A solid-state synaptic memory matrix (10) having switchable weakly conductive connections at each node (24) whose resistances can be selectably increased or decreased over several orders of magnitude by control signals of opposite polarity, and which will remain stable after the signals are removed, comprises an insulated substrate (16), a set of electrical conductors (14) upon which is deposited a layer (18) of an organic conducting polymer, which changes from an insulator to a conductor upon the transfer of electrons, such as polymerized pyrrole doped with 7,7,8,8-tetracyanoquinodimethane (TCNQ), covered by a second set of conductors (20) laid at right angles to the first

DiStefano, Salvador

Moacanin, Jovan

Nagasubramanian, Ganesan

NASA Technical Reports Server

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US005272359A 
United States Patent [19] (111 Patent Number: 5,212,359 
Nagasubramanian et al. 1451 Date of Patent: Dec. 21, 1993 
(541 REVERSIBLE NON-VOLATILE SWITCH 
BASED ON A TCNQ CHARGE TRANSFER 
COMPLEX 
[75] Inventors: Ganesan NagasubrmPnian, 
LaCrescenta, Salvador DiStefano, 
Alhambra; Jovan Moacanin, Los 
Angeles, all of Calif. 
(731 Assignee: California Institute of Technology, 
Pasadena, Calif. 
[21] Appl. No.: 631,230 
[22] Filed: Dec. 21,1990 
Related U.S. Application Data 
1631 Continuation of Ser. No. 178,482, Apr. 7, 1988, aban- 
doned. 
[51] Int. Cl.5 ............................................. HOlL 29/28 
[52] U.S. C1. ........................................... 257/40; 257/2 
(581 Field of Search ................................... 357/8, 8.15 
1561 References Cited 
US. PATENT DOCUMENTS 
3,341,362 9/1967 Hacskaylo ........................ 357/15 X 
3,519,999 7/1970 Gregor ............................. 357/15 X 
4,507,672 3/1985 Potember et al. .................. 357/8 X 
4.761,677 8/1988 Sasaki ...................................... 357/8 
4,806,995 2/1989 Day et al. ............................... 357/8 
22 
/ 
FOREIGN PATENT DOCUMENTS 
62-222669 9/1987 Japan ...................................... 357/8 
OTHER PUBLICATIONS 
Day et all “Synthesis of Aliphatic Polymers ... ’’ Jour. 
of Polymer Science, Part A, vol. 24, 1986, pp. 645-654. 
Reynolds, “Advances in the Chemistry of Conducting 
Organic Polymers,” Jour. of Molecular Electronics, 
primary Examiner-Sara W. Crane 
Attorney. Agent, or Firm-Beman & Collins 
1571 A B S T R A c r  
A solid-state synaptic memory matrix (10) having 
switchable weakly conductive connections at each node 
(24) whose resistances can be selectably increased or 
decreased over several orders of magnitude by control 
signals of opposite polarity, and which will remain sta- 
ble after the signals are removed, comprises an insulated 
substrate (16), a set of electrical conductors (14) upon 
which is deposited a layer (18) of an organic conducting 
polymer, which changes from an insulator to a conduc- 
tor upon the transfer of electrons, such as polymerized 
pyrrole doped with 7,7,8,8-tetracyanoquinodimethane 
(TCNQ), covered by a second set of conductors (20) 
laid at right angles to the first. 
VO~.  2, 1986, pp. 1-21. 
4 Claims, 1 Drawing Sheet 
22 
/ 
12 
https://ntrs.nasa.gov/search.jsp?R=20080005942 2019-08-30T02:58:51+00:00Z
U,S, Patent Dec. 21, 1993 5,272,359 
:* 22 / 
5,272,359 
1 2 
be reversed when desired (ON-->OFF->ON etc.). 
A further desireable feature would allow control over 
the non-volatile resistance value so as to be able to 
predetermine a desired ON-state resistance value. Ap- 
5 plication Ser. No. 07/133,897, filed Dec. 16, 1987, and 
=signed to the same assignee as the present application 
discloses an EPROM having these characteristics, 
adapted especially for use in synaptic memory matrices. 
That System employs a resistive switching element that 
This is a continuation of co-pending application Ser. lo acts by transfer of ions from one layer to another. In 
some cases, it would be desirable to perform such 
switching in a faster manner than is possible with ion 
transfer, and to employ a switch with fewer layers- TECHNICAL FIELD 
The present invention relates broadly to electronic thus simpler and therefore cheaper to fabricate. 
