Abstract:
A contact structure, including a first conducting region having a first thin portion with a first sublithographic dimension in a first direction; a second conducting region having a second thin portion with a second sublithographic dimension in a second direction transverse to said first direction; the first and second thin portions being in direct electrical contact and defining a contact area having a sublithographic extension. The thin portions are obtained using deposition instead of lithography: the first thin portion is deposed on a wall of an opening in a first dielectric layer; the second thin portion is obtained by deposing a sacrificial region on vertical wall of a first delimitation layer, deposing a second delimitation layer on the free side of the sacrificial region, removing the sacrificial region to form a sublithographic opening that is used to etch a mold opening in a mold layer and filling the mold opening.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a small area contact region, a high efficiency phase change memory cell and a fabrication method thereof. 
     2. Description of the Related Art 
     As is known, phase change memory cells utilize a class of materials that have the unique property of being reversibly switchable from one phase to another with measurable distinct electrical properties associated with each phase. For example, these materials may change between an amorphous disordered phase and a crystalline, or polycrystalline, ordered phase. A material property that may change and provide a signature for each phase is the material resistivity, which is considerably different in the two states. 
     Specific materials that may be suitably used in phase change cells are alloys of elements of the VI group of the periodic table, such as Te or Se, also called chalcogenides or chalcogenic materials. Therefore, hereinafter, the term “chalcogenic materials” is used to indicate all materials switchable between at least two different phases where they have different electrical properties (resistances) and include thus the elements of the VI group of the periodic table and their alloys. 
     The presently most promising chalcogenide is an alloy of Ge, Sb and Te (Ge 2 Sb 2 Te 5 ) which is already widely used for storing information in overwritable optical disks. 
     As indicated, for microelectronics applications, the interesting parameter is resistivity that varies of two or more orders of magnitude when the material transforms from the amorphous phase (more resistive) to the crystalline phase (more conductive) and vice versa. Thus a thin film of chalcogenic material may be employed as a programmable resistor, switching between a high and a low resistance condition, with a resistance change ratio higher than 40, as shown in  FIG. 1 , wherein Vr indicates the read voltage. 
     Phase change may be obtained by locally increasing the temperature. Under 150° C., both phases are stable. Over 200° C., nucleation of crystallites is fast and if the material is kept to the crystallization temperature for a sufficient time, it changes phase and becomes crystalline. In order to change the phase back to the amorphous state, its temperature is brought over the melting point (about 600° C.) and rapidly cooled. 
     From an electrical point of view, it is possible to reach both critical temperatures (crystallization and melting temperatures) using an electric current flow through a resistive electrode in contact or close proximity with the chalcogenic material and heating the material by Joule effect. 
     A chalcogenic element  1  based on the above is shown in  FIG. 2 , and comprises a resistive electrode  2  and a chalcogenic region  3 . The chalcogenic region  3  is generally in the crystalline state to allow good current flow. A portion of the chalcogenic region  3  is in direct contact with the resistive electrodes and forms a phase change portion  4 . 
     By passing an electrical current of suitable value through the resistive electrode  2 , it is possible to selectively heat the phase change portion  4  to the crystallization or melting temperatures and cause a phase change. 
       FIG. 3  shows the plots of the required temperature versus time when a phase change from the crystalline to the amorphous status is desired (curve A) and a phase change from the amorphous to the crystalline status is desired (curve B). T m  indicates the melting temperature and T x  indicates the temperature at which crystallization begins. As shown, amorphization requires a short time (reset pulse) but a high temperature; furthermore the material should be cooled in a very short time (t 1 ) to maintain the atomic disorder and avoid recrystalization of the material. Crystallization requires a longer time t 2  (also called set pulse) to allow nucleation and crystal growing. 
     The state of the chalcogenic material may be read applying a sufficiently small voltage so as not to cause a sensible heating and measuring the current passing through it. Since the current is proportional to the conductance of the chalcogenic material, it is possible to discriminate between the two states. 
     Of course, the chalcogenic material may be electrically switched between different states intermediate between the amorphous and the crystalline states, thereby giving rise to a multilevel storing capability. In the following however, for sake of clarity, the binary situation will be considered, without the invention being limited thereto. 
