U.S. patent application number 11/759727 was filed with the patent office on 2007-10-04 for technique to control tunneling currents in dram capacitors, cells, and devices.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Salman Akram, Leonard Forbes.
Application Number | 20070228438 11/759727 |
Document ID | / |
Family ID | 25482949 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070228438 |
Kind Code |
A1 |
Forbes; Leonard ; et
al. |
October 4, 2007 |
TECHNIQUE TO CONTROL TUNNELING CURRENTS IN DRAM CAPACITORS, CELLS,
AND DEVICES
Abstract
Structures and methods are provided for the use with PMOS
devices. Materials with large electron affinities or work functions
are provided for structures such as gates. A memory cell is
provided that utilizes materials with work functions larger than
n-type doped polysilicon (4.1 eV) or aluminum metal (4.1 eV) for
gates or capacitor plates.
Inventors: |
Forbes; Leonard; (Corvallis,
OR) ; Akram; Salman; (Boise, ID) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Micron Technology, Inc.
|
Family ID: |
25482949 |
Appl. No.: |
11/759727 |
Filed: |
June 7, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11267009 |
Nov 4, 2005 |
7235837 |
|
|
11759727 |
Jun 7, 2007 |
|
|
|
10721585 |
Nov 25, 2003 |
6979607 |
|
|
11267009 |
Nov 4, 2005 |
|
|
|
09945310 |
Aug 30, 2001 |
6664589 |
|
|
10721585 |
Nov 25, 2003 |
|
|
|
Current U.S.
Class: |
257/296 ;
257/E21.648; 257/E27.085 |
Current CPC
Class: |
H01L 27/10852 20130101;
H01L 27/10805 20130101; H01L 27/10873 20130101; H01L 28/65
20130101; H01L 27/1085 20130101; G11C 7/02 20130101; G11C 11/401
20130101; G11C 11/404 20130101 |
Class at
Publication: |
257/296 ;
257/E27.085 |
International
Class: |
H01L 27/108 20060101
H01L027/108 |
Claims
1. A memory cell, comprising: a storage capacitor, including: a
first plate and a second plate separated by a dielectric; wherein
at least one of the plates includes a p-doped silicon carbide
material; and an access transistor coupled to the storage
capacitor.
2. The memory cell of claim 1, wherein the dielectric has an
equivalent oxide thickness of less than 20 angstroms.
3. The memory cell of claim 1, wherein the access transistor
includes a gate formed from a material with a work function greater
than 4.1 eV.
4. The memory cell of claim 3, wherein the gate material includes a
metal selected from a group consisting of cobalt, nickel,
ruthenium, rhodium, palladium, iridium, platinum, and gold.
5. A memory cell, comprising: a storage capacitor, including: a
first plate and a second plate separated by a dielectric; wherein
at least one of the plates includes a p-doped silicon oxycarbide
material; and an access transistor coupled to the storage
capacitor.
6. The memory cell of claim 5, wherein the dielectric has an
equivalent oxide thickness of less than 20 angstroms.
7. The memory cell of claim 5, wherein the access transistor
includes a gate formed from a material with a work function greater
than 4.1 eV.
8. The memory cell of claim 7, wherein the gate material includes a
metal nitride selected from a group consisting of titanium nitride,
tantalum nitride, tungsten nitride, and molybdenum nitride.
9. A memory cell, comprising: a storage capacitor, including: a
first plate and a second plate separated by a dielectric; wherein
at least one of the plates includes a p-doped gallium nitride
material; and an access transistor coupled to the storage
capacitor.
10. The memory cell of claim 9, wherein the dielectric has an
equivalent oxide thickness of less than 20 angstroms.
11. The memory cell of claim 9, wherein the access transistor
includes a gate formed from a material with a work function greater
than 4.1 eV.
12. The memory cell of claim 11, wherein the gate material includes
a p-doped material selected from a group consisting of p-doped
silicon, p-doped germanium, p-doped silicon germanium, p-doped
silicon carbide, p-doped silicon oxycarbide, p-doped gallium
nitride, and p-doped gallium aluminum nitride.
13. A memory cell, comprising: a storage capacitor, including: a
first plate and a second plate separated by a dielectric; wherein
at least one of the plates includes p-doped gallium aluminum
nitride material; and an access transistor coupled to the storage
capacitor.
14. The memory cell of claim 13, wherein the dielectric has an
equivalent oxide thickness of less than 20 angstroms.
15. The memory cell of claim 14, wherein the access transistor
includes a gate formed from a material with a work function greater
than 4.1 eV, the gate being formed over a gate dielectric with an
equivalent oxide thickness of less than 20 angstroms.
16. A dynamic random access memory device comprising: an array of
memory cells, wherein the memory cells include: a storage
capacitor, including: a first plate and a second plate separated by
a capacitor dielectric, wherein the capacitor dielectric has an
equivalent oxide thickness of less than 20 angstroms; wherein at
least one of the plates includes a material with a work function
greater than 4.1 eV; a number of PMOS access transistors coupled to
the storage capacitors of the memory cells, wherein the PMOS access
transistors include: a first source/drain region; a second
source/drain region; a channel region coupled between the first and
second source/drain regions; and a gate located over the channel
region and separated from the channel region by a gate dielectric,
wherein the gate includes a material with a work function greater
than 4.1 eV.
17. The dynamic random access memory device of claim 16, wherein
the gate dielectric has an equivalent oxide thickness of less than
20 angstroms.
