# Patent

## Title
Programmable resistive RAM and manufacturing method

## Abstract

Programmable resistive RAM cells have a resistance that depends on the size of the contacts. Manufacturing methods and integrated circuits for lowered contact resistance are disclosed that have contacts of reduced size.

## Background
REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No. 60/757,368, filed 9 Jan. 2006 by inventors ChiaHua Ho, Erh-Kun Lai and Kuang-Yue Hsieh entitled Process of 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 characteristics of the electrical pulse depend in part on the resistance of the programmable resistive RAM cells, it is critical to manufacture the programmable resistive RAM cells with the correct resistance.
The resistance of the programmable resistive RAM cells is reduced by shrinking the size of the electrical contacts that connect the programmable resistive RAM cells to the remainder of the integrated circuit. A traditional solution to forming small electrical contacts is to rely on a photolithographic mask that defines the small electrical contacts. 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 electrical contacts for programmable resistive RAM without relying on an aggressive photolithography mask to define the small electrical contacts.
SUMMARY OF THE INVENTION
Embodiments of the technology include a self-aligned method of forming an integrated circuit with nonvolatile memory cells. The following steps are included:
The step of forming conductive rows accessing the nonvolatile memory cells by row.
The step of forming dielectric layers above the conductive rows. In one embodiment, these dielectric layers include multiple layers, at least two of which have an etching selectivity difference between the layers.
The step of forming interlayer contacts through the dielectric layers to conductively connect with the conductive rows.
The step of reducing a cross-section of a part of the interlayer contacts in a self-aligned process. This reduction process is performed in some embodiments by forming dielectric structures at least partly covering the interlayer contacts, and reducing a cross-section of a part of the interlayer contacts by removing material from a part of the interlayer contacts uncovered by the dielectric structures. One example of reducing the cross-section is performed as follows. A dielectric layer is exposed by the interlayer contacts, by removing another dielectric layer at least by the interlayer contacts. A new dielectric layer is formed at least partly covering the interlayer contacts. Only part of the new dielectric layer covering the interlayer contacts is removed, to leave dielectric structures at least partly covering the interlayer contacts. One example of removing the new material is by wet etching part of the new dielectric layer for a duration, which controls a critical dimension of the interlayer contacts achieved by reducing the cross-section. In one embodiment, the dielectric structures have a substantially triangle-shaped cross-section. Finally, a cross-section of a part of the interlayer contacts is reduced by removing material from a part of the interlayer contacts uncovered by the dielectric structures. In one embodiment, the cross-section of the interlayer contacts is reduced by dry etching the part of the interlayer contacts uncovered by the dielectric structures. In one embodiment, the critical dimension of the interlayer contacts is controlled to be less than or equal to about 60 nm.
The step of forming programmable resistive elements of the nonvolatile memory cells to conductively connect with the interlayer contacts. Example materials for forming the programmable resistive elements are: a chalcogenide, PrxCayMnO3, PrSrMnO3, ZrOx, a two-element memory compound, TCNQ, and PCBM.
The step of forming conductive columns accessing the nonvolatile memory cells by column to conductively connect with the programmable resistive elements.
Some embodiments include the step of: surrounding the interlayer contacts with additional dielectric structures to fill gaps between the interlayer contacts and the dielectric layers. The gaps result from the step of reducing the cross-section. The additional dielectric structures have a thermal conductivity sufficiently low to reduce a reset current.
Further 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 said conductive rows, programmable resistive elements of the nonvolatile memory cells above the dielectric layers, interlayer contacts through the dielectric layers to conductively connect with the conductive rows, and conductive columns accessing the nonvolatile memory cells by column to conductively connect with the programmable resistive elements.
Each of the interlayer contacts has a first part and second part. The first part is adjacent to and conductively connects to at least one of the programmable resistive elements. In some embodiments, the cross-section of the first part has a critical dimension less than or equal to about 60 nm. The second part is adjacent to and conductively connects to the first part. The second part is also conductively connected to the conductive rows. The second part has a cross-section larger than the cross-section of the first part. The cross-section of the second part is substantially uniform between the first part and the conductive rows.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view showing transistor structure to select particular programmable resistive RAM cells by row.
FIG. 2 is a cross-sectional view showing the removal of a top layer of dielectric, stopping on the next layer of dielectric.
FIG. 3 is a cross-sectional view showing the formation of a new layer of dielectric, including dielectric structures on the contacts having a substantially triangle-shaped cross-section.
FIG. 4 is a cross-sectional view showing the removal of part of the new layer of dielectric, including part of the dielectric structures on the contacts having a substantially triangle-shaped cross-section.
FIG. 5 is a cross-sectional view showing the removal of part of the contacts uncovered by the remainder of the substantially triangle-shaped dielectric structures.
FIG. 6 is a cross-sectional view showing the formation of dielectric in the gaps around the contacts resulting from the prior removal of part of the contacts, and removal of material to flatten the surface.
FIG. 7 is a cross-sectional view showing the formation of resistive memory elements and top electrodes.
FIG. 8 is a cross-sectional view showing the formation of bit lines and an additional layer of dielectric.
FIG. 9 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 (RAM), polymer memory, and phase change memory (PCRAM). The cross-section is reduced of an upper portion of the interlayer contacts of programmable resistance RAM.
FIG. 1 is a cross-sectional view showing transistor structure to select particular programmable resistive RAM cells by row. 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, and 24 cover the substrate 8 and the gates 10 and 12. Interlayer contacts 28 and 30 physically and electrically connect to regions 14 and 18 through dielectric layers 20, 22, and 24. Example materials of dielectric layer 20 and 24 are oxides such as SiOx 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 layer 24 is 100-200 nm. An example material of dielectric layer 22 is SiN, and an example thickness of dielectric layer 22 is 30 nm. Example materials of interlayer contacts 28 and 30 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 showing the removal of a top layer of dielectric, stopping on the next layer of dielectric. Dielectric layer 24 is removed, exposing dielectric layer 22 and an upper portion of the interlayer contacts 28 and 30. Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods to remove dielectric layer 24. 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 22 and dielectric layer 24 is sufficiently high, such that the removal of material stops at dielectric layer 22.
