U.S. patent application number 11/290713 was filed with the patent office on 2007-05-31 for phase change material and non-volatile memory device using the same.
Invention is credited to Dong Ho Ahn, Byung-ki Cheong, Jeung-hyun Jeong, Dae-Hwan Kang, In Ho Kim, Ki Bum Kim, Won Mok Kim, Tae-Yon Lee, Taek Sung Lee.
Application Number | 20070120104 11/290713 |
Document ID | / |
Family ID | 38086567 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070120104 |
Kind Code |
A1 |
Ahn; Dong Ho ; et
al. |
May 31, 2007 |
PHASE CHANGE MATERIAL AND NON-VOLATILE MEMORY DEVICE USING THE
SAME
Abstract
The present invention provides a phase change memory cell
comprising
(Ge.sub.ASb.sub.BTe.sub.C).sub.1-x(R.sub.aS.sub.bTe.sub.C)x solid
solution, the solid solution being formed from a Ge--Sb--Te based
alloy and a ternary metal alloy R--S--Te sharing same crystal
structure as the Ge--Sb--Te based alloy. A nonvolatile phase change
memory cell in accordance with the present invention provides many
advantages such as high speed, high data retention, and multi-bit
operation.
Inventors: |
Ahn; Dong Ho; (Seoul,
KR) ; Lee; Tae-Yon; (Seoul, KR) ; Kim; Ki
Bum; (Seoul, KR) ; Cheong; Byung-ki; (Seoul,
KR) ; Kang; Dae-Hwan; (Seoul, KR) ; Jeong;
Jeung-hyun; (Seoul, KR) ; Kim; In Ho; (Seoul,
KR) ; Lee; Taek Sung; (Seoul, KR) ; Kim; Won
Mok; (Seoul, KR) |
Correspondence
Address: |
David A. Einhorn, Esq.;Anderson Kill Olick, P.C.
1251 Avenue of the Americas
New York
NY
10020
US
|
Family ID: |
38086567 |
Appl. No.: |
11/290713 |
Filed: |
November 29, 2005 |
Current U.S.
Class: |
257/2 ; 257/4;
257/5; 257/E45.002 |
Current CPC
Class: |
H01L 45/06 20130101;
H01L 45/126 20130101; H01L 45/1233 20130101; H01L 45/144
20130101 |
Class at
Publication: |
257/002 ;
257/004; 257/005 |
International
Class: |
H01L 29/02 20060101
H01L029/02 |
Claims
1. A non-volatile phase change memory cell comprising a compound
having the following formula:
(Ge.sub.ASb.sub.BTe.sub.c).sub.1-x(R.sub.aS.sub.bTe.sub.c)x
wherein, Ge is germanium; Sb is antimony; Te is tellurium; R is an
element selected from the elements belonging to the IVB group in
the periodic table; S is an element selected from the elements
belonging to the VB group in the periodic table; A, B, C, and a, b
and c are atomic mole ratios satisfying the condition that the
R--S--Te alloy part is stoichiometrically equivalent to the
Ge--Sb--Te alloy part; x is a mole fraction in the range of 0 to 1;
R.sub.aS.sub.bTe.sub.c has the same crystal structure as
Ge.sub.ASb.sub.BTe.sub.C; and at least one element of R and S has a
higher atomic number and a smaller diatomic bond strength than that
of the corresponding element in the GeSb portion of Ge--Sb--Te.
2. The non-volatile phase change memory cell of claim 1, wherein R
is Sn or Pb and S is Bi.
3. The non-volatile phase change memory cell of claim 1, wherein a
first combination of A, S and C and a second combination of a, b
and c are the same with each other, and selected from the group
consisting of (4, 1, 5), (2, 2, 5), (1, 2, 4) and (1, 4, 7), the
combination being limited to the sequence expressed in parenthesis
in that order.
4. The non-volatile phase change memory cell of claim 3, wherein
both the first and the second combinations are (1, 2, 4).
5. The non-volatile phase change memory cell of claim 4, wherein X
is around 0.2.
