Patent Document

RELATED APPLICATIONS 
     The present invention is a divisional of U.S. patent application Ser. No. 10/095,901 filed Mar. 11, 2002, now U.S. Pat. No. 6,661,042, and is related to commonly owned, co-filed U.S. patent application Ser. No. 10/095,984 filed Mar. 11, 2002, now U.S. Pat. No. 6,686,624. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dynamic random access memory (DRAM) cell, as well as methods for operating and fabricating a DRAM cell. More specifically, the present invention relates to a one-transistor floating-body DRAM cell formed using a process compatible with a bulk CMOS process, wherein charge is stored inside an electrically isolated body region underneath the transistor channel region. 
     2. Related Art 
     conventional one-transistor, one-capacitor (1T/1C) DRAM cells require a complex process for fabrication. Moreover, significant area is required to form the capacitor needed for storage of signal charge. Recently, one-transistor, floating-body (1T/FB) DRAM cells using partially-depleted silicon-on-insulator (PD-SOI) processes have been proposed, in which a signal charge is stored inside a floating body region, which modulates the threshold voltage (V T ) of the transistor. As a result, the separate capacitor of a 1T/FB DRAM cell can be eliminated, thereby resulting in reduced cell area and higher density. Periodic refresh operations are still required for these 1T/FB DRAM cells to counteract the loss of stored charge through junction leakage, gate tunneling leakage and access-induced hot-carrier injections (HCI). 
       FIG. 1  is a cross-sectional view of a conventional 1T/FB DRAM cell  100  fabricated using a PD-SOI process. DRAM cell  100  includes silicon substrate  101 , buried oxide layer  102 , oxide regions  103 - 104 , N++ type source and drain regions  105 - 106 , N+ type source and drain regions  107 - 108 , P type floating body region  109 , gate oxide  110 , gate electrode  111  and sidewall spacers  112 - 113 . Floating body  109  is isolated by gate oxide  110 , buried oxide layer  102  and the source and drain depletion regions  107 ′ and  108 ′. The partially-depleted floating body  109  is used for storing signal charges that modulate the threshold voltage (V T ) of DRAM transistor  100  differently when storing different amount of charge. The source node  105  is typically grounded. 
     A logic “1” data bit is written into DRAM cell  100  by biasing drain node  106  at a high voltage and gate node  111  at a mid-level voltage to induce hot-carrier injection (HCI), whereby hot-holes are injected into floating body node  109 , thereby raising the voltage level of floating body node  109 , and lowering the threshold voltage (V T ) of cell  100 . Conversely, a logic “0” data bit is written into DRAM cell  100  by biasing drain node  106  to a negative voltage while gate node  111  is biased at a mid-level voltage, thereby forward biasing the floating body-to-drain junction and removing holes from floating body  109 , thereby raising the threshold voltage (V T ) of cell  100 . 
     A read operation is performed by applying mid-level voltages to both drain node  106  and gate node  111  (while source node  105  remains grounded). Under these conditions, a relatively large drain-to-source current will flow if DRAM cell  100  stores a logic “1” data bit, and a relatively small drain-to source current will flow if DRAM cell  100  stores a logic “0” data bit. The level of the drain-to-source current is compared with the current through a reference cell to determine the difference between a logic “0” and a logic “1” data bit. Non-selected DRAM cells in the same array as DRAM cell  100  have their gate nodes biased to a negative voltage to minimize leakage currents and disturbances from read and write operations. 
     One significant disadvantage of conventional 1T/FB DRAM cell  100  is that it requires the use of partially depleted silicon-on-insulator (PD-SOI) process, which is relatively expensive and not widely available. In addition, the floating body effect of the SOI process, although utilized in the 1T/FB DRAM cell advantageously, complicates circuit and logic designs significantly and often requires costly substrate connections to eliminate undesired floating body nodes not located in the 1T/FB DRAM cells. Further, with a PD-SOI process, the device leakage characteristics can be difficult to control due to the lack of effective back-gate control of the bottom interface of the silicon layer that includes silicon regions  107 - 109 . 
