U.S. patent application number 08/940897 was filed with the patent office on 2002-01-03 for memory cell for dynamic random access memory (dram).
Invention is credited to GALL, MARTIN, SCHELLER, GERD, STENGL, REINHARD J..
Application Number | 20020001900 08/940897 |
Document ID | / |
Family ID | 25475604 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001900 |
Kind Code |
A1 |
SCHELLER, GERD ; et
al. |
January 3, 2002 |
MEMORY CELL FOR DYNAMIC RANDOM ACCESS MEMORY (DRAM)
Abstract
A memory cell, which includes a transistor and a capacitor, for
use in a DRAM uses a silicon-filled vertical trench as the
capacitor and a vertical transistor superposed over the vertical
trench in a silicon chip. An epitaxial layer is formed at the top
of the fill in the trench to impart seed information to the
primarily polysilicon silicon fill in the trench. A polysilicon
layer is deposited over the top surface of the chip, is apertured
over the top of the trench, and has its sidewalls oxidized. The
opening is then refilled with epitaxial silicon in which there is
created in operation an inversion layer that serves as the channel
of the transistor, and the deposited polysilicon layer serves as
the word line. Another silicon layer is deposited over the
epitaxial layer to serve as the bit line. The source/drain regions
of the transistor are formed at the merger of the deposited layer
with the fill in the trench and the merger with the polysilicon
layer that serves as the bit line.
Inventors: |
SCHELLER, GERD; (THALWIL,
CH) ; GALL, MARTIN; (SOUTH BURLINGTON, VT) ;
STENGL, REINHARD J.; (STADTBERGEN, DE) |
Correspondence
Address: |
Jerome J. Norris
1901 Pennsylvania Avenue, Suite 305
Washington
DC
20006
US
|
Family ID: |
25475604 |
Appl. No.: |
08/940897 |
Filed: |
September 30, 1997 |
Current U.S.
Class: |
438/244 ;
257/E21.652; 438/245 |
Current CPC
Class: |
H01L 27/10864
20130101 |
Class at
Publication: |
438/244 ;
438/245 |
International
Class: |
H01L 021/8242 |
Claims
What is claimed is:
1. A process for making a memory cell comprising the steps of:
forming a trench in a silicon chip of one conductivity type;
forming a dielectric layer over the walls of the trench; filling
the trench with polysilicon of a conductivity type opposite that of
the chip; growing an epitaxial silicon layer over the surface of
the chip of sufficient thickness for forming over the top of the
trench a layer of essentially monocrystalline silicon of a
conductivity that is opposite that of the chip for serving as a
first source/drain; patterning the eptiaxial silicon layer to form
a mesa over the trench; forming a first dielectric layer over the
surface of the chip, covering the expitaxial silicon mesa; forming
spacers on the side of the mesa, the mesa comprising a material in
which the first dielectric layer can be removed selectively to the
spacers; removing the first dielectric layer from the surface of
the chip and top of the epitaxial silicon mesa, wherein the spacers
protect the first dielectric layer from being removed from the
sides of the mesa; forming a second dielectric layer over the
surface of the chip; removing the sidewall spacers, leaving the
first dielectric layer on the side of the mesa, the first
dielectric layer on the sides of the mesa serving as gate oxide;
forming a frist polysilicon layer of the conductivity type opposite
that of the chip over the surface of the second dielectric layer;
forming a third dielectric layer over the surface of said
polysilicon layer; planarizing the third dielectric layer to expose
the surface of the epitaxial silicon mesa; and forming and
patterning a second polysilicon layer of the conductivity opposite
that of the chip to from a polysilicon mesa over the epitaxial
silicon mesa.
Description
FIELD OF THE INVENTION
[0001] This invention relates to memory cells for use in a dynamic
random access memory (DRAM) and, more particularly, to a memory
cell that uses a dielectrically-filled vertical trench as a storage
node and a vertical transistor as a switch located over the
trench.
