Abstract:
A phase-change memory cell, comprising: a substrate housing a transistor, for selection of the memory cell, that includes a first conduction electrode; a first electrical-insulation layer on the selection transistor; a first conductive through via through the electrical-insulation layer electrically coupled to the first conduction electrode; a heater element including a first portion in electrical contact with the first conductive through via and a second portion that extends in electrical continuity with, and orthogonal to, the first portion; a first protection element extending on the first and second portions of the heater element; a second protection element extending in direct lateral contact with the first portion of the heater element and with the first protection element; and a phase-change region extending over the heater element in electrical and thermal contact therewith.

Description:
BACKGROUND 
     Technical Field 
       [0001]    The present disclosure relates to a phase-change memory cell and to a method for manufacturing the phase-change memory cell. In particular, the present disclosure relates to production of a heater of the phase-change memory cell. 
       Description of the Related Art 
       [0002]    As is known, phase-change memories use a class of materials having the property of switching between two phases having distinct electrical characteristics, associated to two different crystallographic structures of the material, and precisely a non-orderly amorphous phase and an orderly crystalline or polycrystalline phase. The two phases are thus associated to values of resistivity that differ considerably from one another, even by two or more orders of magnitude. 
         [0003]    Currently, the elements of Group XVI of the periodic table, such as for example Te or Se, also known as chalcogenide materials or chalcogenides, may be used in phase-change memory cells. As is known, for example, from P. Zuliani, et al., “Overcoming Temperature Limitations in Phase Change Memories With Optimized Ge x Sb y Te z ”, IEEE Transactions on Electron Devices, Volume 60, Issue 12, pages 4020-4026, Nov. 1, 2013, it is possible to use alloys of Ge, Sb, and Te (Ge x Sb y Te z , for example Ge 2 Sb 2 Te 5 ) optimized by appropriately choosing the percentages of the elements that form said alloys. 
         [0004]    The temperature at which phase transition occurs depends upon the phase-change material used. In the case of Ge 2 Sb 2 Te 5  alloy, for example, below 150° C. both the amorphous phase and the crystalline phase are stable. If the temperature is increased beyond 200° C., there is noted a fast re-arrangement of the crystals, and the material becomes crystalline. To bring the chalcogenide into the amorphous state, one can increase further the temperature up to melting point (approximately 600° C.) and then cool it rapidly. 
         [0005]    Numerous memories are known that exploit phase-change materials as elements for storage of the two stable states (amorphous and crystalline states), which may each be associated to a respective bit at “1” or at “0”. In these memories, a plurality of memory cells are arranged in rows and columns to form an array. Each memory cell is coupled to a respective selection element, which may be implemented by any switching device, such as PN diodes, bipolar junction transistors, or MOS transistors, and typically includes a chalcogenide region in contact with a resistive contact, also known as heater. A storage element is formed in a contact area between the chalcogenide region and the heater. The heater is connected to a conduction terminal of the selection element. 
         [0006]    In fact, from an electrical standpoint, the crystallization temperature and the melting temperature are obtained by causing flow of an electric current through the resistive contact that extends in direct contact with or is functionally coupled to the chalcogenide material, thus heating it by the Joule effect. 
         [0007]    According to the prior art, various processes of production of phase-change memory cells are known, which, however, present some disadvantages and limitations. In particular, processes of a known type normally require numerous manufacturing steps to form the selection elements, the chalcogenide regions, the heaters, and the contacts for connecting the selection elements and the storage elements to the bitlines and to the wordlines. An example of embodiment of a phase-change memory device of this type is described, for example, in U.S. Pat. No. 7,422,926. 
         [0008]    These problems have been partially solved by techniques of self-alignment of the chalcogenide regions, of the heaters, and of the contacts. However, the manufacturing steps, and in particular the precision for producing the heaters, as well as solutions for minimizing the contact area between the heaters and the chalcogenide regions (formation of heaters having a thickness and/or diameter of sublithographic dimensions) render the process for manufacture of this type of memory cells problematical, long, and easily subject to errors. 
