Method of manufacture of programmable conductor memory

[0001] 1. Field of the Invention

[0002] This invention relates generally to a method of manufacture of memory devices for integrated circuits and more particularly to programmable conductor memory arrays comprising glass electrolyte elements.

[0003] 2. Description of the Related Art

[0004] 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.

[0005] 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 laptop 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.

[0006] 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. 5,761,115; No. 5,914,893; and No. 6,084,796), which are incorporated herein by reference. The programmable conductor cell of Kozicki et al. (also referred to by Kozicki et al. 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 on another. The glass/ion element shall be referred to herein as a glass electrolyte, or, more generally, cell body. When a voltage is applied to the anode and the cathode, a non-volatile conductive pathway (considered a sidewall dendrite by Kozicki et al.) grows from the cathode through or along 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 longer the 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.

[0007] In the case of a dielectric material, programmable capacitance between electrodes are programmed by the extent of dendrite growth. In the case of resistive material, programmable resistances are also programmed in accordance with the extent of dendrite growth. The resistance or capacitance of the cell thus changes with changing dendrite length. By completely shorting the glass electrolyte, the metal dendrite can cause a radical change in current flow through the cell, defining a different memory state.

[0008] The recent trends in memory arrays generally have been to form first a via, then fill it with a memory storage element (e.g., capacitor) and etch back. It is simple to isolate individual memory cells in this way. Programmable memory cells have been fabricated also using a container configuration, wherein the electrodes and body layers are deposited into a via etched into an insulating layer. The dendrites were believed by Kozicki et al. to grow along the interface between the cell and the via wall. Typically, the memory cell is formed in an array having a conventional DRAM-type circuit design. For example, in a conventional cross-point circuit design, memory elements are formed between upper and lower conductive lines at intersections. Typically, 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.

[0009] Heretofore known structures for PMC memory arrays are rather limiting. For example, it has often been found difficult to deposit component films for a PMC device to fill small vias. Accordingly, a need exists for novel methods and structures for exploiting the potential of PMC technology.

[0010] Accordingly, a need exists for improved methods and structures for forming integrated programmable conductor memory arrays.

[0011] In accordance with one aspect of the invention, a method is provided for fabricating programmable conductor memory cells on a substrate using blanket deposition of layers. The fabrication method comprises forming a bottom electrode in contact with a conductive region in a semiconductor base material, providing a glass electrolyte layer having metal ions diffused therein and being capable of growing conductive pathway(s) therethrough under the influence of an applied voltage and forming a top electrode layer. Subsequently, the top electrode layer, the glass electrolyte layer and the bottom electrode are patterned and etched to define separate pillars or cells of stacked materials.

[0012] In a preferred embodiment, a liner layer of an insulating material that conforms to the cells and the semiconductor base material is deposited over the cells. This liner layer acts as a barrier to prevent diffusion of the metal away from the cell. Remaining regions between the cells are filled with an insulating layer. At least some of the insulating layer and some of the liner layer are removed to make contact to the top electrode layer of the cell and to the substrate.

[0013] In accordance with another aspect of the invention, a memory array structure is described wherein each programmable conductor memory cell or pillar comprises a bottom electrode, a cell body formed of a glass electrolyte layer having metal ions diffused therein and a top electrode.

[0014] These and other aspects of the invention will be better understood in view of the detailed description below and the appended drawings, which are meant to illustrate and not to limit the invention, and in which:

[0015] FIG. 1 is a schematic cross-section of a partially fabricated integrated circuit, showing blanket layers deposited on a semiconductor substrate according to a preferred embodiment of the present invention.

[0016] FIG. 2 is a schematic cross-section of the structure of FIG. 1 after patterning to define a programmable memory cell in a stud configuration.

[0017] FIG. 3 is a schematic cross-section of the programmable memory cell of FIG. 2 after deposition of a thin, conformal sealing layer.

[0018] FIG. 4A is a schematic cross-section of two cells in an array of programmable memory cells, similar to that of FIG. 3, after deposition of a layer of insulating material to fill the spaces around them.

[0019] FIG. 4B is a schematic cross-section of the cells of FIG. 4A after electrical contacts to the electrodes of the cells have been made.

[0020] The preferred embodiment discloses a method of fabricating a programmable memory cell in a stud configuration. Blanket layers of electrode and body materials are deposited and subsequently patterned and etched, leaving pillars or studs as individual devices. The blanket deposition of the layers avoids problems associated with via filling and allows more flexibility in cell configuration.

[0021] With reference to FIG. 1, a cross section of deposited blanket layers is shown. A semiconductor base material 100 containing a heavily doped (e.g., n) conductive region 102 comprises the substrate 101 upon which the memory cells are fabricated. In general, the substrate comprises the lowest layer of semiconductor material from which devices are formed in the integrated circuit. In the illustrated embodiments, the substrate comprises a single-crystal silicon wafer. In other arrangements, an epitaxial silicon layer or another semiconductor, such as a III-V material, can be used.