18. The dynamic random access memory device of claim 16, wherein
both the first plate and the second plate of the storage capacitor
include a material with a work function greater than 4.1 eV.
19. The dynamic random access memory device of claim 16, wherein at
least one of the plates includes p-doped gallium aluminum nitride
material.
20. The dynamic random access memory device of claim 16, wherein at
least one of the plates includes p-doped silicon oxycarbide
material.
21. The dynamic random access memory device of claim 16, wherein at
least one of the plates includes a p-doped silicon carbide
material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 11/267,009, filed Nov. 4, 2005; which is a continuation of U.S.
application Ser. No. 10/721,585, filed Nov. 25, 2003, now issued as
U.S. Pat. No. 6,979,607; which is a divisional of U.S. application
Ser. No. 09/945,310, filed Aug. 30, 2001, now issued as U.S. Pat.
No. 6,664,589; each of which is incorporated herein by
reference.
[0002] This application is related to the following commonly
assigned U.S. patent applications: L. Forbes, "P-CHANNEL DYNAMIC
FLASH MEMORY CELLS WITH ULTRATHIN TUNNEL OXIDES," 09/514,627, filed
Feb. 28, 2000, now issued as U.S. Pat. No. 6,384,448; L. Forbes,
"STATIC NVRAM MEMORY CELL WITH ULTRATHIN ULTRA THIN TUNNEL OXIDES,"
Ser. No. 09/515,630, filed Feb. 29, 2000, now issued as U.S. Pat.
No. 6,639,835; L. Forbes and K. Y. Ahn, "LOW VOLTAGE FIELD (IN
SYSTEM) PROGRAMMABLE LOGIC ARRAY PLA'S WITH ULTRATHIN TUNNEL
OXIDES," Ser. No. 09/515,759, filed Feb. 29, 2000, now issued as
U.S. Pat. No. 6,605,961; L. Forbes and k. Y. Ahn, "LOW VOLTAGE
PROGRAMMABLE LOW VOLTAGE MEMORY ADDRESS AND DECODE CIRCUITS WITH
ULTRATHIN TUNNEL OXIDES FOR FAULT CORRECTION," Ser. No. 09/515,115,
filed Feb. 20, 2000, now issued as U.S. Pat. No. 6,351,428; each of
which disclosure is herein incorporated by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to integrated
circuits, and in particular to techniques to control tunneling
currents in DRAM capacitors, cells, and devices.
BACKGROUND OF THE INVENTION
[0004] Field-effect transistors (FETs) are typically produced using
a standard complementary metal-oxide-semiconductor (CMOS)
integrated circuit fabrication process. As is well known in the
art, such a process allows a high degree of integration such that a
high circuit density can be obtained with the use of relatively few
well-established masking and processing steps. A standard CMOS
process is typically used to fabricate FETs that each have a gate
electrode that is composed of--type conductively doped
polycrystalline silicon (polysilicon) material or other conductive
materials.
[0005] The modern memory cell is composed of one transistor, such
as the above described FET, and one capacitor. This modern form of
the memory cell is referred to as dynamic random access memory
(DRAM). In a DRAM, stored charge on the capacitor represents
represent a binary one or zero while the transistor, or FET, acts
as the switch interposed between the bit line or digit line and
capacitor. The capacitor array plate or common node is typically
charged to Vcc/2 (Vcc also written as Vdd), and therefore the
charge stored on the capacitor for logic 1 is q=C*Vcc/2 and for a
logic zero the stored charge is q=-C*Vcc/2 the charge is negative
with respect to Vcc/2 common node voltage. The bit line or digit
line connects to a multitude of transistors. The gate of the access
transistor is connected to a word or row line. The wordline
connects to a multitude of transistors.
[0006] In conventional DRAMS using NMOS access transistors, the
transmission of a 1 or Vcc in writing a 1 into the capacitor (i.e.,
charging the capacitor to Vcc, though the total voltage across the
capacitor is Vcc/2 as the array plate is kept at Vcc/2) is degraded
unless a gate voltage higher than Vcc or Vdd is used. If the gate
voltage was just kept at Vdd or Vcc the amount of voltage on the
capacitor plate connected to the transistor would only be Vdd-Vtn
(where Vtn is the threshold voltage). Using an n-channel access
devices requires the gate voltage of the n-channel transistor be
raised to Vdd+Vtn where Vtn is the threshold voltage of the NMOS
transistor. This will allow the capacitor plate to see a full Vdd,
e.g. [(Vdd+Vtn)-Vtn]=Vdd. Similarly for the PMOS access transistors
the transmission of a zero or Vss is degraded, and the voltage of
the PMOS gate has to be lowered to Vss-Vtp. The preferred voltage
applied to the gate of the PMOS device when turned on in this
invention is -Vtp, or more negative than Vtp. Applying this voltage
to the PMOS transistor turns it on and therefore a 1 or a 0 can be
written into the capacitor. If the plate connected to the PMOS is
charged to Vcc or Vdd then the capacitor stores a 1, and if the
plate connected to the PMOS is charged Vss then the capacitor
stores a 0. Normally the array plate of the capacitor is tied to
Vcc/2 and the voltage across the capacitor is Vcc/2.
