Doping of Alumina Substrates for Laser Induced Selective Metallization

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ScienceDirect Procedia CIRP 68 (2018) 772 – 777

19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain

Doping of alumina substrates for laser induced selective metallization Philipp Ninz a*; Frank Kern a; Eugen Ermantraut b; Hagen Müller c; Wolfgang Eberhardt c; André Zimmermann b,c and Rainer Gadow a a

Institute for Manufacturing Technologies of Ceramic Components and Composites (IFKB), University of Stuttgart, Allmandring 7b, 70569 Stuttgart, Germany b Institute for Microintegration (IFM), University of Stuttgart, Allmandring 9b, 70569 Stuttgart, Germany c Hahn-Schickard-Institute for Microassembly Technology, Allmandring 9b, 70569 Stuttgart, Germany

* Corresponding author. Tel.: +49 711 685 68225; fax: +49 711 685 58225. E-mail address: [email protected]

Abstract Laser induced selective activation and metallization of ceramics is a novel additive metal plating process enabling the application fine metallic paths or metallic surfaces on complex three-dimensionally shaped ceramic substrates. Metal deposition by electroless plating occurs selectively where the substrate has been locally activated by a preceding selective laser activation. The main influences on the process are the ceramic substrate material, its microstructural and optical properties as well as the laser type, laser process parameters and metallization parameters. Recent positive results with activation by a green picosecond laser were obtained only with oxygen vacancy containing alumina substrates sintered in hydrogen atmosphere. In order to be able to apply inexpensive conventional state-of-the-art sintering in air it was tried to modify the composition by doping with different oxides. 2 mass-% of fine particles of neodymium-, manganese-, samarium-, chromium-, nickel- iron-, ceria- and antimony doped tinoxide were introduced into an alumina matrix by mixing and milling. Slip cast samples were sintered at 1500°C in air and machined. Laser activation was performed on polished sample surfaces using a Nd:YVO4 laser with a wavelength of 532 nm and a pulse length of 10 ps. Electroless plating was performed with commercially available copper electrolytes. Microstructural properties, laser-matter interaction and metallization effectivity were investigated. Doping of alumina substrates with antimony doped tin-oxide, chromium-, iron- and nickel-oxide results in very good metallization of laser activated areas. Dopants enable plating larger areas and writing fine circuit paths to integrate microelectronic devices. Other additives were ineffective. The activation mechanisms triggered by the dopants is not fully understood yet. Separation and identification of single mechanisms of the dopants is required. The influence of dopants and laser activation on the mechanical properties is to be studied. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2018 2018The The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining

Keywords: Ceramics; alumina; slip casting; electroless plating; laser activation; 3D interconnect device

1. Introduction Laser induced activation and subsequent autocatalytic metallization of ceramics (LIAM-C) is an additive method suitable for the application of electronic circuits on complex, three-dimensionally shaped ceramic substrates. The process offers various advantages over established methods for ceramic metallization such as direct bonded copper (DBC) and is already established for polymer substrates and the manufacturing of dree-dimensional molded interconnect devices (3D-MID) [1,2]. In comparison to polymers ceramic substrates offer specific benefits such as high thermal stability,

chemical resistance and adjusted thermal expansion coefficient. This technology can enlarge the application range of MIDs and offer new possibilities in the integration and functionalization of ceramic parts. Autocatalytic plating of catalytically active surfaces is one way to apply metallic circuits on ceramics. The currentless metallization process is based on an electrolyte bath containing metal ions as well as a reducing agent. The reduction of the metal salt and hence metal deposition is enabled by the contribution of electrons from the reducing agent and initiated by the existence of catalytically active surface sites. Such catalytically active sites can be generated by laser treatment of