BACKGROUND OF THE INVENTION
[0002] DRAMS have become among the most important of the integrated
circuit devices and are the source of continuing research and
development which, in particular, aims to increase their storage
capacity and writing and reading speeds. This has necessitated the
use of smaller and more closely spaced memory cells for use in the
memory arrays. Of increasing significance are memory cells in which
the storage node is provided by a polysilicon-filled trench in a
silicon chip and the switching transistor is a vertical transistor
located in the chip over the trench. It is known that a DRAM that
uses a MOSFET as the switching transistor. The two output current
terminals of the transistor alternate between source and drain
roles as the storage node is charged and discharged. As such, each
of these terminals can be described as a source/drain and a
drain/source, as is appropriate for the particular role. For
purposes of discussion, these terminals are simply referred to as a
source/drain. The vertical transistor is located over the storage
node in a manner which renders the surface area of the chip used by
the cell to be essentially the same as that used by the vertical
trench. Ideally, a cell using a vertical transistor can provide
higher packing densities than a cell that uses a horizontal
switching transistor that is positioned adjacent to the trench that
provides the storage node. One type of a vertical transistor over a
vertical trench cell is described in U.S. patent application Ser.
No. 08/770,962, filed on Dec. 20, 1996, in which Norbert Arnold is
the inventor and the assignee is the same as that of the present
application.
SUMMARY OF THE INVENTION
[0003] A memory cell of the present invention has a unique
structure fabricated by a novel process. In one embodiment, a
semiconductor chip is first provided with a vertical trench to be
used in forming the storage capacitor of the cell. After the trench
is made, the dielectric of the capacitor is formed by coating its
walls with a dielectric material. The storage node of the capacitor
is provided by a doped polysilicon fill of the trench. The top
portion of the trench is provided with essentially monocrystalline
silicon, suitable for forming one source/drain of the vertical
transistor. An additional silicon layer, intermediately placed
between two dielectric layers, is subsequently deposited over the
chip. The three layers are apertured in the region over the trench
to expose the top of the fill. Typically, this layer is
polysilicon. The walls of the additional polysilicon layer that
were exposed in the aperturing operation are oxidized to form the
gate dielectric of the transistor. The aperture is then filled with
silicon that is appropriate to form the intermediate layer of the
transistor in which there is to be created during operation the
inversion layer that forms the channel between the source/drain
regions of the transistor. Ultimately an additional silicon layer
is formed that will form with this intermediate layer the second
source/drain region of the transistor. A bit line connection is
made to this last-mentioned layer, and a word line is provided by
the apertured polysilicon layer.
[0004] Another embodiment of the present invention is directed to a
memory cell that is for use in a memory array of rows and columns
of memory cells within a monocrystalline bulk portion of a silicon
chip and that is addressed by word lines and bit lines. The memory
cell comprises a capacitor, a vertical transistor, a word line, and
a bit line. The capacitor comprises a vertical trench filled with
silicon and having a layer of dielectric along its wall that
isolates the silicon fill from the bulk portion of the chip. The
vertical transistor is superposed over the trench and has a first
source/drain merged with the silicon at the top of the trench, an
intermediate silicon layer that is merged with the silicon fill at
the top of the trench to form the first source/drain region, and in
which an inversion layer is to be created to form a conductive
channel, a second source/drain region overlying the intermediate
silicon layer, a gate dielectric layer surrounding the intermediate
silicon layer, and a gate surrounding the gate dielectric layer and
extending along the surface of the chip and dielectrically
insulated therefrom and being coupled to a word line. The bit line
is in electrical contact with the second source/drain and otherwise
extends over the surface of the trench and is electrically
insulated from the word line and from the chip.
[0005] In another embodiment, the invention is a novel process of
fabricating the cell that includes imparting seed information to
the polysilicon deposited in the trench. This seed information
makes it possible to provide a semiconductor intermediate layer
wherein the channel of the transistor can be created.
