Abstract:
A SRAM memory cell including an access device formed on a storage device is described. The storage device has at least two stable states that may be used to store information. In operation, the access device is switched ON to allow a signal representing data to be coupled to the storage device. The storage device switches to a state representative of the signal and maintains this state after the access device is switched OFF. When the access device is switched ON, the state of the storage device may be sensed to read the data stored in the storage device. The memory cell may be formed to be unusually compact and has a reduced power supply requirements compared to conventional SRAM memory cells. As a result, a compact and robust SRAM having reduced standby power requirements is realized.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of pending U.S. patent application Ser. No. 09/757,683, filed Jan. 8, 2001, now U.S. Pat. No. 6,489,192 which is a divisional of U.S. patent application Ser. No. 09/268,823, filed Mar. 16, 1999, issued Nov. 6, 2001 as U.S. Pat. No. 6,313,490. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to memory circuits and in particular to improved static random access memory cells. 
     BACKGROUND OF THE INVENTION 
     Random access memory (“RAM”) cell densities have increased dramatically with each generation of new designs and have served as one of the principal technology drivers for ultra large scale integration (“ULSI”) in integrated circuit (“IC”) manufacturing. The area required for each memory cell in a memory array partially determines the capacity of a memory IC This area is a function of the number of elements in each memory cell and the size of each of the elements. State-of-the-art memory cells for gigabit memory ICs using dynamic RAM (“DRAM”) technology have cell areas approaching six minimum feature dimensions squared, or 6F 2 , where F represents a minimum feature size for photolithographically-defined features. Static RAM (“SRAM”) densities, while increasing less dramatically than densities for DRAM technologies, have nevertheless also increased substantially. 
     A traditional six-device SRAM cell contains a pair of cross-coupled inverters, forming a latch circuit having two stable states. The minimum memory cell size attainable for this type of SRAM is approximately 120F 2 , as described in “CMOS Technology for 1.8V and Beyond,” by Jack Y.-C. Sun, 1997 Int. Symp. on VLSI Tech., Syst. and Apps., Digest of Tech. Papers, pp. 293-297. Achieving further size reduction requires a new mechanism of memory cell operation. 
     Tunnel diodes have also been employed to provide negative differential resistance for SRAM cell operation. U.S. Pat. No. 5,390,145, entitled “Resonance Tunnel Diode Memory”, issued to Nakasha et al., describes a memory cell using pairs of GaAs tunnel diodes coupled in series and providing memory cells having an area of about 30F 2 . “RTD-HFET Low Standby Power SRAM Gain Cell”, IEEE El. Dev. Lett. Vol. 19, No. 1 (Jan., 1998), pp. 7-9, by J. P. A. van der Wagt et al. describes successful operation of memory cells using III-V semiconductor resonant tunnel diodes and separate read and write devices. However, GaAs devices are expensive to manufacture. 
     Base current reversal in bipolar transistors also can permit data storage. Base current reversal occurs when impact ionization occurring at a p-n junction between a base and a collector in the transistor results in minority carrier generation sufficient to cancel or exceed majority carrier injection from an emitter to the base. The base terminal then displays two or more stable states that do not source or sink current, and the transistor may be used to store information as represented by the state of the base terminal. FIG. 1 is a graph showing a simplified current-voltage characteristic for a storage device employing base current reversal, in accordance with the prior art. 
     As base voltage is increased from zero volts, base current is initially increased also, as shown in a first portion of a current-voltage characteristic  21  (to the left of a point marked “B”). A first stable state, at a point denoted “A,” where no current passes through the base terminal corresponds to a base voltage of zero volts. As the base voltage increases, the number of electrons injected into the base and then diffusing into a depleted portion of the collector increases. These electrons are accelerated through the depleted portion of the collector. At the point marked “B” on the first portion  21  of the base-emitter current-voltage characteristic, holes created through impact ionization in the collector region and that are swept into the base begin to outnumber electrons injected from the emitter in forming a base terminal current I B . As base-emitter voltage further increases, the number of holes created by impact ionization also increases (dashed portion of curve  21 ) until the net base terminal current I B  becomes zero at the point marked “C” in FIG. 1, at a base emitter voltage of slightly less than 0.6 volts. This portion  21  of the current-voltage characteristic corresponds to a base current flowing in a direction normally associated with a base current for a NPN bipolar transistor. 
