Abstract:
Phase change devices, and particularly multi-terminal phase change devices, include first and second active terminals bridged together by a phase-change material whose conductivity can be modified in accordance with a control signal applied to a control electrode. This structure allows an application in which an electrical connection can be created between the two active terminals, with the control of the connection being effected using a separate terminal or terminals. Accordingly, the resistance of the heater element can be increased independently from the resistance of the path between the two active terminals. This allows the use of smaller heater elements thus requiring less current to create the same amount of Joule heating per unit area. The resistance of the heating element does not impact the total resistance of the phase change device. The programming control can be placed outside of the main signal path through the phase change device, reducing the impact of the associated capacitance and resistance of the device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of prior U.S. patent application Ser. No. 11/267,788, filed on Nov. 3, 2005, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to phase change devices (PCDs) used in logic and memory applications. 
     2. Description of the Related Art 
     A two-terminal phase change device (PCD)  10  is shown in  FIG. 1 , and consists of a heating element  14  connected to a control terminal B. Heating element  14  acts as both a heater and an electrically conducting node in thermal and electrical contact with phase change material (PCM)  12 , which connects to another metal terminal A which is simply an electrically conducting node connecting the PCD  10  to other circuitry (not shown). An electrical schematic of PCD  10  is shown in the right-hand side of  FIG. 1 , with the heating element  14  being represented as a resistance R heater , and the PCM  12  being represented as a resistance R PCM . 
     In operation, a programming pulse is applied to PCD  10  in such a way as to selectively create a high resistance state or a low resistance state in the PCM  12 , as explained with reference to  FIGS. 2 and 3A . In particular, phase change materials are a class of materials which can change phase from crystalline structures to amorphous structures or back when under different thermal treatments, supplied in this example by heating element  14  by way of control terminal B. When phase change material  14  is heated above its crystallization temperature and cooled down gradually, it tends to form a crystalline phase and exhibits low electrical resistance (SET). When the phase change material is heated above its melting temperature and cooled down abruptly, it forms amorphous phase and exhibits high electrical resistance (RESET). Essentially, the material operates as a programmable resistor with two distinct electrical resistance values. Phase change materials may contain atom elements in group 4, 5 and 6 such as Ge, As, Se, Te. 
       FIG. 3A  is a graph of the temperature profile for crystalline and amorphous phase change. Ta and Tx are melting and transition temperatures. t 1  and t 2  are time control periods for amorphous and crystallize state formation. 
     The two-terminal device of  FIG. 1  consists of a volume of phase change material contacted on one end by a low resistance metal and on the other end contacted to a higher resistance interface. In order to program the device an electrical current is passed through the higher resistance interface into the phase change material. The heat Generated in the high resistance interface along with the current injected into the phase change material causes the phase change material to change state. The way in which the current is removed will determine the final state of the material. By rapidly quenching the phase change material the material will be left in an amorphous state. If the materials temperature is slowly brought through the phase transition region the material will be left in a crystalline state. 
     Phase change materials have found their applications in optical disk memory such as CD-RW and DVD-RW based on its optical index change properties between crystalline and amorphous phases. In optical disk memory applications, a laser beam is used to introduce heat into the materials to switch between crystalline and amorphous states which have different refractive index. In integrated circuit applications, electric current is used to introduce joule heating into the phase change memory to switch between crystalline and amorphous states which have different resistance. 
     A problem associated with the two-terminal device structure depicted in  FIG. 1  is that the heating element must be incorporated at one end of the device, which means that the heating element is directly in the electrical path between the two terminals of the device which directly affects the electrical characteristics of the device. This causes contention between the electrical characteristics of the device during its programming state and its read or non-programming state. In order to Set or Reset the PCD  10 , the joule heating via the heating element  14  is used to transition the PCM  12  to the appropriate temperature. This requires that a trade-off be made between the electrical resistance of the PCD  10  and the thermal resistance of the PCD due to the heating element  14 . In typical circuit application, designers are accustomed to being able to adjust resistance by making the material in the conductive path large. In this case growing the conductive path would also mean changing the amount of joule heating per unit area that is applied to the PCM  12  given that the programming circuitry does not change. The conducting path through the device incorporates the heater element which inherently needs to be of high resistance in order to heat at low enough currents. This means that a substantially high series resistance is included in the path between the two nodes. Thus there are three variables with strong interdependence that can not be decoupled in the 2 terminal device: resistance in the on state, current, resistance in the off state (the last to relate directly to the heating of the Phase change material). 
