Abstract:
A method of forming a programmable conductor memory cell array is disclosed wherein metal and chalcogenide glass are co-sputtered to fill an array of cell vias in a prepared substrate. The prepared substrate is heated above room temperature before the metal and chalcogenide glass film is deposited, and the heating is maintained throughout the deposition. The resulting metal/chalcogenide glass film has good homogeneity, a desired ratio of components, and has a regular surface.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. application Ser. No. 10/164,646, by Li, et al., filed on Jun. 6, 2002, entitled ELIMINATION OF DENDRITE FORMATION DURING METAL/CHALCOGENIDE GLASS DEPOSITION, with attorney docket no. MICRON.250A, the entirety of which is hereby incorporated by reference. This application is also related to the disclosure of U.S. application Ser. No. 10/164,429 of Li et al., filed Jun. 6, 2002, entitled CO-SPUTTER DEPOSITION OF METAL-DOPED CHALCOGENIDES, attorney docket no. MICRON.243A. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to methods of forming metal-chalcogenide glass structures and more particularly to a method of preventing formation of extraneous metal dendrite structures during fabrication of programmable conductor memory cells using metal-chalcogenide glass systems.  
           [0004]    2. Description of the Related Art  
           [0005]    The digital memory most commonly used in computers and computer system components is the dynamic random access memory (DRAM), wherein voltage stored in capacitors represents digital bits of information. Electric power must be supplied to the capacitors to maintain the information because, without frequent refresh cycles, the stored charge dissipates, and the information is lost. Memories that require constant power are known as volatile memories.  
           [0006]    Non-volatile memories do not need frequent refresh cycles to preserve their stored information, so they consume less power than volatile memories. The information stays in the memory even when the power is turned off. There are many applications where non-volatile memories are preferred or required, such as in lap-top and palm-top computers, cell phones or control systems of automobiles. Non-volatile memories include magnetic random access memories (MRAMs), erasable programmable read only memories (EPROMs) and variations thereof.  
           [0007]    Another type of non-volatile memory is the programmable conductor or programmable metallization memory cell, which is described by Kozicki et al. in U.S. Pat. No. 6,084,796 and is included by reference herein. The programmable conductor cell of Kozicki et al. (also referred to as a metal dendrite memory) comprises a glass ion conductor, such as a chalcogenide-metal ion glass, and a plurality of electrodes disposed at the surface of the fast ion conductor and spaced a distance apart from one another. The glass/ion element shall be referred to herein as a “glass electrolyte,” or, more generally, a “cell body.” When a voltage is applied across the anode and the cathode, a non-volatile metal dendrite grows from the cathode along the surface of the cell body towards the anode. The growth of the dendrite depends upon applied voltage and time; the higher the voltage, the faster the growth rate; and the more time, the longer the dendrite. The dendrite stops growing when the voltage is removed. The dendrite shrinks, re-dissolving metal ions into the cell body, when the voltage polarity is reversed.  
           [0008]    The programmable conductor memory cell can serve as a “one” state when the conductive path has grown all the way from the cathode to the anode, thus providing a low resistance metallic conduction path. The programmable conductor memory cell is in a “zero” state when the conductive path is at least partially dissolved, and the metallic conduction path is broken and the resistance of the cell is several orders of magnitude higher. In other arrangements, varying extent of the conductive path can be used in forming variable capacitors or variable resistors.  
           [0009]    The recent trends in memory arrays generally have been to first form a via, then fill it with a conventional memory storage element (e.g., a capacitor) and etch back. It is simple to isolate individual memory cells with this container structure. Following this trend, programmable memory cells have been also fabricated using this sort of container configuration, wherein the electrodes and body layers are deposited into a via etched into an insulating layer. Under normal operating conditions, conductive paths can grow between the electrodes, such as along the interface between the cell and the via wall. Typically, the memory cell is formed in an array having a conventional memory array circuit design. For example, in a conventional cross-point circuit design, memory elements are formed between upper and lower conductive lines at intersections. Typically, after forming the lower set of lines, a via is formed in an insulating layer and filled with the memory element, such as a glass electrolyte or glass fast ion diffusion (GFID) element.  
