Copper plating on and electrical investigation of a low-permittivity cycloolefin-copolymer

Polymer Testing 22 (2003) 657–661 www.elsevier.com/locate/polytest

Material Performance

Copper plating on and electrical investigation of a lowpermittivity cycloolefin-copolymer Andrea Edit Pap, Krisztia´n Korda´s ∗, Heli Jantunen, Esa Haapaniemi, Seppo Leppa¨vuori Microelectronics Laboratory and EMPART Research Group of Infotech Oulu, University of Oulu, P.O. Box 4500, FIN-90570 Oulu, Finland Received 3 October 2002; accepted 4 December 2002

Abstract Adhesive ( ⬎ 5 MPa) and conductive (ρ~2 µ⍀ cm) copper films with a thickness of ~200 nm were deposited by chemical means on a recently developed low-k cycloolefin-copolymer (COC). The steps of the metallization procedure (chemical etching, metal seeding and copper plating) were optimized. Raman spectroscopy, SEM and AFM were used in the investigation of surface structures formed in the course of processing. Since this polymer is a good candidate for high-frequency microelectronics applications, electrical measurements up to 3 GHz were performed as well.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Low-k polymers; Metal deposition; Electroless plating; Copper; Palladium

1. Introduction In recent micro- and opto-electronics applications, fast electronic signal transmission is indispensable. Since the dimensions of micromodules are continuously shrinking, the density of wiring is increasing, causing undesired side effects such as inductive cross-talk as well as parasitic capacitance between adjacent lines. The former effect results in unreliable data transport, while the latter causes RC-delay of signals. The aforementioned problems can be decreased with proper design (e.g., shortdistance interconnections, straight wiring, etc.), and with the application of advanced materials. Manufacturers have already been urged to replace Al lines with Cu, reaching the theoretical limits in improvement of conductivity. A further RC-delay reduction can be achieved by utilizing low permittivity interlayer dielectrics. Thus, since the beginning of the 1990s, major development of

low-k materials has begun, resulting in the birth of a number of novel polymers (polyimides, polyquinolines, polyarylene ethers, cycloolefin-copolymers, polytetrafluoroethylenes, etc.) with low relative permittivity values (from 3.5 to 1.9) [1–3]. The recently developed cycloolefin-copolymers (COCs) own most of the physical (low permittivity, low water absorption, good process ability, low density, etc.), and chemical features (resistant to the most commonly used chemicals) needed to be an excellent candidate for interlayer dielectrics in microelectronics packaging technology [4]. Thus, the aim of this work is to investigate COC in terms of its metallization and electrical characterization.

2. Experimental 2.1. Electroless plating on COC

∗ Corresponding author. Tel.: +358-8-5532728; fax: +3588-5532723. E-mail address: [email protected] (K. Korda´s).

After degreasing the original polymer sheets (COC, Topas 8007 with a thickness of 140 µm, by Ticona GmbH, Germany), the samples were subjected to differ-

0142-9418/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9418(02)00172-1

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ent solutions, such as (i) 15 M HNO3, (ii) 7 M NH4OH, (iii) 2 M NaOH and (iv) 0.3 M K2Cr2O7 in 17 M H2SO4, in order to get their surfaces etched. Seeding with palladium was executed by submerging the etched polymers into a solution of 0.005 M Pd(CH3COO)2 0.2 M NH4OH and 1.2 M HCOH (60 min @ 60 °C). Thin copper films were deposited by autocatalytic electroless plating on the pre-treated (degreased, chemically etched and Pdseeded) COC surfaces using a plating bath of 0.014 M CuSO4, 0.08 M KNaC4H4O6 (Rochelle-salt), 0.1 M NaOH and 0.25 M HCOH (5 min @ 20 °C). After each step of metallization, the samples were flushed in distilled water and dried with pressurized N2. 2.2. Surface characterization and electrical measurements AFM (Nanoscope II) and SEM (Jeol JSM-6400F) were employed in the investigation of surface morphology. Chemical analyses of the samples were carried out by Raman spectroscopy (Bruker FRA 106). Adhesion between Cu and COC was measured by 90° pull-tests using a Sebastian five unit. For dielectric measurements, capacitor structures were made by plating on both sides of the polymer sheets. Relative permittivity of COC was determined up to 1 MHz by using a precision RCL meter (HP 4284A equipped with a 16047A test fixture) and up to 3 GHz by a network analyser (HP 8719C).

