US3574679A - Process for embedding or encircling polycrystalline materials in single crystal material - Google Patents

Abstract

CRYSTALLINE GROWTH AT NUCLEATION SITES ON THE POLYCRYSTALLINE BODY DOES NOT OCCUR. IN AN EXEMPLARY EMBODIMENT, EPITAXIALLY GROWTH MONOCRYSTALLINE FERRITE EMBEDS POLYCRYSTALLINE METALLIC CONDUCTORS TO FORM A STRUCTURE USEFUL AS A MAGNETIC MEMORY.

A PROCESS FOR ENCIRCLING A POLYCRYSTALLINE BODY IN SINGLE CRYSTAL MATERIAL. A SINGLE CRYSTAL SEED IS PLACED IN A REACTOR ADJACENT THE POLYCRYSTALLINE BODY. CHEMICAL VAPOR DEPOSITION IS USED TO DEPOSIT EPITAXIALLY A SINGLE CRYSTAL MATERIAL ON THE SEED, GROWTH BEING CONTINUED UNTIL THE DEPOSIT BUILDS ONTO AND AT LEAST PARTIALLY SURROUNDS THE BODY. THE RATE OF DEPOSITION IS CONTROLLED SO THAT POLY-

Description

April 13, 1971 R PULLIAM ETAL 7 3,574,679-

PROCESS FOR EMBEDDING 0R ENCIRCLING POLYCRYSTALLINE MATERIALS IN SINGLE CRYSTAL MATERIAL Filed Jan. 25, 1965 2 Sheets-Sheet l l I w I 4 k u I I 5 I II. I III I il I III I I! |l| I" l I IN'VENTORS I GEORGER. PULLIAM JOHN L- ARCHER BY 2 5C fimf w ATTORNEY Aprnl 13, 1971 R PULUAM ETAL 3,574,579

PROCESS FOR EMBEDDING OR EN IRGLING POLYCRYSTALLINE MATERIALS IN SINGLE CRYSTAL MATERIAL 7 Filed Jan. 25, 1965 2 Sheets-Sheet 2 FIG.4 l4

FIG. 3

- INVENTORS GEORGE R PULLI AM JOHN L. ARCHER ATTORNEY United States Patent 3,574,679 PROCESS FOR EMBEDDING OR ENCIRCLING POLYCRYSTALLINE MATERIALS IN SINGLE CRYSTAL MATERIAL George R. Pulliam and John L. Archer, Anaheim, Calif.,

assignors to North American Rockwell Corporation Filed Jan. 25, 1965, Ser. No. 427,804 Int. Cl. C23c 11/08 US. Cl. 117-212 Claims ABSTRACT OF THE DISCLOSURE A process for encircling a polycrystalline body in single crystal material. A single crystal seed is placed in a reactor adjacent the polycrystalline body. Chemical vapor deposition is used to deposit epitaxially a single crystal material on the seed, growth being continued until the deposit builds onto and at least partially surrounds the body. The rate of deposition is controlled so that polycrystalline growth at nucleation sites on the polycrystalline body does not occur. In an exemplary embodiment, epitaxially grown monocrystalline ferrite embeds polycrystalline metallic conductors to form a structure useful as a magnetic memory.

This invention relates to a deposition process for embedding or encircling polycrystalline materials in single crystal materials.

Previous attempts to either embed or encircle a polycrystalline material in a single crystalline material have resulted in a polycrystalline deposition on the surface of the encapsulated polycrystalline material. In effect, the polycrystalline material was encapsulated in a polycrystalline deposition. Thus, the advantages of a single crystal embedment or encirclement were not realized.

For example, as the name implies, a single crystal material has one continuous crystalline structure and has uniform characteristics throughout its structure. Also, characteristics from one production run to another are uniform. The opposite result is true of polycrystalline production runs. It is difiicult to produce substantially identical characteristics in polycrystalline materials. On the other hand, the properties of single crystals are more predictable and, thus, their controllability is enhanced.

Single crystal magnetic materials are anisotropic and susceptible to easy magnetization in a given crystallographic direction. Single crystal materials may, therefore, be used as magnetic memory devices.

