Method of fabricating magnetic bubble circuits

United States Patent m

Sandfort

[li] 4,104,422 [45] Aug. 1, 1978

[54] METHOD OF FABRICATING MAGNETIC BUBBLE CIRCUITS

[75] Inventor: Robert Melvin Sandfort, St. Charles, Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[21] Appl. No.: 582,787

[22] Filed: Jun. 2, 1975

Related U.S. Application Data

[62] Division of Ser. No. 455,219, Mar. 27, 1974, Pat. No. 3,921,157.

[51] Int. C1.2 B05D 5/12

[52] U.S. CI 427/123; 427/130;

427/131; 427/132; 427/264; 427/265; 427/271 [58] Field of Search 427/123, 125, 130, 131,

427/132, 264, 265, 271, 47, 48; 156/3, 18, 652, 656; 340/174 TF, 174 BA

[56] References Cited

U.S. PATENT DOCUMENTS

3,677,843 7/1972 Reiss 427/130

3,723,716 3/1973 Bobeck et al 340/174 TF

3,919,055 11/1975 Urban 427/131

3,932,688 1/1976 Sugita 427/132

3,967,002 6/1976 Almasi et al 427/132

4,013,803 3/1977 Josephs 427/125

Primary Examiner—Cameron K. Weiffenbach
Attorney, Agent, or Firm—Lane, Aitken, Dunner &
Ziems

[57] ABSTRACT

In a bubble chip, increased spacing between a circuit overlay and a sheet of magnetic bubble material is provided at selected locations for particular circuit elements from which bubbles are to be deflected by magnetic repulsion. Differential spacing is achieved in one embodiment by means of a stratified spacing layer composed of the conventional nonmagnetic spacing layer plus a layer of nonmagnetic metal etched to form a mesa pattern which may include electrical control leads where necessary.

3 Claims, 5 Drawing Figures

METHOD OF FABRICATING MAGNETIC
BUBBLE CIRCUITS

This is a division, of application Ser. No. 455,219 filed 5 Mar. 27, 1974, now U.S. Pat. No. 3,921,157.

BACKGROUND OF THE INVENTION

The invention relates generally to the field of magnetic bubble technology (MBT), and more particularly 10 to the structure and fabrication of bubble devices, especially those in which bubble logic is performed.

MBT involves the creation and manipulation of magnetic bubbles in specially prepared magnetic materials. The word "bubble" used throughout this text, is in- 15 tended to encompass any single-walled magnetic domain, defined as a domain having an outer boundary which closes on itself. The application of a static uniform magnetic bias field orthogonal to a sheet of magnetic material having suitable uniaxial anisotropy causes 20 the normally random serpentine pattern of magnetic domains to shrink into isolated, short cylindrical configurations or bubbles whose common polarity is opposite that of the bias field. The bubbles repell each other and can be moved or "propagated" by a magnetic field in 25 the plane of the sheet.

Many schemes exist for propagating bubbles along predetermined channels at a precisely determined rate so that uniform data streams of bubbles are possible in which the presence or absence of a bubble at a particu- 30 lar position within the stream indicates a binary "1" or "0". MBT was originally envisioned in the form of a mass memory, but some of the most difficult problems have been encountered in the basic memory function of readout. It is possible, however, to minimize readout to 35 a great extent by incorporating logic in the memory so that the magnetic bubbles representing information can be logically manipulated before readout is necessary to increase the quality or informational content of each bit of readout. The use of bubbles themselves as logic vari- 40 ables for performing logic operations is based on the fact that close magnetic bubbles tend to repell each other. Thus, if alternate paths with varying degrees of preference are built into a propagation system, the direction which a bubble on one channel ultimately takes 45 may be influenced by the presence or absence of a corresponding bubble on another closely spaced channel.

Conventional bubble devices are prepared in composite monolithic structures referred to as "chips" in analogy to the structure of integrated electrical circuits. In 50 field-accessed propagation systems, a thin overlay pattern of soft ferromagnetic elements is formed on a spacing layer over a sheet of magnetic bubble material. Nonmagnetic electrical conductors are also bonded to the spacing layer for use in bubble generation, transfer, 55 readout and annihilation, for example. The thickness of the spacing layer has critical margins. It is well known that the spacing layer effects the degree of interaction or "coupling" between the bubbles and the overlay circuit. Magnetic bubbles tend to stay under the soft 60 ferromagnetic elements of the circuit overlay because of the energy minimizing effect of containing the magnetic lines of flux emanating from the bubble domain, much like a keeper on the poles of a magnet. If the overlay material is too close to the bubble material, 65 however, spurious bubble generation or "nucleation" can occur. If the spacing is too great, the interaction between the overlay and the bubbles may not be signifi

2

cant enough to be useful. The aim is thus to have the overlay close enough to the bubble material to guide the bubbles without adversely affecting them. There are optimal spacings for different kinds of circuit elements as well as for different materials, thickness, widths and other parameters.

Generally speaking, the degree of coupling in a circuit designed only for storage or memory should be high since the basic characteristic of field-accessed bubble memory is orderly propagation, and a high magnetic interaction between the overlay and the bubbles serves to keep the bubble from straying from the propagating track. On the other hand, in circuits designed to perform logic by means of bubble-to-bubble repulsion, there are occasions when it is desirable to deflect one bubble onto an adjacent track, and in this case the deflected bubble should be less strongly attracted or coupled to its original circuit element while the deflecting bubble should be strongly coupled to its circuit element.

SUMMARY OF THE INVENTION

The general object of the invention is to provide an efficient technique for fabricating bubble chips which permits a variation in the thickness of the spacing layer to accommodate different degrees of coupling between bubbles and the overlay circuit to facilitate logic operations without degrading the integrity of normal propagation or storage operations.

According to the invention, a spacing layer between the bubble material and the circuit overlay is designed with a special topography providing raised plateaus or mesas at selected locations where it is desired to weaken the coupling between bubbles and a particular overlay element or series of elements. The nonuniform thickness of the spacing layer facilitates logic operations by making bubbles more prone to deflection at selected locations of logic interaction while preserving the optimum spacing for orderly propagation in other areas of the same bubble chip.

In one embodiment the nonuniform spacing layer is a composite comprised of adjacent strata or layers of different material. The first layer adjacent to the bubble material is, for example, silicon oxide applied in the conventional manner to a thickness which would permit strong enough coupling with a circuit overlay for normal propagation, i.e., simple shift register operation, or for holding a bubble on a circuit element where that bubble is to perform the function of deflecting another bubble from a different circuit element. A nonmagnetic electrically conductive layer is applied over the first layer to the appropriate thickness to reduce magnetic coupling in order to facilitate deflecting a bubble from a particular overlay element or elements. A pattern is defined and etched in the metal layer to serve as a thickness mesa in those areas where it is desired to reduce coupling to overlay elements. The circuit overlay of soft ferromagnetic material is deposited and etched over the mixed topography of the composite spacing layer such that circuit elements from which bubbles can be deflected are located on the raised mesa portions of the stratified spacing layer; elements carrying out normal propagation or guiding deflector bubbles are located on the lower level at closer spacing to the magnetic bubble material to enhance coupling.

In this embodiment the upper portion of the stratified spacing layer can simultaneously be etched to provide the electrical control leads normally required for functions like transfer generation, readout and annihilation.