Abstract:
A two-dimensional array of single polysilicon DRAM cells is disclosed. The array comprises a plurality of DRAM cells arranged in a two-dimensional matrix, wherein each of the DRAM cells comprises: a deep n-well in a silicon substrate; a p-well within the deep n-well; a gate structure over and straddling the deep n-well and the p-well; and a n +  region within the p-well and adjacent to a sidewall of the gate structure. The array is connected together by a plurality of column bitlines, each of the column bitlines connected to the n +  regions of all of the DRAM cells that are in a common column. Further, a plurality of row wordlines are provided, each of the row wordlines connected to the gate structures of all of the DRAM cells that are in a common row.

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. patent application Ser. No. 09/170,863 filed Oct. 13, 1998 entitled “Single Polysilicon DRAM Cell with Current Gain”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to DRAM memory cells, and more particularly, to a DRAM memory cell that can be made with a single polysilicon layer and with current gain. 
     BACKGROUND OF THE INVENTION 
     The most commonly used DRAM cell structure is the one transistor/one capacitor cell. This DRAM cell structure typically requires the deposition of three layers of conductive polysilicon: one layer for the gate of the transistor, one layer for the bottom storage node of the capacitor, and a third layer for the top storage node of the capacitor. The relatively complex process required to form modern DRAM cells causes practical incompatibility with standard logic processes that typically use only a single polysilicon layer. 
     Nevertheless, with the trend towards “system-on-a-chip” devices where memory and logic are placed onto a single chip, it is important to develop a DRAM cell structure that will be compatible with logic. There have been prior art attempts to design a DRAM cell structure that can store information without the benefit of a capacitor. An example of such a DRAM cell is disclosed in “A Novel Merged Gain Cell for Logic Compatible High Density DRAMs,” by Mukai et al., Symposium on VLSI Technology Digest of Technical Papers, 1997, at page 155. The DRAM cell disclosed in the Mukai et al. reference shows a single transistor structure that uses n +  and p +  regions formed in p-well and n-wells, respectively. Although this proposed DRAM cell design does address some of the problems of embedded DRAM design, the DRAM cell design proposed by the Mukai et al. reference requires very precise manufacturing processes to ensure that the DRAM cell will operate correctly. In addition, the fabrication process is still relatively complicated. 
     What is needed is a new design for a DRAM cell that can be used in embedded logic applications. 
     SUMMARY OF THE INVENTION 
     A two-dimensional array of single polysilicon DRAM cells is disclosed. The array comprises a plurality of DRAM cells arranged in a two-dimensional matrix, wherein each of the DRAM cells comprises: a deep n-well in a silicon substrate; a p-well within said deep n-well; a gate structure over and straddling said deep n-well and said p-well, said gate structure being a stack of a thin gate oxide layer and a conductive layer; and a n +  region within said p-well and adjacent to a sidewall of said gate structure. The array is connected together by a plurality of column bitlines, each of the column bitlines connected to the n +  regions of all of the DRAM cells that are in a common column. Further, a plurality of row wordlines are provided, each of the row wordlines connected to the gate structures of all of the DRAM cells that are in a common row. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIGS. 1-4 illustrate in schematic cross-section the steps in forming a DRAM cell in accordance with the present invention; 
     FIG. 5 illustrates in schematic form a completed DRAM cell in accordance with the present invention; 
     FIG. 6 illustrates an electrical schematic of a row of DRAM cells during the reset operation; 
     FIG. 7 illustrates a top view of an integrated circuit for the row of DRAM cells of FIG. 6; 
     FIG. 8 illustrates a DRAM cell in accordance with an alternate embodiment of the present invention 
     FIG. 9 illustrates a top view of a row of DRAM cells of FIG. 8; 
     FIG. 10 illustrates a cross section view taken along line B-B′ of FIG. 9; and 
     FIG. 11 is a electrical schematic of an array of DRAM cells formed from the cells of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description includes two embodiments of a DRAM cell. The first embodiment is described in conjunction with FIGS. 1-7 and the second embodiment is described in conjunction with FIGS. 8-11. 
     First Embodiment: 
     The method of manufacturing a first embodiment of the DRAM cell of the present invention is described in conjunction with FIGS. 1-4. The operation of the DRAM cell is described in conjunction with FIGS. 5-7. 
