Copper ion binding to N-phenylphthalamic acid studied by 13Cnuclear magnetic resonance and electron paramagnetic resonance: model interaction of polyamic acid with copper Toshifumi Hiraoki,"" Noriyuki Kinjo,tb Kunio Miyazaki,b Osamu Miurab and Akihiro Tsutsumi*" aDepartment of Applied Physics, Hokkaido University, Sapporo 060, Japan Hitachi Research Laboratory, Hitachi Ltd., Hitachi 319-12, Japan The interaction of copper with N-phenylphthalamic acid (PPA) cured at various temperatures was investigated by 13C NMR and EPR spectroscopies, as the model system of polyamic acid on copper. EPR spectra prove that copper is dissolved into the PPA- N-methylpyrrolidone solution, producing paramagnetic Cu2 ions. The 13C NMR resonances of the phthalic group of PPA are + selectively broadened due to the paramagnetic interaction between the 13C nuclei and Cu2+ ions, showing binding of Cu2+ to the carboxylate group. Cu2+ ion has no effect on the amide and phenyl groups.Cu2+ ion exchanges rapidly between the carboxylate groups at an exchange rate > lo3s-' at 23 "C. PPA is imidized to N-phenylphthalimide (PPI)at 150 "C, accompanied by the dissociation of Cu2+. Paramagnetic effects from the Cu2+ ions is not exerted on PPI. The results obtained are compared with the interfacial interaction between polyamic acid and copper. Polyimides are one of the most important classes of high- performance polymers.' Owing to their excellent electrical, thermal and high-temperature mechanical properties, polyim- ides have found many applications, in particular for advanced microelectronics.The polyimide-metal interface characteristics, such as chemical, adhesion and electrical properties are very important for microelectronic devices. A number of extensive studies of polyimide-metal interface interactions have been performed.'q2 Understanding the nature of the interacting processes is essential to basic device reliability. There are basically two types of interface between polyimide and metal: metal-on-polymer and polymer-on-metal. The metal-on-polymer type of interface, normally formed by vapour deposition of metal films onto polyimide, has been charac- terized extensively by surface spectros~opies.~-~ The polymer- on-metal type of interface is created by coating a polyimide precursor onto a metal followed by curing to form the polyimide. Among the metals investigated, copper has been found to exhibit an important differen~e.~~' Copper is commonly used as the metal in microelectronic applications because of its high conductivity and low cost.Two reactions have been reported to occur at the interface between pyromellitic dianhydride- oxydianiline (PMDA-0DA)-derived polyamic acid and copper during ~uring.~-'~ At first, the polyamic acid reacts directly with copper or copper oxide and the copper-polyamic acid complex is proposed to form. Secondly, this complex decom- poses to the polyimide during subsequent curing. Both reac- tions are controlled by the curing temperature and the oxygen level supplied to the interface.The thermal imidization in the presence of oxygen was found to produce a cuprous oxide (Cu20), which degrades the polyimide ~atalytically.~-'~ The reverse, copper-on-polyimide, interface, however, does not produce CU,O.~ The nature of the interactions at the interface still remains to be understood concerning the binding site and exchange rate of copper. In the copper-polyamic acid complex the copper ion is believed to bind the carboxylate group of the polyamic We have shown that N-phenylphthalamic acid (PPA; Fig. l), as a model compound of the main compo- -f Present address: Ibaraki Research Laboratory, Hitachi Chemical Co., Ltd., Higashi 4-13-1, Hitachi 317, Japan. nent of polyamic acid, interacts with copper to form the PPA-copper ion complex in N-methylpyrrolidone (NMP), and that the copper ion binds to the carboxylate group of PPA.12 The present study clarifies the interaction of PPA with copper and the curing of the PPAXopper complex, as a model system of polyamic acid on copper, by means of I3C NMR and EPR spectroscopies. The interfacial interaction of polyamic acid with copper will be discussed, and compared with the present study.Experimental N-phenylphthalamic acid (PPA) was synthesized from phthalic anhydride and aniline in N-methylpyrrolidone (NMP).I2 PPA solutions in NMP (10%) were treated at 50, 100 and 150°C with and without copper particles for 3 h each, and then poured into water. The resultant precipitates were fil- tered off, washed several times with water, and dried in U~CUO at room temperature.The copper content of each sample was obtained using a Hitachi 2-8000 atomic absorption spectrometer. 