Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating selected area
Reexamination Certificate
1998-12-03
2001-03-27
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating selected area
C205S123000, C205S114000, C205S118000, C205S136000, C205S147000, C205S317000
Reexamination Certificate
active
06207034
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to a method of manufacturing polymer transistors using electrochemical scanning tunneling microscopy techniques.
BACKGROUND OF THE INVENTION
The operation of a polymer transistor is described with reference to FIG.
1
. The transistor, designated generally by numeral
10
, has opposing conductors, namely source
12
and drain
14
, separated by a junction region
16
of conducting polymer.
Conducting polymers feature a conjugated carbon backbone. Some common conducting polymers are polyaniline, polypyrrole and polyacetylene. These materials are semi-conductors. However, upon oxidation or reduction of the polymer, conductivity is changed. The oxidation or reduction leads to a charge imbalance which, in turn, results in a flow of ions into the material in order to balance the charge. These ions or dopants enter the polymer from a surrounding, ionically conductive medium, such as a gel, a solid electrolyte or a liquid electrolyte. If ions are already present in the polymer, they may exit when it is oxidized or reduced.
The electrical resistance of junction
16
is a function of the oxidation state of the conducting polymer. Polymer
16
is immersed in an electrochemical solution
18
, or gel electrolyte, containing mobile ions, and the oxidation state of junction
16
is varied by changing the potential applied to a gate electrode
19
in solution
18
to drive ions into or out of the junction
16
. The state of higher conductivity (or, equivalently, lower resistivity) of the junction
16
is referred to as the ‘ON’ state, while the state of lower conductivity (or higher resistivity) is referred to as the ‘OFF’ state. Of course, description in terms of binary states is merely a descriptive convenience and more states of intermediate conductivity may be defined or employed.
Relevant characteristics of a polymer transistor include the junction resistance in both the low- and high-resistance states, switching speed of the transistor and the amount of charge needed to switch the resistance of the device. It is desirable to provide a method for manufacturing a polymer transistor which allows a small junction size to be fabricated controllably and reliably, so as to decrease resistance in the ON state (thereby minimizing energy loss in the transistor), increase switching rates, and decrease the amount of charge needed for switching. Current techniques provide junctions having widths no smaller than on the order of 50 nanometers. Polymer transistors are currently fabricated using lithographic processes such as described by E. Paul, et al., “Resistance of Polyaniline Films as a Function of Electrochemical Potential and the Fabrication of Polyaniline-Based Microelectronic Devices,” 89
J. Phys. Chem.,
1441-47 (1985), which is incorporated herein by reference. C. Kranz, et al. in “Lateral Deposition of Polypyrrole Lines over Insulating Gaps,”
Advanced Materials
v.7, n.6, 568-71 (1995) describes employing a scanning microscopy tip to electrochemically deposit lines of polypyrrole on a two-dimensional surface and creating a transistor by connecting the lines. Junction or gap dimensions are not controllable with this method.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, in a preferred embodiment, there is provided a method of manufacturing a polymer transistor. The method has the steps of positioning a conducting tip proximate to a conducting surface so as to form a gap, introducing an electrochemical medium in contact with the conducting tip and the conducting surface, and applying an electrical potential across the electrochemical solution so as to deposit a conductive polymer that electrically bridges the gap. The surface may be non-planar or may be a second conducting tip. The electrochemical medium may be an electrochemical solution or a gel electrolyte. In accordance with an alternate embodiment of the invention, an additional step of immersing the conductive polymer in an electrolyte solution is included. The step of positioning, may in other embodiments, include monitoring presence of tunneling current between the conducting tip and the conducting surface, and, alternatively, include monitoring a position of electrical contact between the tip and the surface. The step of applying an electrical potential may include depositing a conductive polymer on both the conducting tip and the conducting surface. In accordance with another embodiment, the applying step may include maintaining an electrochemical potential difference between the tip and the surface. The method may, additionally, have the step of immersing a counter electrode in the electrochemical solution, in which case the step of applying an electrical potential includes maintaining the conducting tip and the conducting surface at the same electrochemical potential and providing a return path for current through the counter electrode.
In accordance with a further, alternate embodiment of the present invention, the step of applying an electrical potential may include maintaining an electrochemical potential difference between the conducting tip and the conducting surface. In this embodiment, a further step may include separating the conducting tip from the conducting surface by a predetermined distance from a position of onset of tunneling current between the conducting tip and the conducting surface. In accordance with yet another embodiment, the conducting tip may be repositioned after applying the electrical potential.
In accordance with another aspect of the present invention, there is provided a polymer transistor manufactured by a method of positioning a conducting tip proximate to a conducting surface so as to form a gap, introducing an electrochemical medium in contact with the conducting tip and the conducting surface, and applying an electrical potential across the electrochemical medium so as to deposit a conductive polymer that electrically bridges the gap. In a further embodiment, a method of manufacturing a plurality of conducting polymer transistors on a single conducting surface is provided.
REFERENCES:
patent: 4968390 (1990-11-01), Bard et al.
patent: 5130380 (1992-07-01), Carew
patent: 5500537 (1996-03-01), Tsumura et al.
patent: 5641391 (1997-06-01), Hunter et al.
patent: 5710051 (1998-01-01), Park et al.
Jones, et al., “Preparation and Characterization of Molecule-Based Transistors with a 50-nm Source-Drain Separation with use of Shadow Deposition Techniques: Toward Faster, More Sensitive Molecule-Based Devices”, J. Am. Chem. Soc., v. 109, pp. 5526-5528 (1987), no month available.
McCoy, et al., “Potential-Dependent Conductivity of Conducting Polymers Yields Opportunities for Molecule-Based Devices: A Microelectrochemical Push—Pull Amplifier Based on Two Different Conducting Polymer Transistors”, Chemistry of Materials, v. 5, pp. 914-916 (1993), no month available.
Kranz, et al., “Lateral Deposition of Polypyrrole Lines over Insulating Gaps. Towards the Development of Polymer-Based Electronic Devices”, Advanced Materials, v. 7, No. 6, pp. 568-571 (1995), no month available.
Madden, et al., “Three Dimensional Microfabrication by Localized Electrochemical Deposition”, Journal of Microelectromechanical Systems, IEEE vol. 5, No. 1, Mar. 1996, pp. 24-32.
Hunter Ian W.
Madden John
Madden Peter
Bromberg & Sunstein LLP
Gorgos Kathryn
Keehan Christopher M.
Massachusetts Institute of Technology
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