Structure for solid state switch

Certain structures are described for solid state electrical switches which employ electrochromic material. These structures involve use of a common base contact for both switching and readout circuits. The structures are particularly easy to fabricate using integrated circuit techniques and exhibit reduced electrical shorts due to reduced migration of metallic ions.

TECHNICAL FIELD 
The invention involves certain structures for solid state electrical 
switches. 
BACKGROUND OF THE INVENTION 
Electrical switching is extensively used for electrical apparatus and 
electrical circuitry for a variety of applications. For example, switches 
are used to turn on and off certain circuits including individual 
telephone lines to various customers. Inherent in such switching 
properties are amplifier properties and electrical energy (current) 
control properties. Desirable characteristics for electrically operated 
switches in modern electrical apparatus are high on/off resistance ratio, 
low insertion loss, low electrical control energy, and permanent latching 
on removal of control or switching energy. In addition to these desirable 
properties it is often advantageous in modern electrical apparatus to have 
extremely high reliability, large packing density and easy fabrication, 
particularly for large numbers of switches. 
SUMMARY OF THE INVENTION 
The invention is a solid state switch. This solid state switch contains 
electrochromic material (generally tungsten trioxide) that changes from 
insulator to metallic conductor on injection of certain ions under the 
influence of an electrical field. The structure is such that a common 
contact (called here the base contact) is used for both the input signal 
circuit and the readout circuit. Also, the readout contacts and the base 
contacts are in the same plane. Various geometries and fine line 
photolithographic techniques may be used to increase the length of 
interface between coplanar readout contacts and base contacts so as to 
reduce insertion loss. The coplanar configuration of base and readout 
contacts is advantageous for a number of reasons. First, these types of 
solid state switches may be fabricated using integrated circuit 
techniques. Insertion loss and resistance ratio are easily traded off by 
suitable changes in geometry. Also, metalion migration that may occur 
during the control signal pulse will be orthogonal to the readout contact. 
This essentially eliminates the development of shorts between the base and 
readout contacts.

