Abstract:
A non-volatile memory cell having enhanced protection against mobile ions. The electric field within the memory cell is controlled in a manner that minimizes migration of mobile ions toward the floating gate. Each conductive layer in the memory cell is biased to reduce the flow of mobile ions toward the floating gate. The memory cell is preferably manufactured using a conventional logic process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Non-volatile memory devices may be fabricated with either single or multiple polysilicon layers using a conventional logic process. As used herein, a conventional logic process is defined as a semiconductor process that implements twin-well or triple well technology and uses only one or more layers of polysilicon. The floating gates in a non-volatile memory store electrical charge. Considerable efforts have been directed toward preventing charge loss through the charge-injection oxide. However, charge loss can occur when mobile ions permeate the charge storage area of non-volatile devices. 
         [0002]    In some cases, mobile ions move from the interlevel dielectric layers which leads to severe charge loss at high temperatures. The mobile ion movement is due to the presence of the electric field formed by the charge stored in the floating gate and the biases applied during operation or standby. Usually small positive ions such as Na + and K + migrate from the upper layers of the chip toward the silicon substrate. Attracted by the cells that have a high threshold voltage (excess negative charge on the floating gate), the ions move toward the floating gates of these cells and ultimately neutralize the effect of electrons within the floating gates. Consequently, the mobile ion migration cancels the negative charges on the floating gate, resulting in superficial charge loss and data retention failures. 
         [0003]    Under certain bias conditions, the movement of mobile ions toward the floating gate may accelerate, causing devices to fail HTOL (High Temperature Operating Life) tests. For example, in the case where the first metal layer is used to route a control gate signal, mobile ions will accelerate toward the floating gate when the control gate is biased at a high voltage. Because the HTOL failure is accelerated as a result of such bias, the reliability failure is not always predictable during wafer sort or wafer-level data retention testing. 
         [0004]    Single-poly cells and cells having a large portion of their floating gate uncovered by the control gate are more susceptible to charge loss. As devices scale downward, they become more vulnerable to failure from mobile ions, since the number of electrons stored on the floating gate scales with the cell dimension scaling. One way to counteract mobile ions is through the use of a gettering layer. 
         [0005]    A gettering layer involves incorporating phosphorus in dielectric layers to immobilize mobile ions. However, a gettering layer may not completely confine mobile ions within the layer. Despite the use of gettering layers, mobile ions still affect the device characteristics and result in data retention failures. With the continued increase in cell density there is an increase in vulnerability to charge loss. Therefore, a need exists for a non-volatile memory cell that has an improved resistance to data retention failures. 
       SUMMARY 
       [0006]    The present invention provides a non-volatile memory cell with enhanced reliability protection during operation. The memory cell includes a semiconductor substrate having a tunnel oxide layer, upon which is disposed a floating gate. A control gate is separated from the floating gate by a dielectric layer. Two conductive layers (“conductors”) are included in the memory cell, each having a bias voltage different from the other during operation. Further, the bias polarity carried by a pair of conductors during standby may be opposite that of a pair of conductors during a read operation. For example, in a selected state, the first conductor may have a high polarity and the second conductor a low polarity. Then, in an unselected state, the first conductor will have a low polarity and the second conductor a high polarity. During standby, the memory cells are typically unselected, and will be biased in accordance with the predetermined bias polarity level. 
         [0007]    Each conductor is connected to either a source/drain region or a control gate, depending on their respective bias voltages during standby. During a standby mode, if the first conductor is biased at a lower voltage relative to the second conductor, then the first conductor is connected to the source/drain region and the second conductor is connected to the control gate. 
         [0008]    The invention also provides a non-volatile memory cell having a floating gate that is configured to operate at a potential greater than the bias voltage of the first conductor. With this configuration, mobile ions are influenced to migrate away from the floating gate. As a result, the non-volatile memories of the invention exhibit enhanced data reliability. 