     A second portion  23  of the current-voltage characteristic corresponds to a base current flowing in the opposite of the direction illustrated in the first portion  21 . The second portion  23  corresponds to holes being created by impact ionization at the collector-base junction of the transistor, where the holes collected by the base outnumber electrons emitted from the emitter and collected by the base. The base current becomes increasingly negative until the point marked “D” on the curve  23 . At the point marked “D,” electrons injected into the base from the emitter begin to dominate the base terminal current I B , and the base terminal current I B  again becomes very small (dashed trace). 
     The base terminal current I B  again becomes zero at a point marked “E” in FIG. 1, corresponding to a base-emitter voltage of about 0.9 volts. 
     As base-emitter voltage is increased even further, a third portion  25  of the current-voltage characteristic corresponds to a base terminal current I B  flowing in the same direction as the first portion  21 . The base terminal current I B  then behaves conventionally with further increases in base emitter voltage. 
     At the points “A,” “C” and “E,” the net base terminal current I B  is zero. Significantly, the transistor is stable at these points. As a result, opening a switch coupled to the base results in the transistor staying at one of these points and allowing a state of the transistor to be determined by measuring the base-emitter voltage, (i.e., a “read” of the data stored in the transistor). 
     U.S. Pat. No. 5,594,683, entitled “SRAM Memory Cell Using A CMOS-Compatible High Gain Gated Lateral BJT”, issued to M.-J. Chen and T. S. Huang, describes a memory employing base current reversal for data storage. FIG. 2 is a simplified schematic diagram of a generic memory cell  30  formed from a storage device  32  and an access element  34 , in accordance with the prior art. The storage device  32  is represented as a NPN bipolar transistor in FIG. 2, however, the storage device  32  may be formed from a structure corresponding to a NMOS FET and may be capable of operating as either an NPN transistor or a NMOS FET, as described in “High-Gain Lateral Bipolar Action in a MOSFET Structure” by S. Verdonckt-Vandebroek et al., IEEE Trans. El. Dev., Vol. 38, No. 11, Nov. 1991, pp. 2487-2496. 
     The memory cell  30  is read by turning the access element  34  ON through application of a suitable signal to a word line driver  36 . A sense amplifier (not shown in FIG. 2) is coupled to the storage device  32  through a bit line  38  and the access element  34 . 
     Data can be written to the storage device  32  by applying a write pulse to a control electrode of a bit line switch  40  and also turning ON the access element  34  as described above. The data bit to be written to the storage device  32  is coupled through the bit line switch  40  to a control electrode of the storage device  32 . The access element  34  is then turned OFF, electrically isolating the storage device  32  from the bitline  38  and storing the data bit in the memory cell  30 . Compact memory cells  30  drawing as little as 1 nanoampere of standby current can be designed using this approach. However, the memory cell described in U.S. Pat. No. 5,594,683 requires an area of at least 8F 2 . 
     There is therefore a need for a compact and robust memory cell having reduced standby power draw requirements. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes a memory cell. The memory cell is formed from semiconductor material and includes a vertical access element formed on a storage device. The storage device has a control electrode, a first current-carrying electrode coupled to a first reference voltage and a second current-carrying electrode coupled to a second reference voltage. The access element has a control electrode coupled to a first selection line, a first current-carrying electrode coupled to the control electrode of the storage device and a second current-carrying electrode coupled to a second selection line. The control electrode of the storage device can be set to one of several predetermined voltages by turning ON the access element and applying a signal to the control electrode of the storage device. Memory cells fabricated by forming the access element on the storage device allow very high density SRAMs to be manufactured. 