3. Results and discussion 3.1. Improvement of adhesion Besides the electrical performance of a realized circuit board, adhesion between the metallization and substrate is also a key issue. In general, adhesion depends on the polarity and roughness of the surface to be metallized. Therefore, especially for most polymers, which are known to be apolar and smooth materials, it is essential to change these surface properties to achieve better adhesion. Wettability and adhesion can be improved by applying either chemical or physical methods. In the case of chemical treatment, the surfaces to be metallized are exposed to various acids, alkalis or oxidizing chemicals in order to get their surfaces etched [5]. Physical processing includes roughening, using either fine sandpaper or plasma etching [6]. Lasers and excimer lamps can be utilized as well to induce changes in surface polarity and/or roughness [7,8]. In this work, chemical methods were investigated. Cleaned and dried COC samples were immersed for 5 min in various solutions (listed in section 2.1.) at 20 and 60 °C, and the change in surface wettability was monitored. The samples were resistant to nitric acid, sodium

hydroxide and ammonium hydroxide solutions, as well. Treatment with chromosulphuric acid caused a significant decrease in the contact angle (of a water droplet) from 75 down to 43° (Fig. 1). Investigations using AFM revealed that the treatment resulted in degradation (erosion) of the polymer surface. The originally smooth surface turned into a rough texture. The height (or depth) and lateral dimensions of surface features increased from ~5 – ~20 nm and from ~50 – 200 nm, respectively (Fig. 2). Chemical analyses of the polymer surfaces caused by chromosulphuric acid were studied by Raman spectroscopy (Fig. 3). The appearance of characteristic alcohol (CH3(CH2)11–17OH) peaks in the Raman-spectrum at 900–1500 cm⫺1 indicates oxidation of the surface. The new C=O, C=C and C=C=O peaks at 1650–2450 cm⫺1 indicate ring openings and/or chain scissions followed by strong oxidation. The increased peak intensities at 2800– 3000 cm⫺1 and 200–550 cm⫺1 are due to the formation of new olefin compounds on the surface of COC. With the appearance of polar functional groups, such as hydroxyl (O–H) and carbonyl (C=O), the acid-treated surface turns into hydrophilic resulting in the increase of wettability. 3.2. Seeding and electroless plating Surface activation (catalyst deposition or seeding) is needed prior to electroless plating on polymers. For this, thin films or seed layers of metals such as Ag, Cu or Pd are applied most frequently. These metals can be deposited using a number of methods, such as chemical reduction of metal ions/complexes from solutions [5,9,10], by pyrolytic and/or photolytic decomposition of metal-organic films [11], by laser-assisted methods [12– 14], by sputtering, evaporization, etc. In this work, ammonia-complexed palladium ions were chemically reduced with formaldehyde using the solution specified in section 2.1. Without heating, depending on the ambient temperature, 10–14 h are needed for the reaction to be completed. In order to accelerate the process, the liquid (together with the

Fig. 1. Water droplets on COC and their contact angles before (upper picture) and after (lower picture) treatment with chromosulphuric acid (0.3 M K2Cr2O7 in 17 M H2SO4) for 5 min at 20 °C.

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Fig. 2. Texture of COC surface (a) before and (b) after etching with chromosulphuric acid (0.3 M K2Cr2O7 in 17 M H2SO4) for 5 min at 20 °C.

Fig. 3.