         [0009]    Further, there are increasingly efforts aimed at integrating phase-change memories in CMOS platforms provided with logic devices having a wide range of functions (e.g., micro-controllers), thus providing devices or circuits of an embedded type. 
       BRIEF SUMMARY 
       [0010]    At least some embodiments of the present disclosure provide a phase-change memory cell and a method for manufacturing the phase-change memory cell that overcome the drawbacks set forth above. 
         [0011]    At least one embodiment is a phase-change memory cell that includes: 
         [0012]    a substrate; 
         [0013]    a selection transistor in the substrate and including a first conduction electrode; 
         [0014]    a first electrical-insulation layer on the selection transistor; 
         [0015]    a first conductive through via through the electrical-insulation layer and electrically coupled to the first conduction electrode; 
         [0016]    a heater element including a first portion in electrical contact with the first conductive through via and a second portion that extends in electrical continuity with, and orthogonal to, the first portion; 
         [0017]    a first protection element extending on the first and second portions of the heater element; 
         [0018]    a second protection element extending in direct lateral contact with the first portion of the heater element and with the first protection element; and 
         [0019]    a phase-change region extending over, and in electrical and thermal contact with, the heater element. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]    For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
           [0021]      FIG. 1  is a perspective view of a portion of a wafer that houses a phase-change memory and a logic device, in an initial step of a manufacturing process; 
           [0022]      FIG. 2  is a top plan view of the portion of wafer of  FIG. 1 ; 
           [0023]      FIG. 3  is a lateral cross-sectional view of the portion of wafer of  FIGS. 1 and 2 , taken along the line of section of  FIG. 2 ; 
           [0024]      FIG. 4  is a perspective view of the portion of wafer of  FIG. 1 , in a subsequent manufacturing step; 
           [0025]      FIGS. 5-13  show an enlarged detail of the portion of wafer of  FIG. 4 , and regard manufacturing steps subsequent to that of  FIG. 4 ; 
           [0026]      FIG. 14  reproduces the same perspective view as that of  FIGS. 1 and 4 , and illustrates a manufacturing step subsequent to that of  FIG. 13 ; 
           [0027]      FIG. 15  illustrates the portion of wafer of  FIG. 14  in lateral cross-sectional view; 
           [0028]      FIG. 16  shows, in perspective view, the portion of wafer of  FIG. 14  in a manufacturing step subsequent to that of  FIG. 14 ; 
           [0029]      FIG. 17  illustrates the portion of wafer of  FIG. 16  in lateral cross-sectional view; 
           [0030]      FIG. 18  is a perspective view of the portion of wafer of  FIG. 16  in a manufacturing step subsequent to that of  FIG. 16 ; 
           [0031]      FIG. 19  illustrates the portion of wafer of  FIG. 18  in lateral cross-sectional view; 
           [0032]      FIG. 20  is a lateral cross-sectional view of the portion of wafer of  FIG. 19  in a manufacturing step subsequent to that of  FIG. 19 ; 
           [0033]      FIG. 21  is a lateral cross-sectional view of the portion of wafer of  FIG. 20  in a manufacturing step subsequent to that of  FIG. 20 ; and 
           [0034]      FIG. 22  is a schematic representation of a system that uses the phase-change memory device according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Illustrated with joint reference to  FIG. 1  (perspective view),  FIG. 2  (top plan view), and  FIG. 3  (cross-sectional view along the line of section of  FIG. 2 ) is a wafer  1  (in particular a portion of a wafer  1 , for simplicity of representation). The wafer  1  is represented in a triaxial system X, Y, Z, in which the axes X, Y, and Z are mutually orthogonal. 
