U.S. patent application number 09/939498 was filed with the patent office on 2002-02-14 for vertically stacked field programmable nonvolatile memory and method of fabrication.
Invention is credited to Cleeves, James M., Farmwald, Paul Michael, Johnson, Mark G., Lee, Thomas H., Subramanian, Vivek.
Application Number | 20020018355 09/939498 |
Document ID | / |
Family ID | 46278050 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018355 |
Kind Code |
A1 |
Johnson, Mark G. ; et
al. |
February 14, 2002 |
Vertically stacked field programmable nonvolatile memory and method
of fabrication
Abstract
A very high density field programmable memory is disclosed. An
array is formed vertically above a substrate using several layers,
each layer of which includes vertically fabricated memory cells.
The cell in an N level array may be formed with N+1 masking steps
plus masking steps needed for contacts. Maximum use of self
alignment techniques minimizes photolithographic limitations. In
one embodiment the peripheral circuits are formed in a silicon
substrate and an N level array is fabricated above the
substrate.
Inventors: |
Johnson, Mark G.; (Los
Altos, CA) ; Lee, Thomas H.; (Cupertino, CA) ;
Subramanian, Vivek; (Menlo Park, CA) ; Farmwald, Paul
Michael; (Portola Valley, CA) ; Cleeves, James
M.; (Redwood City, CA) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
46278050 |
Appl. No.: |
09/939498 |
Filed: |
August 24, 2001 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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09939498 |
Aug 24, 2001 |
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09714440 |
Nov 15, 2000 |
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09714440 |
Nov 15, 2000 |
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09469658 |
Dec 22, 1999 |
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6185122 |
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09469658 |
Dec 22, 1999 |
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09192883 |
Nov 16, 1998 |
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6034882 |
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Current U.S.
Class: |
365/103 ;
257/E27.073 |
Current CPC
Class: |
G11C 11/5692 20130101;
G11C 2213/71 20130101; H01L 27/2481 20130101; H01L 27/101 20130101;
G11C 17/165 20130101; G11C 13/003 20130101; H01L 45/1233 20130101;
G11C 17/06 20130101; G11C 17/14 20130101; H01L 45/16 20130101; H01L
27/1021 20130101; G11C 2213/76 20130101; G11C 17/16 20130101 |
Class at
Publication: |
365/103 |
International
Class: |
G11C 017/00 |
Claims
We claim:
1. A memory cell comprising: a steering element for providing
enhanced current flow in one direction through the steering
element; a state change element for retaining a programmed state,
connected in series with the steering element such that the
steering element and state change element provide a two terminal
cell; the steering element and state change element being
vertically aligned with one another.
2. The cell defined by claim 1 wherein the steering element is
fabricated from polysilicon.
3. The cell defined by claim 2 wherein the polysilicon is doped so
as to form a diode.
4. The cell defined by claim 1 wherein the steering element is a
metal-semiconductor Schottky diode.
5. The cell defined by claim 1 wherein the steering element is a
junction field-effect transistor with a gate connected to one of a
source or drain region.
6. The cell defined by claim 1 wherein the steering element is a
field-effect transistor having an insulated gate connected to one
of a source or drain regions.
7. The cell defined by claim 1 wherein the steering element is a PN
junction diode formed in amorphous semiconductor.
8. The cell defined by claim 1 wherein the steering element is a
Zener diode.
9. The cell defined by claim 1 wherein the steering element is an
avalanche diode.
10. The cell defined by claim 1 wherein the steering element is a
tunnel diode.
11. The cell defined by claim 1 wherein the steering element is a
four-layer diode (SCR).
12. The cell defined by claim 1 wherein the state change element is
an antifuse.
13. The cell defined by claim 12 wherein the antifuse is formed
from polysilicon.
14. The cell defined by claim 12 wherein the antifuse includes
silicon dioxide.
15. The cell defined by claim 1 wherein the state change element is
a dielectric-rupture antifuse.
16. The cell defined by claim 1 wherein the state change element is
a polycrystalline semiconductor antifuse.
17. The cell defined by claim 1 wherein the state change element is
an amorphous semiconductor antifuse.
18. The cell defined by claim 1 wherein the state change element is
a metal filament electromigration fuse.
19. The cell defined by claim 1 wherein the state change element is
a polysilicon resistor-fuse.
20. The cell defined by claim 1 wherein the state change element
employs trap-induced hysteresis.
21. The cell defined by claim 1 wherein the state change element
employs a ferroelectric capacitor.
22. The cell defined by claim 1 wherein the state change element
employs a Hall effect device.
23. The cell defined by claim 1 wherein the steering element
comprises a diode and the state change element comprises an
antifuse, and where the diode is able to carry a sufficient current
to change the state of the antifuse.
24. The cell defined by claim 1 wherein the steering element
comprises a recrystallized semiconductor.
25. The cell defined by claim 1 wherein the steering element and
state change element include amorphous silicon.
26. The cell defined by claim 1 wherein one of the terminals of the
cell is connected to a word line.
27. The cell defined by claim 26 wherein the other of the terminals
of the cell is connected to a bit line.
28. A memory cell comprising: a pillar having a generally
rectangular cross-section and having at one end a steering element
which more readily conducts current in one direction, and a state
change element located at the other end of the pillar, the state
change element for recording a state.
29. The memory cell defined by claim 28 wherein the steering
element comprises a diode.
30. The memory cell defined by claim 29 wherein the diode comprises
polysilicon.
31. The memory cell defined by claim 28 wherein the state change
element comprises a dielectric rupture antifuse.
32. The memory cell defined by claim 31 wherein the antifuse
comprises a silicon dioxide layer sandwiched between two layers of
polysilicon.
33. The memory cell defined by claim 28 having a first conductor in
contact with the steering element, the first conductor having a
width approximately equal to one dimension of the rectangular
cross-section.
34. The memory cell defined by claim 33 having a second conductor
in contact with the state change element, the second conductor
having a width approximately equal to the other dimension of the
rectangular cross-section.
35. A memory array comprising: a first plurality of spaced-apart,
parallel, substantially coplanar conductors; a second plurality of
spaced-apart, parallel, substantially coplanar conductors disposed
generally vertically above and spaced-apart from the first
conductors, said first and second conductors being generally
orthogonal to one another; and a plurality of first memory cells,
each cell disposed between one of the first and one of the second
conductors and located where a vertical projection of the first
conductors intersects the second conductors, the cells being
vertically aligned with at least one of the conductors.
36. The array defined by claim 35 wherein the cells are aligned
with both the first and second conductors.
37. The array defined by claim 35 wherein the cells each comprise a
steering element and a state change element.
38. A memory array comprising: a first plurality of spaced-apart,
parallel, substantially coplanar conductors; a second plurality of
spaced-apart, parallel, substantially coplanar conductors disposed
generally vertically above and spaced-apart from the first
conductors, said first and second conductors being generally
orthogonal to one another; and a plurality of first memory cells,
each cell disposed between one of the first and one of the second
conductors and located where a vertical projection of the first
conductors intersects the second conductors; a third plurality of
spaced-apart, parallel, substantially coplanar conductors disposed
generally vertically above and spaced-apart from the second
conductors, the third conductors running in the same direction as
the first conductors; a plurality of second memory cells, each cell
disposed between one of the second conductors and one of the third
conductors and located where a vertical projection of the second
conductors intersects the third conductors.
39. The array defined by claim 38 wherein first cells and second
cells are in vertical alignment with one another.
40. The array defined by claim 38 wherein the first cells and
second cells are staggered from one another.
41. The array defined by claim 38 wherein each of the first cells
and the second cells comprise a steering element and a state change
element.
42. The array defined by claim 41 wherein the steering elements of
the first and second cells are connected to the second
conductors.
43. The array defined by claim 41 wherein the state change elements
of the first and second cells are connected to the second
conductors.
44. The array defined by claim 38 wherein the first and second
cells have a generally rectangular cross-section.
45. The array defined by claim 38 wherein the first and second
cells comprise polysilicon and silicon dioxide.
46. The array defined by claim 38 wherein the first and second
cells comprise polysilicon.
47. The array defined by claim 38 wherein the array is fabricated
on a silicon substrate.
48. The array defined by claim 47 includes a first contact
extending from one of the second conductors to a first region in
the substrate.
49. The array defined by claim 48 including a second contact
extending from one of the first conductors to a second region in
the substrate.
50. The array defined by claim 38 wherein the first and second
conductors include a refractory metal.
51. The array defined by claim 50 wherein the refractory metal is
tungsten.
52. The array defined by claim 38 wherein the first and second
conductors are a silicide.
53. A memory array comprising: a plurality of conductors on levels
1, 2, 3, 4 . . . where the levels are parallel and spaced-apart,
the conductors in the odd numbered levels 1, 3 . . . running in a
first direction, the levels in the even numbered levels 2, 4 . . .
running in a second direction, generally perpendicular to first
direction, and a plurality of memory cells each having an input
terminal and an output terminal, the cells being disposed between
conductors in each of the levels 1, 2, 3, 4 . . . .
54. The array defined by claim 53 wherein the input terminals of
the cells are connected to the conductors in the odd numbered
levels 1, 3 . . . and the output terminals of the cells are
connected to the conductors in the even numbered levels 2, 4 . . .
.
55. The array defined by claim 53 wherein the output terminal of
the cells are connected to the conductors in the odd numbered
levels 1, 3 . . . and the input terminal of the cells are connected
to the conductors in the even numbered levels 2, 4 . . . .
56. The array defined by claims 54 or 55 wherein the cells include
a steering element and a state change element coupled between the
input and output terminals.
57. The array defined by claim 53 wherein the cells have a
generally rectangular cross-section.
58. The array defined by claim 53 wherein the cells comprise
silicon and silicon dioxide.
59. The array defined by claim 53 wherein the conductors of levels
1, 3 . . . are in vertical alignment with one another.
60. The array defined by claim 53 wherein the conductors in one of
the sets of levels 1, 3 . . . and levels 2, 4 . . . are staggered
in the vertical direction.
61. The array defined by claim 53 wherein the conductors comprise
polysilicon.
62. The array defined by claim 53 wherein the array is fabricated
on a silicon substrate.
63. The array defined by claim 62 including a contact extending
from one of the conductors in the odd numbered levels 1, 3, . . .
to the substrate.
64. The array defined by claim 62 including a contact extending
from one of the conductors in the even numbered levels 2, 4, . . .
to the substrate.
65. The array defined by claim 62 including at least one contact
extending from the highest level to the substrate.
66. The array defined by claim 62 wherein a contact is made up of
several contacts, one for each of the layers.
