WO1994006844A1

WO1994006844A1 - Fast-curing adhesive of adjustable viscosity

Info

Publication number: WO1994006844A1
Application number: PCT/US1993/008757
Authority: WO
Inventors: My Nhu Nguyen; Michael B. Grosse
Original assignee: Johnson Matthey Inc.
Priority date: 1992-09-16
Filing date: 1993-09-16
Publication date: 1994-03-31
Also published as: AU4925893A

Abstract

An adhesive which is capable of rapid curing and, after curing, is thermally stable up to temperatures of up to 460 °C. The viscosity of the adhesive may be varied to formulate a liquid adhesive or a solid adhesive such as a film.

Description

FAST-CURING ADHESIVE OF ADJUSTABLE VISCOSITY

Field of the Invention

The present invention relates to an adhesive formulation which includes different types of cyanate esters and which is useful in bonding a semiconductor device to a substrate. One embodiment is directed to a thermally stable, fast-curing adhesive which is sufficiently stable to resist degradation of adhesive properties. The viscosity of the adhesive of the invention may be adjusted, enabling it to be formulated as a liquid or solid, e.g., a film.

Background of the Invention

It has been recently proposed to formulate adhesive compositions containing cyanate esters. Such adhesive compositions have the capability of being cured in a short time at relatively low temperatures. Prior to use of cyanate ester-containing adhesive formulations, organic adhesives have been used to bond semiconductor devices, also known as "dies," to a substrate such as a lead frame. Organic adhesives such as epoxy polyamide material suffer from various shortcomings and cyanate ester- containing adhesive formulations are an improvement over organic adhesive formulations used prior thereto.

The cyanate esters used are resins of a family of aryl dicyanate monomers and their pre-polymers which contain reactive cyanate functional groups. When heated, the cyanate functionality undergoes exothermic cyclotrimerization reaction to form a triazine ring connection unit which result in gelation and formation of thermoset polycyanurate plastics. The cyanate ester-containing adhesive formulations frequently include metal curing catalysts such as naphthenates, acetylacetonates or chelates of zinc, copper and cobalt, which may or may not be dissolved in alkylphenols, such as nonylphenol, and fillers comprising particles of electrically and/or thermally conductive material, such as silver flake and/or powder, nickel, copper, aluminum oxide, aluminum nitride, etc. Cyanate ester-containing adhesive formulations of the type described are generally useful in applications where they are used as a paste and/or where it is not necessary to expose the adhesive formulation to temperatures above about 400 °C such as is used in connection with glass sealing of ceramic packages.

Summary of the Invention

In accordance with the present invention there is provided an adhesive formulation which includes a cyanate ester vehicle whose viscosity can be adjusted to produce the adhesive in the form of a liquid or a solid such as a film. In one embodiment the adhesive formulation is thermally stable and capable of rapid curing.

In this embodiment, the adhesive comprises about 5 to 40 wt.% phenolic cyanate ester monomer, 0 to 35 wt. % of a viscosity-lowering diluent and 10 to 90 wt.% of an electrically and/or thermally conductive filler. When present, advantageously, an amount of diluent is used to provide a ratio of diluent.phenolic cyanate ester monomer sufficient to provide the desired viscosity, e.g., up to 4:1. The electrically and/or thermally conductive filler may be used in amounts of from 10 to 90 wt.% and the filler preferably comprises one of silver, nickel, copper or gold, and alloys thereof, silicon, silica, diamond, alumina, silicon carbide and aluminum nitride. Fillers may be present as bimodal or polymodal flakes and/or powder. Optionally, a small but effective amount of a curing catalyst, such as those described previously, may be included to minimize shrinkage during curing.

The useful diluents include reactive diluents such as dicyanate ester, diallyl phthalate, triallyl cyanurate, castor oil and liquid polyester oligomers as well as nonreactive diluents such as dimethyl phthalate, and dibutyl phthalate. Optional additions include 0 to 10 wt.% of a curing catalyst and 0 to 10 wt.% of alkylphenol.

