Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-06-08
2002-01-01
Tolin, Michael A. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S272200, C156S273300, C156S275700, C156S327000, C156S330000
Reexamination Certificate
active
06334926
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for the low temperature lamination of metals, and especially copper, to the surface of inert fluoropolymers. More specifically, the invention relates to a method for surface modification of fluoropolymers by thermal graft copolymerization with concurrent lamination of copper metal in the presence of a functional monomer.
When thermal grafting or graft copolymerization occurred on the pre-activated surface and interface of a fluoropolymer in contact with a metal surface, lamination occurs simultaneously. The surface and interfacial graft copolymerization with concurrent lamination of the metal are carried out under atmospheric conditions, and in the complete absence of an added polymerization initiator or system degassing. The process and properties can be imparted on the inert surfaces of most fluoropolymers in contact with a preferred metal.
Thermal grafting or graft copolymerization with concurrent lamination can be carried out in the presence of one or more functional monomers including, but not limited to, 1-vinyl imidazole (VIDZ), 1-allyl imidazole, 2-vinyl pyridine (2VP), 4-vinyl pyridine (4VP), 2,4,6-triallyloxy-1,3,5-triazine, 1,2,4-trivinylcyclohexane and triallyl-1,3,5-benzenetricarboxylate, epoxide-containing monomers, hydroxy-containing monomers, amine-containing monomers, monomers of polyelectrolyte and monomers of polyampholyte.
The lap shear adhesion interface can readily exceed the tensile yield strength of the substrate polymer film. The lap shear adhesion strength of the so-laminated fluoropolymer-copper interface can readily exceed the tensile yield strength of the substrate film. The T-peel strengths of the so-laminated fluoropolymer-metal interfaces are in excess of 8 N/cm, with delamination occurs via cohesive failure of the polymer film. The strong adhesion between the polymer and the metal arises from the covalent bonding of the grafted functional chains on the fluoropolymer surface on one hand, and the strong adhesion, with or without charge transfer interaction, of the grafted functional chains to the metal surface on the other.
The present invention distinguishes itself from the prior art in that the grafting/lamination process is carried out at temperatures substantially below the melt processing or sintering temperature of the fluoropolymer and no adhesive is required to effect the lamination process under atmospheric conditions.
2. Description of Related Arts
One of the most important requirements for the fastest microelectronic devices of the near future will be the reduction of the signal interconnection delay time to a small fraction of total switching delay time. One method of lowering this delay time relates to the use of multilayer devices incorporating highly conductive metal, such as copper, and low capacitance dielectrics, such as the fluoropolymers.
Of all the dielectric materials, the fluoropolymer, in particular poly(tetrafluoroethylene) (PTFE) and its derivatives, are ideal dielectric materials, from the standpoints of electrical and thermal properties, for the packaging of microelectronics. With improved adhesion between fluoropolymer surfaces and at the interface of a fluoropolymer and a metal, the application of fluoropolymers in multi-chip module(MCM) packaging, for example, may become a reality. The physical and chemical inertness associated with most of the fluoropolymer, however, dictates the use of more drastic means for achieving the required surface modifications. The strategies of surface chemical and physical modification have been widely implemented for fluoropolymers and hydrocarbon polymers alike.
One of the major drawbacks of the most commonly utilized technique of plasma treatment is that the physicochemical characteristics of the modified polymer surfaces, including surface compositions, are time-ependent. Chain and polar group reorientation in the surface region can result in gradual deterioration of the surface reactivity. Furthermore, anomalous changes in oxygen and fluorine contents, and therefore also surface compositions, may result from the presence of surface hydrocarbon contamination during plasma treatment, as suggested by M. A. Golub, E. S. Lopata, L. S. Finney,
Langmuir
, 10, 3629 (1994). To overcome the time-dependent surface characteristics, the plasma-treated fluoropolymers have been subjected to further surface modification via graft copolymerization, as shown in K. L. Tan, L. L. Woon, H. K. Wong, E. T. Kang and K. G. Neoh,
Macromolecules
, 26, 2832 (1993); E. T. Kang, K. G. Neoh, W. Chen, K. L. Tan, C. C. Huang and D. J. Liaw,
J. Adhesion Sci. Technol
., 10, 725 (1996); and Tie Wang, E. T. Kang, K, G, Neoh, K. L. Tan, C. Q. Cui and T. B. Lim,
J. Adhesion Sci. Tech
., 11, 679 (1997). The factors affecting the adhesion of fluoropolymer composites to commercial copper foils have been summarized by L. J. Jimarez, L. J. Matienzo and A. A. Mehta in
Transactions of the ASME
, 115, 256 (1993). The fluoropolymer metallization for microelectronics application has also been reviewed recently by E. Sacher in
Prog. Surf. Sci
. 47 (3), 273 (1994).
The patent literature contains numerous disclosures of surface modification of fluoropolymers for adhesion enhancement. However, most of the cases are related to plasma or chemical surface treatment. A few cases are related to surface modification via graft copolymerization. Almost no case study is directly related to the modification of fluoropolymer via surface graft copolymerizaiton for the improvement of adhesion between two fluoropolymer surfaces, between a fluoropolymer surface and a conjugated polymer surface, or between a fluoropolymer surface and a metal surface. Throughout our exhaustive patent literature search, there is no relevant process which involves the simultaneous modification of a fluoropolymer surface via grafting of graft copolymerization and the simultaneous lamination of a metal in the complete absence of an adhesive.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new method for the low temperature direct lamination of copper metal to fluoropolymer surfaces under atmospheric conditions. It is also an object of the present invention to effect the said lamination in the absence of an added adhesive. These and other objects and advantages of the present invention are obtained by providing a method for the modification of fluoropolymer via, first plasma pretreatment, followed by low temperature thermal graft copolymerization of an appropriate functional monomer at the lapped interface between the fluoropolymer and the selected metal. Preferably, a low grafting/lamination temperature is selected to be substantially below the melting or sintering temperature of the fluoropolymer. Desirably, radio frequency argon plasma with low plasma power is selected for the pretreatment of the fluoropolymer to minimize the undesirable over-oxidation, etching or sputtering of the fluoropolymer surface.
The objects and advantages of the present invention can be achieved when the monomers used for surface graft copolymerization with concurrent lamination are selected form a group of vinyl monomers which contain nitrogen heteroatoms or nitrogen functionalities in the pendant group or groups. The monomers are also selected from the family containing multiple vinyl group functionalities, which can also promote chain crosslinking, as well as from the family which contains epoxide functional groups.
The objects and advantages of the present invention can be achieved when the monomer concentrations used for graft copolymerization range from 2 to 100 weight percent. Desirable solvents are selected from the group which provide good solubility for the vinyl monomer, and which promote free radical polymerization.
The objects and advantages of the present invention can be achieved on virtually all fluoropolymer substrates. The maximum objects and advantage are realized on fluoropolymer substrates, such as, but not limited to, poly(tetrafluoroethylene) (PT
Cui Cheng Qiang
Kang En Tang
Lim Thiam Beng
Neoh Koon Gee
Shi Jian-Li
National University of Singapore and Institute of Microelectroni
Tolin Michael A.
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