Abstract:
A memory structure has an access transistor connected in series with a programmable resistive element, wherein the programmable resistive element comprises on a semiconductor substrate; an insulated layer with a cavity comprising: a first layer lining the lateral surfaces and the bottom of the said cavity and impermeable to the diffusion of metal; a second layer made of porous material on the said first layer; a third layer of metallic material allowing to realize a contact electrode susceptible to spread within the said formed porous material of the second layer. Diffusion of metallic ions within the said second layer is controlled by the joint action of an electric field and temperature. A manufacturing process is also described.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to the electronic memories domain and more specifically to a memory structure with an access transistor (TR) connected in series with a programmable resistive element. 
     2. Description of the Related Art 
     Programmable electronic memories for a number of years have undergone significant development, in particular with the growth of non-volatile memories which keep the information even when they are not powered and which are widely used in portable electronic devices. 
     It is desirable to integrate those non-volatile memories into integrated circuits comprising CMOS logic in order to realize full systems (called “system on chip” in the Anglo Saxon literature), including in a same semiconductor product, electronic logic and memory circuits. 
     The category of non-volatile memories subdivides in three sub-categories, that is rewritable ROM memories (Read Only Memory including PROM, EPROM, EEPROM, and FLASH memories), non-re-writable ROM memories, and memories called RAM (Random Access Memory). 
     Research regarding RAM memories has enabled the development of ferroelectric-type memories (FeRAM) and magneto resistive memories (MRAM), the latter based on a resistive material controlled by a magnetic field and, more recently, phase change memories (PCM in the Anglo Saxon literature). 
     Phase change memories (PCM) are based on the use of chalcogenide alloys (Ge 2 , Se 2 , Te 2 ) which present two possible phases, respectively disorderly (amorphous) or more orderly (crystalline), distinguished from one another by different resistivity. The transition from one phase to another is controlled by a cycle of heating/cooling and enables then to fix the material in a predetermined phase representative of a given information, a “0” or a “1”. 
     U.S. Pat. Nos. 3,271,591 and 3,530,441 describe the use of phase change material (PCM) to realize programmable memory elements. 
     Another known technique allowing realization of a programmable resistive memory element is based on the use of a solid electrolyte and the dissolution of metal in the electrolyte using a current or an electric field. The international application WO 2005/124788 entitled “ Nanoscale Programmable Structures and methods of forming and using the same ” from M. KOZICKI, illustrates such a technique of realization of a programmable memory cell. 
     The use of techniques outlined above has interesting advantages because of significant space savings offered by the memory cell which may be formed with one access transistor. Drawbacks remain, in particular related to the high number of photolithography steps used by these techniques. 
     For these reasons in particular, alternative techniques to achieve cells and memory circuits based on programmable resistive elements are the subject of much research. 
     BRIEF SUMMARY 
     One embodiment is an alternative technique enabling realization of a non-volatile memory cell based on a programmable resistive material. 
     One embodiment realizes a non-volatile memory that can be easily integrated with logic circuits within a same semi conductor product and that, in addition, achieves high density, rapid memory access, and a limited power consumption. 
     One embodiment is a memory structure with an access transistor (TR) connected in series with a programmable resistive element (EM), wherein the programmable resistive element includes:
         on a semiconductor substrate an insulating layer ( 170 ) having a cavity;   a first layer ( 140 ) lining the lateral surfaces and the bottom of the cavity and impermeable to the diffusion of metal;   a second layer ( 150 ) made of porous material on the first layer;   a third layer ( 160 ) of metallic material allowing to realize a contact electrode susceptible to spread within the said formed porous material of the second layer.       

     Diffusion of metallic ions within the second layer is controlled by the joint action of an electric field and temperature. 
     In a particular mode of realization, the first layer is made of tantalum nitride/tantalum (TaN—Ta) or titanium nitride/titanium (TiN—Ti). 
     In a particular mode of realization, the second layer is made of silicon oxycarbide (SiOC) or germanium oxycarbide (GeOC). 
     In one embodiment the third layer is a metal with high diffusion, such as copper for example. 
     In a particular mode of realization, an intermediate layer is arranged under the memory point, and includes a metallic via, for example in tungsten, insulated by a barrier layer of Ta—N. 