         [0036]    The wafer  1 , comprising a substrate  2 , for example of a P type, is subjected to front-end processing steps of a standard type, in particular manufacturing steps of a CMOS process. In particular, formed in the substrate  2  are insulation regions (not illustrated), which delimit active areas  4 . Then formed (e.g., implanted) in the active areas  4  are drain regions  5 , source regions  8 , and gate regions  9  of respective MOS transistors. 
         [0037]    Next, one or more dielectric layers  10  are deposited and planarized, for electrical insulation of the gate regions  9 , typically a pre-metal dielectric (PMD) layer. Openings are formed in the dielectric layer  10  over the drain regions  5  and the source regions  8 , and said openings are filled with tungsten to form a plurality of plugs, having the function of drain contacts  11   a  and source contacts  11   b  of the aforementioned MOS transistors. The drain contacts  11   a  are, in particular, in electrical contact with the implanted drain regions  5 , and the source contacts  11   b  are in electrical contact with the implanted source regions  8 . 
         [0038]    In a per se known manner, the openings formed in the dielectric layer  10  may be covered by a first barrier layer, for example a Ti/TiN layer, before being filled with tungsten. 
         [0039]    The left-hand side of the representation of the wafer  1  in  FIG. 1  is dedicated to creation of a phase-change memory and will consequently be identified, in the sequel of the description, as memory side  1 ′; the right-hand side of the representation of the wafer  1  in  FIG. 1  is dedicated to creation of a logic device  16 , which is to form an embedded circuit integrated in the same chip as the one that houses the phase-change memory and will consequently be identified, in the sequel of the description, as logic side  1 ″. It is evident that the use of the terms “right-hand” and “left-hand” has exclusively purposes of description with reference to the view of the figures and is in no way limiting for the purposes of the present disclosure. 
         [0040]    The drain regions  5 , the source regions  8 , and the gate regions  9  formed on the memory side  1 ′ form selection transistors  15  of an nMOS type for cells of the phase-change memory, whereas the source regions  8  and the gate regions  9  formed on the logic side  1 ″ form transistors of the logic device  16 . 
         [0041]    As may be noted, the source contact  11   b  of each selection transistor  15  extends in a continuous way in the direction of the axis Y, parallel to the gate regions  9 . This embodiment presents the advantage of enabling electrical contact of the gate regions  9  and of the source contacts  11   b  in a dedicated area of the wafer  1 , thus simplifying routing of the metal levels of the memory. 
         [0042]    The drain contacts  11   a  extend in the form of pillars and so that drain contacts  11   a  belonging to one and the same selection transistor  15  are aligned with respect to one another in the direction Y. Drain contacts  11   a  belonging to different selection transistors extend aligned with respect to one another in the direction X. 
         [0043]    Once the steps for formation of the selection transistors  15  (memory side  1 ′) and of the transistors of the logic device  16  (logic side  1 ″) are completed, a protective layer  20 , for example of silicon nitride Si 3 N 4 , and a dielectric layer  21 , for example of silicon oxide SiO 2 , are deposited on the wafer  1  and then defined by lithographic and etching steps to form trenches  24  on the memory side  1 ′. The trenches  24  have a main (major) extension along Y and a secondary (minor) extension along X. 
         [0044]    In each trench  24  there are exposed respective top faces of drain contacts  11   a , which are aligned with respect to one another along one and the same direction parallel to the direction Y. 
         [0045]    More in particular, the steps of lithography and etching of the protective layer  20  and of the dielectric layer  21  are carried out so that a side wall  24   a  of each trench  24  extends alongside, or partially over, the top faces of the drain contacts  11   a . The latter are thus completely or partially exposed through the respective trench  24 . The fact that the top faces of the drain contacts  11   a  are exposed only partially guarantees a certain safety margin in the case of alignment errors. In this way, the problems regarding the fact that the wall  24   a  could extend at an excessive distance, in the direction X, from the top faces of the drain contacts  11   a  are solved. It is in fact convenient for the side wall  24   a  of each trench  24  to extend (even in the case of misalignments) adjacent to, or in the proximity of, respective drain contacts  11   a . Acceptable distances between the wall  24   a  and the center (or centroid) of the drain contacts  11   a , measured along X, are, for example, comprised between 0 nm (condition of contiguity or partial overlapping) and 30 nm. 