[0022] A bottom electrode 103 makes contact with the substrate. The bottom electrode 103 can include more than one layer. In the illustrated embodiment the bottom electrode 103 comprises a layer of polysilicon 104, a layer of tungsten nitride 106 and a layer of tungsten 108. Preferably, the polysilicon layer 104 is between about 25 nm and 75 nm thick, the tungsten nitride layer 106 is between about 5 nm and 40 nm thick and the tungsten layer 108 is between about 50 nm and 100 nm thick.

[0023] The body 110 of the programmable memory cell is deposited over the electrode 103. The memory cell body 110 comprises a glass material, preferably a chalcogenide glass selected from the group consisting of sulfur, germanium, selenium and tellurium, more preferably a glass comprising germanium (Ge) and selenium (Se), in which the ratio of Ge:Se can vary (e.g., Ge₄Se₆, Ge₃Se₇ or Ge₂Se₈) but is preferably targeted for about Ge₂₅Se₇₅). The thickness of the germanium selenide layer is preferably between about 25 nm and 75 nm, with a target thickness of about 50 nm in the illustrated embodiment. Preferred methods of depositing the GeSe material include sputtering and evaporation. More complicated structures for the cell body are also contemplated, as discussed below.

[0024] A metal film (not shown) is deposited over the memory cell body 110 as a source of metal ions for diffusion into the memory cell body. Preferably, the metal film includes a metal from Group B or Group IIB (e.g., silver, copper or zinc), and more preferably the metal is silver. Alternatively, the metal film may be co-sputtered tungsten-silver. Energy is provided to diffuse metal from the film into the memory cell body 110, preferably by photodissolution, a self-limiting reaction. Take, for example, a silver layer of 30 nm over a GeSe layer 110 of 50 nm, exposed to ultraviolet radiation, particularly ultraviolet radiation with a wavelength of less than 500 nm at about 4 mW/cm² for about 15 min. Sufficient silver dissolves into the glass layer 110 to saturate the glass by forming a glass electrolyte ternary compound with about 30 atomic percent silver. It is preferred that there be some silver film left over the cell body 110 after the diffusion occurs. In one example, the 50-nm layer of GeSe consumes about 15 nm of silver, leaving a film 112 of about 15 nm over the memory cell body after the photodissolution reaction.

[0025] In other arrangement, metal for the programmable conductor memory cell body is supplied by other means. For example, the metal and glass material can be co-sputtered or deposited from a source that contains all species, so no metal deposition and drive-in steps are needed. This fast ion conducting element with metal ions diffused therein will be referred to herein as the memory cell body 110.

[0026] In still other arrangements, the cell body 110 can include multiple layers. For example, the cell body 110 in one embodiment includes a first GeSe layer (e.g., Ge₄Se₆), a metal selenide layer (e.g., Ag₂Se) thereover, and a second GeSe layer (e.g., Ge₄Se₆) over the metal selenide layer. The skilled artisan will appreciate that the embodiments discussed below are equally applicable to any of a variety of other programmable conductor arrangements. In the GeSe/metal selenide/GeSe sandwich structure described in this paragraph, the intermediate metal selenide layer provides metal to the cell body 110 for formation of conductive pathways under the influence of applied electrical fields.

[0027] After dissolution, if used, an additional silver layer 112 may be deposited particularly if the original silver layer was fully consumed in the dissolution process. Preferably the top electrode further comprises a tungsten layer 114 with a thickness between about 5 nm and 25 nm and is formed over any remaining or added metal film 112.

[0028] The stack of blanket layers is patterned using a mask that forms pillars, preferably less than about 250 nm in width and preferably separated from one another by at least about 300 nm, using standard photolithographic techniques as are known in the art. The layers from the bottom electrode 103 to the top electrode 114 are then etched down to the substrate 101, preferably including a physical etch component, more preferably using Ar/Cl sputtering or reactive ion etching.

[0029] As shown in FIG. 2, after patterning and etching, separate pillars 120 (one shown) of the layered materials shown in FIG. 1 remain. Each pillar 120 is an individual programmable memory cell. Preferably, the width of each cell 120 is less than about 250 nm, and the spaces between the cells are at least about 300 nm in width. The skilled artisan will readily appreciate that such spacing issues will depend upon the critical dimension for any particular generation of integrated circuit processing. Currently, state-of the-art fabrication techniques employ 0.13 m line widths, although this critical dimension is expected to continue to shrink with each new generation of integrated circuit technology.

[0030] FIG. 3 shows an exemplary memory cell 120, covered by a blanket liner layer 200 of insulating material, preferably between about 5 nm and 50 nm in thickness, and also preferably comprising a material that can serve as a diffusion barrier. The liner layer 200 is selected to prevent diffusion of metal from metal film 112 away from the cell 120 and into other areas of the integrated circuit, where it could cause severe problems. In the illustrated embodiment, the liner layer 200 comprises silicon nitride that conforms over the cells and the semiconductor base material.

