Abstract:
There is disclosed a multi-bit flash memory cell and programming method using the same. In order to solve the problems that the size of a cell per unit is increased, reliability of a device is degraded due to a high operating voltage and a circuit necessary for driving the cell becomes complicated, the multi-bit flash memory cell and programming method using the same according to the present invention stores information of various states, by interchangeably programs a drain and a source in a cell array of virtual ground type, in a structure in which that two types of cells look like connected serially by doping a floating gate in a flash memory cell with two regions of a N type and a P to type.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to a flash memory cell. More particularly, it relates to a multi-bit flash memory cell and programming method using the same capable of storing information of more than 2 states at a single flash memory cell. 
     BACKGROUND OF THE INVENTION 
     In a conventional memory cell, 1 bit, i.e., only 2 states could be stored at a single memory cell. Thus, in order to reduce the size of a chip, the size of the chip per unit must be reduced. In order to reduce the size of the chip per unit, however, difficulties in various processes are accompanied its cost is increased. Therefore, if data of more than two bits is stored at a single cell, the size of the cell could be reduced to more than half. 
     In a flash memory cell, there have attempted to develop thus multi-bit cell. Recently, cells implemented in various methods have been reported. As one of the methods reported conventionally, there exists a method by which a floating gate is divided into more than two and the number of electrons in the floating gate is adjusted depending on the programming condition to implement a multi-bit cell. This method, however, is difficult in process and increases the size of the cell per unit, thus counterbalancing the advantage of implementing a multi-bit cell. Also, there is a difficulty in constructing a circuit because the method requires a high voltage upon programming. Further, there is a possibility that reliability of the device is degraded because the method uses a high voltage inside the circuit. 
     As another method for implementing a multi-bit cell, there is a method of manufacturing a cell, by defining a program verification condition corresponding to  4  states while controlling a program bias or a pulse. This method, however, makes circuits for implementing various program conditions complicated. Further, if the program verification condition is subdivided to perform a program, there is a disadvantage that the time required for the program verification becomes long. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a multi-bit flash memory cell and programming method using the same, which can store information of various states by interchangeably programming a drain and a source in a cell array of virtual ground type, in a structure in which that two types of cells look like connected serially by doping a floating gate in a flash memory cell with two regions of a N type and a P type. 
     In order to accomplish the above object, a multi-bit flash memory cell according to the present invention is characterized in that it comprises a floating gate electrically isolated from a semiconductor substrate by a gate oxide film, one side of the floating gate has a first doping region and the other side of the floating gate has a second doping region, a control gate electrically isolated from the floating gate by a dielectric film and overlapped with the floating gate in a self alignment manner, a first junction region formed at the semiconductor substrate outside the first doping region in the floating gate, and a second junction region formed at the semiconductor substrate outside the second doping region in the floating gate. 
     Also, a method of programming a multi-bit flash memory cell according to the present invention, having a structure in which a floating having N-type and P-type doping regions is formed, a gate electrode consisted of a control gate formed on said floating gate is formed and first and second junction regions are formed on both sides of said gate electrode is characterized in that assuming that an initial state where a programming operation is not performed is a ‘00’ state, the first junction region and the second junction region are programmed with a ‘01’ state and a ‘11’ state, respectively, by switching a source voltage and a drain voltage applied to the first and second junction regions, respectively; and the first junction region and the second junction region are programmed with a ‘11’ state by applying a programming voltage for a longer time than when the programming into the ‘10’ state is performed. 
     The present invention forms a floating gate wherein two regions; a N-type doping region and a P-type doping region are divided when forming polysilicon used for a floating gate. In case that a gate material of a N-type is used, the threshold voltage can be differently adjusted since its work function is different from that of a substrate. In this case, the programming efficiency of the N-type doping side and the P-type doping side is different. Thus, if they are programmed under same conditions, their thresholds to be attained are different. The present invention can use the difference of the threshold voltages thus implemented differently as the threshold voltage in the intermediate step and uses a general erase state and a general program state as an erase state and a complete program state, respectively. The general program state performs the programming using the junction having good program efficiency as a drain side. 