67. A process for fabricating a memory array comprising: (a)
forming a first layer of conductive material; (b) forming a
plurality of second layers for defining memory cells on the first
layer; (c) patterning the second layers into a plurality of
parallel, spaced-apart strips; (d) etching the first layer in
alignment with the strips formed from the second layers; (e)
forming insulating material between the strips of first layer
material and the strips formed from the second layers; (f) forming
a third layer of conductive material on the insulating material and
the strips formed from the second layer; (g) forming a plurality of
fourth layers for defining memory cells of the second level; (h)
patterning the fourth layers into a plurality of parallel,
spaced-apart strips, the strips formed from the fourth layers
running generally perpendicular to the strips formed from the first
layer; (i) etching the third layer and the strips formed from the
second layers in alignment with the strips formed from the fourth
layers.
68. The process defined by claim 67 including forming a fifth layer
of conductive material on the strips formed from the fourth layer;
patterning the fifth layer into a plurality of parallel,
spaced-apart conductors running generally perpendicular to the
strips in the fourth layer; and, etching the strips formed from the
fourth layer in alignment with the conductors thereby defining
additional memory cells.
69. The process of claim 67 repeating steps (a) through (i).
70. The process defined by claim 69 with the steps of claim 68.
71. The process defined by claim 67 repeating the steps (a) through
(i) a plurality of times.
72. The process defined by claim 71 with the steps of claim 68.
73. The process defined by claim 67 wherein a dielectric is applied
and a planarization step occurs after the etching of the first
layer and prior to the forming of the third layer.
74. The process defined by claim 73 wherein the planarization
comprises chemical-mechanical polishing.
75. The process defined by claim 67 including depositing an
insulator and etching it back to planarize the structure and
opening electrical contacts to the strips formed from the second
layer between steps (d) and (e).
76. The process defined by claim 67 wherein the plurality of second
layers and plurality of fourth layers each comprise layers of
polysilicon and silicon dioxide.
77. The process defined by claim 76 wherein the polysilicon is
doped such that each memory cell includes a diode.
78. The process defined by claim 76 wherein the silicon dioxide in
each layer forms part of an antifuse.
79. The process defined by claims 67 or 73 wherein a planarization
step occurs after the forming of the second layers and before the
forming of the third layer.
80. The process defined by claim 79 wherein the planarization is
performed by chemical-mechanical polishing.
81. The process defined by claim 79 including forming openings for
contacts after the planarization.
82. The process defined by claim 76 wherein the polysilicon is
deposited at low temperature using chemical vapor deposition.
83. The process defined by claim 67 including the deposition of a
barrier metal layer after the forming of the first layer and prior
to the forming of the second layers.
84. The process defined by claim 67 including the deposition of a
barrier metal layer after the forming of the third layer and prior
to the forming of the fourth layer.
85. The process defined by claim 67 wherein the fabrication
includes the fabrication of contacts and where contact openings are
made followed by the deposition of silicon into the openings.
86. The process defined by claim 67 including the use of ion
implanted silicon.
87. The process defined by claim 67 including the use of in-situ
doped silicon.
88. The process defined by claim 87 wherein the silicon is
deposited using LPCVD.
89. The process defined by claim 87 wherein the silicon is
deposited using PECVD.
90. The process defined by claim 87 wherein the silicon is
deposited using PVD.
91. The process defined by claim 87 wherein the silicon is
deposited using UHVCVD.
92. A memory comprising: a monocrystalline silicon substrate with
row address decoders and column address decoders and input/output
circuitry formed on and in the substrate; a memory array having a
plurality of column conductors and row conductors formed above the
substrate and being electrically coupled to the decoders and
input/output circuitry; the array comprising a first plurality of
levels each having spaced-apart, parallel, generally coplanar row
conductors; a second plurality of levels interleaved with the first
plurality of levels, each having spaced-apart, parallel, generally
coplanar column conductors generally perpendicular to the row
conductors; and a plurality of memory cells between each of the
levels, each cell being connected to one of the row conductors and
one of the column conductors.
93. The memory defined by claim 92 wherein the column decoders and
input/output circuitry are folded under the array and coupled to
the column conductors.
94. The memory defined by claim 92 wherein the array is divided
into a plurality of subarrays.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to low cost, high density
semiconductor memories and, in particular, to semiconductor
memories whose contents are "nonvolatile": data stored in the
memory is not lost or altered when electrical power is removed.
[0003] 2. Background of the Invention
[0004] There is an ever-increasing demand for ever-denser
semiconductor memories, and customers continue to purchase these
memories in ever-greater quantities, even as the number of bits per
chip quadruples (approximately) every three years. Increasingly
higher densities are required, at ever lower costs, to meet the
needs of the marketplace.
[0005] Semiconductor nonvolatile memories may be divided into two
categories: (1) those in which data is permanently written during
the manufacturing process and whose contents cannot be subsequently
changed, called "mask ROMs" or "factory programmed ROMs"; (2) those
in which data may be supplied after the finished memory device
leaves the factory. This latter category is called "field
programmable memories" because their contents may be written, by
the user, when the semiconductor memory chip is deployed to its
final application, "in the field".
[0006] Field programmable memories are further subdivided into
"write once" memories and "write/erase/rewrite" memories. Those
written once are referred to as "PROM" (programmable read only
memories) or "OTP ROM" (one time programmable read only memories).
And those memories that provide write/erase/rewrite capabilities
have been referred to as "UVEPROM" (ultraviolet erasable
programmable read only memories) or "EEPROM" (electrically erasable
programmable read only memories) or "Flash EEPROM" (fast and
flexible EEPROMs). In contrast, the contents of mask ROMs are
permanently stored during manufacture, therefore mask ROMs are not
erasable and are effectively "write only once, at the factory"
memories.
[0007] Field programmable memories are much more flexible than mask
ROMs, since they allow system product makers to inventory a single
general part-type for many applications, and to personalize
(program the memory contents of) this one part-type in numerous
different ways, much later in the system product flow. This
flexibility lets system manufacturers more easily adapt to
fluctuations in demand among different system products, and to
update or revise system products without the expense of scrapping
(discarding) existing inventories of pre-programmed mask ROMs. But
this flexibility has a cost: field programmable memories generally
achieve lower densities (fewer bits per chip) and higher cost
(larger price per bit) than mask ROMs. Customers would prefer to
buy something that offered the flexibility and convenience of a
field programmable memory, while achieving the cost and density of
a mask ROM. Unfortunately, such a device has yet not been
available.
[0008] There are two reasons why mask ROMs have been denser and
cheaper than field programmable memories. First, since mask ROMs do
not support erase or rewrite functions, their peripheral circuits
need not contain any dedicated circuitry or I/O terminals for
input-data steering, for write timing, or for write control. Thus
the peripheral circuits of a mask ROM may be smaller than those of
a field programmable nonvolatile memory. This reduces the die size
of a mask ROM, compared to the die size of a field programmable
nonvolatile memory, allowing more mask ROM chips to fit on a
semiconductor wafer, which lowers costs.
[0009] Second, since mask ROMs are written only at the factory,
their memory cells may be designed and optimized for read
operations exclusively, and generally their memory cells consist of
only a single circuit element (e.g. a single MOS transistor). But
the memory cell of a field programmable nonvolatile memory must
include support for write operations. Therefore, field programmable
memory cells generally contain more than one circuit element:
generally a second tunnel oxide floating gate, or a write/erase
series transistor, is added to the single MOS transistor needed for
reading. The extra element(s) in the field programmable cell
consume additional silicon area, making the memory cell area larger
than the area of a mask ROM memory cell. Thus the density of field
programmable nonvolatile memories has been lower than the density
of mask ROMs.
[0010] Field programmable memories having write/erase/rewrite
capabilities offer yet more flexibility. They permit product
upgrades, field reconfiguration, and enable a host of new
applications such as digital photography, solid state disks, et
cetera. Unfortunately, these devices have generally suffered from
lower density and higher cost than one-time programmable
memories.
[0011] Turning now to the design of the memory cell used in these
memories, most nonvolatile memory cells have employed semiconductor
devices such as MOS field-effect transistors, junction transistors,
or junction diodes, built in a planar monocrystalline semiconductor
substrate. This approach allows only very limited integration
vertically into the third dimension (i.e. perpendicular to the
plane of the substrate), since each memory cell contains some
elements built in the substrate.
[0012] Conventional nonvolatile memory cells are manufactured using
a number of sequential photolithographic steps, which define the
geometric shapes of the cell features. For example, fabrication of
the prior art mask ROM cell shown in FIG. 1 requires at least five
photolithographic masking steps: (a) nitride-LOCOS patterning; (b)
polysilicon gate patterning; (c) contact patterning; (d) metal
patterning; (e) programming with ion implant patterning. These
steps are performed sequentially, and care is taken to align each
subsequent layer to earlier layer(s) already patterned on the
memory circuit, to ensure that the geometric features of each layer
will be printed in their desired spatial locations. For example, in
the cell 10 of FIG. 1 the ion implant layer would conventionally be
aligned to the polysilicon layer, which was patterned
previously.
[0013] Unfortunately, photolithography machines used in high volume
semiconductor manufacturing do not perform these alignments
perfectly. They have a "layer misalignment tolerance" specification
which expresses the alignment error that may result when aligning a
new layer to a previously existing layer on the memory circuit.
These misalignment tolerances force memory cell designers to use
larger feature sizes than otherwise would be necessary if alignment
errors were negligible.
[0014] For example, if a certain feature on the metal layer were
required to completely overlap a feature on the contact layer, the
geometric overlap between these two features would have to be
designed at least as large as the misalignment tolerance between
the contact layer and the metal layer. For another example, if a
certain feature on the polysilicon gate layer were required to
avoid and not touch a feature on the LOCOS layer, the geometric
spacing between these two features would have to be increased to be
at least as large as the misalignment tolerance between the
polysilicon gate layer and the LOCOS layer.
[0015] Memory cell sizes are enlarged by these misalignment
tolerances, which increase die size, decrease density, and increase
cost. If a new memory cell structure could be found which required
fewer sequential photolithographic steps, this cell would include
fewer misalignment tolerances in its feature sizes, and it could be
made smaller than a cell with more photolithographic steps.
[0016] And if a new memory cell structure could be found which had
no alignment requirements at all (a "selfaligned" cell), in either
the X- or Y-directions, it would not need to include any alignment
tolerances in its feature sizes. The new cell could be made smaller
than a corresponding non-selfaligned memory cell.
[0017] FIG. 1 depicts a very popular circuit design used in mask
ROMs. It is an example of the "virtual ground" class of ROM
circuits as taught, for example, in U.S. Pat. No. 4,281,397. Its
memory cell such as cell 10, consists of a single MOS transistor
built in the planar semiconductor substrate, connected to a
polysilicon wordline (such as WL1, WL2), a metal bitline (such as
BL1, BL2), and a virtual ground line (such as VG1, VG2). The cell
is programmed by a mask which greatly increases the threshold
voltage of the MOS transistor, e.g. by ion implantation. For
instance, if implanted, the cell holds a logic-one, and if not
implanted, the cell holds a logic-zero.
[0018] FIG. 2 shows a field programmable nonvolatile memory as
taught, for example, in U.S. Pat. No. 4,203,158. Its memory cell 12
contains a wordline, a program line, a floating gate, a bit line,
and a ground line. By application of suitable voltages on the bit
line and program line, this cell can support write operations,
erase operations, and rewrite operations as well as reading.
