Abstract:
A phase changeable memory cell is disclosed. In an embodiment of the invention, a phase changeable memory cell is formed with an ultra-small contact area to reduce the programming current. This contact area between heater electrode and phase changeable material is limited by the thickness of thin films rather than lithographic critical dimension in one dimension. As a result, the contact area is much less than the square of lithographic critical dimension for almost every technology node, which is helps in reducing current. To further reduce the current and improve the heating efficiency, heater electrode is horizontally put with its length being tunable so as to minimize the heat loss flowing through the heater to the terminal that connects to the front end switch device. In addition, above and below the heater layer, low-thermal-conductivity material (LTCM) is used to minimize heat dissipation. This results in reduced power consumption of the phase changeable memory cell with improved reliability.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of U.S. Provisional Application Ser. No. 61/786,609, filed on Mar. 15, 2013, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Resistive-type non-volatile memories (NVMs), such as phase change random access memories (PCRAMs), employ memory elements having different stable resistive states. Such memory elements enable data corresponding to the different resistive states to be stored. The memory element switches from one resistive state to another. For PCRAMs, the switching between the states involves switching between an amorphous and a crystalline phase. The switching between two phases is achieved by heating the memory element using a heater. 
         [0003]    However, conventional resistive NVMs have drawbacks. For example, conventional resistive NVMs, such as PCRAM, require large programming currents to switch from one resistive state to the other. To produce the necessary programming currents, a large transistor is needed. This results in a large cell size. Furthermore, the memory elements are disposed in close proximity with various heat sinks, for example metal lines as well as top and bottom electrodes, contributing to undesired heat loss and low heating efficiency. Inefficient heating and heat loss, as well as the proximity effect on the neighboring cells result in a decrease in reliability and performance and an increase in power consumption due to large programming current requirements. 
         [0004]    Therefore, there is a need to improve resistive NVMs. 
       SUMMARY 
       [0005]    Embodiments generally relate to semiconductor devices and method of forming semiconductor devices. In one embodiment, a method of forming a memory cell is disclosed. The method includes providing a substrate and forming a storage unit. The storage unit includes at least one cell stack having first and second opposing ends. A storage layer may be formed on the first end of the cell stack. A top electrode may further be formed on the first end of the cell stack. The top electrode is disposed over the storage layer. In addition, a bottom electrode may be formed on the second end of the cell stack. 
         [0006]    In another embodiment, a memory cell is disclosed. The memory cell includes a storage unit disposed in an interlevel dielectric layer between first and second metal levels. The storage unit includes at least one cell stack having first and second opposing ends. A storage layer is disposed on the first end of the cell stack. The storage unit further includes a top electrode on the first end of the cell stack. The top electrode layer is disposed over the storage layer. The storage unit also includes a bottom electrode on the second end of the cell stack. 
         [0007]    In yet another embodiment, a method of forming a memory cell is disclosed. The method includes providing a substrate and forming a storage unit. First and second cell stacks having a first and second opposing ends may be formed. The second ends of the cell stacks are adjacent to each other. A storage layer may be formed on the first ends of the cell stacks. A top electrode may further be formed on the first ends of the cell stacks. The top electrode is disposed over the storage layer. A bottom electrode may be formed on the second ends of the cell stacks. 
         [0008]    These and other advantages and features of the embodiments herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the disclosure. In the following description, various embodiments of the present disclosure are described with reference to the following 
           [0010]      FIGS. 1   a - b  show schematic diagrams in accordance with embodiments of a device; 
           [0011]      FIGS. 1   c - d  show portions of a memory cell array; 
           [0012]      FIG. 1   e  shows an embodiment of temperature-time plot; 
           [0013]      FIGS. 2   a - b  show schematic diagrams in accordance with embodiments of a device; 
           [0014]      FIGS. 2   c - d  show portions of a memory cell array; 
           [0015]      FIG. 3   a  shows exemplary embodiment of a device; 
           [0016]      FIGS. 3   b - d  show plan views of embodiments of a device; 
           [0017]      FIG. 4   a  shows another exemplary embodiment of a device; 
           [0018]      FIGS. 4   b - d  show plan views of embodiments of a device; and 
           [0019]      FIGS. 5   a - j  show cross-sectional views of an exemplary process of forming a device. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Embodiments generally relate to semiconductor devices. More particularly, some embodiments relate to memory devices, such as non-volatile memory (NVM) devices. Such memory devices, for example, can be incorporated into standalone memory devices, such as USB or other types of portable storage units, or ICs, such as microcontrollers or system on chips (SoCs). The devices or ICs can be incorporated into or used with, for example, consumer electronic products, or relate to other types of devices. 
         [0021]      FIG. 1   a  shows a schematic diagram of an embodiment of a device. In one embodiment, the device includes a memory cell  100 . The memory cell, in one embodiment, is a resistive-type memory cell. As shown, the memory cell includes a resistive element R1. Providing a memory cell with a plurality of resistive elements may also be useful. A resistive element is employed for storing information. 
         [0022]    A resistive element is a programmable resistive element. The programmable resistive element has multiple stable resistive states. In one embodiment, the resistive element is a bi-stable resistive element having first and second stable resistive states. For example, the resistive element has a stable high resistive state and a stable low resistive state, with one corresponding to a logic “0” and the other corresponding to a logic “1”. For example, the high resistive state may represent a logic 0 while the low resistive state may represent a logic 1. Having the high resistive state representing a logic 1 and the low resistive state representing a logic 0 may also be useful. Other configurations of data storage for the resistive element may also be useful. For example, the resistive element may have more than two stable resistive states. For example, the resistive element may have 2″ resistive states, where n is a whole number greater than 1. For example, the resistive element may have 4 resistive states (n=2), representing logics 00, 01, 10 and 11. Other number of resistive states may also be useful. 
         [0023]    In one embodiment, a resistive element is a phase change (PC) resistive element to form a PCRAM. For example, the resistive element includes a PC material (PCM). The PCM, in one embodiment, is a chalcogenide material. In one embodiment, the chalcogenide material is a GeSbTe alloy. In another embodiment, the chalcogenide material may be Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed. Other types of thermal based elements forming other types of RAMs may also be useful. 
         [0024]    The PC material has stable first and second phases. For example, the PCM can be in a first or a second phase, corresponding to a resistive state. For example, the first phase is a high resistive state and the second phase is a low resistive state. In one embodiment, the high resistive state is an amorphous phase and the low resistive state is a crystalline phase. One resistive state represents a logic “0” while the other resistive state represents a logic “1”. For example, the high resistive state may represent a logic “0” while the low resistive state may represent a logic “1”. Other configurations of PCMs may also be useful. 
         [0025]    The PCM, in either phase or state, is stable until reset or set. The PCM, for example, is stable at below a threshold temperature. For example, the threshold temperature is about 85° C. Other threshold temperatures may also be useful and may depend on the type of PCM employed. The threshold temperature, for example, should be above normal operating temperature of the device. The retention of the PCM should be, for example, 10 years. The PCM may be reset to the amorphous phase from the crystalline phase by exposing it to a reset condition or set from the amorphous phase to the crystalline phase by exposing it to a set condition. 
         [0026]    In one embodiment, the set condition includes heating the PCM at its amorphous phase to a crystallization temperature for a sufficient time to transform it to a crystalline phase. On the other hand, the reset condition includes heating to melt the crystalline PCM and rapidly cooling it so it becomes amorphous. Heating the PCM includes appropriately passing current to a heater or a heating element. The resistive element and heating element, for example, form a storage unit  110  of the memory cell. 
         [0027]    The resistive element includes first and second resistive element terminals (RETS). In one embodiment, the first RET forms a first node N1 of the storage unit; the second RET forms a second node NB of the storage unit. The second node serves as a bitline node. A bitline BL is coupled to NB. As for N1, it is coupled to a cell selector  170 . The cell selector, in one embodiment, is a transistor. As shown, the cell selector is a metal oxide semiconductor (MOS) transistor. The transistor, for example, may be a planar transistor. Other types of MOS transistors, such as fin-type MOS transistors may also be useful. Other types of transistors or cell selectors may also be useful. The transistor includes a gate and first and second transistor terminals  173 ,  175  and  177 . The first transistor terminal is coupled to N1. As for the second transistor terminal, it is coupled to a select line SL while the gate terminal is coupled to a word line (WL). 
         [0028]    In other embodiments, the cell selector may be a bipolar junction transistor (BJT). For example, a first terminal of the BJT may be coupled to N1, a second terminal may be coupled to SL while a base of the BJT is coupled to WL. The first and second terminals are the collector and emitter terminals of the BJT. For example, in the case of a NPN BJT, the collector is coupled to the N1 and the emitter is coupled to SL. Other types of cell selectors may be employed. For example, in some cases, a diode may be used as a cell selector, as shown in  FIG. 1   b . In such cases, the diode is provided between N1 and WL, forming a cross-point type of memory cell. For example, a first terminal  175  of the diode is coupled to N1 while a second terminal  177  of the diode is coupled to WL. No SLs are employed for cell selectors which are diodes. 
         [0029]    Although, as described, the resistive element is a PC resistive element, other types of resistive elements may also be useful. For example, the resistive element may be a thermal-based resistive element. The thermal-based resistive element may include a thermal assisted switching (TAS) magnetic material, forming a TAS magnetic RAM (TAS-MRAM). Other types of thermal based resistive elements, such as thermal chemical (TC) material, forming a TCRAM, may also be useful. 
         [0030]      FIG. 1   c  shows a portion of a memory cell array  105 . The portion, as shown, is a 3×3 array of memory cells  100 , as described in  FIG. 1   a . For example, a memory cell includes a cell selector which is a transistor. The memory cells are interconnected by WLs, BLs and SLs. The array, for example, includes WLs and SLs in a first or row direction and BLs in a second or column direction. The first and second directions, for example, are orthogonal. Providing non-orthogonal first and second directions may also be useful.  FIG. 1   d  shows a portion of a memory cell array  105  for the case where a cell selector  170  is a diode. As shown, the cell selector is coupled to the storage unit of a memory cell and a WL. 
         [0031]    An active signal at the WL selects a row of cells associated with the activated WL. To select a cell within the row of the activated WL, an appropriate BL voltage is provided at the BL associated with the selected cell. For example, an active signal at WL, and an appropriate BL voltage at BL, selects memory cell  100 s. The appropriate voltage, in one embodiment, has a magnitude which is larger than the sum of the threshold of the cell selector and the threshold of the PCM. 
         [0032]    Table 1a below shows signals at BL, WL and SL for a program access for the selected memory cell: 
         [0000]                                        TABLE 1a                       Program Access   BL   WL   SL                           Bit = 0   RESET Pulse   V WLA     GND               (R: high Resistance)           Bit = 1   SET Pulse   V WLA     GND               (R: low Resistance)                        
Table 1a, as shown, reflects signals for logic 1 as being low impedance and logic 0 as being high impedance. In the case that logic 1 corresponds to high impedance and logic 0 corresponds to low impedance, the SET and RESET signals may be switched. To perform a read access, the signals applied to the bitlines (read pulses) may be similar, but lower than the threshold voltage of the PCM. For example, the magnitude of the read voltage applied to the BL is less than the melt and crystallization temperature of the PCM. As for V WLA  signal, it is equal to the active signal at the WL to select a row of cells. For non-selected rows, an inactive WL signal V WLI  is provided. For example V WLA  may be a logic 1 signal and a V WLI  is a logic 0 signal. For example, in the case of  FIG. 1   c , V WLA  is applied to WL n , and SET and RESET signals are applied to BL i . For BLs of unselected cells, they are floated or in a high impedance state (Z state). As for the SLs, they may be coupled to the substrate well or ground, effectively forming a common SL. Other configurations of signals may also be useful.
 
