Background of The Invention Silicon wafers carrying integrated circuits are typically cut into a plurality of dies, and the dies are subsequently attached to a packaging material to physically protect, and electrically connect the integrated circuit on the die to a plurality of electrical contacts.

Various configurations and methods of attaching a die (i. e., die attach) are known in the art, however, all or almost all of them suffer from one or more disadvantages.

For example, in one method a die is attached to a substrate with a polymeric adhesive such as an epoxy resin or a cyanate ester resin. Polymeric adhesives frequently exhibit several desirable qualities. Among other things, cyanate ester-containing die attach adhesives can be cured within a relatively short time (i. e., within several minutes) at a temperature typically below 200°C (see e. g., U. S. Pat. Nos. 5,150,195,5,195,299,5,250,600,5,399,907, and 5,386,000). Furthermore, some polymeric adhesives retain structural flexibility after curing to allow die attach of integrated circuits onto flexible substrates as disclosed in U. S. Pat. No.

5,612,403 to Nguyen et al. However, many polymeric adhesives tend to produce resin bleed, potentially leading to undesirable reduction of electrical contact of the die with the substrate, or even partial or total detachment of the die. Moreover, application of polymeric adhesives to the substrate and/or die typically requires the use of organic solvents that tend to generate inhomogeneity in the adhesive when the solvent is removed.

To circumvent at least some of the problems with resin bleed, die attach adhesives comprising a silicone composition may be employed. For example, in U. S. Pat. No.

5,982,041, Mitani et al. describe a silicone composition that cures both via a free radical reaction of an acrylic-functional organopolysiloxane and via a hydrosilylation reaction between an alkenyl-functional organopolysiloxane and a silicon-bonded hydrogen-functional organopolysiloxane. Mitani's silicone composition advantageously suppresses the

outmigration of low-molecular-weight silicone oil, thereby alleviating potential problems, including impaired wire bondability to the semiconductor chip or lead frame, and defective bonding between the resin sealant and the semiconductor chip, substrate, package, and/or lead frame. However, the curing process for Mitani's silicone composition requires a source of high-energy radiation, which may add significant cost to the die attach process. Moreover, Mitani's silicone composition still requires a solvent.

In another method, (e. g., U. S. Pat. No. 4,459,166 to Dietz et al.), a glass paste comprising a high-lead borosilicate glass with a softening temperature of approximately 325°C-425°C is employed to couple a die to a substrate. After placing the glass paste between the die and the substrate, the glass paste is dried and heated to permanently bond the die to the substrate. Glass pastes typically avoid problems associated with bleeding of low molecular weight components, however, they often require temperatures of 425°C and higher to permanently bond the die to the substrate. Moreover, glass pastes frequently tend to crystallize during heating and cooling, thereby reducing the adhesive qualities of the bonding layer.

In order to circumvent at least some of the problems with high firing temperatures, an improved glass paste composition including V2Os, Ti203, P205, and Ag can be employed as disclosed in U. S. Pat. No. 5,076,876 to Dietz et al. Such compositions advantageously reduce the firing temperature (i. e. the temperature required to achieve permanent bonding) to a temperature of less than 350°C while simultaneously reducing the undesired incidence of crystallization. However, heating an electronic device to 350°C might still be undesirably high in some applications. Furthermore, the use of the improved glass paste composition generally requires a solvent, which may be difficult to remove, especially where the die is relatively large.

In yet another method, a die is attached to a substrate by soldering. Soldering a die to a substrate has various advantages, including relatively simple processing, solvent-free application, and especially when a lead-based solder is utilized, relatively low cost. However, the use of lead may pose environmental and health problems not only during manufacturing, but also after an electronic device that includes a lead-based die attach has been disposed of

(e. g., in a landfill). To reduce the toxicity of lead-based solders, lead has been replaced in Sn- based alloys by various low melting point metals such as bismuth to form a eutectic alloy replacement (see e. g., U. S. Pat. No. 4,929,423 to Tucker et al.) However, eutectic alloy replacements typically exhibit melting temperatures well below 270°C, which is especially problematic when the assembled electronic device (i. e., the die attached to the substrate) is exposed to temperatures above the melting point of the eutectic alloy replacement in a downstream processing step (e. g., wave soldering).

Although various methods for die attach are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need for improved methods and compositions for die attach.

Summary of the Invention The present invention is directed to an electronic device in which a die is attached to a metallic contact of a substrate via a coupling layer, wherein the coupling layer comprises an alloy of at least two metals selected from the group consisting of Zn, Al, Mg, and Ga, and wherein the alloy further comprises at least one sacrificial chemical element other than Zn, Al, Mg, and Ga in more than a trace amount (i. e., greater than 10 ppm) that has an oxygen affinity higher than the alloy. Alternative contemplated alloys comprise at least two metals selected from the group consisting of Sn, Ag, Bi, Zn, and Cu, wherein the alloy further comprises at least one sacrificial chemical element other than Sn, Ag, Bi, Zn, and Cu in more than a trace amount (i. e., greater than 10 ppm) that has an oxygen affinity higher than the alloy. Still further alternative alloys comprise at least two metals selected from the group consisting of Sn, Sb, Bi, and Ag, wherein the alloy further comprises at least one sacrificial chemical element other than Sn, Sb, Bi, and Ag in more than a trace amount (i. e., greater than 10 ppm) that has an oxygen affinity higher than the alloy. The die preferably comprises a semiconductor or an integrated circuit, and the metallic contact preferably comprises copper or a copper alloy.

