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Microstructured metallization of insulating polymers
Electrochimica Acta 48 (2003) 3021 /3027 www.elsevier.com/locate/electacta
Microstructured metallization of insulating polymers T.T. Mai a,*, J.W. Schultze a, G. Staikov b a
Institut fu ¨ r Physikalische Chemie und Elektrochemie, Heinrich-Heine Universita ¨ t Du ¨ sseldorf, D-40225 Du ¨ sseldorf, Germany b Institut fu ¨ r Schichten und Grenzla ¨ chen, Forschungzentrum Ju ¨ lich, D-52425 Ju ¨ lich, Germany Received 20 December 2002; received in revised form 11 April 2003; accepted 14 April 2003
Abstract Direct Ni electrodeposition on insulating polymers by the so-called PLATO technique is studied and the application of this technique for microstructured metallization is investigated. Propagation behavior, surface morphology, conductivity and thickness of a deposited metal layer are characterized using microscopy, AFM, four-point conductivity and XPS sputter measurements. Two layers are formed during metal deposition: primary layer and secondary layer. Both layers propagate with constant rates during the first 60 s and the propagation rates are influenced by the metallization potential. The primary layer has hemispherical morphology, low conductivity and an uneven thickness of about 25 /100 nm. The secondary layer has the repetition morphology of the primary 8 10 sec and higher roughness (/Rprim a / /40 nm, Ra / /150 nm), higher conductivity (ssec/sprim /10 /10 ) and a thickness of 100 /200 nm. The high lateral propagation rate of the metal strip during metal deposition offers possibilities for metallization of insulating microstructures. Routines for microstructured metallization using PLATO technique are proposed and examples for the applications are demonstrated. # 2003 Elsevier Ltd. All rights reserved. Keywords: Metal deposition; Insulating polymers; Cobalt sulphide; Microstructuring; PLATO technique
1. Introduction Microstructured metallization of insulating substrates by localization of electrical contact is an interesting prospective in microsystem technology [1]. Until now, the procedures used for metallization of insulating substrates in microsystems must follow several steps: pre-deposition of the metal by physical vapor deposition (PVD) or electroless metal deposition to form a conducting layer and subsequent electrodeposition. The microstructuring is achieved by mask and lithography [2,3]. Recently, new methods using a local electrical contact for direct electrodeposition of metals onto modified insulating substrates (direct galvanic metallization) have been proposed [4 /10]. The application of the local contact offers possibilities for localization of the
* Corresponding author. Tel.: /49-211-8113697; fax: /49-2118112803. E-mail address: [email protected] (T.T. Mai). 0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00369-4
metal deposition in microsystems. The key step in the direct metallization methods consists of the activation of the surfaces by a non-metallic substance which is a metal sulphide in the so-called PLATO technique [8 /10]. An interesting feature of the activated surface is that the metal sulphide is not an intact, continuous film covering completely the polymer surface, and the degree of activation is crucially influenced by pre-conditioning and by the nature of the polymer [11]. Therefore, the beginning stages of metallization do not follow the wellstudied mechanisms of metal deposition on metal substrates. This leads to characteristic properties of the deposited metal layer and to specific electrochemical behaviors of the metal deposition processes in macroand microscales. In this paper, we show studies of Ni layers deposited on acrylonitrile /butadiene /styrene (ABS) polymers using cobalt sulphide activation. The electrochemical behavior of the deposition process is examined. The application of PLATO technique for microstructured metallization is also reported.