The viscosity of the adhesive may be adjusted by varying the amount of diluent, selection of cyanate ester, use of partially polymerized cyanate ester monomer, the application of heat, or addition of at least one reactive or nonreactive plasticizer, or a combination of one or more of the foregoing.

Brief Description of the Drawings

Figure 1 is a graph showing the relationship between viscosity and temperature for various types of cyanate esters;

Figure 2 shows the effect of time at temperature on the trimerization of cyanate;

Figure 3 shows an example of silver flake particles of polymodal distribution; Figure 4 is a diagram describing the phenolic cyanate monomer curing reaction; and

Figure 5 is a graph showing the effect of temperature on adhesion strengths for a ceramic, dual, in-line package.

Description of Preferred Embodiments

The present invention is directed to an adhesive formulation which includes a cyanate ester vehicle with adjustable viscosity and comprises different types of cyanate esters. Alkylphenols and metal curing catalysts as used in prior cyanate ester- containing formulations are optional and among the electrically and/or thermally conductive fillers which can be used to make paste formulation, include particulate fillers of polymodal distribution.

By adjustment of viscosity, a die attach adhesive in accordance with the invention may be in the form of either a liquid or solid at room temperature. Solid adhesives are typically used in the form of a thermosetting film. Such thermosetting films are melted at temperatures ranging from about 50°C to 200°C. Figure 1 shows the relationship between viscosity and temperature for various types of cyanate esters.

As indicated previously, liquid or film adhesives can be formulated by adjusting the viscosity of the cyanate ester vehicle. Adjustment of viscosity can be achieved by the following:

(1) use of different types of cyanate esters;

(2) partial polymerization of die monomer, e.g., to a 20%-50% conversion level, by application of heat, (Figure 2 shows the effect of time at temperature on the trimerization of cyanate); (3) addition of reactive or nonreactive plasticizers; and

(4) combination of one or more of the foregoing.

It has been determined that both liquid and solid forms of the adhesive can be rapidly cured, that is, adhesives of the invention have the property of being able to be cured in less than about 5 minutes at 200°C. Notwithstanding this property, i.e., capability, other curing cycles may be employed if desired.

The following examples shown in Table 1 illustrate the wide variation in adhesive formulations which are possible in practicing the invention. For example, liquid or solid adhesives can be formulated with 20 wt.% cyanate ester monomer as a vehicle with 80 wt.% silver, or silver and 20% partially polymerized (40% polymerized) cyanate ester vehicle. A formulation comprising 100% of partially polymerized cyanate ester vehicle 50% or 20% polymerized may be, respectively, a solid or liquid at room temperature. Tahle 1

Table 2 below identifies some of the commercially available dicyanate ester monomers usable in adhesive formulations, with their respective commercial designations, and including physical properties and a key monomer/homopolymer feature. These cyanate esters are believed to be available from Hi-Tek Polymers (Ciba-Geigy), Louisville, Kentucky. Another useful cyanate ester is a phenolic cyanate ester, 200 cps at 80°C, shown below, which is believed to be available from Allied- Signal Co. and Rhone-Poulenc Co.

The optional filler includes any of the electrical and/or thermally conductive fillers known to be useful in die attach adhesives. Such fillers are used in particulate form and the particle distribution may be bimodal or polymodal and in a broad range of particle size. Use of such fillers of which silver is a typical example, may be desirable to enhance certain performance properties, for example, to maximize filler loading in a paste formulation, to provide enhanced electrical and/or thermal conductivity to improve adhesive strength and to eliminate crack formation. Figure 3 shows an example of the silver flake particles of polymodal distribution.

An adhesive can be formulated according to one embodiment of the inventor which is both fast-curing and sufficiently thermally stable to resist degradation of adhesive properties at temperatures up to 460°C. Such an adhesive is not only suitable for bonding a semiconductor device to a substrate, because of its thermal stability at elevated temperatures, it can be used in ceramic packages for semiconductor devices that employ glass seals. As can be seen in Figure 4, the application of heat to phenolic cyanate monomer, such as during curing, results in polytriazine, a resin. The cured resin is purely aromatic and highly cross linked which gives it higher thermal stability than conventional cyanate esters.