[0007] The use of PMOS devices in DRAM memory cells is in itself
not new, in fact the original patent (U.S. Pat. No. 3,387,286
"FIELD EFFECT TRANSISTOR MEMORY," R. H. Dennard, 4 Jun. 1968)
described both the use of NMOS and PMOS devices. In 1970, the newly
formed Intel Company publicly released the 1103, the first DRAM
(Dynamic Random Access Memory) chip (1K bit PMOS dynamic RAM ICs),
and by 1972 it was the best selling semiconductor memory chip in
the world, defeating magnetic core type memory. The first
commercially available computer using the 1103 was the HP 9800
series. These devices however were based on an old technology with
gate oxides in the range of 1000 angstroms, 0.1 micron, or 100 nm.
PMOS devices were used because of the normally accumulated surface
on n-type wafers, techniques had not yet been fully developed to
control the surface inversion in the field regions of p-type
wafers. With such thick gate insulators and capacitor dielectrics
there was and is little consideration and concern about tunneling
leakage currents.
[0008] With the development of the LOCOS process and field
implantations to control surface inversion on p-type wafers the
industry changed to NMOS technology and then CMOS technology on
p-type wafers. Subsequent developments and scaling of devices to
below 0.1 micron, or 100 nm, dimensions have resulted in the use of
ultrathin gate oxides and capacitor dielectric insulators, as low
as 12 angstroms, or 1.2 nm. Such ultrathin insulators can result in
large tunneling currents, in the case of silicon oxide as large as
1.0 A/cm.sup.2 (S. M. Sze, "Physics of semiconductor devices,"
Wiley, N.Y., 1981, pp. 402-407; T. P. Ma et al., "Tunneling leakage
current in ultrathin (<4 nm) nitride/oxide stack dielectrics,"
IEEE Electron Device Letters, vol. 19, no. 10, pp. 388-390, 1998).
While such leakage or tunneling currents may not cause faults in
microprocessors and logic circuits (R. Chau et al., "30 nm physical
gate length CMOS transistors with 1.0 ps n-MOS and 1.7 ps p-MOS
gate delays," IEEE Int. Electron, Devices Meeting, San Francisco,
pp. 45-48, December 2000) they are intolerable in DRAM devices,
capacitors and cells.
[0009] FIG. 1A, illustrates a conventional DRAM cell 100. As shown
in FIG. 1A, the conventional DRAM cell includes a transistor 101
and a capacitor cell 102. A gate 103 for the transistor 101 is
separated from the channel 104 of the transistor 100 by an
insulator 106, such as an oxide. The channel region 104 of the
transistor separates a source region, or first source/drain region
108 from a drain region, or second source/drain region 110. As
shown in FIG. 1A, the drain region 110 is coupled to a first plate
or capacitor plate 112 of the capacitor cell 102. A second plate,
or array plate 114 of the capacitor cell 102 is coupled to Vdd/2.
As stated above, these cells depend upon charge storage on
capacitance nodes. FIG. 1A illustrates tunneling currents which are
leakage currents that will discharge the cells resulting in
shortened retention times and/or lost data and faults. FIG. 1A
further illustrates that a cause of leakage currents is tunneling
from the source/drain of the transfer device which is connected to
the capacitor plate to the gate of the transistor when the
transistor is off.
[0010] As illustrated in FIG. 1B, if a zero is stored in the
capacitor cell 102, then the drain 110 of the transistor will be at
zero or ground potential, but the gate 103 of the transistor when
turned off will be a potential Vdd. This results in a large
positive potential between the source/drain at ground, e.g. drain
110 and the gate 103 at potential+Vdd which can result in tunneling
leakage currents. These leakage currents would tend to make the
capacitor electrode more positive and can result in data
errors.
[0011] Also, tunneling leakage currents from the gate 103 to
substrate/channel 104 when the transistor 101 is turned on with a
large negative gate 103 to source 108 voltage will result in
excessive gate currents. While the tunneling current of one gate
103 may be very small, modern DRAM arrays have a large number of
capacitor cells 102 and transfer devices 101. Summed over an entire
array, this leakage current, which may be up to 1 A/cm.sup.2, will
result in excessive power supply currents and power
dissipation.
[0012] Therefore, there is a need in the art to provide improved
techniques for controlling tunneling currents in DRAM capacitors,
cells and devices. Such improved techniques should take into power
supply currents and power dissipation issues.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGS. 1A-1B, illustrate a conventional DRAM cell.
[0014] FIG. 2 is an energy band diagram illustrating the "mid-gap"
metals used for optimizing NMOS and PMOS transistor threshold
voltages according to the prior art.
[0015] FIG. 3A is an energy band diagram illustrating direct band
to band tunneling with low voltages across the gate oxides or gate
insulators in conjunction with a p-type semiconductor gate or
p-type capacitor storage nodes having large electron affinities or
work functions according to the teachings of the present
invention.
[0016] FIG. 3B is an energy band diagram illustrating direct band
to band tunneling with low voltages across the gate oxides or gate
insulators in conjunction with metal gate or metal capacitor
storage nodes having large work functions according to another
embodiment of the present invention.
[0017] FIG. 4 is a graph plotting electron affinity versus band gap
energy of silicon, carbide, and oxygen related compounds.
[0018] FIG. 5 is a graph plotting work function versus atomic
number for large work function materials.
[0019] FIG. 6 illustrates one embodiment for DRAM device, or
transistor according to the teachings of the present invention.