[0006] In another embodiment, the present invention is directed to
a process for making a memory cell. The process comprises the steps
of forming a trench in a semiconductor chip of one conductivity
type; forming a dielectric layer over the walls of the trench;
filling the trench with polysilicon of a conductivity type opposite
that of the chip; growing an epitaxial silicon layer over the
surface of the chip of sufficient thickness for forming over the
top of the trench a layer of essentially monocrystalline silicon of
a conductivity that is opposite that of the chip for serving as a
first source/drain; forming a first dielectric layer over the
surface of the chip; forming a polysilicon layer of the
conductivity type opposite that of the chip over the surface of the
first dielectric layer; forming a second dielectric layer over the
surface of said polysilicon layer; etching an opening through the
first and second dielectric layers and said polysilicon layer to
bare the essentially monocrystalline silicon over the top of the
trench; forming a silicon oxide layer selectively along the
sidewall of the opening in the polysilicon layer; growing
monocrystalline silicon of the one conductivity type in the opening
for forming an intermediate layer in which there will be formed the
channel of a vertical transistor in which the silicon oxide layer
over the sidewall of said polysilicon layer of the opening serves
as the gate dielectric; and depositing a conductive layer of the
opposite conductivity type over the top surface of the chip that
contacts the intermediate silicon layer for serving as a second
source/drain and bit line of the cell.
[0007] The invention will be better understood from the following
more detailed description taken in conjunction with the
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an electrical circuit schematic of a standard
memory cell that includes a transistor and a capacitor of the kind
used in DRAMS;
[0009] FIG. 2 is a cross section of a portion of a silicon chip
including a memory cell that has the electrical schematic of the
cell of FIG. 1 and comprises a vertical trench for a capacitor and
a superposed vertical transistor, in accordance with the present
invention;
[0010] FIG. 3 is a top of a memory array using vertical transistors
superposed over vertical trenches, in accordance with the present
invention; and
[0011] FIGS. 4-10 show a portion of a silicon chip in various
stages of the formation therein of a memory cell of the kind shown
in FIG. 2 by one process in accordance with the present
invention.
DETAILED DESCRIPTION
[0012] FIG. 1 shows an electrical circuit schematic of a memory
cell 10. Such a cell is employed, for example, in a random access
memory (RAM) integrated circuit (IC) or chip. Such a cell can also
be used in a dynamic RAM (DRAM), a synchronous DRAM (SDRAM), or
other memory chip. The cell includes a MOS transistor 12 in series
with a capacitor 14. Transistor 12 has first and second output
current electrodes 12A and 12B, respectively, and a gate electrode
12C. Gate electrode 12C of the switching transistor is connected to
a word line of the DRAM array. Electrode 12A serves as the drain of
the transistor when positive current flows therefrom through
transistor 12 and into electrode 12B, which then serves the source.
This occurs when logic information (data, signal bits, i.e., a "1"
or a "0") is read into or refreshed into memory cell 10. Electrode
12B serves as the drain when information is read out of memory cell
12 as current flows from electrode 12B through transistor 12 to
electrode 12A which serves as the source. However, as mentioned
earlier, for simplicity, each will be described as a source/drain
of transistor 12. Capacitor 14 has first (14A) and second (14B)
plates. Plate 14B is typically coupled to a reference voltage,
which is shown as ground 17 in the drawing. In some instances, it
may be desirable to use some other reference voltage on plate 14B,
such as Vpp/2. Transistor 12 is switched on to facilitate current
flow from a bit line 18, which is connected to electrode 12A, to
capacitor 14. It is switched off to isolate capacitor 14 from the
bit line 18. A signal corresponding to a bit of information (logic
information) is stored as charge on capacitor 14. When appropriate
signals are applied to the word line 19 and to the bit line 18,
data ("1" or a "0") is stored as charge on the capacitor 14 where
it is held for a useful time. Because of leakage from storage node
16, it is generally necessary to refresh the stored data
periodically.