         [0033]    Table 1b shows various voltage values for the different signals for BLs, WLs and SLs of a memory array. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1b 
               
               
                   
                   
               
               
                   
                 Signals 
                 Value (V) 
               
               
                   
                   
               
             
             
               
                   
                 V WLA   
                 About 1.8 
               
               
                   
                 V WLI   
                 0 
               
               
                   
                 V SL   
                 0 
               
               
                   
                 V BL (unselected) 
                 Floating or high-Z state 
               
               
                   
                 V BL  RESET Pulse 
                 About 50 ns duration with peak V of 
               
               
                   
                   
                 about 1.8~2.5 V 
               
               
                   
                 V BL  SET Pulse 
                 About 100~300 ns duration with 
               
               
                   
                   
                 peak V of about 1.8~2.8 V 
               
               
                   
                 V BL  Read Pulse 
                 About 50~150 ns duration at about 
               
               
                   
                   
                 1.2~1.5 V 
               
               
                   
                   
               
             
          
         
       
     
         [0034]    The values of the signal provided in Table 1b are exemplary. These values may change, depending on, for example, the technology. The various signals should be sufficient to achieve the objective, such as performing the memory access (e.g., programming and reading). Furthermore, it is understood that the stimuli for setting and resetting is the current which is produced by the voltage pulses. The voltage pulses are provided to produce the desired SET and RESET currents. For example, in the case of a SET current, it may be about 120 μA while the RESET current is about 200 μA for 40 nm node. Other SET and RESET current values may also be useful. As for the Read pulse, it may have a shorter duration, such as about 12˜25 ns for a resistive element set at about 50˜1000 ohms. Other Read pulse durations may also be useful. 
         [0035]      FIG. 1   e  shows an embodiment of temperature-time plot  108  for the SET, RESET and READ pulses. Referring to  FIG. 1   e , curve  111  is the SET pulse. As shown, the SET pulse has a slow rise time to above the crystallization temperature of the PCM. The SET pulse, once it reaches the peak temperature, has a linear portion and a slow fall time to room temperature. This is to ensure that the PCM sufficiently crystallizes after being above the crystallization temperature. As for the RESET pulse  112 , it has a fast rise time to above the melting temperature of the PCM and a fast fall time to room temperature. This is to ensure that the PCM melts and does not have a chance to crystallize. As for the read pulse  113 , it has a peak voltage below the crystallization temperature. The read pulse remains at the peak read voltage sufficiently long to perform the read access. This ensures that the phase of the PCM is not changed. 
         [0036]      FIG. 2   a  shows a schematic diagram of an embodiment of a device. In one embodiment, the device includes a memory cell  200 . The memory cell is similar to that described in  FIGS. 1   a - b . Common elements may not be described or described in detail. The memory cell, in one embodiment, is a resistive-type memory cell. 
         [0037]    As shown, the memory cell includes a plurality of resistive elements. In one embodiment, the memory cell includes first and second resistive elements R1 and R2. Providing other number of resistive elements may also be useful. The resistive elements are employed for storing information. In one embodiment, the resistive elements include PC elements. A storage unit includes a PC element and a heating element. The resistive elements and heating elements, for example, form a storage unit  210  of the memory cell. The storage unit includes first and second sub-storage units. Providing a storage unit with other number of sub-storage units may also be useful. 
         [0038]    A resistive element includes first and second resistive element terminals (RETs). In one embodiment, first RETs of the resistive elements are commonly coupled and form a first node N1 of the storage unit. The second RET of R1 forms a first bitline node NB1 of the storage unit and the second RET of R2 forms a second bitline node NB2 of the storage unit. A first bitline BL1 is coupled to NB1 and a second bitline BL2 is coupled to NB2. As for N1, it is coupled to a cell selector  170 . The cell selector, in one embodiment, is a transistor. As shown, the cell selector is a metal oxide semiconductor (MOS) transistor. The transistor includes a gate and first and second transistor terminals  173 ,  175  and  177 . The first transistor terminal is coupled to N1. As for the second transistor terminal, it is coupled to a select line SL while the gate terminal is coupled to a WL. 
         [0039]    In other embodiments, the cell selector may be a bipolar junction transistor (BJT). For example, a first terminal of the BJT may be coupled to N1, a second terminal may be coupled to SL while a base of the BJT is coupled to WL. The first and second terminals are the collector and emitter terminals of the BJT. For example, in the case of a NPN BJT, the collector is coupled to the N1 and the emitter is coupled to SL. Other types of cell selectors may be employed. For example, in some cases, a diode may be used as a cell selector, as shown in  FIG. 2   b . In such cases, the diode is provided between N1 and WL, forming a cross-point type of memory cell. For example, a first terminal  175  of the diode is coupled to N1 while a second terminal  177  of the diode is coupled to WL. No SLs are employed for cell selectors which are diodes. 
         [0040]    In one embodiment, a plurality of storage units may be coupled in parallel through N1 to a cell selector. For example, the cell selector can be used to select a plurality of storage units. Each storage unit (or sub-storage unit) is coupled to its respective BL. This enables a cell selector to select a plurality of storage units. Providing multiple storage units coupled to a cell selector is described in, for example, co-pending U.S. patent application Ser. No. 13/802,899, titled “Stackable Non-Volatile Memory”, which is herein incorporated by reference for all purposes. 
         [0041]      FIG. 2   c  shows a portion of a memory cell array  205 . The portion, as shown, is a 3×3 array of memory cells  200 , as described in  FIG. 2   a . For example, a memory cell includes a cell selector which is a transistor. The memory cells are interconnected by WLs, BLs and SLs. A memory cell is coupled to a WL, SL and first and second BLs. The array, for example, includes WLs and SLs in a first or row direction and BLs in a second or column direction. The first and second directions, for example, are orthogonal. Providing non-orthogonal first and second directions may also be useful.  FIG. 2   d  shows a portion of a memory cell array  205  for the case where a cell selector  170  is a diode. As shown, the cell selector is coupled to the storage unit  210  of a memory cell and a WL. 
         [0042]    The memory cells, as described in  FIGS. 2   a - b , may be configured in different modes. For example, the memory cell may be configured as a multi-level cell (MLC). A memory cell includes multiple bits. For example, a dual-level cell (DLC) includes two bits, associated with BL1 and BL2. Alternatively, the memory cell may be configured as a single-level cell (SLC). In the SLC, the resistive elements are in opposite states. For example, the first and second resistive elements represent one bit of the memory cell. 
         [0043]    An active signal at the WL selects a row of cells associated with the activated WL. To select a cell within the row of the activated WL, appropriate voltages are provided at the BLs. BLs of non-selected bits or cells are floated or in a high impedance state. 
         [0044]    Table 2a below shows signals at BLs, WL and SL for a program access for a memory cell with first and second resistive elements configured in the SLC mode: 
         [0000]                                    TABLE 2a               Program Access   BL1   BL2   WL   SL                   Bit = 0   RESET Pulse   SET Pulse   V WLA     GND           (R1: high R)   (R2: Low R)       Bit = 1   SET Pulse   RESET Pulse   V WLA     GND           (R1: low R)   (R2: high R)                    
Table 2a, as shown, reflects signals for logic 1 as being low impedance and logic 0 as being high impedance and BL1 is the true port and BL2 is the complementary port. In the case that logic 1 corresponds to high impedance and logic 0 corresponds to low impedance, the SET and RESET signals may be switched. To perform a read access, the signals applied to the bitlines (read pulses) may be similar, but lower than the threshold voltage of the PCM. For example, the magnitude of the read voltage applied to the BLs is less than the melt and crystallization temperature of the PCM. As for V WLA  signal, it is equal to the active signal at the WL to select a row of cells. For non-selected rows, an inactive WL signal V WLI  is provided. For example V WLA  may be a logic 1 signal and a V WLI  is a logic 0 signal. For BLs of unselected cells, they are floated or in a high impedance state (Z state). As for the SLs, they may be coupled to the substrate well or ground, effectively forming a common SL. Other configurations may also be useful. In the SLC mode, read pulses are applied to BLs of the memory cell (e.g., BL1 and BL2).
 