T.T. Mai et al. / Electrochimica Acta 48 (2003) 3021 /3027
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2. Experimental The deposition was performed on ABS substrates supplied by Good Fellows. The process of direct metallization consists of three main steps: etching, activation and metal deposition. The activation is based on the so-called adsorption concept and includes two steps: adsorption of cobalt(II) and sulphidication. The conditions of the process steps are given in Table 1. After each step, the samples were thoroughly cleaned in Millipore† water. The lateral propagation of the metal layer was studied using optical microscope (PM-6; Karl Suss, USA). The topography and roughness of the deposited Ni surfaces were measured by atomic force microscopy (AFM) using commercial equipment (PicoSPM, Molecular Imaging, and Nanoscope E Controller; Digital Instruments, USA). The conductivities were measured using a four-point method with the amperometer having a current limitation of 1012 A. The surface composition and the nature of the chemical states were investigated by X-ray photoelectron spectroscopy (XPS) using a modified ESCALAB-V (VG-Instruments, UK) with polychromatic AlKa radiation, CAE mode, a pass energy of 50 eV for survey scans and 20 eV for specific element scans. The base pressure of 10 10 mbar with polychromatic Al Ka radiation (1486.6 eV), CAE mode, a pass energy of 50 eV for survey scans and 20 eV for specific element scans. Ion etching for sputter profile measurement was carried out by an AG-S2 ion source with an Ar pressure of 1.5 /10 5 mbar and an acceleration potential of 4 kV. The sputter rate was 1.6 nm/min, based on Ta2O5 standard. A computer program for peak deconvolution (Unifit V.2.4; University of Leipzig, Germany) was used to analyze quantitatively XPS peaks. The interface of the deposited Ni layer and the polymer substrate was characterized by a cross section prepared by chemical etching (acetic acid / perchloric acid). Electrochemical metal deposition was performed in a conventional three-electrode electrochemical cell with a Hg/Hg2SO4/1 M Na2SO4 reference electrode and a Niplate counterelectrode. All electrode potentials are referred to the potential of the standard hydrogen electrode (SHE). A Pt tip was used to contact the
activated polymer surface and to induce electrodeposition. The contact area of the Pt tip was shielded by an appropriate insulation with Apiezon† wax so that only the contact area was exposed. The masks for microstructuring were laser-structured polyimide (PI), supplied by the company Bartels GmbH (Dortmund, Germany).
3. Results and discussion 3.1. Characterization of electrodeposited metal layers The electrodeposition of Ni on the activated surface takes place by fast lateral propagation of the overlayer. As shown in the micrograph (Fig. 1), two layers are generally formed during the propagation: a primary and a secondary layer. Fig. 2 shows the changes of the distances to the Pt tip from the edge of the primary and of the secondary layers during propagation at E //0.6 V. Results show that both distances increase linearly with time after the first 60 s, indicating that the propagations of the primary and the secondary layers take place with constant rates V ?x and V ƒx ; respectively. It should be noted that after extended deposition (t /180 s), propagation rates of the primary and the secondary layers are no longer constant and decrease with time. This can be explained by the increasing potential drop under these conditions. Fig. 3 shows the influence of the
Fig. 1. Micrographs of the primary and the secondary layers during metal deposition (E //0.6 V, t /5 s).
Table 1 Conditions for process steps of the technique Process
Solution
Temperature (8C)
Etching Activation Adsorption Sulphidication Electrodeposition
CrO3/H2SO4
65
PLATOTM Aktivierung (CoSO4 ×/7H2O, NH3, oxidizer) PLATOTM Vernetzung (Na2S) 150 g/l NiSO4 ×/6H2O, 30 g/l NiCl2 ×/6H2O, 30 g/l H3BO3
Room temperature Room temperature 45
pH
Time (min)
/
5
10 / 4
3 1 /
T.T. Mai et al. / Electrochimica Acta 48 (2003) 3021 /3027
Fig. 2. (a) Schematic description of the Ni layer propagations. (b) The change of the distance from Pt tip to the end of the primary and the secondary layers during metal deposition (E //0.6 V, t /5 s).
Fig. 3. Influence of the metallization potential on the propagation rates of the primary layer (/V ?x ); the secondary layer (/Vƒx ) and the growth of metal in the z -direction (Vz ).
