US3609469A

US3609469A - Voltage-controlled ionic variable resistor employing material transfer

Info

Publication number: US3609469A
Application number: US692738A
Authority: US
Inventors: Charles Feldman
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-12-22
Filing date: 1967-12-22
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28

Abstract

An ionic variable resistor comprises a base metallic film, an intermediate film of a solid ionic conductor containing a cation similar to the metal comprising the base layer, and an upper film layer of the same metallic material as the base layer. A source of potential connected across the device causes transference of material from the upper layer to the lower layer, or vice versa, depending on the polarity, to effect a change in resistance in the metal films. The variable resistor may be advantageously applied by vacuum deposition to microelectronic wafers, thin film substrates, or circuit boards.

Description

United States Patent [72] Inventor Charles Feldman 7400 Rebecca Drive, Alexandria, Va. 22307 [21] Appl. No. 692,738 [22] Filed Dec. 22, 1967 (45] Patented Sept. 28, 1971 [54] VOLTAGE-CONTROLLED IONIC VARIABLE RESISTOR EMPLOYING MATERIAL TRANSFER 16 Claims, 5 Drawing Figs.

[52] U.S.Cl 317/231, 317/237, 317/235 [51] lnt.Cl ll0lg9/14 [50] Field of Search 338/20, 32,

[56] References Cited UNITED STATES PATENTS 2,075,733 3/1937 Lazarus 338/20 2,751,477 6/1956 Fitzgerald 338/20 2,773,250 12/1956 Aigrain et al.. 317/235 3,307,089 2/1967 Yamashita 317/235 3,355,637 11/1967 Johnson 317/235 3,436,668 4/1969 Russell 317/235 830,924 9/1906 Pawlowski 317/237 1,891,097 12/1932 Krauss 317/237 1,930,519 10/1933 lrion 317/237 1,900,018 3/1933 Lilienfeld 317/231 Primary Examiner.lames D. Kallam Attorney-Larson, Taylor and Hinds ABSTRACT: An ionic variable resistor comprises a base metallic film, an intermediate film of a solid ionic conductor containing a cation similar to the metal comprising the base layer, and an upper film layer of the same metallic material as the base layer. A source of potential connected across the device causes transference of material from the upper layer to the lower layer, or vice versa, depending on the polarity, to effect a change in resistance in the metal films. The variable resistor may be advantageously applied by vacuum deposition to microelectronic wafers, thin film substrates, or circuit boards.

SOURCE PATENTEnsiPzslsn $609,469

8 SOURCE 4Q ELECTRICAL 3O CIRCUIT F/G. 4 I l A I R5 SOURCE-I ELECTRICAL J CIRCUIT INVENTOR CHARLES FELDMAN BY Jug Ja W ATTORNEY 5 VOLTAGE-CONTROLLED IONIC VARIABLE RESISTOR EMPLOYING MATERIAL TRANSFER FIELD OF THE INVENTION The present invention relates to variable resistance devices and more particularly to variable resistance devices usable in microelectronic circuits.

BACKGROUND OF THE INVENTION AND THE PRIOR ART The present trend toward miniaturization of electronic components has resulted in heavy demands for components corresponding to standard-sized components which may be adapted for use in the reduced-size circuits. Furthermore, there have been demands for new types of microminiature components capable of being adjusted or altered by control signals transmitted from a distance for space applications and the like. One particularly difficult problem has been that of providing a satisfactory variable resistance element and particularly of providing a resistance element that may be reversibly varied, i.e., a resistance element whose resistance may be selectively increased or decreased. A second problem is that of providing a satisfactory trimming or tuning resistor used in increasing the precision of microcircuits, e.g., in achieving a precision frequency filter.

ln large-sized systems, a number of resistance varying techniques are available, e.g., hand adjustable rheostats, and servomotors connected to potentiometers, but in microminiature systems which are too small for knobs, techniques available for variation of resistance are very few in number and in general rather crude. Typical microcircuit techniques include scratching or abrading of an exposed film resistor, by use of a suitable tool. It will, of course, be appreciated that the original resistance value of the resistor cannot be restored once part of the resistance material has been scraped off and thus this technique provides variation of resistance in one sense only.