         [0045]    Table 2b below shows signals applied to the WL, SL and BLs for a program access for a memory cell configured in the MLC mode: 
         [0000]                                    TABLE 2b               Program Access   BL1   BL2   WL   SL                   Bit 1 = 0   RESET Pulse   Floating or   V WLA     GND           (R1: high R)   High Z       Bit 1 = 1   SET Pulse   Floating or   V WLA     GND           (R1: low R)   High Z       Bit 2 = 0   Floating or   RESET Pulse   V WLA     GND           High Z   (R2: high R)       Bit 2 = 1   Floating or   SET pulse   V WLA     GND           High Z   (R2: low R)                    
Table 2b, as shown, reflects signals for logic 1 as being low impedance and logic 0 as being high impedance. In the case that logic 1 corresponds to high impedance and logic 0 corresponds to low impedance, the SET and RESET signals may be switched. To perform a read access, the signals applied to the bitlines (read pulses) may be similar, but lower than the threshold voltage of the PCM. For example, the magnitude of the read voltage applied to the BLs is less than the melt and crystallization temperature of the PCM. As for V WLA  signal, it is equal to the active signal at the WL to select a row of cells. For non-selected rows, an inactive WL signal V WLI  is provided (e.g., ground). In the MLC mode, a read pulse is applied to BL of the selected bit of the memory cell (e.g., BL1 or BL2). The values of the various access signals in Tables 2a-b may be similar to those provided in Table 1b and they are exemplary. These values may change, depending on, for example, the technology.
 
