Metallization of Polymers - American Chemical Society

Chapter 17

Novel Process for Surface Modification of Polyimide N. L. D. Somasiri, T. A. Speckhard, and R. L. D. Zenner

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch017

Industrial and Electronic Sector Laboratory, 3M Company, St. Paul, MN 55144-1000

Surface modification (texturing) of polyimide through a metal clustering and migration process is reported. This process involves heat treatment of polyimide coated with a thin copper layer. Subsequent metallization of the textured surface leads to improved adhesion due to mechanical anchoring. Adhesion values of 7-10 lbs/in (ambient) and 46 lbs/in (after solder float) have been obtained.

Kapton polyimide has been widely used in the electronic industry because of its low dielectric constant, good mechanical properties and high thermal stability. Many applications require good adhesion between Kapton polyimide film and metal. Various processes to improve adhesion of metal to Kapton polyimide have been reported in the literature. DeAngelo et al., (JJ describe a process to form metal oxides on the surface of polyimide to improve adhesion. Other efforts to improve adhesion of a metal layer involve roughening of the surface of polyimide substrate by methods such as cathodic sputtering (2), chemical attack (2, 5), and reactive ion etching (1,1).

Recently Krause et al., (6) have reported a novel method for metallizing Kapton polyimide utilizing an aqueous based reversible charge transfer process. The Kapton polyimide film is first reduced to the radical anion state (which exhibits a characteristic green color) in the absence of oxygen by exposure to a reducing agent. Upon immersion in certain metal salt solutions, the reduced polyimide acts as a reducing agent to deposit a thin film of metal on the polymer surface. The metal film that is deposited (e.g., copper) then serves as a seed layer for subsequent electroless and electrolytic plating to the desired metal film thickness. Adhesion values in excess of 6 lbs/in have been obtained under ambient conditions using this process but adhesion values fall to less than 1 lb/in when the samples are subjected to the solder float test at 280°C.

0097-6156/90/0440-0235$06.00A) © 1990 American Chemical Society Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

METALLIZATION OF POLYMERS

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Wc have recently modified the Krause process by incorporating a heat treatment step which results in a novel process for surface texturing of Kapton polyimide (Somasiri, N. L . D.; Speckhard, T. A. U. S. Patent Applied For). When the textured polyimide is subsequently metallized, again using the Krause process, good adhesion is obtained after exposure to the îolder float test.


Experimental Kapton polyimide 300VN purchased from DuPont (Wilmington, Delaware) was reduced to a green colored film and a seed layer of copper (-80 Angstrom) was deposited as described by Krause et al., (6). After washing and drying in air, the seed copper coated Kapton polyimide was heated to 450°C in air for 3-5 minutes. More seed copper was deposited as described previously, and the thickness of copper was then increased to 2000 Angstrom by electroless copper plating (Cuposit CP-78, Shipley Chemical Co., Newton, MA). Final thickness of copper was increased to 1 mil (25 micron) by electroplating (Harshaw Cu-Tronix acid plating bath). The electroplated metallized Kapton polyimide was initially dried in air and then dried at 135°C for one hour. The metal surface was masked with 1/16 inch wide 3M brand masking tape. Exposed metal was etched by immersing in aqueous 3M FeCl3 solution. After thorough rinsing, the masking tapes were removed and 90° peel test was performed (using an Instron device) according to I.P.C. test method 650-2.4.9 Method A (Z) in order to characterize the adhesive force between copper and Kapton polyimide. The material was then subjected to the solder float test (I.P.C. test method 650-2.4.9 Method C.; ref. 2) and 90° adhesion was measured immediately and after 24 hours exposure to laboratory air (50% relative humidity) at room temperature. Surface topography of Kapton polyimide as-received, seeded with copper, after the 450°C heat treatment, and after removal of copper oxide by acid etching was examined by scanning electron microscopy. Crosssectional analysis of Kapton seeded with copper and after 450°C heat treatment was carried out by transmission electron microscopy. Results and Discussion We have discovered that heating Kapton polyimide with a seed coating of copper in air to 450°C leads to surface texturing. Scanning electron micrographs of Kapton polyimide, as-received, after seeding with copper, after 450°C heat treatment, and after removal of copper oxide by acid etching are shown in Figures la, b, c, and d, respectively. The as-received and copper coated films are smooth except for protrusions due to slip agent particles. After heating, the film exhibits a textured surface on the scale of about 0.5 micron. This roughness is due to the underlying polyimide and not metal oxide formation as can be seen in Figure Id where the metal oxide has been etched away. Although at present the processes leading to the textured surface are not completely understood, preliminary experiments suggest that the copper atoms migrate and form clusters which then serve as catalytic sites for polyimide degradation. As shown in Figure 2, transmission electron micrographs of Kapton seeded with copper and after 450°C heat treatment reveal evidence for copper clustering. We have also observed a weakening of the copper signal in x-ray

Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Surface Modification ofPolyimide


17. SOMASIRIETAL.

Figure 1. Scanning electron micrographs of Kapton polyimide: (a) as-received, (b) seeded with copper, (c) after 450°C heat treatment, and (d) after heat treatment and removal of a l l copper oxide.


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Figure 1 Continued.



Figure 2. Transmission electron micrographs of Kapton polyimide (magnification 36000X): (a) seeded with copper and (b) a f t e r 450°C heat treatment.


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photoelectron spectroscopic elemental analysis following treatments of 15, 75, and 135 seconds at 450°C further illustrating that copper migrates deeper into the Kapton. In effect, the valleys in this "hill-and-valley" type morphology are created by preferential degradation of polyimide in the vicinity of copper/copper oxide clusters. Some support for this mechanism can be found in the literature. Ho et al., (SJ have reported the formation of copper clusters when annealing copper coated polyimide to 300°C. Copper is known to catalyze the thermal oxidation of polyethylene (9) and several metals such as silver (10), copper (10). and cobalt (11) have been reported to catalyze the thermal decomposition of polyimide. When subject to 90° peel test, adhesion values of 7-10 lbs/in have been obtained under ambient conditions. Interestingly, the adhesion after the solder float test retained 4-6 lbs/in. This high adhesion is believed to be primarily due to the mechanical interlocking facilitated by the textured Kapton polyimide. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Cited

DeAngelo, Μ. Α.; Sharp, D. J. U. S. Patent 3 562 005, 1971. Schoenaich, D. B.; Boeblingen P. F.; Gaertringen, W. K.; Schwerdt, F.; Thelen, U.; Holzgerlingen, T. V. U. S. Patent 4 152 195, 1979. Ruoff, A. L.; Kramer, E. J.; Li, C. Y. IBM J. Res. Develop. 1988, 32(5), 626. Dunn, D. S.; Grant, J. L.; McClure, D. J. J. Vac. Sci. Technol. 1989, A7(3), 1712. Walsh, D. P. U. S. Patent 4 806 395, 1989. Krause, L. J.; Rider, S. A. U. S. Patent 4 710 03, 1987. IPC Standard IPC-FC-FLX, The Institute for Interconnecting and Packaging Electronic Circuits, April, 1988. Ho, P. S.; Hahn, P. O.; Bartha, J. W.; Rubloff, G. W.; LeGowes, F. K.; Silverman, B. D. J. Vac. Sci. Technol. 1985, A3(3), 739. Chan, M. G.; Allara, D. L. Polymer Engineering and Science 1976, 14(1), 12. Mittel, K. L. In Polyimides Synthesis, Characterization and Application; Plenum Press, New York, 1984; Vol. 2, ρ 871-887. Anderson, S. G.; Meyer, III, Η. M.; Weaver, J. H. J. Vac. Sci. Technol. 1988, A6(4), 2205.

RECEIVED May 16, 1990


Metallization of Polymers - American Chemical Society

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