6. The non-volatile phase change memory cell of claim 4, wherein X
is around 0.1.
7. The non-volatile phase change memory cell of claim 1, wherein
the compound presents two crystalline phases of a
face-centered-cubic (fcc) and a hexagonal states.
8. The non-volatile phase change memory cell of claim 1, wherein
the compound presents three phases of an amorphous, a crystalline
face-centered cubic (fcc) and a crystalline hexagonal state.
9. A memory device comprising the phase change memory cell of claim
1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a non-volatile memory
device using a phase change material.
BACKGROUND OF THE INVENTION
[0002] In recent years, there has been a renewal of interest in
phase change random access memory (PCRAM) as a promising candidate
for next generation nonvolatile memory device because of many
advantages such as non-volatility, fast operation property, process
simplicity and possibility of multi-bit operation.
[0003] Traditionally, PCRAM employs a chalcogenide-based phase
change material such as a stoichiometric Ge--Sb--Te alloy like
Ge.sub.2Sb.sub.2Te.sub.5. A Ge--Sb--Te based alloy is capable of
storing information in a binary form by electrically switching
between the amorphous and crystalline states in a reversible
manner.
[0004] Despite its merits as nonvolatile phase change memory
material, however, a Ge--Sb--Te based alloy is disadvantageous as
it tends to yield slow writing speed. For instance, it takes about
100 ns for the completion of the phase change from the amorphous
(high resistance) to the crystalline (low resistance) states when a
Ge--Sb--Te based alloy is employed. It takes ordinarily less than
100 ns in the reverse direction. On the other hand, conventional
DRAM (dynamic random access memory), SRAM (static random access
memory) and MRAM (magnetic random access memory) show the writing
time of .about.50 ns, .about.8 ns and .about.10 ns, respectively.
Therefore, efforts should be made if PCRAM is to be used for high
speed applications.
[0005] In addition, there is a stability problem associated with
thermal interference between adjacent memory cells.
[0006] To store information in a binary form, memory cell exploits
the difference in electrical resistance between crystalline and
amorphous states. Specifically, in order to write `1` state (reset
state) in a single cell, an electric voltage or current pulse is
applied between the top and bottom electrodes contacting a phase
change material, which induces direct or indirect heating on the
phase change material for melting thereof. Upon termination of the
electric pulse, the molten phase change material is quenched to an
amorphous state, thereby writing the state `1` in a single
cell.
[0007] With density of PCRAM growing higher, binary data stored in
amorphous memory cells may be corrupted with ease by unintended
crystallization as a result of the heat generated in an adjoining
memory cell which undergoes melting during a reset process
thereof.
[0008] Nitrogen or silicon may be added to a Ge--Sb--Te based alloy
for raising the crystallization temperature thereof. However, the
addition of impurities may slow the crystallization process (B. J.
Kuh et al, EPCOS 2005).
[0009] Further, integrating the memory device by sizing down the
cell area is inherently bound by the limits of photolithographic
techniques. In U.S. Pat. No. 5,414,271, it is disclosed that data
can be stored in multi-bit forms by controlling the ratio between
the amorphous and crystalline states in a single cell unit.
However, it is extremely hard to control the dispersion between
these two states.
[0010] Accordingly, it is imperative to find a way for storing
multi-bit information in a single cell unit.
SUMMARY OF THE INVENTION
[0011] It is, therefore, an object of the present invention to
provide a non-volatile phase change memory cell devoid of at least
one of the aforementioned problems, and a memory device using the
same.