         [0046]    The various signals should be sufficient to achieve the objective, such as performing the memory access (e.g., programming and reading) on the desired bit of the memory cell. Furthermore, it is understood that the stimuli for setting and resetting is the current which is produced by the voltage pulses. The voltage pulses are provided to produce the desired SET and RESET currents. For example, in the case of a SET current, it may be about 120 μA while the RESET current is about 200 μA for 40 nm node. Other SET and RESET current values may also be useful. As for the Read pulse, it may have a shorter duration, such as about 12˜25 ns for a resistive element set at about 50˜1000 ohms. Other Read pulse durations may also be useful. 
         [0047]      FIG. 3   a  shows a cross-sectional view of an embodiment of a device  300 .  FIGS. 3   b - d  show top views of various embodiments of the device. The cross-sectional view, for example, is along a first or wordline direction. A perpendicular direction to the first direction may be the bitline direction. The device includes a memory cell. In one embodiment, the memory cell is similar to the memory cell described in  FIG. 1   a . Common elements may not be described or described in detail. 
         [0048]    The memory cell is disposed on a substrate  305 . The substrate, for example, is a semiconductor substrate, such as a silicon substrate. For example, the substrate may be a lightly doped p-type substrate. Providing an intrinsic or other types of doped substrates, such as silicon-germanium (SiGe), germanium (Ge), gallium-arsenic (GaAs) or any other suitable semiconductor materials, may also be useful. In some embodiments, the substrate may be a crystalline-on-insulator (COI) substrate. A COI substrate includes a surface crystalline layer separated from a crystalline bulk by an insulator layer. The insulator layer, for example, may be a dielectric insulating material. The insulator layer, for example, is formed from silicon oxide, providing a buried oxide (BOX) layer. Other types of dielectric insulating materials may also be useful. The COI substrate, for example, is a silicon-on-insulator (SOI) substrate. For example, the surface and bulk crystalline layers are single crystalline silicon. Other types of COI substrates may also be useful. It is understood that the surface and bulk layers need not be the same material. 
         [0049]    The memory cell includes a storage unit  310  disposed between two interlevel dielectric (ILD) layers on the substrate. For example, the storage unit is disposed in a cell dielectric layer  360  between upper and lower ILD layers  390 U and  390 L. The cell dielectric layer, for example, may be formed of silicon oxide. Other types of dielectric layers may also be useful. As for the ILD layers, they may be formed of silicon oxide. Other types of dielectric materials may also be useful for the ILD layers. It is understood that the different ILD layers need not be formed of the same material or have the same configuration. 
         [0050]    An ILD layer includes a metal level and a contact level. For example, the upper ILD layer  390 U includes an upper metal level  394 U and an upper contact or via level  392 U while the lower ILD layer  390 L includes a lower metal level  394 L and a lower contact level  392 L. Metal levels include conductors while contact levels include contacts. For example, upper metal level includes upper conductors  395 U, upper contact level includes upper contacts  393 U, lower metal level includes lower conductors  395 L and lower contact level includes lower contacts  393 L. The conductors and contacts may be formed of a metal, such as copper, copper alloy, aluminum, tungsten or a combination thereof. Other suitable types of metal, alloys or conductive materials may also be useful. In some cases, the conductors and contacts may be formed of the same material. For example, in upper metal levels, the conductors and contacts may be formed by dual damascene processes. This results in the conductors and contacts having the same material. In some cases, the conductors and contacts may have different materials. For example, in the case where the contacts and conductors are formed by single damascene processes, the materials of the conductors and contacts may be different. 
         [0051]    In one embodiment, the lower metal level is the first metal level M1 and the upper metal level is a second metal level M2 of the device. In such case, the lower contact level is a pre-metal dielectric (PMD) or contact (CA) level and the upper contact level is a first via level V1. Contacts at the PMD level may be tungsten contacts while contacts at the V1 level may be copper or copper alloy. Other types of conductive materials, such as aluminum, may also be useful. Other configurations of contacts at the contact levels may also be useful. 
         [0052]    In general, the storage unit may be disposed between any two metal levels of the device. For example, the storage unit may be disposed between any two metal layers, such as M2 and M3. In such case, a contact and contact pad may be provided in CA and M1 to facilitate electrical connections to, for example, the cell selector. In some instances, the contact may be a direct contact which extends through CA and M1. Other contact configurations may also be useful. Disposing the storage unit between other metal levels may also be useful. Preferably, the storage unit is disposed between two adjacent metal layers. Disposing the storage unit between non-adjacent metal layers may also be useful. For example, the storage unit may be disposed between M1 and M3. Disposing the storage unit between other non-adjacent metal levels may also be useful. Contacts may be provided in one or more intermediate ILD levels to facilitate connections between two non-adjacent metal layers (e.g., M1 and M3). The contact may be a direct contact or a contact at the via level (e.g., V1) and a metal pad at the metal level (e.g., M2). Other contact configurations may also be useful. 
         [0053]    The storage unit is disposed within a cell dielectric layer  360  located between the lower metal level and upper contact level. The cell dielectric layer may be formed of silicon oxide. Other types of dielectric materials may also be useful for the cell dielectric layer. The cell dielectric layer may be formed of different dielectric layers. The different dielectric layers may be formed of the same material. Providing the dielectric layers with different materials may also be useful. For example, low k dielectric materials may be used. In some cases, bad thermal conducting dielectric materials may be used, for example, to reduce thermal cross-talk. Other configurations of the dielectric layers may also be useful. The cell dielectric layer, for example, may be considered as part of the upper ILD layer. 
         [0054]    In one embodiment, a dielectric liner  361  is disposed between the cell dielectric layer and the lower ILD layer. The dielectric liner, for example, serves as an etch stop layer. In one embodiment, the dielectric liner is formed of a low k dielectric. For example, the dielectric liner may be nBLOK. Other types of dielectric materials for the dielectric liner may also be useful. 
         [0055]    The storage unit includes a cell stack  350 . The width and length of the cell stack, for example, are greater than the minimum lithographic critical dimension (CD). The cell stack includes a programming stack  351  disposed between lower and upper stack dielectric layers  366  and  368 . The cell stack dielectric layers, for example, may be silicon oxide. Other types of dielectric materials may also be useful. For example, the dielectric layers may be silicon oxynitride, silicon nitride or a combination thereof. In one embodiment, the programming stack includes a heater layer  352 . The heater layer, for example, may be formed of an electrical connection material (ECM). The heater, in one embodiment, includes a high conductivity ECM, such as TiN. Other types of ECMs with appropriate or high conductivity may also be useful. The type of ECM may depend on, for example, the storage layer. 
         [0056]    The heater layer has a thickness T p . The thickness T p  of the heater layer, in one embodiment, is less than the lithographic CD. The thickness T p , for example, may be about 5-10 nm or less. Other thicknesses for T p  may also be useful. 
         [0057]    The heater layer is disposed between lower and upper heat shield layers  354  and  356 . In one embodiment, the heat shield layers are formed of a PCM. For example, the heat shield layers are PC layers. The PCM, in one embodiment, is a chalcogenide material. In one embodiment, the chalcogenide material is a GeSbTe alloy. In another embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs as well as other types of dielectric layers with low thermal conductivity may also be employed. The heat shield layers sandwich the heater, preventing heat loss therefrom. The thickness of the heat shield layers may be about 20-100 nm. Other thicknesses may also be useful. 
         [0058]    The heat shield layers may include a capping or protective layer. For example, a capping layer may be on a surface not in contact with the heater. Providing capping layers on both surfaces of a heat shield layer may also be useful. Other configurations of capping layers may also be useful. The capping layer may be employed to reduce oxidation or poor interface viscosity between layers. The capping layer, for example, may be silicon nitride or silicon oxynitride. Other types of capping layers may also be useful. The thickness of the capping layer, for example, may be about 20 nm. Other thicknesses may also be useful. 
         [0059]    The cell stack includes opposing first ends along the first direction. For example, the cell stack includes opposing first ends along the wordline direction. A storage liner  338  lines at least the sidewall of the cell stack at one of the opposing first ends. For example, the storage liner lines at least the sidewall of the first opposing first end of the cell stack. As shown, the storage liner lines the sidewall of the cell stack at the first opposing first end and lower ILD layer, forming an L-shaped storage liner. For example, the storage liner lines the sidewall of the cell stack at the first opposing first end and the dielectric liner above the lower ILD layer. In one embodiment, the width of the storage liner is less than a width of the first opposing first end. For example, a width of the storage liner may be about the lithographic CD while the width of the first opposing first end is greater than the lithographic CD. Providing other widths for the storage liner may also be useful. The thickness of the storage liner may about 5-10 nm or less. Other thicknesses may also be useful. 
         [0060]    In one embodiment, the storage liner is formed of a PCM. For example, the storage layers are PC layers. The PCM, in one embodiment, is a chalcogenide material. In one embodiment, the chalcogenide material is a GeSbTe alloy. In another embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed. 
         [0061]    The storage liner may include a capping or protective layer. For example, a capping layer may be on a surface not in contact with ILD or cell stack. For example, the capping layer may be provided on the top surface of the storage liner. Providing it on the bottom surface of both surfaces of the storage liner may also be useful. The capping layer may be employed to reduce oxidation or poor interface viscosity between layers. The capping layer, for example, may be silicon nitride or silicon oxynitride. Other types of capping layers may also be useful. The thickness of the capping layer, for example, may be about 20 nm. Other thicknesses may also be useful. 
         [0062]    In one embodiment, the sidewall of the cell stack may include a thin insulator layer. For example, the thin insulator layer is disposed between the cell stack and storage liner. The thin insulator layer may be a thin silicon oxide layer. Other types of insulators may also be useful. The thin insulator layer may serve as a high voltage breakdown layer for initializing the procedure of the memory cell. Such a high voltage breakdown will introduce a conductive path, the cross section area of which is usually much smaller than the product of litho CD and heater layer thickness. For example, the initialization procedure forms dielectric or heating islands. Dielectric or heating islands are disclosed in, for example, U.S. patent application Ser. No. 13/802,841, titled “Resistive Non-Volatile Memory”, which is herein incorporated by reference for all purposes. 
