US 3854892 A
A method is described for direct bonding of metallic members to other metallic members with a metal-gas eutectic. The method comprises placing a metal member such as copper, for example, in contact with another metal member, such as nickel, for example, heating the metal members to a temperature slightly below the melting point of the lower melting point metal, e.g., approximately 1072 DEG C. for copper, the heating being performed in a reactive atmosphere, such as an oxidizing atmosphere, for a sufficient time to create a metal-gas eutectic melt which, upon cooling, bonds the metal members together. Various metals and reactive gases are described for direct bonding.
Description (OCR text may contain errors)
United States Patent [19 Burgess et a1.
m I 3,854,892 1 Dec. 17, 1974  Filed:
[ DIRECT BONDING OF METALS WITH A METAL-GAS EUTECTIC  Assignee: General Electric Company,
Mar. 1, 1973 21 Appl. No.: 337,143
Related U.S.- Application Data  Division of Ser. No. 245,890, April 2(1, 1972, Pat.
 U.S. Cl 29/l96.l, 29/194, 29/1962, 29/l96.3, 29/l96.6,' 29/196, 29/197,
 Int. Cl B32b 15/00, B32b 15/20  Field of Search 29/494, 196.1, 196.2, 196.3,
 References Cited Simpelaar 29/194 X 3,093,885 6/1963 Morrison et al. 29/l96.2 3,322,517 5/1967 Miller 29/l97.5 3,667,110 6/1972 Primary ExaminerC. Lovell Assistant ExaminerArthur J. Steiner Attorney, Agent, or FirmPaul F. Wille; Joseph T. Cohen; Jerome C. Squillaro [571 ABSTRACT A method is described for direct bonding of metallic.
members to other metallic members with a metal-gas eutectic. The method comprises placing a metal member such as copper, for example, in contact with another metal member, such as nickel, for example, heating the metal members to a temperature slightly below the melting point of the lower melting point metal, e.g., approximately 1072C. for copper, the heating being performed in a reactive atmosphere, such as an oxidizing atmosphere, for a sufficient time to create a metal-gas eutectic melt which, upon cooling, bonds the metal members together.- Various metals and reactivegases are described for direct bond- 7 Claims, 3 Drawing Figures Gwyn 29 494 PATENTS} B531 3,854,892
PLACE METAL MEMBERS //V CONTACT WITH EACH OTHER HEAT //v REACT/V5 ATMOSPHERE 7'0 1 FORM EUTECT/C EUTECT/C MELT WETS METAL MEMBERS COOL TO FORM BOND BE TWE E /V METAL ME MBE RS GAS OUTLET DIRECT BONDING OF METALS WITH A METAL-GAS EUTECTIC This is a division, of application Ser. No. 245,890, filed Apr. 20, 1972 now U.S. Pat. No. 3,744,120.
The present invention relates to improved bonds and methods of directly bonding two or more metallic members together. This application relates to our concurrently filed application Ser. No. 245,889, now US. Pat. No. 3,766,634, of common .assignee. The entire disclosure of which is incorporated herein by reference thereto.
The formation of bonds between metallic members is achieved in various, ways. For example, certain metals can be bonded together with the use of solders. Other metals are bonded together by welds, such as are welds or spot welds. Where certain metals cannot be directly bonded to each other, generally intermediate metallic members are used to form the bond. The need for simple methods of forming bonds between similar and dissimilar metals still exists. For example, in the fabrication of semiconductor integrated circuits, tenacious bonds between various metals arerequired. In addition, it is desirable to provide low ohmic contact between such metals. The foregoing methods are frequently not compatible with integrated circuit fabrication and even if compatible arefrequently economically unacceptable. Accordingly, a need for a simple and'economically acceptable method of forming bonds between metallic members is still desired.
It is therefore an object of this invention to provide a method of forming bonds between metallic members with a metal-gas eutectic composition.
It is yet another object of this invention to provide a method of bonding metallic members together without the use of intermediate metal layers.
Another object of this invention is to provide a method of bonding metallic members together in a simple heating step without the need for intermediate flux.
Yet another object of this invention is to provide a tenacious bond and a method of forming this bond between metallic members which bond exhibits low ohmic resistance and is compatiblewith the fabrication of semiconductor integrated circuit modules.