     One embodiment is an electrically programmable non-volatile memory device comprising a memory map formed of a matrix of memory cells each formed of one access transistor (TR) and one memory element (EM i,j). The memory element includes:
         on a semi conductor substrate an insulated layer ( 170 ) with a cavity;   a first layer ( 140 ) lining the lateral surfaces and the bottom of the cavity and impermeable to the diffusion of metal;   a second layer ( 150 ) made of porous material on the said first layer;   a third layer ( 160 ) of metallic material allowing to realize a contact electrode susceptible to spread within the said formed porous material of the second layer.       

     Diffusion of metallic ions within the said second layer is controlled by the joint action of an electric field and temperature. 
     The matrix comprises:
         first groups of cells (CLii) all extending according to a first direction, each first group comprising cells having transistors that are connected together by a first metallization (WLAi), and memory elements (EMi) with upper electrodes that are connected together by a second metallization (WLPi),   second groups of cells (CLj) all extending according to a second direction, each second group comprising cells having transistors with sources that are connected together by a third metallization (BLj).       

     The device has means of control, able to apply selected voltages on the first, second and third metallization, in order to selectively program each cell. 
     One embodiment provides a manufacturing process of a programmable memory structure comprising the following steps:
         providing a substrate;   depositing an insulated layer with a cavity;   setting a first layer lining the lateral surfaces and the bottom of the said cavity and impermeable to the diffusion of metal;   setting a second layer made of porous material on the said first layer,   setting a third layer realized in metallic material presenting a power of diffusion within the said second layer,   applying an electric field together with a change in temperature to cause a controlled diffusion of metal atoms of the said third layer within the said second layer.       

     In one embodiment, the process includes the provision of a first layer of pre-metal dielectric, which is hollowed out to make a well exposing a determined surface of substrate. The process sets down in that cavity a thin layer of TiN lining the sidewalls as well as the bottom of the well in order to realize a first protective barrier. Then the process fills in the well with a metal, such as tungsten, for instance, and performs an operation of chemical mechanical planarization (CMP). The process then sets down a second barrier of material SiC, then a second layer of silicon oxycarbide. The process then realizes the second cavity of the second layer of silicon oxycarbide with a greater cross-section than that of the first cavity. The process sets down then on the inner walls of that second well a third barrier made of TaN—Ta or TiN—Ti and a third thin layer of silicon oxycarbide intended to serve as a programmable resistive element. The remaining cavity is then filled with copper, for example, in order to form an upper contact electrode for the programmable resistive element. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other characteristics will appear from reading the description and the drawings below, given only as non-exhaustive examples. 
         FIG. 1  illustrates a semiconductor product comprising a logic circuit with a matrix of memory cells. 
         FIG. 2  illustrates the microelectronic structure of a memory element. 
         FIG. 3  illustrates the schematic diagram of the association of the  FIG. 2  structure to form a memory cell. 
         FIG. 4  illustrates the equivalent electric diagram of a memory cell. 
         FIG. 5  illustrates a matrix organization of memory cells to realize a memory map MM. 
     
    
    
     DETAILED DESCRIPTION 
     We describe now how to realize a memory element based on a programmable resistive material to achieve an onboard non-volatile RAM, such as that represented in  FIG. 1  which illustrates a semiconductor product with a matrix of memory cells  20 , arranged in rows and columns, and communicating with logic circuits  30  through circuits  40 . The realization of such an “onboard” non-volatile memory RAM on a system on chip therefore represents only one particular embodiment. One skilled in the art may adjust the teaching of the present application to realize a semiconductor product with a single memory. 
     In reference to  FIG. 2 , the structure of a particular mode of realization of a memory element and its manufacturing process is described. Although the description below will describe the fabrication of a single memory element, it is clear that many elements can be made as needed. 
     First, the process prepares a layer of semiconductor substrate  110 , made of silicon (Si), GaS, or SOI (Silicon on insulator in the Anglo-Saxon literature) which may also serve as the medium to the access transistor (shown in  FIG. 4 ) as well as to metallizations forming the worldlines and bitlines of  FIG. 4 . 
     In a first step an optional layer  120  of pre-metal dielectric having a conductor via  130  is realized. 