In a polycrystalline material the easy direction of magnetization for all of crystals are not uniformly oriented and, therefore, point in random directions. In single crystal materials the easy direction of magnetization points in precise and known directions and is more predictably rotated in response to an electrically produced magnetic force.

Since the single crystal material is more uniform throughout, in its magnetic characteristics, than polycrys talline material, less power vis required in magnetizing a selected area. Saturation of a single crystal material is achieved more easily than in polycrystalline materials and, thus, less current is required. For example, if the material is used to produce the core of a miniaturized inductor by encapsulating a polycrystalline conductor, a higher and more uniformly repeatable inductance could be obtained from a single crystal device as compared with a polycrystalline device using the same amount of current.

It is also possible to embed or encircle a polycrystalline material with in a single crystal structure and subsequently dissolve the encapsulated polycrystalline substance leaving holes, openings, or channels in the single crystal. Such structures might be useful in laser devices where single "ice crystals are frequently used. The holes or channels might be used for circulating coolants through the crystal for removing heat.

Embed or encircle in addition to the ordinary meaning of such words includes encapsulate and enclose wherein, for example, a polycrystalline substance is entirely surrounded by a single crystal. The terms also include situations wherein the polycrystalline material is not entirely surrounded.

Another use of the process is for forming metal base transistors having single crystal semiconductor layers on each side.

Other circuit schemes and devices may also be produced by the embedding or encircling process.

Accordingly, it is an object of this invention to embed or encircle polycrystalline materials in single crystal materials.

It is another object of this invention to :produce improved magnetic devices by use of a process for embedding or encircling polycrystalline materials in single crystal materials.

It is still another object of this invention to provide processes for producing a magnetic memory device.

A process has been invented for embedding or encircling polycrystalline material in single crystal material. It has been demonstrated that when a polycrystalline material and a single crystal substrate material are placed in close proximity in a chemical vapor deposition chamber, and if the crystal structure of the single crystal substrate material is similar to that of the depositing reaction product, the single crystal substrate will be the preferred deposition site. Furthermore, it has been demonstrated that a critical rate of reaction may be established such that all of the deposition occurs on the preferred single crystal substrate material, thus allowing no polycrystalline deposition on the polycrystalline material. Therefore, the critical rate of reaction may be established empirically by observing deposition within an environment. The rate of reaction may be changed by varying the gas fiow rates of the reacting elements until a single crystal growth rate is established which predominates over the nucleation rate.

A critical rate of reaction may be determined for each material being deposited and for each substrate. Each combination of deposited material and substrate material has a different critical reaction rate. That is to say that each combination of deposited material and substrate material has a growth rate at which the deposition is single crystal in nature rather than of a polycrystalline nature. Thus, no material is deposited on a polycrystalline material which might be present in the reaction chamber since this would require nucleation. Although the polycrystalline material is placed in the path of the crystal growth of the depositing material, the crystal structure of the deposit is influenced entirely by the single crystal substrate and the polycrystalline material is embedded or encircled in the single crystal deposit with no polycrystalline deposit inclusions.

In one embodiment of this invention, a polycrystalline material and a single crystal substrate material are placed in close proximity in a vapor deposition chamber. The crystal structure of the single crystal substrate material is similar to that of a deposition reaction product so that the single crystal substrate is the preferred deposition location. A critical rate of reaction is established and all of the deposition occurs on the preferred single crystal substrate material and no polycrystalline deposition occurs on the polycrystalline material.

In another embodiment, a single crystal material is first deposited on a single crystalline substrate and subsequently the polycrystalline material is deposited. Following deposition of the polycrystalline material at a designated location on the single crystal material, the single crystalline deposition is continued until the poly crystalline material is embedded or encircled, as desired.

In one specific application of the process, a magnetic memory device is prepared by producing a single crystal ferrite body which is magnetically anisotropic and is susceptible to easy magnetization in given crystallographic direction. In one embodiment of the produced device, a plurality of electrical conductors in insulated crossover relationship are incorporated and substantially surrounded by the ferrite body so that a magnetic orientation may be obtained in the ferrite by passing electric current through one or more of such conductors.

