CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending USSN 07/881,926 filed

May 12, 1992.

FIELD OF THE INVENTION

The present invention relates generally to metallization for aluminum nitride ceramics having the properties of low thermal expansion and high thermal conductivity.

More particularly, this invention relates to a metallization structure formed on an aluminum nitride substrate and a method of producing the metallization structure on the aluminum nitride substrate.

BACKGROUND OF THE INVENTION

Ceramic substrates are widely used as substrates for electronic circuits, and more recently as substrates for hybrid integrated circuits. In manufacturing a hybrid integrated circuit, it is necessary to mount an integrated circuit chip and other metal members such as bonding wires on the ceramic substrate by brazing or soldering. However, these members cannot be directly bonded to the ceramic substrate. It is therefore generally practiced to first form an electroconductive metallized layer on the ceramic substrate and then bond the metal members to the metallized layer.

Because of its high thermal conductivity, excellent heat dissipation and electric insulating properties, aluminum nitride (AIN) substrates have been used as substrates for electronic packages such as hybrid integrated circuits. AIN is used in place of alumina (AI2O3) because of AlN's improved heat dissipation capabilities, and in place of beryllia (BeO) because unlike AIN, BeO is toxic and, thus difficult to handle.

When an electronic substrate is used, the substrate is usually joined with a metal layer,- so that a conductive metallized structure is formed on the surface of the AIN substrate. Conventionally for AIN, this metallized structure has been a layer of Cu, Au

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or Ag-Pd formed by the direct bond copper (DBC) method or by the thick film method. However, this method of metallizing AIN does not result in a well-adhered metallized layer, particularly when the package is subsequently processed at high temperatures. When a wire or semiconductor element is brazed onto the metallized layer at a high temperature, the adhesion strength between the metallized layer and the AIN substrate is greatly lowered and the metallized layer along with the wire or semiconductor element may eventually peel off of the AIN substrate. Because of the problems associated with the DBC method, metallization of AIN substrates has been attempted in a number of other ways. U.S. Patent No. 3,716,759 discloses a bonding system for use with an aluminum nitride body and a semiconductor crystal providing contact metallization by depositing in a vacuum a thin layer of a refractory metal such as chromium, tungsten, or molybdenum followed by a thin layer of nickel which is in turn followed by a thin layer of silver. A conventional soft solder is then utilized capable of alloying with silver which bonds directly to leads and heat sink as well as the contact metallization.

U.S. Patent No. 4,761,345 discloses an aluminum nitride substrate having a metallized layer containing titanium nitride (TiN) and at least one selected from the group Mo, W, Ta, an element of group Ilia, nib, and IVb of the periodic table, a rare earth element and an actinide element. The metallized layer is formed by dispersing the powder of the respective elements of the metallization composition in a binder to form a paste, attaching the paste onto the surface of the AIN sintered substrate by dipping or coating, followed by calcination by heating. The resulting metallized layer may comprise, for example, W-TϊN. A protective layer of nickel may then be electrolessly plated or electrolytically plated onto the metallized layer. U.S. Patent No. 4,770,953 discloses an aluminum nitride sintered body having a metallized layer formed from simultaneously sintering a paste or liquid containing a conductive element belonging to a first group and an element belonging to a second group. The conductive element of the first group may be tungsten or molybdenum, among others and the element of the second group may be titanium, hafnium, or zirconium, among others. When titanium is the element of the second group, titanium exists in the metallized layer as TiN.

U.S. Patent No. 4,873,151 discloses an aluminum nitride substrate having a

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conductive material bonded to the AIN substrate through a metallized layer formed on the AIN substrate. The metallized layer contains at least one element selected from Mo, W, Ta and at least one element selected from the lib, Ilia, Illb and JNb group elements, and rare earth elements. It is not disclosed how this metallized layer is formed. The conductive material which is bonded to the metallized layer has a thermal expansion coefficient of between 2 x 10 ^_ 6 to 6 x 10"6/°C. This thermal expansion coefficient range limits the usefulness of metallized A1Ν, since the typical conductive materials for electronic circuits are Cu, Ag, Au and typical lead frame materials include Fe-Νi-Co alloys. These materials have thermal expansion coefficients substantially higher than 6 x 10-6/°C.

U.S. Patent No. 4,876,119 discloses a method of coating a nitride ceramic substrate by bringing a metal vapor into contact with the surface of the substrate. The metal vapor reacts with an element present in the nitride ceramic substrate to form a metallized layer on the substrate surface. For example, when Ti vapor is brought into contact with the surface of an aluminum nitride substrate, it is disclosed that a layer of TiN forms on the surface of the substrate.

