Abstract:
A cell array of a NAND type ferro-dielectric memory is disclosed. The cell array of ferro-dielectric memory system, including: a plurality of unit cell strings coupled to one bit line; and a plurality of string selectors between each of the unit cell strings and the bit line, wherein only one unit cell string is connected to the bit line through one string selectors. The present invention can decrease a size of cell by eliminating a bit line contact formed in cells and controls a bit line capacitance by using selection transistor, therefore, the present invention can control optimum bit line capacitance by gaining the maximum sense margin.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a semiconductor device; and, more particularly, to a cell array of a NAND type ferroelectric random access memory. 
   DESCRIPTION OF RELATED ARTS 
   Recently, a limitation of refresh in a dynamic random access memory (DRAM) has been overcome by using ferro electric thin film as a dielectric film of a capacitor and it has been caused to increase a capacity of a memory. The ferroelectric Random Access Memory. (FeRAM) is one of nonvolatile memories having ferro electric thin film. It has several advantages such as to nonvolatile and fast access time. Therefore, it has been spotlighted as next generation memory. 
     FIG. 1  is a circuit diagram illustrating a conventional cell array of ferro electric memory. 
   Referring to  FIG. 1 , unit cells are arranged at a location where a plurality of word line WL and one bit line BL are crossed. Each unit cell includes a transistor and a ferroelectric capacitor FC. A drain of the transistor is coupled to the bit line BL and a gate of the transistor is coupled to the word line WL. The ferroelectric capacitor FC has a first electrode coupled to a source of the transistor M and a second electrode coupled to a cell plate line CP. 
   A cell array is consisted of the plurality of the above mentioned unit cells as forming a matrix-type and a cell string is coupled to one bit line. 
   Since an active area and bit line of the conventional cell array of ferroelectric memory is separated, a bit line contact is necessary in every unit cell to transmit a signal to bit line. Therefore, a predetermined space is required for the bit line contact by considering a contact size, an active area, and an overlap margin of a contact and a word line. 
   Therefore, there is a limitation to integrate the conventional memory system because-the active area and the bit line are separated in the conventional cell array. 
   For overcoming the limitation of the conventional cell array of memory, another conventional cell array of memory is introduced as shown in FIG.  2 . 
     FIG. 2  is a circuit diagram depicting another conventional cell array of a NAND type ferroelectric memory system. 
   Referring to  FIG. 2 , the cell array has a plurality of cell strings STR 1 ˜STRN and each cell string has same structure. 
   The cell string includes a plurality of unit cells corresponding to the number of desired bits and each unit cells includes a depletion mode transistor (D), an enhancement mode transistor (N) and a ferro capacitor (FC). 
   A plurality of enhancement mode transistors is coupled in series. A source of the enhancement mode transistor is coupled to a source of depletion mode transistor and two nodes of capacitor are connected to a drain and a cell plate line, respectively. Gates of the enhancement mode transistor N and the depletion mode transistor D are commonly connected to a word line WL. 
     FIG. 3  shows a layout of NAND type cell array in FIG.  2 . 
   Referring to the  FIG. 3 , a cell array includes a first active area A 1 , a second active area A 2 , a word line WL, a storage node contact electrode SN and a ferroelectric capacitor FP. 
   Gates D 0 , D 1 , . . . D 15  are passed over the first active area A 1 . The second active area A 2  is coupled one side of the active area A 1  and gates of the enhancement mode transistor N 0 , N 1 , . . . N 15  is passed over the second active area A 2 . The word line WL is coupled to the gates of depletion mode transistor D 0 , D 1 , . . . , D 15  and gate of the enhancement mode transistor N 0 , N 1 , . . . , N 15 . A ferro electric capacitor includes a storage node contact electrode SC contact electrode coupled to a drain of the enhance mode transistor through a drain contact and a cell plate electrode, coupled to a cell plate line. 