In another embodiment of the memory device produced by the inventive process, after the electrical conductors are disposed on the single crystal ferrite surface 'having a preselected direction relative to the crystal ferrite and approximately insulated at the crossover point, additional single crystal ferrite material is deposited thereover so as to encapsulate at least the crossover point in single crystal ferrite material. For example, the direction of the conductors might be parallel, orthogonal or at an angle to a selected crystal direction.

It is a preferred feature of the process of the present invention that the ferrite body is formed by reacting selected metal halides and water vapor to epitaxially deposit the ferrite material in a preselected crystallographic form.

In the following description and examples, the process is described in terms of producing a magnetic memory device by encapsulating polycrystalline conductors in a single crystal ferrite material. It should be understood that the description and examples are for the purpose of illustrating embodiments of the broad inventive process for embedding or encircling polycrystalline materials in single crystal materials and that the description and examples are not exhaustive of the uses nor the scope of the invention. As indicated above, the process may have many applications and may produce many differing devices.

Other objects and features of the invention will become apparent from the following description taken in light of the figures in which:

FIG. 1 is a representation of an apparatus used in embedding or encircling polycrystalline material in single crystal material.

FIG. 2 is a cross-sectional view of the apparatus of FIG. 1 taken on line 2-2.

FIG. 3 is a cross-section of a portion of a magnetic memory device which may be produced by the present invention.

FIG. 4 is a pictoral representation of a memory system utilizing a magnetic memory device produced by the present invention.

FIG. 5 is a cross-section view of the memory device showing insulation between the conductors.

Referring now to FIG. 1, the apparatus of the present invention comprises a chamber 1, preferably of T shape and including inlet means 2 for injecting gases into one end of the chamber 1, and an exhaust 18 at the other end of member 4. The cross member 4 is surrounded by a heating element to control the temperature within the member 4. Supported inside chamber 1 are a plurality of spaced crucibles 5. The spaced crucibles are each attached to a central holder rod 6 but may be maintained in a central position by any well known means. Adjacent to the outer surface of chamber 1 and positioned to surround each crucible 5 is a heater element 7. Each element 7 may be controlled independently so that the zone in which each crucible 5 is located may be heated to a preselected temperature. Within the cross member 4 is a quartz holder 8 on which substrates or crystals 10 are supported. The crystals 10 are located along cross member 4 from a point where chamber 1 joins the cross member 4 toward the exhaust port so that each is exposed 4 to a mixture of the gases flowing from inlets 2 and 3. Inlet 2 is connected to a source (not shown) of a dry mixture of He and Ar. Inlet 3 is connected to a source (not shown) in which helium, argon and oxygen are bubbled through water to produce a mixture of inert gases, Water vapor, and oxygen, e.g., He, Ar, H 0, and 0 The substrates 10 on which the ferrite crystal is deposited or grown are comprised of material having crystal structure similar to that to be deposited. For the purpose of describing the ferrite memory produced by the invention, the substrate material selected is MgO, although other substrate materials, e.g., MgAl O A1 0 and other materials having the formula MeMe "O may also be utilized in producing tre memory. The terms, Me and Me are defined hereinafter.

It ShOuld be pointed out that the process is not limited to use of a particular substrate material or crystal orientation. It is not limited to the use of spinel materials set forth herein. The process and materials used in the process are limited only by the crystal orientation of the deposited material and the process used in the deposition. For example, the material may be deposited by chemical vapor techniques. If so, then various materials may be deposited. The substrate material is then selected with a crystal orientation which will permit single crystal deposition thereon. The deposition rate is dependent on the type of substrate and deposited material.

The substrates 10 may be prepared by cleaving optical grade MgO along a cleavage plane or by cutting the MgO into plates of desired configuration along its other crystal faces. The plates are ground flat to produce a plate having a selected size and a selected crystallographic plane and then chemically polished in an acid etch solution.

The substrates 10 are supported on holder 8 inside the cross member 4 and a ferritic layer may be deposited as described in detail hereinafter.