[0020] FIG. 7 illustrates a memory cell according to the teachings
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] In the following detailed description of the invention,
reference is made to the accompanying drawings which form a part
hereof, and in which is shown, by way of illustration, specific
embodiments in which the invention may be practiced. The
embodiments are intended to describe aspects of the invention in
sufficient detail to enable those skilled in the art to practice
the invention. Other embodiments may be utilized and changes may be
made without departing from the scope of the present invention. In
the following description, the terms wafer and substrate are
interchangeably used to refer generally to any structure on which
integrated circuits are formed, and also to such structures during
various stages of integrated circuit fabrication. Both terms
include doped and undoped semiconductors, epitaxial layers of a
semiconductor on a supporting semiconductor or insulating material,
combinations of such layers, as well as other such structures that
are known in the art. The following detailed description is not to
be taken in a limiting sense, and the scope of the present
invention is defined only by the appended claims.
[0022] In the past we have disclosed the use of this direct band to
band tunneling current in PMOS devices for flash memory type
devices and cells (L. Forbes, "P-CHANNEL DYNAMIC FLASH MEMORY CELLS
WITH ULTRATHIN TUNNEL OXIDES," Ser. No. 09/514,627, filed Feb. 28,
2000, now U.S. Pat. No. 6,384,448; L. Forbes, "STATIC NVRAM MEMORY
CELL WITH ULTRA THIN TUNNEL OXIDES," Ser. No. 09/515,630, filed
Feb. 29, 2000, now U.S. Pat. No. 6,639,835; L. Forbes and K. Y.
Ahn, "LOW VOLTAGE PLA'S WITH ULTRATHIN TUNNEL OXIDES," Ser. No.
09/515,759, filed Feb. 29, 2000, now U.S. Pat. No. 6,605,961; L.
Forbes and K. Y. Ahn, "PROGRAMMABLE LOW VOLTAGE MEMORY ADDRESS AND
DECODE CIRCUITS WITH ULTRATHIN TUNNEL OXIDES," Ser. No. 09/515,115,
filed Feb. 20, 2000, now U.S. Pat. No. 6,351,428), the intent there
being to increase the endurance of flash memory type cells since
direct band to band tunneling will not result in electron
collisions in the oxide and damage to the oxide as occurs in F-N
tunneling.
[0023] Prior art in the use of metals with work functions larger
than aluminum or n-type polysilicon have been directed at so called
"mid-gap" work functions which make the threshold voltages for both
NMOS and PMOS devices symmetrical (see generally, B. Maiti and P.
J. Tobin, "Metal gates for advanced CMOS technology," Proc.
Microelectronics Device Technology III, Santa Clara, Calif., 22-23
September, 1999, Soc. of Photo-Optical Instrumentation Engineers,
Bellingham Wash., pp. 46-57) or the same magnitude or numeric
value. This is illustrated in FIG. 2 showing the position of the
Fermi level in the metal falling in the center of the silicon
bandgap with no potential difference across the gate insulator.
Common mid-gap metal work functions are provided by the refractory
metals tungsten, W, and molydenum, Mo.
[0024] FIG. 2 is an energy band diagram illustrating the "mid-gap"
metals used for optimizing NMOS and PMOS transistor threshold
voltages according to the prior art. FIG. 2 is used to illustrate a
metal gate 209, such as Tungsten W (work function 4.6 eV) or
Molybdenum Mo (work function 4.7 eV) in a DRAM cell separated by an
oxide 201 from a channel region 207 in either an n-type (PMOS
transistor) or a p-type (NMOS transistor) substrate. As shown in
FIG. 2, the metal/semiconductor work function differences can be
expressed as follows. For NMOS devices 4.7V-5.0 eV=-0.3 eV, for
PMOS devices 4.7-4.4 eV=+0.3 eV. FIG. 2 illustrates the position of
the Fermi level in the metal falling in the center of the silicon
bandgap with no potential difference across the gate insulator. As
stated above, common mid-gap metal work functions are provided by
the refractory metals tungsten, W, and molybdenum, MO. However,
these prior art techniques still do not solve the problem of low
leakage current, which may be up to 1 A/cm.sup.2, and will result
in excessive power supply currents and power dissipation.
[0025] In the present invention, the intent is to utilize the
larger tunneling barriers and lower voltages than used in flash
memory devices to limit these tunneling leakage currents to levels
which are acceptable in DRAM devices, cells and capacitors.
[0026] FIG. 3A is an energy band diagram illustrating direct band
to band tunneling with low voltages across the gate oxides or gate
insulators in conjunction with a p-type semiconductor gate or
p-type capacitor storage nodes 309 having large electron affinities
or work functions according to the teachings of the present
invention. FIG. 3A illustrates a p-type semiconductor gate or
capacitor storage node/plate 309 separated by an insulator, e.g. an
oxide 301, from a channel region/substrate 307 or second capacitor
storage node/plate 307. According to the teachings of the present
invention, the p-type semiconductor gate or capacitor 309 includes
polycrystalline semiconductor plates selected from the group
consisting of p-doped silicon, p-doped germanium, p-doped silicon
germanium compounds, p-doped silicon carbide, p-doped silicon
oxycarbide compounds, p-doped gallium nitride compounds, and
p-doped gallium aluminum nitride compounds. According to the
teachings of the present invention, the tunneling barriers in the
structure of FIG. 3A are much larger than in conventional NMOS
devices and capacitor plates doped n-type. As one of ordinary skill
in the art will understand upon reading this disclosure, these much
larger tunneling barriers will result in tunneling currents which
are orders of magnitude smaller at the same electric fields across
the gate and/or capacitor insulators 301.