           [0010]    Deposition of metal and chalcogenide glass is not simple. At normal deposition temperatures, metal atoms are highly mobile and can diffuse away from the glass or can form extraneous metal-rich structures, thus altering the component proportions in the deposited mixture and, thus, the performance of the programmable conductor memory cells.  
           [0011]    Accordingly, a need exists for improved methods for depositing metal and chalcogenide glass to form integrated programmable conductor memory arrays.  
         SUMMARY OF THE INVENTION  
         [0012]    In accordance with one aspect of the present invention, a method is provided for forming a programmable conductor memory cell. The method includes sputtering metal and chalcogenide glass onto a prepared substrate. The substrate is maintained at a temperature higher than room temperature during sputtering.  
           [0013]    In accordance with another aspect of the present invention, a method is provided for depositing a metal/chalcogenide glass film with a desired metal to glass ratio onto a substrate. The method includes pre-heating the substrate to a desired temperature between about 40° C. and 130° C. Metal and chalcogenide glass are co-sputtering metal and chalcogenide glass onto the substrate after pre-heating. The substrate is held at the desired temperature during the co-sputtering.  
           [0014]    In accordance with another aspect of the invention, a method is provided for co-sputtering a homogeneous metal/chalcogenide glass layer onto a irregular surface. The irregular surface is kept at an elevated temperature, above room temperature, before and during the co-sputtering. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic cross-section of a via formed in an insulating layer, within which via chalcogenide glass elements are to be deposited.  
         [0016]    [0016]FIG. 2A is a schematic drawing of an array of vias that have been filled with silver and germanium selenide sputtered at room temperature.  
         [0017]    [0017]FIG. 2B is a schematic drawing of one isolated via filled with silver and germanium selenide sputtered at room temperature.  
         [0018]    [0018]FIG. 3A is a schematic drawing of an array of vias that have been filled with silver and germanium selenide sputtered at 50° C.  
         [0019]    [0019]FIG. 3B is a schematic drawing of one isolated via filled with silver and germanium selenide sputtered at 50° C. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    Metal/chalcogenide glass films form the key functional structure of programmable conductor memory cells. It is within these films that metal dendrite growth or shrinkage occurs in response to an applied voltage. Reproducible conductive path growth and shrinkage are essential to the proper functioning of these memory devices. The programmable conductor memory cell can serve as a “one” state when the conductive has grown all the way from the cathode to the anode, thus providing a metallic path for easy conduction. The programmable conductor memory cell can serve as a “zero” state when the conductive path is at least partially dissolved, and the short from electrode to electrode is broken.  
         [0021]    Often, the metal and chalcogenide glass are sputtered to fill an array of device vias, as one or more step(s) in fabricating a programmable conductor memory cell array. However, because of the high mobility of the metal atoms, metal dendrites can form around the device vias during the sputter deposition. The dendrites seem to nucleate at top edges of the vias and can extend for several microns along the surrounding surface. These dendrites are extraneous and should not be confused with the conductive paths that grow and shrink within the programmable conductor memory cells under normal operating conditions.  
         [0022]    Extraneous dendrite formation during metal/chalcogenide glass deposition can cause a number of problems. The metal contained within the extraneous dendrites is intended to be included in the as-deposited metal/chalcogenide glass mixture that forms the programmable conductor memory cells. The metal that forms the dendrites is removed from the metal/chalcogenide glass mixture by the migration of metal during extraneous dendrite formation and the component proportions are thus different than intended. Controlled and consistent composition is important for reproducible functioning of the device. If there is less metal in the cell mixture, it may take a higher voltage and/or a longer time for a cell dendrite to grow across the cell and to switch the cell from a “zero” state to a “one” state. The extraneous dendrites also cause changes in the surface morphology around the device vias. This non-planar topography can cause problems during subsequent process steps, such as photolithography and etch.  