2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.037

Philipp Ninz et al. / Procedia CIRP 68 (2018) 772 – 777

the substrate. This laser induced activation has become a standard technology for producing molded interconnected devices from special doped polymers [3]. Laser activation processes and mechanisms differ significantly depending on the substrate material. Activation mechanisms for ceramics are not comparable to those of polymers. Shafeev [4,5,6] investigated the activation mechanisms of various ceramics in the context of autocatalytic metal plating revealing different interacting mechanisms depending on the ceramic material. For alumina, he identified following, laser induced and probably interacting mechanisms: x Oxygen vacancies x Residual stresses The formation of metallic Al on treated surfaces was not confirmed [6]. Recent investigations on alumina show that the presence of oxygen vacancies (by sintering in H2) or Ȗ-alumina (produced by thermal spraying) are not a sufficient criteria to enable autocatalytic deposition [7]. This is shown in the image of a H2 sintered, laser activated (Nd:YVO4 picosecond laser, wavelength 532 nm) and subsequently electroless plated alumina sample in Fig.1. Metal is only deposited on the laser irridated areas although the whole surface exhibits oxygen vacacies due to H2 sintering.

earth oxides may form catalytic centers by laser treatment suitable to enable autocatalytic metallization. This approach is similar to the procedures used in polymers where e.g. antimony doped tin oxide or copper(I) oxide are used, here upon laser irradiation metallic copper or antimony particles are formed as active species [1]. For ceramics the dopants must however be able to survive the high firing temperatures. The choice of the dopants used in this study is based on thermodynamic considerations. At high temperature the higher oxidation states are less stable as the release of oxygen increases the entropy of the system. The validity of this principle is well known from petrology [8]. It can therefore be assumed that e.g. transition metal oxides are reduced by heating to high temperatures (by laser irradiation) and that either oxides with lower valence or even the metals are formed. Metals, especially if less noble than copper will trigger copper deposition. Rare earth compounds such as neodymia ceria and samaria were tried as they also have different oxidation states and may form hexaaluminates introducing residual stess into the alumina matrix and change the band structure of alumina. The quality of the activation and metallization process with regard to subsequent manufacturing processes for electronics as well as for the most probable applications (MIDs) can be defined by the following criteria: x Structuring accuracy: sharpness of edges, recast x Structuring depth x Deposition effectivity x Deposition efficiency x Deposition thickness x Deposition accuracy: metallization overlap, sharpness of metallic edges) x Roughness x Specific electrical resistivity x Adhesion x Reliability For electronic parts these criteria influence application range, system reliability, bonding to electronic parts etc. In this study a first dopant screening is carried out to identify suitable dopants enabling laser activation of alumina sintered in air for subsequent electroless plating. Nomenclature

Fig. 1 Metal paths applied by electroless plating on injection molded, H2 sintered alumina substrate (Nd:YVO4 picosecond laser, wavelength 532 nm,) [7]

On the other hand laser irradiation alone applied to surfaces of alumina sintered in air is also insufficient to enable deposition of connected metal layers. Thus it seems that in case of pure alumina the activation requires both oxygen vacancies and laser irradiation to trigger nucleation sites for the electroless deposition of copper. Hydrogen sintering furnaces due to their high investment and operation cost are not widespread in the ceramics industry beyond manufacturers of translucent alumina for medical or lighting applications. It is therefore highly desirable to develop a process based on alumina sintered in air which can be manufactured by any producer without additional investment. The idea behind this investigation is that dopants such as transition metal or rare

773

LIAM LMC A-Ni A-Cr A-Fe A-Sn A-Ce A-Mn A-Nd A-Sm EI KIC P

Laser Induced Activation and Metallization Laser-activatable and Metalizable Ceramic Alumina 2 wt% nickel oxide Alumina 2 wt% chromium oxide Alumina 2 wt% iron oxide Alumina 2 wt% tin oxide (5 wt% Sb in SnO) Alumina 2 wt% ceria Alumina 2 wt% manganese Alumina 2 wt% neodymium Alumina 2 wt% samarium Indentation modulus Indentation toughness Laser power