     In another aspect, the present invention includes a method of operating a memory device. The method includes coupling a first voltage to a control electrode of an access element that is part of a memory cell to turn the access element ON. The method also includes sensing a voltage at a control electrode of a storage device that is coupled to a first current-carrying electrode of the access element. The voltage represents data stored in the memory cell. The present invention further permits application of a voltage to the storage device control electrode to set the storage device to one of several stable states to store data in the memory cell. The method also permits coupling a second voltage to the access element control electrode to turn the access element OFF. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing a simplified current-voltage characteristic for a storage device, in accordance with the prior art. 
     FIG. 2 is a simplified schematic diagram of a generic memory cell formed from a storage device and an access element, in accordance with the prior art. 
     FIG. 3 is a simplified isometric view of an embodiment of the memory cell of FIG. 2, in accordance with an embodiment of the present invention. 
     FIG. 4 is a simplified schematic diagram for the memory cell of FIG. 3, in accordance with an embodiment of the present invention. 
     FIG. 5 is a simplified cross-sectional view of a semiconductor substrate that can be processed to form the memory cell of FIGS. 3 and 4, in accordance with an embodiment of the present invention. 
     FIG. 6 is a simplified cross-sectional view of the substrate of FIG. 5 at a later point in processing, in accordance with an embodiment of the present invention. 
     FIG. 7 is a simplified cross-sectional view of the substrate of FIG. 6 at a later point in processing, in accordance with an embodiment of the present invention. 
     FIG. 8 is a simplified isometric cross-sectional view of the substrate of FIG. 7 at a later point in processing, in accordance with an embodiment of the present invention. 
     FIG. 9 is a simplified cross-sectional view of the substrate of FIG. 8 at a later point in processing, in accordance with an embodiment of the present invention. 
     FIG. 10 is a simplified cross-sectional view of the substrate of FIG. 9 at a later point in processing, in accordance with an embodiment of the present invention. 
     FIG. 11 is a simplified block diagram of an SRAM that can be formed using the memory cell of FIGS. 3 and 4, in accordance with an embodiment of the present invention. 
     FIG. 12 is a simplified block diagram of a computer system including one or more memories using the SRAM FIG. 11, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 is a simplified isometric view of an embodiment of the generic memory cell  30  of FIG. 2, in accordance with an embodiment of the present invention. A substrate  40  has a contact stud  42  formed on it. In one embodiment, the substrate  40  is formed from n+-type semiconductor material such as silicon. In another embodiment, the substrate  40  is formed from p+-type semiconductor material. In either case, the contact stud  42  forms a low resistance contact to the substrate  40 , which acts as a power supply return electrode. 
     A dielectric  44  is also formed on the substrate  40  and electrically isolates the substrate  40  from power supply busses  46  and from memory cells  48 . Each of the memory cells  48  includes a storage device  50  and an access element  52  formed on the storage device  50 . A word line  54  is capacitively coupled to the access elements  52  through a dielectric  56  surrounding the word line  54 . The dielectric  56  also electrically isolates the word line  54  from the power supply busses  46 . 
     FIG. 4 is a simplified schematic diagram for the memory cells  48  of FIG. 3, in accordance with an embodiment of the present invention. Many of the elements described in conjunction with the schematic diagram of FIG. 4 are identical to elements described in conjunction with in the embodiment illustrated in FIG.  3 . Therefore, in the interest of brevity, these elements have been provided with the same reference numerals, and an explanation of them will not be repeated. 
     Data is coupled between the memory devices  50  and a selected bit line  58  by a signal coupled through one of the word lines  54  that turns a selected group of the access elements  52  on. In one embodiment, the access elements  52  are vertical PMOS FETs having gates formed by the word line  54  and the storage devices  50  are bipolar transistors formed in sources of the PMOS FETs forming the access elements  52 . 
     FIG. 5 is a simplified cross-sectional view of a semiconductor substrate  40  that can be processed to form the memory cells  48  of FIGS. 3 and 4, in accordance with an embodiment of the present invention. Axes labeled “x” and “z” are also shown in FIG. 5 to clarify relationships between the various Figures. 