13C NMR spectra in solution were obtained on a Bruker AM-500 spectrometer operating at 125.76 MHz at 23 "C un- 38 PPA 02 II PPI Fig. 1 Structures of N-phenylphthalamic acid (PPA) and N-phenyl- phthalimide (PPI) J. Muter. Chem., 1996, 6(5),727-731 727 less otherwise stated The sample concentrations used were 0 1-0 05 mol dm-3 solutions in deuteriated dimethyl sulfoxide [(CD3)ZSO, MSD Isotopes] Chemical shifts are expressed relative to the resonance of (CD,),SO at 6 39 5 Two-dimen-sional 'H 2QF-COSY,15 l6 C-H C0SYl7 and HMBCI8 experi- ments were performed to obtain resonance assignments EPR spectra in the solid state were obtained at 9 25 GHz and a microwave power of 1mW using a JEOL FES-1XG spec- trometer at room temperature The 100 kHz field modulation width was 1 mT Results and Discussion Paramagnetic copper ion The copper concentrations of PPA cured at 50,100 and 150 "C in the presence of copper were estimated to be 3 6, 3 0 and 0 16%, respectively Fig 2 shows the EPR spectra of PPA cured at three different temperatures with copper The spectra show that the cunng of PPA in the presence of copper produces paramagnetic Cu2+ ions Analogous results have been obtained from visible spectra in NMP l2 Paramagnetic effect on 13CNMR spectra of PPA Resonance assignments of the 13C NMR spectra of PPA without copper were made with the two-dimensional NMR spectroscopies described in the Experimental section The chemical shifts of PPA obtained are summarized in Table 1 The paramagnetic interaction between the 13C nucleus and the Cu2+ ion is clearly observable in NMR spectra Resonances around the metal-ion binding site will be broadened because J I I I I300 400 BlmT Fig.2 EPR spectra of copper with PPA in the solid state at 23°C Cunng temperature and recording gain of each spectrum (a) 50°C and 6 3, (b) 100"Cand 5, (c) 150"C and 5 Table 1 I3C NMR chemical shifts (6) for PPA and PPI at 296 K PPA PPI carbon atom 6 carbon atom 6 C' 138 92 C' 131 54 CZ 129 97 c3 129 54 CZ 123 72 c4 129 38 c3 134 72 c5 131 72 C6 127 79 c7 16740 c4 167 03 C8 167 48 c9 139 58 c5 131 91 C'O 119 53 C6 127 42 C" 128 63 c7 128 86 CIZ 123 32 C8 128 08 of the hyperfine interaction between a nucleus and an unpaired electron with a large magnetic moment l9 24 Since the Cu+ ion contains no unpaired electron, its complex is always diamagnetic Therefore, for Cu +,paramagnetic broadening is not observed in the NMR spectrum and the EPR spectrum is silent The solution of PPA containing copper was added to the PPA solution without copper, both of which were cured at 50 "C Fig 3 and 4 show the copper concentration dependence of the aromatic and carbonyl regions, respectively, of the 13C NMR spectra of PPA The resonance of the carboxylate car- bon Cs of PPA is at first selectively broadened and its intensity decreases remarkably with the addition of copper Simultaneously, the C1 and C2 resonances are considerably broadened This observation illustrates that the paramagnetic Cu2+ ion interacts with PPA and is located close to the carboxylate group of PPA The addition of increasing concen- trations of copper ions results in the broadening of the C3, C6 and C4 resonances and the disappearance of the C1, C2 and Cs resonances Copper ion exhibited hardly any influence on the carbons of the phenyl group, C5 of the phthalic group and the amide group at the concentration used This is confirmed by 'H NMR spectra, which show that the amide proton resonance is not affected by the presence of Cu2+ ion (data not shown) These results show that Cu2+ ion is bound to the carboxylate group and forms a complex in solution The paramagnetic line broadening effect of copper ions on the carbon resonances of PPA is of the following order Cs >C2> C1 >C3> C6 > C4 No paramagnetic shift was observed for resonances of PPA within the copper concen- tration used The paramagnetic contribution of the line width, Avp, is I (4I I I I I 1 140 I35 130 125 120 6 Fig.3 Aromatic region of 13C NMR spectra of PPA Cu PPA molar ratio (a) 0, (b) 17 x 10 3, (c) 3 4 x 10 3, (d) 5 1 x 10 3, (e) 8 4 x 10 3, (f) 16 x lo-' Resonances in (a) are labelled with the PPA carbon number as shown in Fig 1 728 J Mater Chem, 1996,6(5), 727-731 A-8 ,7I I I I68 1676 Fig.4 Carbonyl region of I3C NMR spectra of PPA us. Cu: PPA molar ratio: (a) 0, (b) 1.7 x (c) 3.4 x Resonances in (a) are labelled with the PPA carbon number as shown in Fig. 1. I I I I TPC Fig. 5 Temperature dependence of Av, for C4 (0)and C6 (0)reson-ances of PPA. Cu: PPA molar ratio, 1.6 x lop2. defined as shown in eqn. ( 1):24 AvP = AVobs -AVO (1) where Avobsand Avo are the observed linewidth in the presence and absence of copper ion, respectively. Fig. 5 shows the temperature dependence of