Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-04-16
2002-04-09
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S334000, C428S338000, C428S339000, C428S458000, C428S469000, C428S472000
Reexamination Certificate
active
06368705
ABSTRACT:
BACKGROUND
This invention relates to metal-insulator-metal diodes.
A metal-insulator-metal diode (M-I-M) diode exhibits current-voltage characteristics similar to a semiconductor diode. One distinction from semiconductor diode current-voltage characteristics is that the current-voltage characteristics of a M-I-M diode are symmetrical. Typical the current-voltage characteristic curve of an M-I-M diode is bipolar. That is, at a predetermined negative voltage and substantially the same predetermined positive voltage, the M-I-M diode can switch between non-conducting and conducting states.
One type of metal-insulator-metal diode includes a composite metal-insulating layer that spaces two conductive layers, i.e., electrodes. The composite metal/insulating layer has an insulating binder, suspending metal particles having a relatively thick, thermally grown or deposited oxide layer on the metal particles. In a thesis of Jaeyong Park entitled “All Printed Bistable Reflective Displays: Printable Electrophoretic Ink and All Printed Metal-Insulator-Metal Diodes” Massachusetts Institute of Technology June 1998, two types of processes for producing such metal-insulator-metal diodes are described. The thesis describes a thermal annealing process and an anodized process.
In thermal annealing, a thick deliberately deposited tantalum oxide layer is formed about the tantalum particles. An ink is produced by incorporating the tantalum particles with the deliberately grown oxide into a binder. The tantalum oxide powder is prepared, prior to mixing with the binder by thermally annealing tantalum powder in an oven for a period of time at a temperature between 250 and 400° C. in an oxidizing atmosphere. Once the powder is oxidized it is mixed with a binder and screen printed onto a copper foil substrate. That assembly is dried and after cooling, a chromium ink layer is placed on the tantalum-binder ink layer through screen printing.
In the anodized process, a tantalum powder is mixed with a polymer binder and printed on top of the copper surface. This assembly is placed in an electrolyte solution to anodize i.e., grow an anodic oxide layer over the exposed surfaces of the tantalum material, that is, those portions of the tantalum that are not submerged in the polymer binder. A chromium layer is printed on top of the anodic oxide.
The current-voltage characteristic curves of either device provide a distinctive symmetrical diode response, where the switching voltages of the response is dependent upon the oxide thickness, how long it is heated and at what temperature it is heated to in the oven. Generally the reported switching voltages are in the range of 50 to 80 volts.
SUMMARY
The switching voltage characteristic of known M-I-M devices is too high for many applications that require low switching voltages. The diodes described herein can achieve switching voltages less than those of the prior art. In particular, the diodes can achieve switching voltages less than volts, generally less than 2 volts, and in particular less than 1 volt down to 0.5 volts or so.
According to an aspect of the invention, a metal insulator device includes a conductive layer and a metal-insulator layer of particles of a metal that forms a surface layer of an intrinsic oxide when exposed to an ambient of oxygen. The particles having the intrinsic oxide are suspended in a dielectric binder.
The device can also includes a second conductive layer disposed to be in direct contact with the metal-insulator layer. The particles can be any metal that form oxides that are self-limiting, stable, and have a suitable dielectric constant for the application. Examples of metals include tantalum and niobium.
According to a further aspect of the invention, a metal insulator diode device includes a conductive layer and a metal-insulator layer comprising particles of a metal that forms an intrinsic oxide surface layer over the particles when exposed to an ambient of oxygen, with the particles suspended in a dielectric binder vehicle. The device also includes a second conductive layer disposed to be in direct contact with the insulator metal layer, with said insulator metal further comprising a dielectric filler material.
According to a still further aspect of the invention a method of fabricating a metal-insulator-metal device includes printing over a first conductive layer, a layer comprised of a binder and tantalum particles having oxide surface layers with a thickness in a range of one to several, e.g., 10 or so monolayers.
One or more of the following advantages may be provided by one or more aspects of the invention. Using an intrinsic oxide coating for the tantalum metal provides an oxide layer of relatively uniform thickness. Bulk thermal oxidation of the tantalum can be very inconsistent producing a very surface oriented device having electrical characteristics that may be difficult to predict or control. In a bulk sample, tantalum on the surface may be oxidized more than the tantalum material on the bottom of the sample.
The intrinsic oxide is easier to control and will not go all the way through to 100% oxidation which would yield 100% tantalum oxide powder. The combination of the metal particles having intrinsic oxide and the binder also avoids potential formation of pinholes in the oxide layer which is a problem for conventional M-I-M diodes. Pinholes in the tantalum oxide layer can cause shorting of a diode having the tantalum oxide layer. The tantalum oxide layer is uniform allowing excellent contact to conductive electrodes.
Typically, in conventional M-I-M diode devices, a thick oxide is deposited on the first conductor to minimize pinhole formation. This thick oxide provides diodes that switch at the relatively high switching voltages of 50 to 80 volts mentioned above. The arrangement of the present diode allows for much lower switching voltages than those of the prior art.
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patent: WO 99/05745 (1999-04-01), None
“All Printed Bistable Reflective Displays: Printable Electrophoretic Ink and All Printed Metal-Insulator-Metal Diodes”, Massachusetts Institute of Technology, 6/98, pp. 1-19.
“Electrophoretic Displays”, J.C. Lewis, 1976, pp. 223-240, No Month.
“Electrophoretic Displays”, A.L. Dalisa, pp. 213-232, No Date.
“The Reinvention of Paper”, Scientific American, 9/98, pp. 36, 40.
“An Electrophoretic Ink for All-Printed Reflective Electronic Displays”, Comiskey et al., Nature, vol. 394, Jul. 16, 1998; pp. 253-255.
“Electronic Ink: A printable display system”, Comiskey et al., 6/97, pp. 1-3.
“Easy Reader”, J. Wilson, Popular Mechanics, Nov. 1998, pp. 94-96, 98.
Gordon Eric S.
Lynch Anne T.
Jones Deborah
Stein Stephen
The Gillette Company
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