Patent Publication Number: US-6222764-B1

Title: Erasable memory device and an associated method for erasing a memory cell therein

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor memories, and, more particularly, to an electrically erasable programmable read only memory (EEPROM). 
     BACKGROUND OF THE INVENTION 
     Non-volatile memories are used in a variety of products because they retain their contents even when power is no longer supplied. An electrically erasable programmable read only memory (EEPROM) is a type of non-volatile memory that permits the contents to be erased and different data to be stored therein. 
     A typical EEPROM device includes an array of memory cells, and, each memory cell includes a floating gate and a control gate over the floating gate. The floating gate is positioned over a channel of the transistor that is defined between spaced apart source and drain regions formed in a semiconductor substrate. Intervening insulating layers are between the channel and floating gate, and between the floating gate and control gate. One type of memory cell configuration is a stacked gate arrangement wherein the control gate is directly over the floating gate. Another type of memory cell configuration is the split gate arrangement wherein the control gate extends over the floating gate, but also extends laterally adjacent the floating gate over a portion of the channel of the transistor. 
     A disadvantage of the stacked and split gate arrangements is that they can not be manufactured through the standard complementary metal oxide semiconductor (CMOS) process. This is due to the standard CMOS process using a single layer polysilicon deposition step whereas the stacked and split gate arrangements require two polysilicon deposition steps for the floating gate and the control gate. 
     A CMOS EEPROM with the control gate and the floating gate formed with a single poly layer is disclosed in U.S. Pat. No. 5,886,376 to Ohsaki and in an article titled “A Single Poly EEPROM Cell Structure For Use In Standard CMOS Process”, by Ohsaki et al., IEEE Journal of Solid-State Circuits, Vol. 29, No. 3, March 1994. The disclosed single poly layer memory cell includes adjacently placed NMOS and PMOS transistors. A common polysilicon gate with respect to the NMOS and PMOS transistors serves as the floating gate, and the well region of the PMOS transistor serves as the control gate for the memory cell. 
     However, there are two problems with erasing a single poly layer memory cell as disclosed in the Ohsaki patent and in the Ohsaki et al. article. One approach to erasing a charge of the floating gate requires a high erase voltage applied to the spaced apart source and drain regions and to the n-well of the PMOS transistor while the NMOS transistor is grounded. When the gate capacitance ratio, i.e., a ratio of the capacitance of the gate of the PMOS transistor to the capacitance of the gate of the NMOS transistor, is much greater than 1, the high erase voltage approaches the junction breakdown voltage of the n-well to the p-well which is about 13 to 15 volts for 0.25 micron technology. A well or tub breakdown reduces device reliability and data retention. 
     Another approach to erasing a charge of the floating gate requires a high erase voltage applied to the spaced apart source and drain regions of the NMOS transistor while the PMOS transistor is grounded. Depending on the gate capacitance ratio, this high erase voltage approaches the junction breakdown of the PMOS transistor, which is about 7 to 9 volts for 0.25 micron technology. A drain voltage close to the junction breakdown voltage during erase can lead to hole injection into the floating gate, and can thus reduce device reliability and data retention. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an erasable memory device and an associated method for erasing a memory cell therein without reducing device reliability and data retention of the erased memory cell. 
     This and other objects, advantages and features in accordance with the present invention are provided by an electrically erasable memory device comprising a substrate and a plurality of memory cells in the substrate. Each memory cell preferably comprises a first region having a first conductivity type in the substrate, and a first MOS transistor in the first region and comprising spaced apart source and drain regions defining a channel therebetween and a gate overlying the channel. A second region having a second conductivity type in the substrate is preferably laterally adjacent to the first region wherein a capacitor comprising a first electrode preferably overlies the second region and an insulating layer is therebetween, and a third region having the first conductivity type is preferably in the second region defining a second electrode. The gate of the first MOS transistor and the first electrode of the capacitor are preferably connected together to define a floating gate, and the second region of the capacitor preferably serves as a control gate. In other words, the floating gate and the control gate of the memory cell are formed as a result of a single poly layer deposition step during the CMOS process. 