[0027] In fact, according to the teachings of the present
invention, the larger barriers and lower operating voltages will
preclude Fowler-Nordheim (F-N) tunneling and the primary tunneling
mechanism will be limited to direct band-to-band tunneling.
[0028] As discussed above, a number of previous works by the
current inventors have disclosed the use of direct band-to-band
tunneling current in PMOS devices for flash memory type devices and
cells. However, there the intent was to increase the endurance of
flash memory type cells since direct band-to-band tunneling will
not result in electron collisions in the oxide and damage to the
oxide as occurs in F-N tunneling. FIG. 3A illustrates the direct
band-to-band tunneling with low voltages across the gate oxides or
gate insulators 301.
[0029] In the present invention, the intent is to use the larger
tunneling barriers and lower voltages than used in flash memory
devices to limit these tunneling leakage currents to levels which
are acceptable in DRAM devices, cells and capacitors.
[0030] FIG. 3B is an energy band diagram illustrating direct band
to band tunneling with low voltages across the gate oxides or gate
insulators in conjunction with metal gate or metal capacitor
storage nodes having large work functions according to another
embodiment of the present invention. FIG. 3B illustrates a metal
gate or metal capacitor storage node/plate 309 separated by an
insulator, e.g. an oxide 301, from a channel region/substrate 307
or second capacitor storage node/plate. According to the teachings
of the present invention, the metal gate or metal capacitor storage
node/plate 309 includes a metal gate selected from the group
consisting of cobalt, nickel, ruthenium, rhodium, palladium,
iridium, platinum and gold. Alternatively, in one embodiment of the
present invention, the metal gate or metal capacitor storage
node/plate 309 includes a metallic nitride gate selected from the
group consisting of titanium nitride, tantalum nitride, tungsten
nitride, and molybdenum nitride.
[0031] In contrast to the previous work, the present invention
utilizes p-type semiconductor or metal gates or capacitor plates
with work functions larger than those of n-type doped polysilicon
(4.1 eV) or the commonly used aluminum metal in MOS technology (4.1
eV). Voltages applied to the gates or plates are lower than 3.2
Volts so the primary tunneling mechanism is restricted to direct
band to band tunneling (see generally, T. P. Ma et al., "Tunneling
leakage current in ultrathin (<4 nm) nitride/oxide stack
dielectrics," IEEE Electron Device Letters, vol. 19, no. 10, pp.
388-390, 1998).
[0032] FIG. 4 is a graph plotting electron affinity versus band gap
energy of silicon, carbide, and oxygen related compounds. As shown
in FIG. 4, silicon dioxide is an insulator with a relative
dielectric constant of 3.9, energy gap of approximately 9.0 eV, and
electron affinity of 0.9 eV. In a conventional flash memory,
electrons stored on the polysilicon floating gate see a large
tunneling barrier of about 3.2 eV. This value is the difference
between the electron affinities of silicon (4.1 eV) and SiO.sub.2
(0.9 eV). This is a relative large barrier which requires high
applied electric fields for electron injection. SiO has a
dielectric constant close to that of SiO.sub.2 which, as stated
above, has a value near 3.9. Also, as shown in FIG. 4, SiO has a
band gap of approximately 3.2 eV and an estimated electron affinity
of 3.5 eV. Accordingly, as shown in FIG. 4, the x in SiOx can be
varied to produce a range of electron affinities and poly-Si/a-SiOx
tunneling barriers from 0.6 eV to 3.2 eV. Finally, Crystalline SiC
has a band gap of 3 eV and an electron affinity of 3.7 eV.
Amorphous SiC or a-SiC and hydrogenated, amorphous a-SiC.sub.x:H
films have relatively low conductivity under modest applied
electric fields (see generally, F. Dimichelis et al., "Doped
amorphous and microcrystalline silicon carbide as wide bandgap
material," Symp. On Wide Band Gap Semiconductors, Mat. Res. Soc.,
Pittsburgh, Pa., pp. 675-680, 1992).
Amorphous-Si.sub.xC.sub.yO.sub.z, or a-Si.sub.xC.sub.yO.sub.z, is a
wide band gap insulator with a low dielectric constant (<4),
comparable to SiO.sub.2 (see generally, T. Furusawa et al., "Simple
reliable Cu/low-k interconnect integration using
mechanically-strong low-k dielectric material: silicon-oxycarbide,"
Proc. IEEE int. Interconnect Technology Conf, pp. 222-224, June
2000). No measurements have been reported on the electron
affinities of amorphous films of silicon oxycarbide but projections
can be made based on the electron affinities and band gaps of
SiO.sub.2 and SiC. As shown in FIG. 4, the electron affinity of
a-Si.sub.xC.sub.yO.sub.z should vary from that of silicon dioxide
(0.9 eV) to that of the silicon carbide (3.7 eV). This means that
the electron barrier between the oxycarbide and silicon (sic, the
difference in electron affinities of the pure silicon dioxide and
pure silicon carbide) can be varied from roughly 0.4 to 3.2 eV.
[0033] FIG. 5 is a graph plotting work function versus atomic
number for large work function materials. FIG. 5 is provided to
note the relationship of work functions to atomic number and
position in the periodic table. FIG. 5 illustrates plots the work
function versus atomic number of p-type silicon, aluminum (Al),
p-type germanium, cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium
(Rh), palladium (Pd), iridium (Ir), platinum (Pt) and gold
(Au).
[0034] According to the teachings of the present invention, the
p-doped silicon and silicon germanium, p-doped large band-gap
semiconductors, metals with large work functions, and metallic
nitrides with large work functions are fabricated using
conventional process techniques.