[0019] FIG. 3 shows a programmable logic array (PLA) semiconductor
structure as taught in U.S. Pat. No. 4,646,266. Its elemental cell
14 consists of a pair of back-to-back diodes, giving four possible
states: nonconductive in either direction, conductive in both
directions, conductive in a first direction but not in a second
direction, and conductive in a second direction but not in a first
direction. This structure is not built in a planar semiconductor
substrate, and it does stack numerous layers of PLA cells
vertically above one another to form a 3 dimensional structure.
[0020] Another type of prior art mask ROM circuit is taught in U.S.
Pat. No. 5,441,907. Its memory cell contains an X conductor, a Y
conductor, and a possible diode. The cell is programmed by a mask
which permits (or blocks) the formation of a "plug" diode at the
intersection of the X conductor and the Y conductor. For instance,
if the diode is present, the cell holds a logic-one, and if it is
absent, the cell holds a logic-zero.
[0021] A field programmable nonvolatile memory cell using both a
fuse and a diode is taught in U.S. Pat. No. 5,536,968. If the fuse
is unblown (conductive), the diode is connected between the X
conductor and the Y conductor, and the cell holds a logic-zero. If
the fuse is blown (not conductive), there is no diode connected
between the X conductor and the Y conductor, and the cell holds a
logic-one.
[0022] A field programmable nonvolatile memory cell using both a
Schottky diode and an antifuse is taught in U.S. Pat. No.
4,442,507. Its memory cell contains an X-conductor made of
polycrystalline semiconductor material, a Schottky diode, an
intrinsic or lightly doped semiconductor that forms an antifuse,
and a Y-conductor made of metal. The intrinsic or lightly doped
semiconductor antifuse has a very high electrical resistance, and
this corresponds to a logic-zero stored in the memory cell. But if
a suitably high voltage is impressed across the cell, the antifuse
switches to a very low electrical resistance, corresponding to a
logic-one stored in the cell.
SUMMARY OF THE INVENTION
[0023] A memory cell comprising a steering element for enhancing
the flow of current in one direction and a state change element is
disclosed. The state change element retains a programmed state and
is connected in series with the steering element.
[0024] An array using these cells is vertically fabricated into
multi-layers of cells. Self alignment methods permit very high
density with a minimum of masking steps. The array may be
fabricated above a silicon substrate, with decoders and I/O
circuitry formed either in the substrate or in thin film
transistors above the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a circuit diagram of a prior art mask ROM.
[0026] FIG. 2 is a circuit diagram of a prior art field
programmable memory.
[0027] FIG. 3 is a circuit diagram of a prior art PLA.
[0028] FIG. 4(a) is a perspective view of one embodiment of a
memory cell built in accordance with the present invention.
[0029] FIG. 4(b) is a schematic of an array using the cell of FIG.
4(a).
[0030] FIG. 5 is a cross-sectional elevation view of an array using
the cell of FIG. 4(a).
[0031] FIG. 6(a) are three cross-sectional views of layers used to
fabricate different embodiments of the cell of FIG. 4(a).
[0032] FIG. 6(b) is a perspective view of a conductor layer and
layer stack used in the fabrication of the cell of FIG. 4(a).
[0033] FIG. 6(c) illustrates the structure of FIG. 6(b) after
patterning.
[0034] FIG. 6(d) illustrates the structure of FIG. 6(c) after an
additional conductor layer and layer stack have been formed.
[0035] FIG. 6(e) illustrates the structure of FIG. 6(d) after
patterning.
[0036] FIG. 6(f) illustrates the structure of FIG. 6(e) after an
additional conductor layer and layer stack have been formed.
[0037] FIG. 6(g) illustrates the structure of FIG. 6(f) after
another patterning step.
[0038] FIG. 7 is a cross-sectional elevation view of an array using
the cell of FIG. 4(a) where the cells are staggered in the vertical
direction.
[0039] FIG. 8(a) is a perspective view of vertically stacked
cells.
[0040] FIG. 8(b) is a schematic of the cells of FIG. 8(a).
[0041] FIG. 9(a) is a plan view of a substrate showing a layout of
circuitry in the substrate.
[0042] FIG. 9(b) is a plan view of a substrate showing another
layout of circuitry in the substrate.
[0043] FIG. 9(c) is a plan view of a substrate showing one layout
of circuitry in a substrate used for the present invention.
[0044] FIG. 9(d) is a plan view of circuitry for an embodiment of
the present invention using a plurality of subarrays.
[0045] FIG. 10(a) is an electrical schematic of peripheral
circuitry coupled to an array.
[0046] FIG. 10(b) is another electrical schematic of peripheral
circuitry coupled to an array.
[0047] FIG. 11 is an electrical schematic of peripheral circuitry
coupled to an array used in one preferred embodiment of the present
invention.
[0048] FIG. 12 is a cross-sectional elevation view of an array
showing a contact between three levels of the memory array.
[0049] FIG. 13(a) illustrates a contact between levels 1 and 3.
[0050] FIG. 13(b) illustrates a contact connecting levels 1, 2 and
4.
[0051] FIG. 13(c) illustrates a contact between levels 1, 3 and
5.
[0052] FIG. 13(d) illustrates a contact between levels 1 through
5.
[0053] FIG. 13(e) illustrates a contact between levels 1 and 3.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0054] A field programmable nonvolatile memory cell and memory
array is disclosed. In the following description numerous specific
details are set forth in order to provide a thorough understanding
of the present invention. However, it will be apparent to one
skilled in the art that the present invention may be practiced
without these specific details. In other instances, well-known
circuits and processes have not been described in detail in order
not to obscure the present invention.
OVERVIEW OF THE PRESENT INVENTION
[0055] The field programmable nonvolatile memory cell of the
present invention is built above, rather than in, a planar
substrate. Therefore, this memory cell can be stacked vertically in
numerous layers to form a three dimensional array. Each layer of
memory cells interacts only with the layer above and the layer
below, which makes vertical stacking of layers quite simple.
[0056] A unique organization of these cells in a three dimensional
memory array disposed above a substrate, with peripheral circuitry
built in the substrate, is also described.
[0057] FIG. 4(a) shows one embodiment of our newly invented memory
cell. It has two explicit terminals: an input terminal 20 and an
output terminal 21. Between these terminals, the memory cell
contains a steering element 22 and a state change element 23
connected in series. Neither the input terminal 20, nor the output
terminal 21, nor the steering element 20 22, nor the state change
element 23 is built in the planar semiconductor substrate.
[0058] The steering element 22 is a device with a strongly
asymmetric current-versus-voltage characteristic; it conducts more
readily in one direction than in the other. The purpose of the
steering element 22 is to ensure that current flow through the
memory cell is substantially unidirectional. This unidirectional
behavior enables the memory decoders to establish a unique circuit
path to each individual memory cell, allowing it to be individually
accessed (for reads and for writes) regardless of the state of all
other cells.
[0059] The state change element 23 is a device which can be placed
in more than one state, and whose state is not lost or altered when
electrical power is removed. One possible implementation among the
many discussed below, is a dielectric-rupture antifuse, having the
states {high impedance} and {low impedance}. These two stored
states accomplish the encoding of one bit of memory.
[0060] As shown in FIG. 4(a), the steering element 22 and the state
change element 23 are stacked vertically in a "pillar" shaped
arrangement having a generally rectangular cross-section. The
pillar is vertical and so is the current flow. Depending on the
orientation of the unidirectional steering element 22, current can
flow either upwards or downwards. In fact, in one embodiment,
current flows upwards in some layers of a vertical stack of cells,
and downwards in the other layers.
[0061] The state change element 23 is chosen so that it can be
switched from its initial state to another state by electrical
means, thereby making the memory field programmable. For example,
the state of a dielectric-rupture antifuse may be changed
electrically by applying a relatively large voltage (when compared
to the voltage used for reading) across the input and output
terminals of the memory cell.
[0062] The memory cell of the present invention is capable of being
fabricated with full selfalignment in both the X (east-west) and Y
(north-south) directions. This means the pillars are defined by,
and are automatically formed by, the intersection of an input
conductor and an output conductor. Thus the cell can be made quite
small, since its feature sizes need not include often used
allowance for misalignment tolerances.
[0063] Furthermore, the number of photolithographic masking steps
needed to build the cell of FIG. 4(a) is small. For a single level
of cells as shown in FIGS. 4(a) and 4(b), three masking steps are
needed: one to pattern the bottom conductor and cell material,
another to pattern the upper conductor and cell material, and a
third to provide contact holes outside the array for vertical
electrical connections. This pattering scheme results in a self
alignment of the cell pillar (i.e., the steering element and the
state change element) to the upper and lower conductors. If a
second level of cells is added vertically above the first level,
only two additional photolithographic steps are needed: one for the
next level conductor and the cell material, and the second for the
contacts outside the array. The top conductor of the lower level of
cells forms the bottom conductor of the top layer of cells. In
general if the array contains (N) levels of cells, there are (N+1)
conductor layers and (N+1) photomasking steps in the fabrication of
the cell array itself. There are also a number of additional
photomasking steps to form contacts. These contacts are outside the
cell array; they make connection between the array conductor layers
and the peripheral circuits.
[0064] The memory cell may also be fabricated using alternative
embodiments; the self-aligned pillar formation described above may
be replaced by a formation involving the use of a pillar formation
photomask. This would eliminate the self-alignment of the pillar to
the conductors, but would be advantageous in fabrication processes
that could potentially exploit the physics of free sidewalls. These
processes include steering element formation using solid-phase
crystallization of amorphous silicon, laser crystallization of
amorphous or polycrystalline silicon, and other processes apparent
to persons skilled in the art. The contact to the upper conductor
layer in both the self-aligned fabrication process and the
non-self-aligned fabrication process described above is exposed by
the planarization of the insulation, requiring no photomask step.
This process may be replaced by a contact formation photomasking
step, as would be apparent to persons skilled in the art.
[0065] Assume the first conductor 25 of FIG. 5 runs east-to-west.
Then the second conductor 26 will run north-to-south
(orthogonally), and memory cell pillars 27 will be formed wherever
a vertical projection of the first conductor intersects the second
conductor. The third conductor 29 will run east-to-west, and memory
cell pillars 30 will be formed wherever the third conductor 29
intersects the second conductor 26. Similarly, the fourth, sixth,
eighth, tenth, . . . conductors will run north-south, and the
fifth, seventh, ninth, eleventh, conductors will run east-west.
Odd-numbered conductors run in one direction, and even-numbered
conductors run in the perpendicular direction. Thus, conductor
number J forms pillars downward (to wiring layer number J-1) and it
forms pillars upward (to wiring layer number J+1).