         [0009]    In another embodiment, the invention pertains to a non-volatile memory system having an array of memory cells, wherein each memory cell includes an access transistor having source/drain regions and a capacitor structure which preferably has a diffusion region of opposite conductivity to the source/drain regions. The access transistor and capacitor structure share a common floating gate. The other electrode of the capacitor is a control gate, whose bias modulates the conductivity of the access transistor by coupling its bias to the floating gate determined by the capacitive coupling ratio between the access transistor and the capacitor. 
         [0010]    Each control gate in the memory system is separated from the floating gate by a dielectric region. If the bit line is biased at a higher voltage than the control gate, then a first conductor is connected to the control gate and the second conductor serves as the bit line. In this embodiment, the bit line is connected to a source/drain region of the access transistor. 
         [0011]    The invention is also directed to a structure for a non-volatile memory system composed of an array of memory cells in which each cell includes an access transistor and a capacitor that share a common floating gate. Specifically, each memory cell has a structure that disposes a bit line between the floating gate and the control gate line. In this embodiment, the bit line is biased at a higher voltage than the control gate, such that a first conductor constitutes the control gate line and is connected to the control gate while a second conductor constitutes the bit line and is connected to one of the source/drain regions. 
         [0012]    The present invention will be more fully understood in view of the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a top view of a non-volatile memory cell having a PMOS access transistor and an NMOS coupling gate in accordance with one embodiment of the present invention; 
           [0014]      FIGS. 2A and 2B  are cross sections of the non-volatile memory cell of  FIG. 1 ; 
           [0015]      FIG. 3  is a schematic diagram of an array of the non-volatile memory cells of  FIG. 2 ; 
           [0016]      FIGS. 4A ,  4 B and  4 C illustrate electrical connections that enhance the reliability of the memory cell of the present invention; and 
           [0017]      FIG. 5  is a cross sectional view of a double-poly non-volatile memory cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  is a top layout view of a non-volatile memory cell  100  in accordance with one embodiment of the present invention. Floating gate  50  extends over access transistor  110  at one end, and over capacitor  120  at the opposite end. Source line and bit line contacts flank the channel region  218  of access transistor  110 . A control gate (CG) for controlling the conductivity of memory cell  100  is shown adjacent to coupling region  221 .  FIG. 2A  is a sectional view of capacitor  120  of  FIG. 1  along section line A--A. 
         [0019]    A control gate line (CGL) supplies a bias voltage to the control gate through a connection that is shown in  FIG. 2A . CGL  44  operates as a word line in a 1T cell. In the described example, non-volatile memory cell  100  is fabricated in a p-type mono-crystalline semiconductor substrate  201  (see  FIG. 2A ).  FIG. 2A  illustrates a cross-sectional view of capacitor  120  of  FIG. 1 . Floating gate extends over p-type substrate  201  and n-type coupling region  221 . N-type coupling region  221  is coupled to n+region  222 . N-type regions  221 - 222 , capacitor oxide  216  and floating gate  50  form an NMOS capacitor structure  120 . NMOS capacitor  120  couples the bias at n+region  222  to floating gate  50 . N-type coupling region  221  is self-aligned with the edge of floating gate  50 . This self-alignment is accomplished by implanting an n-type impurity using the edge of floating gate  50  as a mask, and then diffusing the impurity under the floating gate using an annealing step. N-type coupling region  221  is formed at the same time as the source and drain regions of NMOS logic transistors (not shown). Accordingly, no additional step is required to form n-type coupling region  221 . 