[0012] In accordance with the present invention, there is provided
a non-volatile phase change memory cell comprising a compound
having the formula
(Ge.sub.ASb.sub.BTe.sub.C).sub.1-X(R.sub.aS.sub.bTe.sub.C).sub.X,
wherein Ge is germanium; Sb is antimony; Te is tellurium; R is an
element selected from the elements belonging to the IVB group in
the periodic table; S is an element selected from the elements
belonging to the VB group in the periodic table; A, B, C, a, b and
c are atomic mole ratios; x is a mole fraction in the range of 0 to
1; R.sub.aS.sub.bTe.sub.C has same crystal structure as
Ge.sub.ASb.sub.BTe.sub.C; and at least one element of R and S has a
higher atomic number and thus a smaller diatomic bond strength than
that of the corresponding element in the GeSb portion of
Ge--Sb--Te.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The above and other objects and features of the present
invention will become apparent from the following description of
preferred embodiments given in conjunction with the accompanying
drawings, in which:
[0014] FIG. 1 describes a schematic diagram of a phase change
memory cell including a material in accordance with the present
invention;
[0015] FIG. 2 shows a planar view of 70 nm contact pore by SEM;
[0016] FIG. 3 illustrates sectional SEM picture of a phase change
memory cell including a material in accordance with the present
invention;
[0017] FIGS. 4a, 4b and 4c offer DC I-V characteristics of
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(R.sub.1S.sub.2Te.sub.4).sub.0.2,
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.9(R.sub.1S.sub.2Te.sub.4).sub.0.1
and Ge.sub.1Sb.sub.2Te.sub.4, respectively;
[0018] FIGS. 5a, 5b and 5c delineate resistances of memory cells
having
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(R.sub.1S.sub.2Te.sub.4).sub.0.2,
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.9(R.sub.1S.sub.2Te.sub.4).sub.0.1
and Ge.sub.1Sb.sub.2Te.sub.4, respectively;
[0019] FIG. 6 demonstrates relationship between SET pulse voltage
characteristics and SET pulse width; and
[0020] FIG. 7 outlines change in sheet resistance with respect to
the temperature of heat treatment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The preferred embodiment of the present invention will now
be explained.
[0022] A phase change memory cell according to the present
invention comprises a ternary alloy of R--S--Te which forms a
homogeneous pseudo-binary solid solution with a Ge--Sb--Te
alloy.
[0023] Specifically, the phase change memory cell of the present
invention comprises a composition having the formula
(Ge.sub.ASb.sub.BTe.sub.C).sub.1-x(R.sub.aS.sub.bTe.sub.C).sub.X,
wherein Ge is germanium; Sb is antimony; Te is tellurium; R is an
element selected from the elements belonging to the IVB group in
the periodic table; S is an element selected from the elements
belonging to the VB group in the periodic table; A, B, C, a, b and
c are atomic mole ratios; x is a mole fraction in the range of 0 to
1; R.sub.aS.sub.bTe.sub.C has the same crystal structure as
Ge.sub.ASb.sub.BTe.sub.C; and at least one element of R and S has a
higher atomic number and thus a smaller diatomic bond strength than
that of the corresponding element in the GeSb portion of
Ge--Sb--Te.
[0024] In one embodiment, the Ge--Sb--Te alloy may be a
stoichiometric compound alloy of Ge, Sb and Te, preferably selected
from the group of Ge.sub.4Sb.sub.1Te.sub.5,
Ge.sub.2Sb.sub.2Te.sub.5, Ge.sub.1Sb.sub.2Te.sub.4, and
Ge.sub.1Sb.sub.4Te.sub.7. Therefore, it is prefera that a
combination of A, B and C is selected from the group consisting of
(4, 1, 5), (2, 2, 5), (1, 2, 4) and (1, 4, 7), the combination
being limited to the sequence expressed in parenthesis in that
order.
[0025] In one embodiment, the R--S--Te ternary alloy is
stoichiometrically equivalent to the Ge--Sb--Te alloy.
[0026] A stoichiometric compound alloy tends to have fast kinetics
of an amorphous to crystalline transformation for the following
reasons: the alloy tends to have a high atomic mobility effected by
its large thermodynamic driving force of an amorphous to
crystalline transformation; the alloy tends to crystallize into a
single phase, requiring only short-range atomic reconfiguration
with no need of long-range atomic diffusion indispensable to phase
separation. Therefore, it is preferable that R--S--Te alloy has the
same stoichiometric composition as the compound Ge--Sb--Te alloy so
that an amorphous to crystalline transformation in a solid solution
of R--S--Te and Ge--Sb--Te alloys would proceed rapidly
likewise.
