Abstract:
A vertical semiconductor material mesa upstanding from a semiconductor base that forms a conductive channel between first and second doped regions. The first doped region is electrically coupled to one or more first silicide layers on the surface of the base. The second doped region is electrically coupled to a second silicide layer on the upper surface of the mesa. A gate conductor is provided on one or more sidewalls of the mesa.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. 12/469,563, filed concurrently herewith, the specification of which is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The embodiments disclosed herein relate generally to the field of semiconductor selection devices and, more particularly, to access devices for semiconductor memory devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    A non-volatile memory device is capable of retaining stored information even when power to the memory device is turned off. Traditionally, non-volatile memory devices occupied large amounts of space and consumed large quantities of power. As a result, non-volatile memory devices have been widely used in systems where limited power drain is tolerable and battery-life is not an issue. However, as systems requiring non-volatile memories have continued to shrink in size, improvements in non-volatile memory devices have been sought in order to make these devices more suitable for use in portable electronics or as substitutes for frequently-accessed volatile memory devices. Desired improvements include decreasing the size and power consumption of these memories and improving the memory access devices. 
         [0004]    Improved non-volatile memory devices under research include resistive memory cells where resistance states can be programmably changed. Resistive memory cells store data by structurally or chemically changing a physical property of the memory cells in response to applied programming voltages, which in turn changes cell resistance. Examples of variable resistance memory devices being investigated include memories using variable resistance polymers, perovskite materials, doped amorphous silicon, phase-changing glasses, and doped chalcogenide glass, among others. Phase change memory (“PCM”) cells have varying resistances as a result of changes in the crystal phase of the cell material. Spin-tunneling random access memory (“STRAM”) cells have varying resistances as a result of changes in current induced magnetization of the cell material. 
         [0005]    For many resistive memory cells, changing the cell resistance is accomplished by passing an electrical current of sufficient strength through the resistive memory cell. For phase change memory cells and spin-tunneling memory cells, for example, programming and reset currents of 50 to 100 μA are not uncommon. However, these high currents result in extremely high current densities as the size of the memory cells continues to shrink. For example, for a 20×20 nm 2  memory cell, the resulting current density is of the order of 1×10 7  A/cm 2  or greater. For such high current densities, improved memory access devices are desired to provide high currents and low “off” state leakage. 
         [0006]    Improved access devices such as those desired for use with resistive memory cells could also be used to provide high currents to any type of memory or semiconductor circuit that requires a high current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a vertically-oriented semiconductor transistor device. 
           [0008]      FIGS. 2A and 2B  illustrate a memory cell and a memory access device, according to one or more embodiments of the disclosure. 
           [0009]      FIGS. 3A ,  3 B and  3 C illustrate an array of memory cells and memory access devices, according to one or more embodiments of the disclosure. 
           [0010]      FIG. 4  illustrates a processing system utilizing a memory array, according to one or more embodiments of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Traditional memory access devices are planar in nature, meaning that the access devices are formed layer by layer within the plane of the underlying structure. The underlying structure includes a substrate that is a base material and layers formed on the surface of the substrate. The substrate and overlaying layers on top of the substrate are flat or planar. The access devices are formed within these layers so that the resulting devices are also laid out in a planar arrangement. As a specific example, a planar field-effect transistor (“FET”) is a FET with a conductive channel that is within the layers of the underlying structure. Planar access devices have a relatively large footprint since area is required for source and drain contacts as well as isolation between the contacts. 
         [0012]    Non-planar access devices are alternatives to planar devices. Non-planar access devices are access devices that are not flat or planar and can be oriented in a vertical direction from a substrate. These devices can include raised portions that extend above the planar surface of the underlying structure. An example of a non-planar access device is a fin-FET. A fin-FET is a FET that includes thin vertical “fins” of the underlying substrate material that act as the transistor body. The source and drain of the fin-FET are located at the ends of the fin, while one or more gates are located on a surface of the fin. Upon activation, current flows through the fin. The thin vertical structure results in significant space savings over traditional planar access devices. 