The higher thermal stability of an adhesive formulation broadens the range of applications to which it is suitable. For example, a high temperature adhesive is needed for a glass sealed ceramic packages because glass sealing typically requires heating the glass to sealing temperatures from about 430°C to 460°C.

The phenolic cyanate monomer used in accordance with the present invention is available from Allied-Signal, Inc. of Morristown, New Jersey under the trade name "Primaset." The curing chemistry of this monomer is governed by the cyclotrimerization reaction which as discussed above, forms a highly cross-linked polytriazine network under the influence of heat, and with or without catalysts. The curing reaction may be accelerated with curing catalysts used to cure dicyanate ester- containing adhesive formulations. The use of a small but effective amount of a curing catalyst, e.g., 100-1000 ppm, has been shown to be useful to minimize weight loss during curing and/or subsequent processing of packages or other devices in which the adhesive formulation has been used, from volatilization of unreacted monomer.

The formulation of phenolic cyanate monomer and electrically conductive filler tends to be highly viscous and for some applications it may be advantageous to include a diluent which will reduce the viscosity at room temperature to render the adhesive formulation useful in a wide range of semiconductor processing procedures. To accomplish satisfactory reduction of viscosity for this purpose, the techniques previously described may be used. One technique is to include a diluent. Among the diluents which may be used are reactive diluents that co-react with the polytriazine base resin, such as dicyanate ester, diallyl phthalate, triallyl isocyanurate, and liquid polyester oligomers. Also suitable are nonreactive diluents such as dimethyl phthalate and dibutyl phthalate. The diluent may be present in an amount of up to 35 wt.% and in a ratio of diluent to the phenolic cyanate ester monomer of up to 4: 1, preferably up to 3:1, for the purpose of reducing viscosity.

The amount of electrically and/or thermally conductive filler is chosen to satisfy the required end uses of the adhesive formulation. Generally, 10 to 90 wt.% fillers are present, preferably 60 to 90 wt. % for some applications, and the particular filler selected depends upon the intended use and expected processing of the semiconductor to which the adhesive formulation has been applied. Polymodal and bimodal particulate forms of fillers may be used.

To illustrate the practice of the invention, the examples described in Table 2 were prepared in the amounts, in wt.%, indicated therein. As can be seen, each of the examples contains phenolic cyanate ester monomer ("PCEM") and silver flake as an illustrative filler. Example A is provided for comparison purposes of a formulation without a diluent and Examples B-E contain varying amounts of dicyanate ester ("L-10 Resin") as diluents. Example E includes a small amount of a curing catalyst, copper naphthenate. The formulation of Example A is a solid and Examples B-E are pastes.

Table 2

Also shown in Table 2 are the measured weight loss after 10 minutes at 225 °C and 8 minutes at 430°C. No weight loss occurred at 225^CC and modest weight loss occurred at 430°C. Adhesion testing indicated that adhesive strength of the sample dies (300 x 300 mil.) attached on an alumina substrate was acceptable in all cases. Similarly, the moisture content, which is in the cavity of a glass sealed package using the adhesive formulation at 430°C for 8 minutes, was also acceptable. It was additionally determined that the adhesive formulations are capable of rapid curing using curing schedules of 5 minutes or less at temperatures up to 460 °C. The curing temperature to be selected will depend upon the overall processing involved.

Additional examples appear in the following Table 3, which shows Example F wherein dimethyl phthalate was used as the diluent, Example G wherein triallyl isocyanurate and "L-10" are used as diluents, and Example H wherein allyl ethylate and "L-10" are used as diluents. All Examples F-H contained a curing catalyst, Cu naphthenate, and silver. The adhesive strength following curing was also determined to be satisfactory.