         [0046]    It should be noted that the steps of deposition of the protective layer  20  and of the dielectric layer  21  are carried out over the entire wafer  1 , and thus also on the logic side  1 ″ of the wafer  1 . The portions of the protective layer  20  and dielectric layer  21 , which extend on the logic side  1 ″, will then be removed. 
         [0047]    Illustrated with reference to  FIGS. 5-9  is a method of production of a heater within the trenches  24 , according to one aspect of the present disclosure. For simplicity of representation,  FIGS. 5-9  regard a portion of a trench  24 . It is evident that what has been described with reference thereto applies to all the trenches  24  provided in the wafer  1  on the memory side  1 ′. 
         [0048]    First of all ( FIG. 5 ), a step of deposition of a resistive layer  26 , for example silicon and titanium nitride (TiSiN), is carried out to cover the wafer  1  and in particular the walls and the bottom of the trench  24 . The resistive layer  26  extends over the side wall  24   a  and in direct contact with the top face of the drain contacts  11   a  exposed through the trench  24 . 
         [0049]    Since the material used for the resistive layer  26  tends to undergo fast oxidation in air and thus its own electrical characteristics tend to deteriorate, a step is carried out of deposition of a protective layer  28 , for example of dielectric material such as silicon nitride (Si 3 N 4 ), on the resistive layer  26 , in particular on the side wall  24   a  of the trench  24 . The protective layer  28  has a thickness, measured along X on the side wall  24   a , of some tens of nanometers, for example between 20 and 100 nm, or in any case a thickness greater than the distance, along X, between the side wall  24   a  and the drain contacts  11   a  that extend in the trench  24  considered. 
         [0050]    Then ( FIG. 6 ), a first step of dry etching of the protective layer  28  is carried out, for example anisotropic plasma etching, in the direction of the arrows  29  (i.e., in the direction Z). This first etch enables removal of portions of the protective layer  28  that extend parallel to the plane XY, maintaining the portions thereof that extend parallel to the plane YZ, i.e., on the side walls inside the trench  24  and in particular on the side wall  24   a , substantially unaltered. The protection walls  32 ′ and  32 ″ of  FIG. 6  are thus formed. 
         [0051]    Via an appropriate choice of the thickness of the protective layer  28 , after the etching step of  FIG. 6 , the extension in the direction X of the protection wall  32 ′ on the side wall  24   a  of the trench  24  is such as to overlie at least in part (in top plan view) the top faces of the drain contacts  11   a . In this way, during subsequent removal of selective portions of the resistive layer  26 , also the regions of the latter that extend underneath the protection walls  32 ′,  32 ″ will at least in part overlie (and, more in particular, will be in direct electrical contact with) the drain contacts  11   a . This step is illustrated with reference to  FIG. 7  and may be carried out simultaneously with the step of etching of the protective layer  28  or else in a separate and subsequent etching step. Selective portions of the resistive layer  26  are thus removed from the wafer  1  except for the regions thereof protected (masked) by the protection walls  32 ′,  32 ″. 
         [0052]    Resistive regions  34 ′ and  34 ″ are thus formed, which are, in lateral cross-sectional view in the plane XZ, substantially L-shaped and cover the side walls of the trench  24  (longer leg of the L) and, in part, the bottom of the trench  24  (shorter leg of the L). The resistive region  34 ′ extends over the side wall  24   a  of the trench  24  and proceeds, with electrical continuity, until it electrically contacts, at least partially, the drain contacts  11   a . Preferably, the resistive region  34 ′ extends over the bottom wall of the trench  24  entirely covering the drain contacts  11   a . The resistive regions  34 ′,  34 ″ present, following upon the step of  FIG. 7 , exposed regions at the bottom of the trench  24 , where coverage of the protection walls  32 ′,  32 ″ is not present. 
