Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-12-11
2004-06-29
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
Reexamination Certificate
active
06756296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to molecular electronic devices that can be utilized for memory storage, logic circuitry or signal routing. More specifically, the present invention relates to improved methods for making such devices wherein the critical dimensions of the devices are measured in nanometers.
2. Description of Related Art
Molecular electronic devices have been demonstrated to be capable of performing many of the same tasks that are commonly performed by semiconductor (e.g. silicon, gallium arsenide, etc.) devices. These tasks include signal rectification, signal switching and simple logic functions. Such devices are described in: “Molecular Wire Crossbar Logic” (U.S. Ser. No. 09/282,045); “Molecular Wire Crossbar Interconnect” (U.S. Ser. No. 09/280,225); “Demultiplexer for a Molecular Wire Crossbar Network” (U.S. Ser. No. 09/282,049); “Chemically Synthesized and Assembled Electronic Devices” (U.S. Ser. No. 09/292,767); and “Electrically Addressable Volatile and Non-Volatile Molecular Based Switching Devices (U.S. Ser. No. 09/459,246). Molecular electronic devices are also described in U.S. Pat. Nos. 6,128,214 and 6,159,620.
An advantage of molecular electronic devices is that the device performance characteristics originate from molecular properties. This has several implications. First, it means that the devices can potentially scale down in size to nanometer dimensions without significant change in device performance. Second, it also means that the unique electronic properties that can be designed into molecular structures can also be designed into solid state devices. These advantages are not characteristic of semiconductor devices. However, many molecular electronic devices that have been fabricated to date involve fairly awkward device processing steps. As one example of this awkward processing, electrical connections to the molecules are often evaporated through contact shadow masks, meaning that a thin metal foil that has been previously patterned with holes of various shapes is placed in contact with the molecular thin film and metal electrodes are deposited by directing a metal vapor through the open pattern. This technique has serious limitations in terms of the size resolution and complexity of electrode patterns that can be deposited. For example, it is very difficult and expensive to fabricate shadow masks that have patterned features that are smaller than a couple of micrometers in size.
As a second example of the current art that is utilized for molecular electronic devices, a nanometer scale wire is defined using electron beam lithography and great effort is made to fabricate a wire that has a very thin cross-section. The wire is then “broken” in a manner similar to how a fuse is blown. Under appropriate conditions, the broken junction can be designed to have a gap that is of molecular dimensions, so that molecules can be chemically attached to bridge across the junction. Once again, while it is possible to make electrical contact to the molecules in this way, the device processing steps are just awkward, and the performance characteristics of such a device are difficult to reproduce across many devices.
Nevertheless, the above-described procedures have been developed because, while at thin film of organic molecules may have desirable characteristics for electronic device applications, it is also an inherently delicate film. This is because such a film may melt, flow, or be otherwise damaged by low-temperature processing steps such as the spin-coating of a photolithographic resist materials. A set of processing techniques for dealing with such films have not been developed. It would be desirable to provide improved processing techniques that would eliminate the problems associated with existing processing technology and allow the production of molecular electronic devices having electrode patterns with nanometer scale dimensions.
DEFINITIONS. The following definitions apply to the present invention:
“Mol-RAM” in this context refers to molecular-switch based array of memory cells.
“Molecular electronic devices” in this context refers to devices in which some critical component of the device, such as the wire or the switch, is a molecule or a collection of molecules.
A “memory bit” in this context refers to a physical device that can exist in two states (‘0’ or ‘1’) that can be distinguished from one another by electrically probing the device.
“Lithographic processing” in this context refers to any procedure in which light or electron beams are used to produce a chemically or materially differentiated pattern onto a substrate.
A “switch” in this context refers to a physical device that can switch between two states, such as ‘open’ and ‘closed,’ and the difference between the two states can be probed electrically. The difference between the two states of a switch is typically greater than for a memory bit. For example, if the electrical property that is measured to determine the state of the switch is the resistance of the device, then a memory bit may be characterized by a 20% change in resistance, while a switch may be characterized by a 200% change in resistance. A switch can be used as a memory bit, but a memory bit may not necessarily be useful as a switch.
“Self-assembled” in this context refers to a system that naturally adopts some geometric pattern because of the identity of the components of the system; the system achieves at least a local minimum in its energy by adopting this configuration. For example, a self-assembled molecular monolayer is a geometrically arranged monolayer film of molecules that is formed when certain molecules chemically bind to a certain surface. The organization of such a self-assembled monolayer is controlled by both the geometric registry of the molecules with the atomic structure of the underlying surface, as well as the interactions between neighboring molecules in the monolayer.
“Singly configurable” in this context means that a switch can change its state only once via an irreversible process such as oxidation or reduction reaction; such a switch can be the basis of a programmable read only memory (PROM), for example.
“Reconfigurable” in this context means that a switch can change its state multiple times via a reversible process such as an oxidation or reduction; in other words the switch can be opened and closed multiple times, e.g., the memory bits in a random access memory (RAM).
A “crosspoint memory” in this context means a memory circuit that consists of a grid of crossed wires. At each junction of the grid is a memory bit, in which some material, such as switching molecules, are sandwiched between the electrodes. The ‘0’ or ‘1’ state of the memory bit may be set electrically, and that state of the memory bit may be probed electrically. The electrical setting or probing of the bit is carried out by electrically addressing the two wires of the crosspoint memory that form the intersection.
“Redox active” in this context means that a molecule or molecular junction can be electrochemically reduced or oxidized, meaning that electrical charge can be added or taken away from the molecules or molecular junction.
A “wetting film” in this context refers to a film that completely and uniformly covers another film. This term does not imply that the wetting film is liquid, it only refers to how the wetting film coats an underlying substrate. If a top material does not uniformly wet a bottom material, then that top material will instead form islands and patches of coverages.
“Micron-scale dimensions” refers to dimensions that range from 1 micrometer to a few micrometers in size.
“Sub-micron scale dimensions” refers to dimensions that range from 1 micrometer down to 0.04 micrometers.
“Nanometer scale dimensions” refers to dimensions that range from 1 nanometers up to 50 nanometers (0.05 micrometers).
“Micron-scale wires” and “submicron-scale wires” refers to rod or ribbon-shaped conductors of semiconductors with widths or diameters having the dimensions of
Beck Patricia A.
Collier Charles P.
DeIonno Erica
Heath James R.
Luo Yi
California Institute of Technology
Le Thao P.
Nelms David
Shapiro & Dupont LLP
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