         [0020]      FIG. 2B  is a cross sectional view of the access transistor  110  of  FIG. 1  along section line B-B of  FIG. 1 . In  FIG. 2B , substrate  201  is a p-type semiconductor. Non-volatile memory cell  100  of  FIG. 1  includes a PMOS access transistor  110 . Access transistor  110  includes p-type source region  211  and p-type drain region  212 , which are formed in n-well  202 . In  FIG. 2B , source region  211  includes lightly doped source  211 A and p+source contact region  211 . Source region  211  is connected to a bit line (BL) and drain region  212  is connected to a source line (SL). However, the connections may be reversed, such that source region  211  is connected to a source line, and drain region  212  may instead be connected to a bit line. Drain region  212  includes lightly doped drain  212 A and p+drain contact region  212 . An n-type channel region  218  is located between source region  211  and drain region  212 . Field oxide  214  is located between the source and drain regions as illustrated ( FIG. 2B ). Field oxide  214  is planarized, such that the upper surface of field oxide  214  and the upper surface of substrate  201  are located in substantially the same plane. A thin tunnel oxide layer  215  is located over the channel region  218 . Tunnel oxide layer  215  has the same thickness as the gate oxide layers used in the logic transistors (not shown) fabricated in substrate  201 . A conductively doped polycrystalline silicon floating gate  50  is located over thin tunnel oxide  215 . Sidewall spacers  206  and  228  are typically formed from silicon nitride or silicon oxide, are located at the edges of floating gate  50 . 
         [0021]    Similarly, control gate  222  is self-aligned with the edge of sidewall spacer  228 . This self-alignment is accomplished by implanting an n-type impurity using the edge of sidewall spacer  218  as a mask, and then diffusing the impurity under the sidewall spacer using an anneal step. The control gate  222  is formed at the same time as the n+contact regions of NMOS logic transistors (not shown). Accordingly, no additional step is required to form n+control gate  222 . 
         [0022]    The total capacitance of NMOS coupling capacitor structure  120  is preferably significantly larger than the gate capacitance of the PMOS access transistor  110 . In preferred embodiments, the capacitance of NMOS coupling capacitor structure  120  is about four to ten times larger than the gate capacitance of PMOS access transistor  110 . Non-volatile memory cell  100  can be fabricated using a conventional logic process, without any process modifications or special implants. 
         [0023]      FIG. 3  is a schematic diagram of a 2×3 array of non-volatile memory cells  100 ,  200 ,  300 ,  400 ,  500  and  600 . Non-volatile memory cells  200 ,  300 ,  400 ,  500  and  600  are identical to above-described non-volatile memory cell  100 . Thus, non-volatile memory cells  200 ,  300 , 400 ,  500  and  600  include PMOS access transistors  210 ,  310 ,  410 ,  510  and  610  respectively, and NMOS capacitor structures  220 ,  320 ,  420 ,  520  and  620 , respectively. The sources of PMOS access transistors  110  and  410  are commonly connected to a first bit line BL 0 . The sources of access transistors  210  and  510  are commonly connected to a second bit line BL 1 . The drains of PMOS access transistors  110  and  410  are commonly connected to a first source line SL 0  whereas, the drains of PMOS access transistors  210  and  510  are commonly connected to a second source line SL 1 . The drains of access transistor  310  and  610  may be connected to a separate source line (not shown) parallel to SL 1 . Alternatively the drains of access transistors  310  and  610  may share source line SL 1  with the drains of access transistors  210  and  510 . In addition, the sources of transistors  310  and  610  are connected to BL 2 . It is not essential for source lines to run vertically as shown in  FIG. 3 . The source lines may instead run horizontally. For example, cells  100 ,  200 , and  300  may share one source line along a horizontal axis, and cells  400 ,  500  and  600  may share a different source line along a second horizontal axis. Alternatively, a source line can be shared by two adjacent rows of cells. In this embodiment, an even and an odd row of cells along a common bit line may share one drain contact. NMOS capacitor structures  120 ,  220  and  320  are commonly connected to a first control gate line CGL 0 . Similarly, NMOS capacitor structures  420 ,  520  and  620  are commonly connected to a second control gate line CGL 1 . Although the described array has two rows and three columns, it is understood that arrays having other sizes can be implemented by one of ordinary skill in the art. 