         [0053]    Next ( FIG. 8 ), a step of deposition on the wafer  1  of a further protective layer  38 , for example silicon nitride (Si 3 N 4 ), is carried out. The protective layer  38  has a thickness, measured along X on the side wall  24   a , of some tens of nanometers, for example between 10 and 60 nm, and in any case a thickness such as not to occlude the trench  24  completely. 
         [0054]    Then ( FIG. 9 ), a dry etching step is carried out to remove the protective layer  38  from the front of the wafer  1  and partially from the trench  24  except for portions of the protective layer  38  that extend coplanar to the protection walls  32 ′,  32 ″. 
         [0055]    Further protection walls  40 ′,  40 ″ are thus formed, which extend in the trench  24  in contact with the protection walls  32 ′,  32 ″ and with the exposed portions of the resistive regions  34 ′,  34 ″ that derive from the previous etching step. In this way, the resistive regions  34 ′,  34 ″ are effectively and completely protected from oxidation phenomena. 
         [0056]    Next ( FIG. 10 ), a step is carried out of deposition of dielectric material, in particular silicon oxide, SiO 2 , on the wafer  1 , to form a filling layer  42  that extends over the wafer  1  and fills the trench  24  completely. Portions of the filling layer  42  that extend outside the trench  24  are removed by a step of chemical mechanical polishing (CMP). CMP is carried out over the entire wafer  1 . 
         [0057]    With reference to  FIG. 11 , the CMP step completely removes the filling layer  42  that extends outside the trench  24  and thus completely removes, from the entire wafer  1 , also the dielectric layer  21 , stopping at the protective layer  20 . If the CMP step proceeds beyond the dielectric layer  21 , any possible removal of a minimal top portion of the protective layer  20  does not involve significant problems. During the step of removal of the dielectric layer  21 , the CMP technique is not, in practice, selective in regard to the Si 3 N 4  layers and to the material used for the resistive layer  26  that extend in the trench  24 , which are thus also removed partially until the maximum height, along Z, to which the protective layer  20  extends is reached. The thickness of the protective layer  20  and the duration of the CMP process thus define the maximum extension, along Z, of the resistive regions  34 ′,  34 ″ and of the protection walls  32 ′,  32 ″ and  40 ′,  40 ″. 
         [0058]    This is followed by formation ( FIG. 12 ), in a per se known manner, of a layer of phase-change material (in what follows, “PCM layer”)  50 , for example by depositing a chalcogenide, such as a GST (Ge—Sb—Te) compound, e.g., Ge 2 Sb 2 Te 5 . Other phase-change materials may be used. Formation of the PCM layer  50  is carried out over the entire wafer  1 . A barrier layer  51 , of metal material, for example TiN, is formed on the PCM layer  50  to protect the PCM layer  50  from oxidation phenomena and likewise to form a low-resistivity layer for subsequent electrical-contact steps. 
         [0059]    This is then followed ( FIG. 13 ) by deposition of an etch-protection layer, or “hard mask”,  52  (made, for example, of silicon nitride) and by lithographic and etching steps in order to remove selective portions of the barrier layer  51  and of the PCM layer  50  exposed through the hard mask  52  to create resistive bitlines  54  on the memory side  1 ′. Etching proceeds in the direction Z with removal of exposed portions of the protective layer  20  between adjacent resistive bitlines  54 . In this step, selective portions of the resistive regions  34 ′,  34 ″, of the protection walls  32 ′,  32 ″, of the protection walls  40 ′,  40 ″, and of the filling layer  42  that extend, in top plan view XY, between one resistive bitline  54  and an adjacent one are likewise removed. 
         [0060]      FIG. 14  is a perspective view, which reproduces the view of  FIGS. 1 and 4 , of the wafer  1  after the manufacturing steps described with reference to  FIG. 13 , and  FIG. 15  is a lateral cross-sectional view in the plane XZ, which reproduces the view of the portion of the wafer  1  illustrated in  FIG. 14 . 