[0066] Since the memory cells need not contact a monocrystalline
semiconductor substrate, a substrate beneath the memory cell array
is available for use other than for defining the memory cells. In
one embodiment of the present invention, this area may be used to
good advantage by laying out substantial portions of the row
decoders, column decoders, I/O multiplexors, and read/write
circuits directly beneath the memory cell array. This helps to
minimize the fraction of the die surface area not devoted to memory
cells, which increases the figure of merit known as "array
efficiency": 1 Array Efficiency = ( total area devoted to memcells
) ( total area devoted to memcells ) + ( total area devoted to non
memcells ) As can be
[0067] As can be seen, a decrease in (total are devoted to
non-memcells) results in an increased array efficiency.
MEMORY CELL: PILLAR
[0068] In the embodiment of the invented memory cell shown in FIG.
4(a), there are two explicit local terminals: an input terminal 20
(also called a wordline), and an output terminal 21 (also called a
bitline). In addition the cell may also contain "implicit" or
"widely shared" terminals which are unavoidable consequences of its
construction, and which are common to large groups of cells at
once. One example of an implicit terminal is the semiconductor
substrate, which forms a parasitic capacitance to each memory cell.
To simplify the figures and the discussion, these implicit
terminals are omitted, but as will be appreciated these implicit
terminals might affect the functionality and performance of the
memory cell. Thus the invented memory cell is referred to as a "two
terminal structure", meaning there are two explicit, local,
terminals, possibly with additional terminals which are implicit
rather than explicit.
[0069] Between its input terminal and output terminal, the memory
cell consists of a series connection of a steering element and a
state change element. In some embodiments, the steering element may
be connected to the input terminal (and the state change element
connected to the output terminal), and in other embodiments they
may be reversed: the state change element may be connected to the
input terminal and the steering element connected to the output
terminal.
[0070] The steering element is a semiconductor element that has a
strongly asymmetric current-versus-voltage characteristic; it
conducts much more readily in one direction than in the other. Some
possible implementations of the steering element are (i) a PN
junction diode, in amorphous, microcrystalline, polycrystalline or
single crystal semiconductor (e.g. Si, Ge, SiGe, GaAs, InP, etc.);
(ii) a metal-semiconductor Schottky diode; (iii) a junction
field-effect transistor with gate connected to source (or to
drain); (iv) a MOSFET with gate either floating, or connected to
source or connected to drain; (v) a Zener diode, avalanche diode,
or tunnel diode; (vi) a four-layer diode (SCR); (vii) a P-I-N diode
in amorphous, microcrystalline, polycrystalline or single crystal
semiconductor; and others that will be readily apparent to those
skilled in the art.
[0071] For descriptive purposes in this disclosure the two ends of
the steering element are referred to as "anode" and "cathode",
arranged so that conventional current flows more readily from
"anode" to "cathode" than from "cathode" to "anode". These labels
are consistent with standard terminology for a PN junction diode:
conventional current in a PN junction diode flows from anode to
cathode. Of course the present invention is not limited to the use
of a PN junction diode for its steering element (as was discussed
in the preceding paragraph); the adoption of the same terminal
labeling as a diode is merely for convenience and familiarity.
Further, if the voltage on the steering element's anode is larger
than the voltage on its cathode, the steering element is "forward
biased." But when the cathode voltage exceeds the anode voltage, we
will say the steering element is "reverse biased." These phrases
are also borrowed from standard diode terminology, again for
convenience and familiarity.
[0072] The steering element can be oriented two different ways: (1)
with its anode facing the input terminal and its cathode facing the
output terminal; (2) with its cathode facing the input terminal and
its anode facing the output terminal. Either orientation can be
made to function correctly, by appropriate design of the memory
decoders and read/write circuits, and neither orientation is
strongly preferred over the other.
[0073] The state change element is where data is actually stored in
the memory cell. It is a device that can be placed in more than one
state, and is so chosen that its state is not lost or altered when
electrical power is removed.
[0074] Some examples of the types of states that may be employed in
a state change element according to the present invention, are (i)
(high impedance state) and (low impedance state); (ii) (state with
peak capacitance at voltage V1) and (state with peak capacitance at
voltage V2); (iii) (state with Hall effect voltage positive) and
(state with Hall effect voltage negative); (iv) (state with
polarization vector pointing up) and (state with polarization
vector pointing down)and others.
[0075] Some possible realizations of the state change element
include, but are not limited to, (a) dielectric-rupture antifuses;
(b) intrinsic or lightly-doped polycrystalline semiconductor
antifuses; (c) amorphous semiconductor antifuses; (d) metal
filament electromigration fuses, either of the reversible (U.S.
Pat. No. 3,717,852) or irreversible type; (e) polysilicon
resistor-fuses, either of the reversible (U.S. Pat. No. 4,420,766)
or irreversible type; (f) ferroelectric capacitors; (g) capacitors
with trap-induced hysteresis; (h) coulomb blockade devices; and
others.
[0076] During integrated circuit manufacturing, the state change
element of the memory cell is fabricated and placed in a certain
one of its possible states; this is called the "initial state." For
example, if the state change element is a dielectric-rupture
antifuse having the two states (ruptured dielectric) and (intact
dielectric), the initial state of this element is (intact) after
manufacturing and before programming. Other embodiments of state
change elements will have different sets of states and thus
different initial states. By convention this initial state, the
"logic zero" state denotes the initial value stored in the memory
cell during semiconductor manufacturing. But of course other
conventions, calling the initial state e.g. "logic one," would be
equally valid, and the choice is merely a matter of preference or
convenience rather than technological necessity.
[0077] The memory cell is programmed by causing the state change
element to transition from its initial state into a new state. Many
embodiments of the state change element can be caused to change
state by applying a suitably large voltage across the memory cell,
from input terminal to output terminal. For example if the state
change element is embodied as a dielectric-rupture antifuse, it is
programmed by applying a large voltage across the cell's terminals
(or by forcing a large current through the cell), with the polarity
chosen such that the steering element is forward biased. This
places a large electric field directly across the dielectric
antifuse, which ruptures the dielectric, thus changing the state of
the state change element.
[0078] One possible method for programming a dielectric-rupture
state change element is to ground the memory cell's output terminal
and simultaneously raise its input terminal to a large positive
voltage (assuming the steering element is so oriented that its
anode faces the input terminal and its cathode faces the output
terminal, i.e., steering element is forward biased when the input
terminal is at a higher voltage than the output terminal). If the
steering element is oriented the other way, with anode facing the
output terminal and cathode facing the input terminal, the designer
can simply reverse the programming voltages and keep the steering
element forward biased during programming: ground the input
terminal and simultaneously raise the output terminal to a large
positive voltage. Many other voltage arrangements for forward
biasing the steering element and programming a dielectric-rupture
state change element will be readily apparent to those skilled in
the art.
[0079] Other embodiments of the state change element can be caused
to change state by forcing a suitably large current through the
memory cell, rather than forcing a large voltage across the memory
cell. For example, if the state change element is embodied as a
polysilicon-resistor fuse, it may be programmed by connecting a
current source to its input terminal and simultaneously grounding
its output terminal (assuming this polarity forward biases the
steering element). Assuming the current is large enough, it alters
the resistance of the polysilicon-resistor fuse, thus changing the
state of the state change element and programming the cell.
[0080] During programming, it is possible for nonselected memory
cells to be reverse-biased by the full programming voltage.
Accidental writes of nonselected memory cells might occur, if the
reverse leakage current of the steering element exceeded the
programming current necessary to change the state of the state
change element. Thus, the characteristics of the steering and state
change elements should be matched to one another; a state change
element that requires a large current to program (e.g., an
instrinsic poly fuse) can be used with a rather high-leakage
steering element, while a state change element that programs at
very low current (e.g., a dielectric rupture antifuse) requires a
low-leakage steering element.
[0081] The invented memory cell can be embodied either as a
one-time programmable nonvolatile memory, or as a
write/erase/rewrite nonvolatile memory, depending on the state
change element selected. In a first example, if a thin, highly
resistive, polycrystalline silicon film antifuse is employed as the
state change element (as taught in U.S. Pat. No. 4,146,902), its
programming operation is irreversible and the cell is one-time
programmable. After manufacturing and before programming, all cells
contain "logic zero". Those cells whose desired contents are "logic
one" are programmed, irreversibly, by forcing the state change
element into a new state. Logic zeroes may become logic ones (by
programming), but logic ones may NOT become logic zeroes (since
programming is irreversible in this type of state change
element).
[0082] In a second example, if a metal-via-insulator-silicon
filament fuse is employed as the state change element (as taught in
U.S. Pat. No. 3,717,852), its programming operation is reversible
and the cell may be written, erased, and rewritten. After
manufacturing and before programming, all cells contain "logic
zero". Those cells whose desired contents are "logic one" are
programmed. However, for this state change element, programming is
reversible and logic values may be changed from zero to one and
back from one to zero, if desired.
[0083] In a third example, a state change element having a
write/erase/rewrite capability may be employed, whose programming
operation is electrical but whose erase operation is not
necessarily electrical. The erase operation may be selectively
applied to a single memory cell, or it may be applied to all memory
cells at once, "in bulk," such as by exposing them to a strong
source of ultraviolet light as is done with UVEPROM memories. Or a
bulk erase operation may be initiated by heating the integrated
circuit, either from a heat source external to the IC or from a
heater directly on the IC. Or a bulk erase might be initiated by
placing the state change elements in a strong magnetic field.
[0084] While the above discussion is based on a state change
element that has two states, this is not necessary. An antifuse
that can provide a predetermined range of resistance where for
instance it is partly fused, would provide a three state element. A
floating gate MOS device allows numerous possible implementations
of multi-level storage, providing more than 2 states for a state
change element, as is well known in the art.
MEMORY CELL: CONDUCTORS
[0085] As shown in FIG. 4(a), the field programmable nonvolatile
memory cell consists of a vertical pillar, with a conductor at the
bottom of the pillar and another conductor at the top.
[0086] The bottom conductor is a relatively long conductor line or
wire on a first conductor layer. This conductor runs in a certain
direction (for example, east-to-west). The top conductor is a
relatively long conductor line or wire on a second conductor layer,
vertically above the layer that forms the bottom conductors. The
top conductors run in another direction (for example,
north-to-south). The angle between the top and bottom conductors is
preferably ninety degrees (i.e. it is preferred they are
orthogonal) but this is not mandatory. The memory cell pillar is
located at the intersection where the top conductor crosses over a
projection of the bottom conductor.
[0087] In practice the conductors on each level are parallel spaced
apart conductors where for instance, the space between each
conductor is equal to the conductor's width.
[0088] The first conductor layer ("conductors1") contains a large
number of parallel conductors all running in the same direction,
for example, east-to-west. And the second conductor layer
("conductors2") also contains a large number of parallel conductors
all running in the same direction, for example, north-to-south,
preferably perpendicular to the conductor direction of the first
conductor layer as shown in FIG. 5. Wherever a conductor on
conductors2 crosses over (or "intersects") a conductor on
conductors1, one of our field programmable nonvolatile memory cells
is fabricated. This is shown in FIG. 4(b).