DISCLOSURE OF INVENTION memories based on neural networks, and, more particu- larly, to reversible switches employing organic con- 
ducting polymers. In accordance with the invention, a controllable, 
reversible, variable non-volatile resistance switching 
element is provided. The switching element is based on BACKGROUND ART 
Neural network models offer a totally new approach 2o a doped organic conducting polymer material which 
to  intelligent information processing that is robust, fault controllably changes from an insulator to a conductor 
tolerant, and can be extremely fast. Such neural net- upon the transfer of electrons under the impetus of a 
works are discussed, for example, in A. p. Thakoor et control signal, and which can be reversed to the insulat- 
al, “Binary Synaptic Connections Based on Memory ing State by another signal. ’ 
Switching in a-Si:H”, Neural Nemorks for Computing, J. 25 in particular, a solid-state synaptic memory matrix 
S. Denker, Ed., American Institute of Physics Confer- having switchable wedy conductive at 
ence Proceedings # 15 1, Snowbird, Utah, PP. 426-43 1 each node whose resistances can be selectably increased 
or decreased over several orders of magnitude by con- (1986). 
The aforementioned features originate directly from trol signals of opposite polarity, and which will remain the massive interconnectivity of neurons (the decision- 30 stable after the signals are removed, comprises an insu- 
making elements) in the brain and its ability to store lated substrate, a set of electrical conductors upon information in a distributed manner as a large number of which is deposited a layer of an organic conducting synaptic interconnects of varying strengths. Hardware 
implementations of neural network concepts, therefore, polymer, which changes from an insulator to a conduc- 
are attracting considerable attention. Such artificial 35 tor such as polymerized 
neural networks are expected, for example, to function Pyrrole doped with 7,7~8~8-tetrac~anoq~inodimethane 
as high speed, content addressable, associative memo- pcNQ)* ‘Overed by a second set Of conductors laid at 
ries in large knowledge bases for artificial intelligence right 
BRIEF DESCRIPTION O F  THE DRAWING applications and robotics or to perform complex com- 
putational tasks such as combinatorial optimization for 40 
autonomous systems. The sole FIGURE is a cross-sectional view, depict- 
In particular, electronic implementation of an asso- h g  one configuration employing the reversible, non- 
ciative memory based on neural network models re- volatile switch of the invention. 
BEST MODES FOR CARRYING OUT THE quires large arrays of extremely simple, binary connec- 
INVENTION tion elements or synaptic interconnects. Information is 45 
essentially stored in the binary states of the intercon- 
nects. Non-volatile storage of information therefore can In recent Years, organic conducting polymers (ocp )  
take place at the time of fabrication of the synaptic array have generated widespread interest, having been pro- 
by making the resistive state of the desired interconnect posed as materials of choice for a variety of applications 
“ O N  or “ O F F .  Alternatively, significantly more use- 50 including use as surface passivants, corrosion protection 
ful memory systems can be built if programmable, bi- coatings on silicon h photoelectrochemical cells, elec- 
nary resistive thin film devices (non-volatile micro- trochromic displays, batteries, and electrochemical sen- 
switches) are used as synaptic interconnects. sors. The potential of these OCP materials to cycle 
Non-volatile, associative electronic memories based between conducting and insulating forms makes them 
upon neural network models, with dense synaptic inter- 55 attractive for reversible switching applications, since 
connection arrays in thin-film form, have recently been this reversible nature is not inherent in inorganic 
developed using hydrogenated amorphous silicon (a- Switching materials such as amorphous silicon hydride. 
Si:H). Similarly, using conventional photolithography, In spite of this salient feature, the emergence of the 
manganese dioxide based synapses have also been fabri- conducting organic polymers as possible candidate ma- 
cated. Irreversible memory switching in hydrogenated 60 terials for switching applications has been deterred by 
amorphous silicon (OFF->ON) and manganese diox- the presence Of liquid electrolytes (which act as source 
ide (ON->OFF) makes them ideally suited only for and sink for the counter ions). This usually involves 
use as programmable, binary, weak synaptic connec- bulky cell arrangements for containment. 
tions in associative programmable read-only memories In accordance with the invention, a solid state OCP 
(PROMS) based on neural networks models. For many 65 switching material without any liquid electrolyte is 
applications in computing and memory operations, it is provided which lends itself to the fabrication of high- 
also desirable to obtain the equivalents of erasable density, thin film devices-particularly to an erasable 
PROMS (EPROMs), such that the switching state can read-only-memory (EPROM) especially suitable for use 
REVERSIBLE NON-VOLATILE SWITCH BASED 
ON A TCNQ CHARGE TRANSFER COMPLEX 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected to retain title. 