     At this time, the verification condition is same to the complete program state. Thus implemented cell performs the programming operation while the source and the drain are switched when viewed from the unit cell. Therefore, it can select the source and the drain depending on the program condition, since the array is constructed in a virtual ground type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned aspects and other features of the present invention will be explained in the following description, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view of a multi-bit flash memory cell according to the present invention; 
     FIG. 2 shows an equivalent circuit diagram of a multi-bit flash memory cell according to the present invention; 
     FIG. 3 shows a structure of a multi-bit flash memory cell according to the present invention; and 
     FIGS. 4A to  4 D are a programming condition depending on respective states of multi-bit flash memory cells and a graph showing the relationship between a gate voltage and a drain current at this time. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described in detail by way of a preferred embodiment with reference to accompanying drawings. 
     FIG. 1 is a cross-sectional view of a multi-bit flash memory cell according to the present invention, which shows a flash memory cell of a stack-gate type. 
     As shown, the multi-bit flash memory cell according to the present invention includes a floating gate  12  electrically isolated from a semiconductor substrate  11  by a gate oxide film (not shown) and one side of which has a first doping region  12 A and the other side of which has a second doping region  12 B, a control gate  13  formed on and electrically isolated from the floating gate  12  by a dielectric film (not shown) and overlapped with the floating gate  12  in a self alignment manner, a first junction region  14  formed at the semiconductor substrate  11  outside the first doping region  12 A in the floating gate  12  and a second junction region  15  formed at the semiconductor substrate  11  outside the second doping region  12 B in the floating gate  12 . At this time, the first doping region  12 A and the second doping region  12 B have the same lengths. That is, if the length of the floating gate  12  is L, the floating gate  12  is doped so that the length of the first doping region  12 A and the second doping region  12 B is L/ 2 , respectively. The first doping region  12 A may be, for example, N-type, and the second doping region  12 B may be, for example, P-type. 
     A hot carrier injection is used to program the multi-bit flash memory cell having this structure and a channel erase using a F-N tunneling is also used to erase it. At this time, the erase is almost same to a method of erasing a typical flash memory cell and is same to a typical programming method. However, the programming efficiency is different in the case that the first junction region  14  is used as a drain and the case that the second junction region  15  is used as a drain. Thus, if they are programmed by the same program bias for same time, the threshold voltage to be attained will be different. As mentioned above, the present invention implements a 2-bit cell using the two different threshold voltages as two states in the intermediate step. In this programming method, the reason why the program efficiencies are different, when they are programmed by changing their junctions each other, will be below explained by reference to FIG.  2 . 
     FIG. 2 shows an equivalent circuit diagram of a multi-bit flash memory cell according to the present invention. 
     The flash memory cell has a structure in which two cells C 1  and C 2  each having different threshold voltages are serially connected between a bit line BL 1  and a bit line BL 2 , as shown in FIG.  2 . The control gates of the first and second cells C 1  and C 2 , connected each other, are connected to a word line WL. For example, the first cell C 1  formed by the first doping region  12 A doped with a N-type impurity has a low voltage while the second cell C 2  formed by the second doping region  12 B doped with a P-type impurity has a high voltage. Due to the threshold voltage between the first cell C 1  and the second cell C 2 , the carrier concentration in the junction edge portion where channel concentration and hot carrier injection are generated under a programming condition, is different. Therefore, as the probability by which the hot carrier injection is generated and the probability by which the carriers pass over the barrier into the floating gate are different, their final programming efficiencies become different. At this time, it can be considered that the junction structure per cell is changed to differentiate the programming efficiency using this principle. However, it should be noted that the size of the cell might be increased since both the junctions become asymmetric. 