[0089] Vertically from bottom to top, the invented memory cell
contains a conductor, then a pillar, then another conductor:
conductors1.fwdarw.pill- ar.fwdarw.conductors2. Conductors1 is on
the bottom and conductors2 is on the top. But then conductors2 is
the bottom of a new level of memory cells, vertically stacked above
the first level: conductors1.fwdarw.pilla-
r1.fwdarw.conductors2.fwdarw.pillar2.fwdarw.conductors3. The
present invention stacks multiple levels of memory cells above one
another: a vertical stack having (N) levels of memory cells
contains (N) levels of pillars and (N+1) layers of conductors. (It
takes (N+1) conductor layers to make (N) levels of cells: one
conductor on the bottom of each level of pillars, and then one more
conductor on the top of the array). FIG. 5 shows a portion of a
three dimensional memory array according to the present invention,
having N=6 levels of memory pillars and (N+1)=7 conductor layers. A
vertical stack of (N) pillars uses 1/N as much surface area as an
assembly of (N) pillars that are not stacked vertically; vertical
stacking gives an N-fold improvement in density.
[0090] A memory pillar's bottom conductor is the top conductor of
the memory pillar below, and a memory pillar's top conductor is the
bottom conductor of the memory pillar above. This makes stacking
especially simple and flexible.
[0091] In one embodiment, the two conductors at either end of a
memory pillar are perpendicular. And since conductors are shared
between levels of pillars, the result in this embodiment is that
even-numbered conductors run in one direction, and odd-numbered
conductors run in the perpendicular direction. For example, suppose
conductors1 runs east-to-west. Conductors2 would be perpendicular
to conductors1, so conductors2 would run north-to-south.
Conductors3 would be perpendicular to conductors2, so conductors3
would run east-to-west. Conductors4 would run north-to-south
(perpendicular to conductors3), and so forth. Thus conductors 1, 3,
5, . . . run east-to-west, and conductors 2, 4, 6, . . . run
north-to-south (in this example).
FABRICATION
[0092] In one embodiment of the present invention, a conductor
layer (say, conductor layer number J) runs north-to-south, and
adjacent conductor layers (numbers J-1 and J+1) run east-to-west.
Wherever a conductor's vertical projection on layer (J) crosses
over a conductor on layer (J-1), a memory cell pillar is created.
Similarly, wherever a conductor's projection on layer (J+1) crosses
a conductor on layer (J), a memory cell pillar is created. Memory
cell pillars are defined and patterned by the intersection
(crossover) of the conductors, and so the pillars are selfaligned
to the conductors. Selfalignment is an extremely important
advantage, because it lets the photolithographic patterns of the
memory cell be designed without including any extra allowance for
misalignment tolerances. Thus the pattern features of our
selfaligned memory cell may be made smaller, resulting in a smaller
cell area, which gives higher density and lower cost.
[0093] For purposes of illustrating the selfaligned fabrication of
these pillars, consider an embodiment which uses four sequential
layers of material (a "layer stack") to fabricate the steering
element and the state change element. In this illustrative example
the steering element consists of a polycrystalline silicon PN
junction diode, and the state change element consists of a
poly-oxide-poly dielectric rupture antifuse. Other embodiments are
set forth in the body of this application.
[0094] In this embodiment, a pillar contains four layers of
material in a layer stack, deposited sequentially as shown in FIG.
6(a): (1) a layer of P+doped polysilicon 40; (2) a layer of N-doped
polysilicon 41; (3) a layer of silicon dioxide 42; (4) a layer of
N+doped polysilicon 43. Layers (40) and (41) form a PN junction
diode (the steering element), and layers (41-43) form a
poly-oxide-poly dielectric rupture antifuse. In this embodiment the
stack of four materials which together create the memory cells are
referred to as the "layer stack" 45. There are also a conductor
layer below and above the layer stack 45 which is patterned as will
be described. These are shown as conductors 46 and 48 in FIG.
6(a).
[0095] An alternate stack is shown in FIG. 6(a) as stack 450. Again
it includes conductors at the ends of the stack, specifically 460
and 480 which may be fabricated from any conductive material such
as a metal or a polysilicon. The steering element in stack 450
comprises a first layer 400 of P+doped semiconductor such as
microcrystalline silicon, and a second layer 410 of N doped
semiconductor such as microcrystalline silicon.
[0096] The state change element comprises the layer 420. Layer 420
may be an amorphous silicon layer used to form an antifuse. This
layer has a nominal high resistance, however, after a large current
is passed through it for programming, its resistance will be
substantially lower. The layer 430 is shown as an N+layer to
provide good electrical contact to the overlying conductor 480.
Layer 430 could be amorphous, microcrystalline or polysilicon but
the processing methods need to be low temperature to maintain the
amorphous structure in layer 420.
[0097] Another stack 405 is also shown in FIG. 6(a). It comprises
an N-polysilicon layer 400, a silicon dioxide layer 402 and an
N+polysilicon layer 403. Again, the layers 400 or 403 could be
microcrystalline or amorphous semiconductor layers. The stack 405
is sandwiched between the conductors 406 and 408. Here the steering
element is a Schottky diode formed by the metal of conductor 406
and the layer 400. The state change element is an antifuse formed
by layer 402. By way of example, layers 406 and 408 may be titanium
silicide or aluminum with a thickness of approximately 1000A. The
layers 400, 402 and 403 may be 500A, 80A, and 500A in thickness,
respectively.
[0098] The fabrication sequence for the memory cell is
schematically illustrated in FIGS. 6(b)-6(g). After deposition and
before patterning, the layer stack 45 (or the stacks 450 and 405)
is a continuous sheet that extends across the entire integrated
circuit (indeed across the entire wafer) such as shown in FIG.
6(b). Conceptually the selfalignment method is a two-etch-step
procedure: In the first etch step, this layer stack (a continuous
sheet) is patterned into long straight strips running (say)
east-to-west, by etching them with the same patterning step that
etches the east-to-west conductors on the conductor layer below.
After deposition and planarization of an interlevel dielectric, a
second conductor and layer stack is deposited. This stack is
patterned into long straight strips running north south. Etching
used to pattern the north-to-south lines continues until the first
layer stack has also been etched through the steering element. This
results in pillars formed on the east-to-west running lines. The
resulting pillars are perfectly aligned to both the conductor below
and the conductor above since both the pillars and the conductors
are etched simultaneously. In alternate embodiments the
semiconductor layers within the layer stack (45 or 450 or 405) may
be deposited as microcrystalline or polycrystalline, and then laser
treated to improve crystallinity and enhance the dopant
activation.
[0099] The cross-section of the pillar will be rectangular with one
dimension being equal to the width of the bottom conductors and the
other dimension equal to the width of the top conductors. If these
conductors have equal width then the cross-section will be
square.
[0100] The patterning in both east-to-west and north-to-south uses
well-known photolithographic steps widely used in the semiconductor
industry and may use either wet or dry etching. Also, the silicon
used in the cells and when used for the conductors may be doped
insitu or after being deposited, for example, by ion
implantation.
[0101] Of course other patterning technologies may be used rather
than etching, for example "liftoff" technology or "Damascene"
technology or an additive rather than subtractive patterning
technology may be employed instead of etching. But ideally the
layer stack should be patterned in two separate steps, once with
the mask that defines the conductors below, and again with the mask
that defines the conductors above. This holds true regardless of
the specific fabrication techniques used to pattern the various
layers.
[0102] In practice a large number of vertically stacked memory
cells are built, and each conductor layer is selfaligned to both
the layer stack below, and the layer stack above. Therefore the
etching steps which selfalign the conductors to the pillars, must
etch away material from three different layers: the layer stack
above, the conductor layer, and the layer stack below.
[0103] The processing may begin with a wafer that may have received
prior processing steps, for example, CMOS transistors may be
fabricated in the monocrystalline substrate for the peripheral
circuitry. An insulator then is deposited , and preferably,
planarized (using chemical-mechanical polishing ("CMP"), resist
etchback planarization, or any of a number of other technologies
for planarization). The first conductor layer is deposited such as
layer 46 of FIG. 6(b), and then the first layer stack 45 is
deposited. FIG. 6(b) shows the wafer at this stage.
[0104] Next, the mask which defines the features on the conductors1
layer is applied, and these features are etched into both the
pillar layer stack 45 and the conductors1 layer 46 below. An
insulator is deposited on the wafer and planarized, using CMP or
other planarizing technology. FIG. 6(c) shows the wafer at this
stage. Note in particular that the pillar layer stack and bottom
layer have, been etched into long continuous strips (46a and 45a)
and (46b and 45b), not isolated individual pillars. Also note that
the edges of the pillar layer stack 45a and 45b are aligned to the
edges of the conductor 46a and 46b layer, since both were etched at
the same time with the same mask. Note the conductors generally
comprise coplanar conductors, such as aluminum or other metals,
suicides, or doped silicon conductors, for each level.
[0105] While not shown in FIG. 6(c) or the other figures, the
dielectric fills the voids between the strips (and pillars) and
thus adds support to the array. Also it should be noted that the
planarization must reveal the upper surface of the strips so that
the conductor layer that follows contacts the strips. The
planarized dielectric also forms the layers through which the vias
and vertical conductors of FIG. 13 pass.
[0106] Next, the second conductor layer 50 ("conductors2") is
deposited, and the second pillar stack 51 ("stack2") is deposited.
FIG. 6(d) shows the wafer at this stage. Note that the
planarization automatically gives a selfaligned contact between a
pillar layer stack (such as 45b) and the subsequent conductor layer
(such as 50) above it.
[0107] Now, the conductors2 mask is applied, and its features are
etched downward into three distinct strata: pillarstack2 (51),
conductors2 layer 50, and pillarstack1 (45a and 45b). (This etch
stops below the steering element within 45a and 45b., providing a
unique circuit path through the memory cell). An insulator is
deposited on the wafer and planarized (using CMP or other means).
FIG. 6(e) shows the wafer at this stage. Note that the conductors2
mask+etch has completed the definition of the individual pillars
(45a1, 45a 2, 45b 1 and 45b 2) in the layerstack1. Also note that
these pillars in the layerstack1 layer are aligned to both the
conductors1 layer (46a, 46b) and to the conductors2 layer (50a,
50b), thereby achieving the goal of selfalignment.
[0108] Next, the third conductor layer 52 ("conductors3") is
deposited, and the third pillar layerstack 53 ("layerstack3") is
deposited. FIG. 6(f) shows the wafer at this stage.
[0109] Now, the conductors3 mask is applied, and its features are
etched downwards into layers stack3, conductors3, and stack2. (This
etch stops below the steering element of layer stack 2 and is
intended to leave the conductor2 layer intact.) An insulator is
deposited on the wafer and planarized (using CMP or other means).