         [0023]    A method of depositing metal/chalcogenide glass films to form programmable conductor memory cells without forming extraneous dendrites is needed. The aforementioned need is satisfied by the process of the preferred embodiment, which includes conducting the deposition process under conditions of elevated temperatures.  
         [0024]    The preferred embodiments are illustrated in the context of co-sputtering of metal and chalcogenide glass for a programmable conductor memory cell array, for which the preferred embodiments have particular utility. The skilled artisan will readily appreciate, however, that the materials and methods disclosed herein will have application in a number of other contexts where prevention of dendrite formation is desirable. For example, in one embodiment, chalcogenide glass (e.g., Ge x Se 1-x ) is first deposited, followed by sputtering metal (e.g., Ag). The glass element can then be doped with metal by photodissolution, as is known from prior disclosures. In such an embodiment, elevated temperatures are preferably maintained, as disclosed below, at least during the metal sputtering phase.  
         [0025]    These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.  
         [0026]    There are a number of possibilities that might explain extraneous on surface dendrite formation during deposition. One possible mechanism is that the step edge, where the via meets the uppermost surface of the surrounding insulating layer, is energetically favorable for atom absorption. Therefore, highly mobile metal atoms have a high probability of being absorbed at the step edge. Metal atoms cluster together at the step edge, forming a nucleation site for dendrite growth. There are many free metal atoms available from the surrounding layer surface to contribute to a growing dendrite.  
         [0027]    If a metal with low mobility were used, there would be less likelihood of forming extraneous dendrites, but this would work against the proper functioning of a chalcogenide glass-based memory device. A metal with low mobility would also be less likely to form a conductive path within the memory cell when a voltage is applied, which is necessary for the cell to function. Metals with high mobility are therefore most desirable for use in chalcogenide glass-based memory devices.  
         [0028]    Experiments were done to try to reduce or eliminate extraneous dendrite formation. Test structures were formed on production grade silicon wafers  4  covered by 600 Å of tungsten  6 , followed by a 500 Å layer of silicon nitride  8 , to simulate actual device array constructions. Individual isolated vias  9  and arrays of vias were patterned and etched into the nitride layer to a depth of 50 nm. The wafers were heated to various test temperatures and held on the heated wafer stage for at least 5 minutes to obtain a stable and uniform temperature distribution across the wafer. Layers of silver and germanium-selenide were co-sputtered to a thickness of 50 nm. Co-sputtering was performed in accordance with the teachings of U.S. application Ser. No. 10/164,429 of Li et al., filed Jun. 6, 2002, entitled CO-SPUTTER DEPOSITION OF METAL-DOPED CHALCOGENIDES, attorney docket no. MICRON.243A, the disclosure of which is incorporated herein by reference. Results from these experiments are shown in FIGS. 2A, 2B,  3 A, and  3 B for vias having a width of about 0.5 μm. To make comparisons simple, the figures have been drawn to approximately the same magnification.  
         [0029]    [0029]FIG. 2A is a schematic drawing made from an electron micrograph that shows a via array structure  10  after deposition of silver and germanium-selenide, co-sputtered at room temperature. There are a number of dendrites  12  growing out from the periphery of the array. In addition, in the outermost columns, nearly every via  14  has the beginning of a silver dendrite structure  16  formed along an edge. These were all seen clearly in the original micrograph whose magnification was 5000×.  
         [0030]    [0030]FIG. 2B is a schematic drawing made from an electron micrograph that shows an isolated via  20  in another region of the test structure after deposition of silver and germanium-selenide, co-sputtered at room temperature. Extensive silver dendrite growth can be seen in the micrograph whose magnification is 2600×. In one example, three long dendrites  22  extend outward from the test via  20 , and additional silver precipitation  24  has occurred along the rim of the via.  