     The electrically erasable memory device preferably further comprises an erasing circuit for selectively erasing at least one of the memory cells by supplying a first voltage reference of a first polarity to the spaced apart source and drain regions of the first MOS transistor, a second voltage reference of a second polarity to the first region, and a third voltage reference of the second polarity to the second electrode in the third region. 
     The first and second voltage references advantageously bias the first MOS transistor so that the third voltage reference for erasing the selected memory cell does not cause a junction breakdown of the first MOS transistor. In other words, the bias voltage applied to the first MOS transistor allows a lower erase voltage to be applied to the memory cell for removing a charge of the floating gate. In addition, when the bias voltage is applied to the first MOS transistor, the time required for erasing the data is significantly reduced. This aspect of the invention is particularly advantageous when the gate capacitance ratio, i.e., a ratio of the capacitance of the capacitor to the capacitance of the gate of the first MOS transistor, is greater than 1. 
     The first voltage reference preferably has an absolute value less than about 7 volts, the second voltage reference preferably has an absolute value in a range of about 0 to 2 volts, and the third voltage reference has an absolute value in a range of about 3 o 7 volts. Depending on the technology size of the electrically erasable memory device, the third voltage reference is such that it does not cause a junction breakdown of the first MOS transistor as a result of the first and second voltage references. The first conductivity type is preferably P conductivity type, and the second conductivity type is preferably N conductivity type. 
     Another aspect of the present invention relates to a method for erasing a single poly layer memory cell in an electrically erasable memory device. The method preferably comprises the steps of supplying a first voltage reference of a first polarity to spaced apart source and drain regions of a first MOS transistor in a first region of a first conductivity type of the single poly layer memory cell; supplying a second voltage reference of a second polarity to the first region; and supplying a third voltage reference of the second polarity to an electrode of a capacitor in a second region laterally adjacent the first region. 
     The first and second voltage references preferably bias the first MOS transistor so that the third voltage reference for erasing the single poly memory cell does not cause a junction breakdown of the first MOS transistor. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a single poly memory cell and an associated erase circuit of an EEPROM in accordance with the present invention. 
     FIG. 2 is a graph of the erase voltage versus time of the single poly memory cell illustrated in FIG. 1 for various bias voltage levels. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Referring to FIG. 1, an electrically erasable memory device, generally indicated at  10 , is described in accordance with the present invention. The memory device  10  includes a substrate  12  and a plurality of memory cells formed in the substrate, with only one memory cell  14  being illustrated for clarity. The memory cell  14  includes a first region  16  having a first conductivity type in the substrate  12 . In the illustrated embodiment, the first region  16  is of N conductivity type. A first MOS transistor  18  is formed in the first region  16  and comprises spaced apart source and drain regions  20 ,  22  defining a channel  24  therebetween and a gate  26  overlying the channel. An insulating or gate oxide layer  28  separates the gate  26  and the channel  24 . The insulating layer  28  may have a thickness in a range of about 5 to 12 nm. 
     The memory cell  10  further comprises a second region  30  having a second conductivity type in the substrate  12  laterally adjacent to the first region  16 . In the illustrated embodiment, the second region  30  is of P conductivity type. A capacitor  32  comprising a first electrode  34  is formed overlying the second region  30  and an insulating or oxide layer  36  is also provided therebetween. A second electrode of the capacitor  32  is formed in the second region  30  by at least one third region  38  having a first conductivity type. The gate  26  of the first MOS transistor  18  and the first electrode  34  of the capacitor  32  are connected together to define a floating gate, indicated generally at  40 , and the second region  30  serves as a control gate. 
     The memory device further includes an erasing circuit for selectively erasing at least one of the memory cells by supplying specific voltages as will now be described. The erasing circuit is provided by the schematically illustrated three voltage sources and associated switches  42   a - 42   c.  These voltage sources can be provided by on-chip or external circuitry, or a combination thereof, as will be appreciated by one skilled in the art. One or more external input pins may be provided within an overall integrated circuit package and connected to respective pads of the integrated circuit to receive external voltage(s). 