         [0063]    A top electrode (TE) layer  330  is disposed on the storage liner. The TE layer, for example, is formed of an ECM. The ECM, for example, has good conductivity and good interface with the PCM. In one embodiment, the TE layer is formed of an ECM with bad thermal conductivity to serve as a heat shield. For example, the TE layer may be a TaN layer. Other types of ECMs may also be useful. The TE layer is coupled to a bitline BL. For example, the TE layer is coupled to the BL by a contact. The contact may extend from the upper contact level through the cell dielectric layer. The TE layer provides connection for the bitline to the storage unit. In the case where the TE layer is an ECM with bad thermal conductivity, it also serves as a heat shield. 
         [0064]    In some embodiments, the TE layer may be a TE stack having multiple layers. For example, the TE stack may include first and second TE layers  332  and  334 . The first TE layer, in one embodiment, contacts the BL. The second TE layer is disposed below that first TE layer and contacts the storage liner. In one embodiment, the first TE layer is formed of an ECM with good conductivity and low thermal conductivity. For example, the first TE layer is formed of TaN, serving as a heat shield and providing connection for the BL to the storage unit. Other types of ECMs may also be useful. As for the second TE layer, it is formed of an ECM which provides good conductivity and good interface with the PCM. For example, the second TE layer may be formed of TiN. Other types of ECMs may also be useful. Other configurations of the TE stack may also be useful. The TE and storage liner may be collectively referred to as TE storage stack. 
         [0065]    A bottom electrode (BE) layer  340  at least lines the sidewall of the cell stack at the other of the opposing first end. For example, the BE layer at least lines the sidewall of the cell stack at the second opposing first end. In one embodiment, the BE layer lines the sidewall of the cell stack at a second opposing first end and lower ILD layer. In one embodiment, the cell dielectric includes an opening exposing the drain pad. The BE layer may line the sides of the opening, including second opposing side of the cell stack, and bottom, contacting a drain pad  395 L disposed in the lower metal level  394 L. For example, the dielectric liner includes an opening to expose the drain pad, enabling the BE layer to electrically couple to it. The BE layer, for example, is formed of an ECM with good electrical conductivity and bad thermal conductivity, such as TaN. Other types of ECMs may also be useful. Preferably, the BE is formed of the same material as the TE (or the first TE) layer. Other configurations of the electrode liner may also be useful. 
         [0066]    The cell stack  350  may have various shapes from the top view. For example, the cell stack may have a rectangular shape, as shown in  FIG. 3   b . Other shapes may also be useful. For example, the cell stack may have a polygonal or a trapezoidal shape, as shown in  FIGS. 3   c - d . For non-rectangular shaped cell stacks, the narrower portion is disposed adjacent to the TE side while the wider portion is disposed adjacent to the BE side. The TE storage stack may have a rectangular shape. Other shapes may also be useful. The TE storage stack may have a width W1 along the BL direction. The width W1, for example, may be equal to the lithographic resolution. The width W1, for example, may depend on the technology node. For example, W1 may be about 40 nm for a 40 nm node. Other widths for W1 may also be useful. The width W1 is preferably as small as possible to minimize contact cross-section between the storage liner and heater element. In some cases, if current is sufficiently small, W1 may be larger than lithographic CD. This reduces cost due to non-CD mask or reticle. 
         [0067]    As described, the TE storage stack has a width W1. The width W1 may be equal to CD to minimize contact area with the heater element to improve heating efficiency. It is understood that the width W1 is only related to the portion which contacts the sidewall of the first opposing first end. It is understood that the width of the TE stack may have a wider width W1c for forming contacts to the BL at a contact portion of the TE stack. The width W1c may be sufficient to accommodate a contact. This improves electrical connectivity to the BL. 
         [0068]    The width W2 of the BE, as shown, is wider than W1. For example, W2 is larger than the lithographic CD. As shown, W2 is wider than the width of the cell stack. This is because BE serves as a connection to the drain pad. There is no need to minimize the cross-sectional contact area to the heater as in the case of the storage liner. Providing a larger surface area increases current capacity. This reduces current crowding. Other widths for W2 may also be useful. For example, W2 may be the same as the width of the cell stack. Providing W2 which is narrower than the cell stack may also be useful. 
         [0069]    A cell selector is provided. The cell selector may be provided on the substrate. In one embodiment, the cell selector is a MOS transistor. The MOS transistor may be a planar transistor. Providing a fin-type MOS transistor may also be useful. Other types of cell selectors, such as BJT or diodes, may also be useful. The MOS transistor includes a gate and first and second S/D regions in the substrate adjacent to the gate. The S/D regions may include lightly doped (LD) regions. The first S/D region  175 , for example, serves as a drain while the second S/D region serves as a source. In one embodiment, the source is a common source region for an adjacent transistor of an adjacent row. The gate may include dielectric sidewall spacers. 
         [0070]    In one embodiment, the source is coupled to a SL. For example, the source is coupled to the SL via a contact. As for the drain, it is coupled to the BE layer. For example, as shown in  FIG. 3   a , a drain contact  393 L at the via or contact level and a drain pad  395 L at the metal level may be provided to couple the cell stack to the drain. Other configurations of coupling the drain and cell stack may also be useful. In the case of a BJT, for example, NPN BJT, the collector is coupled to BE, the base is coupled to WL and the emitter is coupled to SL. In the case of a diode as a cell selector, the drain pad may be a wordline forming a row of memory cell. 
         [0071]    Isolation regions  380  are provided. In one embodiment, the isolation region is a shallow trench isolation (STI) region. Other types of isolation regions may also be useful. The isolation regions are provided to separate columns of memory cells in the bitline direction and adjacent drain regions of select transistors of memory cells in the wordline direction. 
         [0072]    Memory cells, as described, can be arranged in rows and columns, forming an array. Rows may correspond to wordlines and columns may correspond to bitlines. Cell selectors of two adjacent rows may be configured to share a common source region while the drain regions are not common, forming a transistor pair. A plurality of transistor pairs is arranged in the row and column directions. The transistor pairs are separated from each other by isolation regions. Gate conductors are disposed on the substrate and traverse the transistor pairs in the row direction. The gate conductors, for example, are polysilicon gate conductors. 
         [0073]    In the case where a large number of memory cells are provided in a row, strap cells may be provided. Strap cells, for example, are contacts which connect wordlines in a metal level to the gate. The use of strap cells to stitch connections between the wordlines and gate increases array performance. Strap cells, for example, may be provided at every 16 or 32 cells in the rows. Providing strap cells at other number of cells may also be useful. 
         [0074]    In the case where strap cells are employed, it is preferable to avoid SLs and BLs in the same level. For example, WLs are provided in M1, SLs in M2 and BLs in M3, with storage units disposed between M2 and M3. Additional storage units may be stacked in metal levels above M3. Other configurations of WLs, SLs, BLs and storage units may also be useful. In the case of an array without strap cells, such as in the case of a small array, WLs are the gates, SLs are located in M1 and BLs are located in M2, with the storage units disposed between M1 and M2. Additional storage units may be stacked in metal levels above M2. Other configurations of WLs, SLs, BLs and storage units may also be useful. 
         [0075]    As described, the storage element R1 of the memory cell is located in the storage liner at about the interface with the heater layer. The cross-sectional area of the interface of the resistive element and the heater can be designed to be small. For example, the cross-sectional area of the interface is equal to W1×T p . Since T p  is less than the minimum lithographic resolution and W1 can be equal to the minimum lithographic resolution, the cross-sectional area can be designed to be smaller than that of conventional devices. For example, the minimum cross-sectional area of the interface for conventional devices is equal to about the minimum lithographic resolution×minimum lithographic resolution. Providing a small cross-sectional area for the interface increases heating efficiency, therefore reducing programming current necessary for programming R1. This reduces power consumption of the device. Additionally, this may reduce cell size since the transistor required for generating the programming current can be smaller. This results in reduced manufacturing cost. 
         [0076]    A plurality of storage units may be coupled in parallel to a cell selector. For example, the cell selector can be used to select a plurality of storage units. Each storage unit (or sub-storage unit) is coupled to its respective BL. This enables a cell selector to select a plurality of storage units. The storage units may be stacked one above another. For example, storage units may be disposed between adjacent metal levels and interconnected to the drain pad by contacts and pads. Stacking of storage units, for example, are described in co-pending U.S. patent application Ser. No. 13/802,899, titled “Stackable Non-Volatile Memory”, which is already herein incorporated by reference for all purposes. 
         [0077]      FIG. 4   a  shows a cross-sectional view of an embodiment of a device  400 .  FIGS. 4   b - d  show top views of various embodiments of the device. The cross-sectional view, for example, is along a first or wordline direction. A perpendicular direction to the first direction may be the bitline direction. The device includes a memory cell. In one embodiment, the memory cell is similar to the memory cell described in  FIG. 1   a ,  FIG. 2   a  and  FIGS. 3   a - d . Common elements may not be described or described in detail. 
         [0078]    Referring to  FIG. 4   a , the memory cell is disposed on a substrate  305 . The substrate, for example, is a semiconductor substrate, such as a silicon substrate. Other types of substrates may also be useful. The memory cell includes a storage unit  410  disposed in a cell dielectric layer  360 . The cell dielectric layer is disposed between two interlevel dielectric (ILD) layers on the substrate. For example, the storage unit is disposed between upper and lower ILD layers  390 U and  390 L. The ILD layers may be formed of silicon oxide. Other types of dielectric materials may also be useful for the ILD layers. It is understood that the different ILD layers need not be formed of the same material or have the same configuration. 
         [0079]    In one embodiment, the lower metal level  394 L is the first metal level M1 and the upper metal level  394 U is a second metal level M2 of the device. In such case, the lower contact level  392 L is a pre-metal dielectric (PMD) or contact (CA) level and the upper contact level  392 U is a first via level V1. Contacts at the PMD level may be tungsten contacts while contacts at the V1 level may be copper or copper alloy. Other types of conductive materials, such as aluminum, may also be useful. Other configurations of contacts at the contact levels may also be useful. 