         [0031]    [0031]FIG. 3A is a schematic drawing made from an electron micrograph that shows a via array structure  10  after deposition of silver and germanium-selenide, co-sputtered at 50° C. No dendrite formation can be seen even at an electron micrograph magnification of 15,000×.  
         [0032]    [0032]FIG. 3B is a schematic drawing made from an electron micrograph that shows an isolated via  20  in another region of the test structure after deposition of silver and germanium-selenide, co-sputtered at 50° C. No dendrite formation or silver precipitation can be seen even at an electron micrograph magnification of 50,000×.  
         [0033]    The preferred embodiments of the current invention provide a method of forming a programmable conductor memory cell that minimizes or, preferably, eliminates extraneous dendrite formation.  
         [0034]    In a preferred embodiment, a metal and a chalcogenide glass are sputtered onto a prepared substrate that is maintained at a temperature higher than room temperature during the sputtering. Preferably, the metal and the chalcogenide glass are co-sputtered, or sputtered concurrently from separate sputtering targets in the same physical vapor deposition (PVD) or sputtering chamber. The chalcogenide glass can be sputtered using a pulsed DC power supply.  
         [0035]    Generally, for fabrication of programmable conductor memory cells having a container configuration, the prepared substrate comprises a top insulating layer with vias therein. It should be understood that, although the illustrated embodiments of the current invention have been described for a container configuration, they may be useful also for other configurations of programmable conductor memory cells and for other applications wherein layers of metal-diffused chalcogenide glass without dendrites are desired.  
         [0036]    The chalcogenide glass can comprise chalcogens such as sulfur, selenium, or tellurium. Preferably, the chalcogenide glass also comprises selenium and germanium in a ratio of Ge:Se between about 4:6 and 2:8. The metal can comprise any metal that is a fast diffuser in chalcogenide glass, including but not limited to silver or copper. Preferably, sputtering the metal and chalcogenide glass results in a programmable conductor memory cell with a desired ratio of components.  
         [0037]    More generally, it will be understood that the memory cell formed of metal-doped chalcogenide glass comprises constituents in proportions to maintain the cell body in an amorphous state. The skilled practitioner will appreciate that the amorphous state of the metal-doped chalcogenide glass such as germanium selenide (Ge x Se 1-x ) depends upon both the value of x and the relative concentration of the metal within the memory cell.  
         [0038]    A phase diagram can be used to select a value for x and to select the amount of metal that is to dope the chalcogenide glass such that the chalcogenide glass remains amorphous. Such a phase diagram can be found in Mitkova et al., “Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses,”  Physical Review Letters , Vol. 86, No. 19 (Nov. 8, 1999), pp. 3848-3851. FIG. 1 of Mitkova illustrates two glass forming or amorphous regions for germanium selenide (Ge x Se 1-x ) doped with silver (Ag). In one example, where x is 30, i.e., 0.30, so that the germanium selenide glass is Ge 30 Se 70 , the amount of silver (Ag) used to dope the germanium selenide should fall within about 0% to 18% or within about 23% to 32% by atomic percentage versus the amount of selenide (Se).  
         [0039]    Preferably, the prepared substrate is maintained at a temperature between about 30° C. and 150° C., more preferably between about 40° C. and 130° C. and most preferably, between about 45° C. and 60° C. during the sputtering. In some arrangements, the prepared substrate is maintained at the desired temperature for more than 1 minute and preferably for at least about 5 minutes before sputtering the metal and chalcogenide glass.  
         [0040]    Hence, although the foregoing description of the preferred embodiments of the present invention has shown, described and pointed out the fundamental novel features of the invention, it will be understood that various omissions, substitutions and changes in the form of the detail of the method as illustrated as well as the uses thereof may be made by those skilled in the art, without departing from the spirit of the present invention. Consequently, the scope of the present invention should not be limited to the foregoing discussion, but should be defined by the appended claims.