THE DIRECT BONDING OF METALS TO CERAMICS AND APPLICATION IN ELECTRONICS

The direct bonding of metals to ceramics is possible utilizing a gas metal eutectic. The mechanism of direct bonding of copper foil to ceramics in a slightly oxidizing atmosphere is presented. It involves the formation of eutectic melt between copper and oxygen at a temperature slightly below the melting point of copper, which serves to bring the foil into intimate contact with the substrate. Metals to which technique is applicable include Cu, Fe, Ni, Co, Ag, Cr, Mo and A1. A brief review of other metal-ceramic bonding techniques is given for comparison. The process for fabricating direct bonded copper structures is given, and properties of the bond are discussed. The use of the directed bonding process in a number of electronic applications such as hybrid packages and power device heat sinks is indicated.


INTRODUCTION
Strong bonding of metals to ceramics generally does not take place unless an intermediary layer is present, or forms due to diffusion or reaction.The adhesion of the metal to this intermediary is better than to the ceramic directly.However, an intermediary layer is undesirable in many applications.This is true, for instance, where high heat conductivity is required.The presence of an intermediary layer generally reduces heat conductivity, especially if the layer is non-metallic.Further, in high frequency applications, where the current flows in a thin skin at the metal-ceramic boundary, intermediary layers are undesirable because they are generally more resistive than the metal to be bonded.Finally, intermediary layers are often less corrosion resistant and lead to premature failure.
Unfortunately, strong direct metal to ceramic bonds are not common, either because the bond is inherently weak, or because of the difficulty of bringing the metal into intimate contact with the ceramic.In this paper, a new method of direct bonding of metals to ceramics will be given.For a better perspective, a short summary of the mechan- isms of presently used metal-ceramic bonding tech- niques is given below.

BONDING MECHANISMS INVOLVING INTERMEDIARY LAYERS
Some of the more pertinent literature in this area has been reviewed by C. I. Helgesson, and a short review was recently given by Loasby et al. 2 Commonly used intermediary layers are glass layers, where the glass is intentionally added or is originally present in the ceramic itself, or polycrystalline semi-conducting compounds formed by the reation between the metal or metal oxide and the ceramic itself.
2.1 Glassy Intermediary Layers Glassy phases are usually present in the grain bound- aries of impure (<99.8%)ceramics.Thus, A12 O a or BeO ceramics usually contain glassy mixtures of MgO, SiO2 and CaO concentrated in the grain boundaries.
When a metallizing mixture is applied to such a ceramic, and the temperature is raised to sintering tem- peratures, the glass from the ceramic penetrates into the (porous) metal layer and locks it to the ceramic after cooling by mechanical forces.
More commonly, glass is intentionally added to the metallizing mixture to form the intermediate layer.This is particularly true if the ceramic is pure.For example, to take advantage of the high thermal conductivity of BeO ceramic, glass must be kept out of the grain boundaries.Therefore, the common metallizing mixtures, such as Mo-Mn, when used on beryllia, contain up to 20% SiO2.Thus the compos- ition of a typical mixture is 71.5% Mo, 10.7% Mn, and 17.8% SiO2, fired on BeO at 1300C.The manganese diffuses into the glass.
Another commonly practiced method of bonding through an intermediary glass phase involves glass frit additions to the finely divided metal in the metallizing mixture.Thus, the majority of the so-called "thick film" conductor pastes contain a glass frit, generally chosen for low melting point and thermal expansion match, in addition to dispersed metal (normally noble metals) in an organic carrier.The glass frit melts and reacts with the ceramic (usually A1203), and it also forms a network within the metal conductor.Thus, the bond between the metal and the ceramic is due to mechanical locking by the reacted glass.It should be noted that the bond strength of glass frit compositions is best on alumina; for beryllia, cracking of the glass and spalling is generally observed.mediate layer compound MnO'A1203.The molyb- denum, which is unable to diffuse deeply into pure AIO3, will diffuse readily into the manganese aluminate spinel.This leads to good adhesion of the Mo layer.When MoO3 alone is used, some pene- tration of alumina by MoO3 occurs, especially in hydrogen atmospheres at high humidities.A com- pound between Mo and AI O 3, perhaps AIMoO4)3, 3,4 is formed to provide the inter- mediary bonding layer.

