Patent Application
20080152792
The present invention relates generally to methods of manufacturing switching devices having an organic switching layer, and to methods of manufacturing arrays of microelectronic switches.
The present invention is related to attorney's docket number CML03613T, Bistable Microelectronic Switch Stack, filed on even date herewith and having common assignee.
Organic devices promise to revolutionize the extent of, and access to, electronics by providing extremely inexpensive and lightweight components that can be fabricated onto plastic, glass or metal sheets. Data storage is a basic necessity for large area, flexible, electronic assemblies. There has been research in the area of organic printed memory for high density and low cost devices, but most efforts have been focused on using silicon-based technology and processes. Prior art processes generally all require a high precision deposition process to apply the organic molecules and the semiconducting polymer on top of each electrode. This is a limiting step in the process of low cost memory devices. The use of nanotechnology and associated nanomaterials and molecules are good alternate candidates for this application, but the processability and cost of the nanomaterials presents a significant challenge. A simplified, low cost alternative to these prior art techniques would be a significant addition to the art.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method and apparatus components related to manufacturing a stack for a bistable microelectronic switch. Accordingly, the apparatus components and methods have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional elements, materials, or processes, that, combined in a novel manner, provide a manufacturing method for a bistable microelectronic switch described herein. Thus, methods and means for these functions have also been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such stacks with minimal experimentation.
A stack for a bistable microelectronic switch is fabricated by providing a first metal electrode on a supporting substrate. A bistable macrocyclic compound, such selected from the porphyrin family, is printed over the first electrode using a high speed printing process. An electrically conducting material, for example a conductive polymer is then printed over the bistable porphyrin compound using a high speed printing process, and a second electrode is then created on the conductive polymer, by printing or other conventional means. One embodiment of the switch prints a layer of copper phthalocyanine as the bistable compound, and prints a layer of poly-(3,4-ethylenedioxythiophene) and poly-(styrenesulphonic acid) as the conductive polymer. One application of the switch is to form an array of switches to create a random access memory device. When a voltage less than a switching voltage is applied between the two electrodes, the resistance is very high, and when a voltage greater than the switching voltage is applied, the resistance is generally two orders of magnitude lower.
Referring now to
The conducting polymer 140 is then printed 235 on the organic bistable layer 130 using a high speed printing process, as described above for printing the layer 130. The type of process used can be the same, or it can be a different technique, as dictated by manufacturing and material concerns. Some examples of suitable printing processes are screen printing, gravure printing, offset printing, ink jetting, and flexography. As previously described, the conducting polymer to be printed is an admixture in a liquid state, such as a solution of the desired material in one or more solvents, or a suspension or slurry of the material particles in a liquid, and generally needs a finite period of time to dry 245, depending on the amount and type of carrier solvents used. In one embodiment, the conducting polymer 140 is printed in a continuous sheet over the bistable layer 130, and in another embodiment, the conducting polymer is printed as one or more discrete areas in a pattern. We find that a preferred conducting polymer is a combination of poly-(3,4-ethylenedioxythiophene) and poly-(styrenesulphonic acid), also known as PEDOT:PSS.
Overlying the conductive polymer layer 140 is a top electrode 110. In one embodiment, the electrode 110 is printed 255 on the conductive polymer layer using an ink having silver, carbon, carbon nanotube, copper, gold or aluminum filler as the conductive material. In another embodiment, the electrode 110 is formed 255 by vacuum depositing one or more metals such as silver, carbon, carbon nanotube, copper, gold, or aluminum.
When a voltage less than a switching voltage, defined herein as between about 1.5 volts and 2 volts, is applied between (i.e. across) the electrodes 110, 120, the impedance, and therefore the amount of current that can be conducted between the electrodes, is very low. At voltages between zero and the switching voltage, the impedance remains relatively constant until, at the switching voltage, the bistable macrocyclic compound undergoes a reversible electrochemical redox reaction and the impedance changes significantly. We have observed impedance changes from about two orders of magnitude (100 times) to about 4 orders of magnitude (10,000 times). The amount of current that can be conducted across the two electrodes increases in a step function manner by multiple orders of magnitude at the switching voltage, then proceeds to climb in a linear fashion as the voltage is increased further.
Below the switching voltage, the stack 100 behaves electrically like a diode, that is, essentially non-conducting. Once the switching voltage is reached, the stack changes and now behaves electrically like a fixed resistor, where the amount of current that can be conducted is a direct function of the voltage, per Ohm's Law. Once the voltage field is removed, the stack remains at the “switched” behavior in a resistive state. That is, the bistable macrocyclic compound does not revert, until a lower voltage is presented.
Referring now to
An organic bistable layer 330 is then printed over the array of electrodes, and optionally, on portions of the substrate, using a high speed printing process such as screen printing, gravure printing, offset printing, ink jetting, and flexography. During the printing process, the material to be printed is an admixture in a liquid state, such as a solution of the desired material in one or more solvents, or a suspension or slurry of the material particles in a liquid. Generally, the printed liquid material needs a finite period of time to dry, depending on the amount and type of carrier solvents used. In one embodiment, the layer 330 is printed in a continuous sheet over the electrodes, so that it is common to each of the individual electrodes in the array. In another embodiment, the layer is printed as one or more discrete areas in a pattern. The organic bistable layer 330 is a macrocyclic compound, generally of the porphyrin family. We find that 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) and salts thereof are particularly effective.
A layer of conducting polymer 340, such as PEDOT:PSS, is deposited on top of the organic bistable material 330 using for example, a high speed printing process, as described above. In one embodiment, the conducting polymer 330 is printed in a continuous sheet over the bistable layer, and in another embodiment, the conducting polymer is printed as one or more discrete areas in a pattern.
A top or upper array of electrodes 310 is then formed on the conducting polymer 340. In one embodiment, the electrodes 310 are printed on the conductive polymer layer 340 using an ink having silver, carbon, carbon nanotube, copper, gold or aluminum filler as the conductive material. In another embodiment, the electrodes 310 are formed by vacuum depositing one or more metals such as silver, carbon, carbon nanotube, copper, gold, or aluminum. In the case where the bottom electrodes 320 are arranged in a one dimensional array, (i.e. a series of parallel lines or strips), the top electrode array is, preferably, likewise a one dimensional array, with the lines situated orthogonally to the bottom electrodes. Optionally, one could arrange the lines at other angles that are not right angles. This “crossbar” arrangement provides a matrix of uniquely addressable locations at the intersection of each upper and lower electrode. Due to a donor-acceptor charge transfer mechanism, an anisotropic conduction path where a lower electrode intersects an upper electrode is created via a “most preferred” path. That is, the conductive path is vertical between the electrodes at the intersection, and does not cross horizontally or at an angle to another adjacent electrode on either layer. Devices built in this manner with common bistable layer 330 and common conducting polymer layer 340 were subjected to writing cycles at locations “A” and “B” in
In summary, a method of manufacturing a bistable microelectronic switch uses high speed printing processes to print a layer of a porphyrin compound and a conductive polymer that are sandwiched between two electrodes. When a voltage greater than zero and less than about 2 volts is applied between the first electrode and the second electrode, the resistance across the two electrodes is very high, and when a voltage of greater than about 2 volts is applied, the resistance is generally two orders of magnitude lower. It can be used in arrays to form a memory device.
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