     The possibility of changing the phase from the amorphous to the crystalline state using electrical pulses is indeed not immediately obvious, since, in the amorphous state, the material resistivity is very high and the current flowing through the chalcogenic material would not allow a sufficient dissipation and thus a sufficiently high temperature. However, chalcogenic materials have the property that they change their transport characteristics as a function of the applied electric field. This is shown in  FIG. 4 , plotting the curve of the current as a function of the voltage for a structure formed by a chalcogenic material arranged between two metal electrodes. 
     As visible, above a threshold voltage Vth, the structure begins to conduct not following a phase change, but because of a change in the electronic conduction mechanism. This behavior is called “electronic switching”; accordingly, biasing the chalcogenic structure to a voltage higher than the threshold voltage, it is possible to considerably increase the current flow. By causing this current to pass through a suitable neighboring series resistor, that operates as a heater, it is thus possible to obtain, by Joule effect, a sufficient heating of the chalcogenic material up to the crystallization temperature. 
     The use of the chalcogenic element of  FIG. 2  has been already proposed to form a memory cell. To avoid disturbances caused by adjacent memory cells, the chalcogenic element is generally coupled with a selection element, such as a MOS transistor, a bipolar transistor or a diode. 
     All known approaches however are disadvantageous due to the difficulty of implementing solutions that satisfy present requirements regarding current and voltage withstand capability, functionality, as well as compatibility with current CMOS technologies. 
     In particular, technological and electrical considerations impose a limit onto the maximum value of the current usable to cause phase change in the memory cells. Indeed, considerations about the present current capability of transistors in a technology with a gate length of 130 nm and a gate oxide withstanding a power supply of 3 V, for memory devices of the present generation (working by 8 or 16 or 32 bits), impose maximum current values of about 100–200 μA that in turn require a contact area between the chalcogenic region and the resistive electrode in the range of preferably, at the most, 20 nm×20 nm. The problem is that such dimensions are far ahead of the present optical (UV) lithography that can hardly reach a linear 100 nm definition. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the invention provides a contact region, in particular for a phase change memory cell, and a fabrication method thereof that solve the above problem. 
     In order to manufacture the small contact region between, e.g., the chalcogenic region and the resistive electrode, according to one embodiment of the invention, the contact area is defined by the intersection of two transverse thin portions having a sublithographic dimension, that is a dimension that is lower than a minimum dimension obtainable through optical UV lithography. According to another embodiment of the invention, to form the sublithographic dimensions, the deposition of layers instead of lithography is exploited. This embodiment is based on the fact that actual existing technologies allow very thin layers (having a thickness much lower than the lithographic dimension) to be deposited with a good precision. 
     Preferably, also the height of the phase change portion is determined by the thickness of a suitable deposited film, so that the phase change portion has a very small volume. 
     In particular, the resistive electrode includes a thin wall formed by a deposed resistive layer. For example, the resistive layer is deposed onto at least a side wall surface of a cavity in an insulating layer. The chalcogenic region is obtained by filling a slit formed in a dielectric material. The slit may be simply obtained by removing a sacrificial region deposed on a side wall surface of a first delimitation layer. Thereafter, a second delimitation layer is deposed which, together with the first delimitation layer, laterally delimits a slit. In the alternative, the delimitation layers form a hard mask for an underlying dielectric layer that defines a mold layer and the slit is formed as above indicated in the delimitation layers; the dielectric layer is etched under the slit to form a molding aperture; after removing the hard mask, the molding aperture is filled with the chalcogenide. 
     To improve the adhesion of the chalcogenic material to the underlying structure, in particular to the dielectric material that forms the molding aperture, according to a further embodiment of the invention, an adhesion film is used. Preferably, titanium or polysilicon is used as the adhesion film. 
     The process is studied so as to be fully compatible with a standard and up-to-date CMOS processes. To this end, the phase change memory cells are manufactured after the operations relative to the front-end, wherein the active elements of the circuits (the elementary components such as transistors diodes, resistors, etc.) are formed and before the operations relative to the back-end, wherein the interconnections for electrically connecting the elementary components are formed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       For the understanding of the present invention, a preferred embodiment is now described, purely as a non-limitative example, with reference to the enclosed drawings, wherein: 
         FIG. 1  illustrates the characteristic current-voltage of a phase change material; 
         FIG. 2  shows the basic structure of a known chalcogenic element; 
         FIG. 3  plots the temperature versus time that causes a phase change for the chalcogenic element of  FIG. 2 ; 
         FIG. 4  plots the current versus voltage of a chalcogenic material subject to electronic switching; 
         FIG. 5  shows a contact region between the resistive electrode and the chalcogenic region, in a horizontal cross-section taken along line V—V of  FIG. 6 , according to a first embodiment of the invention; 
         FIG. 6  is a cross-section of the contact area of  FIG. 5 , taken along line VI—VI of  FIG. 5 ; 
         FIG. 7  is a cross-section of the contact area of  FIG. 5 , taken along line VII—VII of  FIG. 5 ; 
         FIGS. 8–25  show cross-sections through a semiconductor wafer in successive manufacturing steps of a phase change memory cell; and 
         FIGS. 26–28  show cross-sections of an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 5–7  illustrates a contact region  30  of, e.g., a phase change memory cell including a resistive electrode  31  and a chalcogenic region  32  in direct electric contact at a contact area  33 . 