         [0013]      FIG. 1  illustrates a vertical FET  100 . The vertical FET  100  includes a thin vertical fin or mesa  120  through which current flows vertically between a source  130  and a drain  140 . The mesa  120  extends above a substrate  555 . In the example vertical FET  100 , the substrate  555  and the mesa  120  are formed of silicon. The source  130  and drain  140  regions are both either n-doped or p-doped, while a vertical current channel  125  is either p-doped or n-doped, accordingly. A gate  150  is formed along a sidewall of the mesa  120 . Additional gates  150  may be formed. In the example of  FIG. 1 , two gates  150  are formed on opposite sidewalls of the mesa  120 , although vertical FET  100  may also be formed with only a single gate  150 . Gates  150  are separated from the sidewalls of the mesa  120  by thin gate insulators  155  such as a gate oxide layer. The thin gate insulators  155  are L-shaped in order to insulate the gates  150  from contact with the mesas  120  and the substrate  555  or any conductor on the substrate  555 . The gates  150  may be formed of polysilicon. When an appropriate bias is applied to one or more of the gates  150 , current flows vertically through the channel  125  from the source  130  to the drain  140 . 
         [0014]    In a disclosed embodiment, the vertical FET  100  may be used as a selection device such as a memory access device  200  for one or more electrical devices, as illustrated in the structure of  FIG. 2A  and the schematic diagram of  FIG. 2B . In  FIG. 2A , a memory cell  220  is electrically coupled to the vertical FET device  200 . The memory cell  220  includes a top electrode  222  and a bottom electrode  224 . The bottom electrode  224  is coupled to a contact  240  for the drain  140 . The source  130  is coupled to a contact  230 . Upon appropriate biasing of the source contact  230 , the gate  150  and the top electrode  222 , the vertical FET  100  is turned “on” and current flows through the channel  125  and memory cell  220 . With appropriate biasing, the current flowing through the memory cell  220  is strong enough to be used as a programming or reset current for the memory cell  220 . 
         [0015]    The memory access device  200  and the memory cells  220  are generally formed in an array of access devices  200  and memory cells  220 . Thus, the source contact  230  may extend a relatively long distance from the source  130  of memory access device  200  to the nearest voltage source. Additionally, source contacts  230  may be shared by multiple access devices. In order to facilitate the shared contacts  230  and to minimize the effect of parasitic resistance, the contacts  230  are formed of metal silicide  250 . In other words, the substrate  555  surface near the bottom of the mesa  120  is silicided with metal such as Ni, Co or Ti. The metal silicide  250  (also known as a salicide) near the bottom of the mesa  120  (or the source metal silicide layer  252 ) acts to reduce the series resistance that results from using a common current source contact for each individual access device  200  in an array. The source contacts  230  may also be formed of heavily doped silicon as long as the resistance of the doped silicon is low enough to carry the required current. 
         [0016]    Additionally, the drain contact  240  is also formed of a metal silicide  250  which helps to reduce contact resistance between the access device  200  and the bottom electrode  224  of the memory cell  220 . The metal silicide  250  formed on the upper portion of the access device  200  is the drain metal silicide layer  251 . 
         [0017]    In a disclosed embodiment, the access devices  200  and the memory cells  220  are arranged in an array  300  as illustrated in  FIG. 3A . In  FIG. 3A , a silicon substrate  555  is shown. Rising from the silicon substrate  555  are four silicon mesas  120 . Other substrate and mesa material, such as Ge, SiC, GaN, GaAs, InP, carbon nanotube and graphene, for example, may be used instead of silicon. Additionally, the array  300  generally includes many more than just four mesas. The illustration of the array  300  is simplified in order to aid explanation. 
         [0018]    The mesas  120  each include source  130 , drain  140  and gate  350  regions. The gate  350  regions are formed on one or more sidewalls of the silicon mesas  120  and are commonly shared between mesas  120 . In the example of  FIG. 3A , gates  350  are formed on two opposite sides of each mesa  120 , thus forming double-gated vertical FETs. Single-gated vertical FETs (i.e., only one gate  350  on a mesa  120 ) or surround-gated vertical FETs (i.e., the gate  350  surrounds mesa  120 ) may also be formed. The sidewall gates  350  extend along the column of mesas  120  so that each column of mesas  120  includes at least one common sidewall gate  350 . The sidewall gates  350  may also be silicided. The source  130  regions of each mesa  120  are electrically coupled with the source metal silicide layer  252  which covers the surface of the silicon substrate  555 . In this way, source  130  regions for multiple mesas  120  are electrically coupled together to form shared sources  130 . Source  130  regions may also merge into a single common source  130 . The drain  140  regions are electrically coupled to the drain metal silicide layer  251  which covers the top portion of the mesas  120 . The gates  350  are insulated from the silicide layers  251 ,  252  by the thin gate insulator  155 . In order to further insulate gates  350  from the silicide layers  251 , gate  350  need not extend all the way to the top edge of the mesas  120 . 