potential E on V x? and V ƒx ; and the growth of the Ni layer in the z -direction, Vz , which is calculated from the current transients and the corresponding current effi-
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ciency [11,12]. Results show that at E //0.8 V, the rate V x? is larger than V ƒx and both layers can be observed in the micrograph (Fig. 1), whereas V x? is equal to V ƒx at E B//0.8 V, leading to the complete covering of the secondary layer on the primary sublayer during propagation and only the secondary layer is observed in microscope. It should be noted that the lateral propagation rates, V x? and V ƒx ; are higher than Vz by a factor of about 104, slightly increasing with decreasing potential. This offers possibilities for metallization of microstructures. Fig. 4 shows typical AFM topographies of the activated area, the primary layer and secondary layer obtained after 60 s deposition at E //0.6 V. Cobalt sulphide clusters with average roughness, Rprim ; of 22 nm a can be observed on the activated surface. The primary Ni layer shows hemispheres with a height of 10/50 nm and a diameter of 0.5 /2 mm. The secondary Ni layer owns a repetition topography of the primary one with a sec higher roughness (/Rprim a / /40 nm, Ra / /150 nm), but lower density of hemispheres. This repetition behavior is expected, since the deposition of the secondary Ni layer follows the mechanism of electrodeposition on a metal substrate. In the absence of additives, the morphological repetition of substrates for electrodeposited metal layers is well-known [13]. The conductivity measurements carried out on surfaces of the three zones are shown in Fig. 5. The conductivity of the secondary Ni layers is clearly higher than that of the primary (ksec/kprim /108/ 1010). This result indicates that the secondary Ni layer is remarkably thicker than the primary one. The conductivity of the activated zone cannot be measured due to the fact that cobalt sulphide is in the form of discrete clusters and not as a continuous layer on the surface [11]. XPS sputter profiles of the primary and secondary Ni layers are shown in Fig. 6. It should be noted that during sputtering, the Ni layer and the polymer surface are etched with different rates (average sputter rates of the Ni layer nNi /1.6 nm/min and of the polymer nNi /6 nm/min). On the unsputtered surface of the primary layer (Fig. 6a), the concentrations of Co and S are zero, indicating that the primary Ni layer is a continuous layer and the activated surface underneath this layer is not exposed. The first Co and S signals are recorded after 15 min of sputtering, corresponding to the removal of the thinnest part with thickness of about 25 nm. It is interesting to note that during the first 15/60 min sputtering, all Ni, Co and S signals appear with concentrations decreasing gradually with time. This result indicates that the thickness of the primary layer is not evenly distributed, e.g. thinner parts were etched away faster. After sputtering for 60 min, Ni is completely removed, corresponding to a thickness of about 100 nm. The corresponding thickness of the primary layer is thus in the range of about 25 /100 nm (Fig. 6a).
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Fig. 4. AFM images of the activated surface, and the primary and the secondary Ni layers. The Ni layer is obtained after deposition with E //0.6 V and t /60 s.
Fig. 5. Conductivities of the activated surface, and the primary and the secondary Ni layers. The Ni layer is obtained after deposition with E //0.6 V and t/60 s.
Further sputtering yields the decreasing of Co and S concentrations until they vanish after 180 min. Fig. 6b shows the sputter profile of the secondary layer. The same sputter behavior of the primary layer is also observed for the secondary layer. However, the time for an appearance of the first Co signal is much higher (after 60 min sputter), corresponding to the higher thickness range of 100/200 nm (Fig. 6b). It is suggested that the primary and secondary Ni layers are formed following different mechanisms. On the primary layer, the appearance of hemispheres indicates that the Ni is deposited preferentially at certain positions. Recent studies show that the reduction of cobalt sulphide at potentials E B//0.4 V catalyzes the deposition reaction of Ni [11]. The formation of the
primary layer is thus attributed to the Ni deposition reaction on cobalt sulphide clusters. The different sizes of the sulphide clusters lead to the uneven distribution of the primary thickness, e.g. the Ni layers are thicker on larger cobalt sulphide clusters. On the other hand, the secondary layer is formed on the primary one by the mechanism of metal deposition on a metal substrate. Since the propagation of the secondary layer occurs very fast at E 5//0.6 V, a thick deposited Ni layer has an even thickness distribution (Fig. 7a), which is required for metallization of insulating microstructures. At more positive potential of E //0.6 V and low degree of activation (coverage of cobalt sulphide, u B/40%), the thickness decreases with distance from contact (Fig. 7b). Therefore, for microstructured metallization, the deposition potential and activation should be optimized in order to obtain metal microstructures with even thickness. 3.2. Microstructured metallization In principle, the microstructured metallization applying PLATO technique can be carried out using poststructuring concept. A typical example for this routine is the damascene process (IBM) [14], in which the direct metallization method is considered as a substitutional method for electroless deposition and PVD. In this study, we propose two new approaches for microstructured metallization considering the specific properties of the direct metallization using metal sulphide (Fig. 8): a) homogeneous activation and selective metallization using a mask and b) microstructured activation (pre-structuring) and subsequent electrodeposition.