U.S. Patent No. 4,980,239 discloses a metallization structure for AIN which includes an intermediate layer of AlTiN formed on me AIN base, a Ti layer formed on the intermediate layer, a heat resistant metallic layer of W or Mo formed on the Ti layer and a layer of Ni formed on the heat resistant metallic layer for soldering or brazing. It is disclosed that the heat resistant metallic layer prevents inner diffusion between Ti layer and the Ni layer.

U.S. Patent No. 5,063,121 discloses a metallized AIN substrate having a metallized layer formed by first coating a paste containing compounds of yttria and alumina onto the substrate, followed by coating a metallized paste of Mo or W, Tiθ2 and a binder onto the yttria and alumina coating. The coated substrate is then fired to form a metallized layer of TiN and Mo or W on the AIN substrate.

Whereas there have been several methods proposed for metallizing AIN, few of these methods result in a ductile metallization structure having strong ceramic to metal adhesion. Ductility is an important property because the metallization structure must be able to withstand the stress caused by thermal cycling or mechanical vibration during manufacture and operation of the electronic package. TiN, which is used in most

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metallization methods, is a brittle compound and its presence in the metallization structure lowers the fracture energy of the metal/ceramic interface and thus, can lead to catastrophic failure when the metal/ceramic interface is stressed.

Furthermore, the metal/ceramic interface must be resistant to embrittlement by hydrogen. Hydrogen is used in the various operations of electronic packaging such as brazing and annealing. In addition, hydrogen is a by-product of electrodeposition. A metallization structure composed of Ti metal is not resistant to hydrogen embrittlement.

Thus, it is an object of the present invention to provide an adherent metallization structure for aluminum nitride ceramics. It is another object of the present invention to provide a metallization structure for AIN ceramics that is resistant to embrittlement by hydrogen. It is yet another object to provide a metallization structure that is sufficiently ductile to withstand the stresses associated with electronic package manufacture and operation.

SUMMARY OF THE INVENTION

The present invention achieves a metallization structure for AIN ceramics that is well adhered to the AIN ceramic, is resistant to embrittlement by hydrogen, and is sufficiently ductile to withstand the stresses caused by electronic package manufacture and operation. An aluminum nitride metallized structure of the present invention includes a substrate comprising an AIN sintered body and a metallization structure formed on the substrate comprising a first layer deposited on the sintered body and a second layer deposited on the first layer. The first layer comprises an alloy having the general formula based on atomic percent wherein X is at least one member selected from the group consisting of Ti, Zr, Hf, and the rare earth elements, Z is at least one member selected from the group consisting of

Mo, W, Cr, Nb, V and Ta, and 10 < x <60 atomic %. The second layer comprises at least one member selected from the group consisting of Au, Co, Cu, Ni, and Fe. The present invention further includes an aluminum nitride substrate comprising an AIN sintered body and a metallic alloy bonded to the substrate comprising at least one member selected from the group consisting of Au, Co, Cu, Ni, and Fe; at least one

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member selected from the group consisting of Ti, Zr, Hf, and the rare earth elements; at least one member selected from the group consisting of Mo, W, Cr, Nb, V and Ta; and less than about 1% of nitrides or aluminides of members selected from the group consisting of Ti, Zr, Hf, and the rare earth elements; and having an elongation at room temperature greater than 5 % .

The present invention also includes an aluminum nitride substrate having a lead frame bonded thereto by a metallic alloy structure, wherein said metallic alloy structure comprises i) greater than 80 weight percent silver and copper, ii) less than 2 weight percent of an element selected from at least one of Ti,

Zr, Hf and the rare earth elements, iii) at least one element selected from Mo, W, Cr, Nb, V, and Ta. The metallic alloy structure may further comprise at least one element selected from Au, Co, Ni, and Fe.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of the aluminum nitride metallized structure of the present invention before brazing.

FIG. 2 is a cross-sectional view of the aluminum nitride metallized structure of the present invention including an additional ductile layer before brazing.

FIG. 3 is a cross-sectional view of the aluminum nitride metallized structure of the present invention after brazing and including an attached lead frame.

FIG. 4 is a cross-sectional view of a further embodiment of the aluminum nitride metallized structure of the present invention after brazing and including an attached lead frame.

DETAILED DESCRIPTION OF THE INVENTION

A description of the preferred embodiment of the present invention is given with reference to Figures 1-3. Referring to Figure 1, the preferred embodiment of the present invention comprises an aluminum nitride ceramic substrate 11, a first thin film layer 12 formed on AIN substrate 11, and a second thin film layer 13 formed on first thin film layer 12.