     The resistive electrode  31  is formed by a thin region having a thickness L, extending in a first dielectric layer  34 . The resistive electrode  31  is of a material with selected resistivity (e.g., with a resistivity of 0.1–100 mΩ cm, preferably 1–10 mΩ cm), stable with temperature (at least up to 700–800° C., that is just above the phase change material melting temperature), which is CMOS-compatible and has a good interface with the phase change material (chalcogenic material). For example, the resistive electrode  31  may be of TiSiN, TiAlN or TiSiC. 
     Below the resistive electrode  31  is a resistive support region  35 , also surrounded by the dielectric layer  34  and connected to a selection element (not shown); on top of the first dielectric layer  34  there extends a second dielectric layer  38  covered by an adhesion layer  39 . The second dielectric layer  38  and the adhesion layer  39  form a slit or opening  40  extending, at least near the contact area  33 , perpendicularly to the resistive electrode  31 . The slit  40  may have any shape, for example circular (here approximated by a straight line) and is slightly larger at the adhesion layer  39 . In  FIG. 7 , the upper surfaces of the second dielectric layer  38  and of the adhesion layer  39  are indicated by dotted lines, for better comprehension. 
     The chalcogenic region  32  extends on the adhesion layer  39  and fills the slit  40 , thereby contacting the resistive electrode  31  with a thin chalcogenic portion  32   a,  the width whereof is determined by the width of the slit  40  (indicated by W in  FIG. 5 ). The intersection between the thin chalcogenic portion  32   a  and the resistive electrode  31  thus forms the contact area  33  and the volume of the thin chalcogenic portion  32   a  extending above the contact area  33  forms a phase change region  41 , whose base is W×L and whose height H equals the thickness of the second dielectric layer  38 , as shown in  FIG. 6 . 
     According to one embodiment of the invention, the dimensions W, L and H are determined by the thickness of deposited layers, as below explained by way of an example for an embodiment of a manufacturing process. In particular, dimensions W, H, L are lower than 100 nm, preferably lower than 50 nm and more preferably of about 20 nm, as it will be clear from the following description, made with reference to  FIGS. 8–25 , wherein the different layers and structures are not drawn in scale. The described exemplary process is directed to the manufacture of an electronic device including circuitry transistors made in the CMOS technology and a phase change cell memory. 
     Initially a wafer  43 , comprising a substrate  44  of P + -type and an epitaxial layer  45  of P − -type silicon, is subject to standard front-end steps, as rapidly summarized hereinbelow. 
     Referring to  FIG. 8 , active areas are defined in the substrate  45 . To this end, insulating regions  46  are formed within the substrate, e.g., using the “shallow trench” technology. Then, in the array portion (on the right in the Figure) buried subcollector regions  47  of P + -type are implanted to extend down to the substrate  44 . After annealing, N-wells  48  and P-wells  49  are implanted, using separate masks. Furthermore, doping of the channels of the periphery transistors is carried out. 
     Thereafter, an oxide layer  50  is grown onto the entire surface of the epitaxial layer  45  and a polysilicon layer of preferably 250 nm is deposed. The polysilicon layer is then defined, to form gate regions  51  of the periphery transistors. After source and drain reoxidation, LDD implants are carried out for both P-channel and N-channel periphery transistors, to form P-type, light doped regions  52  and analogous, not shown, N-type light doped regions. 
     Then, a salicide protection mask (not shown) is formed over the array portion; a dielectric material (oxide) with a thickness of 220 nm is deposed onto the whole wafer  43  and etched, to form spacers  53  on the sides of the gate regions  51 ; source and drain regions  55 ,  56  for the N-channel and P-channel periphery transistors are implanted and salicide regions  57  are grown over the gate regions  51  and the source and drain regions  55 ,  56 , in a per se known manner. 