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Fig. 7. (a) Even thickness distribution of an electrodeposited Ni layer obtained at E //0.6 V, t /120 min. (b) Thickness profile obtained on surface low coverage of cobalt sulphide (u/40%) at E//0.5 V and t /120 min. Fig. 6. XPS sputter profile of (a) the primary layer and (b) the secondary layer. The Ni layer is obtained after deposition with E // 0.6 V and t /60 s.
Fig. 9 demonstrates the example of direct microstructured metallization using mask (routine (a) in Fig. 8).
The mask, which has the shape of a Christmas tree, was fixed on the activated ABS surface (coverage of cobalt sulphide, u/84%). The metallization was carried out using a local contact at the top of the tree. This
Fig. 8. Routines for microstructured metallization using PLATO technique: (a) selective activation and subsequent metallization and (b) homogeneous activation and selective metallization using masks.
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through-hole metallization was carried out successfully by placing a contact tip on the surface of the sample. The uneven distribution of the thickness (Fig. 10) due to the diffusion of the electrolyte in the hole can be further optimized by applying suitable technological parameters (additive, convection, etc.).
4. Conclusions
Fig. 9. Metallized microstructure of a homogeneously activated ABS substrate (coverage of cobalt sulphide, u /84%). For metallization, an excimer laser-structured PI mask was applied (E//0.8 V, t /3 min).
experiment shows that the propagation during metal deposition can also take place in small and complex geometries with the width of 40 mm and the length of several millimeters. Bended shapes with angles B/308 are well reproduced. However, similar to electrodeposition through masks on metal substrates, concentration and current distributions should be taken into account [15,16]. Due to the fast propagation rate and the flexibility of the activation process, the direct metallization is a good method for metallization of insulating microstructures with complex geometries. Fig. 10 demonstrates a through-hole metallization of an insulating structure with aspect ratio A /5. The activation was applied on the surface of the sample and on the hole walls. The
The electrodeposition of Ni on insulating ABS polymers activated by cobalt sulphide takes place by the propagation of primary and secondary layers. The propagation rates of both layers are constant during the first 60 s and are dependent on the metallization potential. The primary layer has a topography characterized by hemispheres with different sizes (10 /50 nm height and 0.5 /1 mm diameter), which correspond to the cobalt sulphide clusters underneath. The relatively low conductivity of the primary layer could be related to the insufficient electrical contact between Ni hemispheres. The morphology of the secondary layer repeats the primary layer, however, with a higher roughness (/Rsec a / / 150 nm compared with Rprim / /40 nm) and lower density a of hemispheres. The conductivity of the secondary layer is much higher than that of primary layer (ksec/kprim / 108/1010). This result is explained by the higher thickness of the secondary layer. The high lateral propagation rate of the overlayer offers possibilities for metallization of insulating microstructures. This behavior can be successfully applied for metallization of insulating polymers with complex microstructures. Different concepts for
Fig. 10. Cross section of the through-hole-metallized structure on ABS substrate applying PLATO technique (E//0.8 V, t /5 min).
T.T. Mai et al. / Electrochimica Acta 48 (2003) 3021 /3027
microstructured metallization using PLATO technique are proposed and applications of those concepts are demonstrated.
Acknowledgements We thank the Ministerium fu¨r Schule, Weiterbildung, Wissenschaft und Forschung des Landes NordrheinWestfalen, for the financial support of this work. The technical supports of the companies ENTHONE GmbH (Langenfeld, Germany) and Bartels GmbH (Dortmund, Germany) are gratefully acknowledged.
References [1] J.W. Schultze, A. Bressel, Electrochim. Acta 47 (2001) 3. [2] E. Sacher, J.J. Piraeux, S.P. Kovalczyck, Metallization of Polymers, American Chemical Society, Washington, DC, 1990.