Avp for the C4 and C6 resonances. Both Avp values decrease with increasing temperature, showing the fast chemical exchange between free and bound Cu2+ to PPA on the NMR timescale in the temperature range investi- gated.This observation implies that copper ion does not remain bound to a particular carboxylate group, but migrates rapidly between different carboxylate groups. The simultaneous imidization of PPA was not detected in the spectrum during the experiment at 80 "C. In the case of no paramagnetic shift, Avp is expressed as follows [eqn. (2)]:24 nAvp = fq/( GM + zM) (2) where f is the Cu : ligand molar ratio, q the number of ligands attached to the copper ion, GMthe spin-spin relaxation time of the nucleus of the molecule bound to copper ion, and T~ the mean lifetime of a nucleus in the bound state. As the present system is undergoing rapid exchange, GM>> z~. Eqn. (2) is thus simplified to: GM= fq/nAvp >> TM (3) We can estimate the upper limit value of T~ from eqn.(3) iff and q are known. Although no data is available for the binding constant of Cu2+ ion to PPA, all Cu2+ ions are assumed to bind to PPA. zM is roughly evaluated to be << 1 ms for f = 1.6 x in solution at 23 "C,assuming q = 1-4. It is known that the scalar interaction between a paramag- netic centre and a remote nucleus is the major contributor to GM,rather than the dipole-dipole interaction, in Cu2+ ion Paramagnetic broadening of the signals of the C4 and C6 carbons, as shown in Fig. 3, indicates that the unpaired electron of Cu2+ is transmitted to both carbons through the chemical bonds. Curing effect on PPA Fig.6 shows the effect of curing on PPA in the absence of copper.As resonances of imidized PPA, namely N-phenyl- phthalimide (PPI), are not detected in the spectrum shown in Fig. 6(a), PPA is judged not to be imidized at 50°C. With increasing curing temperature, new resonances of PPI as well as resonances of PPA are observed simultaneously in Fig. 6(b) and only the resonances of PPI appear in Fig. 6(c). The chemical shifts of PPI are summarized in Table 1. These results indicate that curing at 150°C induces PPA to imidize com- pletely to PPI. The spectrum in Fig. 6(b) is composed of resonances of both PPA and PPI. As expected, the C7 and C8 resonances of PPA and the C4 resonance of PPI are simultaneously observed in the carbonyl region. The degree of imidization of PPA can be determined from a comparison of the relative integrated inten- sities for the corresponding signals between PPI and PPA. The amount of PPI is estimated to be approximately 17% at a curing temperature of 100 "C.Fig. 7 shows the 13C NMR spectra of PPA cured in the presence of copper. Note that the signal-to-noise ratio of the spectrum of PPA cured at 50°C is quite poor, as shown in Fig. 7(a). Very broad and sharp signals are observed simul- taneously. The latter can be assigned to signals of PPI, from comparison with chemical shifts and the spectrum shown in Fig. 6(c). The former can be assigned to the CIO, C'l and C12 resonances of the phenyl group of PPA in the same way, 34 I 6 12 lo II 91 5 2 (a 1 A (I I I I I I 140 I35 130 6 12s 120 Fig.6 13C NMR spectra of PPA in the absence of copper at different curing temperatures: (a) 50°C, (b) 100°C, (c) 150°C. The numbered resonances in (a) and (c) correspond to the PPA and PPI carbon numbers shown shown in Fig. 1. J. Mater. Chem., 1996, 6(5), 727-731 729 I 1;D 135 1io 1Is 1io 6 Fig. 7 13C NMR spectra of PPA in the presence of copper at different cunng temperatures (a) 50 "C, (b) 100 "C, (c) 150 "C Arrows see text indicated by the arrows in Fig 7(a) and (b) However, no other signals belonging to PPA can be observed PPA cured at 50 "C contains copper at a Cu PPA molar ratio of 0 14 There is a large excess of paramagnetic Cu2+ ions in this system, though we could not determine quantitatively the amount of paramag- netic species present in the sample Therefore, all 13C NMR signals of PPA are broadened out and there is no selectivity of broadening of the signals for PPA This bulk paramagnetic effect is observed in the slightly broadened solvent signal as well 23 The spectrum of PPA cured at 100"C is similar to that at 50°C The appearance of PPI in Fig 7(a) implies that the presence of copper may stimulate the imidization of PPA to PPI, as resonances of PPI cannot be observed for PPA cured in the absence of copper at 50 "C It is difficult to estimate the amount of PPI from the spectrum, because signal intensities of PPA cannot be obtained owing to marked line broadening The relative amount of PPI in the sample cured at 100"C is clearly more than that at 50 "C, by comparison of the signal intensities of PPI and PPA The spectrum shown in