[0013] FIG. 2 shows a structure of a memory cell 11 in accordance
with the invention. A plurality of memory cells are interconnected
to form an array or arrays. Such arrays are used to form memory
integrated circuits such as DRAMs, synchronous DRAMs (SDRAMs), or
other memory ICs. The memory cell 11 is shown formed in a portion
of a semiconductive chip 20, such as a semiconductor wafer. The
wafer comprises, for example, silicon. Other semiconductor wafers,
including silicon on insulator (SOI) or gallium arsenide, are also
useful. The wafer can be undoped or doped lightly or heavily with
dopants having a first conductivity. In one embodiment, the chip
includes a bulk portion that is advantageously a p-type
monocrystalline silicon. Memory cell has the electrical schematic
as that shown in FIG. 1. The chip 20, which includes a bulk
monocrystalline portion 21, comprises a trench 22. In one
embodiment, the trench includes a substantially square cross
section filled with, for example, heavily doped n+ type
polycrystalline silicon 23 that serves as the storage node 16 (FIG.
1) of memory cell 11. Trenches with other shaped cross section are
also useful. The n+polysilicon fill also corresponds to one plate
14A (FIG. 1) of the capacitor 14 merged with a
source.backslash.drain 12B of the transistor 12.
[0014] A dielectric layer 24 surrounds the sidewalls and bottom of
the trench 22 and serves as the dielectric of the capacitor 14.
Optionally, a heavily doped n+type layer 26 surrounds the outside
of the trench 22 insulated from its fill 23 by the dielectric layer
24. Layer 26 serves as the other plate 114B (FIG. 1) of the
capacitor 14. The p-type bulk 21 of the chip 20 typically is
maintained at a reference potential, typically ground, as discussed
above. Other reference potentials, such as Vpp/2, are also
useful.
[0015] Located over the trench 22 is a vertical transistor that
corresponds to transistor 12. The vertical MOSFET transistor
includes n+type layers 34 and 37, each generally circular in cross
section, that correspond to source/drain 12B and source/drain 12A
of FIG. 1 and so form the two output current terminals of the
transistor. Intermediate between these extend the p-type layer 30
within which there will be created the n-type inversion layer (not
shown) to serve as the conductive channel between the layers 34 and
37 when the gate voltage is such as to put the transistor in its
closed switch conductive state. The gate dielectric of the
transistor is provided by a silicon oxide layer 32 that surrounds
layer 30. Layer 34 will merge with a conductive layer 36 that
serves as a bit line 52 of the DRAM and runs over the surface of
the trench in the direction normal to the plane of the drawing.
Layer 37 corresponds to the storage node 16 (FIG. 1) of the memory
cell 10.
[0016] A word line 50 is provided by the segmented n+ type layer,
shown as portions 38A, 38B, that also runs over the top surface of
the chip 20 orthogonal to a bit line 36, as will appear from the
discussion of FIG. 3 below. Layer portions 38A and 38B serve as the
gate electrode 12C of transistor 12. The p+ type intermediate
region 30 and the gate dielectric layer 32 essentially are stitched
in between the two segments 38A and 38B of the word line 50. An
oxide layer 40 insulates a bottom surface of the word line 50 from
a top surface of the silicon chip 20.
[0017] It is desirable that the intermediate region 30 be of high
mobility for the charge carriers, such as electrons in the
NMOS-type transistor, because of its potential for higher switching
speed. For this reason, it is desirable that the intermediate
region comprises essentially monocrystalline. In accordance with
one embodiment of the invention, a substantially monocrystalline
intermediate region 30 is provided between the polycrystalline fill
23 of the trench 22 that serves as one source/drain 12A (FIG. 1) of
the switching transistor and the silicon layer 34 over the region
30 that serves as the other source/drain 12B (FIG. 1) of the
switching transistor. As shown, the intermediate region merges with
the bit line 36 of the cell.
[0018] In FIG. 3, there is shown a top view of a two dimensional
rectangular array of memory cells 11 of FIG. 2. As seen in FIG. 3,
the columns of word lines 50 are running vertically, and the rows
of bit lines 52 are running horizontally. Insulated crossovers are
provided where the two sets of lines would otherwise intersect. The
larger tilted squares 54 represent the outline of the vertical
trenches 22, and the smaller squares 56 enclosed within the larger
tilted square 54 are the outlines of the vertical transistors.