[0035] FIG. 6 illustrates one embodiment for DRAM device 600
including a transistor 601 according to the teachings of the
present invention. As shown in FIG. 6, a transistor 601 is provided
having a first source/drain region 608 and a second source/drain
region 610. According to the embodiment shown in FIG. 6, the first
608 and the second 610 source/drain region include source/drain
regions, 608 and 610, formed of a material having a large work
function. A channel 607 is located between the first and the second
source/drain regions, 608 and 610. A gate 609 opposes the channel
607. According to the teachings of the present invention, the gate
609 includes a gate 609 formed of a material having a large work
function. A gate insulator 613 separates the gate from the
channel.
[0036] Several embodiments of the present invention can be
described in connection with the transistor 601 in FIG. 6. These
several embodiments include a gate 609 having a large work function
where the gate material is formed from the group consisting of
p-type doped polycrystalline semiconductor material, large work
function metals, and large work function metallic nitrides.
[0037] As stated above, one of the several embodiments of the
present invention, includes the gate 609 of the transistor 601,
formed of a material having a large work function, being a p-type
doped polycrystalline semiconductor gate. The embodiment of the
present invention, having a large work function, p-type doped
polycrystalline semiconductor gate 609 can further include several
subsets to this embodiment. In one subset embodiment, the large
work function, p-type doped polycrystalline semiconductor gate 609
includes p-doped germanium. In another subset embodiment, the large
work function, p-type doped polycrystalline semiconductor gate 609
includes p-doped silicon germanium compounds. In another subset
embodiment, the large work function, p-type doped polycrystalline
semiconductor gate 609 includes p-doped silicon carbide.
[0038] FIG. 6 can similarly illustrates another embodiment for DRAM
device 600, or transistor 601 according to the teachings of the
present invention. As shown in FIG. 6 a transistor 601 is provided
having a first source/drain region 608 and a second source/drain
region 610. According to the embodiment shown in FIG. 6, the first
608 and the second 610 source/drain region include source/drain
regions, 608 and 610, formed of a material having a large work
function. A channel 607 is located between the first and the second
source/drain regions, 608 and 610. A gate 609 opposes the channel
607. According to the teachings of the present invention, the gate
609 includes a gate 609 formed of a material having a large work
function. In this alternative embodiment, the gate includes a metal
gate selected from the group consisting of cobalt, nickel,
ruthenium, rhodium, palladium, iridium, platinum and gold. Further,
the metal gate can include a metallic nitride gate selected from
the group consisting of titanium nitride, tantalum nitride,
tungsten nitride, and molybdenum nitride.
[0039] As shown in FIG. 6, the above described transistor 601 forms
part of a memory cell 600. According to the teachings of the
present invention, the memory cell is a DRAM cell. In one
embodiment of the present invention, the gate insulator is less
than 20 Angstroms thick.
[0040] As shown in FIG. 6, the memory cell 600 includes a capacitor
602 coupled to the second source/drain region 610 wherein a first
612 and a second plate 614 of the capacitor include first and
second plates, 612 and 614 respectively, having a large work
function. In one embodiment according to the teachings of the
present invention, at least one of the first and the second plates,
612 and 614 include p-type polysilicon polycrystalline
semiconductor plates. In this embodiment, the polycrystalline
semiconductor plates, 612 and 614, are selected from the group
consisting of p-doped silicon, p-doped germanium, p-doped silicon
germanium compounds, p-doped silicon carbide, p-doped silicon
oxycarbide compounds, p-doped gallium nitride compounds, and
p-doped gallium aluminum nitride compounds.
[0041] In an alternative embodiment, at least one of the first and
the second plates, 612 and 614 respectively include metal plates.
In this embodiment, the metal plates include metal plates selected
from the group consisting of cobalt, nickel, ruthenium, rhodium,
palladium, iridium, platinum and gold. In still another embodiment
of the present invention, the first and the second plates, 612 and
614 include metallic nitride plates. In this embodiment, the
metallic nitride plates, 612 and 614 include metallic nitride
plates selected from the group consisting of titanium nitride,
tantalum nitride, tungsten nitride, and molybdenum nitride.
[0042] FIG. 7 illustrates a memory cell 700 according to the
teachings of the present invention. As shown in FIG. 7, memory cell
700 includes a PMOS transistor 701 formed in an n-type well 731. As
shown in FIG. 7, the PMOS transistor 701 includes a first
source/drain region 708, and a second source/drain region 710,
where the first and the second source/drain region, 708 and 710
include source/drain regions having a large work function. A
channel is 707 located between the first and the second
source/drain regions, 708 and 710. A gate 709 opposes the channel
707. The gate includes a gate having a large work function. A gate
insulator 706 separates the gate from the channel 707. In one
embodiment, the gate insulator 706 is less than 20 Angstroms thick.
As shown in FIG. 7, the memory cell 700 further includes a
capacitor 703 coupled to the second source/drain region 710 wherein
a first and a second plate of the capacitor, 712 and 714 includes
first and second plates having a large work function.
[0043] As described above, embodiments of the present invention
include first and the second p-type polysilicon polycrystalline
semiconductor plates selected from the group consisting of p-doped
silicon, p-doped germanium, p-doped silicon germanium compounds,
p-doped silicon carbide, p-doped silicon oxycarbide compounds,
p-doped gallium nitride compounds, and p-doped gallium aluminum
nitride compounds.