Table 3

The information reported in Figure 5 shows the effect of temperature on adhesion. For the data reported in Figure 2, a 200 x 200 mil. die was provided with the adhesive formulation and then subjected to exposure for 7 minutes as simulated ceramic dual in-line package sealing temperatures of 430°C, 440°C and 450°C. The data in the left portion of the graph is a sample of the composition described in Example A and the data in the right portion of the graph is a sample of the Example F composition. As can be seen, in the data reported for Example A the die shear strength, which is the measure of the adhesion effectiveness, remains high through 440 °C but drops significantly at 450 °C. In contrast, the data reported for Example F shows that the die adhesion strength remains substantially the same through 450°C. The dark portion of the graph in both examples of the chart is the strength (in kg) after curing at 220 °C and the remaining data reports die shear strength at simulated glass sealing temperatures of 430°C, 440°C, and 450°C, respectively.

Adhesive formulations employing dicyanate esters may be used in environments where the adhesive formulation is subjected to temperatures up to 360°C. The new adhesive formulation employing phenolic cyanate monomers may be exposed to temperatures up to 460°C after curing without suffering significant degradation in adhesive properties. The "significant degradation" as used herein means without a deterioration of adhesive strength that would impair the effectiveness of the die adherence to a substrate. However, dicyanate esters, among other materials, are useful as diluents for the adhesive formulation. In addition, it should be noted mat the adhesive formulation is capable of being cured in a short time, i.e., 5 minutes or less, at curing temperatures up to 460°C, although, the adhesive formulation is also curable in a short time at temperatures lower than 460°C. An important advantage of the formulation is its ability to withstand die elevated temperatures mentioned without significant degradation of adhesiveness. Some hermetic ceramic packages for die attach assemblies are typically sealed widi glass compositions. The sealing temperatures for glass seals on such packages is generally in the range of 430° to 460^CC. Therefore, in such applications it is necessary to employ an adhesive formulation that is able to sustain these elevated temperatures without significant degradation of adhesive properties and remain stable.

It is apparent from the foregoing that various changes and modifications may be made without departing from the scope of the invention. Accordingly, the invention should be limited by the appended claims wherein what is claimed is:

Claims

Claims : "10- 1. An adhesive capable of rapid curing comprising a cyanate ester with adjustable viscosity to enable the adhesive to be used in solid or liquid form, said adhesive comprising 5 to 90 wt. % cyanate ester monomer, an amount of a thermally stable viscosity-lowering diluent up to 35 wt. % effective to alter the viscosity of the adhesive, 10 to 90 wt. % of electrically and/or thermally conductive filler, 0 to 10 wt. % of a curing catalyst and 0 to 10 wt. % of alkylphenol, said adhesive having 0 wt.% loss after heating at 225°C for 10 minutes and being substantially thermally stable at temperatures up to about 460 °C without significant degradation of adhesive properties. 2. An adhesive according to claim 1 wherein the viscosity is adjusted by one or more of the following: varying the amount of diluent, addition of a cyanate ester other than the cyanate ester of the adhesive, addition of partially polymerized cyanate ester monomer, the application of heat to polymerize the cyanate ester monomer, addition of at least one reactive or nonreactive plasticizer. 3. An adhesive according to claim 1 wherein said filler is a polymodal or bimodal particulate material.

4. An adhesive according to claim 1 having 60 to 90% filler.

5. An adhesive according to claim 4 wherein said filler is at least one from the group consisting of silver, copper, nickel, gold and alloys thereof, silica, silicon, alumina, silicon carbide, diamond and aluminum nitride.

6. An adhesive according to claim 1 wherein the diluent comprises about 5 to 90 wt. % phenolic cyanate ester monomer.

7. An adhesive according to claim 6 wherein the ratio of phenolic cyanate ester monomer to cyanate ester monomer is up to 4:1. 8. An adhesive according to claim 1 wherein said curing catalyst is present in an amount to minimize shrinking during curing and to impart to the adhesive the property of being able to be cured in not more than 5 minutes at 200°C.

9. An adhesive according to claim 1 wherein said diluent is from the group consisting of dicyanate ester monomer, diallyl phthalate, triallyl isocyanurate, liquid polyester oligomers, dimethyl phthalate and dibutyl phthalate.