Although voltage variable resistance elements per se are known, such elements are not in general suitable for microminiature use because of their sizes and shapes. One such known device employs a liquid electrolyte which must be encapsulated thus generally limiting the use of the device to larger sized circuits. Other presently known so-called miniature variable or reversible resistors require the use of miniature sockets or screws.

BRIEF SUMMARY OF THE INVENTION The present invention provides a variable resistance device which is particularly adapted for use in microelectronic circuitry. The resistance device of the invention is of completely solid state construction, generally comprising layers of thin films, and thus the space requirements are extremely low.

In accordance with a presently preferred embodiment of the invention there is provided a base resistance layer comprising a thin metallic film mounted on a nonconducting substrate, an intermediate layer comprising an ionic conductor containing a metal ion or cation similar to the metal of the resistance layer and an upper layer comprising a metallic film of the same material as the resistance layer. In operation, the resistance of the device is altered by applying a potential thcreacross such that material from the upper layer is transferred by ionic conduction through the intermediate conductor to the base layer. As the material builds up on the base layer the resistance thereof decreases. Similarly with a potential of opposite polarity applied across the device, material will be removed from the base layer and resistance of the base layer will be increased. It should be noted that it is also possible to utilize an ionic conductor having a mobile anion rather than a mobile cation.

In accordance with one feature of the invention both the base layer and the upper layer may be used as circuit elements, the resistance of one layer increasing while the other decreases and vice versa (as in a potentiometer).

Other features and advantages of the present invention will become apparent upon consideration of the following description of the drawings wherein:

FIG. 1 is a schematic representation of a resistance device in accordance with the present invention,

FIG. 2 is a perspective view of an embodiment of the invention wherein the resistance device of FIG. I located in a microminiature circuit board arrangement,

FIG. 3 is a perspective view of a thin film circuit embodiment of the invention wherein the resistance device acts as a trimmer for a further resistance,

FIG. 4 is a schematic circuit diagram of a further embodiment of the invention, and

FIG. 5 is a schematic representation of yet another embodiment of the invention.

Referring to FIG. 1 of the drawings, there is shown a solid state variable resistance device generally denoted RI which comprises a base layer I mounted on a nonconductive substrate 2, an intermediate layer 3 and an upper layer 4. It will be understood that FIG. 1 is merely illustrative of the construction of resistance device R1 and that the thicknesses and proportions shown therein are not to be taken as definitive.

Base layer 1 is constructed of a metallic resistance material and if formed on substrate 2 as a very thin film by a suitable method such as vacuum deposition. The metallic material used may, for example, be a metal such as silver or copper. It is noted that base layer 1 may itself be made up of a plurality of layers of material. For example, base layer 1 may comprise a base layer of relatively stable resistive metal such as platinum or chromium and an upper layer of a relatively unstable resistive metal such as silver. In this way a more stable overall resistance device is produced and the value of the resistance may be varied only within a relatively narrow range for purposes of trimming. Alloys or mixtures of metals may also serve as the base layer. As will become clear from the description of the operation of the device as set forth hereinbelow it is essential that the layer of base layer 1 which is in contact with intermediate layer 3 be of a metal that gives up its metal ions relatively freely.

Base resistance 1 may be made extremely thin and may even be made lacunary." The term lacunary structure as used herein means a layer so thin that it is not continuous, the layer constituting, in effect, islands of conductive material. An electrical resistor including such a conductive layer is disclosed in my earlier U.S. Pat. No. 2,984,589. Such layers are extremely sensitive to thickness changes.

Intermediate layer 3 comprises an ionic conductor which contains a metal ion or cation similar to the metal comprising the base layer 1. The ionic conductor 3 can, for example, be a silver or copper halide or sulfide or a mixture thereof. Possible combinations include a mixture of silver iodide with sliver chloride. Other possibilities include a mixture of copper bromide and copper iodide and mixtures of various halides of thallium although in general any stable ionic conductor wherein the metal ion is capable of movement may be used. It is noted that the additive of an impurity such as Tellurium to a material such as AgCl also improves the cation mobility. Intermediate layer 3 may be vacuum deposited on base layer 1.