     Another issue with two-terminal devices is that the circuitry needed to program the device is directly connected to one or both of the terminals of the device. This means that the control function that determines the state of the device is also part of the nodes that are used to read the device or in any other non-programming state. This also can add capacitive loading or current paths to the device that would be seen in normal operations at the two terminals. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a multi-terminal phase change device (PCD) is provided, allowing separation of the Set/Reset controls from the target electrical path. One way of accomplishing this is by constructing a device that consists of a phase change material (PCM) that connects to a conducting terminal(s) in an electrical path. Other terminals) used to control the Set/Reset operations through heater element(s) are connected separately to the PCM. This structure allows an application in which an electrical connection can be created between two terminals, with the control of the connection being effected using a separate terminal or terminals. The benefits are manifold. The resistance of the heater element can be increased independently from the resistance of the path between the two conductive terminals. This allows the use of smaller heater elements thus requiring less current to create the same amount of Joule heating per unit area The resistance of the heating element does not impact the total resistance of the PCD. As the improvements are done to the heater element the resistance across the two conducting elements of the switch is not increased. The programming control can be placed outside of the main signal path through the PCD, reducing the impact of the associated capacitance and resistance of the device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements, and wherein: 
         FIG. 1  is schematic diagram of a two-terminal phase change device and an equivalent electrical schematic thereof; 
         FIG. 2  is a graph of the temperature profile for crystalline and amorphous phase change; 
         FIG. 3A  shows IV curves of a phase change material for crystalline (low resistance) and amorphous (high resistance) states; 
         FIG. 3B  is a plot relating to the programming steps by which the two states—Set and Reset—of a PCD are realized; 
         FIG. 4A  is a diagram of a three-terminal phase change device in a stacked arrangement; 
         FIG. 4B  is a diagram of a three-terminal phase change device in a lateral arrangement; 
         FIG. 4C  is a circuit diagram modeling the devices of  FIGS. 4A and 4B ; 
         FIG. 5  is a diagrammatic illustration of the phase behavior of a self-isolating device; 
         FIG. 6A  is a schematic diagram of a stacked arrangement of a four-terminal phase-change device having, in addition to two active terminals, a pair of control terminals; 
         FIGS. 6B and 6C  are schematic diagrams of stacked arrangements of four-terminal phase-change devices; 
         FIG. 6D  is a circuit diagram modeling the devices of  FIGS. 6A-6C ; 
         FIG. 7  is a schematic diagram depicting the use of phase change devices in a cross bar switching structure; 
         FIGS. 8A-8J  are schematic representations of steps of a first process flow directed at fabricating a multi-terminal; 
         FIG. 9  is a schematic view of a four-terminal top-connectivity device; 
         FIG. 10  is a schematic view of a four-terminal bottom-connectivity device; 
         FIG. 11  is a layout of a four-terminal device having a mixed top/bottom connectivity pattern; 
         FIG. 12  is a layout of a three-terminal device; 
         FIGS. 13A and 13B  are layouts of a two-terminal device, in cross-sectional and top plan views, respectively; 
         FIG. 14  is top plan view of an arrangement showing multiple PCDs used to obtain a 3:1 multiplexer device fabricated in accordance with the first process flow; 
         FIGS. 15A-15M  relate to a second process flow for fabricating phase change devices in accordance with the invention; 
         FIG. 16  is top plan view of an arrangement showing multiple PCDs used to obtain a 3:1 multiplexer device fabricated in accordance with the second process flow; 
         FIGS. 17A-17C  are schematic diagrams of phase change devices having one or more isolated control/programming nodes; 
         FIG. 17D  is an electrical schematic of the devices of  FIGS. 17A-17C ; 
         FIG. 18A  depicts stacked-typed arrangement of a multiplexer with a three-terminal device; 
         FIG. 18B  depicts a lateral-type arrangement of a multiplexer with a three-terminal device; 
         FIG. 18C  is a schematic diagram of the two arrangements of  FIGS. 18A and 18B ; 
         FIGS. 19A and 19B  depict stacked-type ( FIG. 19A ) and lateral-type ( FIG. 19B ) arrangements of pass gates using a three-terminal device in accordance with an aspect of the invention; 
         FIG. 19C  is an electrical schematic of the devices of  FIGS. 19A-19B ; 
         FIG. 20  is a schematic diagram of a memory array in which three-terminal phase change devices are used; and 
         FIG. 21  is a schematic diagram of a memory array in which three-terminal phase change devices are used, wherein one of the terminals is not connected, and the control node is used for programming and reading. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4A  shows an example of a three-terminal PCD (phase change device)  400  in accordance with the invention. PCD  400  includes a pair of active electrically conductive terminals,  404 ,  406 , bridged together by phase change material (PCM)  402 . Terminals  404  and  406  connect PCD  400  to other circuit devices (not shown) whose exact nature depends on the particular application. 
