fig4 a 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 fig3 b . 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 fig4 a , 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 fig4 b . 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 fig4 c , 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 fig7 . 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 ore 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 fig7 , 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 fig5 . 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 fig1 a - c , and an equivalent schematic is provided in fig1 d . fig1 a relates to a stacked - type of arrangement , while fig1 b and 17c relate to a lateral - type of arrangement . other devices can be realized with more than three terminals . fig6 a 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 fig6 a - 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 fig6 a . in particular , as shown in fig6 b and 6c , lateral arrangements having different input and output terminals can be realized . an equivalent electrical representation is shown in fig6 d . 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 fig6 a ) is being operated in the conductive ( polycrystalline ) state , but turning it on ( short - circuiting the path to ground ) when the terminal 611 ( in fig6 a ) 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 . fig1 a depicts stacked - typed arrangement of a multiplexer with a three - terminal device . fig1 b depicts a lateral - type arrangement . a schematic illustration of these two arrangements is provided in fig1 c . fig1 a and 19b depict stacked - type ( fig1 a ) and lateral - type ( fig1 b ) arrangements of passgates using a three - terminal device in accordance with an aspect of the invention . fig2 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 fig2 , 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 fig8 a - 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 fig8 j . 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 fig8 a . this is usually the case for both alcu ( aluminum - copper ) and cu ( copper ) beol ( back - end of the line ) flows . the cross - section in fig8 a 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 fig8 b . 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 fig6 b 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 . fig8 c 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 fig8 d . next , in step 7 ( fig8 e ), 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 fig8 f ). in step 9 , phase change material ( pcm ) 109 a is deposited into hole 109 , as illustrated in fig8 g . 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 fig8 h . in step 11 , ild oxide 111 is deposited and planarized , as shown in fig8 i . 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 fig8 j 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 fig9 , and a four - terminal bottom - connectivity device 100 bot illustrated in fig1 . fig1 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 ; 20 f 2 . this is three to four times smaller than the area of prior art memory cells using two terminal phase change devices . fig1 illustrates the cross - sectional view of a three - terminal device 100 three . fig1 a illustrates a cross - sectional view of a two - terminal device using a similar process flow . fig1 b is a multi - terminal layout view of a device fabricated in accordance with the first process flow . fig1 illustrates the layout of a 3 : 1 multiplexer device fabricated in accordance with the first process flow . fig1 a - 15 sm relate to a second process flow for fabricating phase change devices in accordance with the invention . beginning with step 1 , described with reference to fig1 a , 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 ( fig1 b ), 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 fig1 c . 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 fig1 d . 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 tan or tin of 250 å is deposited and patterned to cover the via materials 206 a , as seen in fig1 e . 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 ( fig1 f ), 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 ( fig1 g ), a pcm layer 208 a is deposited and planarized using cmp ( chemical - mechanical polishing ). step 8 , illustrated in fig1 h , 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 fig1 i . 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 fig1 j . the excess film from the surface can be removed by cmp . in step 10 , a heater top electrode barrier cap ( tin 250 å ) 211 is deposited and patterned , as shown in fig1 k . then , in step 11 , ild oxide is deposited and planarized . at this point the standard beol processing for defining vias 212 ( fig1 l ) and metal 213 can be resumed , to yield the dual damascene structure 200 shown in fig1 l ( cross - section ) and fig1 m ( 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 imd 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 ; 20f 2 . 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 ( 100b ), as shown in fig1 ( 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 .