Briefly, our invention relates to bonds and methods of bonding together metallic members by placing at least two metallic members in contact with each other and elevating the temperature of the members in a reactive atmosphere of selected gases and at controlled partial pressures for a sufficient time to produce a metal-gas eutectic composition on the surface of at least one of the metallic members. This eutectic composition or melt forms at a temperature below the melting point of one of the metallic members and wets both metallic members so that upon cooling, a tenacious bond is formed between the metallic-members. By way of ex-' become more apparent to those skilled in the art from the following detailed description taken in connection with the accompanying drawings in which:
FIG. 1 illustrates a typical bond between two metallic members formed in accord with our invention;
FIG. 2 is a flow diagram illustrating the process steps for forming tenacious metal-to-metal bonds in accord with our invention; and i a FIG. 3 schematically illustrates a horizontal furnace usefulin practicing our invention.
FIG. 1 illustrates, by way of example, a typical bond 11 between metallic members 12 and 13. The bond 11 comprises a eutectic composition formed with at least one of the metallic members and a reactive gas in accord with the novel aspects of our invention.
As used herein, the term metallic member or material is intended to include such materialsas copper, nickel, iron, cobalt, chromium, silver, aluminum, alloys of the aforementioned elemental materials, and stainless steel. As will become more apparent'from the following description, still other metallic materials, such as beryllium-copper, for example, may also be, advantageously employed, if desired.
,The term eutectic'or metal-gas eutectic composition as used herein means a mixtureof atoms of the metallic member and the reactive gas or'compound formed between the metal and the reactive gas; but does not include eutectics formed by the reaction or mixing of two metals rather than a reaction between a metal and a ,component of a gas. For example, where the-metallic member is copper and the reactive gas is oxygen, the
member is cobalt and the reactive gas is a sulfur-' bearing gas, the eutectic is formed between cobalt and cobalt sulfide.
The novel process for making tenacious bonds between metallic members 12 and I3 is illustrated inthe flowchart of FIG. 2. More specifically, FIG. 2'illustrates the practice of our invention by placing two metallic members in contact with each other, such as one member overlying another. These members are then placed in a suitable furnace, such as is described below, which includes a reactive atmosphere 'such that upon heating of the metallic members, a.metal-gas eutectic composition forms. The temperature-at which the desired eutectic composition forms and the partial pressure of the'reactive gas necessary to form the desired eutectic composition depend upon the selected metallic members and the reactive gas. lngeneral, however; the partial pressure of the reactive gas must exceed the equilibriumpartial pressure of the reactive gas in the metal at or above the eutectic temperature. For example, when bonding copper members together, a reactive atmosphere including oxygen, for example, re-
quires a partial pressure of oxygen in excessof 1.5 X
' 10' atmospheres at the eutectic "temperature of l065C. Other metallic materialsand other reactive gases require different partial pressures and different temperatures to form the desired eutectic.
Table I is a representative listing of typical eutectic compositions which are useful in practicing our invention. These eutectics are formed by reacting themetallic members to be bonded with a reactive gas controllably introduced into an oven or furnace.
. TABLE 1 Per Cent by Weight Metal-Gas Eutectic of Reactive Gas Eutectic Temperature, C. at Eutectic Composition iron-Oxygen v l523 O. l 6 O Copper-Oxy en lO65 0.39 Chromiumygen l800 0.6 0 Chromium-Sulfur 1550 2.2 S Copper-Phosphorus 714 8.4 P Nickel-Oxygen 1438 0.24 I O Nickel-Phosphorus 880 11.0 P Molybdenum-Silicon 2070 5.5 Si Silver-Sulfur 906 1 .8 V S Silver-Phosphorus 878 1 .0 P Copper-Sulfur 1067 0.77 S Co alt-Oxy en l45l 0.23 0
Aluminumilicon 577 11.7 Si
The eutectics listed in Table I are formed by reacting the metallic membersin an oxygen-bearing gas,-such as oxygen, a sulfur-bearing gas, such as hydrogen sulfide, a phosphorus-bearing gas, such as phosphine, or a silicon-bearing gas, such as silane. At the eutectic temperature of the selected. metallic member and the reactive gas, such as those temperatures listed inTable l, the eutectic composition becomes a' liquid and wets the adjoining member so that upon cooling, the metallic members become tenaciously bonded together.
Table, ll illustrates, by way of example, typical metalto-metal bonds formed in accord with our invention and the conditions under which the bonds are formed. For these conditions, the reactive gas is oxygen.