     PCM  402  is in thermal and electrical contact with a heating element  408 , which is part of a control terminal  410  through which current is applied to the heating element to activate same. When activated in a controlled manner as explained further below, heating element  408  induces a phase change in PCM  402 . Two important phases are a high resistance amorphous phase, which causes the PCM  402  to be effectively high resistance, thereby establishing a virtual open circuit condition between active terminals  404  and  406 , and a low resistance polycrystalline phase, which electrically connects the active terminals  404  and  406  to one another in a closed circuit configuration. The two conditions are designated Reset (amorphous phases) and Set (polycrystalline phase). It should also be appreciated that there are many states of resistance between the highest resistance state and the lowest resistance state that may be useful for many types of applications. 
     PCM  402  is selected from a class of well-known chalcogenide materials, which include those containing atom elements in group 4, 5 and 6 such as Ge, As, Se, Te and behaving in the manner described—that is, as programmable resistors with two different electrical resistance values. The programming steps by which these two states—Set and Reset—are realized are further described with reference to the characteristics curves of PCM shown in  FIG. 3B . First, a voltage is applied at the control terminal ( 410 ), bringing the PCM  402  to what is referred to as the snap back region, carrier avalanche generation. At this point a small amount voltage can dramatically increase the current through the device. At a given current the temperature of the heating element ( 408 ) can rise to the point at which PCM  402  reaches its phase transition temperature. After reaching this temperature, the mechanism for cooling the PCM  402  determines the state in which the PCM remains after conclusion of programming. If the PCM  402  is cooled slowly, by slowly ramping the applied voltage or current down, PCM  402  will assume the crystalline low resistance form (Set). If the temperature of PCM  402  is reduced rapidly (the voltage quickly removed), PCM  402  will retain an amorphous, high resistance form (Reset). Thus a typical programming regime simply requires that the appropriate voltage waveform be applied to the device  400 . It will be appreciated that during programming—that is, in a programming mode when a control signal is being applied to the control terminal  410 —an electrical path is established between the control terminal and one of the active terminals  404  or  406 , in the form of a current passing between the control terminal and one the active terminals (in either direction), for example by way application of a voltage (positive or negative) across the control terminal and one of the active terminals. This electrical path activates heating element  408 , which presents a finite resistance, and which induces the phase change in the PCM  402 . Conversely, when not in the programming mode—that is, when the PCD is operating in a field mode after it has been programmed—the control terminal  410  is substantially electrically inactive. It is removed or isolated from the PCD, and ideally does not draw current or add current, or present any loading on the circuit. 
     In the arrangement of  FIG. 4A , the device is herein referred to as a stacked three-terminal PCD. Alternatively, a lateral three-terminal PCD arrangement can be provided, as shown in  FIG. 4B . Lateral three-terminal PCD  430  has a different physical layout, consisting of first ( 414 ) and second ( 416 ) active terminals that overlie one another, and that are bridged by a PCM  412  in thermal contact with a heating element  418 . A control terminal  420  activates the heating element  418 , causing the change in phase of the PCM  412  in the manner described above. 