FIG. 3 is a cross-sectional view showing the formation of a new layer of dielectric, including dielectric structures on the contacts having a substantially triangle-shaped cross-section. High-density plasma (HDP) oxide layer 32 and structures 34 and 36 having a substantially triangular-cross section are deposited. An example thickness range of oxide layer 32 is 150-300 nm. Structures 34 and 36 cover the interlayer contacts 28 and 30. Alternatively, part of contacts 28 and 30 remain exposed.
FIG. 4 is a cross-sectional view showing the removal of part of the new layer of dielectric, including part of the dielectric structures on the contacts having a substantially triangle-shaped cross-section. Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods. In one example, dilute HF (DHF) or buffer HF (BHF) are used to wet etch a silicon oxide layer 32 and structures 34 and 36. Removal of material from structures 34 and 36 shrinks their triangular cross-section, resulting in structures 38 and 40. Contacts 28 and 30, which were formerly completely covered by structures 34 and 36 are now partly exposed by structures 38 and 40. Alternatively, an already exposed part of contacts 28 and 30 is increased in area. The etching selectivity difference between contacts 28 and 30, and layer 32 and structures 34 and 36 is sufficiently high to prevent significant etching of contacts 28 and 30. However, layer 32 is etched during the etching of structures 34 and 36.
FIG. 5 is a cross-sectional view showing the removal of part of the contacts uncovered by the remainder of the substantially triangle-shaped dielectric structures. Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods. In one example, reactive-ion-etch with SF6 based chemistry is used for dry etching the contacts 28 and 30. In effect, the substantially triangular-shaped structures 38 and 40 are used a as a hard mask to prevent etching of the parts of contacts 28 and 30 under structures 38 and 40. The etching selectivity difference between contacts 28 and 30, and structures 38 and 40, is sufficiently high to prevent significant etching of structures 38 and 40. The etching time is controlled to etch the contacts 28 and 30 to a suitable depth of for example 200 nm. Contacts 28 and 30 now have respective portions 42 and 44 with smaller cross-sections.
FIG. 6 is a cross-sectional view showing the formation of dielectric in the gaps around the contacts resulting from the prior removal of part of the contacts, and removal of material to flatten the surface. Dielectric structures 46 and 48 are deposited into the volume left open by the removal of part of the contacts 28 and 30. Thus, dielectric structures 46 and 48 surround contact portions 42 and 44. Examples of materials for dielectric structures 46 and 48 are low-k material, HDP oxide, and Accuflow with a suitable post-annealing temperature of for example 400° C.
Chemical mechanical polishing (CMP) planarizes the surface and opens the contact portions 42 and 44 covered by the formation of dielectric structures 46 and 48. An example critical dimension of the contact portions 42 and 44 is 60 nm.
FIG. 7 is a cross-sectional view showing the formation of resistive memory elements and top electrodes.
The programmable resistive elements 50 and 52 physically and electrically connect with contact portions 42 and 44.
Embodiments of the memory cell include phase change based memory materials, including chalcogenide based materials and other materials, for the resistive element. 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 TeaGebSb 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 '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 be 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 Ge2Sb2Te5.
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; PrxCayMnO3, PrSrMnO3, ZrOx, 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, C60-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 GexSbyTez 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, N2, 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 collimator can be used simultaneously.
A post-deposition annealing treatment in vacuum or in an N2 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 PrxCayMnO3 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, N2, O2, 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 (YBaCuO3, 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 NixOy; TixOy; AlxOy; WxOy; ZnxOy; ZrxOy; CuxOy; 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, N2, O2, and/or He, etc. at the pressure of 1 mTorr  ̃100 mTorr using a target of metal oxide, such as NixOy; TixOy; AlxOy; WxOy; ZnxOy; ZrxOy; CuxOy; 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, 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 N2 ambient or O2/N2 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/O2, Ar/N2/O2, pure O2, He/O2, He/N2/O2 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 collimater can be used simultaneously.
A post-deposition annealing treatment in vacuum or in an N2 ambient or O2/N2 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 O2 or N2/O2 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 O2 or Ar/O2 mixed gas or Ar/N2/O2 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, C60, 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 N2 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.
The conductive top electrodes 54 and 56 physically and electrically connect with programmable resistive elements 50 and 52. Example materials of top electrodes 54 and 56 are single layer and multilayered. Single layered top electrodes 54 and 56 have low thermal conductivity, and are, for example, TiN, TaN, LaNiO3, etc. Multilayered top electrodes 54 and 56 are, for example, TiN/AlCu, TaN/Cu, etc.
Finally, the stacks of programmable resistive elements 50 and 52 and conductive top electrodes 54 and 56 are etched. The stack etch of programmable resistive elements 50 and 52 and conductive top electrodes 54 and 56 can be defined by a non-aggressive mask, because the stack feature sizes are larger than the critical dimension of the contact portions 42 and 44. Wet etching, dry etching, or some combination of wet etching and dry etching, are alternative methods. For example, dry etching uses chemistries such as CL2, BCl3, etc. The etching selectivity difference is sufficiently high that the oxide layer 32 is not significantly etched.
FIG. 8 is a cross-sectional view showing the formation of bit lines and an additional layer of dielectric. Intermetal dielectric 58 is deposited, which can be silicon oxide, HDP oxide, plasma enhanced (PE) oxide, etc. Vias 60 and 62 are formed to physically and electrically connect with the top electrodes 54 and 56. Vias 60 and 62 are filled, and metal bit lines 64 that access the programmable resistive RAM cells by column. Example materials for the vias 60 and 62 are W and for the metal bit lines 64 are AlCu. An all Cu processes is another alternative.
FIG. 9 is a block diagram of the integrated circuit including an array of nonvolatile programmable resistive memory cells, and other circuitry.
The integrated circuit 950 includes a memory array 900 implemented using memory cells with resistive elements on a semiconductor substrate. The memory array 900 has contacts with a narrowed cross-section as described herein. Addresses are supplied on bus 905 to column decoder 903 and row decoder 901. Sense amplifiers and data-in structures in block 906 are coupled to the column decoder 903 via data bus 907. Data is supplied via the data-in line 911 from input/output ports on the integrated circuit 950, or from other data sources internal or external to the integrated circuit 950, to the data-in structures in block 906. Data is supplied via the data-out line 915 from the block 906 to input/output ports on the integrated circuit 950, or to other data destinations internal or external to the integrated circuit 950. The integrated circuit 950 may also include circuitry directed a mission function other than the nonvolatile storage with resistive elements (not shown).
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.

