Adhesion of Metals to Thin-Film Fluorocarbon Polymers - American

Chapter 30

Adhesion of Metals to Thin-Film Fluorocarbon Polymers Chin-An Chang, Yong-Kil Kim, and A. G. Schrott

Downloaded by MONASH UNIV on November 14, 2015 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch030

T. J. Watson Research Center, IBM Corporation, Box 218, Yorktown Heights, NY 10598

Adhesion of metal films to several fluorocarbon polymer films is studied for Cu, Cr, Ti, Al, and Au. The polymers include polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), and fluorinated ethylene perfluoroalkyl vinyl ether copolymer (PFA), a l l deposited on Cr/SiO substrates by spin coating. Peel strengths of the metal strips on these polymers are compared, and are taken as measures of the metal-polymer adhesion. Among the polymers, FEP gives the highest peel strengths to metals, with PTFE the least. Among the metals, Ti gives the highest peel strength for each polymer, followed by Cr, with Cu being the lowest. The peel strengths for Ti on FEP, PFA, and PTFE are 85, 75, and 20 g/mm, respectively. Those for Cu to the polymers are around 5 g/mm or less. The results are correlated with the differences among the polymers and metals, and the mechanisms involved are suggested. 2

Metal-polymer adhesion has been of interest in recent years for the electronic packaging technology. A wide range of studies has been devoted to the understanding of interactions between metals and polymers, mostly on polyimide (1-3). In this paper, we report the studies on the adhesion between metals and fluorocarbon polymer films. Fluorocarbon polymer has a dielectric constant of 2.1, lower than that of polyimide, 3.2-3.5, and is attractive to packaging. We have studied the adhesion of Cu to bulk Teflon, a polytetrafluoroethylene (PTFE) polymer, and found enhanced adhesion using a presputtering treatment of the Teflon prior to the deposition of Cu (A). Further analysis shows that the morphological changes of the Teflon due to the sputtering treatment could be a major contributor to the enhanced adhesion observed (5). 0097-6156/90/044(M)416$06.00/0 © 1990 American Chemical Society In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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EXPERIMENTALS AND RESULTS Three fluorocarbon polymers are compared here, including polytetrafluoroethylene (PTFE), fluoroethylenepropylene (FEP), and a copolymer containing a perfluoroalkoxy group (PFA). Both PTFE and FEP contain only f l u o r i n e and carbon atoms, the only difference being the presence of isopropylene for FEP. PFA contains, i n addition, an oxygen atom for each of the isopropylene unit, connecting an a l k y l group to the polymer backbone. The polymer films were made by a spin-coating technique using polymer dispersions. Upon spinning the polymer d i s persion on the Cr/SiO^/Si substrates, the samples were dried and cured at d i f f e r e n t temperatures to remove the solvents, and to s i n ter the polymer p a r t i c l e s . The f i n a l curing temperature i s 370-390 C. Multiple coatings were made, with a f i n a l thickness around 10 um for the polymer f i l m s . Afterwards, d i f f e r e n t metal layers of 2000A were deposited by electron beam evaporation, followed by thick Cu s t r i p s of 10 um with a defined geometry. Several metals were compared, including Cu, Cr, T i , Au, and A l . A 90 peel tester was used to peel the Cu s t r i p s , and the peel strength i s taken as a measure of adhesion between the metal and the polymer. Bonding between the metal and polymer i s studied using x-ray photoelectron spectroscopy (XPS). Figure 1 shows the peel strengths for the d i f f e r e n t metal/polymer systems. T i gives the highest peel strength for each polymer, followed by Cr, with Cu and Au being the lowest. For the polymers, FEP and PFA show much higher peel strengths than those of PTFE. XPS analysis of the metal-polymer bonding i s shown i n Figs. 2-4. Weak interactions are seen between Cu and the polymers. Cr and T i , on the other hand, show strong interactions, as manifested by the carbide-like peaks shown. DISCUSSION We have compared the peel strengths of d i f f e r e n t metals deposited on several fluorocarbon polymer films. Strong peel strengths exceeding 80 g/mm have been obtained f o r the Ti/FEP system. The difference among the polymers i s probably related to the s t r u c t u r a l differences. Both FEP and PFA have branched structures, with some carbon atoms being surrounded by three carbon neighbors. PTFE, on the other hand, i s l i n e a r , with a l l the carbon atoms, except the end ones, surrounded by two carbon neighbors. Such a difference i s consistent with the much lower peel strength using PTFE than those of both FEP and PFA. It i s also i n agreement with the observation of strong carbide-like XPS peaks observed for FEP and PFA, but not for PTFE. The mechanism involved, however, can be complicated. One possible mechanism i s suggested below. For the carbon atoms with d i f f e r e n t neighbors, the consideration of electronegativity provides a guideline of the r e a c t i v i t y of these carbon atoms toward the metals. Fluorine i s more electronegative than carbon; the e l e c t r o n e g a t i v i t i e s are 4.0 and 2.5 f o r f l u o r i n e and carbon, respectively (8). Carbon atoms with more f l u o r i n e n e i ghbors are expected to be more electronegative than those with less

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


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METALLIZATION OF POLYMERS

100

80 60 40 20 h

5-

Au

Figure 1. Peel strengths of different metals adhered to PTFE, FEP, and PFA fluorocarbon polymerfilms.(Reprinted with permissionfromChang, C.-A. et al. J. Vac. Set 1990, 5(4), 3306. Copyright 1990 American Institute of Physics.)