     FIG. 3 shows a structure of a multi-bit flash memory cell according to the present invention. 
     As mentioned above, if the programming is to be performed by changing the drain and the source, the unit cell array must have a virtual ground array structure so that the programming can be performed while the ground lines are changed interchangeably. The virtual ground array structure is one in which cells neighboring the same lines (bit lines) are used as a source or a drain. A small cell array may be implemented using this cell array structure. 
     FIGS. 4A to  4 D are a programming condition depending on respective states of multi-bit flash memory cells and a graph showing the relationship between a gate voltage and a drain current at this time. 
     As shown, the flash memory cell according to the present invention has a structure in which a floating gate  42  having first and second doping regions  42 A and  42 B is formed on a semiconductor substrate  41 , a control gate  43  is formed on the floating gate  42  to form a stack gate, and first and second junction regions  44  and  45  are formed on both sides of the gate electrode  42 . 
     FIGS.  4 A˜ 4 D show ‘01’, ‘10’ and ‘11’, respectively, where the initial state is ‘00’ when a programming is not performed. In order for the multi-bit flash memory cell according to the present invention to have the ‘01’ state, a programming operation by which a voltage of 0V is applied to the source, a voltage of 3V˜8V is applied to the drain and a voltage of 5˜15V is applied to the control gate, is performed using the junction region  45  formed on the semiconductor substrate outside the second doping region  42 B (P-type doping region) having a high threshold voltage in the floating gate  42 , as the drain, and using the junction region  44  as the source. Also, in order for the multi-bit flash memory cell to have the ‘10’ state, a programming operation by which a voltage of 0V is applied to the source, a voltage of 3V˜8V is applied to the drain and a voltage of 5˜15V is applied to the control gate, is performed using the junction region  44  formed on the semiconductor substrate outside the first doping region  42 A (N-type doping region) having a low threshold voltage in the floating gate  42 , as the drain, and using the junction region  45  as the source. Finally, in order for the multi-bit flash memory cell to have the ‘11’ state, a programming operation by which a voltage of 0V is applied to the source, a voltage of 3V˜8V is applied to the drain and a voltage of 5˜15V is applied to the control gate, for a longer time than the case where the programming is performed in the ‘10’ state, using the junction region  44  formed on the semiconductor substrate outside the first doping region  42 A (N-type doping region) having a low threshold voltage in the floating gate  42 , as the drain, and using the junction region  45  as the source. Under these programming conditions, the conditions on the ‘10’ state and the ‘01’ state can be exchanged. FIG. 4D shows the relationship between the control gate voltage Vcg and the drain current Id depending on respective states. 
     The present invention basically uses the difference of the programming efficiency in the source side and the drain side in order for the flash memory to store information of more than 1 bit. However, the present invention is not limited to the above embodiment but it can be applied to a method of generating the difference of the programming efficiency by changing the junction structure. In other words, the method makes the junction structure of the source and the drain different, thus making different their programming efficiency (one side is an abrupt junction and the other is a graded junction). Also, though the above-mentioned embodiment has described a stack gate type flash memory cell as an example, it may be also applied to a split gate cell. 
     As described above, the present invention can reduce the area of a cell array with respect to the same cell size to its half since it can store information of 2 bits per cell. Also, the present invention can reduce the processes since it can manufacture a multi-level cell by adopting the so method of manufacturing a conventional NOR type cell intact and can also implement a multi-level cell using one type of program bias. Further, the present invention can prevent a leak current by increasing the threshold voltage in some channel regions of a cell using a P-type impurity doped polysilicon as a gate. Additionally, there are problems that the leak current of a cell is increased due to a weaken drain turn-on phenomenon or source/drain characteristic, etc. However, the present invention can reduce the leak current in the same channel concentration by using some of the floating gate as a P-type impurity doped polysilicon. 
     The present invention has been described with reference to a particular embodiment in connection with a particular application. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and applications within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications, and embodiments within the scope of the present invention.