DIRECT BONDING
Direct bonding here implies the absence of a readily identifiable intermediate phase between the metal and the ceramic.It does not exclude, however, the presence of a transition layer one or two monolayers thick, such as perhaps oxygen bridges between the metal and ceramic.
A method of direct bonding which does not depend on any kind of intimate bond between metal and ceramic is that involving mechanical locking or keying.
In this process the metal is screened on the ceramic in the form of an organometallic paste.The paste decomposes on heating in air, with the result that metal particles are now locked in pores in the ceramic surface to give a mechanical bond.Films of noble metals, such as platinum, are typically applied in this way.However, poor adhesion is obtained in the mechanical locking process.Clearly, in order to get good adhesion by direct bonding it is required that: 2.2 Polycrystalline Intermediary Layers Such layers are usually compounds formed between the metallizing mixture and the ceramic.In one method, the metal oxide is applied to the ceramic, and sintered with the ceramic to form a mixed oxide compound.For instance, such a compound is formed between copper oxide and alumina, possible CuA102, when they are sintered together at 1150C in air.
Subsequent reduction gives an adherent metal layer bonded on the mixed oxide phase.This method is also applicable to cobalt, nickel, iron and silver.
The widely used moly-manganese process also falls into this category.In this process, a mixture of molybdenum and manganese oxide is fired on alumina (not beryllia) at 1400C in an atmosphere of wet hydrogen.The hydrogen reduces the molybdenum oxide to the metal, but not the manganese oxide, which reacts with the alumina to form the inter- 1) The metal must be brought into intimate contact with the ceramic over the entire area on an atom to atom basis.
2) The metal must form a strong bond with the ceramic, be it by Van der Waal's or chemical bonding, without extensive reaction or diffusion of the two phases into each other.
To get intimate contact between metal and a ceramic surface, the metal could be brought into the liquid state, by melting.The liquid will replicate the ceramic surface where it wets it, and if a strong bond is formed, good adhesion is obtained after cool-down.
For instance, when copper is melted in hydrogen on an alumina substrate, a molten droplet is formed with a melting angle of approximately 120 , which after cool-down to room temperature adheres strongly to the ceramic.The problem with simply melting the metal is that one or more droplets are formed, and the metal member to be bonded entirely loses its shape.This can be avoided by the gas metal eutectic method.

DIRECT BONDING BY THE GAS-METAL EUTECTIC METHOD
The basic idea here is to form a liquid skin around the metallic member to be bonded.The liquid must wet both the metallic member and the ceramic, and form a strong bond after cool-down.The melting point of this liquid should be near the melting point of the metal member itself, say within 50C, so that the metal member becomes soft and pliant and conforms easily to the shape of the substrate surface.Further, the predominant constituent of this liquid should be the same element as makes up the metallic member, and any additional constituent should be present in only small amounts or be easily removable after bonding.The thickness of the molten skin must be kept small in comparison to the thickness of the member to be bonded.Ideal for use as the skin material are eutectics formed by the flow gas in a bonding system with the metal member itself.Such a eutectic exists, for instance, between copper and oxygen, at 0.39% weight percentage oxygen, The melting point of this eutectic is 1065C, as compared to 1083C for pure copper.Thus, if the oxidizing conditions in the flow gas are chosen properly, one may have the molten copper-oxygen eutectic mixture and solid copper (containing a small amount of dissolved oxygen) coexist together, in the temperature interval 1065C <T < 1083C.It is this copper-oxygen eutectic skin which is in effect used as a glue to bond the solid copper member to the ceramic, without the Cu member losing its shape.Thus, in order to bond copper foil to ceramic, the process is as follows.
The Cu foil is laid on top of the ceramic in a furnace containing a flow gas atmosphere consisting primarily of an inert gas, such as argon or nitrogen, with a small addition of oxygen, typically of the order of a few hundredth of a percent.Copper initially reacts during the heat-up period with the oxygen in the flow gas.A small amount of oxygen dissolves in the copper, but most of it reacts to form Cu20 around the foil.When 1065C or above is reached, a liquid phase of or near the eutectic composition forms a skin around the copper.The thickness of this molten skin depends on the 02 partial pressure and the oxidation time.If the partial 02 pressure in the flow gas is less than 1.5 x 10 -6 arm.(the equilibrium partial pressure over Cu20 at 1065C), Cu20 will not form and the eutectic phase also will not form.The formation of a molten layer between the Cu foil and the substrate serves to bring the copper in intimate contact with the ceramic, by wetting it over the entire interface.Once in intimate contact with the ceramic, copper forms a strong bond with it.The temperature must remain below 1083C, the melting point of copper, since otherwise the foil loses its structural integrity and liquid drops are formed.Similarly, if the partial pressure of oxygen is too high, all copper present is converted into eutectic melt.Thus, an intermediate zero partial pressure is required where both phases are present simultaneously.On cool-down below 1065C the eutectic segregates into Cu and Cu20.Evidence of this is shown in Figure 1, which is a micrograph through a section of bonded copper foil, and indi- cates the presence of a second phase.This oxide phase may now be reduced in hydrogen at low temperature, without loss of adhesion.The bonding process is schematically illustrated in Figure 2.
Copper has been bonded successfully by the gas-metal eutectic methods on alumina, beryllia, silica, various other spinels, other metals and to itself.
It does not bond, however, to boron nitride or carbon.The bond strength to oxygen-containing ceramics can be in excess of 20,000 psi.Removal of  the copper by etching in nitric acid or ferric chloride solution leaves no copper residue.Since the copper itself does not melt during bonding, pre-cut or stamped foils of copper of the desired shapes can be bonded.In addition, holes previously cut into ceramic substrates can be covered hermetically, and the copper can overhang the ceramic.Bonds between copper and other metals, and between two copper members, can also be made by this method.A photograph of various direct bonded copper struc- tures prepared in this laboratory are shown in Figure 3.
among various conductor layers; the inclusion of a Cu backplate for heat sinking; and external package leads in one single step.
3) Since the Cu-ceramic bond does not require an intermediate interfacial layer, better heat sinking is possible.
4) Thick f'n conductor screening and firing steps can be entirely eliminated.
5) The Cu-ceramic bond is exceptionally strong.6) Fabrication costs are considerably below con- ventional methods because of process simplicity and lower materials cost. 5PROPERTIES OF DIRECT BONDED COPPER STRUCTURES Some properties of direct bonded copper on ceramic structures are listed in Table I.

