference cell sensing scheme of the present invention, the time required to reach the same z, 11  is decreased. Therefore the dual reference cell sensing scheme significantly improves read speed especially in low voltage application.  
         [0035]    [0035]FIG. 4 is an alternative embodiment of the present invention using in a voltage-based dual reference cell sensing scheme. Reference numerals  402 ,  406 , and  410  denote a resistor connected to an operation voltage (VDD) of a memory cell, to a high voltage (HVT) reference cell, and to a low voltage (LVT) reference cell respectively. The resistors  402 ,  406 , and  410  connected to VDD are individually coupled to current sources  404 ,  408 , and  412  for converting the voltage into charges. The current source  404  generates a signal CMI to a positive input of a sense amplifier  414  and to a negative input of a sense amplifier  416 . The HVT reference cell generates a signal TREF HVT to a negative input of the sense amplifier  414 . The LVT reference cell generates a signal TREF LVT to a positive input of the sense amplifier  416 . The output of the sense amplifiers  414  and  416  are sent to a sense amplifier  418  to determine the status of the memory cell by comparing the signals          A1 and          A2, respectively from the sense amplifiers  414  and  416 . The equations for calculating          A1 and z, 12  A2 are the same as equations (3) and (4). The voltage diagrams of the voltage-based dual reference cell sensing scheme is identical to the current-based dual reference cell sensing scheme so FIG. 3 and FIG. 4 also apply to the voltage-based dual reference cell sensing scheme in FIG. 4.  
         [0036]    [0036]FIG. 5 is an alternative embodiment of the present invention using ground as the high voltage reference cell. A resistor  512  and a current source  514  supply a potential to the memory cell and output a signal CMI. The signal CMI is then sent to a positive input of a sense amplifier  522  and to a negative input of the sense amplifier  524 . The high voltage reference (HVT) cell is a ground signal coupled to a negative input of the sense amplifier  522 . A resistor  518  connected to VDD and a current source  510  supply a potential to the low voltage reference cell and output a signal TREF LVT to a positive input of the sense amplifier  514 . The output of the two sense amplifiers  522  and  524  are connected to inputs of a sense amplifier  526  to determine the status of the memory cell by comparing the signals          1 and          2. The equations for calculating          A1 and          2 are the same as equations (3) and (4). The voltage diagrams of the voltage-based dual reference cell sensing scheme is identical to the current-based dual reference cell sensing scheme so FIG. 3 and FIG. 4 also apply to the voltage-based dual reference cell sensing scheme in FIG. 5.  
         [0037]    [0037]FIG. 6 is a circuit implementation of the current-based dual reference cell sensing scheme in FIG. 1. Reference numeral  116  denotes the sense amplifier  116  which is coupled to the high voltage (HVT) reference cell and the memory cell in FIG. 1. Reference numeral  114  denotes the sense amplifier  114  which is coupled to low voltage (LVT) reference cell and the memory cell in FIG. 1. Some transistors are incorporated herein for controlling input signals of the sense amplifiers. The bias voltage levels of the sense amplifiers are predetermined in this embodiment. The output of the sense amplifiers  114  and  116  are connected to the sense amplifier  118  through two transistors  610  and  620 , respectively. The sense amplifier  118  receives and compares the signals          1 and          2 to determine the signal SA 1 OFF or SA 2 OFF to switch off the input transistors of the sense amplifier  118 . The final signal OUT determines if the data in the memory cell is 1 or 0.  
         [0038]    [0038]FIG. 7 is a circuit implementation of the current-based dual reference cell sensing scheme with adjustable bias Av in FIG. 1. Reference numeral  116  denotes the sense amplifier  116  which is coupled to the high voltage (HVT) reference cell and the memory cell in FIG. 1. Reference numeral  114  denotes the sense amplifier  114  which is coupled to low voltage (LVT) reference cell and the memory cell in FIG. 1. Some transistors are incorporated herein for control input signals of sense amplifiers. The bias Av of the sense amplifiers is adjustable in this embodiment. Each input signal of both sense amplifiers  114  and  116  can accept an adjustable bias Av value (the adjustable bias Av value is denoted as “BiasA” shown in the FIG. 7). The output of the sense amplifiers  114  and  116  are coupled to the sense amplifier  118  through two transistors  710  and  720 , respectively. The sense amplifier  118  receives and compares the signals          A1 and          A2 to determine the signal SA 1 OFF or SA 2 OFF to switch off the input transistors of the sense amplifier  118 . The final signal OUT determines if the data in the memory cell is 1 or 0.  
         [0039]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure and method of the present invention without departing from the scope or spirit of the present invention. In view of the foregoing description, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.