     Turning to FIG. 1, a silicon substrate (preferably p-type having a &lt;100&gt; orientation) is provided for formation of the DRAM cell in accordance with the present invention. Field oxide isolation regions (FOX)  101  are formed using conventional masking and oxidation steps. For example, the FOX regions  101  can be formed via photolithography and dry etching steps to etch a silicon nitride-silicon dioxide composition layer. After the photoresist is removed and the substrate wet cleaned, the FOX regions  101  are grown in an oxygen-steam environment, at a temperature between 850-1050° C., to a thickness of about 4000-6000 angstroms. 
     After the creation of the FOX regions  101 , the silicon nitride-silicon dioxide composition layer is removed by using hot phosphoric acid solution for the silicon-nitride layer, and a buffered hydrofluoric acid solution for the silicon-dioxide. The area between the FOX regions  101  is referred to herein as the “active area” and is the location where the DRAM cell is formed. 
     Next, a deep n-well  103  is formed in the substrate under the active area by conventional masking, ion implantation, and drive-in. The deep n-well  103  preferably has a depth of 3 microns and has a dopant concentration of approximately 10 16 /cm 3 . Next, a p-well  105  is formed wholly within the deep n-well  103 . The p-well  105  is formed using conventional masking and ion implantation techniques. Preferably, the p-well  105  has a depth of 1.5 microns and has a dopant concentration of 10 17 /cm 3 . 
     Next, a gate structure  107  is formed in the active region between the FOX  101 . The gate structure  107  is of conventional design and comprises a gate oxide layer  109  and a conductive polysilicon layer  111 . The gate structure  107  straddles the lateral termination of the p-well  105  in the deep n-well  103 . In other words, the p-well  105  extends under and terminates under the gate structure  107 . 
     As noted above, the gate structure  107  is of conventional design and can be easily formed by the deposition or growth of the thin gate oxide layer  109  followed by the CVD deposition of a polysilicon layer  111 . Preferably, the thin gate oxide layer  109  is under 100 angstroms in thickness to take advantage of the GIDL current effect. Next, conventional photolithography and etching is used to form the gate structure  107 . The resultant structure is seen in FIG.  2 . 
     Turning to FIG. 3, a further ion implantation is performed to form an n +  region  113  wholly within the p-well  105 . The n +  region  113  is formed between the FOX  101  and one edge of the gate structure  107 . Indeed, the n +  region  113  is self-aligned to the gate structure  107  and the FOX  101 . The formation of the n +  region  113  can be performed by conventional photolithography masking and ion implantation. Preferably, the n +  region  113  has a depth of 0.4 microns with a dopant concentration of 10 20 /cm 3 . 
     Turning to FIG. 4, sidewall spacers  115  are formed on the sidewalls of the gate structure  107 . The sidewall spacers  115  are preferably formed from an oxide and can be formed using conventional techniques such as the deposition of an oxide layer followed by an isotropic etching back process. It can be appreciated that the formation of sidewall spacers  115  is a well known technique. 
     Finally, turning to FIG. 5, the DRAM cell is completed by forming various interconnections with the cell to signal lines or metal interconnects. In particular, a bit line  121  is connected to the n +  region  113 . A word line  123  is connected to the gate structure  107 . A reset p +  junction  124  is connected to the p-well  125  via a transistor switch  127 . Finally, a Vcc line  125  is connected to the deep n-well  103 . It should be noted that the various connection lines are only shown in electrical schematic. However, it can be appreciated that the physical formation of the lines would require the deposition of an insulative oxide layer, followed by contact etching, and finally metal filling of the contact holes and formation of the metal interconnect structure. Notice that the above fabrication process is the same as a typical CMOS logic flow with only a single extra masking step for the n +  region  113  implant. 
     The operation of the present invention will now be described in conjunction with FIG.  5 . The DRAM cell of the present invention takes advantage of the phenomena known as gate-induced drain leakage (GIDL) current. The GIDL current typically occurs in thin gate oxide MOS devices and is current between the drain and the substrate. The basis of the GIDL current is band-to-band tunneling that occurs on the surface of the gate-to-drain overlap region. Additional information on GIDL current may be found in “Design for Suppression of Gate-Induced Drain Leakage in LDD MOSFET&#39;s Using a Quasi-2-Dimensional Analytical Model,” by Parke et al., IEEE Transactions on Electron Devices, Vol. 39, No. 7, July 1992, pp. 694-1702. In that article, the authors explain that an n +  region underneath a gate edge produces a high vertical electrical field that results in hole generation on the surface of an n+ region underneath the gate by band-to-band tunneling in the device. 