     After removing the salicide protection mask, collector regions  60  of P-type and base regions  61  of N-type are implanted over the buried sub-collector regions  47 ; then a first nitride layer  62  with a thickness of 60 nm is deposed. Thus, the structure of  FIG. 8  is obtained. 
     Thereafter,  FIG. 9 , a first dielectric layer  63  (preferably, USG—Undoped Silicate Glass—with a thickness of 700 nm) is deposed and planarized down to about 600 nm. Then first dielectric layer  63  and first nitride layer  62  are etched where contact are to be formed so as to uncover portions of salicide regions  57  and base region  61 . 
     Then,  FIG. 10 , a base contact region  64  of N + -type and, subsequently, an emitter region  65  of P + -type are implanted into base region  61 . Base region  61 , base contact region  64  and emitter region  65  form a diode; in particular, base region  61  forms the cathode and emitter region  65  forms the anode of the diode. 
     A first barrier layer  66 , e.g., a multiple Ti/TiN layer with a thickness of about 70 nm, is deposited onto the whole wafer  43 ,  FIG. 11 , thereby conformally covering all sides of the first dielectric layer  63  and the exposed portions of the epitaxial layer  45 . A W layer  67  with a thickness of about 300 nm is then deposited and fills the openings in the first dielectric layer  63 . The wafer  43  is then planarized, e.g., by CMP (Chemical Mechanical Polishing), to obtain a total thickness of layers  63  and  62  of about 500 nm, measured from the surface of the epitaxial layer  45 ,  FIG. 12 . The remaining portions of the first barrier layer  66  and W layer  67  form first-level plugs  68 ; in particular a storage element plug  68   a  is formed on emitter region  65  and a word-line plug  68   b  is formed on base contact region  64 . Then a second nitride layer  70  with a thickness of 60 nm and, thereover, a second dielectric layer  71 , e.g., USG with a thickness of 240 nm, are deposited. 
     Thereafter,  FIG. 13 , an opening  72  is formed in the second dielectric layer  71  and in the second nitride layer  70 , to reach the surface of the first dielectric layer  63 . Opening  72  has dimensions given by the lithography, for example it may have an annular shape with a minimum dimension of 0.2 μm. Then, a heater layer  73  is deposed. The heater layer  73 , intended to form resistive electrode  31  of  FIGS. 5–7 , has a thickness of 10–50 nm, preferably 20 nm, and the above discussed properties as resistivity, temperature stability, and good compatibility with CMOS processes and chalcogenides. For example, TiSiN, TiAlN or TiSiC is used, which conformally covers the side wall surface and the bottom surface of the opening  72 . Then a third dielectric layer  74  is deposed and completely fills the opening  72 . The material of the third dielectric layer  74 , as well as the material of the second dielectric layer  71 , is preferably a low conductivity or “low-K” type, to limit heating diffusion (as indicated, the heater should be able to generate as high temperatures as 600° C., for ensuring melting of the chalcogenic materials and transition to the amorphous phase). The third dielectric layer  74  may be for example USG with a thickness of 300 nm, or other porous dielectric material (e.g., xerogels). 
     Then,  FIG. 14 , the wafer  43  is planarized, e.g., by CMP, to obtain a total thickness of the second nitride layer  70  and second dielectric layer  71  of about 200 nm. Thus, the heater layer  73  is completely removed from above the second dielectric layer  71 , and remains only in the opening  72 , defining a cup-shaped heater, extending, in a top view, along a closed line and housing the remaining portion  74   a  of the third dielectric layer  74 . 
       FIG. 15  represents a detail of  FIG. 14 , in enlarged scale, to better show a contact area analogous to the reduced contact area  33  of  FIG. 5 . 
     In sequence,  FIG. 16 , a mold layer  76  of dielectric material, for example USG or a low-K material with a thickness of 20 nm, an adhesion layer  77 , for example Ti or Si with a thickness of 5 nm, and a first delimitation layer  78 , for example nitride or other material that allows selective etching with respect to the mold layer  76  and adhesion layer  77 , are deposited. First delimitation layer  78  has a thickness of, e.g., 150 nm. 
     Using a mask, part of the first delimitation layer  78  is dry etched to expose the adhesion layer  77 ; thereby a step is formed having a vertical side  79  extending above portion  74   a,  along about a central line thereof. 