         [0024]    In one embodiment, non-volatile memory cells  200  and  300  may be read by holding CGL 0  at  0  Volts, source lines SL 0 -SL 1  at 0 V (or some other voltage level to suppress leakage current), n-well  202  at 1.0 V, and p-type substrate  201  at 0 V. Bit lines BL 0 -BL 1  are pre-charged to 1.0 V (or some other voltage higher than the control gate bias). Memory cells  200  and  300  may instead be read by precharging the source lines to 1 V, and the bit lines to 0 V. Under these conditions, read current will flow through the access transistors of programmed cells, without disturbing the data stored in the access transistors of non-programmed (erased) cells. 
         [0025]    The control gate line CGL 1  associated with the unselected cells is held at a positive voltage such as 4V in the normal read mode. A 4V voltage is sufficient to turn off access transistors  410 ,  510 , and  610 . When access transistors  410 ,  510 ,  610 , are turned off current will not flow through these transistors into bit lines BL 0  and BL 1 , even if any of these transistors are programmed. As a result, cells  400 ,  500  and  600  do not interfere with the bit line signals from selected cells  100 ,  200  and  300 . 
         [0026]      FIG. 4A  illustrates a PMOS access transistor  110  located in an n-well that is in a p-substrate. Floating gate  50  is disposed on tunnel oxide layer  52 , and bit line  42  is separated from floating gate  50  by dielectric region  47 . Also shown in  FIG. 4A  is dielectric region  48 , which separates bit line  42  from second conductor  44 . To minimize the impact of mobile ions on the floating gate during standby, the bias voltages of the conductors during standby determine which conductor is connected to which cell region. In addition, the speed, power, and reliability desired for a specific memory device will determine the appropriate bias to select for the bit line. Specifically,  FIG. 4A  illustrates the situation where bit line  42  is biased between 1V-2V. In this embodiment, N-well  415  is preferably biased at the same or higher bias than the bit line in order to avoid forward-biasing the P+/N-well diode. Second conductor  44  is preferably biased at a voltage sufficient to turn off unselected programmed cells. The above bias conditions for the cell shown in  FIG. 4A  result in bit line  42  having a lower bias voltage than conductor  44  (typically 4V or higher during standby). In accordance with the invention, bit line  42  is connected to source or drain region  212 , and conductor  44  is connected to control gate  55 . Then, when memory cell  110  is programmed, floating gate  50  has a higher potential during standby than the bit line bias. Consequently, the electric field between the lower positively-biased conductor  42  and floating gate  50  is reduced compared to the electric field generated by the voltage difference between the control gate and the floating gate. 
         [0027]    In  FIG. 4A , mobile ions within the memory cell migrate in the direction shown since floating gate  50  has a higher voltage than conductor  42  (bit line). As a result, mobile ions migrate away from the floating gate and avoid significantly disturbing its contents. 
         [0028]    During a standby operation for the PMOS transistor of  FIG. 4A , n-well  415  is held at 1V and p-type substrate  405  is held at 0 V. Cells which are in a programmed state preferably have the electric field potential of floating gate  50  at a potential higher than 1V (the BL bias), bit line  42  is biased at 1V, and second conductor (control gate line)  44  is biased at 0 V. Since the floating gate in the PMOS transistor is more positive than bit line  42  in  FIG. 4A , mobile ions in dielectric region  47  will be forced by the electric field to migrate away from the floating gate. Mobile ions also exist within dielectric region  48 . Ions in the electric field of region  48  travel downward away from the control gate because conductor  44  has a higher potential than bit line  42 . However, such ions are prevented from accumulating around floating gate  50  because of the field created by the voltage difference between floating gate  50  and bit line  42 . Consequently, mobile ions will not accumulate around floating gate  50  of the PMOS transistor during standby. 
         [0029]    The standby operation for NMOS transistor  105  will now be described in conjunction with  FIG. 4B . Memory cell  105  is programmed to provide the floating gate with a potential higher than the control gate during standby. In one embodiment, the cells in their programmed state are configured to place floating gate  50  at a potential between 1-2V. Preferably, control gate line  42  is biased at 0V during standby. Under the above conditions, the electric field (shown by arrows) causes ions to migrate away from floating gate  50 . During a read operation, however, the selected control gate line has a potential of 4V, which is higher than the bit line bias and the floating gate. The voltage difference between the control gate line and the floating gate will cause a slight migration of ions toward the floating gate, but the ions will migrate in the opposite direction, i.e., away from the floating gate, for a much longer period when unselected or during a standby. 