TABLE'II Time at Temperature, Elevated Metals 1 Thickness C Temperature Cu Cu 5 mils lO72C 0.5 hrs. Cu Ni 5 mils 1072C LO hrs. Cu-Stainless Steel 5 mils 1072C 1.0 hrs. I i i IO mils 1445C 1.0 hrs.
' Fe Fe It) mils 1530C 1.0 hrs.
- Co C0 l5 mils 1458C in'Table ll are byway of example',and not by way of tallic member, for example, themetallic member tends to plastically conformto the shape of theother member and thereby produce better bonds than those eutectics which become liquids at temperatures greaterthan approximately 50C. belowthe meltingpoint of the metallic member/The uniformity -of the bond therefore appears to be related to the creep of the metal which becomes considerableonly near the melting point.
' From Table -I, for example,-it canbe seenthat-the folour invention and the methods of forming metal-tometal bonds, apparatus useful in'practicing our invention along with more specific details of theproceSs will now be describedwith'reference to FIG. 3.-
FIG. 3 illustrates a horizontal furnacecomprising an elongated quartz tube 22, for example, having 'a gas other end. The quartz tube'22 also includes an opening Y The examplesof metal-to-metal bonding illustrated limitation. In general, most metals which form a metah gas eutectic .in a reactive atmosphere are useful in forming metal-to-metal bonds.- The bonding can be between like metals, dissimilar metals, or even alloys. For
example, where like metals are bonded together, the
eutectic forms on both surfaces of the members. Where dissimilar metals are bonded together, the eutectic generally forms on at least one surface of the metal having the lower eutectic-forming temperature. For example, as in the case of copper-nickel, the eutectic forms with the copper. The eutectic then wa both metal surfaces thereby forming the desired bond. Where alloys are" employed, such as the various alloys of nickel, iron, cobalt, copper, silver, chromium'and aluminum are em ployed,-the eutectic composition is believed to form' with one of. the elemental metals, generally the one withthe lower melting point;
inlet 23 atone end thereof and a gas outlet24 at the or port 25 through which materials are placed into and removed from the furnace. Materials are placed on a' holder 26 having a push *rod- 27-extending'throughone endof the.furnace so that the holder and materials placed thereon may be introduced and removed from thefurnace. r
. The furnace 21 .is also providedwith suitable heating .elements, illustrated-inFlG.-3'.as electrical wires 28 which surround the quartz: tube 22 in the region to be' heated. The electrical wires 28 may, for example, be
. connected to a suitable current source, such as a 220- volt alternating currentsource. The electrical wires 28 may then be surrounded by suitable insulatin'grnaterial Y 29 to confine the heat generated by the electrical'wires to the region within, the quartz tube. Obviously those skilled in the art can appreciate that" other. heati-ng' means may also be employed, if desired, and that FIG.
3 is merely illustrative of one such heating means T'he temperature of the furnace is detected by' a suitable thermocouple 29 whichextends through an opening in the quartz tube soth'at electricalconnections can be made thereto. FIG. 3 also illustrates a metallic .member 12 positioned on theholder 26 and a metallic member One factor which appears to affect the tenacity and uniformity of metal-to-metal bonds formed in accord with our invention-is the relationshipbetween the melting point of the metallic member andtheeutectic tem-' perature. Where the eutectic temperature is within apflow of approximately 4 cubic feet per hour of nitrogen l3 overlying the member 12. These metallic members are introduced into the quartz tube through theopening 25 which is then sealed by suitablestopper means.
The quartz tube 22 is then purged with a reactive gas and-0.02 cubic feet per hour of oxygen, for example; As
-' used herein, reactive gas flow or. atmosphere means a mixture of an inert gas such as argon, helium, or nitrothe thickness of the materials, and the gas flow rate, in
a manner more fully described below. In general, however, the partial pressure of the reactive gas must exceed the equilibrium partial pressure of the reactive gas in the metal at or above the eutectic temperature. As
pointed out above, when bonding copper members together, a reactive atmosphere including oxygen, for example, the partial pressure of oxygen must be in excess of 1.5 X atmospheres at the eutectic temperature of 1065 C.
After purging the quartz tube, the furnace is then i brought to a temperature sufficient to form a eutectic melt at the metal-to-metal interface. For example, for
a copper-nickel bond with oxygen as the reactive gas,
the temperature of the furnace is brought to approximately lO72C. At this temperature, a copper-copper oxide eutectic forms on the copper member and wets the copper member and the nickel member so that upon cooling, a tenacious bond is formed between the two metals.