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[3] K.K.H. Wong, S. Kaja, P.W. De Haven, IBM J. Res. Dev. 42 (1998) 587. [4] J. Hupe, W. Kronenberg, Patent Application by Blasberg Oberfla¨chentechnik, PCT No. PCT/EP89/00204, 1989. [5] D. Schattka, S. Winkels, J.W. Schultze, Metalloberfla¨che 5 (1997) 1823. [6] S. Pilyte, G. Valiuliene, A. Zieliene, J. Vinkevicius, J. Electroanal. Chem. 436 (1997) 127. [7] J. Vinkevicius, I. Mozginskie, V. Jasulaitiene, J. Electroanal. Chem. 442 (1998) 73. [8] A. Mo¨bius, P. Pies, A. Ko¨nighofen, Metalloberfla¨che 54 (2000) 3. [9] A. Mo¨bius, J. Oberfla¨chetechnik 1 (2000) 14. [10] A. Mo¨bius, et al., Patent Application by Enthone OMI, PCT No. PCT/US99/26066, 2000. [11] T.T. Mai, G. Staikov, J.W. Schultze, Sol. Stat. Electrochem., submitted for publication. [12] A. Brenner, Electrodeposition of Alloys: Principle and Practice, Academic Press, New York, 1963. [13] G. Staikov, E. Budevski, W.J. Lorenz, Electrochemical Phase Formation and Growth, VCH, Weinheim, 1996. [14] C. Andricacos, C. Uzoh, J.O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Dev. 42 (1998) 567. [15] M. Ku¨pper, J.W. Schultze, Electrochim. Acta 42 (1997) 3033. [16] L.T. Romankiw, Electrochim. Acta 42 (1997) 2985.
Microstructured metallization of insulating polymers T.T. Mai a,*, J.W. Schultze a, G. Staikov b a
Institut fu ¨ r Physikalische Chemie und Elektrochemie, Heinrich-Heine Universita ¨ t Du ¨ sseldorf, D-40225 Du ¨ sseldorf, Germany b Institut fu ¨ r Schichten und Grenzla ¨ chen, Forschungzentrum Ju ¨ lich, D-52425 Ju ¨ lich, Germany Received 20 December 2002; received in revised form 11 April 2003; accepted 14 April 2003
Abstract Direct Ni electrodeposition on insulating polymers by the so-called PLATO technique is studied and the application of this technique for microstructured metallization is investigated. Propagation behavior, surface morphology, conductivity and thickness of a deposited metal layer are characterized using microscopy, AFM, four-point conductivity and XPS sputter measurements. Two layers are formed during metal deposition: primary layer and secondary layer. Both layers propagate with constant rates during the first 60 s and the propagation rates are influenced by the metallization potential. The primary layer has hemispherical morphology, low conductivity and an uneven thickness of about 25 /100 nm. The secondary layer has the repetition morphology of the primary 8 10 sec and higher roughness (/Rprim a / /40 nm, Ra / /150 nm), higher conductivity (ssec/sprim /10 /10 ) and a thickness of 100 /200 nm. The high lateral propagation rate of the metal strip during metal deposition offers possibilities for metallization of insulating microstructures. Routines for microstructured metallization using PLATO technique are proposed and examples for the applications are demonstrated. # 2003 Elsevier Ltd. All rights reserved. Keywords: Metal deposition; Insulating polymers; Cobalt sulphide; Microstructuring; PLATO technique
1. Introduction Microstructured metallization of insulating substrates by localization of electrical contact is an interesting prospective in microsystem technology [1]. Until now, the procedures used for metallization of insulating substrates in microsystems must follow several steps: pre-deposition of the metal by physical vapor deposition (PVD) or electroless metal deposition to form a conducting layer and subsequent electrodeposition. The microstructuring is achieved by mask and lithography [2,3]. Recently, new methods using a local electrical contact for direct electrodeposition of metals onto modified insulating substrates (direct galvanic metallization) have been proposed [4 /10]. The application of the local contact offers possibilities for localization of the
* Corresponding author. Tel.: /49-211-8113697; fax: /49-2118112803. E-mail address: [email protected] (T.T. Mai). 0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00369-4
metal deposition in microsystems. The key step in the direct metallization methods consists of the activation of the surfaces by a non-metallic substance which is a metal sulphide in the so-called PLATO technique [8 /10]. An interesting feature of the activated surface is that the metal sulphide is not an intact, continuous film covering completely the polymer surface, and the degree of activation is crucially influenced by pre-conditioning and by the nature of the polymer [11]. Therefore, the beginning stages of metallization do not follow the wellstudied mechanisms of metal deposition on metal substrates. This leads to characteristic properties of the deposited metal layer and to specific electrochemical behaviors of the metal deposition processes in macroand microscales. In this paper, we show studies of Ni layers deposited on acrylonitrile /butadiene /styrene (ABS) polymers using cobalt sulphide activation. The electrochemical behavior of the deposition process is examined. The application of PLATO technique for microstructured metallization is also reported.