     For this purpose, the process sets down on the substrate  110 , by any means such as a CVD operation (Chemical Vapor Deposition), a layer  120  of pre-metal dielectric (PMD), made of a relevant dielectric such as an oxide, SiOC, BPSG (Boron Phosphorous Silicon Glass), PSG (Phosphorus Silicon Glass) or a silicon nitride. 
     The layer  120  is then hollowed out to achieve a cavity to expose a determined surface of the substrate  110 . The cavity can present any cross-section, rectangular or circular for example and any well-known method of etching can be used. 
     The process then sets down a thin layer  180  of TiN on the sidewalls and the bottom of the well to achieve a barrier that protects layers  110  and  120  from contamination by a metal compound—such as tungsten (W) for example, that is set down in the cavity to form a via  130  referenced in  FIG. 2 . Alternatively, the following metals can be used to realize the via  130 : nickel, molybdenum, platinum, metal silicides and the following compounds SiOC, BPSG (Boron Phosphorous Silicon Glass), PSG (Phosphorus Silicon Glass). 
     Once via  130  is realized, an operation of mechanical planarization of type CMP (Chemical Mechanical Planarization) is performed, for example, or by any other operation know to those skilled in the art. 
     As it has been mentioned above, the layer  120  comprising the metallic via (W) is an option in the process described. Those of skill in the art will be able to adapt the embodiments without realizing the layer  120  and its metallic well  130 , but there is some interest to have this intermediate layer  120  with the metallic via  130 . 
     First, an interest comes from the fact that this layer  120  enables the development of structures of transistors that may constitute, in particular, the amplification circuits and selection lines of the memory. 
     Second, it can be advantageous to use the metallic via  130  to guarantee some thermal inertia as well as good heat conductibility. 
     Despite technical advantages resulting from the combination of layer  120  and via  130 , it should be noted that embodiments are not limited to the use of this layer  120 . 
     Then the process sets down on layer  120  or, when this one is not present, directly on the substrate  110 , a layer of material  190  with low permittivity (low-K). The layer  190  can include a material chosen among the family SiC, SiOC, OCGe, GeSiO and satisfying the following three functions: 
     1) ensure a low permittivity (low-K) to avoid the capacitive type effects in particular; 
     2) provide some electrical insulation; 
     3) a stop layer for later etching. 
     Once layer  190  is in place, the process then sets down a layer  170  of an insulating material with low permittivity, such as a dielectric porous oxide like for example SiOC selected in particular to reduce the dielectric constant value. 
     A second cavity is then formed within the layer  170 . In one embodiment the second cavity has in a larger cross-section than that of the first cavity used to realize the metallic via  130 . In one embodiment, this second cavity also presents a circular cross-section like that of via  130 . 
     This second cavity shows a predetermined surface of the PMD layer  120  comprising the metallic via  130 . 
     A thin barrier layer  140  is then deposited on the inner walls of the second cavity, for example of TaN—Ta intended to avoid any diffusion of copper or metal, through the sidewalls of the second cavity, towards the SiOC of layer  170 . Alternatively TiN—Ti may be used. 
       FIG. 2  illustrates a particular embodiment in which the barrier layer  140  covers the entire inner surface of the second cavity realized within the layer  170 , but it is not a limiting element. 
     The process then arranges, within the second cavity, a second thin layer  150  of porous SiOC or GeOC intended to form a sock to receive or accommodate a metal contact  160 . 
     Contact  160  is selected from any metal whose atoms are likely to disseminate within material SiOC. In a particular mode of realization, contact  160  is realized in copper. Alternatively, it might be realized in another metal at high diffusion, such as Ag, Al . . . . 
     As seen in  FIG. 2 , a contact in copper  160  is thus realized that can diffuse partially within the layer of SiOC  150  depending on settings, in order to modify the characteristics of resistivity of the electric circuit created between the contact  160  and the metallization layers set down (and not shown) on substrate  110 . 
     Contact  160  realizes the upper electrode of a programmable resistive element, which has a second electrode which is the metal or material of via  130 . 
     The structure of contacts coupled to the electrodes can be completed to facilitate the connection of the structure to the outside. Contacts can be made by any metallic material, such as gold, aluminum and its alloys, tungsten or copper. 