DETAILED DESCRIPTION 
The invention is a solid state switch employing a material which changes 
from an insulating state to a conducting state on injection of positive 
ions under the influence of an electric field. In particular, the 
invention is directed to certain structural characteristics of the solid 
state switch. The solid state switch is composed of input circuit or 
signal circuit and output circuit or readout circuit. An important 
characteristic of these particular switches is that the input and output 
circuits have a common base contact. In addition, the base contact and 
readout contact are located on the same plane (usually an insulating 
substrate) and usually parallel to one another. This provides a reasonable 
amount of interface between base contact and readout contact so the 
resistance with the switch in the "on" condition is minimized. Various 
interlacing configurations may be used to increase the amount of interface 
between base contact and readout contact. These contacts may be made of 
any conductive material (nickel, copper, aluminum, platinum, palladium, 
gold, etc., and alloys of these metals) but gold (or gold covered metal 
contacts) or tin oxide containing semiconducting contacts such as indium 
tin oxide or antimony doped tin oxide are preferred because of resistance 
to oxidation and excellent electrical and contact properties. The 
electrochromic material is located above the base and readout contacts. 
Any material which undergoes such an insulator to conductor transformation 
may be used including amorphous, polycrystalline and crystalline. 
Particularly convenient, are electrochromic materials such as tungsten 
oxide (WO.sub.3) and molybdenum oxide (MoO.sub.3). Many such materials 
have been described in the scientific literature. Exemplary articles are: 
U.S. Pat. No. 3,521,941, issued July 28, 1970to S. K. Deb et al; 
"Electrochromism in a WO.sub.3 Amorphous Films" by Brian W. Faughnan et 
al, R.C.A. Review Vol. 36, p. 177 (March 1975); "A Solid State 
Electrochromic Cell--The RbAg.sub.4 I.sub.5 /WO.sub.3 " system by M. Green 
and D. Richman, Thin Solid Films 24, S45 (1974); "Coloration in WO.sub.3 
Film" by Y. Hajimoto and T. Hara, Applied Physics Letters 28, p. 228 
(February 1976); and "Optical and Photoelectric Properties of Colour 
Centres in Thin Films of Tungsten Oxide" by S. K. Deb, Philosophical 
Magazine 27, p. 801 (1973). The transformation from insulating to 
conducting state is controlled by the injection of positive ions under the 
influence of an electric field. The source of the positive ions is an 
electrolyte substance located above the electrochromic material. Various 
positive ions may be useful, particularly including protons and lithium 
positive ions as well as sodium and potassium positive ions. Source of 
such positive ions may be moisture in the air (for protons) as well as 
various insulating films or solid or liquid electrolytes. Various acids 
(e.g., H.sub.2 SO.sub.4) as well as other substances can be used as 
electrolytes. For purposes of this invention, the exact nature of the 
electrolyte is not critical provided it supplies injection ions capable of 
transforming the electrochromic material from insulating state to 
conducting state. 
A typical procedure for preparing a solid state switch is as follows: a 
substrate is used for convenience to mount one or more switches. The 
substrate is usually made of non-conducting material (glass, ceramic, 
etc.). Typical electrolytes may be aqueous solutions of various kinds, 
including acidic solutions such as aqueous nitric acid, aqueous sulfuric 
acid, etc. Aqueous sulfuric acid is preferred because of stability and the 
fact that sulfuric acid does not evaporate. Other liquid electrolytes may 
be used such as solutions of lithium salts (i.e., lithium bromide, lithium 
perchlorate, etc.) in water or various organic solvents. Also a variety of 
solids may be used, for example, lithium fluoride, magnesium fluoride, and 
lanthanum fluoride as well as other rare earth fluorides and silicon 
dioxide have useful properties because water from the atmosphere is 
absorbed by films of these compounds and serves as a source of protons. It 
should be realized that a large variety of other electrolyte systems which 
serve as a source of protons or lithium ions are also useful. 
The signal control contact is located on top of the electrolyte. This 
contact or electrolyte may be made of any productive material as 
enumerated above. As in the case of other contacts in this device, gold or 
semiconducting tin oxide containing materials are preferred because of 
good electrical contact properties and chemical inertness. 
A brief description of the operation of the solid state switch is useful in 
an understanding of the invention. A voltage applied to the input signal 
contact drives proton ions out of the electrolyte and into the 
electrochromic material. This ion injection converts the electrochromic 
material from insulator to conductor. This permits electrical conductivity 
between the readout contact and base contact. The result is that the solid 
state switch becomes conductive. Switching voltages (that is input circuit 
voltages) may vary over large limits depending on electrode spacing and 
the resistances desired in the "on" and "off" positions of the readout 
circuit. Typical switching voltages are 10.sup.-3 to 10 volts (see page 
181 of the B. W. Faughnan reference) with electrochromic material 
thickness (or electrode spacings) of 0.1 to 100 microns (see column 4, 
lines 29-31 of S. K. Deb et al, U.S. Pat. No. 3,521,941). Typical 
conductor thicknesses vary from 100 Angstroms for transparent gold 
electrodes (column 8, line 2 of the Deb et al reference) to much thicker 
gold layers where transparency is not important. 
FIG. 1 shows in cross-section a solid state switch 10 made in accordance 
with the invention. The solid state switch is mounted on a substrate 11. 
Conducting paths 12 and 13, are mounted on top of the substrate. One set 
of these conducting paths are the base contacts 12 and the other are the 
readout contacts 13. Interspaced between these conducting paths and above 
these conducting paths is the electrochromic material 14 generally 
comprising tungsten trioxide. Located above the electrochromic material is 
the electrolyte 15 generally comprising lithium fluoride. Above the 
electrolyte is the input signal contact 16 preferably comprising gold 
sealed down with a thin layer of lead fluoride. 
FIG. 2 shows a top view of the lower conducting paths 20 showing both the 
base contact 21 and the readout contact 22. The length of contact in this 
configuration indicates a desire to increase the amount of interface 
between base contact and readout contact. Spacing between base contact 21 
and readout contact 22 is typically between 0.1 and 100 microns. 
FIG. 3 shows similar contacts as in FIG. 2 but with a configuration which 
further increases the amount of interface between base contact 31 and 
readout contact 32. It involves a zig-zag configuration with base contact 
running parallel to readout contact. Also shown is the connecting leads 
for the base contact 33 and readout contact 34. 
FIG. 4 shows another configuration 40 for base contact 41 and readout 
contact 42. This type of configuration substantially increases the amount 
of interface between base contact and readout contact. It involves 
interlacing the base contact and readout contact. Again electrical leads 
to the base contact 43 and readout contact 44 are shown. 
FIG. 5 shows a spiral configuration 50 and readout contact 52. Such a 
configuration is highly useful where large amounts of interface and base 
contact is desirable. Such a situation is often desirable where insertion 
loss is to be minimized. The figure also shows the electrical connection 
to the base contact 53 and readout contact 54.