[0044] Alternatively, the first and the second plates 712 and 714
include metal plates selected from the group consisting of cobalt,
nickel, ruthenium, rhodium, palladium, iridium, platinum and gold.
Alternatively still, the first and the second plates 712 and 714
include metallic nitride plates selected from the group consisting
of titanium nitride, tantalum nitride, tungsten nitride, and
molybdenum nitride.
[0045] Also, as described above, embodiments of the present
invention include a gate 709 that includes a metal gate 709
selected from the group consisting of cobalt, nickel, ruthenium,
rhodium, palladium, iridium, platinum and gold. Alternatively, the
gate 709 includes a metallic nitride gate 709 selected from the
group consisting of titanium nitride, tantalum nitride, tungsten
nitride, and molybdenum nitride. Alternatively still, the gate 709
includes a p-type polycrystalline semiconductor gate selected from
the group consisting of p-doped silicon, p-doped germanium, p-doped
silicon germanium compounds, p-doped silicon carbide, p-doped
silicon oxycarbide compounds, p-doped gallium nitride compounds,
and p-doped gallium aluminum nitride compounds.
[0046] In one embodiment of FIG. 7, the n-type well 731 is tied to
a positive voltage which is less than a power supply voltage. In an
alternative embodiment of FIG. 7, the n-type well 731 is tied to a
voltage which is equal to a power supply voltage. In still another
embodiment of the present invention, the n-type well 731 is tied to
a voltage which is greater than a power supply voltage.
[0047] Using an array plate voltage of Vcc/2 serves to reduce the
electric field across the capacitor dielectric to reduce dielectric
leakage currents, like tunneling and reduce the probability of
dielectric breakdown. Here however if the plates are made of
different materials the array plate might be tied to Vcc and the
large work function of the other individual capacitor plate used to
reduce the tunneling leakage currents since there would either be
no electric field or voltage across the dielectric when a 1 was
stored in the capacitor or only a negative potential of magnitude
Vcc when a zero is stored in the capacitor. If only a negative
potential difference is used then the large work function plate
material will reduce the tunneling leakage current. If both the
array plate and capacitor plate are made of the same large work
function material then an array plate potential of Vcc/2 can be
used since either plate will have a large work function and it will
be difficult to cause electron tunneling from either. If the array
plate is not of the same material and is not a high work function
material then an intermediate value of array plate potential other
than Vcc/2 might be an optimum choice. The disclosure is not so
limited. For purposes of illustration we will assume the plates are
of the same material and the array plate is at a potential of
Vcc/2. By using capacitor plate materials with large work
functions, as shown in FIGS. 6 and 7, the tunneling leakage
currents of the storage capacitors can be eliminated.
[0048] The use of a PMOS transfer device with a P+ source/drain
region with a large work function will result in minimal tunneling
currents to the gate or eliminate tunneling leakage currents.
According to the teachings of the present invention, the P+
source/drain region can be formed of any of the semiconductor
materials described herein having a large work function.
[0049] Tunneling leakage from the gate can be avoided or eliminated
by using gate materials with a large work function. These tunneling
currents can also be reduced by biasing the n-well at a more
negative potential or less than the power supply voltage Vdd;
however, this can result in extra transistor subthreshold leakage
since the source to n-well will be forward biased. Biasing the
n-well to a potential less than Vdd will however result in less
source/drain junction leakage when a zero is stored in the cell
since the reverse bias between the source/drain and well will be
smaller. Biasing the n-well more positive or above Vdd will result
in less junction leakage tending to reduce the cell capacitor plate
to voltages below Vdd when a one is stored on the capacitor plate.
A variety of well potentials can thus be employed to meet different
application requirements and the disclosure is not so limited by
any particular n-well potential.
[0050] The use of large work function capacitor electrode plate
materials, large work function gate materials, and large work
function P+ source/drain regions have been described for the
elimination of direct band to band tunneling leakage currents in
DRAM memory cells.
METHODS OF THE INVENTION
[0051] As will be understood by one of ordinary skill in the art
upon reading and studying this disclosure the following methods are
included as part of the scope of the present invention. These
methods can be fully understood and practiced in reference to the
Figures described in detail above. A first method includes a method
of forming a memory cell. This method includes forming a PMOS
transistor an n-type well. Forming the PMOS transistor includes
forming a first and a second source/drain regions separated by a
channel. Forming the first and the second source/drain regions
includes forming source/drain regions having a large work function.
The method further includes forming a gate opposing the channel.
According to the teachings of the present invention, forming the
gate includes forming a gate having a work function of greater than
4.1 eV. A gate insulator is formed separating the gate from the
channel. In one embodiment, the gate insulator is formed to a
thickness of less than 20 Angstroms. Finally, the method includes
forming a storage device coupled to the second source/drain region.
Forming the storage device includes forming a storage device having
a first and a second storage node separated by a dielectric.
According to the teachings of the present invention at least one of
the first and the second storage nodes has a large work function.
In one embodiment, the at least one storage node having a large
work function includes a work function greater than 4.1 eV.
[0052] In one embodiment of the above method, forming the PMOS
transistor in an n-type well includes coupling the n-type well to a
positive voltage which is less than a power supply voltage. In
another embodiment of the above method forming the PMOS transistor
in an n-type well includes coupling the n-type well to a voltage
which is equal to a power supply voltage. In yet another embodiment
of the above method forming the PMOS transistor in an n-type well
includes coupling the n-type well to a voltage which is greater
than a power supply voltage.