FIG. 6(g) shows the wafer at this stage. The conductors3 mask+etch
has completed the definition of the individual pillars in the
layerstack2 layer (such as 51a 1, 51a 2, 51b 2). FIG. 6(g) shows
that (N+1)=3 conductor layers and hence (N+1)=3 masking steps, are
required to pattern (N=2) layers of pillar layerstack (not counting
the interlevel via layers which are used in the peripheral circuits
but not in the memory array). The wafer is now ready to receive
more stack layers and conductor layers, at the discretion of the
manufacturer.
[0110] In one possible embodiment of an array of the invented
memory cells the pillars are vertically stacked directly above one
another as illustrated in FIG. 6. Note that pillars are lined up in
vertically aligned stacks. However, because of selfalignment, this
vertical stacking of pillars directly above one another is not a
requirement.
[0111] Memory cell pillars are automatically formed wherever a
conductor on conductor layer (J+1) crosses over a conductor on
conductor layer (J). This is true even if the conductor layers are
not lined up directly above one another, giving vertical stacks of
pillars. In fact it may be preferred that the pillars not be
stacked vertically; that is they are offset from one another, as
illustrated in FIG. 7. Compare FIG. 5 (vertical stacks of pillars)
to FIG. 7 (pillars offset from one another) to see the effect.
Offset or staggered pillar placement such as shown in FIG. 7, may
be advantageous in practice. It may help give a smoother wafer
surface, more suited to planarization and polishing.
[0112] In the foregoing sequence of steps, electrode or conductor
material is etched along with device material. Since most plasma
metal etches also etch polysilicon, a practical combination of
materials that enables such dual etching would be aluminum and
polysilicon, for example. Control of the etching process may be
effected, if desired, through the use of etch chemistries that are
selective (e.g., preferentially etching polysilicon, but stopping
on aluminum), or through the use of barrier materials that are not
etched by the etchants that remove electrode and device material.
The state change element may also be used as an etch stop,
particularly if it is an oxide rupture type.
[0113] Refractory metals such as molybdenum and tungsten are
compatible with conventional CVD deposition temperatures for Si and
may be used for the conductors. Metal suicides are compatible with
even higher temperatures used to activate dopants in Si. Even
heavily doped Si itself can be used as a conductor. The choice may
be dictated based on resistivity and integration concerns including
etch characteristics.
[0114] The planarization described after the first half-step of the
foregoing is necessary to form self-aligned contacts to the
half-etched cells (i.e., the lines running in the east-west
direction in the foregoing example). Such planarization may be
effected through a variety of means well known in the art, such as
chemical-mechanical polishing (CMP), etched-back spin-on dielectric
layers, and etched-back spin-on polymers, to cite three well-known
examples. To tolerate the possibility of excessive over-polishing
or over-etching that may occur during planarization, a second
planarization may be performed after deposition of an electrode
layer to insure a planar electrode surface for subsequent
deposition of device material layers.
[0115] The foregoing process sequence exploits self-alignment to
reduce the required alignment tolerances between the pillar and the
conductors. This embodiment may be substituted with an embodiment
involving one or more additional photomasking steps to explicitly
define the pillar itself, rather than defining it using the
intersection of two conductor photomasking steps, as is done in the
self-aligned process. This may be advantageous in various processes
that could exploit the explicitly defined sidewalls that would
result from such a process. For example, solid-phase
crystallization of amorphous silicon could be used to form the
steering element layer stack. The free energies of the sidewalls
would be expected to favor the formation of a single crystal or
grain within the steering element, which may be advantageous in
some system embodiments.
[0116] Another process that could exploit explicitly defined
sidewalls is laser-induced crystallization. Again, the free
energies of the sidewalls would be expected to favor the formation
of a single crystal or grain within the steering element.
[0117] In processes involving the explicit definition of the
pillar, a photomasking step would be used to define a bottom
conductor. This would be etched. Then, the layer stack required to
form the state change and steering elements would be deposited.
Another photomasking step would be used to define the pillar, which
would be etched. After this etch, an insulating material would be
deposited and planarized as in the self-aligned cell, exposing the
top of the pillar to form a self-aligned contact. The top conductor
would then be deposited and the process would be repeated for
subsequent levels of cells as required.
[0118] The order of masking steps in the above process could also
be reversed. For example, the pillar could be formed prior to
patterning the bottom conductor. In this process, the entire layer
stack for the bottom conductor, the steering element, and the state
change element would be deposited. The pillar would then be
lithographically defined and etched down through the steering
element. The bottom conductor would then be defined and etched.
This structure would be passivated using a planarized insulator
contacting scheme, as described above. In all three processes, the
self-aligned contact could also be replaced by an explicit contact
forming photomasking step.
[0119] The various device fabrication steps may result in the
presence of residual chemicals or dangling bonds that may degrade
device characteristics. In particular, device leakage can result
from the presence of such dangling bonds or chemicals (e.g.,
incompletely removed photoresist). A low-temperature (e.g., <400
C.) plasma oxidation exposure may be used to grow a clean-up oxide
on the edges of the device pillar, thereby passivating edge traps.
The growth of the oxide is self-limiting because the oxygen species
diffuse only slowly through previously grown oxide, resulting in
extremely uniform oxide thickness and, therefore, improved
manufacturability. (Plasma oxidation may also be used to form an
anti-fuse layer.) Oxide deposition may also be used to passivate
the surface, for example, either alone or in conjunction with a
grown oxide.
[0120] Because, in the foregoing for some embodiments, device
material (e.g., polysilicon) is deposited after electrode material
(e.g., metals), it is desirable to deposit and process the device
material at the lowest practical temperatures to widen the
selection of suitable metals. As an example, in-situ doped
polysilicon may be deposited at low temperatures using LPCVD (low
pressure chemical vapor deposition), PECVD (plasma-enhanced
chemical vapor deposition), PVD (physical vapor deposition), or
UHVCVD (ultra high vacuum chemical vapor deposition). An
alternative is to deposit undoped polysilicon, followed by doping
and activation using a low temperature process. (Traditional
activation steps such as long thermal anneals expose the wafer to
potentially unacceptably high temperatures.) It may also be
desirable in some cases to substitute microcrystalline or amorphous
silicon or crystallized amorphous silicon for the polysilicon to
enable low temperature fabrication.
[0121] Another concern is the possibility of diffusion of electrode
material (e.g., metal) into the device layer during processing. Low
temperature processing helps to reduce the severity of this
problem, but may be insufficient to solve it completely. To prevent
this problem, a number of barrier materials may be employed.
Examples include titanium nitride (TiN), tantalum (Ta) or tantalum
nitride (TaN), among many that are well known to the art.
[0122] In one embodiment of the cell, a thin dielectric layer is
employed as an antifuse element. In such a cell, good uniformity of
dielectric thickness, as well as a low film defect density (e.g.,
of pinholes in the dielectric) are among highly desirable
properties. The quality of the dielectric may be enhanced through a
variety of means, such as rotating (continuously or periodically)
the substrate and/or source during deposition; forming the
dielectric by thermal means using plasmas or low-temperature growth
chemistries; or by employing liquid-phase dielectric deposition
means.
[0123] It is desirable to reduce the number of masking steps that
involve critical alignment tolerances. One method for reducing the
number of masking steps is to employ vias that interconnect several
electrode layers. The vias may be rectangular, rather than square,
to allow a relaxation in alignment tolerances. For example, to
interconnect metal lines in several layers running in the
x-direction, the x-edge via size may be made substantially looser
than the pitch of the x-lines in the y-direction, resulting in a
rectangular via. Vias are discussed in conjunction with FIGS. 12
and 13.
CONTACT FORMATION
[0124] As previously pointed out, approximately one masking step
per layer is needed to form the cells in the memory layer.
Additional masking, however, is needed to form contacts, vias and
vertical conductors (collectively sometimes referred to as
contacts) to the conductors in the array as will be discussed
below. First it should be recalled that only one contact need be
made to each of the array conductors. Thus, if the contacts are at
the ends of the array conductors, the contacts for every other
conductor at a given level may be on opposite sides of the array.
This is important since it provides more area for the contacts.
Additionally, the conductors on the same level need not be of the
same length. That is, for instance, they can be progressively
shorter, or longer, or longer in some layers and shorter in others,
to allow area on the periphery of the array for contacts. These
contacts can reach down to lower levels, for instance, every other
lower level without interfering with conductors in the intermediate
layer.
[0125] Contacts are required outside the array to connect the
conductors in the array to the drive circuitry. Transistors built
into the substrate will typically provide drive. Drive transistors
could also be built above the substrate using materials common to
the array. The simplest implementation of contacts is to have a via
mask for each level of the array. These contacts are used to
connect an upper level through all the levels below it to
electrically connect to the substrate. These contacts are built
either stacked directly over one another or staggered, both methods
being common in the semiconductor industry.
[0126] In general, the vias and contacts are used to provide
conductive paths between the conductors in the array and the
periphery circuitry. For instance, contacts are formed in the
periphery of the array to contact the decoders, column I/O
circuitry and row address decoders shown in FIGS. 9(a), 9(b) and
9(c). In another embodiment it may be desirable to fabricate the
array on, for instance, a glass substrate and to form the
peripheral circuitry on a layer employing thin film transistors
with the contacts providing conductive paths from that layer to the
conductors in the array. In another embodiment, the upper most
layer may be used for power distribution.
[0127] One straight-forward plan for making contact, with each of
the levels is to use one masking and etching step per level, which
step occurs before the formation of the layer used to define the
conductors. This masking step forms openings to the layer beneath
and provides contacts as needed.
[0128] An example of this is shown in FIG. 12. Starting from the
base of the structure a contact 110 is masked and etched through
the substrate isolation 100 to the substrate contact 101 prior to
beginning the fabrication of the array.
[0129] Conductor layer 106 is deposited prior to the memory stack
131. The lower level of the memory stack 107 is a heavily doped
semiconductor in this example. This is important in this example
because the heavily doped semiconductor will provide an ohmic
connection and therefore does not need to be completely removed
from the conductor layer.
[0130] Region 120 and the area over contact 110 are formed during
the formation of the strips that make up level 1. In this case, 120
is electrically isolated from the other conductors on level 1 by
virtue of the level 1 mask layout. Dielectric is then deposited and
planarized to expose the top surface of level 1. Contact opening
111 is then formed through the layers of level 1 at least down to
the heavily doped layer 107.
[0131] Level 2 conductor 122 and memory stack layers are then
deposited and patterned in the same way as level one was patterned.
Again, the mask is used to isolate this region from the conductors
of the level 2 array. Dielectric is again deposited and etched back
to expose the top surface of level 2. Just as in level 1, a contact
mask is used to form opening 112 through the memory cell elements
down to the heavily doped material. Finally, level 3 conductor is
deposited into the opening 112 to form a continuous electrical
connection from level 3 to the substrate.
[0132] From the above description, it will be apparent that
contacts from any level may be made to a region in the substrate
with one additional masking step per layer. In another embodiment,
less than one masking step per layer is used to form the conductive
paths to the substrate. This is possible in cases where more than
one conductor contacts a single substrate region. Note in FIG.