         [0019]    The memory cells  220  are electrically coupled via a bottom electrode  224  to the drain metal silicide layer  251  located on the upper surfaces of the mesas  120 . The top electrode  222  of each memory cell  220  is electrically coupled to a conductor  322 . In one embodiment, conductor  322  may extend horizontally in a direction perpendicular to the direction that the sidewall gates  350  extend. Other array layouts are contemplated where conductor  322  may extend in a direction other than perpendicular to sidewall gates  350 . 
         [0020]    A simplified top view of the array  300  is illustrated in  FIG. 3B . From the top view, it is apparent that all access devices  200  and memory cells  220  share a common source metal silicide layer  252  that surrounds the base of each mesa  120 . Access devices  200  in the same column share a common gate  350 . Additionally, gates  350  may be formed on all sides of each access device  200 , resulting in a surround-gated vertical FET, as illustrated in  FIG. 3C . Memory cells  220  in the same row share a common conductor  322 . The common conductor  322  may be made of metal, but may also be made of other conductive materials such as polysilicon, for example. Memory cells  220  are coupled to the upper portion of each mesa  120  via a drain metal silicide layer  251 . 
         [0021]    Individual memory cells  220  are activated (meaning that a desired current flows through the memory cell  220 ) by the appropriate biasing of the source  130 , the respective gate  350  and the respective conductor  322 . While biasing the source  130  or any one of the gates  350  or conductors  322  may affect multiple memory cells  220 , activation of a specific memory cell  220  is only accomplished through the appropriate biasing of that cell&#39;s gate  350  and conductor  322 . 
         [0022]    By using a common source  130  and by surrounding the base of each mesa  120  with a metal silicide  250  (the source metal silicide layer  252 ), the parasitic resistance in the source is reduced. The source metal silicide layer  252  provides additional current paths, resulting in higher current flow. In this example, because every mesa  120  shares a common source  130 , a dedicated contact is not required for any specific strip of source metal silicide layer  252 . Thus, efficiency of current flow through the source metal silicide layer  252  to a specific mesa  120  may be improved. Additionally, by using a drain metal silicide layer  251  on the top surface of each mesa  120 , the contact resistance between the access device  200  and the bottom electrode  224  of each memory cell  220  is reduced. 
         [0023]    The memory access devices of array  300  are able to provide large amounts of current through any selected memory cell  220 . In array  300 , access devices share common sources  130  because of the source metal silicide layers  252 . Mesas  120  in the array  300  share a common source  130 . Thus, the source metal silicide layer  252  helps to facilitate a larger source current. 
         [0024]    It should be appreciated that the array  300  may be fabricated as part of an integrated circuit. The corresponding integrated circuits may be utilized in a processor system. For example,  FIG. 4  illustrates a simplified processor system  700 , which includes a memory device  702  that includes array  300  in accordance with any of the above described embodiments. A processor system, such as a computer system, generally comprises a central processing unit (CPU)  710 , such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device  720  over a bus  790 . The memory device  702  communicates with the CPU  710  over bus  790  typically through a memory controller. 
         [0025]    In the case of a computer system, the processor system  700  may include peripheral devices such as removable media devices  750  (e.g., CD-ROM drive or DVD drive) which communicate with CPU  710  over the bus  790 . Memory device  702  can be constructed as an integrated circuit, which includes one or more phase change memory devices. If desired, the memory device  702  may be combined with the processor, for example CPU  710 , as a single integrated circuit. 
         [0026]    The above description and drawings should only be considered illustrative of exemplary embodiments that achieve the features and advantages described herein. Modification and substitutions to specific process conditions and structures can be made. Accordingly, the claimed invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.