The source materials 11 for producing the ferrite deposit on the substrate 10 are placed in containers 5 and heated to vaporization by various heater elements 7. The source materials placed inside containers 5 may be Me'X Me"X or a mixture thereof where Me may be Li, Mg, Mn, Fe, Co, Ni, Cu, Zn, or Cd; where Me" may be Al, Cr, Mn, Fe, or Ti; and where X is a halide (F, Cl, Br, I), provided that one of the Me or Me" is Fe, and the subscript a is either 1, 2, 3, or 4 to correspond with the valence of the cation. Simultaneously with the heating of material 11, the above described carrier gases are admitted through inlets 2 and 3.

The following reaction takes place at the surface of the substrates 10 to deposit a ferrite film on the MgO substrate crystals:

MO'XZ 2Me"X.. 31120 oz inert gases Me(Me Fe) 04 GHX inert gase The term ferrite as used herein refers to compositions of iron oxide either alone or in chemical combination with at least one other metallic oxide to form a magnetic material. Thus, the expression Me'(Me"lFe) O refers generally to these ferrite materials without reference necessarily to any particular stoichiometric or empirical composition. Most ferrites of commercial interest consist of one or two metallic oxides in chemical combination with iron oxide. Thus, the ferrites formed are preferably those of commercial interest having low coercive forces, e.g., MnFe O NlFCgO MgFe204, ZnFe O (3111 6204, and combinations of these compounds. Other ferrite material may be used depending upon the desired application.

After the first ferrite layers are deposited on the sub strates, the substrates are removed and placed in a con ventional vacuum deposition chamber (not shown). In the chamber, one or more polycrystalline conductors comprising a first array of parallel conductors are deposited on the substrate surfaces in a preselected direction relative to the crystalline structure of the ferrite, by methods well known in the art, e.g., vacuum deposition or sputtering. Electrically conductive materials such as gold, silver, platinum,'or copper may be used, however gold is preferred because of its excellent electrical characteristics. These conductors are preferentially oriented with respect to the crystal plane to provide modes of magnetization along different directions. (See Smith et al., Ferrites]ohn Wiley & Sons, 1959.)

After the first array of conductors is deposited, patterns of insulation material are deposited in a selected pattern to coat portions of the conductors of the first array. Such insulation materials as MgO, A1 and BaF may be deposited by standard deposition techniques. Subsequently, one or more conductors constituting a second array of conductors are vacuum deposited in a manner so as to crossover at a preselected angle to the first array and yet be insulated therefrom by the insulation materials previously deposited.

After the conductor arrays have been deposited with appropriate insulation at each crossover point, the substrate is preferably placed in the chamber and a second ferrite body is deposited on this existing ferrite layer and around the conductors to encircle at least the crossover points of the conductor arrays within the single crystal ferrite mass. The entire conductor may, of course, be encircled.

Also, instead of embedding or encircling a plurality of conductor patterns or arrays, it may be preferable in some instances to encircle a single conductor array and then deposit a second array and encircle it. In that embodiment, single crystal ferrite would separate the conductor arrays whereas in the previous embodiment, the conductor arrays were separated by an insulation material other than the ferrite.

FIG. 2 is a cross-sectional view of the chamber illustrating with greater clarity the position of the cross member 4 and crystals 10 inside the chamber.

In FIG. 3, a cross-section of an encapsulated conductor fabricated in accordance with the above described preferred processes is illustrated. As shown therein, layer 12a represents the first ferrite deposition layer on substrate 13, conductor array 14 represents one of the conductors of an array, while the second conductor array runs orthogonal to conductor array 14 and cannot be seen in this cross-section. The ferrite material 12b provides the completion of encirclement. A pictorial representation of the device shown in FIG. 3 is shown in FIG. 4, which shows the plurality of conductor arrays 14 and 16 angularly disposed with each other whose encirclement is completed by the ferrite 12b. Ferrite has been deposited on a substrate 13 and the conductors are insulated from each other by insulation 17.

FIG. is a cross-section view of a portion of FIG. 4 showing insulation 17 between conductor arrays 14 and 16.

The practice of the inventive method for producing, in one embodiment, an epitaxial ferrite memory device is described more fully with reference to the following examples:

EXAMPLE I The substrate material, MgO, is prepared by cleaving the crystal along a (100) cleavage plane into approximately one-inch square substrates. After cleaving, the substrate is mechanically ground fiat on a series of metallographic papers to the 4/0 size. They are then chemically polished in a 3:1, concentrated H PO concentrated H 80 etching solution heated to 125 C. for two hours. After the two-hours of etching, the substrates are given a thorough hot water rinse.

