Abstract:
Programmable resistive RAM cells have a resistance that depends on the size of the programmable resistive elements. Manufacturing methods and integrated circuits for programmable resistive elements with uniform resistance are disclosed that have a cross-section of reduced size compared to the cross-section of the interlayer contacts.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/757,366, filed 9 Jan. 2006 by inventors ChiaHua Ho, Erh-Kun Lai and Kuang Yeu Hsieh entitled  Process of Self-Align Formation of Bistable Resistance Random Access Memory.    

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to integrated circuit nonvolatile memory. In particular, the invention relates to programmable resistive nonvolatile memory, such as phase change memory. 
   2. Description of Related Art 
   Nonvolatile memory stores data without requiring a continuous supply of power. Programmable resistive memory such as phase change memory is an example of nonvolatile memory. A high current reset electrical pulse melts and quenches the programmable resistive element into an amorphous state, raising the resistance of the programmable resistive element. A low current set electrical pulse crystallizes and lowers the resistance of the programmable resistive element. 
   Because the electrical pulse type determines the data that are stored in the programmable resistive RAM, and the size of the programmable resistive RAM cells helps to determine characteristics of the electrical pulse, it is critical to manufacture the programmable resistive RAM cells with the appropriate size. 
   Generally, a smaller programmable resistive RAM cell is associated with electrical pulses of lower magnitude. Accordingly, a smaller programmable resistive RAM cell is associated with decreased power consumption. A traditional solution to forming small programmable resistive RAM cells is to rely on a photolithographic mask that defines the small cell. However, such a mask is associated with accompanying problems, such as proper alignment of the mask with the already fabricated layers of the planar integrated circuit. 
   Therefore, it would be desirable to manufacture small programmable resistive RAM cells while relying less on aggressive photolithography masks to define the small cells. 
   SUMMARY OF THE INVENTION 
   Embodiments of the technology include an integrated circuit with nonvolatile memory cells. The integrated circuit includes conductive rows accessing the nonvolatile memory cells by row, dielectric layers above the conductive rows, programmable resistive elements of the nonvolatile memory cells, interlayer contacts having a substantially uniform cross-section through the dielectric layers to conductively connect programmable resistive elements with the conductive rows, and conductive columns accessing the nonvolatile memory cells by column. 
   The programmable resistive elements have a cross-section smaller than the cross-section of the interlayer contacts. One end of the programmable resistive elements is adjacent to the interlayer contacts, and the other end of the programmable resistive elements is adjacent to the conductive columns. 
   Other embodiments of the technology include a method of forming an integrated circuit with nonvolatile memory cells, as described herein. In some embodiments, the step of forming interlayer contacts, includes removing upper portions of the interlayer contacts to leave openings having a substantially uniform cross-section. These openings are at least partly filled by the step of forming the programmable resistive elements. 
   Other embodiments of the technology include another method of forming an integrated circuit with nonvolatile memory cells. The method includes the following steps:
         The step of forming conductive rows accessing the nonvolatile memory cells by row.   The step of forming dielectric layers above conductive rows.   The step of forming interlayer contacts having a substantially uniform cross-section through the dielectric layers to conductively connect programmable resistive elements with the conductive rows, which includes the following steps.
           The step of forming the interlayer contacts having the substantially uniform cross-section.   Some embodiments include the step of removing a dielectric layer until exposing at least part of another dielectric layer, thereby exposing at least part of the upper portions of the interlayer contacts. These two dielectric layers have an etching selectivity difference.   Some embodiments include the step of forming sidewall structures adjacent to at least part of the upper portions of the interlayer contacts, which includes the following steps.
               The step of forming a dielectric layer covering and lying adjacent to the upper portions of the interlayer contacts. An etching selectivity difference exists between this dielectric layer and the dielectric layer removed to expose the upper portions of the interlayer contacts.   The step of removing excess material of this dielectric layer, leaving the sidewall structures.   
               The step of removing upper portions of the interlayer contacts to leave openings having the substantially uniform cross-section, whereby the openings are to be filled by the programmable resistive elements. In some embodiments, the upper portions being removed are adjacent to at least one interface between two dielectric layers having an etching selectivity difference, such that a lower layer of the two layers is an etch stop of an upper layer of the two layers. In some embodiments, the upper portions being removed are adjacent to a dielectric layer, such that the interlayer contacts and the first dielectric layer have an etching selectivity difference, and the dielectric layer resists removal during removal of the upper portions of the interlayer contacts.   
               

   The step of, prior to said forming the programmable resistive elements, forming dielectric liner in the openings left from the step of removing the upper portions of the interlayer contacts, and optionally over a dielectric layer as well. Optionally, the following steps are included.