[0053] Further, in one embodiment forming the first and the second
storage nodes includes forming the first and the second storage
nodes of the same material. In one embodiment, forming the first
and the second source/drain regions having a large work function
includes forming the first and the second source/drain regions with
a work function greater than 4.1 eV.
[0054] In one embodiment, forming the first and the second storage
nodes includes forming first and second storage nodes which include
a p-type polysilicon polycrystalline semiconductor material
selected from the group consisting of p-doped silicon, p-doped
germanium, p-doped silicon germanium compounds, p-doped silicon
carbide, p-doped silicon oxycarbide compounds, p-doped gallium
nitride compounds, and p-doped gallium aluminum nitride
compounds.
[0055] Alternatively, in one embodiment, forming the first and the
second storage nodes includes forming the first and the second
storage nodes of a metal selected from the group consisting of
cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum
and gold.
[0056] Alternatively still, in one embodiment, forming the first
and the second storage nodes includes forming the first and the
second storage nodes of a metallic nitride selected from the group
consisting of titanium nitride, tantalum nitride, tungsten nitride,
and molybdenum nitride.
[0057] In one embodiment, forming the gate includes forming a metal
gate selected from the group consisting of cobalt, nickel,
ruthenium, rhodium, palladium, iridium, platinum and gold. In
another embodiment of the above method forming the gate includes
forming a metallic nitride gate selected from the group consisting
of titanium nitride, tantalum nitride, tungsten nitride, and
molybdenum nitride. In another embodiment of the above method
forming the gate includes forming a p-type polycrystalline
semiconductor gate selected from the group consisting of p-doped
silicon, p-doped germanium, p-doped silicon germanium compounds,
p-doped silicon carbide, p-doped silicon oxycarbide compounds,
p-doped gallium nitride compounds, and p-doped gallium aluminum
nitride compounds.
[0058] Another method embodiment of the present invention includes
a method for operating a memory cell. This method includes applying
a negative voltage to a gate of a PMOS transistor formed in an
n-type well. In this embodiment, the PMOS transistor includes a
first source/drain region and a second source/drain region. The
first and the second source/drain region include source/drain
regions having a large work function. A channel is located between
the first and the second source/drain regions. A gate opposes the
channel. In one embodiment, the gate includes a gate having a large
work function. A gate insulator separates the gate from the
channel. In one embodiment, the gate insulator is less than 20
Angstroms thick. The method further includes coupling the n-type
well to a positive voltage which is less than a power supply
voltage. The method further includes reading a charge level of a
storage device. The storage device includes a first and a second
storage node. At least one of the first and the second storage
nodes is formed of a material having a large work function.
[0059] According to the teachings of the present invention,
coupling the n-type well to a positive voltage which is less than a
power supply voltage achieves lower tunneling charge leakage from
the gate.
[0060] Another method embodiment for the present invention includes
a method for operating a memory cell. This method includes applying
a negative voltage to a gate of a PMOS transistor formed in an
n-type well. Again, the PMOS transistor includes a first
source/drain region and a second source/drain region. The first and
the second source/drain region include source/drain regions having
a large work function. A channel located between the first and the
second source/drain regions. A gate opposes the channel. In one
embodiment, the gate includes a gate having a large work function.
A gate insulator separates the gate from the channel. In one
embodiment, the gate insulator is less than 20 Angstroms thick. The
method further includes coupling the n-type well to a voltage which
is equal to a power supply voltage. And, the method includes
reading a charge level of a storage device. The storage device
includes a first and a second storage node. In one embodiment, at
least one of the first and the second storage nodes includes a
storage node formed of a material having a large work function.
[0061] According to the teachings of the present invention,
coupling the n-type well to a voltage which is equal to a power
supply voltage achieves lower tunneling charge leakage from the
gate and lower junction leakage from the second source/drain region
and storage device when the storage device is not charged.
[0062] Another method embodiment of the present invention includes
a method for operating a memory cell. This embodiment includes
applying a negative voltage to a gate of a PMOS transistor formed
in an n-type well. The PMOS transistor includes a first
source/drain region and a second source/drain region. The first and
the second source/drain region include source/drain regions formed
of a material having a large work function. A channel is located
between the first and the second source/drain regions and a gate
opposes the channel. In one embodiment, the gate includes a gate
having a large work function. A gate insulator separates the gate
from the channel. In one embodiment, the gate insulator is less
than 20 Angstroms thick. The method further includes coupling the
n-type well to a voltage which is greater than a power supply
voltage. The method further includes reading a charge level of a
storage device. The storage device includes a first and a second
storage node. In one embodiment, the first and the second storage
nodes includes at least one storage node formed of a material
having a large work function.
[0063] According to the teachings of the present invention,
coupling the n-type well to a voltage which is greater than a power
supply voltage will result in less junction leakage tending to
reduce the cell capacitor plate to voltages below Vdd when a one is
stored on the capacitor plate.
[0064] As such, one of ordinary skill in the art will understand
upon reading this disclosure that a variety of well potentials can
thus be employed to meet different application requirements and
this disclosure in not so limited by any particular n-well 731
potential.
CONCLUSION
[0065] The above structures and fabrication methods have been
described, by way of example and not by way of limitation, with
respect to provide improved techniques for controlling tunneling
currents in DRAM capacitors, cells and devices. Such improved
techniques should take into power supply currents and power
dissipation issues.
* * * * *