Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-05-31
2003-09-02
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C365S153000
Reexamination Certificate
active
06614048
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention lies in the memory technology field. More specifically, the invention the use of molecular or polymeric layers as memory elements and further relates to a memory cell comprising at least two different molecular or polymeric layers which form an electrochemical redox pair, a memory array comprising such memory cells, and a smart card comprising the memory array.
Various nonvolatile memory elements are known which are based on different physical principles. In addition to various advantages such as fast writing and reading times, the memory elements also have disadvantages, however, so that specific memory elements represent the best current solution possibility in each case only for a specific application. Thus, by way of example, SRAMs (synchronous RAMs) require a relatively large amount of space on a silicon substrate, since at least two transistors are required for each bit. In FLASH elements, relatively high voltages are required since charges have to be shot through a barrier. Other memory elements impose extreme requirements on production. Thus, during the fabrication of FRAMs (ferromagnetic RAMs), very high process temperatures of in excess of 700° C. are reached when ferromagnetic ceramics are deposited on the preprocessed silicon wafer. During the fabrication of MRAMs (magnetic RAMs), extremely thin magnetic layer sequences have to be fabricated, and the layer thickness has to be realized with a deviation of less than 0.1 nm. Furthermore, in magnetic memory elements, the memory content can be erased by external magnetic fields.
In addition to the above-described memory elements based on semiconductors, memories based on polymers or special molecules have also been proposed. One concept is concerned with very complex molecules that can assume two different states which are associated with an intramolecular charge flow. Such memory cells can be electrically addressed and also electrically read. (C. P. Collier et al., Science Vol. 285, 391 (1999) “Electronically Configurable Molecular-Based Logic Gate”.) However, they presuppose the synthesis of highly complex molecules which generally have only a limited thermal stability and a limited number of write cycles, since secondary reactions are also possible owing to the complexity of the molecules.
U.S. Pat. No. 5,883,397 and German published patent application DE 44 23 782 A1 disclose a plastic functional element comprising a lower electrode and a transparent upper electrode. Arranged between the two electrodes there is in each case a first membrane made of a first oxidation-reduction material and a second membrane made of a second oxidation-reduction material. A contact area is formed between the two membranes and the contact areas of the electrodes are in each case arranged parallel to the contact area formed between the two membranes. The oxidation-reduction potentials of the two oxidation-reduction materials differ from one another, so that the oxidation state of at least one of the oxidation-reduction materials can be changed by irradiating the contact area formed between the membranes with light or by applying a voltage to the electrodes. This enables storage of information.
In the case of the configuration described in U.S. Pat. No. 5,883,397 and DE 44 23 782 A1 the potential difference between the two oxidation-reduction materials acts as a driving force for the electron transition. Depending on the sign of the potential difference, it is therefore possible with finite probability for an electron transition to take place between the two oxidation-reduction materials even without the presence of an external voltage at the electrodes. Therefore, if the memory cell is stored over a relatively long period of time, the information stored in the memory cell can be lost again, since the oxidation state of the two oxidation-reduction materials undergoes transition again to the thermodynamically most favorable state over a relatively long period of time.
U.S. Pat. No. 4,902,555 describes a hybrid circuit element having an electrical redox element made of redox material and a semiconductor element which is connected to the electrical redox element. The electrical redox element and the semiconductor element are arranged on a common substrate and connected to one another by electrical conductors. By way of example, a biomaterial or a pseudo-biomaterial is used as redox material. In the memory element in U.S. Pat. No. 4,902,555, individual molecules or monomolecular layers are used as materials which can be reversibly oxidized or reduced. Although such layers enable rapid oxidation or reduction of the materials, it is nonetheless only with very great difficulty that electrical contact can be made with these layers, so that these memory elements can only be fabricated in a complicated fashion and with high costs. Moreover, with such a configuration, there is the risk of the redox materials spontaneously undergoing transition to the thermodynamically most favorable state, in a manner dependent on the potential difference, by an electron transition taking place between the different redox materials. This ultimately leads to a loss of stored information if the memory content is not repeatedly refreshed over relatively long periods of time.
The above-described memory elements are either still at the development stage or they are comparatively complicated to fabricate. However, for specific applications, such as e.g. in smart cards, nonvolatile memories are required which can be fabricated at low cost and are comparatively robust in respect of handling. Thus, the memory elements should not require high voltages and should not be able to be overwritten by external magnetic fields. Moreover, the stored information should not be lost even if the smart card is stored over a relatively long period of time. In the case of application in smart cards, somewhat longer access times than usual can also be tolerated, that is to say access times in the range of microseconds instead of nanoseconds.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a useful implementation of molecule or polymer layers as memory elements, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provide for a memory element which can be fabricated simply and cost-effectively, which requires low voltages for writing and read-out, in which an item of information can be stored even over relatively long periods of time, and which cannot be overwritten by external magnetic fields.
With the foregoing and other objects in view there is provided, in accordance with the invention, a memory cell, comprising:
a first layer of electrically conductive material;
a second layer disposed on and electrically connected to the first layer, the second layer containing a first chemical compound adapted to be reversibly converted from a reduced form to an oxidized form;
a third layer disposed on the second layer and containing a second chemical compound adapted to be reversibly converted from an oxidized form to a reduced form; and
a fourth layer of electrically conductive material disposed on and electrically connected to the third layer;
wherein at least one of the second layer and the third layer contain groups acting as proton donors or as proton acceptors.
With the above and other objects in view there is also provided, in accordance with the invention, a memory array, comprising:
a first layer constructed of mutually parallel interconnects;
a second layer disposed on and electrically connected to the first layer, the second layer containing a first chemical compound adapted to be reversibly converted from a reduced form to an oxidized form;
a third layer disposed on the second layer and containing a second chemical compound adapted to be reversibly converted from an oxidized form to a reduced form; and
a fourth layer disposed on and electrically connected to the third layer, the fourth layer being constructed of mutually p
Crane Sara
Locher Ralph E.
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