[0027] In addition, R and S are elements belonging to the IVB and
VB group in the periodic table, respectively, and the diatomic bond
strength of at least one of R and S is smaller than that of the
corresponding element of the Ge--Sb--Te portion. J. H. Coombs, et
al. [J. AppI. Phys., 78, 4918(1995)] studied crystallization
kinetics of Ge--Sb--Te alloys in which a part of Ge is replaced
with Sn, or a part of Te is replaced with Sulfur or Se.
[0028] According to these studies, the nucleation kinetics
increases when a part of Ge is replaced with Sn whose single bond
energy is smaller than that of Ge, whereas the nucleation kinetics
decreases when a part of Te is replaced with Sulfur or Se whose
single bond energy is larger than that of Te. From these studies,
it is apparent that the diatomic bond strength of each constituent
element plays an important role in crystallization kinetics.
[0029] Therefore, in order to increase the crystallization
kinetics, it is preferred that at least one of R and S has a higher
atomic number and thus a smaller diatomic bond strength than that
of a corresponding element in the GeSb portion of Ge--Sb--Te alloy.
Thus, R is preferably Sn or Pb, and S is preferably Bi.
[0030] When the crystalline phases of both R.sub.aS.sub.bTe.sub.C
and Ge--Sb--Te alloys have the same space group symmetry with
slightly different lattice parameters, a pseudo-binary solid
solution can form with a complete solubility between two alloys.
Table 1 shows a list of the preferable R.sub.aS.sub.bTe.sub.C
alloys for each of the stoichiometric compound Ge--Sb--Te alloys.
TABLE-US-00001 TABLE 1 Space Group Sym. (Pearson Lattice Parameters
Compound Symbol) (nm) Ge.sub.4Sb.sub.1Te.sub.5 Fm-3m a = 0.6 (cF8)
.sup.1Pb.sub.4Bi.sub.1Te.sub.5(Pb.sub.39Bi.sub.9Te.sub.52) 0.6415
.sup.2Sn.sub.4Bi.sub.1Te.sub.5(Sn.sub.38Bi.sub.12Te.sub.50) 0.63
Ge.sub.2Sb.sub.2Te.sub.5 P-3ml a = 0.42 c = 1.70 (hP9)
Pb.sub.2Bi.sub.2Te.sub.5 0.446 1.75 Ge.sub.1Sb.sub.2Te.sub.4 R-3m a
= 0.421 c = 4.06 (hR7) Ge.sub.1Bi.sub.2Te.sub.4 0.428 3.92
Pb.sub.1Bi.sub.2Te.sub.4 0.416 3.92 Sn.sub.1Bi.sub.2Te.sub.4 0.4411
4.15 Sn.sub.1Sb.sub.2Te.sub.4 0.4294 4.16 Ge.sub.1Sb.sub.4Te.sub.7
P-3ml a = 0.421 c = 2.37 (hP12) Ge.sub.1Bi.sub.4Te.sub.7 0.4352
2.39 .sup.3Pb.sub.1Bi.sub.4Te.sub.7 0.446 2.36
.sup.4Sn.sub.1Bi.sub.4Te.sub.7(Sn.sub.12Bi.sub.38Te.sub.50) 0.4395
2.44 Note: .sup.1(Bi.sub.2Te.sub.3).sub.x(PbTe).sub.1-x, a =
0.64564-0.64151 nm for x = 0-0.1, .sup.2Bi.sub.1-xSn.sub.xTe.sub.1,
a = 0.6300-0.6316 nm for x = 0.75-1, .sup.3three other crystal
structures are also known, .sup.4Bi.sub.1-xSn.sub.xTe.sub.1, a =
0.4448-0.4395 nm, c = 2.427-2.436 nm for x = 0-0.25;
Sn.sub.1Bi.sub.4Te.sub.7 itself has the symmetry of R-3m(hR7).