     The lateral arrangement behaves in an effectively identical manner to that of the stacked arrangement, and both can be modeled by the circuit shown in  FIG. 4C , which depicts a switch between nodes A and B which can be selectively opened (high resistance, amorphous phase of PCM  412 ) or closed (low resistance, polycrystalline phase of PCM  412 ). However, the physical layout and fabrication processes of the two arrangements are different, and different advantages may inure to each of the arrangements depending on the contemplated application. In the case of the lateral PCD  430 , a very small heating element  418  can be realized, using atomic layer deposition, rather than using via or contact processing techniques which may be required for the stacked arrangement and which are limited by photolithographic and etch techniques. This may require a larger cell area in the X,Y plan but the device scales better in shrinking technologies than the stacked arrangement. 
     The multi-terminal device PCD can be used as a means of creating a cross-bar or switching fabric in a programmable device. The two methods of creating a cross bar structure are a cross-point structure and a mux structure. The cross-point structure simply connects two wires directly via some type of switch, which, in accordance with the invention, can be a multi-terminal device switch  700 , four of which are shown in  FIG. 7 . Each wire-2-wire connection has exactly one switch associated or one cross-point of contact. The mux structure uses multiple switches or pass gates in order to route one of multiple wires to one other wire, and by layer multiple multiplexers together one can achieve a structure that allows one or more wire to connect to one or more other wires. 
     Typically the cross bar functions as a means of electrically connecting one or more nodes to one or more other nodes. The electrical characteristics of the cross bar are defined by the application that it is being used in. Often a very low resistance connecting state is required, less than 1K ohms, a high off-resistance state is desired to reduce leakage through the non-selected lines, and in the configuration of  FIG. 7 , the ability to control the connection with an alternate control node is realized. 
     A multi-terminal phase change device in accordance with the invention is ultimately more useful to such an application than a two terminal device for certain applications. It removes the control/programming element out of the direct path of the electrical connection of the two wires. It also gives a level of electrical separation from the wires that are being connected in the crossbar. 
     In addition to the Set and Reset states, a third state can be induced in the PCD, as explained with reference to  FIG. 5 . This state, herein referred to as self-isolation, can be realized if the Set voltage pulse discussed above is followed by a controlled pulse that leaves portion of the volume ( 503 ) of PCM  502  in the amorphous, high resistance phase, while leaving the larger portion of the material in its low resistance phase. The volume of amorphous material  503  will ideally be adjacent to the heating element  508 , effectively isolating the heater node from the remainder of the device  500 , which may be important for reducing leakage currents or loading by the programming control terminal  520  which thereby disturb the behavior of PCD  500  or other devices (not shown) connected to terminals  504  and  506 . 
     Another method of implementing a mutli-terminal device would be to have one or more of the control/programming nodes isolated with some type of barrier material that under high fields would allow for current to be injected into the substrate to assist the programming operation but under normal operating conditions would not have any current or very minimal current attributed to that node(s). This may be necessary for instances where the heating element is so resistive that not enough current would actually be available to assist in the programming of the device. Such structure is illustrated in  FIGS. 17A-C , and an equivalent schematic is provided in  FIG. 17D .  FIG. 17A  relates to a stacked-type of arrangement, while  FIGS. 17B and 17C  relate to a lateral-type of arrangement. 
     Other devices can be realized with more than three terminals.  FIG. 6A  shows a four-terminal PCD  600  having, in addition to active terminals  604  (In A) and  606  (Out), a pair of control terminals  610  and  611  at least one of which is in thermal communication, by way of a heating element ( 608 ), with PCM  602 . Although only one of the programming nodes in  FIGS. 6A-6C  are shown with at heating element the, device could be constructed with both programming nodes having a heating element. A current path between the control terminals  610  and  611  is thus formed, across PCM  602 . Set/Reset circuitry (not shown) is connected to control terminal  610 , and a suitable current/voltage programming regime is applied in order to transition PCM  602  to the desired phase/resistance state. The use of more than one programming node enhances the ability to more completely heat the PCM and thus reach a better ratio between the high resistance and low resistance states. Using multiple programming nodes also allows for the creating of a conducting path for programming without having to go through any of the terminals that are used in the normal operation. This potentially removes some of the loading on function wires in the device. Further, while only one current sink (terminal  611 ) is shown, the invention is not so limited and multiple current sinks can be provided to thereby achieve an optimum Set/Reset resistance ratio and improve reliability and fabrication yield. Terminal  611  can also be provided with a second heating element (not shown) adjacent PCM  602  to supplement the heating element of terminal  610 . An advantage of a second heating element opposite heating element  608  would be symmetrical heating of PCM  602 , on both sides of the PCM, requiring less power to achieve the desired phase change in the material and less power in the overall system in which multiple PCDs such as PCD  600  are used. 