     The resistive element above described can be associated with any access transistor and the whole realizes then a full memory cell, whose electric diagram is shown in  FIG. 4 . 
     To control the phenomenon of electro-migration of copper atoms within the layer SiOC  150  (and consequently the resistivity of that layer), the power supplied to the memory cell is modified. The power can be modified in particular by applying an electric field, a current, or a stage of heating during a phase of writing and, more specifically, a joint action of an electric field and temperature. 
     The result is a modification of resistivity parameters which is measured by means of amplifiers of appropriate reading. Indeed, when the layer of SiOC  150  is not contaminated by the copper atoms, there is a resistance with a high value. On the contrary, when the copper atoms diffuse within the layer of SiOC  150  the contact is of relatively low resistance between the copper  160  and the TaN barrier layer  140 . 
     That modification of the resistivity of the memory element enables then, to fix information, either 0 or 1 within that element. 
     The memory element described can be used to store information and can then be used to realize electronic memories. In particular, the cell can advantageously be substituted to other know devices, such as DRAM, SRAM, PROM, EPROM, EEPROM memories or any combination of these. 
     To realize the reading operations an access transistor is associated with a memory element, thereby creating a full memory cell. 
     First Mode of Realization: MOS Transistor 
     A first mode of realization of a memory according to one embodiment is based on a structure of type NMOS, allowing to economically manufacture with a certain relative density, and is illustrated in  FIGS. 2 and 3  respectively showing a diagram of structure incorporating the memory to a MOS transistor and its equivalent electrical diagram. 
     In  FIG. 3  the memory cell includes an access transistor, for example NMOS, having a drain, a source and a gate, respectively represented by letters D, S and G in  FIG. 4  and by regions  230 ,  220 , and  210  in  FIG. 3 . 
     Drain  230  is electrically connected to the lower electrode of the memory element  100  whose upper electrode  180  is connected to potential ES. 
     Those of skill in the art may fix values of potential differences according to the technology. 
     As can be seen, such a structure is particularly reduced since one NMOS transistor may achieve a single memory cell. 
     The reading operation of the content of such a memory is performed in a classical way by applying a potential difference between the ES &amp; S terminals and measuring the current flowing through this path. If the current is very low, this means that the memory element  100  has conductivity particularly low, representative of an information type “0” for example; in the opposite case, the conductivity will be higher, and information “1” will be read. 
     Second Mode of Realization: Bipolar Transistor 
     A second mode of realization of a memory according to one embodiment is based on a bipolar transistor structure which enables reduction of the space occupied by the memory cell and has the advantage of a higher density. Clearly, one skilled in the art may adapt the just described scheme by using bipolar transistors, with tunnel effect and diode device. 
       FIG. 5  illustrates a way to organize a set of memory cells, such as those described previously, in order to realize a memory map organized, on one side, in first groups (by rows) and, on the other, in second groups (by columns). 
     As of the first groups, we see that the first row contains the group formed by cells CL 11 , CL 12  and CL 13 , while the second row is formed by cells CL 21 , CL 22  and CL 33  and that the third row is formed by cells CL 31 , CL 32  and CL 33 . 
     Each first group (i.e., group formed by row CL 11 , CL 12  and CL 13 ) comprises cells whose transistor gates are connected together by a first metallization WLi (i=1 for the first row, i=2 for the second row and i=3 for the third row). Moreover, the upper electrodes ES of the memory elements EM(i) are linked together by a second metallization WLPi (i=1 for the first row, i=2 for the second row and i=3 for the third row). 
     As of the second groups, we see that each column contains a second group of memory cells CL. 
     Thus, the first column is formed by the group of cells CL 11 , CL 21 , CL 31 , while the second column is formed by the group CL 12 , CL 22  and CL 32  and finally the third column is formed by the group CL 13 , CL 23  and CL 33 . 
     Each second group has transistors whose sources are linked together by a third metallization BLj (j=1 for the first column, j=2 for the second column and j=3 for the third column). 
     The memory device comprises also a controller  250  able, in the programming or the reading mode, to apply the adequate voltages to rows on the first, second and third metallization WLi, WLPi and BLj, in order to allow the desired reading and writing processes. The way to generate the adequate signal of control is well known by those skilled in the art and consequently the logic circuits and the generation of different voltages will not be described in more detail. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.