     Conventional 1T/FB DRAM cells are described in more detail in “A Capacitor-less 1T-DRAM Cell,” S. Okhonin et al, pp. 85-87, IEEE Electron Device Letters, Vol. 23, No. 2, February 2002, and “Memory Design Using One-Transistor Gain Cell on SOI,” T. Ohsawa et al, pp. 152-153, Tech. Digest, 2002 IEEE International Solid-State Circuits Conference, February 2002. 
     Therefore, one object of the present invention is to provide a 1T/FB DRAM cell that is compatible with a conventional bulk CMOS process, and is compatible with conventional logic processes and conventional logic designs. 
     It is another object of the present invention to provide an electrical isolation junction that can be biased advantageously to minimize sidewall junction leakage and vertical parasitic bipolar leakage currents. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a one-transistor, floating-body (1T/FB) dynamic random access memory (DRAM) cell that includes a field-effect transistor fabricated in a semiconductor substrate using a process compatible with a conventional bulk CMOS process. The field-effect transistor includes a source region and a drain region of a first conductivity type and a floating body region of a second conductivity type, opposite the first conductivity type, located between the source region and the drain region. 
     In accordance with the present invention, a buried region of the first conductivity type is located under the source region, drain region and floating body region. The buried region helps to form a depletion region, which is located between the buried region and the source region, the drain region and the floating body region. The floating body region is thereby isolated by the depletion region. 
     A bias voltage can be applied to the buried region, thereby limiting leakage currents in the 1T/FB DRAM cell. An adjacent well region having the first conductivity type can be used to contact the buried region, thereby enabling the bias voltage to be applied to the buried region via the well region. Alternately, the buried region can be coupled to an underlying deep well region having the first conductivity type, which in turn can be coupled to an adjacent well region of the first conductivity type. In this configuration, a bias voltage can be applied to the buried region via the well region and the deep well region. 
     If the field-effect transistor is an NMOS transistor (i.e., the first conductivity type is N-type), then the bias voltage can be selected to have a nominal voltage between −0.5V to and a V CC  supply voltage. Conversely, if the field-effect transistor is a PMOS transistor (i.e., the first conductivity type is p-type), then the bias voltage can be selected to have a nominal voltage between 0 Volts and a V CC  supply voltage plus 0.5 Volts. In an alternate embodiment, the buried region can be left in a floating state. 
     If the field-effect transistor is an NMOS transistor, a logic “1” data bit is written to the 1T/FB DRAM cell using a hot carrier injection mechanism, and a logic “0” data bit is written to the 1T/FB DRAM cell using a junction forward bias mechanism. 
     In a particular embodiment, the 1T/FB DRAM cell of the present invention includes one or more shallow trench isolation (STI) regions, each having a bottom surface. The STI regions are located adjacent to the source and drain regions. The buried region is formed such that a top interface of the buried region is located at or above the bottom surfaces of the STI regions (but below the source and drain regions of the cell). The buried region is also formed such that a bottom interface of the buried region is located below the bottom surfaces of the STI regions. 
     The present invention also includes a method of fabricating a one-transistor, floating-body (1T/FB) dynamic random access memory (DRAM) cell. This method includes forming a buried region having a first conductivity type below the upper surface of a semiconductor region of a semiconductor substrate, the semiconductor region having a second conductivity type, opposite the first conductivity type. After the buried region has been formed, a field-effect transistor is formed in the semiconductor region over the buried region using conventional CMOS processing steps. The buried region results in the formation of a depletion region between the buried region and source, drain and body regions of the field-effect transistor. 
     In a particular embodiment, the buried region is formed by an ion implantation step, which is performed through a first mask. A threshold voltage adjustment implant for the field-effect transistor can also be performed through the first mask. 
     The method can also include forming a well region having the first conductivity type in the semiconductor substrate, wherein the buried region contacts the well region. Alternately, the method can include forming a deep well region having the first conductivity type in the semiconductor substrate, wherein the deep well region is located below and continuous with the buried region. 
     In accordance with another embodiment, a plurality of the 1T/FB DRAM cells of the present invention can be arranged in an array. An area efficient array layout can be implemented, in which adjacent 1T/FB DRAM cells share a common drain region (and a common drain connection), with the depletion region providing adequate isolation between the two 1T-FB DRAM cells. 