         [0030]    The invention is also directed to a non-volatile memory structure having two conductors that are biased differently during a standby operation. The conductors may be fabricated out of metal or polysilicon. However, it is not essential to use the same material to fabricate each of the conductors. 
         [0031]    In a preferred embodiment, the first conductor  42  is a bit line, and the second conductor  44  is a control gate line. However, it is understood that the roles of conductor  42  and conductor  44  may be reversed, depending on what regions these conductors are connected to in the memory cell. 
         [0032]    The read operation for NMOS access transistor  105  in accordance with another embodiment of the present invention will now be described in accordance with  FIG. 4B . Again, floating gate  50  has an electric potential usually higher than 1V. If conductor  42  is biased at 0V and conductor  44  is biased at 1V, then each conductor will have the electrical connections shown in  FIG. 4B . In this embodiment, conductor  42  functions as a CGL because it is connected to control gate  55 , and conductor  44 , functions as the bit line, as it is connected to the source or drain region  212 . The cell conditions of  FIG. 4B  result in a reduced electric field between the lower positively biased conductor  44  and floating gate  50 . This reduced electric field causes ions to migrate toward floating gate  50  during a read operation. Nevertheless, the reading of memory cell  105  is not expected to cause significant data retention failures, since the non-volatile device is not continually reading the same portions of the memory array. In addition, any charge loss is minimal for the cell of  FIG. 4B  because a read operation is generally interspersed with idle periods during which the memory device is in a standby mode. 
         [0033]    In a read operation, the control gate lines are precharged to an unselected state. When the memory device receives an address for a specific CGL, the selected CGL is discharged to zero and maintained at zero for the first half of the clock cycle period. During the first half of the clock cycle, the read biases are applied to the selected cells until the memory switches to a nonselected state, and then biases the previously selected cells with the voltage for a standby operation during the second half of the same clock period. During the standby mode, the flow of mobile ions in the memory cell is opposite the flow that is present during a read mode. A read operation will occur in a substantially shorter time period than the time that elapses during standby. Thus, the migration of mobile ions toward the floating gate during a read will be immediately reversed by the electric field that is present in the standby mode. Accordingly, the invention will enhance the reliability of the stored data in a non-volatile memory regardless of whether a PMOS or NMOS access transistor is used for the memory cell. 
         [0034]      FIG. 4C  is similar to  FIG. 4B , except for the provision of a P-well within a deep N-well.  FIG. 4C  will generally operate in the same manner as  FIG. 4B . 
         [0035]      FIGS. 2A ,  2 B,  4 A,  4 B and  4 C illustrate a memory cell having a single polysilicon layer. However, the invention may also be implemented using two or more polysilicon layers.  FIG. 5  illustrates a memory cell with two polysilicon layers separated by dielectric layer  216 . The first polysilicon layer forms floating gate  50 , and the second polysilicon layer forms control gate  70 . As in the earlier described embodiments, the memory cell has two dielectric regions  47  and  48 . Mobile ions may exist in each of dielectric regions  47  and  48 . In this embodiment, control gate  70  is connected to conductor  44  (control gate line) and source/drain region is connected to conductor  42  (bit line). Memory cell  125  has a smaller density then the cells of  FIGS. 4A-4C . Memory cell  125  is also generally better protected from mobile ion effects than memory cell  105  or  110 , due to spacers  81  and  82 . The spacers in  FIG. 5  completely surround the lateral portions of floating gate  50  to substantially prevent mobile ions from contacting the floating gate. 
         [0036]    Although the invention has been described in connection with several examples, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. For example, source contacts may be omitted from the memory array and provided at predetermined interface regions that are regularly spaced along row or column directions. Accordingly, the present invention is limited only by the following claims.