T.T. Mai et al. / Electrochimica Acta 48 (2003) 3021 /3027
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2. Experimental The deposition was performed on ABS substrates supplied by Good Fellows. The process of direct metallization consists of three main steps: etching, activation and metal deposition. The activation is based on the so-called adsorption concept and includes two steps: adsorption of cobalt(II) and sulphidication. The conditions of the process steps are given in Table 1. After each step, the samples were thoroughly cleaned in Millipore† water. The lateral propagation of the metal layer was studied using optical microscope (PM-6; Karl Suss, USA). The topography and roughness of the deposited Ni surfaces were measured by atomic force microscopy (AFM) using commercial equipment (PicoSPM, Molecular Imaging, and Nanoscope E Controller; Digital Instruments, USA). The conductivities were measured using a four-point method with the amperometer having a current limitation of 1012 A. The surface composition and the nature of the chemical states were investigated by X-ray photoelectron spectroscopy (XPS) using a modified ESCALAB-V (VG-Instruments, UK) with polychromatic AlKa radiation, CAE mode, a pass energy of 50 eV for survey scans and 20 eV for specific element scans. The base pressure of 10 10 mbar with polychromatic Al Ka radiation (1486.6 eV), CAE mode, a pass energy of 50 eV for survey scans and 20 eV for specific element scans. Ion etching for sputter profile measurement was carried out by an AG-S2 ion source with an Ar pressure of 1.5 /10 5 mbar and an acceleration potential of 4 kV. The sputter rate was 1.6 nm/min, based on Ta2O5 standard. A computer program for peak deconvolution (Unifit V.2.4; University of Leipzig, Germany) was used to analyze quantitatively XPS peaks. The interface of the deposited Ni layer and the polymer substrate was characterized by a cross section prepared by chemical etching (acetic acid / perchloric acid). Electrochemical metal deposition was performed in a conventional three-electrode electrochemical cell with a Hg/Hg2SO4/1 M Na2SO4 reference electrode and a Niplate counterelectrode. All electrode potentials are referred to the potential of the standard hydrogen electrode (SHE). A Pt tip was used to contact the
activated polymer surface and to induce electrodeposition. The contact area of the Pt tip was shielded by an appropriate insulation with Apiezon† wax so that only the contact area was exposed. The masks for microstructuring were laser-structured polyimide (PI), supplied by the company Bartels GmbH (Dortmund, Germany).
3. Results and discussion 3.1. Characterization of electrodeposited metal layers The electrodeposition of Ni on the activated surface takes place by fast lateral propagation of the overlayer. As shown in the micrograph (Fig. 1), two layers are generally formed during the propagation: a primary and a secondary layer. Fig. 2 shows the changes of the distances to the Pt tip from the edge of the primary and of the secondary layers during propagation at E //0.6 V. Results show that both distances increase linearly with time after the first 60 s, indicating that the propagations of the primary and the secondary layers take place with constant rates V ?x and V ƒx ; respectively. It should be noted that after extended deposition (t /180 s), propagation rates of the primary and the secondary layers are no longer constant and decrease with time. This can be explained by the increasing potential drop under these conditions. Fig. 3 shows the influence of the
Fig. 1. Micrographs of the primary and the secondary layers during metal deposition (E //0.6 V, t /5 s).
Table 1 Conditions for process steps of the technique Process
Solution
Temperature (8C)
Etching Activation Adsorption Sulphidication Electrodeposition
CrO3/H2SO4
65
PLATOTM Aktivierung (CoSO4 ×/7H2O, NH3, oxidizer) PLATOTM Vernetzung (Na2S) 150 g/l NiSO4 ×/6H2O, 30 g/l NiCl2 ×/6H2O, 30 g/l H3BO3
Room temperature Room temperature 45
pH
Time (min)
/
5
10 / 4
3 1 /
T.T. Mai et al. / Electrochimica Acta 48 (2003) 3021 /3027
Fig. 2. (a) Schematic description of the Ni layer propagations. (b) The change of the distance from Pt tip to the end of the primary and the secondary layers during metal deposition (E //0.6 V, t /5 s).