     The present invention will be more fully understood in view of the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a conventional 1T/FB DRAM cell fabricated using a PD-SOI process. 
         FIG. 2  is a cross-sectional view of a 1T/FB DRAM cell fabricated in a manner compatible with a bulk CMOS process, in accordance with one embodiment of the present invention. 
         FIG. 3  is a circuit diagram of the 1T/FB DRAM cell of FIG.  2 . 
         FIGS. 4A-4D  are cross sectional views illustrating the manner in which the 1T/FB DRAM cell of  FIG. 2  can be fabricated in a manner compatible with a bulk CMOS process. 
         FIG. 5  is a cross-sectional view of a 1T/FB DRAM cell fabricated in a manner compatible with a triple-well CMOS process, in accordance with another embodiment of the present invention. 
         FIG. 6  is a layout diagram of a repeatable array of 1T/FB DRAM cells, including the 1T/FB DRAM cell of  FIG. 2 , in accordance with one embodiment of the present invention. 
         FIG. 7A  is a cross-sectional view of a 1T/FB DRAM cell along section line A—A of FIG.  6 . 
         FIG. 7B  is a cross-sectional view of a 1T/FB DRAM cell along section line B—B of FIG.  6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a cross-sectional view of an NMOS 1T/FB DRAM cell  200  in accordance with one embodiment of the present invention. Although the present embodiment describes a 1T/FB DRAM cell that uses an NMOS transistor, it is understood that either NMOS or PMOS transistors can be used to form 1T/FB DRAM cells in accordance with the present invention. When a PMOS transistor is used to implement the 1T/FB DRAM cell, the conductivity types of the various elements are reversed. 
     DRAM cell  200  includes P− type silicon substrate  201 , N− type buried region (or back-gate)  202 , depletion regions  203 - 204 , shallow trench isolation (STI) regions  205 , heavily-doped N++ type source and drain regions  206  and  207 , lightly-doped N+ type source and drain regions  208  and  209 , P type floating body region  210 , gate oxide layer  211 , gate electrode  215  and sidewall spacers  221 - 222 . 
     N++ type source region and N+ type source region combine to form n-type source region  211 . Similarly, N++ type drain region and N+ type drain region combine to form n-type drain region  212 . N− type buried region  202  is formed below the transistor as a back-gate node. Under proper bias conditions, depletion region  204  completely isolates the floating body region  210  of 1T/FB DRAM cell  200 . 
       FIG. 3  is a circuit diagram of the 1T/FB DRAM cell  200 . Gate electrode  215  of DRAM cell  200  is connected to a word line WL, drain  212  is connected to a bit line BL and source  211  is connected to a source line SL. The p-type floating body region  210  underneath the channel region is capacitively coupled to the n-type source region  211  through the parasitic capacitance PC 1  of the corresponding PN junction. Similarly, floating body region  210  is capacitively coupled to n-type drain region  212  through the parasitic capacitance PC 2  of the corresponding PN junction. Finally, floating body region  210  is capacitively coupled to buried back-gate region  202  through the parasitic capacitance PC 3  of the corresponding PN junction. 
     1T/FB DRAM cell  200  operates as follows. Source region  211  is maintained at a ground voltage level (0 Volts). Buried back-gate region  202  is biased at a voltage around the mid-point of a high drain voltage (V CC , or 1.2 Volts) and a low drain voltage (−1.0 Volts) to minimize leakage current from parasitic bipolar actions. In a particular embodiment, buried back-gate region  202  is biased at a ground voltage level (0 Volts). The bias level of buried back-gate region  202  can be adjusted to ensure the junction depletion region  204  beneath source  211  and drain  212  completely isolates floating body region  210 , without creating a direct leakage current path from source  211  or drain  212  to back-gate region  202 . 
     A logic “1” data bit is written into DRAM cell  200  by biasing n-type drain region  212  at a logic high voltage of about 1.2 Volts, and gate electrode  215  at a mid-level voltage of about 0.6 Volts, thereby inducing hot-carrier injection (HCI). Under these conditions, hot-holes are injected into p-type floating body region  210 , thereby raising the voltage level of floating body region  210 , and lowering the threshold voltage (V T ) of DRAM cell  200 . 