     In the illustrated embodiment, the switches  42   a - 42   c  are shown in the erase positions. Accordingly, a first voltage reference of a negative polarity is supplied to the spaced apart source and drain regions  20 ,  22  of the first MOS transistor  18  by the first voltage source and associated switch  42   a.  This first voltage reference has an absolute value less than about 7 volts. 
     A second voltage reference is applied to the first region  16  by the second voltage source and associated switch  42   b.  This second voltage reference has an absolute value in a range of about 0 to 2 volts. An N+ region  44  is provided in the first region  16  for coupling the second voltage reference to the first region. In addition, a third voltage reference of a positive polarity is applied to the second electrode  38  in the second region  30  by the third voltage source and associated switch  42   c.  This third voltage reference has an absolute value in a range of about 3 to 7 volts. Those of skill in the art will appreciate that the various semiconductor regions can be reversed, which will also entail a reversal of the polarities of the various voltage sources. 
     Those of skill in the art will also readily appreciate that the memory device  10  can supply programming voltages P 1 -P 3 , and reading voltages R 1 -R 3  from the respective voltage sources and switch circuits  42   a - 42   c.  Accordingly, these voltages and associated circuit portions need no further discussion herein. 
     In accordance with a significant feature of the present invention, the first and second voltage references advantageously bias the first MOS transistor  18  so that the third voltage reference for erasing the selected memory cell  14  does not cause a junction breakdown of the first MOS transistor. In other words, the bias voltages applied to the first MOS transistor  18  allows a lower erase voltage to be applied to the memory cell  14  for removing a charge of the floating gate  40 . In addition, when the bias voltage is applied to the first MOS transistor  18 , the time required for erasing the data is significantly reduced as best shown in FIG.  2 . 
     Referring now to the graph illustrated in FIG. 2, the necessary erase voltage versus time for a selected memory cell  14  will be discussed for various bias voltage levels applied to the spaced apart source and drain regions  20 ,  22  of the first MOS transistor  18 . For example, when −2 volts is applied to the source and drain regions  20 ,  22  as indicated by reference  50  in FIG. 2, an erase voltage of 5.6 volts is required with a corresponding erase time of about 10 seconds. When −3 volts is applied to the source and drain regions  20 ,  22  as indicated by reference  52 , an erase voltage of 5.5 volts is required with a corresponding erase time of about 7 seconds. Similarly, when −5 volts is applied to the source and drain regions  20 ,  22  as indicated by reference  54 , an erase voltage of 5.3 volts is required with a corresponding erase time of about 1 second. 
     As the negative bias voltage applied to the source and drain regions  20 ,  22  of the first MOS transistor  18  becomes more negative, the required voltage for causing the first MOS transistor to conduct, and therefore erase the charge stored of the floating gate  40 , is lowered. The memory cell  14  can thus be erased without causing a junction breakdown of the first MOS transistor  18  since the required voltage is lower as a result of biasing the first MOS transistor. This aspect of the present invention is particularly important when the capacitance ratio, i.e., a ratio of the capacitance of the capacitor  32  to the capacitance of the gate  26  of the first MOS transistor  18 , is greater than 1. The high erase voltage necessary for erasing the memory cell  14  without biasing the first MOS transistor  18  approaches the junction breakdown voltage of the first MOS transistor  18 , which is about 7 to 9 volts for 0.25 micron technology. A drain voltage close to the junction breakdown voltage during erase can lead to hole injection into the floating gate, and can thus reduce device reliability and data retention. 
     Another aspect of the present invention relates to a method for erasing a single poly layer memory cell  14  in an electrically erasable memory device  10 . The method comprises the steps of supplying a first voltage reference of a first polarity to spaced apart source and drain regions  20 ,  22  of a first MOS transistor  18  in a first region  16  of a first conductivity type of the single poly layer memory cell  14 ; supplying a second voltage reference of a second polarity to the first region; and supplying a third voltage reference of the second polarity to an electrode  38  of a capacitor  32  in a second region  30  laterally adjacent the first region. 
     The first and second voltage references preferably bias the first MOS transistor  18  so that the third voltage reference for erasing the single poly memory cell  14  does not cause a junction breakdown of the first MOS transistor. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.