         The step of forming the dielectric liner in the openings. The dielectric liner and the interlayer contacts have an etching selectivity difference. An etching selectivity difference also exists between the dielectric liner and the dielectric layer over which the liner was formed, such that the dielectric layer acts as an etch stop during removal of the liner.   The step of removing at least part of the dielectric liner in the openings to at least partly expose the interlayer contacts, and optionally the dielectric layer over which the liner was formed.       

   The step of forming the programmable resistive elements of the nonvolatile memory cells in the openings left from the step of removing the upper portions of the interlayer contacts. Examples of programmable resistive elements are a chalcogenide, Pr x Ca y MnO 3 , Pr x Sr y MnO 3 , ZrO x , a two-element memory compound, TCNQ, and PCBM. The programmable resistive elements have a cross-section smaller than the cross-section of the interlayer contacts, a first end adjacent to the interlayer contacts, and a second end adjacent to the conductive columns. The following steps are optionally included:
         The step of depositing programmable resistive material to partly fill the openings narrowed by the dielectric liner.       

   The step of removing the dielectric layers at least until the programmable resistive elements are substantially level with surrounding dielectric material. In some embodiments, this is performed by removing a dielectric layer until exposing at least part of another dielectric layer having a polishing sensitivity difference. 
   The step of forming conductive columns accessing the nonvolatile memory cells by column, such that the conductive columns are adjacent to the programmable resistive elements. 
   In some embodiments, the method, or one or more of the steps, is a self-aligned process, such as the step of removing the upper portions of the interlayer contacts, and/or the step of forming the programmable resistive elements. The self-aligned feature reduces cost by reducing the number or precision of photolithographic masks, and correspondingly raises product yield. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view during the manufacturing process that shows the formation of the transistor circuitry, multiple dielectric layers, and interlayer contacts. 
       FIG. 2  is a cross-sectional view during the manufacturing process that shows the removal of an upper dielectric layer to expose part of the upper portions of the interlayer contacts. 
       FIG. 3  is a cross-sectional view during the manufacturing process that shows the formation of the sidewall structures adjacent to the upper portions of the interlayer contacts. 
       FIG. 4  is a cross-sectional view during the manufacturing process that shows the removal of the upper portions of the upper portions of the interlayer contacts. 
       FIG. 5  is a cross-sectional view during the manufacturing process that shows the formation of the dielectric liner in the openings left from removing the upper portions of the interlayer contacts. 
       FIG. 6  is a cross-sectional view during the manufacturing process that shows removal of the dielectric liner portion covering the interlayer contacts. 
       FIG. 7  is a cross-sectional view during the manufacturing process that shows the formation of the programmable resistive material in the openings left from removing the upper portions of the interlayer contacts. 
       FIG. 8  is a cross-sectional view during the manufacturing process that shows removal of the upper layers of the integrated circuit. 
       FIG. 9  is a cross-sectional view during the manufacturing process that shows the formation of the bit lines and upper dielectric layer. 
       FIG. 10  is a block diagram of the integrated circuit including an array of nonvolatile programmable resistive memory cells, and other circuitry. 
   

   DETAILED DESCRIPTION 
   Various embodiments are directed at a manufacturing method for memory, such as nonvolatile embedded memory implementing programmable resistance RAM. Examples of resistance device RAM are resistance memory (RRAM), polymer memory, and phase change memory (PCRAM). The programmable resistive elements are formed to have a cross-section small than the cross-section of the upper portion of the interlayer contacts of programmable resistance RAM. 
     FIG. 1  is a cross-sectional view during the manufacturing process that shows the formation of the transistor circuitry, multiple dielectric layers, and interlayer contacts. 
   The substrate  8  (which may also be an n-well or p-well) has source and drain regions  14 ,  16 , and  18 . The gates  10  and  12  are conductive rows which selectively access the programmable resistive RAM cells, and induce a channel under the respective regions of the substrate  8  under the gates  10  and  12 , between regions  14  and  16  and regions  16  and  18 . Dielectric layers  20 ,  22 ,  24 ,  26 , and  28  cover the substrate  8  and the gates  10  and  12 . Interlayer contacts  32  and  34  physically and electrically connect to regions  14  and  18  through dielectric layers  20 ,  22 ,  24 ,  26 , and  28 . Example materials of dielectric layer  20 ,  24 , and  28  are oxides such as SiO x  and low-k material, and other dielectric materials associated with transistor fabrication. An example thickness of dielectric layer  20  is 600 nm, and an example thickness range of dielectric layers  24  and  28  is 100-200 nm. An example material of dielectric layers  22  and  26  is SiN, and an example thickness of dielectric layers  22  and  26  is 30 nm. Example materials of interlayer contacts  32  and  34  are W, polycrystalline Si without doping or with p or n doping (such as n+ doped polycrystalline Si). 