         [0061]    According to an embodiment alternative to the one illustrated in  FIG. 14 , the resistive bitlines  54  have locally widened regions, i.e., regions having an extension, along Y, that is locally increased. These regions are formed, for example, at the source contact  11   b  on the memory side  1 ′ or in any case in the regions where, in subsequent manufacturing steps (see  FIGS. 18 and 19 ), conductive vias will be formed to provide a top electrical contact in order to enable electrical access to the resistive bitlines  54 . The locally widened regions have the functions of compensating for possible undesired misalignments. 
         [0062]    As illustrated in  FIGS. 13-15 , the resistive bitlines  54 , of chalcogenide, extend electrically separate from one another in the direction X, each of them in thermal contact with a plurality of resistive regions  34 ′. Each of said resistive regions  34 ′ is, in turn, in electrical contact with a respective drain contact  11   a  and forms, in use, a heater designed to generate, when traversed by electric current, heat by the Joule effect having a value such as to cause phase change in a respective portion of the resistive bitline to which it is thermally coupled. 
         [0063]    It may be noted that, since the resistive regions  34 ″ are not electrically coupled to any drain contact  11   a , or to other electrical contacts, they do not play an active role during use of the memory array. 
         [0064]    Following upon the steps of  FIGS. 14 and 15 , the hard mask  52  may then be removed. However, since it does not generate problems during subsequent processing steps, the step of removal of the hard mask  52  is optional. 
         [0065]    Then ( FIGS. 16 and 17 ), deposited on the wafer  1  is a sealing layer  58  of dielectric material, for example silicon nitride, having the function of protection of the chalcogenide material from exposure to air and of electrical insulation between the resistive bitlines  54 . The sealing layer  58  is deposited on the hard mask  52  and in the gaps between one resistive bitline  54  and the adjacent one. The sealing layer  58  is likewise deposited on the logic side  1 ″, on the dielectric layer  10 , and on the drain contacts  11   a  and source contacts  11   b  exposed through the dielectric layer  10  on the logic side  1 ″. 
         [0066]    The resistive bitlines  54  are not suited to being used for conveying electrical signals for selection of the memory cells to be read/written in so far as their resistivity is too high. It is thus expedient to proceed with formation of conductive bitlines, of metal material, in electrical contact with the resistive bitlines  54  through conductive vias. 
         [0067]    For this purpose, as illustrated in  FIGS. 18 and 19 , a dielectric layer  60 , for example a silicon-oxide layer, is deposited on the wafer  1 , over the sealing layer  58 , and, by lithographic and etching steps, a plurality of openings  62   a ,  62   b  are formed in the dielectric layer  60 . The openings  62   a  are formed on the memory side  1 ′, aligned, along Z, to respective resistive bitlines  54 , so that each opening  62   a  forms a path towards a respective resistive bitline  54 . The openings  62   a  are preferably formed at a distance from the heater, for example, at the source contacts  11   b.    
         [0068]    By choosing the materials of the sealing layer  58  and of the dielectric layer  60  such that they may be etched selectively with respect to one another, the sealing layer  58  has the function of etch-stop layer during the step of formation of the openings  62   a ,  62   b.    
         [0069]    Formation of the openings  62   a  thus includes selective removal of the dielectric layer  60  until surface portions of the sealing layer  58  are exposed, and removal of the portions of the sealing layer  58  thus exposed. In the case where the hard mask  52  has not been removed in previous manufacturing steps, it is expedient to remove the portions of hard mask  52  exposed through the openings thus formed, until surface regions of the barrier layer  51  are reached and exposed. 