13(c), for instance, that conductors 1, 3, and 5 are connected to
the same substrate regions.
[0133] Several possible structures for contacts are shown in FIGS.
13(a)-13(e). In FIG. 13(a) an arrangement is shown where contact is
made between conductors in level 1 (or level N) and level 3 (or
level N+2). Note that in this arrangement, the conductors in level
N+1 are made shorter than the conductors in level N and level N+2,
to allow ample space for the contact to be made without interfering
with conductors in level N+1. Here the contact, since it is between
adjacent levels, extends through the memory stack shown in the
crosshatching.
[0134] In FIG. 13(b) a contact is shown from a conductor in level 4
(or level N+3) which contacts conductors in levels 1 and 2 (or
levels N and N+1). Note in this arrangement that the conductor in
level N+2 is shorter than the conductor in level N, allowing the
structure fabricated from level N+3 to reach down and contact two
underlying conductors. Only a single opening needs to be defined in
the insulator to form this contact and the opening is disposed
through the oxide or other insulator used in the planarization
step.
[0135] Another contact is shown in FIG. 13(c) where conductors from
levels 1, 3 and 5 are connected to contact a substrate region. Here
the conductors in levels 2 and 4 are staggered so as not to
interfere with the contact. Again only a single masking step is
used to define this contact.
[0136] In FIG. 13(d) a contact structure is shown where levels 1,
2, 3, 4 and 5 each have a conductor connected to a common substrate
region.
[0137] Finally in FIG. 13(e) a contact from level 3 (or level N+2)
to level 1 (or level N) is shown. Here unlike FIG. 13(a) a single
opening through insulation material is made.
[0138] In forming the structures 13(a)-(e) the resistivity of the
vertical conductors is important. Metals, silicides and insitu
doped silicon can be used. Implanted silicon is not currently
preferred because of the difficulty of doping the silicon on the
sidewalls of the contact.
[0139] It should be noted that in forming the contact of FIG. 13(d)
an opening is first etched from an upper layer through several
lower layers. After the insulation has been etched to expose the
edges of the layers, the memory cell material is then etched
isotropically to expose more of the conductor. In this way, an
isotropic deposition of a material like polysilicon or CVD W can be
used to obtain a large surface area on each conductor to insure low
contact resistance.
[0140] While the contact of FIG. 13(c) uses the same principal,
because of the staggering of the layers only insulating material
needs to be etched isotropically to expose the edges of the level 1
and 3 conductors.
[0141] The techniques shown in FIGS. 13(d) and 13(c) are used to
limit the number of mask steps needed in the process. Use of either
one could reduce the mask count from 2N+1 to N+2 . . .
MEMORY CELL: SMALL FEATURE SIZES
[0142] As was previously discussed, selfalignment permits the
pattern features of the memory cell to be small, since it is not
necessary to allow for misalignment tolerances when laying out the
features. These smaller features allow reduction in the memory cell
area, in fact smaller than it otherwise could be without
selfalignment.
[0143] But there is a second benefit of the memory cell area that
permits additional reduction of the cell: the highly repetitive
pattern of geometric features on each mask layer.
[0144] The geometric shapes in each layer of the invented memory
cell array are especially simple: they are merely a highly
repetitive, regular set of closely spaced, long, straight parallel
conductor lines. Their simplicity and regularity can be exploited
in photolithography, allowing better resolution of smaller feature
sizes than otherwise would be possible with arbitrary-shaped
geometries. For example, if a (wafer stepper and illumination
source and lens and photoresist) system were normally rated for X
micron resolution (e.g. 0.18 microns), the simple and highly
regular shapes of the present invention would permit lines and
spaces substantially smaller than X microns. The present invention
can take advantage of the fact that there are no arbitrary-shaped
geometries; rather there is a highly repetitive, very simple
pattern, which is well known in the field of optics and is called a
"diffraction grating" in textbooks. It will be readily apparent to
those skilled in the art, how to exploit the advantages of a
diffraction grating pattern to achieve better resolution.
3 DIMENSIONAL ARRAY ORGANIZATION
[0145] For a moment assume an embodiment which has six layers of
memory cell pillars, and which therefore has seven conductor layers
of conductors. If the bottom conductor layer (conductors1) runs
east-to-west, then conductors3, conductors5, and conductors7 also
run east-to-west. And conductors2, conductors4, and conductors6 run
north-to-south. For simplicity consider an embodiment in which the
pillars are not offset or staggered; rather, they are stacked
directly above one another. A single vertical stack of six such
pillars is shown in FIG. 8(a).
[0146] FIG. 8(a)'s stack of six memory cell pillars (60-65) is
shown as a circuit schematic diagram in FIG. 8(b). Notice that
conductor layers 1,3,5,7 are spaced apart from one another in the
schematic diagram, but in the physical structure (FIG. 8(a)) they
are vertically stacked directly above one another. Similarly,
conductor layers 2,4,6 are vertically stacked in FIG. 8(a) but
spaced apart in FIG. 8(b).
[0147] There are six memory cell pillars in FIG. 8(a): one where
conductors2 crosses conductors1, one where conductors3 crosses
conductors2, . . . , and one where conductors7 crosses conductors6.
In the schematic of FIG. 8(b) these are shown along a diagonal. At
the bottom left, a memory cell (containing a steering element and a
state change element) is shown between conductors2 and conductors1.
FIG. 8(b) also shows a memory cell where conductors3 crosses
conductors2, another cell where conductors4 crosses conductors3,
etc.
[0148] Adjacent layers of memory cell pillars share a conductor
layer; thus they also share an I/O terminal. In one embodiment,
sharing only occurs between terminals of like type: input terminals
share a conductor layer with other input terminals, and output
terminals share a conductor layer with other output terminals. This
embodiment is advantageous, because it means each conductor layer
is unambiguously either an input layer or an output layer. There is
no mixing as would occur if a conductor layer was shared among
input terminals and output terminals, so the peripheral circuitry
is simplified. Input-terminal-driver circuits and
output-terminal-receiver circuits need not be collocated and
multiplexed onto the same conductor.
[0149] A result of the like-terminals-shared preference is that the
steering elements in the memory cells will be oriented alternately
cathode-up, then cathode-down, then cathode-up, etc. To see this,
suppose conductor layer conductors2 is an output layer; then the
cathodes of pillar60 and pillar61 both connect to conductors2. Thus
pillar60 must be oriented cathode-up and pillar61 is cathode-down.
Continuing, if conductors2 is an output layer, then conductors3 is
an input layer. The anodes of pillar61 and pillar62 connect to
conductors3. So pillar62 is cathode-up. The layers of pillars must
alternate, cathode-up, cathode-down, up, down, up, and so forth
(see FIG. 8(b)) for this embodiment. This means that during
fabrication, the sublayers of the pillar sandwich will be deposited
in a different order. In some pillar layers the anode material
sublayer will be deposited before the cathode material sublayer,
and in the other pillar layers the cathode material sublayer will
be deposited first. Thus the layers shown in FIG. 6(a) will be in
the order shown in alternate array levels and in the opposite order
in the remaining levels. However, it should be recalled that it is
not necessary to alternate the stack material for some
embodiments.
[0150] A further result of the preference for sharing like
terminals of memory cells is that it makes the conductor layers
alternate between input terminals only and output terminals only.
Since successive conductor layers run east-to-west, then
north-to-south, then east-to-west, etc., this means that all input
conductors will run the same direction (e.g. east-to-west), and all
output conductors will run the same direction (e.g.
north-to-south). So it will be especially easy to locate the
input-terminal-driver circuits together (e.g. along the west edge
of the memory array), and to locate the output-terminal-receive- r
circuits elsewhere (e.g. along the south edge of the memory
array).
[0151] This corresponds to standard practice in conventional memory
design: the input-terminal-driver circuitry 67 is located along the
west edge of the array, and the output-terminal-receiver circuitry
68 is located along the south edge of the array, as shown in FIG.
9(a). Sometimes conventional memories put half the
input-terminal-driver circuits along the east edge and half along
the west edge; this is often done when the memory cell row pitch is
very tight. Similarly, conventional memories sometimes place half
the output-terminal-receiver circuits along the south edge and half
along the north edge; this is done when the memory cell column
pitch is very tight. FIG. 9(b) shows a conventional memory with
this splitting performed.
[0152] It is now appropriate to note that the input-terminal-driver
circuitry in a nonvolatile memory (both conventional prior art, and
the present invention) has a shorter and less cumbersome name: "row
address decoder" circuitry. And the output-terminal-receiver
circuitry in a nonvolatile memory (both conventional prior art, and
the present invention) has a shorter and less cumbersome name:
"column address decoder and column I/O" circuitry. In this section
of the disclosure, which discusses array organization outside the
memory cell mats, this shorter name will be used.
[0153] It is possible to fold the row decoder circuits and the
column decoder and column I/O circuits underneath the memory array.
(This is possible because the memory array is above the underlying
monocrystalline substrate and does not contact the substrate.)
Completely folding all of the row decoder circuits and all of the
column circuits underneath the array is not done; such folding
would overlap in the corners. In one embodiment, the column decoder
and column I/O circuits are folded beneath the memory array, but
the row address decoder circuits remain outside the array. In
another embodiment, the column circuits are underneath the array,
and the central portion of the row decoders is folded (where there
is no conflict with the column circuits) under the array. This
gives a layout with small "tabs" of row circuits at the corners, as
shown in FIG. 9(c). These tabs can be interdigitated with the tabs
of other memory arrays, letting four (or more) arrays nestle
closely together, as shown in FIG. 9(d). Other variations on the
theme of partially folding decoders under the array will be readily
apparent to those skilled in the art.
[0154] As the previous paragraph alludes, the field programmable
nonvolatile memory of the present invention includes the
organization of the memory chip into several smaller subarrays,
rather than one single large array. Subarrays give three important
benefits: (1) they allow a simple block-level approach to
redundancy; (2) they increase operating speed; (3) they lower
operating power. Redundancy with subarrays can be quite
straightforward. If the end product is to be a memory having (say)
8N bits, it is a simple matter to build nine subarrays on the die,
each containing N bits. Then one of the nine subarrays can be
defective, yet the die can still be configured and sold as a
working 8N bit memory, by simply bypassing the defective
subarray.
[0155] Dividing the memory into subarrays also increases speed;
this is because the conductors are shorter (decreasing their
resistance), and there are fewer memory cells attached to each
conductor (decreasing the capacitance). Since delay is proportional
to the product of resistance and capacitance, cutting conductor
length in half cuts delay by a factor of four. Thus subarrays
decrease delay, i.e. increase speed.
[0156] Subarrays also provide lower power operation. Since one
important component of power is the capacitive charging and
discharging of conductors in the memory array, decreasing the
conductor capacitance will decrease power consumption. Cutting
conductor length in half cuts capacitance in half, which cuts the
capacitive charging and discharging current in half.