Upper layer 4 may be constructed of the same material as base layer 1 and may be vacuum deposited on intermediate layer 3. An important feature of the present invention is that upper layer 4 need not be and preferably is not deposited over the entire surface of layer 3. To provide trimming" in a manner described hereinbelow layer 4 need only cover a small portion of the base resistor 1 and thus the capacitance of the device may be kept at a minimum.

Resistance device RI further includes a pair of output leads 5 and 6 connected to base resistance 1 as shown and a control lead 7 connected to upper layer 4. Lead 7 connects layer 4 to a variable source of potential denoted 8. Source 8 may, for example, simply comprise a switch and a DC battery although it would be appreciated that source 8 can take a number of other forms.

In the operation of the device of FIG. 1,

resistance layer

1 and 4 serve as electrodes or plates separated by the solid state electrolyte formed by ionic conductor 3. By applying a potential supplied by source 8 across the

layers

1 and 4, a portion of the metallic resistance material from layer 4 may be transferred through ionic conductor 3 to base layer 1. As this material builds up on base layer 1 the resistance of layer 1 decreases. In a like manner, with the potential supplied by source 8 of an opposite polarity, metallic material from layer 1 may be transferred to layer 4. Under these latter circumstances the resistance of layer 1 is increased.

The amount of solid material or metal built up on a layer is governed by well-known laws of electrochemistry. The material transferred, M, is proportional to the total charge, Q, passed according to the relation:

M=Qml VF where F Faradays constant, Q =charge transfer, and m atomic mass of the ion being transferred,

V= valence of the mobile ions.

The charge transferred Q=it=( V/R)t. where 1' current passed, t is time elapsed, and

v voltage and R the resistance of the ionic conducting layer.

In a given layer the mass built up may be controlled by either the voltage or the time, depending on the particular system and the voltage and time available.

For variable resistors, R should be made large, i.e., many times larger per unit area than the under or over lying resistance layers corresponding to

layers

1 and 4 of FIG. 1. In addition R should, of course, be maintained solely an ionic conductor and no electronic conduction should take place.

Because, for most applications, only the relative amounts of material comprising layer 1 are important, the material from layer 4 need not be plated uniformly thereon and there need be no concern with edge effects. This means, as set forth hereinabove, that the configuration of upper layer 4 need not bear any particular relationship to that of base layer 1 and need not cover the entire surface thereof.

FIG. 2 is a largely schematic representation of the resistance device of the present invention embodied in a circuit board assembly. FIG. 2 is not drawn to scale and the relative sizes of the elements shown therein should not be taken to be an accurate depiction of the actual relative sizes. Resistance device R2 corresponds to resistance device R1 of FIG. 1 and corresponding elements thereof have been given the same numbers with primes attached. The assembly as shown is comprised of a pair of circuit boards BI and B2. A plurality of components generally denoted CI and C2 are mounted on boards BI and B2, respectively, in a conventional manner. Resistance device R2 is mounted on lower board B1 and in accordance with an important feature of the invention may be deposited thereon along the other components Cl. It will be further appreciated that because the ionic resistance device may be made very thin it is particularly suitable for use in circuit arrangements utilizing closely stacked mounting boards such as that shown. Variation of the resistance value of the device may be effected by varying the voltage on upper layer 4' as set forth hereinbefore. An input to layer 4' is provided by extending lead 7' through circuit board B2 so that a variable source of potential (not shown) may be applied. The operation of resistance device R2 is the same as described in connection with resistance device R1 of FIG. 1.

FIG. 3 shows an embodiment illustrating the use of the resistance device of the invention in a vacuum deposited thin film circuit. This circuit can be made very small and the ionic resistor of the present invention particularly suited for use therein. The circuit may include a number of various components including a fixed resistor Rf, a capacitor C, a transistor T, and an ionic element R4 according to the invention. All elements may be formed by the same vacuum technique-on a glass or ceramic substrate S. The element of the invention is thus compatible with various curved forms of microelectronic circuits. In this embodiment ionic resistor R4 is connected in parallel with and utilized as a trimming resistor for a main fixed resistor Rf so that the value of resistor Rf may be varied within predetermined narrow limits.