     Four-terminal PCD  600  can have various configurations in addition to the stacked arrangement of  FIG. 6A . In particular, as shown in  FIGS. 6B and 6C , lateral arrangements having different input and output terminals can be realized. An equivalent electrical representation is shown in  FIG. 6D . The Isolate terminal is provided in order to prevent current leakage through the current sink. This is effected by turning the pass gate  612  off (that is, open-circuiting the path to ground) when PCM ( 602  in  FIG. 6A ) is being operated in the conductive (polycrystalline) state, but turning it on (short-circuiting the path to ground) when the terminal  611  (in  FIG. 6A ) is operating as a current sink especially during the Set/Reset programming operations. 
     Depending on the circuitry and architecture of the overall device, it may be better to use PCDs having a stacked structure or a lateral structure or both. The placement of the heating element will also depend on the circuitry and the architecture. In some circumstances it is desired to bring the heating element up to layers above the device in order to get the most efficient use of wires in the upper layers of the integrated circuit. There may be a desire to bring the programming node to the layers below quickly, such as transistors that would be used to drive the current into the heater element. 
     The multi-terminal device can take on the function of a CMOS pass gate, multiplexer, OR gate, AND gate, and so forth.  FIG. 18A  depicts stacked-typed arrangement of a multiplexer with a three-terminal device.  FIG. 18B  depicts a lateral-type arrangement. A schematic illustration of these two arrangements is provided in  FIG. 18C .  FIGS. 19A and 19B  depict stacked-type ( FIG. 19A ) and lateral-type ( FIG. 19B ) arrangements of passgates using a three-terminal device in accordance with an aspect of the invention. 
       FIG. 20  is a schematic diagram of a memory array in which three-terminal PCDs devices (3-T PCD) are used in accordance with the invention. A variation on this arrangement is shown in  FIG. 21 , in which one of the terminals of the 3-T PCDs is not connected, and the control node is used for programming and reading. 
     There are various ways to implement the switch fabric that incorporates a separate set of terminals programming the device. It can also be conceived that the input and the output to the device can be created as a plurality of nodes, such that one or more wires can be connected to more than one output. 
     Phase change devices (PCDs) can be used as memory elements, logic switches or programmable resistors. Each flavor of device may require a different number of terminals and may need a different connectivity strategy, based on the circuit application. For standard integrated circuits all these flavors of devices may be used on the same chip. Therefore, in order to reduce manufacturing costs, it becomes important to develop a manufacturing flow that allows building these devices with the same process steps. The type of device is controlled primarily by layout and programming conditions. PCD fabrication process flows in accordance with the invention may offer one or more of the following advantages:
     1) Allowing fabrication of a multi-terminal phase change device   2) Providing integration above metal  1  and at any metal level (Metalx)   3) Symmetrical placement of heating elements on both side of the PCM material for more efficient Joule heating and better thermal isolation   4) Allows building of devices having different configurations (two terminal or multi-terminal) on the same chip using the same process steps   5) Allows flexible connection to the terminals as required for a more efficient connectivity to the rest of the circuitry   6) Only one or two extra masks needed compared to prior art   

       FIGS. 8A-8I  are schematic representations of steps of a first process flow directed at fabricating a multi-terminal PCD  100  depicted in cross-section and top plan view in  FIG. 8J . The process begins in Step  1  with a wafer after full CMOS processing up to metalx  101 , which is exemplarily any metal layer in the CMOS process. Metalx  101  is surrounded by a dielectric material  102  and is covered with an etch stop layer  103 , as shown in  FIG. 8A . This is usually the case for both AlCu (Aluminum-Copper) and Cu (Copper) BEOL (back-end of the line) flows. The cross-section in  FIG. 8A  is of planar surface that is typical of the Cu BEOL flow. However, similar process steps can be applied to an AlCu flow. 