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AIN ceramic substrate 11 may be produced by a conventional process comprising the steps of forming an AIN powder to obtain a body having a desired shape, and then sintering the formed AIN body. Sintered AIN substrates are commercially available, for example from The Carborundum Company, Niagara Falls, New York. Before first thin film layer 12 is formed, the surface of AIN substrate 11 is cleaned by washing in a solvent such as ethanol or methanol followed by air drying. First thin film layer 12 has a thickness preferably in the range 100-5000A, and more preferably hi the range 250-1500A. The temperature of the AIN ceramic substrate 11 during the deposition of first thin film layer 12 is set within the range of about 25-400°C. First thin film layer 12 is formed on AIN ceramic substrate 11 by a chemical or physical vapor deposition such as sputtering or vacuum evaporation. First thin film layer 12 comprises an alloy having the general formula based on atomic percent _x Zιoo- , wherein X is at least one metal selected from the group consisting of Ti, Zr, Hf, the rare earth elements; Z is at least one metal selected from the group consisting of Mo, W, Cr, Nb, V, Ta; and 10 < x <60. If the first thin film layer 12 comprises Hf or Zr as the X component in the formula X Zιoo_. _x , then preferably 10 < x <30 atomic %.

Next, as shown in Figure 1, a second thin film layer 13 is formed on top of the first thin film layer 12 by a conventional process such as sputtering. Second thin film layer 13 is composed of at least one metal selected from the group consisting of Au, Co, Cu, Ni, and Fe and is about 1-10 microns thick. Preferably, second thin film layer 13 comprises an alloy of Ni and Cu. More preferably, second thin film layer 13 comprises an alloy of Ni and Cu wherein the Cu content is in the range of about 40-90 atomic %. Second thin film layer 13 permits the fastening of a member such as a metallic member to metallized structure 10 by soldering or brazing. Following the deposition of first and second thin film layers 12 and 13, sharp interfaces are observed between the metallic layers and AIN ceramic substrate 11. There is also observed a sharp interface between first thin film layer 12 and second thin film layer 13. Thus, no reaction or mixing between the thin film metallic layers or between the first thin film metallic layer and the AIN ceramic substrate have been observed prior to heat treatment.

The present invention will be described in detail by way of its examples.

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EXAMPLE 1 A sintered AIN substrate measuring 2" x 2" and having a thickness of 25 mils was ultrasonically cleaned in ethanol and dried in air at about 350 °C. A thin film layer of Ti29 ^w 71 was deposited on the substrate by DC magnetron sputtering. The deposition was carried out in a cryopumped (base pressure of 5 x 10~8 torr) chamber using Ar sputter gas (pressure of 3 microns). The rate of deposition was 300 A/min and the temperature within the chamber was in the range 50-100°C. The thickness of the resulting Η29W71 thin film layer was about 0.5 microns.

After the first thin film layer of T_29W7i was deposited, a thin film layer of Ni was deposited by magnetron sputtering. The thickness of the Ni layer was about 1.0 microns.

EXAMPLES 2-9 and COMPARATIVE EXAMPLES a-f

The metallized structures of Examples 2-9 and Comparative Examples a-f were prepared substantially in accordance with Example 1, except the composition of the first thin film layer was varied. A second thin film layer of Ni having a thickness of about 1.0 μm was deposited on the first thin film layer by magnetron sputtering. Table I shows the composition of Examples 2-9 and Comparative Examples a-f. The adhesive strength of the metallization structure 21 , which is comprised of first thin film layer 12 and second thin film layer 13, was measured in a peel test. The test consists of soldering a wire to second thin film layer 13, bending the wire to a 90° angle, and then pulling the wire in the direction perpendicular to the plane of the substrate.

The adhesive strength was measured and compared for various samples of the structure of the present invention before and after exposure to hydrogen gas at 100°C for 24 hours. This exposure to hydrogen gas simulates one of the failure modes that can occur during the processing of an AIN substrate. For example, operations such as brazing are conducted in a hydrogen-containing atmosphere at elevated temperature. In addition, hydrogen can be generated during the electrodeposition of metals such as Ni and Cu. Table I demonstrates the results of the peel test for Examples 1-9 as well as for Comparative Examples a-f.

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We have observed that when the first thin film layer comprises Ti-W alloys having between 10 and 60 atomic % Ti, the peel test values remain greater than 12 lbs force after exposure to hydrogen. The drop in adhesive strength observed when the Ti content exceeds 60 atomic % (examples d and f), may be the result of hydrogen embrittlement of the resulting Ti-W alloy.