     Then,  FIG. 17 , a sacrificial layer  80 , for example TiN with a thickness of 30 nm, is conformally deposited; in particular, sacrificial layer  80  forms a vertical wall portion  80   a  extending against vertical side  79  of first delimitation layer  78 . The height of the vertical wall portion  80   a  (depending on the thickness of the first delimitation layer  78 ) and its width (equal to of thickness of the sacrificial layer  80 ) is a critical parameter in defining the width W of the phase change region  41  ( FIG. 6 ), as it will be clear hereinafter. 
     Thereafter,  FIG. 18 , the sacrificial layer  80  is subject to etch back, thereby removing the horizontal portions of the sacrificial layer  80  and part of the vertical wall portion  80   a  including the upper portion and a thin side portion. By suitably selecting the thickness of the first delimitation layer  78 , the thickness of the sacrificial layer  80 , the material of the sacrificial layer  80 , time and type of etching, the remaining vertical wall portion  80   a  has a lower portion which is rectangular and has a width W1 that equals the desired width W (20 nm) of the phase change region ( 41  in  FIG. 6 ). 
     According to  FIG. 19 , a second delimitation layer  82  of the same material as the first delimitation layer  78  (e.g., nitride with a thickness of 300 nm) is then deposited and covers the adhesion layer  77 , where exposed (on the left in the drawings), the vertical wall portion  80   a  and the first delimitation layer  78 ; then the delimitation layers  78 ,  82  as well as the vertical wall portion  80   a  are thinned to the desired height using CMP. At the end, the remaining portions of the delimitation layers  78 ,  82  form a hard mask  83 , and the remaining portion of the vertical wall portion  80   a  forms a sacrificial region  81 . 
     Thereafter,  FIG. 20 , the sacrificial region  81  is removed, thereby an opening  84  is formed in the hard mask  83  that has equal dimensions as the phase change region to be formed; adhesion layer  77  is then isotropic etched and mold layer  76  is dry etched. As visible from  FIG. 20 , by virtue of the isotropic etching of the adhesion layer  77 , portions of the latter are removed below the delimitation layers  78 ,  82  and the removed portion is larger than the opening  84  in the hard mask  83 ; on the contrary, by virtue of the dry etching, the opening  84   a  formed in the mold layer  76  has the same shape and in particular the same width as the opening  84  in the hard mask  83 . 
     Hard mask  83  is removed,  FIG. 21 , and a chalcogenic layer  85 , e.g., Ge 2 Sb 2 Te 5  with a thickness of 60 nm, is conformally deposed. Preferably, chalcogenic layer  85  is deposed by PVD (Physical Vapor Deposition) at an appropriate temperature to be in the crystalline phase. The portion  85   a  of the chalcogenic layer  85  filling the opening  84   a  in the mold layer  76  is analogous to the thin chalcogenic portion  32   a  of  FIGS. 5 and 7 ; its intersection with the heater layer  73  (not visible in  FIG. 21 ) defines the phase change region  41  of  FIGS. 5 ,  6 . As visible, portion  85   a  of the chalcogenic layer  85  has a height that is equal to the thickness of mold layer  76 ; thus also the height of portion  85   a  depends on the thickness of a deposed layer and may be sublithographic. By virtue of the adhesion layer  77  having a larger opening than opening  84   a,  the portion  85   a  is not in direct contact with the adhesion material and thus cannot either be contaminated thereby or allow heat escape. 
     On top of the chalcogenic layer  85 , a second barrier layer  86 , e.g., two-layer Ti/TiN with a thickness of 70 nm, and thereon a first metal layer  87 , e.g., AlCu with a thickness of 150 nm, are deposed. Also these layers, as well the subsequent ones, are deposed at an appropriate temperature (lower than the chalcogenide melting temperature but above the crystallization temperature), to avoid damage of the chalcogenic layer  85 . 
     As shown in  FIG. 22 , drawn at a smaller scale than  FIG. 21 , the stack formed by first metal layer  87 , the second barrier layer  86 , the chalcogenic layer  85  and the adhesion layer  77  are then defined, using a same mask, to form a bit line. The total thickness of the stack  87 ,  86 ,  85  and  76  is about 300 nm. 