The utilization of the device of the present invention in a completed circuit provides substantial advantages as compared with the conventional scratching or abrading techniques described above. First, the device of the invention provides variation of resistance in both senses, i.e., the resistance of the device may be increased as well as decreased. Second, the change in resistance of the device may be accomplished simply by varying the voltage on the upper layer or plate (element 4 or 4) through an external contact or electrode. This permits final adjustment ofa circuit after packaging or sealing.

Further, in this regard, a plurality of resistance devices may be utilized and under specialized circumstances may be controlled by a single source. A second resistance device R3 is shown in dotted lines in FIG. 2 as being mounted on board B1 and also as having an input lead 7' extending through board B2. Depending, of course, on the nature of the circuit in which they are included, devices R2 and R3 may be simultaneously controlled by a common source of potential. For example. the resistances R2 and R3 might be corresponding elements in like stages of a multistage circuit.

FIG. 4 shows a further embodiment of the invention wherein both resistive layers of the resistance device are utilized as circuit elements. Resistance device R5 corresponds to device R1 of FIG. 1 and corresponding elements thereof have been given the same numbers with double primes attached. Resistance layers 1" and 4" are included in circuits denoted A and B respectively. Because a variation in the potential supplied by source 8" causes a variation in the resistance value of layer 4" as well as layer 1" both layers may be utilized as circuit elements. As set forth above in regard to FIG. I application of a potential of one polarity causes transference of material from the upper layer 4" will increase for this potential while the resistance of base layer 1" decreases. This effect of simultaneously oppositely varying resistances may, for example, be utilized in balancing circuits, and the like.

In accordance with a further variation of the device one or both resistor layers corresponding to

layers

1 and 4 of FIG. 1 may be replaced by a semiconductor layer. The addition of even a small amount of impurity ions to the surface of a semiconductor changes the resistance thereof greatly. The general operation of the unit would be the same as set forth hereinbefore. The semiconductor may conventionally be Si or Ge although CdS-ZnS mixtures or even metal oxides may also be used.

In accordance with a further feature of the present invention the variable resistance device may be heated slightly to increase the ionic conduction of the intermediate layer during trimming. The temperature of the device may be elevated to the l00-200 C. range which in general will not damage the elements of the circuit. This heating will increase the speed of the resistance change. For example, heating Agl above 146 C. decreases the resistance thereof by a factor of about 1,000 due to a crystal transformation.

The stability of the unit after formation, with respect to both time and temperature, is an important factor. The question of whether the ions will diffuse or drift into a desirable position must be attended to. To obtain good stability, at number of steps could be taken as depending on the particular requirements of the system. First, material with very low ionic conductivity can be used so that only upon application of a large voltage would one expect to obtain migration. Second, a barrier consisting of a different material can be formed so that an additional potential above that normally used would be necessary to cause an ion transfer. Third, the ionic conductor can be made thick so that at the voltages usually used (millivolts) no drift would take place. Fourth, ionic conductors may be used which as described previously require thermal activation to permit charge transfer. Fifth, the electrodes of the material,

in some cases, can be connected so that a minimum voltage appears across the ionic conductor during actual use.

In accordance with a specific embodiment of the invention wherein layer 1 is sliver, layer 3 is silver iodide and layer 4 is also silver the following exemplary dimensions, etc., may be observed:

1. Thickness of Agl=lp=l O cm.

2. lonic resistivity of electrolyte lohm/cm.

3. Applied potential l v. (This gives a field of l0v./cm.,

well below the breakdown strength.)

4. Current density at l v. l0 amps/cm.

5. Mass transfer of Ag from Faraday Law under conditions (l), (3) and (4): m==l0 grams/sec. cm. or 100 AU/sec.

Thus a I00 AU silver film would change by 100 percent in one (l) second with l v. applied. This is more than would be required for a simple trimming resistor. The resistance change in thin films will be considerably larger than the mass change since the film does not have bulk behavior but is much more sensitive to thickness variations. The precise change depends on the texture of the film (size and number of grains) and how the new silver deposits on the surface, i.e., whether it builds up on the grain; whether it migrates, etc.