[0019] Referring to FIGS. 4-10, the various steps for forming a
memory cell in accordance with one embodiment are shown.
[0020] Typically a plurality of memory ICs are fabricated in
parallel on a wafer. After the parallel processing, the wafer is
subsequently diced into a number of chips. Each chip typically
houses a single IC, each of which contain many thousands of cells
and the associated auxiliary circuitry for writing in, reading out,
and refreshing. For the sake of simplicity, the discussion of the
processing will be limited to that involved with a single memory
cell.
[0021] As shown, a trench is formed in a slice of a semiconductor
substrate or wafer that serves as the workpiece 60. In one
embodiment, the wafer comprises a silicon lightly doped with p-type
dopants (p-). As is shown in FIG. 4, one typically begins by
forming over the surface of a silicon workpiece (chip) 60, a thin
layer 62 of silicon oxide, generally referred to as the PAD oxide.
This layer primarily serves to protect the surface of the slice 60
during the processing and is removed in the course of the
processing. The PAD oxide 62 is generally covered with a layer 64
of silicon nitride, generally described as the PAD nitride, that
primarily serves as an etch stop in some of the subsequent
processing steps.
[0022] Next there is formed in the silicon workpiece 60 a trench 66
that will subsequently be filled with polysilicon. The polysilicon
is heavily doped with n-typed dopants (n+) and serves as the
storage node of the cell. This can be done in the usual fashion
that requires providing a properly patterned mask layer 65 over the
surface of the slice and using anisotropic reactive ion etching
(RIE) to form a trench in the silicon. The mask, for example,
comprises TEOS. Other types of hard mask for etching the trench are
also useful. Next, if a heavily doped plate region, as represented
by n+ type layer 26 in FIG. 2, is to be included, which is
optional, it would be formed by, for example, introducing a
suitable dopant in the trench and diffusing it out into the
substrate. There are a variety of techniques available to this end,
as by coating the interior of the trench with an arsenic-rich
coating and heating to diffuse the arsenic into the silicon wall of
the trench for doping it n+ type. To simplify the drawing, this
layer is being omitted in this and subsequent figures.
[0023] After the n+ type plate is formed, the walls of the trench
66 are precleaned prior to the formation a dielectric layer 70. The
dielectric layer is formed on the trench walls and over the mask
layer 65. The dielectric layer serves as the dielectric of the
capacitor. The dielectric layer comprises, for example, silicon
oxide, silicon nitride, silicon nitrideoxide, or silicon oxynitride
formed by conventional techniques Next, the trench is filled with
n+ type doped polycrystalline silicon 72 (polysilicon). To get a
good fill, the polysilicon is built up on a top surface of the
workpiece 60 until the trench 22 is filled, as is shown in FIG. 4.
After this, the top surface is subjected to chemical mechanical
polishing (CMP) to planarize the surface of the workpiece 60. In
this operation, the dielectric layer 64 serves as an etch stop to
achieve planarization, as shown in FIG. 4.
[0024] The dielectric layer 70 on the surface and the hard mask
layer (not shown) are then removed, leaving the pad nitride and
residual polysilicon. Removal of the dielectric and hard mask layer
is achieved by, for example, wet chemical etch such as HF. Other
etch processes for removing the layers are also useful. A CMP is
employed to planarize the surface, removing the poly to result in a
planar top surface.
[0025] Optionally, the top portion of the trench is recessed by
RIE. The RIE is selective to the pad nitride and dielectric layers.
The result is shown in FIG. 5. Recess 74 improves the quality of
the epitaxially grown silicon over the polysilicon trench. In one
embodiment, the recess is sufficiently deep so as to enable the
trench to be substantially monocrystalline at about the plane of
the top surface of the silicon workpiece. As such, the recess
improves the quality of the epitaxially grown silicon above the
trench.
[0026] Referring to FIG. 6, the pad nitride and oxide layers are
stripped away to bare the monocrystalline silicon surface of the
workpiece 60 in preparation for the epitaxial growth of silicon
over the trench to form the vertical transistor. The removal of the
pad layers is achieved by, for example, wet chemical etch. The bare
silicon surface imparts seed information for growing silicon
thereon.
