Vertical channel transistor-based semiconductor memory structure
A semiconductor memory structure (e.g., SRAM) includes vertical channels with a circular, square or rectangular cross-sectional shape. Each unit cell can include a single pull-up vertical transistor and either: one pull-down vertical transistor and one pass-gate vertical transistor; or two or more of each of the pull-down and pass-gate vertical transistors. The structure may be realized by providing adjacent layers of undoped semiconductor material, forming vertical channels for vertical transistors, the vertical channels situated on each of the adjacent layers, doping a first half of each of the adjacent layers with a n-type or p-type dopant, doping a second half of each of the adjacent layers with an opposite type dopant to that of the first half, forming wrap-around gates surrounding the vertical channels, and forming top electrodes for the vertical transistors.
1. A semiconductor structure, comprising:
horizontally adjacent layers of conductive material;
a plurality of transistors in process on the horizontally adjacent layers of conductive material, each transistor comprising a vertical channel and a gate electrode wrapped around the vertical channel, each of the adjacent layers of conductive material being a shared bottom source/drain electrode for some of the plurality of transistors;
a cross-coupled contact having at least two portions, each portion on one of the adjacent layers of conductive material;
non-shared top source/drain electrodes on top of each vertical channel and associated gate electrode; and
wherein the plurality of transistors comprises at least two sets of transistors, each of the at least two sets of transistors having only a pull-up transistor, at least two pulldown transistors and at least two pass gate transistors.
2. The semiconductor structure of claim 1 , wherein the adjacent layers of conductive material comprise one of a metal and silicide.
3. The semiconductor structure of claim 1 , further comprising:
an electrical connection between the cross-coupled contact and the gate of each pull-up transistor; and
an electrical connection between the gate of a given pull-up transistor and the gate of an associated pull-down transistor.
4. The semiconductor structure of claim 1 , wherein the semiconductor memory structure is part of a SRAM device.
5. The semiconductor structure of claim 1 , wherein each vertical channel has a circular cross-sectional shape.
6. The semiconductor structure of claim 1 , wherein each vertical channel has a rectangular or square cross-sectional shape.
7. The semiconductor structure of claim 1 , further comprising one or more layers of a conductive material over the bottom source/drain electrodes.
8. The semiconductor structure of claim 7 , wherein the conductive material comprises one of a metal and silicide.
9. The semiconductor structure of claim 1 , wherein each gate electrode comprises:
a wrap-around spacer layer wrapped around the vertical channel;
one or more wrap-around work-function layers wrapped around the wrap-around spacer layer; and
one or more wrap-around conductive metal layers wrapped around the one or more wrap-around work-function layers.
10. The semiconductor structure of claim 1 , further comprising hard masks over the non-shared top source/drain electrodes.
11. The semiconductor structure of claim 10 , further comprising a spacer wrapped around the top source/drain electrodes and corresponding hard masks.
12. The semiconductor structure of claim 1 , further comprising one or more metallization layers over the semiconductor structure.
13. A semiconductor memory structure, comprising:
a plurality of vertical channel transistors that are horizontally adjacent, each vertical channel transistor comprising:
a shared bottom source/drain electrode;
a vertical channel on the shared bottom source/drain electrode;
a gate wrapped around the vertical channel; and
a non-shared top source/drain electrode on the vertical channel and gate;
wherein the plurality of vertical channel transistors are grouped according to each non-shared top source/drain electrode into at least two sets, each of the at least two sets:
having only a pull-up transistor:
at least two pull-down transistors: and
at least two pass-gate transistors.
14. The semiconductor structure of claim 13 , wherein the semiconductor structure is part of a SRAM memory device.
15. The semiconductor structure of claim 14 , further comprising at least one metallization layer over the semiconductor structure.
16. A method, comprising:
providing adjacent layers of undoped semiconductor material;
forming vertical channels for vertical transistors, the vertical channels situated on each of the adjacent layers;
doping a first half of each of the adjacent layers with a n-type or p-type dopant;
doping a second half of each of the adjacent layers with an opposite type dopant to that of the first half;
forming wrap-around gates surrounding the vertical channels; and
forming top electrodes for the vertical transistors: and
wherein the wrap-around gates are self-aligned, wherein forming the vertical channels comprises forming one of a vertical channel having a circular cross-section and a vertical channel having a rectangular cross-section, and wherein the transistors are formed in groups, each group having a single pull-up transistor, at least two pull-down transistors and at least two pass gate transistors.