[0031] According to the present invention, the atomic mole ratios
may deviate from the aforementioned values to the extent that a
solid solution of R--S--Te and Ge--Sb--Te alloys can form a single
crystalline phase, or multiple phases with a predominant
crystalline phase of preferably equal to or more than 90% in
volume.
[0032] R--S--Te alloy sharing the same crystal structure as
Ge--Sb--Te alloy not only accelerates the crystallization of
Ge--Sb--Te based alloy through forming a complete solid solution
alloy therewith but also changes the basic concept of a binary
phase change memory element which has amorphous and crystalline
face-centered-cubic (fcc) states.
[0033] For instance, cell resistance does not return to that of an
original crystalline state (1.sup.st SET value) but rather drops
further to the 2.sup.nd SET value when an electric pulse is applied
to the
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(R.sub.1S.sub.2Te.sub.4).sub.0.2
film in a virgin amorphous state. It was also discovered that the
cell resistance changes reversibly between 1.sup.st SET and
2.sup.nd SET since then.
[0034] Accordingly, data can be stored in different forms of
crystalline phases. Transition of cell resistance from low
conducting crystalline state (fcc) to high conducting crystalline
hexagonal state and vice-versa is very abrupt and fast, and also
each cell resistance value corresponds the resistivity of each
phase. The mechanism is suggested by Dong-ho Ahn et al. (IEEE
electron device letters, Vol. 26, No. 5), which is incorporated
herein by reference.
[0035] Present inventors also discovered that three phases of
amorphous, fcc and hexagonal crystalline states may be accessible
for data storage if the amount of R--S--Te based alloy introduced
is reduced to, e.g.,
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.9(R.sub.1S.sub.2Te.sub.4).sub.0.1.
[0036] Specific aspects of the present invention are further
illustrated through the following Examples, without limiting the
scope thereof.
EXAMPLE 1
[0037] An off-set type phase change memory cell as shown in FIG. 1
was prepared according to the following procedures. 200 nm-thick
SiO.sub.2 film was deposited on silicon substrate. Ti/TiN film, as
a bottom electrode, was deposited thereon at a thickness of 100 nm,
respectively. Next, 100 nm-thick SiO.sub.2 was formed thereon. As
shown in SEM picture of FIG. 2, a contact hole of 70 nm was formed
by electron beam lithography.
[0038] Next, as a phase change material, solid solution of
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(Sn.sub.1Bi.sub.2Te.sub.4).sub.0.2
was deposited in the contact hole by PVD (physical vapor
deposition) at a thickness of 100 nm. As top electrodes, 100
nm-thick TiN and 500 nm-thick Al films were sequentially deposited
on the phase change material.
[0039] FIG. 1 describes a schematic diagram of an off-set type
phase change memory cell including a material in accordance with
the present invention. Transistor part for cell addressing is not
shown in FIG. 1.
[0040] FIG. 3 illustrates sectional SEM picture of a phase change
memory cell including a material in accordance with the present
invention.
EXAMPLE 2
[0041] Procedures in Example 1 were repeated except that a solid
solution of
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.9(Sn.sub.1Bi.sub.2Te.sub.4).sub.0.1
was used instead of
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(Sn.sub.1Bi.sub.2Te.sub.4).sub.0.2.
COMPARATIVE EXAMPLE
[0042] Procedures in Example 1 were repeated except that
Ge.sub.1Sb.sub.2Te.sub.4 alloy was employed as a phase change
material instead of the solid solution.
[0043] Evaluation of DC I-V Characteristics, Resistance and
Operational Speed
[0044] DC I-V characteristics were measured with Agilent 4156C for
the samples prepared in Examples 1, 2 and Comparative Example as
shown in FIGS. 4a, 4b and 4c, respectively.
[0045] In FIGS. 4a to 4c, typical DC I-V curve of the
crystalline-amorphous states was observed when only
Ge.sub.1Sb.sub.2Te.sub.4 was employed (FIG. 4c). For the sample
employing
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(Sn.sub.1Bi.sub.2Te.sub.4).sub.0.2
as the phase change material (Example 1), DC I-V curve revealed
just the features of fcc and hexagonal crystalline states without
negative resistance characteristics of amorphous state (FIG.