CIRCUIT DESIGN: ROW DECODING AND SELECTION
[0157] In one embodiment of the present invention, the rows of a
memory array (also called "wordlines") are the inputs of the memory
cells, and the columns (also called "bitlines") are the outputs of
the memory cells. A forcing function is applied to the memory cell
input (wordline), and for a read the result at the memory cell's
output (bitline) is sensed, while for a write another forcing
function is applied to the memory cell output (thereby forcing both
terminals of the cell). The forcing functions used with the present
invention may be voltage sources, current sources, waveshape
generators (either high impedance or low impedance), charge
packets, or other driving stimuli.
[0158] In order to unambiguously access each individual memory
cell, for both reading and writing, a unique circuit path is
established from the row lines, through the memory cell, to the
column lines. A consequence of the uniqueness requirement is that
all of the row lines cannot be driven simultaneously; this may be
appreciated by considering FIG. 8(b). The row lines (wordlines) in
FIG. 8(b) are on conductor layers 1, 3, 5, and 7. The column lines
(bitlines) are on conductor layers 2, 4, and 6. Recall that FIG. 8
represents a single vertical stack of memory cell pillars; it is
the physical intersection of one single row and one single column.
The drawing in FIG. 8(b) depicts the conductors spaced-apart for
easier viewing, but in reality they are stacked above one
another.
[0159] Suppose that all wordlines were driven simultaneously; for
example, suppose conductor layers 1, 3, 5, and 7 were forced to a
high voltage. There is no unambiguous circuit path to the circuit
outputs (on the bitlines, namely conductor layers 2, 4, and 6), so
the contents of the memory cells cannot be determined. For example,
suppose that sensing circuitry determines that conductors2 is at a
high voltage; what does this mean? It means that either the memory
cell between conductors1 and conductors2 is programmed to a low
impedance state, or the memory cell between conductors2 and
conductors3 is programmed to a low impedance state. Either of these
two possibilities establishes a circuit path from a source of high
voltage (the wordlines) to the bitline on conductors2. But
unfortunately which of these possibilities is in fact true cannot
be determined: there is not a unique circuit path to conductors2.
And this is also the case for the other two bitlines, conductors4
and conductors6.
[0160] Thus all wordlines should not be driven simultaneously; this
produces non-unique circuit paths to the memory array outputs. A
straightforward solution is to only drive a single wordline,
leaving all other wordlines undriven. This is diagrammed in FIG.
10(a). A row decoder 70 selects whether any of the wordlines along
this row should be enabled. And four layer-select signals select
which conductor layer wordline should be enabled in the selected
row. All but one of the layer-select signals are in the deselect
condition (e.g. low voltage), and only one of the layer-select
signals is in the select condition (e.g. high voltage). Thus only
one wordline is driven, and the other three are not driven.
[0161] Clearly the arrangement in FIG. 10(a) establishes a unique
path to the array outputs. Suppose the wordline on conductors5 is
selected, and suppose that sensing circuitry determines that
conductors4 is at a high voltage. There are only two ways for
conductors4 to go high: one is through the memory cell 71 between
conductors3 and conductors4, and the other is through the memory
cell 72 between conductors4 and conductors5. Since conductors5 is
driven and conductors3 is not driven, the only circuit path that
exists is from the wordline on conductors5, through the memory cell
72 between conductors5 and conductors4, and out the bitline on
conductors4. If conductors4 is sensed to be a high voltage, then
this memory cell is programmed to be a logic-zero; otherwise this
memory cell is a logic-one.
[0162] But the arrangement in FIG. 10(a) is costly; it includes a
switching transistor for each of the wordline layers in the memory
array. If there are a large number of vertical layers in the array
(e.g. sixteen layers of memory pillars, requiring nine wordline
conductor layers and eight bitline conductor layers), the switching
transistors consume a lot of silicon area. This degrades die
efficiency, which drives cost up and density down.
[0163] However, we observe that the ambiguity in FIG. 8(b) arises
because there are two paths to each bitline: one from the wordline
on the conductor layer immediately below, and one from the wordline
on the conductor layer immediately above. To avoid ambiguity, all
we must do is guarantee that only one of the two possible paths is
enabled. This is easily accomplished by partitioning the wordlines
into sets: the "first set" and the "second set." Wordlines on
conductor layers conductors1, conductors5, conductors9,
conductors13, conductors17, . . . , etc. are in the first set, and
wordlines on conductor layers conductors3, conductors7,
conductors11, conductors15, . . . , etc. are in the second set. The
key observation is that it is perfectly safe to simultaneously
drive all of the wordlines in the first set, as long as no other of
the wordlines in the second set is driven, and vice versa (FIG.
10(b)).
[0164] The circuit in FIG. 10(b) only includes two switching
transistors 75 and 76, regardless of the number of vertical layers
of memory cells in the array. There is one switching transistor for
the first set of wordlines, and one switching transistor for the
second set. Similarly there are two set-select signals, that
determine which of the two wordline sets are driven. The greater
the number of vertical layers of memory cells in the chip, the
greater are the savings of FIG. 10(b) compared to FIG. 10(a).
[0165] Suppose the first set-select signal is in the select
condition (high voltage) and the second set-select signal is in the
deselect condition. Then the wordlines on layers conductors1,
conductors5, conductors9, . . . , etc are driven, while the
wordlines on layers conductors3, conductors7, conductors11, . . .
are not driven. There is only one (unique) path to the bitline on
conductor2: this is the path from conductors1, through the memory
cell between conductors1 and conductor2, and onto the bitline on
the conductor2 layer. The other possible path, from conductors3,
through the memory cell between conductors3 and conductor2, and
onto conductor2, is disabled because conductors3 is in the second
wordline set and is not driven.
CIRCUIT DESIGN: COLUMN DECODING AND SELECTION
[0166] A consequence of the two-sets-of-wordlines organization
(FIG. 10(b)) is that every bitline will have a memory cell
row-selected onto it. Thus, if there are (N) conductor layers
devoted to bitlines, each selected column could read or write (N)
bits of memory simultaneously. One embodiment of the present
invention does indeed read (and/or write) N bits at once, in each
selected column. Other embodiments introduce column multiplexor
circuitry, which reduces the number of simultaneously accessed
memory cells.
[0167] FIG. 11 shows another embodiment. Each bitline is provided
with its own switching transistor such as transistors 77 and 78;
these transistors connect a bitline to a bidirectional I/O bus if
this column is selected. During read operations, the bitline drives
the I/O bus, but during write operations, the I/O bus drives the
bitline. If there are (N) layers of bitlines, there are (N)
switching transistors and (N) I/O bus conductors. The I/O bus
conductor connects to peripheral circuits, including a sense
amplifier (for reads) and a write driver (for writes).
[0168] This column selection circuitry is far more costly than the
row selection circuits shown in FIG. 10(b). Since there must be a
switching transistor for every bitline, if more and more layers of
memory cells are stacked vertically, there will be more and more
bitlines, hence more and more switching transistors.
[0169] Thus the column selection circuitry will consume more
silicon area than the row selection circuitry, especially when
there are a large number of vertically stacked layers of memory
cells. This is why it is preferred to fold the column select
circuits under the memory array, more so than the row select
circuits, as shown in FIG. 9(c): the column circuits are a lot
bigger. In fact, it would be a reasonable design decision to fold
the column circuits underneath the memory array and completely
forget even trying to fold the row select circuits beneath. The
advantage comes from folding the column selects.
PRECHARGING THE MEMORY ARRAY
[0170] In many cases it is appropriate to "precharge" all wordlines
to an intermediate level such as 0.5 times the supply voltage, and
to "precharge" all bitlines to an intermediate voltage level such
as 0.4 times the supply voltage before commencing a read or write
operation.
CIRCUIT DESIGN: READ/WRITE PERIPHERAL CIRCUITS
[0171] Several embodiments of the present invention use a state
change element whose different states correspond to different
values of impedance. For example, a dielectric rupture antifuse has
two states: very low impedance and very high impedance, in which
the impedances differ by several orders of magnitude. Embodiments
such as these can use a "current-mode read" and a "voltage-mode or
current-mode write," as explained below.
[0172] When reading such a memory cell, a current source can be
selected as the forcing function which drives the wordlines. If the
memory cell is programmed (dielectric ruptured, thus low
impedance), this driving current will pass through the memory cell
and onto the bitline. The selected bitline will be switched onto
the (bidirectional) I/O line, and the driving current will be
passed onto the I/O line. A current-sensing amplifier connected to
the I/O line detects whether or not the driving current is passed
onto the I/O line. If so, the cell being read contains a "logic
one," and if not, the cell contains a "logic zero."
[0173] The main advantage of a current-mode read is speed: by
forcing and sensing current (rather than voltage), the need to
charge and discharge the high-capacitance wordlines and bitlines in
the memory array is avoided, so the wordlines and bitlines do not
swing through large voltage excursions, which speeds up the read
operation. Therefore current-mode reads are preferred in many
embodiments of the present invention.
[0174] In one embodiment of writing the memory cell, a voltage
source can be selected as the forcing function which drives the
wordlines. Additionally, the bidirectional I/O bus can be driven
with another voltage source. The I/O bus will be connected to the
bitline (by the column select switching transistor) in the selected
column, so the selected memory cell (at the intersection of the
selected wordline and the selected bitline) will be driven by two
voltage sources: one on the wordline, the other on the I/O bus. The
large voltage difference between these two sources will be
impressed directly across the selected memory cell, achieving a
voltage-mode (large voltage excursion on the wordlines and
bitlines) write.
[0175] Although voltage-mode writing is slower, since it must
charge and discharge the high capacitance wordlines and bitlines,
it is nevertheless preferable in some embodiments of the present
invention. Voltage-mode writing can, if necessary, provide very
high current through the memory cell, which is advantageous with
several embodiments of the state change element such as an
amorphous-semiconductor antifuse. In some embodiments of
voltage-mode writing, it may be preferable to limit the maximum
current to a particular value. One possible benefit of limiting the
maximum current is to reduce the effect of IR voltage drops along
the conductors of the array to ensure that a consistent programming
energy is delivered to each memory cell, independent of the cell's
location in the array. A consistent programming energy can be
important because the characteristics of some state-change element
materials may be sensitive to programming energy.
[0176] In some embodiments, the voltage necessary to program the
state change element, may exceed the voltage capabilities of the
peripheral transistors. This is particularly true when the
transistors are scaled for small dimensions (for example, channel
length below 0.2 microns). In these cases the peripheral circuits
may be arranged so that during a write cycle, the row decoders
operate from a power supply of +V volts, while the column decoders
and column I/O circuits and write data drivers operate from a power
supply of -V volts. This arrangement puts a voltage difference of
2.times.V volts across the memory cell being written
((+V)-(-V)=2.times.V), while placing at most V volts across any one
transistor.
[0177] Thus, a vertically stacked nonvolatile memory has been
disclosed that permits the fabrication of extremely high density
array.
* * * * *