         [0080]    In general, the storage unit may be disposed between any two metal levels of the device. For example, the storage unit may be disposed between any two metal layers, such as M2 and M3. In such case, a contact and contact pad may be provided in CA and M1 to facilitate electrical connections to, for example, a cell selector. In some instances, the contact may be a direct contact which extends through CA and M1. Other contact configurations may also be useful. Disposing the storage unit between other metal levels may also be useful. Preferably, the storage unit is disposed between two adjacent metal layers. Disposing the storage unit between non-adjacent metal layers may also be useful. For example, the storage unit may be disposed between M1 and M3. Disposing the storage unit between other non-adjacent metal levels may also be useful. Contacts may be provided in an intermediate ILD level to facilitate connections between two non-adjacent metal layers (e.g., M1 and M3). The contact may be a direct contact or a contact at the via level (e.g., V1) and a metal pad at the metal level (e.g., M2). Other contact configurations may also be useful. 
         [0081]    The storage unit is disposed within the cell dielectric layer located between the lower metal level and upper contact level. The cell dielectric layer may be formed of silicon oxide. Other types of dielectric materials may also be useful for the cell dielectric layer. The cell dielectric layer may be formed of different dielectric layers. The different dielectric layers may be formed of the same material. Providing the dielectric layers with different materials may also be useful. Other configurations of the cell dielectric layers may also be useful. The cell dielectric layer, for example, may be considered part of the upper ILD layer. 
         [0082]    In one embodiment, a dielectric liner  361  is disposed between the cell dielectric layer and the lower ILD layer. The dielectric liner, for example, serves as an etch stop layer. In one embodiment, the dielectric liner is formed of a low k dielectric. For example, the dielectric liner may be nBLOK. Other types of dielectric materials for the dielectric liner may also be useful. 
         [0083]    The storage unit includes first and second cell stacks  350   1-2 . A cell stack includes a programming stack  351  disposed between lower and upper stack dielectric layers  366  and  368 . In one embodiment, the programming stack includes a heater layer  352 . The heater layer has a thickness T p . The thickness T p  of the heater layer, in one embodiment, is less than the minimum lithographic dimension or critical CD. Other thicknesses for T p  may also be useful. 
         [0084]    The heater layer is disposed between lower and upper heat shield layers  354  and  356 . In one embodiment, the heat shield layers are formed of a PCM. For example, the heat shield layers are PC layers. The PCM, in one embodiment, is a chalcogenide material. In one embodiment, the chalcogenide material is a GeSbTe alloy. In another embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed as well as other types of dielectric layers with low thermal conductivity. The thickness of the storage layers may be about 20-100 nm. Other thicknesses may also be useful. 
         [0085]    The storage layers may include a capping or protective layer. The capping layer may be employed to reduce oxidation or poor interface viscosity between layers. The capping layer, for example, may be silicon nitride or silicon oxynitride. Other types of capping layers may also be useful. 
         [0086]    The cell stack includes opposing first ends along the first direction. For example, the cell stack includes opposing first ends along the wordline direction. A storage liner  338  lines at least the sidewall of a cell stack at one of the opposing first ends. For example, the storage liner lines at least the sidewall of the first opposing first end of a cell stack. In one embodiment, the first opposing first end is not adjacent to the other cell stack of the storage unit. As shown, the storage liner lines the sidewall of a cell stack at the first opposing first end and lower ILD layer, forming an L-shaped storage liner. For example, the storage liner lines the sidewall of a cell stack at the first opposing first end and the dielectric liner above the lower ILD layer. In one embodiment, the width of the storage liner is less than a width of the first opposing first end. For example, a width of the storage liner may be about the lithographic CD while the width of the first opposing first end is greater than the lithographic CD. Providing other widths for the storage liner may also be useful. The thickness of the storage liner may about 5-10 nm or less. Other thicknesses may also be useful. 
         [0087]    In one embodiment, the storage liner is formed of a PCM. For example, the storage layers are PC layers. The PCM, in one embodiment, is a chalcogenide material. In one embodiment, the chalcogenide material is a GeSbTe alloy. In another embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed. In some embodiments, the storage liner include heater or insulator islands. Providing heater or insulator islands is described in, for example, co-pending U.S. patent application Ser. No. 13/802,841, titled “Resistive Non-Volatile Memory”, which is herein incorporated by reference for all purposes. 
         [0088]    The storage liner may include a capping or protective layer. For example, a capping layer may be on a surface not in contact with ILD or cell stack. For example, the capping layer may be provided on the top surface of the storage liner. Providing it on the bottom surface of both surfaces of the storage liner may also be useful. The capping layer may be employed to reduce oxidation or poor interface viscosity between layers. The capping layer, for example, may be silicon nitride or silicon oxynitride. Other types of capping layers may also be useful. The thickness of the capping layer, for example, may be about 20 nm. Other thicknesses may also be useful. 
         [0089]    In one embodiment, the sidewall of a cell stack may include a thin insulator layer. For example, the thin insulator layer is disposed between the cell stack and storage liner. The thin insulator layer may be a thin silicon oxide layer. Other types of insulators may also be useful. The thin insulator layer may serve as a high voltage breakdown layer for initializing the procedure for the memory cell. Breaking down the dielectric layer forms, for example, heater or insulator islands. Providing heater or insulator islands is described in, for example, co-pending U.S. patent application Ser. No. 13/802,841, titled “Resistive Non-Volatile Memory”, which is herein incorporated by reference for all purposes. 
         [0090]    A top electrode (TE) layer  330  is disposed on the storage liner. The TE layer, for example, is formed of an ECM. The ECM, for example, has good conductivity and good interface with the PCM. In one embodiment, the TE layer is formed of an ECM with bad thermal conductivity to serve as a heat shield. For example, the TE layer may be a TaN layer. Other types of ECMs may also be useful. The TE layer is coupled to a bitline BL. For example, the TE layer is coupled to the BL by a contact. The contact may extend from the upper contact level through the cell dielectric layer. The TE layer provides connection for the bitline to the storage unit. In the case where the TE layer is an ECM with low thermal conductivity, it also serves as a heat shield. 
         [0091]    In some embodiments, the TE layer may be a TE stack having multiple layers. For example, the TE stack may include first and second TE layers  332  and  334 . The first TE layer, in one embodiment, contacts the BL. The second TE layer is disposed below that first TE layer and contacts the storage liner. In one embodiment, the first TE layer is formed of an ECM with good conductivity and low thermal conductivity. For example, the first TE layer is formed of TaN, serving as a heat shield and providing connection for the BL to the storage unit. Other types of ECMs may also be useful. As for the second TE layer, it is formed of an ECM which provides good electrical conductivity and good interface with the PCM. For example, the second TE layer may be formed of TiN. Other types of ECMs may also be useful. Other configurations of the TE stack may also be useful. The TE and storage liner may be collectively referred to as TE storage stack. 
         [0092]    In one embodiment, the TE of the first cell stack is coupled to a first bitline BL1 of the memory cell; the TE of the second cell stack is coupled to a second bitline BL2 of the memory cell. A bitline is coupled to the cell stack by, for example, a via contact  393 U. 
         [0093]    A bottom electrode (BE) layer  340  at least lines the sidewall of the cell stack at the other opposing first end. For example, the BE layer at least lines the sidewall of the cell stack at the second opposing first end. In one embodiment, the second opposing first end is adjacent to the other cell stack. In one embodiment, the BE layer lines the sidewall of the cell stack at a second opposing first end and lower ILD layer. The BE layer is a common BE layer of the first and second cell stacks. In one embodiment, the cell dielectric includes an opening exposing the drain pad. The BE layer may line the sides of the opening, including second opposing sides of the cell stacks, and bottom, contacting a drain pad  395 L disposed in the lower metal level  394 L. For example, the dielectric liner includes an opening to expose the drain pad, enabling the BE layer to electrically couple to it. The BE layer, for example, is formed of an ECM with good electrical conductivity and bad thermal conductivity, such as TaN. Other types of ECMs may also be useful. Preferably, the BE is formed of the same material as the TE (or the first TE) layer. Other configurations of the electrode liner may also be useful. 
         [0094]    The cell stacks  350   1-2  may have various shapes from the top view. For example, the cell stack may have a rectangular shape, as shown in  FIG. 4   b . Other shapes may also be useful. For example, the cell stack may have a polygonal or a trapezoidal shape, as shown in  FIGS. 4   c - d . For non-rectangular shaped cell stacks, the narrower portion is disposed adjacent to the TE side while the wider portion is disposed adjacent to the BE side. The TE storage stack may have a rectangular shape. Other shapes may also be useful. The TE storage stack may have a width W1 along the BL direction. The width W1, for example, may be equal to the lithographic CD. The width W1, for example, may depend on the technology node. For example, W1 may be about 40 nm for a 40 nm node. Other widths for W1 may also be useful. The width W1 is preferably as small as possible to minimize contact cross-section between the storage liner and the heater element. In some cases, if current is sufficiently small, W1 may be larger than lithographic CD. This reduces cost due to non-CD mask or reticle. 
         [0095]    As described, the TE storage stack has a width W1. The width W1 may be equal to CD to minimize contact area with the heater element to improve heating efficiency. It is understood that the width W1 is only related to the portion which contacts the sidewall of the first opposing first end. It is understood that the width of the TE stack may have a wider width W1c for forming contacts to the BL at a contact portion of the TE stack. The width W1c may be sufficient to accommodate a contact. This improves electrical connectivity to the BL. 
         [0096]    The width W2 of the BE, as shown, is wider than W1. For example, W2 is larger than the lithographic CD. As shown, W2 is wider than width of the cell stack. This is because BE serves as a connection to the drain pad. There is no need to minimize the cross-sectional contact area to the heater as in the case of the storage liner. Providing a larger surface area increases current capacity. This reduces current crowding. Other widths for W2 may also be useful. For example, W2 may be the same as the width of the cell stack. Providing W2 which is narrower than the cell stack may also be useful. 