In one aspect of the inventive subject matter, the alloy comprises Zn, Al, Mg, and Ga, preferably with Zn in a range of 85 to 95 wt%, Al in a range of 3 to 8 wt%, Mg in a range of 1

to 5 wt%, and Ga in a range of 1 to 5 wt%. Other particularly contemplated alloys comprise Ag, Cu, Bi, and Zn, preferably at a composition in which Ag is in a range of 1 to 5 wt%, Cu is in a range of 0.5 to 2.5 wt%, Ag and Cu are in a range of 0 to 5 wt% and 0-2.5wt%, respectively, Ag and Bi are in a range of 0 to 5 wt% and 0-6 wt%, respectively, or Zn and Bi are in a range of 0.5 to 2.5 wt% and 0 to 6 wt%, respectively, with the balance being Sn.

Still further especially contemplated alloys comprise Sn, Ag, Bi, and Sb, preferably in a composition in which Sb is in a range of 5 to 15 wt% and Bi is in a range of 0 to 6 wt% and the balance being Sn, or in a composition in which Ag is in a range of 20 to 30 wt% and Sb is in a range of 5 to 15 wt% and the balance being Sn.

In another aspect of the inventive subject matter, the sacrificial chemical element having a oxygen affinity higher than the alloy is Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd, P, Sc, Sr, Ti, Y, or Zr. It is further preferred that the element is present in a concentration between about 10 ppm and 1000 ppm. The melting point of contemplated alloys is at least 270°C, and preferably between 270°C and 330°C.

Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.

Brief Description of The Drawings Fig. 1 is a schematic side view of an electronic device according to the inventive subject matter.

Detailed Description As used herein, the term"alloy"refers to a homogeneous mixture or solid solution of two or more metals, in which the atoms of one metal replace or occupy interstitial positions between the atoms of the other.

As also used herein, the term"oxygen affinity"refers to the propensity of a compound or element to react with radical, molecular, or ionic oxygen to form the respective oxide, which may be chemically stable (i. e., the oxide can be isolated), or may further react into a chemically and/or thermodynamically more stable form. For example, calcium oxidizes under identical reaction conditions at a considerably faster rate than platinum, and has therefore a significantly higher oxygen affinity than platinum under the scope of this definition.

In Figure 1, an electronic device 100 has a substrate 110 onto which an electrical contact 120 is disposed. A coupling layer 130 electrically connects and physically couples the die 140 to the electrical contact 120.

The substrate 110 is typically a material employed in packaging integrated circuit dies, and contemplated materials include molded plastics, laminated plastics, pressed ceramics and laminated ceramics. Many materials and configurations for suitable substrates are known in the art, and all of them are contemplated for use in conjunction with the teachings presented herein. For example, a collection of suitable substrates can be found in Electronic Packaging & Interconnection Handbook by C. A. Harper, published by McGraw-Hill, 2nd edition, 1997, (ISBN 0-07-026694-8).

With respect to the electrical contact 120, it is contemplated that a wide variety of electrical contacts are appropriate, and especially preferred contacts include copper contacts, or copper contacts that are sputtered and/or plated with a gold or silver layer. Where resistance to corrosion is particularly desirable, appropriate contacts may also include noble metals and their alloys, including gold, platinum, silver, etc. While it is contemplated that the contact or contacts are considerably thinner than the substrate (e. g., a conductive trace or thin metal layer of less than 500 microns), pin-shaped contacts or contacts considerably thicker than 500 microns are also contemplated. For example, where the contact is concurrently used as a heat sink, a copper plate with a thickness of about 800-1000 microns or more may be employed. It is further contemplated that the contact may be affixed or deposited on the substrate by various methods, including vapor deposition, electro-and electroless plating, sputtering, photo-chemical deposition or by methods utilizing an adhesive.

The coupling layer 130 is preferably an alloy comprising Zn, Mg, Al, and Ga, wherein Zn is in a range of 85 to 95 wt%, Al is in a range of 3 to 8 wt%, Mg is in a range of 1 to 5 wt%, and Ga is in a range of 1 to 5 wt%. In an even more preferred aspect of the inventive subject matter, the alloy comprises Zn at approximately 90 wt%, Al at approximately 5 wt%, Mg at approximately 3 wt%, and Ga at approximately 2 wt%.