     In one embodiment, an epitaxial layer  60  is grown on the substrate  40 . In one embodiment, the epitaxial layer  60  is a p-type epitaxial layer having a thickness of about 0.4 micron and an acceptor concentration N A  of about 10 17  per cubic centimeter. An epitaxial layer  62  is grown on the epitaxial layer  60 . In one embodiment, the epitaxial layer  62  is a n-type epitaxial layer having a thickness of about 0.2 micron and a donor concentration N D  of about 10 17  per cubic centimeter. An epitaxial layer  64  is grown on the epitaxial layer  62 . In one embodiment, the epitaxial layer  64  is a p+-type epitaxial layer having a thickness of about 0.2 micron and an acceptor concentration N A  of about 10 20  per cubic centimeter. 
     A mask layer  66  is formed on the epitaxial layer  64 . In one embodiment, the mask layer  66  is formed by conventional chemical vapor deposition (“CVD”) of a silicon dioxide layer 10 nanometers thick followed by conventional CVD of a silicon nitride layer 100 nanometers thick. Conventional photolithography and etching are then used to define stripes in the masking layer  66 . The stripes in the masking layer  66  have a width oriented in the “x” direction corresponding to the minimum photolithographic feature size F and a length that corresponds to a “y” direction, i.e., perpendicular to the sheet on which FIG. 5 is printed. The stripes in the mask layer  66  are then used as etch masks for an anisotropic etch to define trenches  68  extending through the epitaxial layers  64  and  62  and into the epitaxial layer  60 , resulting in the structure shown in FIG.  5 . The trenches  68  separate bars  70  of silicon material. In one embodiment, the trenches  68  are etched to a depth of 0.7 micrometers. 
     FIG. 6 is a simplified cross-sectional view of the substrate  40  of FIG. 5 at a later point in processing, in accordance with an embodiment of the present invention. Following etching of the trenches  68 , a thin protective layer  72  is formed over all exposed surfaces of the mask layer  66 , sides of the bars  70  and bottoms of the trenches  68 . In one embodiment, the thin protective layer  72  is formed by conventional CVD of silicon nitride to a thickness of 20 nanometers. A conventional timed anisotropic etch is then used to selectively remove the thin protective layer  72  from the bottoms of the trenches  68  and the tops of the bars  70 . A conventional timed isotropic etch is then used to undercut the bars  70 . The bars  70  of silicon are supported at their ends by portions that are not undercut (not illustrated). In one embodiment, the bars  70  are completely undercut. In another embodiment, the bars  70  are largely undercut. In either case, a conventional thermal oxidation is carried out to form a silicon dioxide layer  74  that supports the bars  70  and that electrically isolates the bars  70  from the substrate  40 , resulting in the structure illustrated in FIG.  6 . 
     FIG. 7 is a simplified cross-sectional view of the substrate  40  of FIG. 6 at a later point in processing, in accordance with an embodiment of the present invention. Conventional photolithography masks the bars  70  and alternate trenches  68 . Conventional anisotropic etching removes the silicon dioxide layer  74  from the bottom of every other trench  68  to expose the substrate  40 , although only two trenches  68  are shown in FIG.  7 . Conventional timed isotropic etching strips the thin protective layer  72  from sidewalls of the trenches  68  and bars  70 . Conventional CVD fills the trenches  68  with n+-type polycrystalline silicon  75 , and conventional chemical-mechanical polishing planarizes the n+-type polycrystalline silicon  75 , stopping on the mask layer  66 . 
     Conventional timed reactive ion etching recesses a top surface of the n+-type polycrystalline silicon  75  to below a top of the epitaxial layer  60  and above a bottom of the epitaxial layer  60 . The substrate  40  is heated to diffuse a portion of the n+-type donor atoms from the n+-type polycrystalline silicon  75  into the epitaxial layer  60  to form regions  76  and  77 , resulting in the structure illustrated in FIG.  7 . The regions  76  correspond to first current-carrying electrodes or emitters of the storage devices  50  of FIGS. 3 and 4, and the regions  77  correspond to second current-carrying electrodes or collectors of the storage devices  50 . 
     FIG. 8 is a simplified isometric cross-sectional view of the substrate  40  of FIG. 7 at a later point in processing, in accordance with an embodiment of the present invention. The structure shown in FIG. 8 is rotated relative to the structures illustrated in FIGS. 5 through 7 as indicated by “x,” “y” and “z” axes shown in FIG.  8 . 