Fig. 3. Influence of the metallization potential on the propagation rates of the primary layer (/V ?x ); the secondary layer (/Vƒx ) and the growth of metal in the z -direction (Vz ).
potential E on V x? and V ƒx ; and the growth of the Ni layer in the z -direction, Vz , which is calculated from the current transients and the corresponding current effi-
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ciency [11,12]. Results show that at E //0.8 V, the rate V x? is larger than V ƒx and both layers can be observed in the micrograph (Fig. 1), whereas V x? is equal to V ƒx at E B//0.8 V, leading to the complete covering of the secondary layer on the primary sublayer during propagation and only the secondary layer is observed in microscope. It should be noted that the lateral propagation rates, V x? and V ƒx ; are higher than Vz by a factor of about 104, slightly increasing with decreasing potential. This offers possibilities for metallization of microstructures. Fig. 4 shows typical AFM topographies of the activated area, the primary layer and secondary layer obtained after 60 s deposition at E //0.6 V. Cobalt sulphide clusters with average roughness, Rprim ; of 22 nm a can be observed on the activated surface. The primary Ni layer shows hemispheres with a height of 10/50 nm and a diameter of 0.5 /2 mm. The secondary Ni layer owns a repetition topography of the primary one with a sec higher roughness (/Rprim a / /40 nm, Ra / /150 nm), but lower density of hemispheres. This repetition behavior is expected, since the deposition of the secondary Ni layer follows the mechanism of electrodeposition on a metal substrate. In the absence of additives, the morphological repetition of substrates for electrodeposited metal layers is well-known [13]. The conductivity measurements carried out on surfaces of the three zones are shown in Fig. 5. The conductivity of the secondary Ni layers is clearly higher than that of the primary (ksec/kprim /108/ 1010). This result indicates that the secondary Ni layer is remarkably thicker than the primary one. The conductivity of the activated zone cannot be measured due to the fact that cobalt sulphide is in the form of discrete clusters and not as a continuous layer on the surface [11]. XPS sputter profiles of the primary and secondary Ni layers are shown in Fig. 6. It should be noted that during sputtering, the Ni layer and the polymer surface are etched with different rates (average sputter rates of the Ni layer nNi /1.6 nm/min and of the polymer nNi /6 nm/min). On the unsputtered surface of the primary layer (Fig. 6a), the concentrations of Co and S are zero, indicating that the primary Ni layer is a continuous layer and the activated surface underneath this layer is not exposed. The first Co and S signals are recorded after 15 min of sputtering, corresponding to the removal of the thinnest part with thickness of about 25 nm. It is interesting to note that during the first 15/60 min sputtering, all Ni, Co and S signals appear with concentrations decreasing gradually with time. This result indicates that the thickness of the primary layer is not evenly distributed, e.g. thinner parts were etched away faster. After sputtering for 60 min, Ni is completely removed, corresponding to a thickness of about 100 nm. The corresponding thickness of the primary layer is thus in the range of about 25 /100 nm (Fig. 6a).
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Fig. 4. AFM images of the activated surface, and the primary and the secondary Ni layers. The Ni layer is obtained after deposition with E //0.6 V and t /60 s.
Fig. 5. Conductivities of the activated surface, and the primary and the secondary Ni layers. The Ni layer is obtained after deposition with E //0.6 V and t/60 s.
Further sputtering yields the decreasing of Co and S concentrations until they vanish after 180 min. Fig. 6b shows the sputter profile of the secondary layer. The same sputter behavior of the primary layer is also observed for the secondary layer. However, the time for an appearance of the first Co signal is much higher (after 60 min sputter), corresponding to the higher thickness range of 100/200 nm (Fig. 6b). It is suggested that the primary and secondary Ni layers are formed following different mechanisms. On the primary layer, the appearance of hemispheres indicates that the Ni is deposited preferentially at certain positions. Recent studies show that the reduction of cobalt sulphide at potentials E B//0.4 V catalyzes the deposition reaction of Ni [11]. The formation of the
primary layer is thus attributed to the Ni deposition reaction on cobalt sulphide clusters. The different sizes of the sulphide clusters lead to the uneven distribution of the primary thickness, e.g. the Ni layers are thicker on larger cobalt sulphide clusters. On the other hand, the secondary layer is formed on the primary one by the mechanism of metal deposition on a metal substrate. Since the propagation of the secondary layer occurs very fast at E 5//0.6 V, a thick deposited Ni layer has an even thickness distribution (Fig. 7a), which is required for metallization of insulating microstructures. At more positive potential of E //0.6 V and low degree of activation (coverage of cobalt sulphide, u B/40%), the thickness decreases with distance from contact (Fig. 7b). Therefore, for microstructured metallization, the deposition potential and activation should be optimized in order to obtain metal microstructures with even thickness. 3.2. Microstructured metallization In principle, the microstructured metallization applying PLATO technique can be carried out using poststructuring concept. A typical example for this routine is the damascene process (IBM) [14], in which the direct metallization method is considered as a substitutional method for electroless deposition and PVD. In this study, we propose two new approaches for microstructured metallization considering the specific properties of the direct metallization using metal sulphide (Fig. 8): a) homogeneous activation and selective metallization using a mask and b) microstructured activation (pre-structuring) and subsequent electrodeposition.