   In a meantime, the storage node contact electrode SN of the ferroelectric capacitor and the second active area A 2  are electrically coupled by a local wiring L 1  between the drain contact DC exposing the second active area A 2  and the storage node contact electrode SC exposing the storage node contact electrode SN. 
   Therefore, the storage node contact electrode SN of the ferroelectric capacitor is coupled to a drain of the enhancement mode transistor and the cell plate line of the ferroelectric capacitor is paralleled with the bit line BL. 
   In the conventional array cells shown in the  FIGS. 2 and 3 , a unit cell string is composed by combining unit cells. The unit cells include two transistors and one ferroelectric capacitor. The unit cell strings STR 1 ˜STRN are coupled to the bit line for transmitting data to a sense amplifier S/A. When a specific ferroelectric capacitor is selected, a capacitance of bit line coupling to the specific ferroelectric capacitor includes a junction capacitance of whole active area included in the unit cell string and a junction capacitance of active area in another unit cell string coupled to selected bit line. 
   Therefore, if a capacitance of whole bit line is increased a lot, it is too difficult to gain an optimized sense margin, which is required to a ferroelectric memory. 
   In other words, an active area of the FeRAM adopting NAND type cell array structure is formed lengthwise comparing to the cell array of the typical ferroelectric memory and as a result, the size of the active area becomes increased and also a junction capacitance and a bit line capacitance are increased too. Therefore, the-sense margin cannot be used at an optimize point. 
     FIG. 4  is a graph showing a relation between a bit line capacitance and a sense margin in a ferroelectric memory. 
   Conventionally, a ferroelectric memory uses polarization value of ferroelectric having a hystericsys curve characteristic. In case of DRAM using a linear capacitance, a sense margin increases linearly according to a capacitance of the bit line. However, in case of the ferroelectric memory device using hysterisys curve characteristic, the sense margin increases corresponding to decrease of the capacitance of bit line for while and then the sense margin decreases again. 
   As shown in a conventional memory of  FIG. 1 , if one bit line is connected to all unit cells, the capacitance of bit line is increased and it becomes larger than optimum bit line capacitance Cb 1 . It is a reason of decreasing the sense margin. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a cell array of ferroelectric memory for constraining reduction of a sense margin caused by increase of a bit line capacitance. 
   It is another object of the present invention to provide a cell array of ferroelectric memory for gaining the optimum sense margin by controlling the bit line capacitance. 
   In accordance with an aspect of the present invention, there is provided a cell array of ferro-dielectric memory system, including: a plurality of unit cell strings coupled to one bit line; and a plurality of string selector, each of which is arranged between each of the unit cell strings and the bit line, wherein one of the string selector couples corresponding unit cell strings to the bit line by selecting said one of string selector. 
   In accordance with an aspect of the present invention, there is also provided a cell array of ferro-dielectric memory, including: a first active area being arranged in one direction and used as a bit line; a gate of a depletion mode transistor crossed over the first active area; a second active area being perpendicularly arranged to the first active area and coupled to one side of the first active area; a gate of an enhancement mode transistor crossed over the second active area; a word line coupled to the gate of the enhancement mode transistor and extended from the gate of the depletion mode transistor; a third active area extended from other side of the first active area; a gate of selection transistor being arranged in parallel with the word line and crossed over the first active area; a ferro capacitance coupled to one side of the second active area; and a cell plate line coupled to the ferroelectric capacitor and being arranged in parallel with the first active area. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a circuit diagram illustrating a conventional cell array of ferroelectric memory; 
       FIG. 2  is a circuit diagram showing another conventional cell array of a NAND type ferroelectric memory; 
       FIG. 3  is a view depicting a lay out of cell array shown in  FIG. 2 ; 
       FIG. 4  is a graph explaining a relation between a bit line capacitance and a sense margin in a ferroelectric memory of a preferred embodiment of the present invention; 
       FIG. 5  is a circuit diagram illustrating a cell array of a ferroelectric memory in accordance with a preferred embodiment of the present invention; and 
       FIG. 6  is a diagram showing a lay out of the cell array of a ferroelectric memory in accordance with the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
     FIG. 5  is a circuit diagram illustrating a cell array of a ferroelectric memory in accordance with a preferred embodiment of the present invention and  FIG. 5  is a diagram showing a lay out of the cell array of a ferroelectric memory in accordance with the preferred embodiment of the present invention. 
