Search

US-12622181-B2 - Phase change memory

US12622181B2US 12622181 B2US12622181 B2US 12622181B2US-12622181-B2

Abstract

An apparatus includes a substrate that has an upper face; a first electrode that is attached to the upper face of the substrate; a second electrode that is attached to the upper face of the substrate at a distance from the first electrode; and a bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first and second electrodes. At least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase. In some embodiments, the apparatus also includes access devices that are disposed between the electrodes and the substrate, with the bridge being electrically connected between the access devices. At least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase.

Inventors

  • Kangguo Cheng
  • Juntao Li
  • Julien Frougier
  • Ruilong Xie

Assignees

  • INTERNATIONAL BUSINESS MACHINES CORPORATION

Dates

Publication Date
20260505
Application Date
20221130

Claims (20)

  1. 1 . An apparatus comprising: a substrate that has an upper face; a first electrode that is attached to the upper face of the substrate; a second electrode that is attached to the upper face of the substrate at a distance from the first electrode; a bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first and second electrodes, wherein at least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase; access devices that are disposed between the first and second electrodes and the substrate, with the bridge being electrically connected between the access devices; and a first resistive liner that extends along a surface of the bridge between the first and second electrodes.
  2. 2 . The apparatus of claim 1 , wherein the liner has an electrical resistance greater than that of the bridge in the low resistance solid phase and less than that of the bridge in the high resistance solid phase.
  3. 3 . The apparatus of claim 2 , wherein the first resistive liner extends along an interface between the bridge and the substrate.
  4. 4 . The apparatus of claim 3 , further comprising a second resistive liner that extends across a surface of the bridge that is opposite the substrate.
  5. 5 . The apparatus of claim 2 , wherein the first resistive liner extends between each end of the bridge and the first and second electrodes.
  6. 6 . The apparatus of claim 2 , wherein the first resistive liner extends between the first and second electrodes and the substrate.
  7. 7 . An apparatus comprising: a substrate that has an upper face; a first electrode that is attached to the upper face of the substrate; a second electrode that is attached to the upper face of the substrate at a distance from the first electrode; a bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first and second electrodes, wherein at least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase; and access devices that are disposed between the first and second electrodes and the substrate, with the bridge being electrically connected between the access devices.
  8. 8 . The apparatus of claim 7 , wherein the access devices are ovonic threshold switches (OTS).
  9. 9 . The apparatus of claim 7 , wherein at least one of the access devices in its ON state has lower electrical resistance and lower thermal conductivity than the bridge.
  10. 10 . The apparatus of claim 7 , wherein the bridge has a smaller cross-sectional area perpendicular to its length compared to larger cross-sectional areas of the first and second access devices perpendicular to their thicknesses.
  11. 11 . An apparatus comprising: a substrate that has an upper face; a first metal word line that extends in a first direction across the substrate at a first distance above the substrate; a metal bit line that extends across the substrate in a second direction that is different than the first direction, at a second distance above the substrate, which is less than the first distance; a first word electrode that is attached to and protrudes from the upper face of the substrate below the first word line; a first contact via that electrically connects the first word electrode to the first word line; a first bit electrode that is attached to and protrudes from the upper face of the substrate below the metal bit line and is electrically connected to the metal bit line; a first bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first word electrode and the first bit electrode; and access devices that are disposed between the first word electrode and the substrate, with the first bridge being electrically connected between the access devices.
  12. 12 . The apparatus of claim 11 , further comprising a second metal word line that extends in the first direction across the substrate at the first distance above the substrate, and is offset by a third distance in the second direction from the first metal word line.
  13. 13 . The apparatus of claim 11 , wherein the first bridge has a smaller cross-sectional area perpendicular to its length compared to larger cross-sectional areas of the first and second access devices perpendicular to their thicknesses.
  14. 14 . The apparatus of claim 11 , wherein the access devices are ovonic threshold switches (OTS).
  15. 15 . The apparatus of claim 11 , further comprising a first resistive liner at a surface of the first bridge, wherein the first resistive liner is electrically connected between the first word electrode and the first bit electrode, wherein the first resistive liner has an electrical resistance greater than that of the first bridge in a low resistance state of the first bridge and less than that of the first bridge in a high resistance state of the first bridge.
  16. 16 . The apparatus of claim 15 , wherein the first resistive liner is atop the first bridge.
  17. 17 . The apparatus of claim 15 , wherein the first resistive liner is between the upper face of the substrate and the first bridge.
  18. 18 . The apparatus of claim 17 , further comprising a second resistive liner across a top face of the first bridge.
  19. 19 . The apparatus of claim 15 , wherein the first resistive liner extends between the bridge and the access devices.
  20. 20 . The apparatus of claim 11 , wherein at least one of the access devices in its ON state has lower electrical resistance and lower thermal conductivity than the first bridge.