A quartz T shaped apparatus of FIG. 1 having a 45 mm. I.D., was utilized as the deposition chamber. A quartz holder is used to support a plurality of MgO crystals in the middle of the cross member 4 of the apparatus. Quartz crucibles 5 supported by quartz rods 6 contained the source materials 11. These crucibles are each carefully located in the chamber 1 with each source material having an individually controlled electrical heater 7. In this example, the source materials are MnBr FeBr and NiBr arranged in the chamber 1 in the stated order from top to bottom. This order is determined by the temperatures necessary to volatilize the various source materials. After the crystals and the containers are placed in the cross member 4, the gas flows are adjusted and the cross member 4- and suspended substrates 10 are heated to the desired temperature of about 1000 C. The various heating elements 7 for the source materials 11 in the chamber 1 are then turned on. The MnBr is heated to about 800 C., the FeBr to about 700 C. and the NiBr to about 600 C.

To obtain maximum interaction between the source material vapors, mixtures of helium and argon are used to carry the source material vapors and the water vapor into contact with the substrate surfaces. A mixture of 5 cubic feet per hour of He and 10 cubic feet per hour of Ar is used to carry the source material vapors. A mixture of 6 cubic feet per hour of He and 3 cubic feet per hour of Ar is used to carry the water vapor. When the desired temperatures are reached, the carrier gases are rechanneled through a water bubbler, thus causing Water Vapor to be carried into the reaction chamber. The fiow rates for the fluids was established empirically to promote single crystal growth rather than nucleation. Ferrous iron (Fe+ is oxidized to ferric iron (Fe+ by the addition of 0.08 cubic feet per hour of O to the reaction chamber at the time the desired temperature is reached.

The reaction to form the ferritic layer was as follows:

1000 C. 0.3MnBr 0.55 NiBlz 2.15 FeBrz 3H2O %O2 In? As a result of this reaction, an epitaxial, single crystal ferrite layer or body is deposited on the MgO substrate.

X-ray examination may be utilized to confirm that the ferrite layer is a single crystal.

The crystal is then placed in a conventional vacuum chamber for deposition of the first gold conductor array through a mask positioned on ferrite layer. This array may be deposited parallel with the l10 or crystal directions.

The gold conductors were 2 mils wide and 0.25 mil thick and were spaced on 10 mil centers on the substrate layer. The conductor size may be varied. The conductor mask is then removed and replaced by a transverse mask for depositing insulation material on the conductors. The insulation, BaF may be limited by a mask or equivalent mechanism to the conductors at the point of crossover. On the other hand, it may also cover the entire array of conductors and etched to produce the desired arrangement of insulation pads. The insulation material may be deposited by any method well known in the art.

After the insulation deposition was completed, a second mask is inserted into the chamber to define a second array of conductors disposed crosswise at a preselected angle to the [first deposited conductor array. The preselected angle of the second array in this example was normal to the first, although other angular relationships may be utilized. In this manner, the second array of deposited conductors overlaps the first conductor array in crossover relation and is insulated therefrom by the insulation material previously deposited.

After the second conductor array is deposited, the substrate with the conductors thereon is placed in the reaction deposition chamber of FIG. 1 and a second layer of single crystal ferrite is deposited as described above. The second layer of ferrite single crystal, together with the first ferrite layer on which it was deposited, encircles the conductor arrays within the single crystal epitaxial ferrite, thereby providing an epitaxial ferrite memory device. In this embodiment, the second layer of ferrite crystal is a continuation of the single crystalline structure of the first layer.

EXAMPLE II Using the apparatus and process steps of Example I, the substrate material is cut so as to provide a (110) crystallographic plane as the substrate surface. The conductor arrays are then deposited on a ferrite layer, as described above, parallel to the 001 direction in the (110) plane or at 45 to that direction. The single crystal ferrite layer was then deposited as described in Example I, thereby providing an epitaxial ferrite memory device.

EXAMPLE 111 Using the apparatus and process of Example I, the substrate material is cut so as to provide a (111) crystallographic plane as the substrate surface. The conductor arrays are then deposited, as described above, parallel to the 111 direction in the (111) plane or parallel to the 112 direction, thereby providing an epitaxial memory device.