[0027] A silicon layer 98 is grown epitaxially over the surface of
the workpiece 60. In one embodiment, the epitaxial silicon layer is
heavily doped with n-type dopants. The n+ silicon layer, for
example, is insitu doped during epitaxial growth. Growth of
epitaxial silicon is described in, for example, U.S. Ser. Nos.
08/667,541 and 08/605,622 which are herein incorporated by
reference for all purposes. The epitaxial layer 98 is grown
sufficiently thick and sufficiently crystalline such that
recrystallization by heating can make it sufficiently
monocrystalline for use as the intermediate layer of the vertical
NMOS transistor that is formed thereof, as is shown in FIG.
[0028] The top surface of the workpiece is now masked so that the
epitaxial layer 98 can be removed except where it overlies the
polysilicon filled trench. After this removal, there remains a mesa
100, as is shown in FIG. 7, located over the polysilicon filled
trench 66A.
[0029] Then over the exposed surface of the mesa 100, there is
formed an oxide layer 102 that is suitable for use as the gate
dielectric of the vertical transistor being formed.
[0030] After this oxide layer 102 is formed over the entire exposed
surface of the mesa, it becomes necessary to remove the oxide 102
selectively from the top surface of the mesa 100. Typically this is
done by first forming a silicon nitride layer 104 over the surface
of the mesa 100, removing it selectively from the top surface of
the mesa, while leaving only nitride spacers over the side wall of
the mesa, and then etching away selectively the exposed oxide on
the top surface of the mesa and the exposed oxide overlying the
remainder of the workpiece 60A.
[0031] Next it is desirable, although not necessary, to etch the
silicon at the top of the mesa to form a shallow recess 106 at the
top of the mesa, as is shown in FIG. 8. There may also be removed
silicon at the surface of the workpiece 60A not protected by the
mesa. This permits the gate oxide to provide source/drain overlap.
This overlap is a factor known to be desirable for high speed
switching of the transistor.
[0032] As is shown in FIGS. 9 and 10, essentially there remains
only to provide the drain/source region and the word and bit lines.
To this end, there is first deposited the first oxide layer 80 that
insulates the surface of the workpiece from the word line layer 82
that will subsequently be deposited over it.
[0033] Before deposit of the word line layer, there needs to be
removed the nitride spacers 102 on the sides of the mesa 100, so
that the polysilicon word line can make physical contact to the
gate oxide 102. This is advantageously done by a wet etch that
attacks only the nitride spacer. After the nitride spacers have
been removed, there are deposited in turn the n+ type conductivity
polysilicon layer 82 and the second oxide layer 84 to arrive at the
structure shown in FIG. 9.
[0034] After the deposition of these two layers, CMP is used to
remove the excess of such layers and to arrive at the structure
shown in FIG. 9, leaving exposed the bared epitaxial silicon at the
top surface of the mesa.
[0035] Referring to FIG. 10, there is deposited over the exposed
surface of the silicon mesa a layer of n+ type conductivity
polysilicon 90 to form the drain/source region and a layer of a
metallic silicide 92 to form a highly conductive bit line.
[0036] There now results a cell that corresponds in essential
details to the cell shown in FIG. 2.
[0037] It is to be understood that the specific embodiments
described are merely illustrative of the general principles of the
invention and various modifications will be possible without
departing from the spirit and scope of the invention. For example,
the monocrystalline silicon in which the memory cell is formed may
be a layer of silicon that has been grown epitaxially on a suitable
crystal of a foreign material, such as sapphire. Still further, the
particular conductivity types can be reversed and the conductivity
of the various silicon layers varied as is known to workers in the
art. Still further, the various processing steps involved, such as
etching and depositions, may be varied. Furthermore, while the
invention has been described in terms of silicon as the
semiconductor, which presently is the preferred choice other types
of substrates are also useful. The scope of the invention should
therefore be determined not with reference to the above description
but with reference to the appended claims along with their full
scope of equivalents.
* * * * *