     In Step  2 , a via hole  104  is patterned into the etch stop layer  103  to open a contact to the metal below. A diffusion barrier metal  104   a  such as TaN or TiN of 250 Å is deposited and patterned to cover the via hole in Step  3 , as seen in  FIG. 8B . W or CoWP can also be used as the diffusion barrier material. The diffusion barrier metal  104   a  represents the interface material to the bottom electrode or Input/Output pin for the multi-terminal PCDa (for example IN A in  FIGS. 6B and 6C ). For more advanced Cu metallization schemes using diffusion barriers selectively grown on top of the Cu, step  2 - 3  in the flow described below can be skipped resulting in further simplification and reduction of process steps. 
       FIG. 8C  relates to Steps  4  and  5 . In Step  4 , a dielectric (˜1000 Å)  105  of the same material typically used for ILD (Inter-Level Dielectric) is deposited. Then, in Step  5 , via holes  106  are etched in the dielectric, landing on the metal below. The via holes are filled with metal  106   a  and planarized. The via holes represent the connection of the program pins to the heater elements and may be needed only when connection to metal below is preferred. Otherwise, this process step can be omitted. 
     In Step  6  a diffusion barrier/etch stop stack  107  is deposited, using for example TaN/SiN (˜200 Å/250 Å), and patterned to define the heater region, or W or CoWP. This is illustrated in  FIG. 8D . Next, in Step  7  ( FIG. 8E ), an additional ILD dielectric layer  108  is deposited (˜1000 Å) on top of the heater stack  107 . In Step  8 , a hole  109  is etched through the heater stack into the dielectric as shown in  FIG. 8F ). 
     In Step  9 , phase change material (PCM)  109   a  is deposited into hole  109 , as illustrated in  FIG. 8G . In order to obtain a good fill, the hole  109  is designed to have a low aspect ratio. This is achieved by limiting the hole height. The deposited material is then planarized. 
     A diffusion barrier/etch stop stack  110  (TiN or TaN/SiN (˜200 Å/250 Å) or W or CoWP) is deposited, in Step  10 , to allow sealing of the PCM and prevent material inter-diffusion at the interface with the top electrode. Step  10  is illustrated in  FIG. 8H . 
     In Step  11 , ILD oxide  111  is deposited and planarized, as shown in  FIG. 8I . At this point standard BEOL processing for defining vias  112  and metal  113  can be resumed. 
     It will be appreciated that the etch stop layer on top of the heater and PCM seal layers help prevent over-etching and punching though the diffusion barrier film while etching the standard Mx to Mx+1 vias. This should help improve via resistance. 
     The device of  FIG. 8J  is configured to have top and bottom connectivity. It will be appreciated that a similar process flow can be used to produce a four-terminal top-connectivity device  100   top  illustrated in  FIG. 9 , and a four-terminal bottom-connectivity device  100   bot  illustrated in  FIG. 10 .  FIG. 11  illustrates the cross-sectional view of a four-terminal device  100   four  having a mixed top/bottom connectivity pattern fabricated using a similar process flow, Device  100   four  has a device floorplan that is &lt;20F2. This is three to four times smaller than the area of prior art memory cells using two terminal phase change devices.  FIG. 12  illustrates the cross-sectional view of a three-terminal device  100   three .  FIG. 13A  illustrates a cross-sectional view of a two-terminal device using a similar process flow.  FIG. 13B  is a multi-terminal layout view of a device fabricated in accordance with the first process flow.  FIG. 14  illustrates the layout of a 3:1 multiplexer device fabricated in accordance with the first process flow. 