In general, the times necessary to form this eutectic melt range between approximately 10 minutes for 1- mil-thick copper members and approximately 60 minutes for 250-mil-thick copper members, for example. For metallic members of other thicknesses and geometric configurations, the times required to form the eutectic melt vary. In general, the longer the metallic members are held at the eutectic temperature, the thicker the eutectic will be. The thickness of the eutectic also depends upon the partial pressure of the reacting gas. As pointed out previously, a partial pressure below the equilibrium partial pressure for the specific eutectic will result in no eutectic formation. Hence partial pressures in excess of this equilibrium value are required to produce the desired eutectic. If the partial pressure of the reacting gas is too high, however, all the metal reacts with the reactive gas and forms, for example, an oxide, sulfide, phosphide, etc. which prevents the formation of the eutectic melt. Thus, an intermediate reacting gas partial pressure is required so that both the eutectic melt phase and the metallic phase-are present simultaneously. Tests have illustrated that extremely strong bonds are achieved when both phases are present. Accordingly, in practicing our invention the partial pressure of the reacting gas must be sufficiently great to permit the formation of a eutectic with the metal but not so great as to completely convert the metal to the oxide, sulfide, phosphide, etc. during the bonding time.
We have found that consistently good bonds are achieved between metallic members so long as the aforementioned conditions are met; However, no bonding occurs where the partial pressure of the reactive gas is less than the equilibrium partial pressure at the eutectic temperature and no bonding occurs where the partial pressure of the reactive gas is such that all the metallic member is converted to an oxide, phosphide, sulfide, etc.
Also, those skilled in the art can appreciate that the gas flow rate is not critical to the practice of our invenof example, selected metals and the percent of reactive tion and may'be-varied over wide ranges without mate- I rially affecting the integrity of the bonds. However,
economic considerations will generally control the acceptable gas flow rates. Further, the partial pressure of the reactive gas-in the inert gas also can be varied, de-
pending in part on the relative sizes of the materials to be bonded. The gas flow rate and the presence of reactive elements in the flow system, such as carbon susceptors, the presence of residual oxygen or water in the bonding system and the bonding time.
. Table III illustrates useful ranges for partial pressures of reactive gases at whichbonding occurs between selected metals in the presence of oxygen-bearing or sulfur-bearing gases. Only those eutectics which exhibit exhibit a eutectic temperature within of the melting point of the metal are listed. I
TABLE III EU'racnc REACTIVE GAS COMPOUND -BY VOLUME a cu-cuo 0.01 0;5 Cu CuS 0.01- 0.5 Ni NiO 0.01 0. 3 C0 C00 0.01 0.4 'FeFeO 0.01-0.3
It is to be understood that Table III illustrates, by way gases in the total gas flow which are-useful in practicing our invention. However, those skilled in the art can readily appreciate that other materials and other reactive atmosphere of oxygen. Additionally, useful bonds are formed with molybdenum or aluminum in a reactive atmosphere including silane. Accordingly, it is to be understood that Table III is merely a partial listing of eutectic compounds and that our invention is not limited solely to those eutectics set forth in Table III.
Those skilled in the art can readily appreciate that the formation of metal-to-metal bonds with a metal-gas eutectic provides an extremely useful capibility in electrical and electronic systems. For example, metal-tometal bonds may be used for interconnections, packaging of electronic components, formation of hermetic seals, electrical crossoves in integrated circuits, to mention only a few. These metal-to-metal bonds are formed without the use of compressive forces on the metal members and do not require the interdiffusion of metals to effect tenacious bonds. Additionally, although the metallic members are illustrated as sheets or plates,
it is to be understood that other configurations may also be used in the practice of our invention. Still other changes and modifications will occur to those skilled in the art and hence, the appended claims are intended to cover all such changes and modifications as fall within a second metal member; I
a eutectic bond between said first and second metal 4. The structure of claim 1 wherein at least said first metal members is copper and said eutectic bond is copper-copper oxide.
5. The structure of claim 4 wherein said second metal 5 member is selected from the group consisting of copper, nickel, cobalt, chromium, iron and alloys thereof. 6. The structure of claim 4 wherein said second metal member is stainless steel.
7. The structure of claim 1 wherein said first metal member is aluminum and said eutectic is aluminum-" aluminum silicide.