It is noted that in the specific embodiment discussed hereinbefore it has been assumed that the cation or metal ion of the intermediate layer is the mobile ion. It is noted that it is possible to achieve the same effect with only the anion mobile. In this latter situation, there is chemical conversion of the film surface rather than plating operation. As stated, the net effect, i.e., variation of the resistance of the sample resistor, would be the same. It is further possible to utilize a system wherein both types of ions, i.e., the cations and ions, are mobile. Examples of some conductors in which only the cation is mobile are silver chloride and silver bromide. Calcium fluoride and lead chloride are examples of conductors in which only the anion is mobile.

It will be appreciated by those skilled in the art that variations of the basic physical construction of the device are possible. For example, a second main resistance layer corresponding to base layer 1 may be utilized. This second layer could be deposited on a second ionic conductor layer deposited on the upper surface of upper layer 4, the resultant device being made up of five layers in all. With this construction variation of the voltage on the central layer (corresponding to layer 4) will cause variation of the resistances of both the base and uppermost resistive layers. Such a construction is shown in FIG. 5, wherein the base and uppermost resistive layers are denoted la and lb, first and second ionic layers are denoted 3a and 3b and the layer corresponding to layer 4 is denoted 4a. It is noted that the intermediate ionic layers may themselves be made up of layers having different conductivities. Thus, referring to H6. 5, ionic layer'3a is made up of

layers

3a and 3a" of different conductivities.

It will be further understood that the variable resistance device of the present invention has many uses, such as in the tuning of circuits, the adjusting of the characteristics of filters, the variation of various circuit voltages and in self adaptive circuits in response to external or internal signals.

Having thus described the invention in the manner required by the Patent Statutes, I wish it to be understood that the foregoing disclosure is illustrative rather than definitive, and that the scope of the invention is defined by the subjoined claims interpreted in the light of the specification and drawing.

lclaim:

l. A variable resistance device comprising a solid state element comprising, an intermediate layer located between first and second outer layers of a metallic material and comprising a solid ionic conductor including a mobile ion, and voltage means for supplying a potential to one said outer layer to cause a transfer of material to one of said outer layers to produce a change in the resistance of the device which persists in the absence ofsaid potential.

2. A variable resistance device in accordance with claim 1 wherein said ionic conductor contains a metal ion similar to the metal comprising one of said outer layers.

3. A variable resistance device in accordance with claim I wherein said layers comprise vacuum-deposited films.

4. A variable resistance device in accordance with claim 3 wherein at least one of said layers is a lacunary structure.

5. A variable resistance device in accordance with claim 1 wherein said outer layers comprise a base layer and an upper layer, wherein said potential is supplied to said upper layer, wherein the metallic material of the base layer and the upper layer is silver, and wherein said intermediate layer includes a halide of silver.

6. A variable resistance device in accordance with claim 1 wherein one said outer layers forms a resistive element in an electrical circuit.

7. A variable resistance device in accordance with claim 6 wherein the other of outer said layer also forms a resistance element in an electrical circuit.

8. A variable resistance device in accordance with claim 1 wherein one of said outer layers includes a relatively stable metal and a metal which gives up its metal ions relatively freely.

9. A variable resistance device in accordance with claim 1 wherein both outer layers are the same material.

10. A variable resistance device in accordance with claim 1 wherein said intermediate layer comprises a further solid ionic conductor of different conductivity from the first-named solid ionic conductor.

11. A variable resistance device comprising a solid state element comprising a base layer of an electronically conductive material, and intermediate layer comprising a solid ionic conductor having a mobile ion and an upper layer of an electronically conductive material, and voltage means for supplying a potential to one of said upper and lower layers to cause a transfer material to one of the layers to thereby produce a change in the resistance which will persist in the absence of said potential, said electronically conductive material of said base layer comprising a semiconductor material.