     Another thin protective layer  78  is formed on the sides of the bars  70 , the recessed surface of the n+-type polycrystalline silicon  75  and the mask layer  66 . In one embodiment, the thin protective layer  78  is formed from silicon nitride having a thickness of 20 nanometers. Conventional CVD fills the trenches  68  and covers the bars  70  with a dielectric  80 . In one embodiment, the dielectric  80  is formed from silicon dioxide formed by conventional TEOS that fills the trenches  68  and covers the bars  70 . In one embodiment, conventional chemical-mechanical polishing planarizes the dielectric  80 , removing the dielectric  80  from tops of the bars  70  and stopping on the mask layer  66 . Another mask layer  82  is formed on the dielectric  80  and the mask layer  66 . In one embodiment, the mask layer  82  is a silicon nitride layer formed by conventional CVD to have a thickness of 100 nanometers. 
     Conventional photolithography defines stripes of resist (not illustrated) at an angle to the stripes of FIGS. 5 through 7. In one embodiment, the mask layers  82  and  66  are etched to define stripes in the mask layer  82  having the minimum photolithographic feature size F along the “y” direction, i.e., forming stripes at right angles to those of FIG.  5 . Conventional anisotropic etching removes exposed portions the dielectric layer  80  from between the stripes of the mask layer  82 , stopping on the thin protective layer  78 . 
     A conventional selective anisotropic etch is used to etch exposed portions of the bars  70 , which are formed from single crystal silicon, stopping at the silicon dioxide layer  74  and at exposed portions of the n+-type polycrystalline silicon  75 . Exposed portions of the thin protective layer  78  are removed using conventional etching to provide trenches  84  in the structure shown in FIG.  8 . 
     FIG. 9 is a simplified cross-sectional view of the substrate  40  of FIG. 8 at a later point in processing, in accordance with an embodiment of the present invention. The dielectric  56  of FIG. 3 is formed on exposed portions of the n+-type polycrystalline silicon  75  (not shown in FIG. 9) and on exposed portions of the silicon dioxide layer  74 . In one embodiment, the dielectric  56  is formed as a conventional CVD oxide that fills the trenches  84  and covers the mask layer  82 . Conventional chemical-mechanical polishing may be used to planarize the dielectric  56  and to remove the dielectric  56  from the mask layer  82 . A conventional timed etch may be used to recess a top surface of the dielectric  56  to a level below a junction between the epitaxial layers  60  and  62  without exposing tops of the n+-type polycrystalline silicon  75 . A conventional thermal oxide  86  may be grown on exposed sides of the epitaxial layers  60 ,  62  and  64 . In one embodiment, the oxide  86  is grown to a thickness of 10 nanometers. 
     P+-type polycrystalline silicon  88  is formed in the trenches  84  and on the mask layer  82  using conventional CVD. In one embodiment, the p+-type polycrystalline silicon  88  is formed to have a thickness of about ⅓ of the minimum photolithographic feature dimension F. A conventional anisotropic etch removes portions of the p+-type polycrystalline silicon  88  from bottoms of the trenches  84  and from tops of the mask layer  82 , leaving p+-type polycrystalline silicon  88  on the thermal oxide  86  and providing the structure shown in FIG.  9 . 
     FIG. 10 is a simplified cross-sectional view of the substrate  40  of FIG. 9 at a later point in processing, in accordance with an embodiment of the present invention. A dielectric  90  is deposited using conventional CVD to fill in spaces between portions of the p+-type polycrystalline silicon  88 . The dielectric  90  is removed from the mask layer  82  and is planarized using conventional chemical-mechanical polishing. Photoresist  92  is conventionally applied and patterned to expose a top surface of the p+-type polycrystalline silicon  88  on one side, but not the other, of each of the trenches  84 . A conventional isotropic etch then removes the p+-type polycrystalline silicon  88  from every other side of the trenches  84  to provide the structure shown in FIG.  10 . The photoresist  92  and the mask layers  66  and  82  are conventionally stripped and conventional microfabrication is used to provide interconnections and other conventional structures. 