   Referring the  FIG. 4 , one bit line BT is coupled to a plurality of unit cell strings STR 1  to STRN and each of unit cell strings includes a plurality of unit cell having two transistors and one ferroelectric capacitor. Selection transistors S 1  to SN couples each of unit cell string STR 1  to STRN and a bit line BL. Cell plate lines CP 1  to CPN are coupled to each of unit cell strings. 
   Two types of transistors are used in the unit cell string. One is a depletion mode transistor (D). The depletion mode transistor (D) is coupled to a plurality of depletion mode transistors (D) in other unit cell strings in series. A gate of the depletion mode transistor is couple to a word line. Other transistor is an enhancement transistor (N) having a source end coupled to a source end of the depletion mode transistor (D) and a drain coupled to one electrode of the ferroelectric capacitor. Other electrode of the ferroelectric capacitor is coupled to the cell plate line CP. The gates of the enhancement mode transistor (N) and the depletion mode transistor (D) are commonly connected to a word line (WL). 
     FIG. 6  is a lay out diagram of FIG.  5 . 
   Referring to  FIG. 6 , a plurality of first active areas A 1  in the unit cell string form the bit line (BL) and a second active area A 2  is formed by being coupled to one side of the first active area A 1 . 
   As showing in  FIG. 6 , the first active area A 1  and the second active area A 2  are perpendicularly arranged. The first active area A 1  forms a line by being coupled to other first active areas of neighbor unit cells in series. The second active area A 2  is formed like a branch from the first active area A 1  by being coupled one side of the first active area A 1  in a direction perpendicular to the bit line BL. The line formed with the first active areas is used as a bit line. 
   One end of the first active area A 1 , which is an end of the bit line, is coupled to a third active area A 3 . A gate (SWL) of a selection transistor S 1  passes over the third active area A 3 . The selection transistor (S 1 ) is used to select only STR 1  among-unit cell strings STR 1  to STRN. 
   Gates D 0  to D 15  of depletion mode transistors are passed over the first active area A 1 , which forms the bit line, and gates N 1  to N 15  of enhancement transistors are passed over junction points of the first active areas and the second active areas A 2 . The gates D 0  to D 15  of depletion mode transistors and gates N 0  to N 15  of enhancement transistors are coupled to a plurality of word lines WL 0  to WL 15 . The plurality of word lines are crossed with one bit line. 
   The third active area A 3  is an active area of the selection transistor. The selection transistor (S 1 ) has a source coupled to a common source of the depletion mode transistor D and enhancement transistor N. A drain of the selection transistor is coupled to the bit line and a gate of the selection transistor receives a select signal SWL 1 . 
   As mentioned above, the unit cell string STR 1  is separated from a bit line contact by controlling on/off of the selection transistor S 1  arranged between the unit cell string and the bit line BL. 
   The present invention uses an active bit line for reducing a loss of cell size caused by forming a bit line above an active area. The active bit line is also used for coupling active areas of cells. 
   When the active area is used as the bit line, data can be transmitted through the bit line, if all of the word lines WL 0  to WL 15  are selected. However, if all the word lines are selected, random accesses of memory cell become impossible. Therefore, for transmitting data without selecting all word lines, the active bit line must be formed with the depletion mode transistor on the active area of each unit cell. The depletion mode transistor always maintains ON state. By implementing the depletion mode transistor&#39;s characteristic, the data passed to the bit line can be transmitted to a sense amplifier in case of “high” by selecting the word line or “low” by not selecting the word line. 
   By implementing the depletion mode transistor, as mentioned above, a bit line contact is no more necessary to be formed in each unit cell. Therefore, an area occupied by the bit line contact, which is required at a contact design rule, can be eliminated. Furthermore, the bit line and cell plate line also are formed in parallel, therefore, an area occupied by a cell plate driver can be eliminated too. As a result, it is possible to decrease a ward line delay. 