Although alloys comprising Zn, Mg, Al, and Ga, are generally preferred, various alternative alloys are also contemplated, so long as the melting temperature is at least 270°C, preferably in a range of about 270°C-450°C, and even more preferably 280°C-380°C.

For example, where economic considerations are particularly relevant, an alternative alloy may comprise Sn, Ag, Bi, Zn, and Cu. In a particularly preferred aspect, the alloy comprises Ag in a range of 1 to 5 wt% with the balance being Sn. Alternatively, Cu may be alloyed with Sn in a range of 0.5 to 2.5 wt%. In still further contemplated alloys, Ag and Cu are in a range of 0 to 5 wt% and 0-2.5 wt%, respectively, Ag and Bi are in a range of 0 to 5 wt% and 0-6 wt%, respectively, or Zn and Bi are in a range of 0.5 to 2.5 wt% and 0 to 6 wt%, respectively, with the balance being Sn.

Alternatively, where inclusion of Zn or Cu is undesirable, contemplated alloys may comprise Sn, Ag, Bi, and Sb, preferably in a composition in which Sb is in a range of 5 to 15 wt% and Bi is in a range of 0 to 6 wt% with the balance being Sn, or in a composition in which Ag is in a range of 20 to 30 wt% and Sb is in a range of 5 to 15 wt% with the balance being Sn.

However, it should be appreciated that although not explicitly excluded, Pb is a less preferred component of contemplated alloys. It should further be understood that all alloy compositions of the inventive subject matter presented herein may include incidental impuri- ties of organic or inorganic components, elements, and their naturally occurring oxides and/or derivatives. For example, impurities may include oils, antimony, borate, etc.

With respect to the sacrificial chemical element having an oxygen affinity higher than the other constituents of the alloy, it is preferred that the sacrificial chemical element is

phosphorus, calcium, or titanium, or any combination thereof. However, various alternative sacrificial chemical elements are also contemplated, so long as the oxygen affinity of the sacrificial chemical element is higher than the oxygen affinity of the alloy. Therefore, suitable sacrificial chemical elements may also include alkali metals, transition metals, and non-trans- ition metals including Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd, P, Sc, Sr, Ti, Y, or Zr. It is particularly contemplated that the element is a"non-essential component"of the alloy, that is, the concentration of the element is less than 1 wt%. Thus, the preferred concentration of the element is between about 10 ppm to about 1000 ppm, and will predominantly be detennined by the particular alloy employed, the amount of oxides present in the metallic contact, and the condition under which the solder step for the die attach is performed. For example, where the die attach is performed under an inert atmosphere, and the metallic contact predominantly comprises gold, relatively low concentrations of approximately 10-100 ppm may be appropriate. Likewise, where the element has a relatively high oxygen affinity (e. g., lithium), concentrations of less than 100 ppm may be required.

On the other hand, where the soldering step is performed in an oxygen containing atmosphere, higher concentrations of the element of 10 ppm to 1000 ppm and more are contemplated. Similarly, where elements with a relatively low oxygen affinity are utilized (e. g., titanium), appropriate concentrations maybe in the range of about 200 ppm-to 1000 ppm and more.

In a still further aspect of the inventive subject matter, it is contemplated that more than one element having an oxygen affinity higher than the oxygen affinity of the alloy may be employed in the coupling layer, and contemplated alloys may comprise binary, ternary, and higher mixtures of the elements. For example, suitable alloys may include mixtures of P and Ti, or Ca, Sr, and P, so long as the mixtures and its individual components have an oxygen affinity that is higher than that of the alloy. It should further be appreciated that not only elements, but also organic and/or organometallic molecules may be employed, provided that such molecules have an oxygen affinity greater than the oxygen affinity of the alloy. For example, TCEP (tris-carboxyethylphosphine) may be utilized, where an organometallic reducing agent (i. e., an agent with relatively high oxygen affinity) is desired.

While not whishing to be bound to any particular theory, it is contemplated that elements having a higher oxygen affinity than the alloy reduce metal oxides that are known to increase the surface tension of a melting or molten solder. Therefore, a decrease in the amount of metal oxides during soldering will generally reduce the surface tension of the molten solder, and thereby significantly increase the wetting ability of the solder.

With respect to the die 140 it is contemplated that the die is fabricated from a silicon or sapphire wafer and processed to comprise a plurality of semiconductor elements. There are many dies known in the art, all of which are contemplated for use herein. Preferred dies comprise an integrated circuit, and may have a size between less than one or two mm2 to several cm2 and more. It is further contemplated that the die has a metal coating on the side that contacts the coupling layer, wherein the metal coating may comprise various metals and metal alloys, including an Au/Si, Ag/Sn, Cu/In, Au/Sn alloy, copper, and gold-or silver coated copper.

Thus, specific embodiments and applications of die attach lead-free alloys have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended contemplated claims. Moreover, in interpreting both the specification and the contemplated claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms"comprises"and "comprising"should be interpreted as referring to elements, components, or steps in a non- exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.