## Claims

1. A method of forming an integrated circuit with nonvolatile memory cells, comprising:
forming conductive rows accessing the nonvolatile memory cells by row;
forming dielectric layers above said conductive rows, the dielectric layers including a first dielectric layer and a second dielectric layer above the first dielectric layer, the first dielectric layer and the second dielectric layer having different etch rates;
forming interlayer contacts through the dielectric layers to conductively connect with the conductive rows;
reducing a cross-section of a part of the interlayer contacts;
forming programmable resistive elements of the nonvolatile memory cells over the interlayer contacts, the interlayer contacts including a first part adjacent to and conductively connected to at least one of the programmable resistive elements, the first part having a top surface, wherein the top surface is higher than the second dielectric layer; and
forming conductive columns accessing the nonvolatile memory cells by column to conductively connect with the programmable resistive elements.

2. The method of claim 1, wherein said forming dielectric layers includes:
forming a first dielectric layer above said conductive rows; and
forming a second dielectric layer at least partly above the first dielectric layer, wherein the first and second dielectric layers have an etching selectivity difference.

3. The method of claim 1, wherein said reducing the cross-section includes:
controlling a critical dimension of the interlayer contacts to be less than or equal to about 60 nm.

4. The method of claim 1, wherein said reducing the cross-section includes:
forming dielectric structures at least partly covering the interlayer contacts prior to said removing part of the interlayer contacts; and
wherein said reducing a cross-section of a part of the interlayer contacts includes removing material from part of the interlayer contacts uncovered by the dielectric structures.