     FIG. 2  is a cross-sectional view during the manufacturing process that shows the removal of an upper dielectric layer to expose part of the upper portions of the interlayer contacts, stopping on the next layer of dielectric. 
   Dielectric layer  28  is removed, exposing dielectric layer  26  and an upper portion of the interlayer contacts  32  and  34 . Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods to remove dielectric layer  28 . One example is wet etching with dilute HF (DHF) or buffer HF (BHF) to wet etch silicon oxide. The etching selective difference between dielectric layer  26  and dielectric layer  28  is sufficiently high, such that the removal of material stops at dielectric layer  26 . 
     FIG. 3  is a cross-sectional view during the manufacturing process that shows the formation of the sidewall structures adjacent to the upper portions of the interlayer contacts. 
   High-density plasma (HDP) oxide layer is formed, and then etched to leave sidewall structures  36  and  38 . The etching selectivity difference between the SiO2 oxide and the dielectric layer  26 , as well as the etching selectivity difference between the SiO2 oxide and the contacts  32  and  34 , are sufficient to selectively remove the excess portions of the SiO2 oxide, without undue etching of either the dielectric layer  26  or the contacts  32  and  34 . 
   Example method of etching is wet etching, dry etching, or some combination of wet etching and dry etching. In one example, dilute HF (DHF) or buffer HF (BHF) are used to wet etch the silicon oxide layer to leave sidewall structures  36  and  38 . 
     FIG. 4  is a cross-sectional view during the manufacturing process that shows the removal of the upper portions of the upper portions of the interlayer contacts. 
   Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods. In one example, SF 6  based chemistry is used to dry etch the contacts  32  and  34 . The etching selectivity difference between contacts  32  and  34 , and structures  36  and  38 , is sufficiently high to prevent significant etching of structures  36  and  38 . Similarly, the etching selectivity difference between contacts  32  and  34 , and dielectric layers  20 ,  22 ,  24 , and  26 , is sufficiently high to prevent significant etching of structures  20 ,  22 ,  24 , and  26 . The etching time is controlled to etch the contacts  32  and  34  to around 100 ˜500 nm, such as below the dielectric layer  22 , for example about 50 nm below dielectric layer  22 . Contacts  32  and  34  have been etched to contacts  40  and  42  with shorter heights. 
     FIG. 5  is a cross-sectional view during the manufacturing process that shows the formation of the dielectric liner in the openings left from removing the upper portions of the interlayer contacts. 
   Dielectric liner  44  is substantially conformal with the exposed surface of the integrated circuit. An example material of dielectric liner  44  is silicon oxide deposited by CVD, which has low thermal conductivity. 
     FIG. 6  is a cross-sectional view during the manufacturing process that shows removal of the dielectric liner portion covering the interlayer contacts. 
   The dielectric liner  44  is etched anisotropically. Inside the openings  46 , 48  formed by removal of the upper portions of the interlayer contacts, the anisotropic etch stops on the interlayer contacts  40  and  42 . Outside the openings formed by removal of the upper portions of the interlayer contacts, the anisotropic etch stops on dielectric layer  26 . 
     FIG. 7  is a cross-sectional view during the manufacturing process that shows the formation of the programmable resistive material  50  in the openings left from removing the upper portions of the interlayer contacts. 
   The programmable resistive elements  52  and  54  physically and electrically connect with interlayer contacts  40  and  42 . 
   Embodiments of the memory cell include phase change based memory materials, including chalcogenide based materials and other materials, for the resistive elements  52  and  54 . Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te a Ge b Sb 100−(a+b) . One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky U.S. Pat. No. 5,687,112 patent, cols 10-11.) Particular alloys evaluated by another researcher include Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky &#39;112 at columns 11-13, which examples are hereby incorporated by reference. 
   Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly. 
   Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge 2 Sb 2 Te 5 . 
   Other programmable resistive memory materials may be used in other embodiments of the invention, including N2 doped GST, GexSby, or other material that uses different crystal phase changes to determine resistance; Pr x Ca y MnO 3 , Pr x Sr y MnO 3 , ZrO x , or other material that uses an electrical pulse to change the resistance state; 7,7,8,8-tetracyanoquinodimethane (TCNQ), methanofullerene 6,6-phenyl C61-butyric acid methyl ester (PCBM), TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ, C 60 -TCNQ, TCNQ doped with other metal, or any other polymer material that has bistable or multi-stable resistance state controlled by an electrical pulse. 