12. A microelectronic circuit comprising a substrate, a plurality of circuit elements deposited on said substrate, said circuit elements including a solid state resistance device comprising a base layer of a metallic material, an intermediate layer of an ionic conductor containing a cation similar to the metal comprising said base layer, and an upper layer of metallic material, and voltage means for supplying a potential to said upper layer to cause a transfer of material through said intermediate layer to one of said other layers to produce a change in the resistance of the device which persists in the absence of said potential.

13. A microelectronic circuit in accordance with claim 12 wherein said substrate comprises a first circuit board and wherein the circuit includes a second circuit board lying above said first circuit board and a connection extending through the uppermost of said circuit boards for connecting said upper layer of said solid state resistor to said voltage means.

14. A microelectronic circuit in accordance with claim 12 wherein said layers are vacuum-deposited films.

15. A microelectronic circuit in accordance with claim 12 wherein said plurality of circuit elements comprise a plurality of solid state ionic resistance devices, said voltage means constituting means for controlling the resistance of said plurality of solid state resistance devices.

16. A variable resistance device comprising a solid state element comprising a base layer of an electronically conductive material, and intermediate layer comprising a solid ionic conductor having a mobile ion and an upper layer of an electronically conductive material, and voltage means for supplying a potential to said upper layer, said electronically conductive material of said base layer comprising a semiconductor material.

Claims

2. A variable resistance device in accordance with claim 1 wherein said ionic conductor contains a metal ion similar to the metal comprising one of said outer layers.
3. A variable resistance device in accordance with claim 1 wherein said layers comprise vacuum-deposited films.
4. A variable resistance device in accordance with claim 3 wherein at least one of said layers is a lacunary structure.
5. A variable resistance device in accordance with claim 1 wherein said outer layers comprise a base layer and an upper layer, wherein said potential is supplied to said upper layer, wherein the metallic material of the base layer and the upper layer is silver, and wherein said intermediate layer includes a halide of silver.
6. A variable resistance device in accordance with claim 1 wherein one said outer layers forms a resistive element in an electrical circuit.
7. A variable resistance device in accordance with claim 6 wherein the other of said outer layer also forms a resistance element in an electrical circuit.
8. A variable resistance device in accordance with claim 1 wherein one of said outer layers includes a relatively stable metal and a metal which gives up its metal ions relatively freely.
9. A variable resistance device in accordance with claim 1 wherein both outer layers are the same material.
10. A variable resistance device in accordance with claim 1 wherein said intermedIate layer comprises a further solid ionic conductor of different conductivity from the first-named solid ionic conductor.
11. A variable resistance device comprising a solid state element comprising a base layer of an electronically conductive material, and intermediate layer comprising a solid ionic conductor having a mobile ion and an upper layer of an electronically conductive material, and voltage means for supplying a potential to one of said upper and lower layers to cause a transfer material to one of the layers to thereby produce a change in the resistance which will persist in the absence of said potential, said electronically conductive material of said base layer comprising a semiconductor material.
12. A microelectronic circuit comprising a substrate, a plurality of circuit elements deposited on said substrate, said circuit elements including a solid state resistance device comprising a base layer of a metallic material, an intermediate layer of an ionic conductor containing a cation similar to the metal comprising said base layer, and an upper layer of metallic material, and voltage means for supplying a potential to said upper layer to cause a transfer of material through said intermediate layer to one of said other layers to produce a change in the resistance of the device which persists in the absence of said potential.
13. A microelectronic circuit in accordance with claim 12 wherein said substrate comprises a first circuit board and wherein the circuit includes a second circuit board lying above said first circuit board and a connection extending through the uppermost of said circuit boards for connecting said upper layer of said solid state resistor to said voltage means.
14. A microelectronic circuit in accordance with claim 12 wherein said layers are vacuum-deposited films.
15. A microelectronic circuit in accordance with claim 12 wherein said plurality of circuit elements comprise a plurality of solid state ionic resistance devices, said voltage means constituting means for controlling the resistance of said plurality of solid state resistance devices.
16. A variable resistance device comprising a solid state element comprising a base layer of an electronically conductive material, an intermediate layer comprising a solid ionic conductor having a mobile ion and an upper layer of an electronically conductive material, and voltage means for supplying a potential to said upper layer, said electronically conductive material of said base layer comprising a semiconductor material.