         [0097]    Furthermore, as discussed, the storage unit may operate in either SLC or MLC mode. In SLC, the resistive elements are in opposite resistances. As such, both resistive elements are programmed at the same time. The hot spots for the resistive elements, for example, are disposed at the sides of the cell stacks which are opposite those adjacent to each other. In other words, the hot spots are located as far away from each other as possible within the storage unit. This reduces or minimizes thermal cross-talk. 
         [0098]    A cell selector is provided. The cell selector may be provided on the substrate. In one embodiment, the cell selector is a MOS transistor. The MOS transistor may be a planar transistor. Providing a fin-type MOS transistor may also be useful. Other types of cell selectors, such as BJT or diodes, may also be useful. The MOS transistor includes a gate and first and second S/D regions in the substrate adjacent to the gate. The S/D regions may include lightly doped (LD) regions. The first S/D region  175 , for example, serves as a drain while the second S/D region serves as a source. In one embodiment, the source is a common source region for an adjacent transistor of an adjacent row. The gate may include dielectric sidewall spacers. 
         [0099]    In one embodiment, the source is coupled to a SL. For example, the source is coupled to the SL via a contact. As for the drain, it is coupled to the BE layer. For example, a drain contact  393 L at the via or contact level and a drain pad  395 L at the metal level may be provided to couple the cell stacks to the drain. The drain, as shown, is commonly coupled to the first and second cell stacks. Other configurations of coupling the drain and cell stack may also be useful. In the case of a BJT, for example, NPN BJT, the collector is coupled to BE, the base is coupled to WL and the emitter is coupled to SL. In the case of a diode as a cell selector, the drain pad may be a wordline forming of a row of memory cell. 
         [0100]    Isolation regions  380  are provided. In one embodiment, the isolation region is a shallow trench isolation (STI) region. Other types of isolation regions may also be useful. The isolation regions are provided to separate columns of memory cells in the bitline direction and adjacent drain regions of select transistors of memory cells in the wordline direction. 
         [0101]    Memory cells, as described, can be arranged in rows and columns, forming an array. Rows may correspond to wordlines and columns may correspond to bitlines. Cell selectors of two adjacent rows may be configured to share a common source region while the drain regions are not common, forming a transistor pair. A plurality of transistor pairs is arranged in the row and column directions. The transistor pairs are separated from each other by isolation regions. Gate conductors are disposed on the substrate and traverse the transistor pairs in the row direction. The gate conductors, for example, are polysilicon gate conductors. 
         [0102]    In the case where a large number of memory cells are provided in a row, strap cells may be provided. Strap cells, for example, are contacts which connect wordlines in a metal level to the gate. The use of strap cells to stitch connections between the wordlines and gate increases array performance. Strap cells, for example, may be provided at every 16 or 32 cells in the rows. Providing strap cells at other number of cells may also be useful. 
         [0103]    In the case where strap cells are employed, it is preferable to avoid SLs and BLs in the same level. For example, WLs are provided in M1, SLs in M2 and BLs in M3, with storage units disposed between M2 and M3. Additional storage units may be stacked in metal levels above M3. Other configurations of WLs, SLs, BLs and storage units may also be useful. In the case of an array without strap cells, such as in the case of a small array, WLs are the gates, SLs are located in M1 and BLs are located in M2, with the storage units disposed between M1 and M2. Additional storage units may be stacked in metal levels above M2. Other configurations of WLs, SLs, BLs and storage units may also be useful. 
         [0104]    As described, the storage elements R1 and R2 of the memory cell are located in the storage liner of the first and second cell stacks at about the interface with the heater layer. The cross-sectional area of the interface of the resistive elements and the heater can be designed to be small. For example, the cross-sectional area of the interface is equal to W1×T p . Since T p  is less than the minimum lithographic resolution and W1 can be equal to the minimum lithographic resolution, the cross-sectional area can be designed to be smaller than that of conventional devices, which is equal to minimum lithographic resolution×minimum lithographic resolution. Providing a small cross-sectional area for the interface increases heating efficiency, therefore reducing programming current necessary for programming R1 and R2. This reduces power consumption of the device. Additionally, this may reduce cell size since the transistor required for generating the programming current can be smaller. This results in reduced manufacturing cost. 
         [0105]    A plurality of storage units may be coupled in parallel to a cell selector. For example, the cell selector can be used to select a plurality of storage units. Each storage unit (or sub-storage unit) is coupled to its respective BL. This enables a cell selector to select a plurality of storage units. The storage units may be stacked one above another. For example, storage units may be disposed between adjacent metal levels and interconnected to the drain pad by contacts and pads. Stacking of storage units, for example, are described in co-pending U.S. patent application Ser. No. 13/802,899, titled “Stackable Non-Volatile Memory”, which is already herein incorporated by reference for all purposes. 
         [0106]      FIGS. 5   a - i  show cross-sectional views of a process of forming an embodiment of a device  500 . The process includes forming a memory cell. The memory cell, in one embodiment, is a resistive-type memory cell. For example, the memory cell is a PCRAM cell. The memory cell is similar to that described in  FIGS. 1   a ,  2   a ,  3   a - d  and  4   a - d . Common elements may not be described or described in detail. The cross-sectional views are along the wordline direction. Although the cross-sectional views show one memory cell, it is understood that the device includes a plurality of memory cells of, for example, a memory array. 
         [0107]    Referring to  FIG. 5   a , a substrate  305  is provided. The substrate, for example, is a semiconductor substrate, such as a silicon substrate. Other types of semiconductor substrates, including silicon-germanium (SiGe), germanium (Ge), gallium-arsenic (GaAs) or any other suitable semiconductor materials as well as COI, may also be useful. The substrate may include a substrate component level. The substrate component level may include circuit components, such as transistors. Other circuit components may also useful. 
         [0108]    In one embodiment, the substrate includes a cell selector for the memory cell. The cell selector may be a transistor. For example, the cell selector is a MOS transistor. Other types of cell selectors may also be useful. The transistor, in one embodiment, includes a gate, a first diffusion region  175  and a second diffusion region. The diffusion regions, for example, are source/drain (S/D) regions. The first diffusion region, for example, is a drain region and the second diffusion region is a source region. The gate is disposed on the substrate between the first and second S/D regions in the substrate. A channel of the transistor is disposed below the gate between the S/D regions. The gate may be a gate conductor which is a common gate for a row of memory cells in the wordline direction. 
         [0109]    A lower ILD layer  390 L is disposed over the substrate covering the cell selector. The lower ILD layer may include a lower contact level  392 L and lower metal level  394 L. In one embodiment, the lower contact level is the CA level and the lower metal level is the M1 level. Contacts are provided to the S/D regions in the contact level of the lower ILD layer. For example, a SL is disposed in the lower metal level of the lower ILD layer and coupled to the second S/D region by a source contact. As for the first S/D region, a drain contact  393 L in the lower contact level couples it to a drain connector or pad  395 L in the lower metal level. 
         [0110]    Isolation regions  380  are prepared. In one embodiment, the isolation region is a shallow trench isolation (STI) region prepared in trenches formed in the substrate. Other types of isolation regions may also be useful. The isolation regions are provided to separate columns of memory cells in the bitline direction and adjacent drain regions of select transistors of memory cells in the wordline direction. A device well may be provided. The device well, for example, encompasses the transistor. 
         [0111]    In one embodiment, the lower ILD layer is formed of silicon oxide. The lower ILD layer, for example, may be formed by Chemical Vapor Deposition (CVD). Other techniques may also be useful. A planarizing process may be performed. The planarizing process, for example, may include chemical mechanical polishing (CMP). Other types of planarizing process may also be useful. 
         [0112]    Referring to  FIG. 5   b , a dielectric liner  361  is formed over the lower ILD layer. The dielectric liner, for example, serves as an etch stop layer. In one embodiment, the dielectric liner is formed of a low k dielectric. For example, the dielectric liner may be an nBLOK layer. Other types of dielectric liner materials may also be useful. The dielectric liner, for example, may be formed by blanket deposition using CVD. Other techniques of forming the dielectric liner may also be useful. 
         [0113]    In one embodiment, the various layers of a cell stack are formed on the substrate. For example, a lower stack dielectric layer  366 , a lower heat shield layer  354 , a heater layer  352 , an upper heat shield layer  356 , and an upper stack dielectric layer  368  are formed on the substrate. The layers of the cell stack may be formed over the dielectric liner. 
         [0114]    The stack dielectric layers, for example, may be formed of silicon oxide. Other types of dielectric materials may also be useful for the stack dielectric layers. It is understood that the upper and lower stack dielectric layers need not be formed of the same material. The stack dielectric layers, for example, may be formed by CVD. Other techniques may also be useful. 
         [0115]    As for the heat shield layers, in one embodiment, they are form of a PCM. For example, the heat shield layers are PCM layers. The heat shield layers, for example, may be formed of chalcogenide material. In one embodiment, the chalcogenide material is a germanium-antimony-tellurium (GeSbTe) alloy. In one embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed. Providing other types of heat shield layers may also be useful. The heat shield layers may be formed by physical vapor deposition (PVD). Forming the heat shield layers using other techniques, such as CVD, or a combination of techniques may also be useful. 
         [0116]    The heat shield layers may include one or more capping or protective layers on its top and/or bottom surfaces. A protective layer reduces oxidation and/or provides good interface forming. A protective layer, for example, may be silicon nitride or oxynitride. Other types of protective layers may also be useful. The protective layer may be formed by CVD. Other techniques for forming the protective layer may also be useful. 
         [0117]    Regarding the heater layer, it may be formed of an ECM. In one embodiment, the heater layer is formed of an ECM with good or suitable conductivity. For example, the heater layer may be a titanium nitride (TiN) layer. Other types of ECMs may also be useful. For example, the heater may be formed of W or TaN. The type of ECM may depend on, for example, the type of storage layer used. In one embodiment, the heater layer may be formed by CVD. Other techniques for forming the heater layer, such as PVD, are also useful. 
