US3302152A

US3302152A - Cryoelectric device

Info

Publication number: US3302152A
Application number: US390560A
Authority: US
Inventors: Charles M Wine
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1964-08-19
Filing date: 1964-08-19
Publication date: 1967-01-31
Anticipated expiration: 1984-01-31

Description

Jan. 31, 1967 c, M, WINE CRYOELECTRIC DEVICE Filed Aug. 19, 1964 a L 7 mam M NW 7. x my w\flw 1 2 my, 7. r 2 W m 1 D" I R I mw P x WW7 M w. M 4. W 4 0 E 4 WMNW M5 Ww; WW 2 00 N Z 5 Wm W in w o x M R m w 2 W 5 a United States Patent G 3,302,152 CRYOELECTRIC DEVICE Charles M. Wine, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Aug. 19, 1964, Ser. No. 390,560 Claims. (Cl. 338-32) plane in close proximity to the line substantially reduces the inductance of the line, a desirable feature in many circuit applications. The strip line may be formed by vacuum depositing a metal, such as tin or lead, through a mask onto a substrate (the insulated surface). So

made, the line is generally found not to be of uniform cross-section. Its edges taper, that is, they are thinner than the center portion of the line. This is believed to be due, among other things, to penumbra effects during the vapor deposition.

If one assumes the current carried by a superconductor line to distribute uniformly along the width of the line, the current density at the edges of a line such as discussed above is greater than that at the center of the line. Due to this non-uniform current density, at some value of current at which the center portion of the line remains superconducting, the current density at the edges of the line exceeds the critical value, that is, it becomes sufiicient to drive the edges to the normal (resistive) condition. Once the edges are driven normal, the normal region rapidly spreads to the remainder of the line. Therefore, it is ordinarily necessary to operate the line at a lower level of currentone at which the current density at the edges is less than the critical value, than the bulk of the line can support.

The so-called edge effects discussed above are largely avoided according to the present invention by spacing the edges of the line further from the ground plane than the center portion of the line as, for example, by placing insulator strips beneath these edges. This substantially increases the inductance of the line at its edges, and consequently reduces the density of current flow at the edges.

The invention is discussed in greater detail below and is illustrated in the following drawings, of which:

FIGURE 1 is a plan view of a prior art in-line cryotron;

FIGURE 2 is a cross-section taken along line 2-2 of FIGURE 1;

FIGURE 3 is a cross-section through an in-line cryotron according to the present invention; and

FIGURE 4 is 'a graph illustrating the performance of the prior art cryotron as contrasted to the one in the present invention.

In the discussion which follows, a low-temperature environment, such as a few degrees Kelvin, at which superconductivity is possible, is assumed.

The cryotron of FIGURES 1 and 2 includes a ground plane 10, which is formed of a superconductor, such as lead, and a thin insulating film 12, which may be formed of silicon monoxide, located on the ground plane. A gate element 14 is located over the insulated ground plane and a control element 16 is located over the gate element. The gate element may be formed of a superconductor, such as tin, and the control element may be formed of a superconductor, such as lead. The two elements are insulated from one 'another by an insulating layer 18, such as silicon monoxide.

The gate element 14 is for the purpose of carrying a current to some load such as a drive line of a superconductor memory. In one condition, the gate element is superconducting and offers zero resistance to this flow of current. In another condition, current is passed through the control element 16 to produce a magnetic field of sufficient magnitude to drive the gate element from its superconducting to its normal (resistive) condition. In this second condition, the gate element exhibits a finite resistance to the flow of current.

The gate element 14 is normally fabricated by vacuum deposition through a mask. When made in this way, the

edges

20, 20a taper rather than being perpendicular to the insulator surface. A current passed through the gate element 14 tends to distribute uniformly over the width w of the gate element. Since the

edges

20 and 20a are thinner than the remainder of the gate element, the current density at these edges tends to be greater than the current density within the remainder of the gate element. As a result of this non-uniform current density, the

edges

20 and 20a tend to be driven normal at a value of current which the remainder of the gate element could otherwise support while in the superconductive state. As is well understood, these normal areas tend to spread to the remainder of the gate element, driving the entire gate element normal and limiting the current-carrying capacity of the gate element.

In the embodiment of the invention illustrated, strips of

insulating material

22 and 22a, shown in FIGURE 3, are laid down on the insulator 12 prior to the time that the gate element is formed. The gate element is vacuum deposited onto the layer 12 with the

edges

20 and 20a thereof on the insulator strips 22 and 2211. Thus, the

edges

20 and 20a are spaced from the ground plane 10 a substantial distance greater than the bulk 24 of the gate element is spaced from the ground plane. The inductance of the gate element is 'a function of its spacing from the ground plane 10. Accordingly, the inductance of the

edges

20 and 20a of the gate element is substantially greater than that of the remainder of the gate element.

As is well understood in the cryoelectric art, if a current is applied to two superconductors in parallel, the current divides in inverse proportion to the inductance of the two conductors. In the present instance, the conductors which exhibit a high inductance can be considered to be the

edge portions

20 and 20a, and the conductor exhibiting a low inductance can be considered to be the center region. Accordingly, when a current is applied to the gate element, it tends to distribute so that less of the current flows at the edges 29 and 20a than in the bulk 24. Therefore, even though the gate element is thinner at its

edges

20 and 20a and the current density accordingly tends to be greater there, the increased inductance at the edges reduces the amount of current flowing at these edges and tends to reduce the current density. The over-all result is that current flow through the gate element in the arrangement of FIGURE 3 is of more uniform density through out the width of the gate element than in the arrangement of FIGURE 2.