[0031] Silver is a fast diffuser in semiconductor devices, and normally it is used in integrated circuits only with extreme caution. Similarly, all metals suitable for doping the glass electrolyte body 110 will be fast diffusers. Fabrication of the memory cell array of the illustrated embodiment comprises deposition of a blanket layer of silver to a thickness of about 30 nm. Much of the blanket layer is removed during etching to form individual memory cell stacks 120. The silver that remains in the individual cells is encapsulated by the liner layer 200 to prevent it from diffusing away from the cell 120.

[0032] FIG. 4A shows two memory cells 120 in a memory array after blanket deposition of an insulating layer 210 to fill the spaces between the separate cells 120. In the illustrated embodiment, the insulating material 210 is silicon oxide deposited using TEOS, although the skilled artisan will readily appreciate that other insulating materials (e.g., low k materials) known in the art can be used readily with good result.

[0033] Electrical contacts are made to the electrodes of the memory cells 120 as shown in FIG. 4B. At least some of the insulating material 210 and/or liner layer 200 are removed from the upper surface of electrode layer 114. These materials can be removed by chemical mechanical polishing (CMP), arranged to stop on and expose the top surface of electrode layer 114, or alternatively by patterning and etching, preferably dry etching, as shown in FIG. 4B, or any other method known in the art. Note that the drawings are not to scale and that the openings for making contact can be comparable to the dimensions of the cell 120 top surface. In addition, vias are etched through the insulating layer 210 between cells down to the conducting region 102 of the semiconductor substrate 101. A conducting material is used to make contact to both the electrode layers 114 (contacts 220) and the conducting region 102 of the substrate (contacts 230). Contact 230 makes electrical connection to the lower electrode 103 through conducting region 102.

[0034] The preferred embodiment thus involves an array of programmable conductor memory cells wherein each cell comprises stacked materials on a semiconductor substrate. The regions between the cells comprise insulating material, preferably including a thin barrier layer 200 (preferably silicon nitride) conforming to the cells 120 and to the substrate 100 and a thick layer of insulation 210 (preferably a form of silicon oxide) that fills in the remaining space between the cells. Preferably, the barrier layer 200 is between about 5 nm and 50 nm thick. Preferably, the silicon oxide 210 is deposited from tetraethylorthosilicate (TEOS).

[0035] Each programmable conductor memory cell 120 comprises the layers discussed above in reference to FIGS. 1-4. These include a bottom electrode 103 comprising one or more layers, a cell body 110 that includes glass electrolyte layer having metal ions diffused therein and a top electrode 112, 114.

[0036] As described above, the bottom electrode 103 preferably comprises a polysilicon layer 104, a tungsten nitride layer 106 and a tungsten layer 108. Preferably, the polysilicon layer 104 is between about 25 nm and 75 nm thick, the tungsten nitride layer 106 is between about 5 nm and 40 nm thick and the tungsten layer 108 is between about 50 nm and 100 nm thick.

[0037] In the illustrated embodiment, the glass electrolyte material 110 comprises a chalcogenide glass with metal ions diffused therein, more preferably silver germanium selenide, typically of the form Ag/Ge₂₅Se₇₅. The thickness of the silver germanium selenide layer is preferably between about 25 nm and 75 nm. There is some silver film 112 remaining over the glass after silver ions have been driven into the GeSe, whether left over from photodissolution or deposited after forming the electrolyte 110.

[0038] Preferably the top electrode also comprises a tungsten layer 114 over the silver film 112 with a thickness between about 5 nm and 25 nm.

[0039] In operation, the memory cell body 110, i.e., the glass electrolyte material having metal ions therein, forms conductive pathways that grow from the negative electrode to the positive electrode when a first voltage is applied to the electrodes 103, 114. The growth of the conductive pathways depends upon applied voltage and time. Low voltages cause slow growth, whereas higher voltages result in faster growth of conductive path(s) from the negative electrode to the positive electrode. Without being limited by theory, it is believed that the conductive pathways grow by precipitation of cations (e.g., silver cations) from the memory cell body 110. Changes in the extent of the conductive pathways affect the resistance of the devices. The conductive pathway remains intact when the voltage is removed.

[0040] For a binary programmable conductor memory device, the memory has two basic states: 0 and 1. When there is no conductive pathway, the memory cell has high electrical resistance and reads as 0. When the conductive pathway shorts the memory cell body, from the cathode to the anode, the resistance is low and the memory cell reads as 1. The change in resistance of the memory cell with and without a conductive pathway can be as much as two orders of magnitude, i.e., a change from Megaohms to milliohms. Reversing the polarity of the voltage preferably reverses the conductive pathway.

[0041] Alternatively, the memory cell can be programmed into as many as 3 or 4 states by setting the extent of the conductive pathway. These changes can be detected easily by passing current through the bit lines and word lines (not shown) connected to the contacts 220, 230 in the memory array, such that changing the extent of the pathways can serve to change the state of the memory bit.

[0042] Although the foregoing description of the preferred embodiment 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 apparatus 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.