5. The method of claim 1, wherein said reducing the cross-section includes:
forming a second dielectric layer at least partly covering the interlayer contacts;
removing only part of the second dielectric layer covering the interlayer contacts to leave dielectric structures at least partly covering the interlayer contacts; and
reducing a cross-section of a part of the interlayer contacts by removing material from a part of the interlayer contacts uncovered by the dielectric structures.

6. The method of claim 5, wherein said removing only part of the second dielectric layer includes:
wet etching said part of the second dielectric layer.

7. The method of claim 5, wherein said removing only part of the second dielectric layer includes:
wet etching said part of the second dielectric layer for a duration, wherein the duration controls a critical dimension of said part of the interlayer contacts achieved by said reducing the cross-section.

8. The method of claim 5, wherein said forming the second dielectric layer includes:
forming the dielectric structures covering the interlayer contacts, the dielectric structures having a substantially triangle-shaped cross-section.

9. The method of claim 5, wherein said reducing the cross-section includes:
dry etching the part of the interlayer contacts uncovered by the dielectric structures.

10. The method of claim 1, further comprising:
surrounding the interlayer contacts with dielectric structures to fill gaps between the interlayer contacts and the dielectric layers, the gaps resulting from said reducing the cross-section, wherein the dielectric structures have a thermal conductivity sufficiently low to reduce a reset current.

11. The method of claim 1, wherein said forming programmable resistive elements includes:
forming programmable resistive elements including at least one of: a chalcogenide, PrxCayMnO3, PrSrMnO3, ZrOx, a two-element memory compound, TCNQ, and PCBM.

12. A method of forming an integrated circuit with nonvolatile memory cells, comprising:
forming conductive rows accessing the nonvolatile memory cells by row;
forming dielectric layers above said conductive rows, including:
forming a first dielectric layer above said conductive rows; and
forming a second dielectric layer at least partly above the first dielectric layer, wherein the first and second dielectric layers have an etching selectivity difference;

forming interlayer contacts through the dielectric layers to conductively connect with the conductive rows;
reducing a cross-section of a part of the interlayer contacts in a self-aligned process, including:
forming a third dielectric layer at least partly covering the interlayer contacts;
removing only part of the third dielectric layer covering the interlayer contacts to leave dielectric structures at least partly covering the interlayer contacts, including wet etching said part of the third dielectric layer; and
reducing a cross-section of a part of the interlayer contacts by removing material from a part of the interlayer contacts uncovered by the dielectric structures;

forming programmable resistive elements of the nonvolatile memory cells to conductively connect with the interlayer contacts; and
forming conductive columns accessing the nonvolatile memory cells by column to conductively connect with the programmable resistive elements.

13. The method of claim 12, wherein said reducing the cross-section includes:
controlling a critical dimension of the interlayer contacts to be less than or equal to about 60 nm.

14. The method of claim 12, wherein said removing only part of the third dielectric layer includes:
wet etching said part of the third dielectric layer for a duration, wherein the duration controls a critical dimension of said part of the, interlayer contacts achieved by said reducing the cross-section.

15. The method of claim 12, wherein said forming the third dielectric layer includes:
forming the dielectric structures covering the interlayer contacts, the dielectric structures having a substantially triangle-shaped cross-section.

16. The method of claim 12, wherein said reducing the cross-section includes:
dry etching the part of the interlayer contacts uncovered by the dielectric structures.

17. The method of claim 12, wherein said forming programmable resistive elements includes:
forming programmable resistive elements including at least one of: a chalcogenide, PrxCayMnO3, PrSrMnO3, ZrOx, a two-element memory compound, TCNQ, and PCBM.

18. An integrated circuit with nonvolatile memory cells, comprising:
conductive rows accessing the nonvolatile memory cells by row;
a first dielectric layer above said conductive rows;
a second dielectric layer above said first dielectric layer, the second dielectric having a different etch rate from the first dielectric layer;
programmable resistive elements of the nonvolatile memory cells above the second dielectric layer;
interlayer contacts through the dielectric layers to conductively connect with the conductive rows, each including:
a first part adjacent to and conductively connected to at least one of the programmable resistive elements, the first part having a first cross-section, the first part having a top surface, wherein the top surface is higher than the second dielectric layer; and
a second part adjacent to and conductively connected to the first part, the second part conductively connected to the conductive rows, the second part having a second cross-section larger than the first cross-section, the second cross-section being substantially uniform between the first part and the conductive rows; and

conductive columns accessing the nonvolatile memory cells by column to conductively connect with the programmable resistive elements.

19. The circuit of claim 18, wherein the first cross-section of the first part of each interlayer contact has a critical dimension less than or equal to about 60 nm.

20. The circuit of claim 18, wherein the programmable resistive elements includes at least one of: a chalcogenide, PrxCayMnO3, PrSrMnO3, ZrOx, a two-element memory compound, TCNQ, and PCBM.