   The following are short summaries describing four types of resistive memory materials. The first type is chalcogenide material, such as Ge x Sb y Te z  where x:y:z=2:2:5, or other compositions with x: 0˜5; y: 0˜5; z: 0˜10. GeSbTe with doping, such as N—, Si—, Ti—, or other element doping is alternatively used. 
   An exemplary method for forming chalcogenide material uses PVD-sputtering or magnetron-sputtering method with source gas(es) of Ar, N 2 , and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The deposition is usually done at room temperature. A collimater with an aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, a DC bias of several tens of volts to several hundreds of volts is also used. On the other hand, the combination of DC bias and the collimater can be used simultaneously. 
   A post-deposition annealing treatment in vacuum or in an N 2  ambient is optionally performed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges from 100° C. to 400° C. with an anneal time of less than 30 minutes. 
   The thickness of chalcogenide material depends on the design of cell structure. In general, a chalcogenide material with thickness of higher than 8 nm can have a phase change characterization so that the material exhibits at least two stable resistance states. 
   A second type of memory material suitable for use in embodiments is colossal magnetoresistance (“CMR”) material, such as Pr x Ca y MnO 3  where x=0.5:0.5, or other compositions with x: 0˜1; y: 0˜1. CMR material that includes Mn oxide is alternatively used. 
   An exemplary method for forming CMR material uses PVD sputtering or magnetron-sputtering method with source gases of Ar, N 2 , O 2 , and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The deposition temperature can range from room temperature to ˜600° C., depending on the post deposition treatment condition. A collimater with an aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several tens of volts to several hundreds of volts is also used. On the other hand, the combination of DC bias and the collimator can be used simultaneously. A magnetic field of several tens of Gauss to as much as a Tesla (10,000 Gauss) may be applied to improve the magnetic crystallized phase. 
   A post-deposition annealing treatment in vacuum or in an N2 ambient or O2/N2 mixed ambient is optionally used to improve the crystallized state of CMR material. The annealing temperature typically ranges from 400° C. to 600° C. with an anneal time of less than 2 hours. 
   The thickness of CMR material depends on the design of the cell structure. The CMR thickness of 10 nm to 200 nm can be used for the core material. A buffer layer of YBCO (YBaCuO 3 , which is a type of high temperature superconductor material) is often used to improve the crystallized state of CMR material. The YBCO is deposited before the deposition of CMR material. The thickness of YBCO ranges from 30 nm to 200 nm. 
   A third type of memory material is two-element compounds, such as Ni x O y ; Ti x O y ; Al x O y ; W x O y ; Zn x O y ; Zr x O y ; Cu x O y ; etc, where x:y=0.5:0.5, or other compositions with x: 0˜1; y: 0˜1. An exemplary formation method uses a PVD sputtering or magnetron-sputtering method with reactive gases of Ar, N 2 , O 2 , and/or He, etc. at the pressure of 1 mTorr˜100 mTorr, using a target of metal oxide, such as Ni x O y ; Ti x O y ; Al x O y ; W x O y ; Zr x O y ; Zr x O y ; Cu x O y ; etc. The deposition is usually done at room temperature. A collimator with an aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several tens of volts to several hundreds of volts is also used. If desired, the combination of DC bias and the collimater can be used simultaneously. 
   A post-deposition annealing treatment in vacuum or in an N 2  ambient or O 2 /N 2  mixed ambient is optionally performed to improve the oxygen distribution of metal oxide. The annealing temperature ranges from 400° C. to 600° C. with an anneal time of less than 2 hours. 
   An alternative formation method uses a PVD sputtering or magnetron-sputtering method with reactive gases of Ar/O 2 , Ar/N 2 /O 2 , pure O 2 , He/O 2 , He/N 2 /O 2  etc. at the pressure of 1 mTorr˜100 mTorr, using a target of metal oxide, such as Ni, Ti, Al, W, Zn, Zr, or Cu etc. The deposition is usually done at room temperature. A collimater with an aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, a DC bias of several tens of volts to several hundreds of volts is also used. If desired, the combination of DC bias and the collimator can be used simultaneously. 
   A post-deposition annealing treatment in vacuum or in an N 2  ambient or O 2 /N 2  mixed ambient is optionally performed to improve the oxygen distribution of metal oxide. The annealing temperature ranges from 400° C. to 600° C. with an anneal time of less than 2 hours. 