No. 07/178,482 filed on Apr. 7, 1988, abandoned. 
the transfer Of 
to the first. 
5,272,359 
3 4 
in a thin-film synaptic memory matrix, in which the low positive voltage is again applied, on the order of 
change of state is accomplished by electron charge 100 millivolts. This causes negative doping of the poly- 
transfer. This material is polypyrrole doped by TCNQ, pyrrole by the TCNQ. 
a solid compound which may be readily incorporated Alternatively to the last step, TCNQ may be dis- 
into solid state devices. The sole FIGURE illustrates a 5 solved in a bath of acetonitrile and the assembly dipped 
simplified cross-sectional schematic of a Synaptic mem- in for sufficient time to cause the desired degree of 
ory matrix 10 using this material as the resistive revers- doping. 
ible switching element 12 at every node. The TCNQ doping may range from about 5 to 30 
Referring to the FIGURE, a first set of parallel con- mole % of the polypyrrole. Less than a b u t  5 mole % 
ductors 14 is laid down on a substrate 16 by electrode- 10 TCNQ does not provide the desired effect, while 
position, or any other process known to the art. These greater than about 30 mole % cannot be incorporated in 
conductors are preferably fabricated from passive met- the polymer. 
als such as gold, platinum, or other conductive materials ne thickness ofthe film is determined by the amount 
such as glassy carbon. AS shown in the FIGURE, these of current passed in the initial step. However, in gen- 
conductors may form the matrix rOW conductors; alter- 15 eral, the thickness of the film is about 1 Fm. 
natively, they may actually be the column conductors. ON state resistance is a ~ m p l i s h ~  by 
Upon these is deposited a layer 18 ofpolypyrrole doped controlling the doping density during TCNQ doping; 
with TCNQ; this is the resistive switching layer. Next, the figher the concentration of TCNQ and the longer 
a second set of parallel conductors 20 are deposited on the soak time the greater the dopant density the 
right angles to the first set of conductors. Lastly, an role forms complexes spontaneously with TCNQ, the 
insulating and sealing layer 22 is laid down on top of the energy involved in the transition from insulator to con- second set of conductors. The switching areas 24 de- ductor-and back-is very small. This transition occurs fined by the offset intersections of the row and column due to the migration of electrons of TCNQ into the 
conductors form the synapse nodes wherein data is to 25 polypyrrole, and back, in response to external stimulus. 
be stored. In the synaptic memory matrix, this stimulus is a voltage 
applied across the desired row and column conductors, known substrate materials, particularly insulating mate- the design being tailored such that this voltage exceeds rials, such as ceramics, including silicon oxide, alumina, magnesia, and the like, polymers, etc. On the other 30 the normal operating voltage of the circuit. Ideally, for 
conductor substrates for direct interaction therewith. In be carried Out by the imposition Of a positive five 
any event, the compos~t~on of the substrate does not volts to one conductor and a negative five volts to the 
form a part of this invention. other, such that the resulting ten volts causes electron 
The insulating and sealing layer 22 may also comprise 35 charge transfer. To reverse states, the ten volts is reap- 
well-known insulating materials such as silicon oxide, Plied, but with the opposite polarity, such that the origi- 
silicon nitride, silicon oxynitride, and the like. As with nal 
the substrate 12, the composition of the insulating and Briefly, the data may be written and erased as fob 
sealing layer does not form a part of this invention. lows: The resistance between contacts is changed by 
The composition and dimensions of the electrodes 14, 40 applying the Proper Positive Write voltage to a desired 
20 is not particularly critical, other than that the compo- Pair of row and column conductors, then the write 
sition be compatible with the other materials employed volW3e is removed. The area of the resistance between 
in the switch. Dimensions will be largely governed by those conductors acts as the switching element in the 
the particular conductive material selected, its deposi- synapse matrix; the ON state may be represented by a 
tion rate and the ability to control spacing between 45 bwer  resistance while the OFF state may be repre- 
neighboring conductors. sented by a higher resistance, for example. The pre- 
In order to fabricate the switching layer 18, the sub- of one state, typically the ON state, denotes a 
strate containing the first set of conductors is placed in stored bit, which is non-volatile and will stay stored as 
an electrochemical cell, containing a solvent such as long as desired. If the stored bit is to be removed, the 
acetonitrile into which monomeric pyrrole is dissolved. 50 negative P l k t Y  erase simal is applied to the same pair 
Since this solution is neutral, a supporting electrolyte is of conductors until the b h e r  resistance OFF state is 
added to enable conduction of current. This electrolyte atbhed. As with an EPROM, an entire section of mem- 
may be a salt, such as tetrabutylammoniumperchlorate ory may be erased by connecting all conductors simul- 
(TBAP), for example. The concentration of the salt is taneously to the erase voltage. Selected groups of nodes 
that commonly used for electrochemical studies, typi- 55 representing a word m y  be erased together once iden- 
cally about 0.1M. A positive voltage, with respect to a tified. 