   Operations of the cell array of the ferroelectric memory shown in  FIG. 5  are explained as below. 
   In case of selecting a unit cell U, in a unit cell string STR 1 , a selection transistor S 1  is turned on by applying high state of a selection signal SWL 1  among the selection signals SWL 1  to SWLN. As the selection transistor S 1  is turned on, only the unit cell string STR 1  is selected among unit cell strings coupled to the same bit line BL. The above-mentioned operation is for maintaining the bit line in a ground state during bit line pre-charging. 
   Next, in case of stand-by mode, all word lines WL 0  to WL 15  are maintained as low. The word lines WL 0  to WL 15  are still maintained as low during bit line pre-charging. After pre-charging, a word line ‘WL 1 ’ is maintained as ‘High’ and any other word lines are maintained as ‘Low’. At this time, a cell plate line CP 1  is maintained as ‘High’ by selecting the cell plate line CP 1 . The depletion mode transistor ‘D’ and enhancement transistor ‘N’ becomes turned on in case that the word line ‘WL 1 ’ is high, therefore, data of selected ferroelectric capacitor is transmitted to a sense amplifier through the bit line. However, other enhancement transistors of unit cells N 0  and. N 2  to N 15  becomes turned off so data cannot be transmitted trough the bit line. After transmitting the data, the cell plate line CP 1  becomes ‘Low’ and the word line is off. 
   As mentioned above, the unit cell of the present invention is operated identically comparing with the conventional FeRAM or DRAM. That is, reading and writing methods are identical. The depletion mode transistor is used for forming bit line with the active area. 
   Processing steps of implementing the cell array of the present invention is very similar to convention processing step, therefore, detailed explanation of processing step is omitted. The depletion mode transistor and the enhancement mode transistor can be formed by exposing each of areas for depletion mode transistor and enhancement mode transistor and then implementing ion to the exposed areas. Another method produces the cell array by exposing each of areas for depletion mode transistor and enhancement mode transistor and performing implementation of ion for the depletion mode transistor first. After implementing ion for the deletion mode transistor, the area of the enhancement mode transistor is exposed and compensation ion implementation is performed for enhancement mode transistor. 
   As mentioned above, the present invention decreases the capacitance of the bit line shared within one cell by additionally equipping the selection transistor when the cell unit of the ferroelectric memory is being operated by additionally equipping the selection transistor. The bit line capacitance includes junction capacitances related to the unit cell string, a parasitic capacitance, a capacitance of a bit line contact area and a capacitance contained in the sense amplifier. As above mentioned, capacitances of the bit line can be decreased by the present invention. 
   By decreasing capacitance of the bit line, a length of unit cell string can be extended. That is, the number of unit cell string is conventionally limited as 16 to 32, however, in the present invention, the number of unit cell strings can be extended to a range of 64 to 256 by considering characteristics of ferroelectric or CMOS transistor. 
   Therefore, in case of the ferroelectric memory, an amount of bit line capacitance needs to be controlled for gaining optimum sense margin. That is, the capacitance of the bit line needs be controlled according to the number of cells coupled to the bit line for maintaining appropriately the value of the capacitance. 
   As mentioned above, the present invention can decrease a size of unit cell by eliminating the bit line contact in the unit cells. Also, the present invention can reduce the bit line capacitance by controlling the selection transistor. Therefore, the present invention can control the optimum bit line capacitance, thereby gaining the maximum sense margin. 
   And, since the present invention uses the active area as the bit line, a contact is not required to be formed in each unit cell and only line formed by an active area is required. Therefore, the present invention also decrease a size of memory system by eliminating the bit line contact required in a contact design rule. 
   Moreover, the present invention can decrease a delay caused by the word line by eliminating an area used as a cell plate driver since the bit line and the cell plate line are formed in parallel. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.