Description

BACKGROUND The present invention relates to the electrical, electronic, and computer arts, and more specifically, to non-volatile memory devices. Phase-change memory (also known as PCM or CRAM (chalcogenide RAM)) is a type of non-volatile random-access memory. Currently, PCM exploits the behavior of chalcogenide glasses, which exhibit different electrical properties in different solid phases (crystalline and amorphous states). In their amorphous phases, chalcogenide glasses have high resistance that can represent, for example, a binary 0, while in their crystalline phases, chalcogenide glasses have low resistance that can represent, for example, a binary 1. In one kind of PCM cell, an electric current is passed through a heating element (often made of titanium nitride) that is adjacent to a glass structure; the electric current is controlled to either quickly heat and quench the glass, making it amorphous, or to hold the glass in its crystallization temperature range for some time (on the order of 5 ns to 100 ns), thereby annealing it to a crystalline state. Resistance drift refers to the phenomenon that the resistance of the phase change memory does not stay at a constant value after programming, particularly after a RESET operation, after which the phase change material is in amorphous state. Instead, the resistance of the phase change memory changes as a function of time. SUMMARY Principles of the invention provide techniques for phase change memory. In one aspect, an exemplary apparatus includes a substrate that has an upper face; a first electrode that is attached to the upper face of the substrate; a second electrode that is attached to the upper face of the substrate at a distance from the first electrode; and a bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first and second electrodes. At least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase. According to another aspect, an apparatus includes a substrate that has an upper face; a first electrode that is attached to the upper face of the substrate; a second electrode that is attached to the upper face of the substrate at a distance from the first electrode; a bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first and second electrodes; and access devices that are disposed between the electrodes and the substrate, with the bridge being electrically connected between the access devices. At least a portion of the bridge is thermally switchable between a low resistance solid phase and a high resistance solid phase. According to another aspect, an exemplary apparatus includes a substrate that has an upper face; a first metal word line that extends in a first direction across the substrate at a first distance above the substrate; a metal bit line that extends across the substrate in a second direction that is different than the first direction, at a second distance above the substrate, which is less than the first distance; a first word electrode that is attached to and protrudes from the upper face of the substrate below the first word line; a first contact via that electrically connects the first word electrode to the first word line; a first bit electrode that is attached to and protrudes from the upper face of the substrate below the bit line and is electrically connected to the bit line; a first bridge of phase-change-memory material that is attached to and lies along the upper face of the substrate between and electrically connecting the first word electrode and the first bit electrode; and access devices that are disposed between the electrodes and the substrate, with the first bridge being electrically connected between the access devices. In view of the foregoing, techniques of the present invention can provide substantial beneficial technical effects. For example, one or more embodiments provide one or more of: Two access devices that are formed symmetrically at two ends of a phase-change-memory material (PCMM) bridge. Each access device is an ovonic threshold switch (OTS), which serves at least two purposes: (1) as access devices, i.e. controlling delivery of current to the bridge; (2) as thermal barriers to reduce heat loss from the bridge (and thus to reduce PCM programming energy) because of the low thermal conductivity of the access device compared to common metal electrodes. A PCM bridge link connected to two access devices directly. Unlike prior art cross-point PCM in which selector and PCM cell are on the same stack and have the same areas, the PCM bridge allows freedom to customize the area ratio between selector and bridge. For example, the larger area of the selector (access device) allows lower current density (reducing stress on the selector) while the smaller area of PCM