10. An adhesive according to claim 1 having 60 to 90% filler.

1 1. An adhesive according to claim 1 wherein said diluent is phenolic cyanate ester monomer and the ratio of cyanate ester to phenolic cyanate ester monomer is up to 4: 1. 12. An adhesive according to claim 1 wherein said diluent is from the group consisting of dicyanate ester, diallyl phthalate, triallyl isocyanurate, liquid polyester oligomers, dimethyl phthalate and dibutyl phthalate. 13. An adhesive according to claim 1 wherein said filler is in the form of polymodal or bimodal particles.

14. A method of adjusting the viscosity of a cyanate ester monomer- containing adhesive formulation comprising at least one of the following: (1) incorporating into said formulation other cyanate ester compositions than the cyanate ester of the adhesive formulation;

(2) partially polymerizing the cyanate ester monomer of the adhesive by application of heat;

(3) adding to the adhesive formulation at least one reactive or nonreactive plasticizer; and

(4) combination of one or more of the foregoing.

15. A method according to claim 14 wherein the viscosity is adjusted by adding a diluent comprising at least one of the following: dicyanate ester monomer, phenolic cyanate ester monomer, diallyl phthalate, triallyl isocyanurate, liquid polyester oligomers, dimediyl phthalate and dibutyl phthalate.

16. A method according to claim 15 wherein said diluent is phenolic cyanate ester monomer and said diluent is present in a ratio to the cyanate ester monomer in the adhesive of up to 4: 1.

17. A metiiod according to claim 16 wherein the ratio is up to 3: 1. 18. A method according to claim 14 wherein the viscosity is adjusted by adding up to 35% of a diluent to provide a ratio of diluent to cyanate ester monomer in the adhesive of up to 4: 1.

AMENDED CLAIMS

[received by the International Bureau on 24 January 1994 (24.01.94); original claims 1-14 amended; other claims unchanged (2 pages)]

1 An adhesive capable ot rapid curing comprising a cyanate ester with adjustable viscosity to enable the adhesive to be used in solid or liquid torm, said adhesive comprising 5 to 90 wt. % cyanate ester monomer, 0.5 to 35 wt. % of a thermally stable viscosity-lowering diluent effective to alter the viscosity of the adhesive. 10 to 90 wt. % of electrically and/or thermally conductive filler. 0 to 10 wt. % of a curing catalyst and 0 to 10 wt. % of alkylphenol, said adhesive having substantially no weight loss after heating at 225 °C for 10 minutes and being substantially thermally stable at temperatures up to about 460 °C without significant degradation of adhesive properties.

2. An adhesive according to claim 1 wherein the viscosity is adiusted by varying the amount of diluent

3. An adhesive according to claim 1 wherein said filler is a polymodal or bimodal paniculate material .

4. An adhesive according to claim 1 having 60 to 90% filler.

6. An adhesive according to claim I wherein the ratio of diluent to cyanate ester is up to 4: 1.

7. An adhesive according to claim I wherein the ratio of diluent to cyanate ester is up to 3: 1.

8 An adhesive according to claim 1 wherein said curing catalyst is present m an amount to minimize shrinking during curing and to impart to the adhesive th propeπy of being able to be cured in not more than 5 minutes at 200 °C

9. An adhesive according to ciaim I wherein said diluent is from the group consisting of diallyl phthalate. triallyl isocyanurate. liquid polyester oligomers. dimediyl phthalate and dibutyl phthalate.

10 An adhesive according to ciaim 9 having 60 to 90% filler ,

11. A method of adjusting the viscosity of a cyanate ester monomer-containing adhesive formulation comprising adding a diluent to the adhesive formulation comprising at least one reactive or nonreactive plasticizer.

12. A method according to claim 11 wherein the viscosity is adjusted by adding a diluent comprising at least one of the following: diallyl phthalate, triallyl isocyanurate, liquid polyester oligomers, dimethyl phthalate and dibutyl phthalate.

13. A method according to claim 12 wherein said diluent is present in a ratio of up to 4: 1 to the cyanate ester.

14. A method according to claim 13 wherein the ratio is up to 3:1.