       FIGS. 15A-15M  relate to a second process flow for fabricating phase change devices in accordance with the invention. Beginning with Step  1 , described with reference to  FIG. 15A , the process again starts with a wafer after full CMOS processing up to Metalx  201  surrounded by dielectric  202 . Metalx  201  is covered with an etch stop layer  203 . This is usually the case for both AlCu and Cu BEOL flows. The cross-section shows a planar surface that is typical of the Cu BEOL flow. However, the same process steps can be applied also to the AlCu flow. 
     In Step  2  ( FIG. 15B ), an oxide layer  204  of the same kind used for inter-level dielectric (ILD) is deposited (1000 Å), and a heater hole  205  is etched to land on the metal below. Sidewall spacer techniques (not shown) can be used to reduce the hole size below the minimum lithography feature size. This step is an option if the heater-to-PCM interface must be reduced to reduce the programming current. 
     Step  3  is illustrated in  FIG. 15C . In Step  3 , the heater hole  205  is filled with diffusion barrier material  205   a  using a CVD (chemical vapor deposition) technique to achieve conformal step coverage and good fill of the hole  205 . 
     In Step  4 , via holes  206   a  are etched to land on Metalx  201 . Via holes  206   a  are filled with conductive materials  206  using well known techniques, as shown in  FIG. 15D . Excess diffusion barrier left from the deposition in Step  3  is removed during Cu CMP (chemical-mechanical polishing). This is only required when bottom-connectivity is desired. 
     Next, in Step  5 , a diffusion barrier metal cap  207 , such as Tali or TiN of 250 Å is deposited and patterned to cover the via materials  206   a , as seen in  FIG. 15E . W or CoWP can also be used. This metal structure represents the bottom electrode or input/output pin for the multi-terminal device. The diffusion barrier metal caps help prevent material inter-diffusion between PCM and via metals. These steps can be omitted if connectivity at Metalx is not desired. 
     In Step  6  ( FIG. 15F ), a dielectric layer  204   a  is deposited (1000 Å) of the same material as the ILD layer and PCM layer trenches  208  are patterned and etched. 
     Then, in Step  7  ( FIG. 15G ), a PCM layer  208   a  is deposited and planarized using CMP (chemical-mechanical polishing). 
     Step  8 , illustrated in  FIG. 15H , entails deposition and patterning of a PCM interface barrier stack  209  (TiN 200 Å/SiN 250 Å). This step is only required for connectivity from the top. 
     An ILD layer  209   a  is then deposited, and a heater hole  210  is then etched, in Step  9 , to land on the PCM film  208   a , as seen in  FIG. 151 . Sidewall spacer techniques (not shown) can be used to reduce the hole size below the minimum lithography feature size. This is an option if heater-to-PCM interface must be reduced to reduce the programming current. The heater hole  210  is filled with diffusion barrier material  210   a  using a CVD technique (500 Å TiN) to achieve conformal step coverage and good fill of the hole, as seen in  FIG. 15J . The excess film from the surface can be removed by CMP. 
     In Step  10 , a heater top electrode bather cap (TiN 250 Å)  211  is deposited and patterned, as shown in  FIG. 15K . Then, in Step  11 , ILD oxide is deposited and planarized. At this point the standard BEOL processing for defining vias  212  ( FIG. 15L ) and metal  213  can be resumed, to yield the dual damascene structure  200  shown in  FIG. 15L  (cross-section) and  FIG. 15M  (top plan view). It will be appreciated that the etch stop layer on top of the GST seal layers help prevent over-etching and punching through the diffusion barrier film while etching the standard Metalx to Metalx+1 vias. Dielectric layer thicknesses can be varied based on the desired total thickness for the ILD and AO layers. 
     As in the first process flow, by using this flow it is possible to generate devices with connectivity at different metal levels and devices with variable number of terminals, including the more common two-terminal device. This can be accomplished by simple modification of the layout. The device floor plan is &lt;20F2. This is three to four times smaller than the area of prior art memory cells using two terminals phase change devices. 
     The devices can be easily stacked to obtain any desired mux size ( 100   b ), as shown in  FIG. 16  (3:1 mux). 
     The above are exemplary modes of carrying out the invention and are not intended to be limiting. It will be apparent to those of ordinary skill in the art that modifications thereto can be made without departure from the spirit and scope of the invention as set forth in the following claims.