Raman spectra of COC before and after treatment in chromosulphuric acid.

etched polymer sheets) was heated to 60 °C. In 10–30 min, the color of the light yellow solution darkened, and in another ~30 min it turned into a black colloidal sol of palladium nanoparticles with a size of 40–200 nm. Finally, with the aging of the sol, the metal particles attached together and sedimentation started (Fig. 4). Simultaneously, palladium seeds and agglomerates with diameters of 50–500 nm formed on the polymers as well. In the case of etched surfaces, the grown metal seeds were small and well-dispersed, while on nontreated polymers, fewer growth centers but larger grains and also agglomerates of Pd seeds were observed (Fig. 5).

Both types of surfaces were suitable for fast (~20 nm/min) chemical plating of copper with a maximal thickness of ~200 nm. Attempts to deposit thicker metal layers have failed, due to delamination and peeling of the films after 10 min of plating. Electrical resistivity of plated Cu was close to that of bulk (~2 µ⍀ cm). The difference between the original and acid-treated surfaces was striking when adhesion of copper to them was measured with a peeling test (90°). Copper films deposited on non-treated (only Pd-seeded) COC surfaces had rather poor adhesion ( ⬍ 0.5 MPa) compared to the ones first etched with chromosulphuric acid ( ⬎ 5 MPa). Differences in both surface roughness and distribution

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Fig. 6. Schematic drawing of copper plating on palladiumseeded polymer surfaces. When catalyst seeds are closer to each other (e.g., on the etched polymer), copper film grows close to the seeded surface, thus attractive forces between polymer and metal are higher.

Fig. 4. Agglomerated palladium grains at the end of sol aging (120 min at 60 °C).

of catalyst palladium seeds explain this enormous difference in adhesion of chemically plated copper: 앫 Roughening increases the real surface area, thus increasing the energy needed to separate the metal film from the surface. A rough surface also enables even mechanical attachment of metal films to the grooves and bumps formed in the etching process. 앫 The fine dispersion of Pd seeds on chromosulphuric acid-treated COC enhances homogeneous deposition of copper. Indeed, growth of Cu happens physically close to the surface, thus providing higher attractive forces between the polymer and plated copper as illustrated in Fig. 6.

COC plated with 200 nm copper on both sides). Within this frequency range, no significant variation of ⑀(f) was found (⑀ = 2.2 ± 0.1). At frequencies above 1 MHz, c(f) and ⑀(f) were calculated from the resonant frequencies (fr) of LC circuits (fr = (2p)⫺1(L(f)c(f))⫺0.5), measured with a network analyzer. Before measurements, the inductivity dependence of coils, L(f), was determined on a broad frequency interval by measuring the resonant frequencies of serial coils and known reference capacitors, cref(f). Although the accuracy of such a measurement is poor due to cumulative errors ( ± 40%), it enables a rough estimation of c(f) and calculation of ⑀(f). The graph in Fig. 7 shows that ⑀(f) slightly increases at higher frequencies, but its value remains below 5. The geometrical dimensions of the LC-components and the dominance of parasitic capacitances at frequencies above 3 GHz prevented the reasonable application of this setup based on serial coils and capacitors.

3.3. Measurements of dielectric permittivity Depending on the frequency interval, two kinds of measurement setups were used. From 50 Hz up to 1 MHz, according to the equation c(f) = ⑀(f)⑀0A d⫺1, dielectric permittivity ⑀(f) was determined from direct capacity c(f) measurements of capacitor structures (d = 0.14 mm, A = 10 × 10 mm2,

Fig. 5.

4. Summary Chemical metallization of and dielectric permittivity measurements on a low-k cycloolefin-copolymer, COC, were performed in this study. Conductive (with resistivity of 2 µ⍀ cm) copper thin films (200 nm) were

SEM images of (a) original and (b) etched polymer films after 60 min of Pd seeding at 60 °C.