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Fig. 7. (a) Even thickness distribution of an electrodeposited Ni layer obtained at E //0.6 V, t /120 min. (b) Thickness profile obtained on surface low coverage of cobalt sulphide (u/40%) at E//0.5 V and t /120 min. Fig. 6. XPS sputter profile of (a) the primary layer and (b) the secondary layer. The Ni layer is obtained after deposition with E // 0.6 V and t /60 s.
Fig. 9 demonstrates the example of direct microstructured metallization using mask (routine (a) in Fig. 8).
The mask, which has the shape of a Christmas tree, was fixed on the activated ABS surface (coverage of cobalt sulphide, u/84%). The metallization was carried out using a local contact at the top of the tree. This
Fig. 8. Routines for microstructured metallization using PLATO technique: (a) selective activation and subsequent metallization and (b) homogeneous activation and selective metallization using masks.
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through-hole metallization was carried out successfully by placing a contact tip on the surface of the sample. The uneven distribution of the thickness (Fig. 10) due to the diffusion of the electrolyte in the hole can be further optimized by applying suitable technological parameters (additive, convection, etc.).
4. Conclusions
Fig. 9. Metallized microstructure of a homogeneously activated ABS substrate (coverage of cobalt sulphide, u /84%). For metallization, an excimer laser-structured PI mask was applied (E//0.8 V, t /3 min).
experiment shows that the propagation during metal deposition can also take place in small and complex geometries with the width of 40 mm and the length of several millimeters. Bended shapes with angles B/308 are well reproduced. However, similar to electrodeposition through masks on metal substrates, concentration and current distributions should be taken into account [15,16]. Due to the fast propagation rate and the flexibility of the activation process, the direct metallization is a good method for metallization of insulating microstructures with complex geometries. Fig. 10 demonstrates a through-hole metallization of an insulating structure with aspect ratio A /5. The activation was applied on the surface of the sample and on the hole walls. The
The electrodeposition of Ni on insulating ABS polymers activated by cobalt sulphide takes place by the propagation of primary and secondary layers. The propagation rates of both layers are constant during the first 60 s and are dependent on the metallization potential. The primary layer has a topography characterized by hemispheres with different sizes (10 /50 nm height and 0.5 /1 mm diameter), which correspond to the cobalt sulphide clusters underneath. The relatively low conductivity of the primary layer could be related to the insufficient electrical contact between Ni hemispheres. The morphology of the secondary layer repeats the primary layer, however, with a higher roughness (/Rsec a / / 150 nm compared with Rprim / /40 nm) and lower density a of hemispheres. The conductivity of the secondary layer is much higher than that of primary layer (ksec/kprim / 108/1010). This result is explained by the higher thickness of the secondary layer. The high lateral propagation rate of the overlayer offers possibilities for metallization of insulating microstructures. This behavior can be successfully applied for metallization of insulating polymers with complex microstructures. Different concepts for
Fig. 10. Cross section of the through-hole-metallized structure on ABS substrate applying PLATO technique (E//0.8 V, t /5 min).
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microstructured metallization using PLATO technique are proposed and applications of those concepts are demonstrated.
Acknowledgements We thank the Ministerium fu¨r Schule, Weiterbildung, Wissenschaft und Forschung des Landes NordrheinWestfalen, for the financial support of this work. The technical supports of the companies ENTHONE GmbH (Langenfeld, Germany) and Bartels GmbH (Dortmund, Germany) are gratefully acknowledged.
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