         [0118]    A first mask layer  510  is formed on the substrate. The first mask layer, for example, is formed over the cell stack layers. In one embodiment, the mask layer is a soft mask layer, such as a photoresist layer. An exposure source may selectively expose the photoresist layer through a reticle containing the desired pattern. For example, a first reticle or mask is used. After selectively exposing the photoresist layer, it is developed to form openings corresponding to location where the cell stack layers are to be removed. To improve lithographic resolution, an anti-reflective coating (not shown) may be used below the photoresist layer. In other embodiments, the mask layer may be a hard mask layer. For example, the hard mask layer may be TEOS layer. Patterning the hard mask layer may be achieved using a soft mask layer, such as photoresist. 
         [0119]    In  FIG. 5   c , the patterned mask layer is used to pattern the cell stack layers. In one embodiment, an anisotropic etch, such as reactive ion etch (RIE), is used to remove exposed portions of the cell stack layers. Other types of etch processes may also be useful. The etch transfers the pattern of the mask layer to the underlying cell stack layers. Patterning the cell stack layers forms first and second cell stacks  350   1-2 . The cell stack may have various shapes, such as those described in  FIGS. 4   b - d . For example, the cell stack may have a rectangular shape, polygonal shape or a trapezoidal shape. Other shapes may also be useful. 
         [0120]    After the patterning of the cell stack layers, the mask layer is removed, as shown in  FIG. 5   d . In one embodiment, the mask layer is removed by ashing. Other techniques of removing the mask layer may also be useful. For example, in the case of a hard mask, a wet etch may be employed to remove the mask. 
         [0121]    The process continues to form a storage liner  338 . The storage liner, for example, is a PCM liner. The storage liner may be formed of a chalcogenide material. In one embodiment, the chalcogenide material is a germanium-antimony-tellurium (GeSbTe) alloy. In another embodiment, the chalcogenide material is Ge 2 Sb 2 Te 5  (GST). Other types of chalcogenide or PCMs may also be employed. The storage liner may be formed by physical vapor deposition (PVD). Forming the storage liner using other techniques, such as CVD, or a combination of techniques may also be useful. 
         [0122]    The storage liner may include a capping or protective layer. The capping layer may be disposed on the storage liner after deposition. The protective layer reduces oxidation and/or provides good interface forming. The protective layer, for example, may be silicon nitride or oxynitride. Other types of protective layers may also be useful. The protective layer may be formed by CVD. Other techniques for forming the protective layer may also be useful. 
         [0123]    In some embodiments, a thin insulator layer may be formed prior to forming the storage liner. The thin insulator layer, for example, may be silicon oxide. Other types of insulating or dielectric materials may also be useful. The insulator may be formed by, for example, CVD. Other techniques for forming the insulator layer may also be useful. The thin insulator layer is subsequently subjected to an initialization process to electrically break it down to form, for example, heater islands. For example, the insulator layer may be subjected to a high breakdown voltage. The insulator layer may be less than about 10 nm thick. For example, the insulator layer may be about 1-5 nm thick. Other thicknesses may also be useful. 
         [0124]    A top electrode (TE) layer  330  is disposed on the storage liner. The TE layer, for example, is formed of an ECM. In one embodiment, the TE layer is an ECM with poor thermal conductivity. For example, the TE layer is a TaN layer. Other types of ECMs may also be useful. The TE layer may be formed by, for example, PVD. Other techniques, such as CVD, may also be useful. 
         [0125]    In some embodiments, the TE layer may be a TE stack having multiple layers. For example, the TE stack may include first and second TE layers  332  and  334 . The first TE layer is formed over the second TE layer while the second TE layer is formed over the storage liner. The first TE layer, in one embodiment, is formed of an ECM with low thermal conductivity. For example, the first TE layer is formed of TaN, serving as a heat shield. Other types of ECMs may also be useful. The second TE layer, for example, may be an ECM. For example, the second TE layer may be TiN. Other types of ECMs may also be useful. The second TE layer may facilitate good interface forming with the PCM. The TE layers may be formed by, for example, PVD. Other techniques, such as CVD, may also be useful. 
         [0126]    A second mask layer (not shown) is formed on the substrate. The second mask layer, for example, is formed over the TE layer. In one embodiment, the mask layer is a soft mask layer, such as a photoresist layer. Other types of mask layers may also be useful. An exposure source may selectively expose the photoresist layer through a reticle containing the desired pattern. For example, a reticle or mask is used. After selectively exposing the photoresist layer, it is developed to form openings. The openings, for example, correspond to where the TE layer and storage liner are to be removed. To improve lithographic resolution, an anti-reflective coating (not shown) may be used below the photoresist layer. In other embodiments, the mask layer may be a hard mask layer. 
         [0127]    The patterned mask layer is used to pattern the TE layer and storage liner. In one embodiment, an anisotropic etch, such RIE, is used to remove exposed portions of the TE layer and storage liner. Other types of etch processes may also be useful. The etch transfers the pattern of the mask layer to the underlying TE layer and storage liner. The TE layer and storage liner may have a rectangular shape with a width W1 which is narrower than the cell stacks, as shown by  FIG. 5   e , which is a corresponding top view of  FIG. 5   d . For example, the TE layer and storage liner have a width W1 which is equal to about the lithographic resolution. Other widths for W1 may also be useful. Patterning of the TE layer and storage liner exposes portions of the dielectric liner and top of the cell stacks. 
         [0128]    As shown in  FIG. 5   f , a dielectric layer  560  is formed on the substrate. The dielectric layer covers the cell stacks, dielectric liner and TE. The dielectric layer is used to form, for example, a part of the cell dielectric layer. The dielectric layer, for example is silicon oxide. Other types of dielectric materials may also be useful. In one embodiment, the dielectric layer may be the same as the stack dielectric layers. The dielectric layer, for example, may be formed by CVD. Other techniques for forming the dielectric layer may also be useful. A planarization process, such as CMP, may be performed to provide a planar surface. The planar top surface of the dielectric is above the top of the cell stacks. 
         [0129]    The dielectric layer is patterned to expose the connection to the cell selector. For example, the drain pad  395 L coupled to the drain of the cell selector is exposed. Patterning the dielectric layer, for example, is facilitated by a mask  520 . For example, the mask is a third mask, such as a photoresist mask. An ARC layer may be provided below the photoresist to improve lithographic resolution. An anisotropic etch, such as RIE, may be employed, removing the exposed dielectric layer and liner to expose the drain pad. 
         [0130]    After the patterning of the dielectric layer, the mask layer is removed. In one embodiment, the mask layer is removed by ashing. Other techniques of removing the mask layer may also be useful. 
         [0131]    Referring to  FIG. 5   g , a BE layer  540  is formed on the dielectric layer, lining it and the opening between the cell stacks. The BE layer also contacts the drain pad between the cell stacks. The BE layer, for example, is formed of an ECM. In one embodiment, the BE layer is formed of an ECM with low thermal conductivity, such as TaN. Other types of ECMs may also be useful. The BE layer serves as a connection to the heater layer and cell selector, as well as a heat shield. The BE layer may be formed by, for example, PVD. Other techniques, such as CVD, may also be useful. 
         [0132]    A dielectric layer  570  is formed on the substrate, as shown in  FIG. 5   h . The dielectric layer covers the BE layer and fills spaces between the cell stacks. The dielectric layer is used to form, for example, a part of the cell dielectric layer. The dielectric layer, for example is silicon oxide. Other types of dielectric materials may also be useful. In one embodiment, the dielectric layer may be the same as the stack dielectric layers. The dielectric layer, for example, may be formed by CVD. Other techniques for forming the dielectric layer may also be useful. 
         [0133]    In  FIG. 5   i , excess dielectric material may be removed by a planarizing process. The planarizing process, for example, may be CMP. Other types of planarizing process may also be useful. Additionally, excess BE, TE and storage liner materials over the cell stacks are removed. The planarizing forms a planar surface over the substrate. For example, the top surfaces of the dielectric layer, cell stacks, BE, TE and storage liner are co-planar. As such, the planarization process forms a storage unit  410  with first and second cell stacks  350   1-2 , TE, storage liner and BE. The storage unit is disposed in the cell dielectric layer  360 . The storage unit, for example, is similar to that described in  FIG. 4   a.    
         [0134]    Referring to  FIG. 5   j , the process continues to form BL1 and BL2 coupled to the storage unit. In one embodiment an upper dielectric layer ILD layer  390 U is formed over the cell dielectric layer. The upper ILD layer may be silicon oxide. Other types of dielectric layers may also be useful. Various techniques may be used to form the upper ILD layer. For example, the upper ILD layer may be formed by CVD. First and second bitlines BL1 and BL2 are formed in a metal level  394 U of the upper ILD layer. BL1 and BL2 are coupled to TE of the first and cell stacks by, for example, first and second bitline contacts  393 U in an upper via level  392 U. The bitlines and contacts may be formed of copper or copper alloy. Other types of conductive materials may also be useful. The bitlines and contacts may be formed by dual damascene techniques. Other techniques for forming the bitlines and contacts may also be useful. 
         [0135]    The process continues to complete forming the device. The process may include, for example, forming additional metal levels, final passivation, dicing, packaging and testing. Other processes may also be included. 
         [0136]    As described, the process forms a storage unit with first and second cell stacks. The process can be easily adopted to form a storage unit with a single cell stack. For example, the patterning of the cell stack layers can be modified to form a single cell stack instead of first and second cell stacks. Although the process, as shown, includes one memory cell, it is understood that the process may be applied to form a plurality of memory cells interconnected by wordlines and bitlines of a memory array. 
         [0137]    A plurality of storage units may be coupled in parallel to a cell selector. For example, the cell selector can be used to select a plurality of storage units. Each storage unit (or sub-storage unit) is coupled to its respective BL. This enables a cell selector to select a plurality of storage units. The storage units may be stacked one above another. For example, storage units may be disposed between adjacent metal levels and interconnected to the drain pad by contacts and pads. Stacking of storage units, for example, are described in co-pending U.S. patent application Ser. No. 13/802,899, titled “Stackable Non-Volatile Memory”, which is already herein incorporated by reference for all purposes. 
         [0138]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments, therefore, are to be considered in all respects illustrative rather than limiting the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.