     The p+-type polycrystalline silicon  88  forms gates of the access elements  52  of FIGS. 3 and 4, and also forms the word lines  54 . The access elements  52  include a drain formed by the epitaxial layer  64 , a channel formed in the epitaxial layer  62  adjacent the p+-type polycrystalline silicon  88  and a source formed by the epitaxial layer  60 . The oxide  86  forms a gate insulator separating the p+-type polycrystalline silicon  88  from the epitaxial layer  62 . Stacking the access element  52  on top of the storage device  50  to provide the memory cells  48  of FIGS. 3 and 4 and forming the p+-type polycrystalline silicon  88  and the n+type polycrystalline silicon  75  between memory cells  48  allows compact memory cells  48  having an area of four minimum photolithographic feature dimensions F squared to be formed. 
     FIG. 11 is a simplified block diagram of an SRAM  175  that can be formed using the memory cells  48  of FIGS. 3 and 4, in accordance with an embodiment of the present invention. The SRAM  175  includes an address register  177  that receives either a row address or a column address on an address bus  179 . The address bus  179  is generally coupled to a memory controller (not shown in FIG.  11 ). Typically, a row address is initially received by the address register  177  and is applied to a row address multiplexer  183 . The row address multiplexer  183  couples the row address to a number of components associated with either of two memory banks  195 ,  197  depending upon the state of a bank address bit forming part of the row address. Associated with each of the memory banks  195 ,  197  is a respective row address latch  201  which stores the row address, and a row decoder  203  which applies various signals to its respective memory bank  195  or  197  as a function of the stored row address. The row address multiplexer  183  also couples row addresses to the row address latches  201  for the purpose of refreshing memory cells in the memory banks  195 ,  197 . 
     After the row address has been applied to the address register  177  and stored in one of the row address latches  201 , a column address is applied to the address register  177 . The address register  177  couples the column address to a column address latch  215 . The column address from the column address latch  215  is decoded by a column address decoder  217  to address a specific column or columns. The row address corresponds to one or more of the word lines  54  of FIGS. 3 and 4, and the column address corresponds to one or more of the bit lines  58 . 
     Data to be read from one of the memory banks  195 ,  197  is coupled to the column circuitry  225 ,  227  for one of the memory banks  195 ,  197 , respectively. The data is then coupled to a data output register  229  which applies the data to a data bus  231 . Data to be written to one of the memory banks  195 ,  197  is coupled from the data bus  231  through a data input register  233  to the column circuitry  225 ,  227  and then is transferred to one of the memory banks  195 ,  197 , respectively. 
     FIG. 12 is a simplified block diagram of a computer system  250  including one or more memories using the SRAM  175 FIG. 11, in accordance with an embodiment of the present invention. The computer system  250  includes a processor  252  for performing various computing functions, such as executing specific software to perform specific calculations or tasks. The processor  252  includes a processor bus  254  that normally includes an address bus, a control bus and a data bus. 
     In addition, the computer system  250  includes one or more input devices  264 , such as a keyboard or a mouse, coupled to the processor  252  to allow an operator to interface with the computer system  250 . Typically, the computer system  250  also includes one or more output devices  266  coupled to the processor  252 , such output devices typically being a printer or a video terminal. One or more data storage devices  268  are also typically coupled to the processor  252  to allow the processor  252  to store data or retrieve data from internal or external storage media (not shown). Examples of typical storage devices  268  include hard and floppy disks, tape cassettes and compact disk read-only memories (CD-ROMs). 
     The processor  252  is also typically coupled to cache memory  276 , which is usually SRAM and may be the SRAM  175  of FIG. 11, by the processor bus  254 , and to DRAM  278  through a memory controller  280 . The memory controller  280  normally includes a control and address bus  282  that is coupled to the DRAM  278 . 
     When incorporated into the computer system  250 , the SRAM memory  175  of the present invention provides increased data density, high access speed and reduced power consumption. These advantages are particularly useful in the context of handheld or portable products. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.