     Conversely, a logic “0” data bit is written into DRAM cell  200  by biasing n-type drain region  212  to a negative voltage of about −1.0 Volts, while gate electrode  215  is biased at a mid-level voltage of about 0.6 Volts. Under these conditions the PN junction from p-type floating body region  210  to n-type drain region  212  is forward biased, thereby removing holes from floating body region  210 . After a logic “0” data bit has been written, DRAM cell  200  exhibits a relatively high threshold voltage (V T ). 
     A read operation is performed by applying a mid-level voltage of about 0.6 Volts to both drain region  212  and gate electrode  215  (while source region  211  and back-gate region  202  remain grounded). Under these conditions, a relatively large drain-to-source current will flow if DRAM cell  200  stores a logic “0” data bit, and a relatively small drain-to source current will flow if DRAM cell  200  stores a logic “1” data bit. The level of the drain-to-source current is compared with the current through a reference cell to determine the difference between a logic “0” and a logic “1” data bit. Non-selected cells in the same array as 1T/FB DRAM cell  200  have their gate electrodes biased to a negative voltage to minimize leakage currents and disturbances from read and write operations. 
       FIGS. 4A-4D  are cross sectional views illustrating the manner in which 1T/FB DRAM cell  200  can be fabricated using a process compatible with a bulk CMOS process. 
     As illustrated in  FIG. 4A , an n-type well region  401  is formed in a p-type monocrystalline silicon substrate  201 . N-well  401  is formed in accordance with conventional CMOS processing steps. For example, N-well  401  can be fabricated by ion implantation. Various crystal orientations and concentrations can be used in various embodiments of the invention. In addition, the conductivity types of the various regions can be reversed in other embodiments with similar results. 
     In the described embodiment, STI regions  205  are formed using shallow trench isolation (STI) techniques. In STI techniques, trenches are etched in silicon substrate  201 , and these trenches are then filled with silicon oxide. The upper surface of the resulting structure is then planarized, such that the upper surfaces of STI regions  205  are substantially co-planar with the upper surface of substrate  201 . In the described, STI regions  205  have a depth of about 4000 Angstroms. It is understood that this depth is used for purposes of description, and is not intended to limit the invention to this particular depth. Substrate  201  includes p-type region  402  located between STI regions  205  as illustrated. P-type region  402  can be a region of substrate  201 , or a conventional P-well region. 
     As illustrated in  FIG. 4B , a photoresist mask  405  is formed over the upper surface of substrate  201  at locations where 1T/FB DRAM cells are not to be formed. For example, photoresist mask  405  is formed over locations (not shown) where conventional CMOS transistors are to be formed in substrate  201 . Such conventional CMOS transistors can include transistors used for controlling the accessing of the 1T/FB DRAM cells. 
     A high-energy n-type ion implantation is performed through photoresist mask  405  into the cell array area to form n-type buried region  202  (FIG.  4 B). In the described example, n-type buried region  202  extends into N-well region  401 . The depth of n-type buried region  202  is chosen so that the bottom interface of this region  202  is below the depth of STI regions  205 , and the top interface of this region  202  is at or above the depth of STI regions  205  and below the depth of the subsequently formed source and drain junctions  211 - 212 . In the described embodiment, the bottom interface of region  202  is located about 6000 to 8000 Angstroms below the upper surface of substrate  201 , and the top interface of region  202  is located about 3000 to 4000 Angstroms below the upper surface of substrate  201 . Thus, the bottom interface of region  202  is about 2000 to 4000 Angstroms below the depth of STI regions  205 , and the top interface of region  202  is about 0 to 1000 Angstroms above the depth of STI regions  205 . In an alternate embodiment, the top interface of buried region  202  can be located below the depth of STI regions  205 , as long as the associated depletion region  204  is located above the depth of STI regions  205 . 
     The formation of n-type buried region  202  results in the presence of adjacent depletion regions  203  and  406 , as illustrated. (Note that the formation of N-well  401  also contributes to the presence of depletion region  203 .) 