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(contract number 47793) and the Graduate School of Infotech Oulu are acknowledged. Krisztia´ n Korda´ s would like to thank for the grants given by the PohjoisPohjanmaan Foundation, by the Finnish Cultural Foundation and by the Instrumentariumin Tiedesa¨ a¨ tio¨ . References

Fig. 7. Linear plot of relative permittivity as a function of frequency. Measurement data were obtained from resonant frequency measurements of LC-circuits constructed from copper plated COC samples and known inductivity coils.

grown on Pd seeded surfaces. The effect of surface roughness on palladium seeding and on adhesion of plated Cu films to COC was investigated as well. The surface roughness of the samples was modified (enlarged) using chemical treatment in chromosulphuric acid. It was found that on treated surfaces, the distribution of grown Pd grains was more homogeneous than on the non-treated reference samples, resulting in significant improvement in the adherence of copper films (from ~0.5–5 MPa) plated on chemically in a consecutive step. The frequency dependence (between 50 Hz and 3 GHz) of the polymer’s relative dielectric permittivity, ⑀(f), was measured using capacitor structures made by plating copper on both sides of COC samples. At lower frequencies (up to 1 MHz), ⑀(f) was constant (⑀ = 2.2 ± 0.1). Between 1 MHz and 3 GHz, ⑀(f) increased with frequency (⌬⑀ / ⌬f~0.74 / 1 GHz). Acknowledgements The authors are grateful to Esa Haapaniemi (Microelectronics Laboratory, University of Oulu, Finland) and Professor Ja´ nos Mizsei (Department of Electron Devices, Budapest University of Technology and Economics, Hungary) for the valuable discussions. The financial support of both the Academy of Finland

[1] M. Morgen, E.T. Ryan, J.-H. Zhao, C. Hu, T. Cho, S. Ho, Low dielctric constant materials for ULSI interconnects, Annu. Rev. Mater. Sci. 30 (2000) 645. [2] Y. Morand, Copper metallization for advanced IC: requirements and technological solutions, Microelectron Engng. 50 (2000) 391. [3] G. Maier, Low dielectric constant polymers for microelectronics, Prog. Polym. Sci. 26 (2001) 3. [4] Topas COC datasheet by Ticona GmbH, Germany. 2000. [5] L. Na´ nai, K. Korda´ s, S. Leppa¨ vuori, Chemistry of materials metallization, in: T.F. George, X. Sun, G.P. Zhang (Eds.), Modern Topics in Chemical Physics, Research Signpost, Trivandrum, India, 2002. [6] H. Esrom, R. Seebo¨ ck, M. Charbonnier, M. Romand, Surface activation of polyimide with dielectric barrier discharge for electroless metal deposition, Surf. Coat Technol. 125 (2000) 19. [7] S. Petit, P. Laurens, J. Amouroux, F. Arefi-Khonsari, Excimer laser treatment of PET before plazma metallization, Appl. Surf. Sci. 168 (2000) 300. [8] D.A. Wesner, H. Horn, R. Weichenhain, W. Pfleging, E.W. Kreutz, Pretreatment by laser radiation and its effect on adhesion in the metallization of poly(butylene terephthalate), J. Adhesion Sci. Technol. 11 (1997) 1229. [9] R. Furness, The practice of Plating on Plastics, Draper, Teddington, UK, 1968. [10] F.A. Lowenheim, Modern Electroplating, Wiley, New York, 1974. [11] Z.S. Geretovszky, I.W. Boyd, Kinetic study of 222 nm excimer lamp induced decomposition of palladium-acetate films, Appl. Surf. Sci. 138-139 (1999) 401. [12] A.G. Schrott, B. Braren, E.J.M. O’Sullivan, R.F. Saraf, B. Bailey, J. Roldan, Laser-assisted seeding for electroless plating on polyimide surfaces, J. Electrochem. Soc. 142 (1995) 944. [13] K. Korda´ s, J. Be´ ke´ si, K. Bali, R. Vajtai, L. Na´ nai, T.F. George, S. Leppa¨ vuori, UV laser-induced liquid-phase palladium seeding on polymers, J. Mater. Res. 14 (1999) 3690. [14] K. Korda´ s, J. Be´ ke´ si, R. Vajtai, L. Na´ nai, S. Leppa¨ vuori, A. Uusima¨ ki, K. Bali, T.F. George, G. Galba´ cs, F. Igna´ cz, P. Moilanen, Laser-assisted metal deposition from liquidphase precursors on polymers, Appl. Surf. Sci. 172 (2001) 178.