EXAMPDE IV Using the apparatus source materials and process steps described in Example 'I, the temperature of the source material heaters was varied to produce controlled variations in the ferrite compositions. The source temperature range was from about 500 C. to about 900 C. with the ferrite composition varying from high nickel and low manganese ferrites to low nickel and high manganese ferrites. Thus, by appropriate selection of the temperature the deposition may be controlled to provide a desired ferrite composition. All temperature conditions within the range produced single crystal ferrite memory devices of acceptable quality.

EXAMPLE V Using the apparatus and process steps described in Example I, the bromides of Mg and Co were substituted for the bromides of Mn and Ni utilized in Example I. The modified process produced single crystal ferrite memory devices of acceptable quality on a MgAl O substrate.

Other variations may be made without departing from the spirit and scope of the invention. For example, other polycrystalline materials as Well as electrical conductor arrays may be utilized. Conductors or other polycrystalline materials may be embedded or encircled for forming miniaturized inductors. Metal base transistors as well as other circuit components including integrated circuits may be at least partially formed by use of the process. For example, polycrystalline noble metals, copper, and other conductor metals, oxides, and soluble inorganics, may be similarly embedded or encircled in Si, Ge, GaAs, CdTe, CdS, A1 and MgO grown in single crystal form around such polycrystalline material. The invention may be used to produce other devices consistent with encapsulating a polycrystalline material in a single crystal material.

These and other modifications in the processes of the present invention will be apparent to those skilled in the art. Therefore, the present invention is not limited to the specific details of the examples described but only by the appended claims.

We claim:

1. A process for embedding polycrystalline electrical 8 conductors in single crystal ferrite, said process comprising the steps of:

(a) heating in a chamber a single crystal substrate selected from the class consisting of MgO, MgAl O and A1 0 to a temperature on the order of 1,000

(b) vaporizing, at other locations in said chamber, halides of iron and one more source materials selected from the class consisting of Mn, Ni, Fe, Mg and Co,

(c) transporting the vaporized halides to said heated substrate in a carrier gas comprising approximately 5 cubic feet per hour of helium and approximately 10 cubic feet per hour of argon,

(d) transporting water vapor to said substrate in a carrier gas comprising a mixture of approximately 6 cubic feet per hour of helium and 3 cubic feet per hour of argon, whereby said halides and said water vapor react to form an initial layer of single crystal ferrite epitaxially atop said substrate,

(e) depositing an array of gold conductors atop said initial ferrite layer,

(f) reheating the combined substrate, initial layer and conductors in said chamber to a temperature on the order of l,000 C., and

(g) repeating steps c and (1, thereby preferentially depositing additional single crystal ferrite atop said initial layer, said ferrite embedding said conductors.

2. The process defined in claim 1 wherein said substrate comprises MgO, wherein said halides comprise MnBr heated to about 800 C. to achieve vaporization thereof, FeBr heated to about 700 C., and Ni-Br heated to about 600 C., and wherein approximately 0.08 cubic feet per hour of 0 also is introduced into said chamber, whereby the deposited ferrite has the formula 3. The process as defined in claim 1 wherein between steps e and f are added the steps of:

depositing an insulating material comprising BaF atop at least a portion of said conductors, and

depositing a second array of conductors crossing said first array and insulated therefrom by said BaF 4. The process defined in claim 1 wherein said source temperature range is varied from about 500 C. to about 900 C. to control the composition of nickel and manganese in the deposited ferrite.

5. The process defined in claim 1 wherein said source materials comprise MgBr CoBr and FeBr and wherein said substrate comprises MgAl O References Cited UNITED STATES PATENTS 3,189,973 6/1965 Edwards et al. l48174X ALFRED L. LEAVITT, Primary Examiner J. H. NEWSOME, Assistant Examiner U.S. Cl. X.R.

Patent No. 3 574 579 ated April 13 1971 Inventor(s) Pulliam et 1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1 column 8 line 8 after "one" and before "more" insert or Signed and sealed this 4th day of January 1972 (SEAL) gAttest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Pate

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