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~ ~ ι — ι —

1

— Γ


PTFE + 3 0 Â T Ï FEP + 3 0 Â T i

CO

ζ

liJ

1 ι I ι 1 ι I ι I ι—

CO CO

Cls

1

SPUTTERED GRAPHITE CLEAN + 5ÂTÏ

J 296

I

L

_L

JL_i

292 288 284 280 BINDING ENERGY (eV)

L 276

Figure 2. Upper: XPS spectra of Ti deposited on PTFE and FEP, showing carbide-like bondings. Lower: XPS spectra of Ή deposited on graphite. The carbon and carbide-like peaks of the lower spectra help the assignment of peaks in the metal—polymer spectra. (Reprinted with permission from Chang, C.-A. et al.J.Vac Sci 1990,8(4), 3306. Copyright 1990 American Institute of Physics.)


420


1

—ι—'—ι—ι—ι—'—ι—ι—Γ


ι

296

292 288 284 BINDING ENERGY

280 (eV)

276

Figure 3. Upper: XPS spectra of Cr deposited on PTFE Lower: XPS spectra of Cr deposited on FEP. A strong carbide-like bonding is seen for FEP, but a very weak one for PTFE. (Reprinted with permission from Chang, C.-A. et al. J. Vac. ScL 1990, 5(4), 3306. Copyright 1990 American Institute of Physics.)



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292 288 284 280 BINDING ENERGY (eV)

421

276

Figure 4. XPS spectra of Cu, Cr, and Ti deposited on PFA. Little reaction is seen with Cu, but strong reactions are shown with Cr and Ti, with the respective carbide-like peaks detected. The peaks around 284.5 eV are due to the surface carbon. (Reprinted with permission from Chang, C.-A. et al.J.Vac. Sci 1990, 8(4% 3307. Copyright 1990 American Institute of Physics.)


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fluorine neighbors. Assuming equalized electronegativities between carbon and fluorine for each C-F bond (9), the electronegativity of the carbon atoms are 3.5, 3.4, 3.17, 2.8, and 2.5 for those with four, three, two, one, and no fluorine neighbors, respectively. The remaining neighbors are carbon. This is a f i r s t order approximation. It shows that carbon atoms with a CF^ configuration are more reactive toward metals, which are less electronegative than carbon, than those with a CF^ configuration. This could be responsible for the higher peel strengths observed for FEP and PFA than those for PTFE. It is also consistent with the strong bonding from the XPS analysis for the metals on FEP and PFA. We tentatively assign the c a r b i d e - l i k e peaks to the bonding involving the carbon atoms with the CF^ configu r a t i o n . Detailed bonding configuration between the metal and the polymer i s not known at t h i s moment, however. The same consideration of electronegativity also gives some c o r r e l a t i o n with the differences among the metals. The electronegativities are 1.5, 1.5, 1.6, 1.9, and 2.A for T i , A l , C r , Cu, and Au, respec t i v e l y (8). The peel strengths are the highest for T i , followed by Cr, and the lowest for Cu and Au. The two exceptions are A l and Au/PFA. More study i s needed to f u l l y understand the differences among the metals studied here. SUMMARY In summary, several fluorocarbon polymer films are compared for t h e i r adhesion to metals. A strong adhesion, as measured by the peel strength, is obtained for T i and Cr bonding to the polymers with a high concentration of carbon atoms with three fluorine neighbors. Some c o r r e l a t i o n i s observed with the electronegativities among the different carbon s i t e s in the polymers, and among the metals studied. ACKNOWLEDGMENT The authors would l i k e to thank M. C e l i and C. F a r r e l l for the metal deposition. REFERENCES 1. Chou, Ν. J. and Tang, C. Η., J. Vac. Sci. Technol. A2, 751 (1984) 2. Ohuchi, F. S. and Frelich, S. C., J. Vac. Sci. Technol. A4, 1039, (1986). 3. Jordan, J. L., Sanda, P. Ν., Morar, J. F, Kovac, C. A.,Himpsel, F. J., and Pollak, R. Α., J. Vac. Sci. Technol. A4, 1046 (1986). 4. Chang, Chin-An, Baglin, J. Ε. E., Schrott, A. G., and Lin, K. C., Appl. Phys. Lett. 51, 103 (1987). 5. Chang, Chin-An, Appl. Phys. Lett. 51, 1236 (1987). 6. Wheeler, D. R. and Pepper, S. V., J. Vac. Sci. Technol. 20, 442 (1982). 7. Michael, R. and Stulik, D., J. Vac. Sci. Technol. A4, 1861 (1986) 8. Pauling, L., The Nature of the Chemical Bond, 3rd ed., Cornell Press (1960), p.93. 9. Sanderson, R. T., Chemical Bonds and Bond Energy, Academic Press, New York, (1971), p.15. RECEIVED May 16, 1990


Adhesion of Metals to Thin-Film Fluorocarbon Polymers - American

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