Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1997-02-12
2003-03-04
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S390000, C257S529000, C257S537000
Reexamination Certificate
active
06528815
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a write-once read-many electrical memory element which comprises two electrode's between which a layer of a material containing an organic conjugated compound is sandwiched. The invention also relates to the use of a device comprising two electrodes between which a layer containing a conjugated polymer or oligomer as well as a dopant is sandwiched, as a write-once read-many memory element. The invention further relates to an assembly of independently addressable memory elements. The invention also relates to a method of electrically writing information while using such a memory element. The invention finally relates to a method of electrically reading information while using such a memory element.
Write-once read-many electrical memory elements, as well as memories derived therefrom, in particular those in which the memory is an integrated circuit or forms part of an integrated circuit (IC) have been known for a long time. The technology which is customarily used to manufacture such memory elements is based on one crystalline silicon. However, there are applications for which silicon-based IC technology is found to be less suitable, for example “low-end” electronics. “Low-end” electronics is to be understood to mean in this context, electronics which, in terms of performance, does not have to meet high requirements and, consequently, is low in price. An example of such a “low-end” application is electronics intended for one-time use, such as an electronic bar code. An element which could suitably be used in such integrated circuits is a write-once read-many electrical memory element based on an organic semiconductive material.
Such an organic memory element is known per se from U.S. Pat. No. 4,371,883. This known memory element, referred to in this patent as “memory switch”, is built up of a metallic substrate of, preferably, silver or copper, which substrate serves as a first electrode carrying a layer of a deposited stoichiometric metal complex of an organic conjugated electron acceptor, such as tetracyanoquinodimethane or tetracyanoethylene, which layer is in contact with a second electrode. If a sufficiently high electric field is applied, then the resistance of the layer changes. As this change in resistance remains intact for some time after the electric field has been removed, this known element is referred to as a “memory switch”. However, this known memory element has a number of disadvantages. One of the disadvantages is that the change in resistance remains in tact for maximally a couple of days. Another disadvantage is that the choice of the electrode material is limited because at least one of the electrodes must be made from a metal which complexes with the organic compound, such as silver or copper. The other electrode material cannot be freely selected either because it determines the function of the device, i.e. a diode or a memory element. In addition, the materials of the two electrodes are different, which renders the manufacturing process laborious and expensive. Moreover, the known organic layer is polycrystalline and of low molecular weight. In general, such layers are brittle, so that they are less suitable to form memories having large surfaces on flexible substrates. Besides, in general the properties of devices employing organic polycrystalline layers are poorly reproducible.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a write-once read-many memory element which does not have the above drawbacks. The invention specifically aims at a memory element which can suitably be used in an integrated circuit and in which the information remains intact for at least one month. The writing of the information should take place in a non-destructive manner so that damage to any other elements of the circuit of which the memory element could form part is precluded. It should be possible to choose and, if necessary, manufacture the electrode material and the organic layer independently. If necessary, it should be possible to make both electrodes from the same material. Further, the memory element should be stable under ambient conditions. The organic layer should be amorphous and it should be possible to provide said layer by means of simple, inexpensive techniques which are suitable for integration. An example of such a technique is spin coating.
These and other objects are achieved by a memory element of the type mentioned in the opening paragraph, which is characterized in accordance with the invention in that the layer comprises a dopant, the material is soluble, the compound is a polymer or an oligomer, and the electroconductivity of the layer in a written state is permanently lower than in an unwritten state.
As will be described in greater detail hereinbelow, it has been found that a difference in electroconductivity can be brought about by applying a suitable voltage between the electrodes. If the memory element has been subjected to such a voltage, then it is in the written state. If it has not been subjected to such a voltage, then it is in the unwritten state. A degree of conductivity in the written state, which is a factor of ten to one million lower than in the unwritten state can be easily achieved. Surprisingly, it has been found that this low conductivity remains substantially in tact for at least a month, even after the voltage is removed. The low conductivity cannot be undone by applying a voltage of comparable magnitude yet opposite polarity. Optical and electron-microscopic recordings show that the difference in electroconductivity is not accompanied by any difference in morphology of the layer, such as a phase separation or (re-)crystallization. Nor does the change lead to a loss of material from the layer or from parts of the layer. In this respect, the layer has not changed and the intended difference in conductivity has been achieved in a non-destructive manner, so that it is precluded that, if the memory element forms part of a larger assembly, other parts become damaged. The memory element proves to be stable under ambient conditions. Special measures for excluding moisture or oxygen proved unnecessary. The layer is amorphous both in the written and the unwritten state. It has also been found that in the voltage range from 0 V to approximately 1 V, the current voltage characteristic of the memory element is ohmic both in the written and the unwritten state, irrespective of the polarity of the applied voltage.
A number of different variants of the memory element can be manufactured in a simple manner. For example, a memory element can be produced in which the electrodes are coplanar and may be manufactured, if desired, from the same material. In this case, use is made of an electrically insulating substrate, such as quartz, glass or a non-conductive synthetic resin, on which the electrodes are provided in accordance with a pattern by means of known techniques, such as sputtering, CVD or vacuum deposition. Dependent upon the required dimensions, the patterns can be formed by means of known methods such as lithography, screen printing or other graphic techniques. The choice of the electrode material is not essential and, inter alia, gold, aluminum, platinum, copper or indium-tin oxide may be selected. Also conductive polymers, such as highly conductive polyaniline can suitably be used as the electrode material. This has the advantage that also the electrodes can be provided by means of simple methods, such as spin coating. Subsequently, the layer of the soluble material containing the conjugated polymer or oligomer and a dopant is provided thereon. This layer can be provided in a simple manner by means of known techniques, such as spin coating, dip coating or spray coating. Apart from wetting, the applicability of such techniques is substantially independent of the properties of the underlying layer. Layer thicknesses of approximately 30 nm to approximately 10 micrometer can be obtained in a reproducible manner, provided that the layer to be provided
Brown Adam R.
De Leeuw Dagobert M.
Havinga Edsko E.
Jarrett Colin P.
Barlett Ernestine C.
Jackson Jerome
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