     After n-type buried region  202  has been implanted, an additional ion implantation step can be performed through photoresist mask  405  to adjust the threshold voltage of DRAM cell  200 , without introducing additional process complexity or cost. 
     The process steps illustrated in  FIGS. 4C-4D  are conventional CMOS processing steps. As illustrated in  FIG. 4C , gate dielectric layer  211  is formed over the upper surface of the resulting structure. In the described embodiment, gate dielectric layer  211  has an equivalent silicon oxide thickness in the range of about 2 to 4 nm. However, this thickness can vary depending on the process being used. 
     A layer of gate material, such as polycrystalline silicon, is deposited over the resulting structure. This layer of gate material is then patterned to form gate electrode  215 . An N+ implant mask (not shown) is then formed to define the locations of the desired N+ LDD regions on the chip. An N+ implant step is then performed through the N+ implant mask. The implantation is self-aligned with the edges of polysilicon gate electrode  215 . The N+ implant step forms N+ source region  208 , N+ drain region  209  and N+ contact region  409 . Note that N+ source and drain regions  208 - 209  result in an adjacent depletion region. The depletion region between N+ source and drain regions  208 - 209  and N− buried region  202  is labeled as element  407  in FIG.  4 C. 
     As illustrated in  FIG. 4D , dielectric sidewall spacers  221 - 222  are formed adjacent to gate electrode  215  using conventional processing steps. For example, sidewall spacers  221 - 222  can be formed by depositing one or more layers of silicon oxide and/or silicon nitride over the resulting structure and then performing an anistotropic etch-back step. 
     After sidewall spacers  221 - 222  have been formed, an N++ photoresist mask (not shown) is formed to define the locations of the desired N++ regions on the chip. An N++ type ion implant is then performed, thereby forming N++ source region  206 , N++ drain region  207  and N++ contact region  410 . N++ source and drain regions  206 - 207  are aligned with the edges of sidewall spacers  221 - 222 , respectively. 
     Note that the formation of N++ source and drain regions  206 - 207  result in the formation of source and drain regions  211 - 212  and depletion region  204 . P-type floating body region  210  remains in substrate  201  as illustrated in FIG.  4 D. The back-gate bias voltage V BG  is applied to buried back-gate region  202  via N++ contact region  410  and N-well  401 . 
     In an alternate embodiment, a process compatible with a conventional triple-well CMOS process is used to fabricate 1T/FB DRAM cell  200 .  FIG. 5  illustrates a triple-well embodiment, wherein similar elements in  FIGS. 4D and 5  are labeled with similar reference numbers.  FIG. 5  shows a deep N-well region  501 , which is formed beneath buried back-gate region  202 . DRAM cell  200  is formed inside a P-well above the deep N-well region  501 . Buried back-gate region  202  is formed so that the bottom interface of this region  202  is in contact with deep N-well region  501 , and the top interface of region  202  is above the depth of STI regions  205 . 
       FIG. 6  is a layout diagram of a repeatable array  600  of 1T/FB DRAM cells, including 1T/FB DRAM cell  200 .  FIG. 7A  is a cross-sectional view of DRAM cell  200  along section line A—A of FIG.  6 .  FIG. 7B  is a cross-sectional view of DRAM cell  200  along section line B—B of FIG.  6 . Similar elements in  FIGS. 2 ,  6 ,  7 A and  7 B are labeled with similar reference numbers. Thus, the reference number  215  is used to identify gate electrodes in  FIGS. 2 ,  6 ,  7 A and  7 B. Note that drain contacts  209  are illustrated in  FIGS. 6 and 7A . 
     As illustrated in  FIGS. 6 and 7A , drain regions of adjacent DRAM cells are formed as continuous regions. A single drain contact  209  is used to provide connections to adjacent drain regions in array  600 , advantageously reducing the required layout area of array  600 . By biasing buried back-gate region  202  in the manner described above, depletion region  204  provides adequate isolation between the adjacent DRAM cells sharing the same drain region  212 . Because STI regions are not required between these adjacent DRAM cells, the layout area of the array  600  can be made relatively small. 
     Although the invention has been described in connection with several embodiments, 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. Thus, the invention is limited only by the following claims.

Technology Category: h