Fig 7(c) is character- istic of PPI and shows the absence of broadened signals, in spite of the presence of paramagnetic copper ions These results imply that paramagnetic copper ions do not interact with PPI All chemical shifts of PPI shown in Fig 7 are different by 0 28 to 0 02 ppm from the equivalent signals in the absence of Cu2+ ions Since PPI does not interact with Cu2+ ions, this is probably due to the bulk paramagnetic effect of the excess of copper ions in the system23 Comparison of the interactions of PPA-Cu and polyamic acid-Cu The 13C NMR and EPR spectroscopic results presented in this work are compared with the polyamic acid-Cu interaction during imidization of polyamic acid In the PPA-Cu sys-tem, metallic copper or copper oxide is dissolved into the PPA-NMP solution, producing paramagnetic Cu2+ ions Simultaneously, Cu2+ ions form paramagnetic complexes with the carboxylate groups of PPA and exchange between the carboxylate groups at a rate >lo3s-' at room temperature in solution When the PPA-copper complex in solution is heated to 150"C, PPA is imidized to PPI, accompanied by the dis- 730 J Muter Chem, 1996, 6(5),727-731 sociation of the copper ions As the copper ions are reported to interact weakly with the solvent NMP,12 the resultant copper ions are dispersed into the solvent and washed out by water during the purification of samples Therefore, the copper content of PPI is very low, as described previously A similar interaction scheme can be applied to the case of polyimide formation from polyamic acid solution on copper, since it is widely believed that a sequence of reactions occurs when the polyamic acid precursor is coated and cured on a copper surface l425 Copper is dissolved in the polyamic acid-NMP solution7 25 and forms the paramagnetic Cu2+ ion complex with the carboxylate group of the polyamic acid This is confirmed by various surface spectros~opies~ lo I4 as well as our preliminary 13C NMR and EPR measurements26 Since the fast exchange behaviour of resonances is observed in the 13C NMR spectra of the polyamic acid-Cu2+ ion complex,26 the Cu2+ ions can migrate into the polyamic acid from the interface through chemical exchange between carboxylate groups It is furthermore demonstrated that the solvent pro- vides the mobility for the copper ions25 As this process accelerates with increasing temperature, the copper ions can easily percolate deeply into the polyamic acid solution from the interface Therefore, cuprous oxide is found in the polyimide far from the interface l2 When the solution is heated, polyamic acid is dehydrated to become polyimide, eliminating copper ions With simultaneous evaporation of the solvent NMP and water, the eliminated copper ions become cuprous oxide in the polyimide in the presence of oxygen7 Although the precise mechanism of this process is not yet clear, the degradation of the polyimide was found to be reduced significantly by the complete or almost complete exclusion of oxygen14 It was confirmed that cuprous oxide rather than metallic copper has a catalytic effect on polyimide degradation 7-14 Conclusion We have examined the interaction of PPA and PPI with copper by NMR and EPR spectroscopies Paramagnetic Cu2+ ion is produced and exchanges rapidly between the carboxylate groups of PPA in solution As these processes have also been observed in the polyamic acid-copper system by our prelimi- nary I3C NMR measurements, Cu2+ ion could easily percolate into the polyamic acid solution from the interface, and after curing copper particles or Cu20 would be found in the polyimide far from the interface PPA is imidized to PPI with curing at 150 "C This reaction is simultaneously accompanied by the dissociation of Cu2+ ion A paramagnetic effect is not directly exerted on PPI The authors are grateful to S Kawahara for his assistance in EPR measurements References 1 Polyimides Synthesis, Characterization and Applications, ed K L Mittal, Plenum Press, New York, 1984, vol 1and 2 2 L B Rothman, J Electrochem SOC,1980,127,2216 3 N J Chou, D W Dong, J Kim and A C Liu, J Electrochem SOC,1984,131,2335 4 N J ChouandC H Tang,J Vac Sci Technol, A, 1984,2,751 5 J W Bartha, F L P 0 Hahn and P S Ho, J Vac Sci Technol, A, 1985,3,1390 6 P S Ho, P 0 Hahn, J W Bartha, G W Rubloff, F K Legoues and B D Silverman, J Van Sci Technol, A, 1985,3,739 7 Y-H Kim, J Kim, G F Walker, C Feger and S P Kowalczyk, J Adhes Sci Technol, 1988,2,95 8 H G Linde, J Appl Polym Sci ,1990,40,2049 9 M C Burrell, P J Codella, J A Fontana and J J Chera, J Vac Sci Technol, A, 1989,7,1778 10 K Kelley, Y Ishino and H Ishida, Thin Solid Films,1987,154,271 11 Y-H.Kim, G. 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