     Returning to FIG. 5, note that the n +  region  113 , the p-well  105 , and the deep n-well  103  form an n-MOS transistor that is controlled by the word line  123  that straddles the deep n-well  103  and the p-well  105 . Digital information is stored in the DRAM cell as voltage potential in the p-well  105 . The voltage potential in the p-well  105  acts to modulate the threshold voltage (V t ) of the n-MOS transistor. 
     In order to write digital information into the DRAM cell, a two step method is used. First, referring to FIGS. 5-7, the DRAM cell is reset into the “0” state. The reset p +  junction  124  is biased to a negative potential, for example, −V cc . In the preferred embodiment, V cc  is 3.3 volts. 
     As seen in FIG. 7, the transistor switches  127  are implemented as p-channel MOS transistors formed between p-wells of individual DRAM cells. The first transistor switch  127   a  in a row is formed between the reset p +  junction  124  and the p-well of the first DRAM cell in a row. A reset line is operative to control the transistor switches  127 . 
     Typically, the threshold voltage of the transistor switches  127  is enhancement type (i.e. V tp  approximately −1.0 volts with no body bias) in a typical 0.35 micron CMOS technology. The gate of transistor switches  127  are biased low enough using the reset line (e.g. −V cc ) in order to turn on the p-channel switches  127 . It can be appreciated that the above is merely one possible implementation for biasing of the p-wells and other implementations are also possible. 
     FIG. 6 illustrates the reset operation in more detail, where three p-wells  105  for three DRAM cells aligned in a row. The transistor switches  127  are biased to the “on” position (e.g. −V cc ) during the reset step using the reset line. This allows each of the p-wells  105  to be biased by the reset p +  junction  124  which carries a voltage of −V cc . Because there is a voltage drop (V tp ) across the first transistor switch  127   a , the voltage of the first p-well  105  will be less than −V cc  by one V tp  value. Note also that the potential of the following p-wells  105  is the same since there is no V tp  drop on the following parasitic p-channel transistor switches  127 . Because −V cc  is nominally −3.3 volts and because a typical value for V tp  is 1.5 volts (with body bias), the voltage of the p-well  105  will be approximately −1.8 volts or approximated as −V cc /2. After the p-well  105  has reached a steady state voltage, the switches  127  are “opened” using the reset line (e.g. applying 0.0 volts to the reset line), allowing the potential of the p-well  105  to “float” at approximately −V cc /2. 
     Next, in the second step of the write operation, after the DRAM cell has been reset to the “0” state, selected DRAM cells in the DRAM array may be written to a “1” state by applying −V cc /2 on the word line and V cc /2 on the bit line. This causes GIDL current to flow into the p-well  105 , thereby changing the p-well  105  voltage from −V cc /2 to near the bit-line potential of V cc /2. 
     The bit lines of cells that are to remain in the 0 state are biased at 0 volts, such that there is not enough voltage drop from the gate  107  to the n +  region  113  for GIDL current generation. After this writing procedure, those DRAM cells that hold a 0 state have their p-well  105  at a potential −V cc /2 and those cells with a 1 state have their p-well  105  potential at V cc /2. 
     For the read operation, it should be noted that digital information of the 0 state and 1 state in each DRAM cell is represented by the p-well  105  bias at −V cc /2 or V cc /2, respectively. Therefore, the threshold voltage of the parasitic n-channel MOS transistor (n + /p-well/deep n-well) is either large, for example, V cc /2 or small, such as 0.5 volt, due to the “body bias effect” of the p-well potential at −V cc /2 and V cc /2, respectively. Therefore, under the bias of read operation where the bit line is set to Vcc/2 and the wordline is set to V cc , the cell current flowing from the deep n-well  103  through the channel to the bit line is either large or negligible as modulated by the p-well  105  bias. By measuring the amount of current flow, it can be determined the memory state of the DRAM cell. For example, if a large current flows, this indicates that the p-well is biased at V cc /2 and that the memory state is “1”. If a small current flows, this indicates that the p-well  105  is biased at −V cc /2 and that the memory state is “0”. Note also that the read operation is non-destructive, i.e., the charge in the p-well  105  is not consumed by the read operation. 