The elemental films of Cr, Mo and W exhibited poor adhesion to the AIN substrates as demonstrated by Table I, Examples a-c. This poor adhesion may be the result of poor chemical bonding of these metals to the AIN substrate. It is also possible that impurities from the AIN substrate surface, for example oxygen, carbon, or water, are incorporated into the growing film at the metal/AlN interface, thereby rendering it brittle and poorly adhered to the AIN substrate. It is possible to obtain a patterned metallization layer on the AIN ceramic base

11. For example, after the thin film layer 12 and thin film layer 13 have been formed on the AIN ceramic base 11, AIN metallized structure 10 is subjected to a

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conventional patterning process in which an etchant liquid containing a mixed acid of hydrofluoric acid and nitric acid is used.

After deposition of first thin film layer 12, it is preferable that thin film layer 12 have a substantially body-centered-cubic crystal structure. For many of the alloys described above, this is a non-equilibrium structure. However, it is the most ductile and compliant form of the subject alloys, and therefore, the most desirable for the purpose of the present invention. Brittle intermetallic compounds such as Cr^Ti and Cr2Ta are preferably avoided. In applications in which the metallized structure will be exposed to temperatures above 500°C, such as in brazing, it is important that the ductility of first thin film layer 12 persist after exposure to high temperatures. The second thin film layer 13 is preferably ductile, in addition to being relatively free of embrittling agents such as oxygen and carbon. As shown in Figure 2, an additional optional layer, ductile layer 14, is deposited on the thin film layer 13 before brazing. Ductile layer 14 is selected from the group consisting of Ni, Co, Cu, Au and alloys thereof. Preferably, ductile layer 14 comprises electrolytically deposited Ni.

During brazing, first thin film layer 12 reacts with second thin film layer 13 to form metallic alloy structure 15, as shown in Figure 3. Metallic alloy structure 15 comprises an alloy of the metals of the first and second thin film layers 12 and 13 and ductile layer 14, if any. To insure that metallic alloy structure 15 possesses sufficient ductility for reliable performance, an excess of the metal comprising second thin film layer 13 may be added to the metallic structure via the addition of ductile layer 14 before brazing. For reliable performance, metallic structure 15 should preferably have greater than 5% elongation at break, and more preferably greater than 15% elongation at room temperature. To achieve this, the thickness of thin film layer 13 and ductile layer 14 combined should be at least 5 times greater than the thickness of thin film layer 12. In other words, greater than about 80 atomic % of the metallic alloy comprises at least one member of the group consisting of Ni, Cu, Co, Fe and Au. If the composition of second thin film layer 13 and the braze composition form a metallic alloy under the braze conditions used, then after the brazing operation layer 15 will contain the elements of thin film layers 12 and 13 (and optionally ductile layer

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14) as well as the elements in the braze.

For example, in the metallization system of the present invention, if second thin film layer 13 is a nickel-copper alloy having greater than 10 weight percent copper, and the braze used is a silver-copper braze having more than 50 weight percent silver and a thickness of greater than or equal to about 25 microns, then after brazing, layer 15 will be a metallic alloy structure bonded to the substrate comprising i) greater than 80 weight percent silver and copper; ii) less man 2 weight percent of an element selected from Ti, Zr, Hf , and the rare earth elements, iii) less than 2 weight percent of an element selected from the group consisting of Mo, W, Cr, Nb, V, and Ta; iv) less than 20 weight percent Ni; v) less than 1 weight percent (if any) of nitrides or aluminides of Ti, Zr, Hf or the rare earth elements and having an elongation at room temperature greater than 5%.

Alternatively, the second thin film layer before braze may comprise at least one element selected from Au, Co, Cu, Fe and Ni, and these elements may then appear after braze in metallic alloy structure 15.

The adhesive strength of metallic alloy structure 15, the structure formed after brazing, was measured according to the peel test described above. Table π demonstrates the results of the peel test for different embodiments of the present invention as well as for comparative examples.

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Examples 10-15 and Comparative Examples g-j were prepared substantially in accordance with example 1, except that the thickness of the second thin film layer was 2.5 microns. In addition, the second thin film layers of examples 12, 13 and 15 comprise copper.