In plan view, the cryotron of FIGURE 3 has substantially the same appearance as the one shown in FIGURE 1.

Typical dimensions of an in-line cryotron such as shown in FIGURE 3 are:

Gate element 14:

width-l0 mils film thickness5000 A. (Angstroms) Control element 16: width-15 mils film thickness5000 A. insulation layer thickness800() A.

Insulation strips

22 and 22a:

width5 mils. (These strips were overlapped by the edges of the gate element over an 'area 2 mils wide.) thickness-8000 A.

The principles of the invention are applicable not only to in-line cryotrons, but also to strip transmission lines, crossed-film cryotrons, and so on. A crossed-film cryo-- tron is one in which the gate element extends at right an gles to the control element. As in the case of in-line cryo-- trons, the gate element may be formed of a superconductor such as tin, and the control element may be formed. of a superconductor such 'as lead.

To check the operation discussed above, four crossed-- film cryotrons on ground planes were fabricated. A plot: of the resistance of the cryotron vs. the temperature (the temperature being the factor varied) is shown in FIGURE- 4. Two of the cryotrons were made with insulation strips under the edges of the gate electrode to space the: edges further from the ground plane than the center region. of the gate element similarly to what is shown in FIG-- URE 3, and two were conventional cryotrons in which the gate element was uniformly spaced from the ground. plane. Except for the insulation under the edges of the: gate elements in two of the cryotrons, they were in otherrespects substantially identical. The significant improve-- ment in performance obtained with the crossed-film cryotrons of the present invention is believed to be self-evident from the curves.

As the cryotrons built were tested individually (rather than in tree configurations), temperature was made thevariable and resistance was measured by passing a rela-- tively small current through the gate element. This was to prevent run-away effects due to heating of the gate elements. However, as is well understood in this art, simi-- lar curves may be obtained with the temperature maintained constant at some value, such as 3 K. or 3 /2 K. and current employed as the variable or running parameter, providing these temperature effects are elimi nated or compensated for.

What is claimed is: 1. A two-conductor transmission circuit comprising: a superconductor ground plane; and

a thin-film superconductor strip line lying over and insulated from the ground plane both at its center and at its longer edges and exhibiting a substantially greater inductance at its opposite edges, by virtue of increased spacing at said edges from the ground plane, than at its center portion.

2. A two conductor transmission circuit comprising:

a superconductor ground plane;

insulation on one surface of the ground plane which is substantially thicker in certain regions thereof over the ground plane than in other regions thereof over the ground plane, and

a thin film superconductor strip which is thinner at its opposite edges than at its center lying on said insulation, the opposite edges of said strip lying on the thicker regions of the insulation and the center portions of the strip lying on the thinner region of said insulation whereby the strip exhibits a substantially greater inductance at its opposite edges than at its center portion.

3. A transmission circuit comprising:

a superconductor ground plane;

insulation on one surface of the ground plane, two spaced strips of which are substantially thicker than other portions of the insulation on the ground plane, said two spaced strips of insulation and the insulation between these strips lying over the ground plane; and

a thin film superconductor line which is thinner at its opposite edges than at its center lying on the insulation, the edges of said line lying on said thicker insulation strips and the center portion of said line lying on the relatively thinner portion of the insualtion, whereby the relatively thinner edges of said line are spaced substantially further from the ground plane than the center portion of the line.

4. A transmission circuit comprising, in combination:

a superconductor ground plane;

insulation on one surface of the ground plane;

a thin film superconductor line on said insulation, said line being thinner at its edges than at its center; and

additional insulation over the ground plane lying under the edges of the line spacing said edges a substantially greater distance from the ground plane than the center portion of the line.

5. A cryotron comprising:

a superconductor ground plane;

insulation on said ground plane;

a thin film superconductor gate element insulated from and lying over the ground plane, the element being thinner at its edges than at its center;

additional insulation under the edges of said gate element for spacing said edges substantially further from the ground plane than the center portion of the gate element; and

a thin film superconductor control element lying over and insulated from the gate element.

References Cited by the Examiner UNITED STATES PATENTS 2,989,714 6/1961 Park et al. 338-32 2,989,716 6/1961 Brennemann et al. 340-173 X 3,059,196 10/1962 Lentz 33832 3,098,967 7/1963 Keck 338--32 X 3,100,267 8/1963 Crowe 33832 X 3,191,055 6/1965 Swihart et al 307-885 3,207,921 9/1965 Ahrons 340173 X 3,233,199 2/1966 Hagedorn 338-32 3,234,439 2/1966 Alphonse 33832 OTHER REFERENCES Superconductive Computers by Thomas Maguire, Electronics, November 24, 1961, pages 45, '51.

IBM Technical Disclosure Bulletin, by R. E. Jones, Jr, vol. 7, N0. 3, August 1964, pages 269-270.

RICHARD M. WOOD, Primary Examiner,

W. D. BROOKS, Assistant Examiner.

Claims

5. A CRYOTRON COMPRISING: A SUPERCONDUCTOR GROUND PLANE; INSULATION ON SAID GROUND PLANE; A THIN FILM SUPERCONDUCTOR GATE ELEMENT INSULATED FROM AND LYING OVER THE GROUND PLANE, THE ELEMENT BEING THINNER AT ITS EDGES THAN AT ITS CENTER; ADDITIONAL INSULATION UNDER THE EDGES OF SAID GATE ELEMENT FOR SPACING SAID EDGES SUBSTANTIALLY FURTHER FROM THE GROUND PLANE THAN THE CENTER PORTION OF THE GATE ELEMENT; AND A THIN FILM SUPERCONDUCTOR CONTROL ELEMENT LYING OVER AND INSULATED FROM THE GATE ELEMENT.