     In an alternative embodiment, different biasing voltages may be applied. For example, during the write function, the DRAM cell is reset into the “0” state by biasing the gate of the transistor switches  127  (using the reset line) to a negative potential of −(3/2)V cc , which is more negative than the reset p +  junction  124  biased at −V cc  by at least one V tp  value. Because the gate of transistor switches  127  are biased low enough, there is no V tp  drop across the first transistor switch  127   a  and therefore all p-wells are biased the same as the reset p +  junction  124  (i.e. −V cc ). After the p-well  105  has reached a steady state voltage, the switch  127  is “opened” by applying 0.0 volts to the reset line, allowing the potential of the p-well  105  to “float” at approximately −V cc . 
     Next, in the second step of the write operation, after the DRAM cell has been reset to the “0” state, selected DRAM cells in the DRAM array may be written to a “1” state by applying −V cc  on the word line and 0 volts on the bit line. This causes GIDL current to flow into the p-well  105 , thereby changing the p-well  105  voltage from −V cc  to near 0 volts in this alternative embodiment. After the writing procedure, those DRAM cells that hold a 0 state have their p-well  105  at a potential −V cc  and those cells with a 1 state have their p-well  105  potential at near 0 volts. 
     For the read operation, it should be noted that digital information of the 0 state and 1 state in each DRAM cell is represented by the p-well  105  bias at −V cc  or 0 volts, respectively. Therefore, under the bias of read operation where the bit line is set to 0 volts and the wordline is set to V cc , the cell current flowing from the deep n-well  103  through the channel to the bit line is either large or negligible as modulated by the p-well  105  bias. By measuring the amount of current flow, it can be determined the memory state of the DRAM cell. Note also that the read operation is again non-destructive, i.e., the charge in the p-well  105  is not consumed by the read operation. 
     Second Embodiment: 
     The DRAM cell described above can be further improved so that the p-wells of each cell can be reset to −V,. by simply biasing the n +  regions  113  (the bitlines) to −V cc , so that the p-well  105  potential is clamped to −V cc . In this way, the reset line  124  and the parasitic pMOS transistors  127  can be eliminated. This results in a smaller cell size. 
     FIGS. 8-10 show various views of the DRAM cell array of this alternate embodiment. FIG. 8 shows in cross section a single DRAM cell of this alternate embodiment. This embodiment is substantially similar to the embodiment shown in FIG. 5 except that shallow trench isolations (STI)  801  are used to isolate the cells instead of a field oxide (FOX). 
     A deep n-well  803  is formed in the substrate under the active area by conventional masking, ion implantation, and drive-in. The deep n-well  803  preferably has a depth of 3 microns and has a dopant concentration of approximately 10 16 /cm 3 . Next, a p-well  805  is formed wholly within the deep n-well  803 . The p-well  805  is formed using conventional masking and ion implantation techniques. Preferably, the p-well  805  has a depth of 1.5 microns and has a dopant concentration of 10 17 /cm 3 . 
     Next, a gate structure  807  is formed in the active region between the STIs  801 . The gate structure  807  is of conventional design and comprises a gate oxide layer  809  and a conductive polysilicon layer. The gate structure  807  straddles the lateral termination of the p-well  805  in the deep n-well  803 . In other words, the p-well  805  extends under and terminates under the gate structure  807 . 
     A further ion implantation is performed to form an n +  region  813  wholly within the p-well  805 . The n +  region  813  is formed between the STIs  801  and one edge of the gate structure  807 . Indeed, the n +  region  813  is self-aligned to the gate structure  807 . The formation of the n +  region  813  can be performed by conventional photolithography masking and ion implantation. Preferably, the n +  region  813  has a depth of 0.4 microns with a dopant concentration of 10 20 /cm 3 . 
     Next, sidewall spacers  815  are formed on the sidewalls of the gate structure  807 . The sidewall spacers  815  are preferably formed from an oxide and can be formed using conventional techniques such as the deposition of an oxide layer followed by an isotropic etching back process. 
     Next, a bit line  821  is connected to the n +  region  813  and a word line  823  is connected to the gate structure  807 . Finally, a V cc  line  825  is connected to the deep n-well  803 . It should be noted that the various connection lines are only shown in electrical schematic. However, it can be appreciated that the physical formation of the lines would require the deposition of an insulative oxide layer, followed by contact etching, and finally metal filling of the contact holes and formation of the metal interconnect structure. Notice that the above fabrication process is the same as a typical CMOS logic flow with only a single extra masking step for the n +  region  813  implant. 
