Method of producing a ferroelectric memory and a memory device

Details Method of producing a ferroelectric memory and a memory device Method of producing a ferroelectric memory and a memory device

2001-12-04

2004-06-29

Zarabian, Amir (Department: 2822)

Semiconductor device manufacturing: process

Having magnetic or ferroelectric component

C438S287000, C438S785000

Reexamination Certificate

active

06756236

ABSTRACT:

The present invention refers to a method of producing a ferroelectric memory, a method of storing information on a substrate and to a memory device.
The storage of information is becoming increasingly important with the advent of larger datasets. Computer programs and algorithms have become more complex and larger, and therefore the demand for an optimization of storage space has grown, aiming at a miniaturization of the individual unit in which the information is to be stored. Conventional storage methods rely for example on optically or magnetically readable/writeable media which, however, have certain limitations with respect to the minimum size possible.
Electronic computers have grown more powerful as their basic sub-unit, the transistor, has shrunk during the past forty years. However, limitations imposed by quantum mechanics and fabrication techniques are likely to inhibit further reduction in the minimum size of today's bulk-effect semiconductor transistors. It is projected that conventional devices may not function well as the overall size of the semiconductor transistor is aggressively miniaturized to below approximately 0.1 micrometers (100 nm). In order to continue the miniaturization of the circuit elements down to the nanometer scale, perhaps even to the molecular scale, researchers are investigating several alternatives to the transistor. These new nanometer-scale electronic (nanoelectronic) devices perform both as switches and amplifiers just like today's transistors. However, unlike today's transistors, which operate based on the movement of masses of electrons in bulk matter, the new devices take advantage of quantum mechanical phenomena that emerge on the nanometer scale, including the discreteness of electrons.
A group of materials that have been found to be potentially useful for information storage are ferroelectric materials. Just like with ferromagnetic materials, used for example in conventional audio tapes, ferroelectric materials display a hysteresis behavior with regards to their electrical polarization under the influence of an external electrical field.
Ferroelectric materials are a subgroup of noncentrosymmetric crystalline materials that have a spontaneous electrical polarization, the direction of which can be altered by an external electrical field. The polarization P versus applied electrical field E dependency shows hysteresis behavior, with P having two stable remanent values (+P
r
and −P
r
) when E=0. The (reverse) electrical fields that must be applied to annihilate the existing polarization (P=0) are termed the coercive fields (+E
c
and −E
c
). The term “ferroelectric” was coined because the P-E relation of these materials is very similar to the B-H relation of ferromagnetic materials. This hysteresis behavior is the basis of the use of both kinds of materials in memory devices. Ferroelectrics are also analogous to ferromagnets in that they are characterized by a Curie temperature T
c
(above which they become paraelectrical) and that they have an internal domain structure.
The hysteresis loop, shown in
FIG. 1
, is caused by the existence of permanent electrical dipoles. The curve starts at the origin (P=O) when the material is first produced because the ferroelectric domains are randomly oriented. When the external field is applied, the B
4+
ions become displaced in the direction of the field, and domains that are more favorably aligned with the field grow at the expense of those that are not. This procession results in a rapid and major polarizing effect until a saturation level P
s
(=saturation polarization) is reached, when the polarization vector of most of the domains are aligned with the field (dashed curve in FIG.
1
). Removal of the field at this point eliminates any normal ionic polarization, but the B
4+
ions remain in their field-oriented sites, and a remanent polarization +P
r
is observed at E=O. In order to remove this polarization, it is necessary to apply an opposing field to revert half of the domains to favor the new field direction. That condition occurs when the opposing field reaches the material-specific coercive field −E
c
. Continuation of the field cycle inverts the polarization to another saturation level, and removal of the negative field leaves the remanent polarization −P
r
. Further cycles of the electrical field retrace the original path, creating a continuous hysteresis loop. The initial condition of P=O when E=O can only be again achieved by short-circuiting the capacitor and subjecting it to a temperature above T
c
to generate a new system of random ferroelectric domains.
Some inorganic (ceramic) ferroelectric materials have the perovskite structure, i.e. ABO
3
, with A=a large divalent metal cation and B=a small tetravalent metal cation. Examples include BaTiO
3
, PbTiO
3
and PbZr
x
Ti
1−x
O
3
(PZT). The structure consists of 12 coordinated A
2+
ions on the comers of a cube, octahedral O
2−
ions on the faces, and tetrahedral B
4+
ions in the center (see for example FIG.
2
). The Curie temperature in these materials is associated with a structural transition from regular cubic above T
c
to a distorted tetragonal form below T
c
.
The hysteresis loop typical of ferroelectrical materials is the basis for ferroelectric random access memory (FRAM) devices. As ferroelectric materials possess two stable polarization directions at zero field, they can be used as non-volatile memory elements. The direction of polarization is used to store information, a logical “0” corresponding to one direction and a logical “1” corresponding to the other direction. Generally, the device structure used in FRAM cells is either the ferroelectric field-effect transistor (FEFET) or the ferroelectric capacitor (FECAP). Computer memory devices utilizing the electrooptical properties of ferroelectric materials have also been described [Munroe, M. R.; Snaper, A. A.; Gregory, G. D. (1972) U.S. Pat. No. 3,675,220: “Planar random access ferroelectric computer memory.” and Ogdfen, T. R.; Gookin, D. M. (1988) U.S. Pat. No. 4,731,754: “Erasable optical memory material from a ferroelectric polymer.”].
Ferroelectric memory devices are generally susceptible to three forms of degradation in performance during use:
Electrical fatigue, which is defined as a decrease in the magnitude of the switchable polarization with increasing number of switching cycles.
Imprint failure, which is a polarization-driven field-shift of the hysteresis loop.
Aging, which is an ill-defined term generally indicating a degradation of the ferroelectric properties (capacitance and dielectric loss) with time.
These phenomena apparently derive from relaxation processes occurring at crystal boundaries. Fatigue and imprint are related to charge trapping at domain boundaries.
Recent work at the experimental level has shown that nanometer-scale polarization domains can be created in inorganic ferroelectric thin films using a conductive atomic force microscope (AFM). Domains as small as 30 nm diameter can be formed in a ferroelectric organic thin film on a conductive substrate by applying electrical pulses with an Au-conductive AFM tip. Binary information can be “written” by changing the polarity of the applied electrical pulses and “read” by using piezoelectrical measurements [Matsushige, K., Yamada, H.; Tanaka, H.; Horiuchi, T.; Chen, X. Q. (1998) Nanotechnology 9,208-211: “Nanoscale control and detection of electric dipoles in organic molecules.”]
The miniaturization of electronic components by using particle technology on the nanoscale is a way to circumvent some of the physical limits and expense of conventional methods of fabrication [Goldhaber-Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.; Ellenbogen, J. C. (1997)
Overview of Nanoelectronic Devices
; The MITRE Corporation (http;//www.mitre.org/technology
anotech), and Ellenbogen, J. C. (1998)
A Brief Overview of Nanoelec

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