   Yet another formation method uses oxidation by a high temperature oxidation system, such as a furnace or a rapid thermal pulse (“RTP”) system. The temperature ranges from 200° C. to 700° C. with pure O 2  or N 2 /O 2  mixed gas at a pressure of several mTorr to 1 atm. The time can range several minute to hours. Another oxidation method is plasma oxidation. An RF or a DC source plasma with pure O 2  or Ar/O 2  mixed gas or Ar/N 2 /O 2  mixed gas at a pressure of 1 mTorr to 100 mTorr is used to oxidize the surface of metal, such as Ni, Ti, Al, W, Zn, Zr, or Cu etc. The oxidation time ranges several seconds to several minutes. The oxidation temperature ranges from room temperature to 300° C., depending on the degree of plasma oxidation. 
   A fourth type of memory material is a polymer material, such as TCNQ with doping of Cu, C 60 , Ag etc. or PCBM-TCNQ mixed polymer. One formation method uses evaporation by thermal evaporation, e-beam evaporation, or molecular beam epitaxy (“MBE”) system. A solid-state TCNQ and dopant pellets are co-evaporated in a single chamber. The solid-state TCNQ and dopant pellets are put in a W-boat or a Ta-boat or a ceramic boat. A high electrical current or an electron-beam is applied to melt the source so that the materials are mixed and deposited on wafers, There are no reactive chemistries or gases. The deposition is done at a pressure of 10 −4  Torr to 10 −10  Torr. The wafer temperature ranges from room temperature to 200° C. 
   A post-deposition annealing treatment in vacuum or in an N 2  ambient is optionally performed to improve the composition distribution of polymer material. The annealing temperature ranges from room temperature to 300° C. with an anneal time of less than 1 hour. 
   Another technique for forming a layer of polymer-based memory material is to use a spin-coater with doped-TCNQ solution at a rotation of less than 1000 rpm. After spin-coating, the wafer held (typically at room temperature or temperature less than 200° C.) for a time sufficient for solid-state formation. The hold time ranges from several minutes to days, depending on the temperature and on the formation conditions. 
     FIG. 8  is a cross-sectional view during the manufacturing process that shows removal of the upper layers of the integrated circuit. 
   Chemical mechanical polishing (CMP) planarizes the surface down to the dielectric layer  22 , removing dielectric layers  26  and  24 . Parts of the dielectric liner  46  and  48  exceeding the height of dielectric layer  22  are removed, leaving dielectric liner  56  and  58 . Parts of the programmable resistive material  52  and  54  exceeding the height of dielectric layer  22  are removed, leaving programmable resistive material  60  and  62 . The CMP stops at dielectric layer  22 , due to a high selectivity slurry such as CeO 2 , etc., and/or a different endpoint or signal point between the polished materials. The CMP process therefore results in all the programmable resistive elements such as  60  and  62  having the same height, which has the benefit of a low resistance variation across the different nonvolatile memory cells, regardless of the stored logic state. 
     FIG. 9  is a cross-sectional view during the manufacturing process that shows the formation of the bit lines and upper dielectric layer. 
   Metal bit lines  64  that access the programmable resistive RAM cells by column are deposited. Example materials for the metal bit lines  64  are TiN/AlCu/Ti/TiN, TaN/CU, and TiN/W. An all Cu process is another alternative. Intermetal dielectric  66  is deposited, which can be silicon oxide, HDP oxide, plasma enhanced (PE) oxide, etc. 
     FIG. 10  is a block diagram of the integrated circuit including an array of nonvolatile programmable resistive memory cells, and other circuitry. 
   The integrated circuit  1050  includes a memory array  1000  implemented using memory cells with resistive elements on a semiconductor substrate. The resistive elements have a small-cross-section, and are formed partly by the removal of the upper portions of the interlayer contacts, as described herein. Addresses are supplied on bus  1005  to column decoder  1003  and row decoder  1001 . Sense amplifiers and data-in structures in block  1006  are coupled to the column decoder  1003  via data bus  1007 . Data is supplied via the data-in line  1011  from input/output ports on the integrated circuit  1050 , or from other data sources internal or external to the integrated circuit  1050 , to the data-in structures in block  1006 . Data is supplied via the data-out line  1015  from the block  1006  to input/output ports on the integrated circuit  1050 , or to other data destinations internal or external to the integrated circuit  1050 . The integrated circuit  1050  may also include circuitry directed a mission function other than the nonvolatile storage with resistive elements (not shown). Bias arrangement state machine  1009  controls the application of bias arrangement supply voltages. 
   Terms describe positions of layers with respect to each other are relative unless otherwise specified. For example, a layer “covers” another layer or is “above” another layer, even in the presence of an intervening layer (e.g., between the covering layer and the covered layer, or between the “above” layer and the “below” layer. A layer is “between” two other layers, regardless of any intervening layers on either side. 
   While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.