     A top view of a row of DRAM cells of FIG. 8 is shown in FIG.  9 . Note that FIG. 8 is a cross section taken along line A—A′ of FIG.  9 . Similarly, FIG. 10 is a cross section taken along line B—B′ of FIG.  9 . From FIG. 9, it is seen that the reset line  124  and the parasitic transistors  127  of the first embodiment are eliminated. 
     The cell operation is similar to the first embodiment, except for the reset  35  operation. Note that during all operations, the deep n-well is biased to V cc . The p-wells of each cell can be reset to −V cc  by simply biasing the bit-line to −V cc . This results in the p-well potential of all cells in the same bit-line to be pulled down to V cc  due to the forward-biased (p-well to n +  bitline) junction. In this way, the reset line and the parasitic p-well transistors of the first embodiment are eliminated, and the cell size is further reduced. Note that the reset operation of this embodiment resets all cells along the same bitline, while the reset operation in the first embodiment resets all cells along the same wordline. 
     The single cell write operation of the DRAM cell is a two step operation. First, the p-well of the cell to be written to is reset to −V cc  by simply biasing the bitline (V bit ) to −V cc  (i.e. the p-well potential is pulled down to −V cc  due to the forward biased junction). Then, if a “1” is to be written, the wordline V word  is biased to −V cc /2 and the bitline (V bit ) is biased to V cc /2. This results in holes on the n +  region  813  to flow into the p-well until the voltage in the p-well becomes approximately V cc /2. Note that with V word  equal to zero volts, the voltage potential in the p-well is unchanged. 
     The single cell read operation is the same as in the first embodiment of the DRAM cell. Digital information of the 0 state and 1 state in each DRAM cell is represented by the p-well  805  bias at −V cc  or V cc /2 volts, respectively. Therefore, under the bias of read operation where the bit line is set to between 0 to V cc /2 volts and the wordline is set to V cc , the cell current flowing from the deep n-well  103  through the channel to the bit line is either large or negligible as modulated by the p-well  805  bias. Note that in the first embodiment, the bitline was set to V cc /2 corresponding to p-well potentials of V cc /2 and −V cc /2. However, in the second embodiment described here, a larger p-well extreme potentials of −V cc  and V cc /2 are used. This allows the bitline bias set in the range of 0 to V cc /2 and still result in successful read operation. By measuring the amount of current flow, it can be determined the memory state of the DRAM cell. Note also that the read operation is again non-destructive, i.e., the charge in the p-well  805  is not consumed by the read operation. 
     An array of the DRAM cells can be seen in FIG.  11 . Note that the gate structures  807  of all cells in a row are connected to a row wordline. Similarly, the n +  regions  813  of all cells in a column are connected to a column bitline. Each of the cells p-wells  805  are left floating to store the digital information in the form of a body bias voltage. Finally, the common deep n-well to all of the cells are biased to V cc . 
     The array can be written to digital information in one of two schemes. First, the array can be written to bitline by bitline. This scheme is useful for the refreshing operation. Specifically, this scheme begins by writing a “0” to all cells in one column by applying a voltage V BLn  of −V cc  for the selected bitline and zero volts for all other bitlines. Next, a “1” or a “0” can be written to the individual cells in the column by applying a voltage V BLn  of V cc  to the selected bitline (V BLn  is zero volts for all other bitlines). The row wordlines voltages V WLm  are biased to −V cc /2 for writing a “1” and are biased to zero volts for writing a “0”. The above steps are repeated for each column bitline in the array. 
     In the second scheme, the full array can be reset before writing. This scheme is useful for fast page write operation. Specifically, this scheme begins by writing a “0” to all cells in all columns by applying a voltage V BLn  of −V cc  for all of the column bitlines. Next, individual column bitlines in the array are sequentially processed. A “1” or a “0” can be written to the individual cells in the column by applying a voltage V BLn  of V cc /2 to the current selected bitline (V BLn  is zero volts for all other bitlines). The row wordlines voltages V WLm  are biased to −V cc /2 for writing a “1” and are biased to zero volts for writing a “0”. The writing step is repeated for each column bitline in the array. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. For example, the polarity of the silicon can be reversed. The n +  well can be replaced with a p+well and the p-well can be replaced with a n-well.