The metallized substrates of examples 10-15 and g-j were brazed to copper pads for adhesion testing. The metallized substrates were mounted in a furnace with 25 microns thick Ag-27Cu(wt%) braze preforms placed on the metallized substrates, followed by 1mm thick copper pads placed on the metallized substrates. Brazing was carried out in an Ar or H2 atmosphere at 825 °C for 5 minutes, followed by cooling to room temperature over a period of about 20 minutes. Other suitable brazes include Ag, Ni, Cu and Au based brazes, such as, for example, Au-20Ge, Ni-5B, Ag-25Cu-5Sn and Ag-5A1. The elemental films of Cr, Mo and W of Comparative Examples g-i, exhibit unacceptably poor adhesion following brazing. Excellent adhesion after brazing was observed for the materials of the present invention, Examples 10-15. The strength of the metal/AIN interface is measured by the fracture energy of the metal/AIN bond. The failure mechanism for Examples 10-15 following testing was fracture of the AIN ceramic near the metal/AIN interface. This indicates that the interfacial metal/AIN

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bond is actually stronger than the AIN ceramic and thus, represents the maximum useful strength for a metal/AIN bond. This superior bond strength is most evident for compositions in which the value for x representing the content of X in the formula X _x Zιoθ-χ ^{m me} first ^m iπ ^ ^m l ^aver * ^s greater than about 30 atomic %. Following the brazing and testing of the examples of Table II, a number of the examples were microstructurally analyzed using Scanning Auger Microprobe. This testing included both a polished cross-section of the brazed parts as well as depth profiles from the metallized surface down to the AIN surface. In Examples 10-15, a similar microstructure was observed. The first and second layers were found to have extensively reacted to form a metallic alloy rich in the second layer components. Interstitial atoms such as oxygen and carbon have been reduced to low levels at the interface and more significantly, have preferentially reacted with the X component of the Xχ oo-χ first layer alloy to form dispersed oxides or carbides.

Near the interface, X-ray Photoelectron Spectroscopy indicates the presence of both metallic and partially oxidized X component elements, while the other components are in fully metallic states. This insures the ductility of the resulting metallic phase in the metallization. No brittle compounds were observed and substantially no reaction of the metallization components and the AIN ceramic could be detected. Less than about one percent of the metallic alloy consisted of nitrides or aluminides of the X compound of the first layer alloy, the products of potential reactions of the metallization components and AIN. This structure, established by the choice of the first and second layer compositions and their relative thicknesses, determines the excellent performance of these materials.

The desired microstructure of the metallic alloy near the AIN interface following brazing or other high temperature exposures greater than 500°C, based on the present invention, is one in which the bulk of the metallic alloy consists of metallic phases of the components of the first and second layers in which less than 10 percent by volume is intermetallic compounds, such as Cr2Ti or NiTi, and at least 25% of the starting amount of the X component of the first layer alloy is in the metallic state as determined by X-ray Photoelectron Spectroscopy. Further, less than about one percent by volume of the metallic alloy consists of brittle compounds such as TiN and Ni-Al-oxides, and less than 2.5 atomic % oxygen is left in solid solution in the

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metallic alloy. In addition, the majority of the embrittling agents such as oxygen, carbon, and nitrogen are associated with the X component of the first layer alloy in the form of discontinuous, dispersed particles in the metallic alloy. The resultant metallization has an elongation at room temperature greater than 5% and preferably greater than 15% at break.

To examine a brazed, metallized AIN substrate, a sintered AIN substrate was sputtered with a two-tenths (2/10) micron thin film layer of 20 wt% titanium and 80 wt% tungsten, followed by the deposition of a separate two (2) micron layer of 60 wt% copper and 40 wt% nickel. The substrate was not intentionally heated during sputtering, and the maximum temperature experienced by the structure was about 200 °C due to the energy of the deposition. There was no reaction detected during the sputtering process between Ti or W and the components of the substrate, or between the Cu or Ni and the Ti or W. A copper clad lead frame was brazed onto the metallized substrate with a 25 micron thick braze composition of Ag - 27 wt% Cu at between 780°-900°C. The resulting metallic alloy structure had an average composition by weight % of Ag 66 - Cu 29 - Ni4 - TiO.03 - W0.1.

The metallic alloy structure 41 had three compositional zones, as depicted in Fig. 4. In aluminum nitride substrate 40, zone 22 contiguous to substrate 21 and zone 23 contiguous to lead frame 25 were copper-rich, and zone 24 was silver-rich. The copper cladding was consumed in the brazing process, providing more volume to the braze.

The foregoing examples are not intended to limit the subject invention, the breadth of which is defined by the specification and the claims appended hereto, but are presented rather to aid those skilled in the art to clearly understand the invention defined herein.

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