second plate, is applied, causing a current to pass When reading the data, the presence of the lower 
through the conducting solution, which causes a film of rdStanCe nodes is detected. This may be done on a 
polymerized pyrrole to be plated on the substrate and location-addressed basis, he-byline or by associative 
first set of conductors. 
The electrolyte also acts to dope the polypyrrole Preliminary tests reveal a change in resistance from 
negatively; thus a reversed polarity voltage is applied the insulating OFF state to the weakly conducting ON 
for sufficient time to reduce the polymer and remove state of about five orders of magnitude: from about 1012 
this unwanted dopant. The electrochemical technique ohms to about IO7 ohms. The 10 megohm resistance in 
allows one to quantitatively remove all of the dopant 65 the ON state is highly desirable, to allow for a high 
regardless of the voltage (once a threshold voltage is density of nodes while avoiding thermal dissipation 
achieved). At this point, a lithium salt of TCNQ may be problems. Previous thin-film synapse memories have 
added to a new batch of acetonitrile solvent, and a very attained a packing density of 109 bits per cm2. 
Control 
top of the switching layer; its conductors running at 2O lower the ON state resistance. since neutral polypyr- 
The substrate l6 may comprise any Of the 
hand, future generation devices may be formed on semi- a five-vo1t Operating application, switching 
migration is reversed* 
60 recall prompting. 
5,272,359 
5 6 
In contradistinction to the the thin-film EPROM of sidered to fall within the scope of the invention, as 
Ser. No. 07/133,897, the ON state resistance cannot be defimd by the appended C h m .  
varied during operation by application of a control What is 
voltage to a separate gate. H ~ ~ ~ ~ ~ ~ ,  since the charge 1. A resistive reversible, non-volatilememory switch 
transfer mechanism in the EPROM of the aforemen- 5 exhibiting a change in resistivity from a tunable ON 
tioned application is the flow of ions, and that in the (a) a first electrode supported on a substrate; 
instant invention is due to the flow of electrons, this e) an organic polymer formed on the 
surface of said first electrode, said polymer consist- innovation has much faster write and erase times. Early 
tests have shown complete changes of state Well within io ing essentially of polymerized pyrrole doped with 
a 100 millisecond pulse. It is anticipated that the ulti- about 5 to 30 mole % of 7,7,8,8-tet- 
mate response time will be less than a few milliseconds. racyanoquinodimethane; and 
Since it is believed that there is actually some molecular (c) a second electrode formed thereover and oriented 
rearrangement occumng during charge transfer, it is orthogonal thereto, wherein said tunable ON state 
not felt that the ultimate switching times will be short 15 is determined by the amount of doping. 
enough to permit use of this invention as a read-only- 2. The memory Switch of Claim 1 further comprising 
memory, capable of reading and writing at high Speed an insulating and sealing layer coating any exposed 
portions of said first and second electrodes and said clock rates. However, its speed i s  such that it is fully organic polymer. compatible with current transistor EPROM capabdi- 2o wherein said first 
and second electrodes comprise material selected from ties. 
ible, non-volatile switch, adapted for use, for example, bon. 
with neural networks. It will be clear to one skilled in 
the art that various changes and modifications may be 25 ganic conducting polymer is about 1 pm thick. 
made, and all such changes and modifications are con- 
is: 
state to an OFF state comprising: 
3. The memory switch of claim 
Thus, there has been Provided a Solid State, revers- the group consisting of platinum, gold, and glassy car- 
4. The memory switch of claim 1 wherein said or- 
* * * * *  
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Reversible non-volatile switch based on a TCNQ charge transfer complex

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