     According to  FIG. 23 , a first oxide layer  90  and a second oxide layer  91  are deposed at low temperature. Preferably the first oxide layer  90  is a low thermal conductivity oxide (e.g., USG) that has the task of sealing the first metal layer  87  and the chalcogenic layer  85  and is deposed at low temperature (e.g., 300° C.) with a thickness of 150 nm, so as not to cause damage in the chalcogenic layer  85 . Thereby, the manufacturing steps specific for the phase change storage element are ended and standard back end steps are carried out. Thus, the second oxide layer  91  is a standard oxide with a thickness of, e.g., 600 nm. 
     Then the wafer  43  is planarized, e.g., by CMP, to obtain a total height of the stack formed by the second nitride layer  70 , the second dielectric layer  71 , the mold layer  76  and the first and second oxide layers  90 ,  91  (at the side of the stack formed by layers  77 ,  85 – 87 ) of about 800 nm. In practice, the height of the layers above the first metal layer  87  is about 300 nm. 
     Then,  FIG. 24 , contacts are formed to electrically contact the first-level plugs  68 ,  68   b  and the first metal layer  87 . To this end, on top of first-level plugs  68 ,  68   b,  the second oxide layer  91 , the first oxide layer  90 , the mold layer  76 , the second dielectric layer  71  and the second nitride layer  70  are opened and, on top of the storage element plug  68   a,  the second and the first oxide layers  91 ,  90  are opened (oxide etch). Then, a third barrier layer  92 , e.g., of Ti/TiN, is deposed and the openings are filled with a conductive layer  93 , e.g., W with a thickness of 300 nm. The structure is planarized to remove the horizontal portions of the conductive layer  93  and of the third barrier layer  92  and so form second-level plugs  94  in direct electrical contact with the first-level plugs  68 ,  68   b  (as shown in  FIG. 24  for the first-level plug  68   b  in contact with base contact region  64 ) or the first metal layer  87  (over the storage element plug  68   a ). 
     Thereafter,  FIG. 25 , a second metal layer  95 , e.g., a multilayer of Ti/AlCu/TiN is deposed, in a per se known manner, to form a first interconnection level, and second interconnection layer  95  is defined to form connection lines in contact with the second-level plugs  94 , according to the required layout. 
     The manufacture may continue with standard steps, such as deposition of an intermetal dielectric layer, etching and filling of vias; deposition and etching of a second metal level and deposition of the passivation. 
     The advantages of the present invention are clear from the above. In particular, it is outlined that the present memory and manufacturing process allow a very efficient integration, ensuring obtainment of the sublithographic contact area in both directions between the resistive electrode and the chalcogenide material. The presence of adhesion layer  77  ensures good adhesion between the chalcogenide and the underlying material with good thermal insulation properties, necessary to avoid heat dissipation. The process is completely CMOS compatible and allows integration of storage elements requiring small area. 
     The described process ensures formation of the thin portions  31  and  32   a  and contact area  33  of  FIGS. 5–7  with the desired small sublithographic dimensions in a well controllable way using specific steps to be performed between standard front-end and standard back-end steps. The same process may be however used for forming contact regions as shown in  FIGS. 5–7 , whenever it is necessary to obtain sublithographic dimensions. 
     Finally, it is clear that numerous variations and modifications may be made to the contact region, the phase change memory cell and process described and illustrated herein, all falling within the scope of the invention as defined in the attached claims. 
     E.g., by suitably selecting the depth and doping of regions  60 ,  61 ,  64  and  65  it is possible to form a bipolar transistor as the selection element. 
     Furthermore, when an adhesion layer is not necessary, the process may be simplified, forming the delimitation layers directly on the insulating layer accommodating the resistive electrode, as shown, e.g., in  FIGS. 26–28 . In detail, after forming the structure of  FIG. 14  and as shown in  FIG. 26 , a first delimitation layer  97   a  (analogous to first delimitation layer  78  of  FIGS. 16–19 ) is deposited directly on second dielectric layer  71  and is shaped to form a vertical side  98 , analogous to vertical side  79  of  FIG. 16 . Thereafter,  FIG. 27 , a vertical wall portion  99  extending against vertical side  98  is formed as above described for vertical wall portion  80   a  and,  FIG. 28 , a second delimitation layer  97   b  is deposited. Delimitation layers  97   a,    97   b  and the vertical wall portion  99  are thinned to form a hard mask having an opening  97   c  and a sacrificial region; chalcogenic layer  85 , second barrier layer  86 , and a first metal layer  87  are deposed and shaped, analogously to the above embodiment. 
     All of the above 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 Sheetare incorporated herein by reference, in their entirety.