4a).
[0046] On the other hand, all three phases of amorphous, fcc and
hexagonal crystalline states were present as shown in the curve of
FIG. 4b when
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.9(Sn.sub.1Bi.sub.2Te.sub.4).sub.-
0.1 was employed as the phase change material (Example 2).
[0047] Further, resistances of the samples were measured with
Agilent 4156C, results being shown in FIGS. 5a, 5b and 5c.
[0048] As shown in FIG. 5c, the resistances of the amorphous and
fcc states of Comparative Example were each 10.sup.6 ohm and
10.sup.4 ohm, showing two orders of magnitude difference.
[0049] On the other hand, the resistances of the fcc and hexagonal
crystalline states of Example 1 were each 10.sup.3 ohm and 10.sup.2
ohm as shown in FIG. 5a. The resistance gap of sample from Example
1, i.e.
(Ge.sub.1Sb.sub.2Te.sub.4).sub.0.8(Sn.sub.1Bi.sub.2Te.sub.4).sub.0.2,
was smaller than Comparative Example where pure
Ge.sub.1Sb.sub.2Te.sub.4 was employed. However, it still has one
order of magnitude, which is large enough to provide
distinguishable binary memory states.
[0050] As shown in FIG. 5b, resistances of the amorphous, fcc and
hexagonal crystalline states of Example 2 were 10.sup.8, 10.sup.4
and 10.sup.2 ohms, respectively. Accordingly, all three states of
Example 2 were distinguishable by two or four orders of
magnitude.
[0051] FIG. 6 demonstrates relationship between SET pulse voltage
characteristics and SET pulse width as measured with Agilent
81110A.
[0052] SET pulse width in FIG. 6 is equivalent to the speed of the
set operation of the device. The duration for the phase change of
Comparative Example was shown to be 100 ns or more while that of
Example 2 was shown to be 20 ns between its amorphous and fcc
states. Duration of fcc-hexagonal phase change of Example 1 was 70
ns.
[0053] FIG. 7 outlines change in sheet resistance with respect to
the temperature of heat treatment as measured with Agilent 4156C.
Sheet resistance drops at phase change temperature. By this
standard, phase change temperatures of Examples 1, 2 and
Comparative Example were determined to be 210, 230 and 110.degree.
C., respectively.
[0054] Test results of Examples are summarized in Table 2 below.
TABLE-US-00002 TABLE 2 Resistance after Speed of phase Phase change
phase change change temperature A* fcc hex A.fwdarw.fcc
fcc.fwdarw.hex A.fwdarw.fcc fcc.fwdarw.hex Example 1 --
.about.10.sup.3 .about.10.sup.2 -- 70 ns -- 210.degree. C. Example
2 .about.10.sup.8 .about.10.sup.4 .about.10.sup.2 20 ns 70 ns --
230.degree. C. Comparative Example .about.10.sup.6 .about.10.sup.4
-- 100 ns .about.10 .mu.s 110.degree. C. Above 300.degree. C. *A
and hex indicate amorphous and crystalline hexagonal states of
Sn.sub.1Bi.sub.2Te.sub.4, respectively.
[0055] In summary, considerable advantages can be expected by
employing solid solution of
(Ge.sub.ASb.sub.BTe.sub.C).sub.1-x(R.sub.aS.sub.bTe.sub.c).sub.x in
a memory cell in accordance with the present invention.
[0056] For instance, the cell of Example 1 is stable against data
corruption since the phase change temperature is higher than that
in a conventional cell by as much as 100.degree. C.
[0057] If the cell of Example 2 is employed, multi-bit information
storage is enabled with three discrete phases available in a single
cell unit. Both cells in Examples 1 and 2 present fast
recording/deleting of data.
[0058] While the invention has been shown and described with
respect to the preferred embodiment, it will be understood by those
skilled in the art that various changes and modification may be
made without departing from the spirit and scope of the invention
as defined in the following claims.
* * * * *