         [0070]    The openings  62   b  are formed on the logic side  1 ″ so that each opening  62   b  is aligned, along Z, with a respective drain contact  11   a  and source contact  11   b  (there may be used for this purpose alignment marks, in a per se known manner). The openings  62   b  have in fact the function of forming, during subsequent manufacturing steps, conductive paths in electrical contact with the drain contacts  11   a  and source contacts  11   b  on the logic side  1 ″. 
         [0071]    Formation of the openings  62   b  thus includes selective removal of the dielectric layer  60  on the logic side  1 ″ until surface portions of the sealing layer  58  are exposed, and removal of the portions of the sealing layer  58  thus exposed, until the drain contacts  11   a  and source contacts  11   b  are reached and exposed. 
         [0072]    Formation of the openings  62   a  and  62  is advantageously carried out using a single etching mask. 
         [0073]    This is followed by a step of filling with conductive material, for example metal material, of the openings  62   a ,  62   b  to form conductive vias in electrical contact with the resistive bitlines  54  (memory side  1 ′) and with the drain contacts  11   a  and source contacts  11   b  (logic side  1 ″). 
         [0074]    After a step of cleaning of the front of the wafer  1  in order to remove the metal layer formed therein during filling of the openings  62   a ,  62   b , it is possible to proceed with processing steps of a known type. In particular,  FIG. 20 , there are formed conductive bitlines  64  on the front of the wafer  1  (memory side  1 ′) and paths  66  for routing of the signals (logic side  1 ″), according to a desired pattern that does not form the subject of the present disclosure. In particular, each conductive bitline  64  extends parallel and aligned, along Z, to a respective resistive bitline  54 . 
         [0075]    This is followed, as illustrated in  FIG. 21 , by a step of deposition of a further dielectric layer  68  on the wafer  1 , over the conductive bitlines  64  and the paths  66 , and a step of deposition and photolithographic definition of a metal layer, for formation of wordlines  70  on the memory side  1 ′. The wordlines  70  are electrically coupled to the gate regions  9  by conductive vias (not illustrated), which extend through the dielectric layer  68  and the dielectric layer  60 . Metal paths  72  may likewise be formed on the dielectric layer  68  on the logic side  1 ″. 
         [0076]    The thickness of inter-metal layer  60  is not the same in the memory and logic regions. It is defined at the same quota, so a thickness approximately doubled is obtained in the logic. Opening  62   a  and  62   b  are consequently of very different height ( 62   b  twice  62   a ). Furthermore metallization levels  64  and  66  are the same quota, and also levels  70  and  72  are basically the same metal layer. 
         [0077]      FIG. 22  illustrates a portion of a system  200  according to an embodiment of the present disclosure. The system  200  may be implemented in various devices, such as for example PDAs, portable computers, phones, photographic cameras, video cameras, etc. 
         [0078]    The system  200  may include a controller  210  (e.g., a microprocessor), an input/output device  220 , for example a keypad and a display, a chip housing in an integrated form the phase-change memory device  1 ′ and the control logic 1″ (designated as a whole by the reference number  1 ), a wireless interface  240 , and a random-access memory (RAM)  260 , connected together by a bus system  250 . According to one embodiment, the system  200  may be supplied by a battery  280 , or alternatively by a mains supply source. It is clear that the scope of the present disclosure is not limited to embodiments comprising all the components of  FIG. 22 . For example, one or more from among the random-access memory (RAM)  260 , the wireless interface  240 , the battery  280 , and the input/output device  220  may be omitted. 
         [0079]    The advantages of the present disclosure emerge clearly from the foregoing description. 
         [0080]    In particular, formation of the heater with dual protection effectively prevents oxidation thereof during the manufacturing steps. 
         [0081]    Further, by providing the memory in the same wafer as the one that houses the logic circuitry, it is possible to obtain the conductive vias  62   a  for the bitline contacts of the memory simultaneously (i.e., with one and the same mask) with formation of the conductive vias  62   b  for the contacts of the logic circuitry. 
         [0082]    Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure. 
         [0083]    The various embodiments described above can be combined to provide 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.