TABLE
Properties of direct bonded copper structures 1) Electrical conductivity of the direct bonded copper within 5% of pure copper.
2) The hardness of the direct bonded copper is much higher than after hydrogen reduction, because of dispersion hardening due to precipitated Cu: O from the eutectic.
4) In spite of the expansion coefficient difference, repeated thermal cycling from 77K to 600K does not cause mechanical failure.5) Brazing, soft and hard soldering, resistance and elec- tron beam welding are possible to the bonded copper without causing bond failure.6) Holes in the ceramic can be sealed hermetically by direct bonded copper.

APPLICATION OF DIRECT BONDED COPPER IN HYBRID PACKAGING
Figure 4 illustrates the use and versatility of the copper foil bonding process in fabricating electronic packages, including heat sinking, conductor cross- overs and interconnections, and top to bottom straps.
Copper foil and ceramic pieces are previously cut to the desired dimensions and are appropriately located on the ceramic substrate before the furnace pass for one step bonding.Alternatively, the pattern can be etched after bonding.This latter technique is particu- larly suitable for fine and complex geometries.
Figure 5 gives a sequence of steps to fabricate a hermetic ceramic package.Pieces of copper and ceramic are cut to the appropriate size and are bonded directly to give a lead frame, conductor pattern, pellet bonding pad, conductor crossover, and heat sink.The window frame for lid attachment is fired on by means of a glass ceramic frit at 500C < T < 700C.Pellet bonding and lid attach- ment complete the package.The use of directly bonded Cu foil to ceramics for the construction of hybrid packages offers advantages because: 1) Foils from to 250 mils thick or more can be bonded on A12 Oa, BeO or metals without the use of an intermediary layer and without losing the struc- tural integrity of the foil.
2) Several layers of Cu foil and ceramic can be bonded in one single firing step.PROCESS STEPS TO FABRICATE ELECTRONIC PACKAGE BY DIRECT COPPER BO!DING PICESS Process steps to fabricate electronic package by direct bonding process.
When the direct bonded copper foil covers a hole previously cut into the ceramic, a hermetic seal is formed.This allows for the mounting of pins or tabs from the backside by soldering, welding, or plating on copper.Figure 6 illustrates a sequence of steps to fabricate a package in this manner.After direct bonding of a copper foil to a pre-drilled ceramic substrate, the conductor pattern and window frame are photoetched.After nickel plating, the pins are mounted and the pellet is attached.Lid attachment completes the hermetic package.Figure 7 is a photo- graph of a hybrid circuit fabricated by this process.

APPLICATION TO OTHER METAL-CERAMIC SYSTEMS
For the direct bonding of metal by the gas-metal eutectic method, the following prerequisites must be fulfilled: 1) A eutectic must exist between the metal and the flow gas.
2) The percentage of the gas component in the eutectic must be low and/or easily removable after bonding.
3) The eutectic temperature must be at least 10 degrees or so below the melting point of the metal to give a realistic temperature range for bonding.
4) The metal must be pliant at the bonding temperature so that it conforms to the substrate shape.
Inspection of phase diagrams of metals with the more common gases reveals a number of possibilities.
In particular, oxygen, sulphur, and phosphorus form eutectics with several metals.These are listed in Table II, in addition to the eutectic temperature,   the melting point of the metal, and the percentage of the gas component in the eutectic.The flow gas would contain small amounts of oxygen, hydrogen disulfide, phosphine, or silane, as appropriate.However, in some cases a constituent of the flow gas such as silicon may not be easily removable, and the bond in such a case could not, strictly speaking, be called "direct".

FIGURE 1 FIGURE 2
FIGURE 1 Mierograph of section through direct copper bond to alumina, showing the precipitated Cu 20 from the euteetic.

FIGURE 3
FIGURE 3 Direct bonded copper structures.

FIGURE 6 FIGURE 7
FIGURE 6 Process steps to fabricate electronic package by direct copper bonding process.