Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2001-05-19
2002-03-19
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S469000, C438S240000, C438S210000, C438S758000
Reexamination Certificate
active
06358758
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to thin film materials for use in integrated circuits and, more particularly, ferroelectric materials for use in integrated memory circuits. More specifically, the thin film ferroelectric materials are layered superlattice materials that exhibit a low degree of imprinting and polarization fatigue after many repetitions of unidirectional voltage pulses.
2. Statement of the Problem
It is well known that thin film ferroelectric materials may be used in a variety of nonvolatile random access memory devices. For example, U.S. Pat. No. 5,600,587 issued to Koike teaches a ferroelectric nonvolatile random access memory using memory cells consisting of a ferroelectric capacitor and a switching transistor. U.S. Pat. No. 5,495,438 issued to Omura teaches a ferroelectric memory that is formed of ferroelectric capacitors connected in parallel. The capacitors have ferroelectric materials of different coercive field values and, consequently, can use or store multi-value data. U.S. Pat. No. 5,592,409 issued to Nishimura, et al., teaches a nonvolatile memory including a ferroelectric layer that is polarized by the impressed voltage between two gates. The polarization or memory storage state is read as a high or low current flow across the ferroelectric layer, which permits nondestructive readout. U.S. Pat. No. 5,539,279 issued to Takeuchi, et al., teaches a high speed one-transistor-one-capacitor ferroelectric memory that switches between two modes of operation including a dynamic random access memory (“DRAM”) mode and a ferroelectric random access memory (“FERAM”) mode.
FIG. 1
depicts an ideal polarization hysteresis curve
100
for ferroelectric thin films. Side
102
of curve
100
is produced by measuring the charge on a ferroelectric capacitor while changing the applied field E from a positive value to a negative value. Side
104
of curve
100
is produced by measuring the charge on the ferroelectric capacitor while changing the applied field E from a negative value to a positive value. The points −E
c
and E
c
are conventionally referred to as the coercive field that is required to bring polarization P to zero. Similarly, the remnant polarization Pr or -Pr is the polarization in the ferroelectric material at a zero field value. The Pr and -Pr values ideally have the same magnitude, but the values are most often different in practice. Thus, polarization measured as 2Pr is calculated by adding the absolute values of the actual Pr and -Pr values even though these values may differ in magnitude. The spontaneous polarization values Ps and -Ps are measured by extrapolating a linear distal end of the hysteresis loop, e.g., end
106
, to intersect the polarization axis. In an ideal ferroelectric, Ps equals Pr, but these values differ in actual ferroelectrics due to linear dielectric and nonlinear ferroelectric behavior. A large, boxy, substantially rectangular central region
108
shows suitability for use as a memory by its wide separation between curves
102
and
104
with respect to both coercive field and polarization.
Presently available ferroelectric materials depart from the ideal hysteresis shown in FIG.
1
. Researchers have investigated materials for use in integrated ferroelectric devices since the 1970's, but these investigations have not yet been commercially successful due to their departures from the ideal hysteresis. For example, U.S. Pat. No. 3,939,292 issued to Rohrer reports early studies of ferroelectric materials for use in ferroelectric memories were performed on Phase III potassium nitrate. In practice, potassium nitrate materials have such low polarizabilities and are so badly afflicted by fatigue and imprint that the materials are practically useless in microelectronic memories.
It is difficult to find ferroelectrics that meet certain commercial requirements. The best materials for integrated ferroelectric devices are switched using a coercive field that can be obtained from conventional integrated circuit operating voltages, i.e., three to five volts. The materials should have a very high polarization, e.g., one exceeding twelve to fifteen &mgr;C/cm
2
determined as 2Pr, to permit the construction of memories having sufficient densities. Polarization fatigue should be very low or nonexistent. Furthermore, the ferroelectric material should not imprint; i.e., the hysteresis curve should not shift to favor a positive or negative coercive field.
FIG. 2
depicts a hysteresis curve
100
next to curve
200
. Curve
200
shows the effect of fatigue on curve
100
. Fatigue reduces the separation between curves
102
and
104
defining central region
108
. Central region
108
progressively becomes smaller and smaller with additional fatigue. This change in separation is due to the creation of point charge defects arising in the ferroelectric material as a consequence of polarization switching and the associated screening effect of the defects on the applied field. Thus, fatigue causes the ferroelectric material to wear out over time due to repeated polarization switching.
U.S. Pat. No. 5,519,234 issued to Araujo, et al., teaches that the fatigue problem of curve
200
is substantially overcome by the use of layered superlattice compounds, such as the “layered perovskite-like” materials described in Smolenskii, et al., “Ferroelectrics and Related Materials,” Gordon and Breach (1984). The layered superlattice compounds are capable of providing a thin film ferroelectric material wherein the polarization state may be switched up to at least 10
9
times with less than thirty percent fatigue. This level of fatigue endurance provides a significant advance in the art because it is at least an order of magnitude better than the fatigue endurance of other ferroelectrics, e.g., lead zirconium titanate (“PZT”) or lead lanthanum zirconium titanate (“PLZT”).
According to Section 15.3 of the Smolenskii book, the layered perovskite-like materials or layered superlattice compounds are of three general types:
(I) compounds having the formula A
m-1
S
2
M
m
O
3m+3
, where A=Bi
3+
, Ba
2+
, Sr
2+
, Ca
2+
, Pb
2+
, K
+
, Na
+
and other ions of comparable size; S=Bi
3+
; and M=Ti
4+
, Nb
5+
, Ta
5+
, Mo
6+
, W
6+
, Fe
3+
and other ions that occupy oxygen octahedra;
(II) compounds having the formula A
m+1
M
m
O
3m+3
, including compounds such as strontium titanates Sr
2
TiO
4
, Sr
3
Ti
2
O
7
and Sr
4
Ti
3
O
10
; and
(III) compounds having the formula A
m
M
m
O
3m+2
, including compounds such as Sr
2
Nb
2
O
7
, La
2
Ti
2
O
7
, Sr
5
TiNb
4
O
17
, and Sr
6
Ti
2
Nb
4
O
20
.
Smolenskii pointed out that the perovskite-like layers may have different thicknesses, depending on the value of m, and that the perovskite AMO
3
is, in principal, the limiting example of any type of layered perovskite-like structure with m=infinity. Smolenskii also noted that if the layer with minimum thickness (m=1) is denoted by P and the bismuth-oxygen layer is denoted by B, then the type I compounds may be described as . . . BP
m
BP
m
. . . Further, Smolenskii noted that if m is a fractional number, then the lattice contains perovskite-like layers of various thicknesses, and that all the known type I compounds are ferroelectrics.
According to the invention, the layered superlattice materials may be summarized more generally under the formula:
A
1
w1
+a1
A
2
w2
+a2
. . . Aj
wj
+aj
S
1
x1
+s1
S
2
x2
+s2
. . . Sk
xk
+sk
B
1
y1
+b2
B
2
y2
+b2
. . . Bl
yl
+bl
Q
z
−2
, (1)
where A
1
, A
2
. . . Aj represent A-site elements in the perovskite-like structure, which may be elements such as strontium, calcium, barium, bismuth, lead, and others; S
1
, S
2
. . . Sk represent superlattice generator (“S-site”) elements, which usually is bismuth, but can also be materials such as yttrium, scandium, lanthanum, antimony, chromium, thallium, and othe
Arita Koji
Cuchiaro Joseph D.
Hayashi Shinichiro
Paz de Araujo Carlos A.
Anya Igwe
Patton & Boggs LLP
Smith Matthew
Symetrix Corporation
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