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2. Experimental Procedure 2.1. Manufacturing and Preparation of Sample Rectangular plate samples (approx. 25 x 35 mm sintered) were produced by slip casting. An Į-alumina powder with 300 nm grain size (Ceralox APA0.5) was doped with 2 m% of various metal oxide dopants. Neodymia and samaria (99.9 % purity, Chempur, Germany) as well as 5 wt.% antimony doped tin oxide were commercially available powders. The Fe2O3, Mn3O4, CeO2, NiO and Cr2O3 were synthesized from nitrate or acetate salts by combustion of these precursors in air at 600°C for 3 h. Alumina and dopants were dispersed in water using 1 wt. % of dispersant (Zschimmer and Schwarz, CE 64) and 0,5 wt.% of binder (Zschimmer and Schwarz, AC 95), the solid loading was set to 75 %. The dispersions were attrition milled with zirconia milling balls for 2h. Rectangular plates were cast on a pre-dried plaster plate, dried overnight and debindered/ presintered at 800°C/1h in air. Sintering for all samples was performed in air at heating rates of 5 K/min to 800 °C and 2 K/min to 1500 °C with 3 h dwell. Sample surfaces for laser activation, mechanical testing and microstructure characterization were lapped with 15 μm diamond suspension and polished to a 1 μm finish. (Struers, Germany). Bending bars of 4 mm width were cut from polished plates, the sides were lapped and the edges were beveled with 15 μm diamond suspension to remove machining defects. Vickers hardness HV10 (Bareiss, Germany) (6 indents) as well as microhardness and indentation modulus measurement according the universal hardness method (Fischerscope, Germany) (12 indents) were measured on polished samples. The indentation fracture toughness KIC was calculated from the indent size and wing crack length of three HV10 measurements and the indentation modulus according the formula Niihara (median crack geometry) [9]. Materials microstructure was investigated on polished, thermally etched (1350°C, 5min air) and 1 μm platinum coated surfaces as well as on cracked coated surfaces using SEM (Zeiss Gemini, Germany). Qualitative XRD phase analysis of all samples was performed with a D8 Discover DaVinci (Bruker AXS GmbH, Karlsruhe Germany) with theta/2theta horizontal and a KĮ1+2 copper source.

Fig. 2 Applied laser pattern

Influence of the laser parameters on the structure of the activated surfaces are not discussed in this paper. Autocatalytic plating with copper was carried out in a commercially available electrolyte bath. The quality of the metalization process was optically evaluated focussing on effeciency and accuracy of the metallization. It was checked if the activated areas were metallized homogeneously, if the edges were straight and if the single paths were metallized continuously – a criterion necessary to write thin conductor paths. It was also observed if metal was deposited on any nonactivated areas. 3. Results and discussion 3.1. Microstructure and phase analysis Fig. 3 and Fig. 4 show SEM images of polished and thermally etched surfaces featuring the change in microstructure depending on the dopant applied.

2.2. Laser Activation and Autocatalytic Metallization The laser activation was performed with a green Nd:YVO4 picosecond laser with an operational wavelength of 532 nm. The pulse length was 10 ps and the spot diameter was 10 μm. Patterns with a 1300 μm wide pad, four single paths, two 100 μm, 150 μm and 200 μm wide paths with a length of 3 mm, as displayed in Fig. 2 were applied on the pretreated ceramic surfaces. Laser parameters were varied with respect to laser power (2-8 W), frequency (200-1000 kHz), longitudinal pitch (3-7 μm), cross pitch (3-70 μm) and laser speed (6003000 mm/s).

Fig. 3 Microstructure on polished samples

A-Ce shows an inhomogeneous microstructure with a braod range of grain size up to 20 μm, intragranular porosity and cerium hexaaluminate platelets. A-Mn forms a spinel with a homogeneously stalky structure and intra- as well as intergranular porosity. The microstructure of A-Nd is also inhomogeneous. It shows large grains of

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10-20 μm with intragranular porosity, plate shape neodymium aluminate, small inclusions are probably unreacted neodymia. A-Sm shows a homogeneous coarse grained structure with intragranular porosity and samaria inclusions of 0,5 to 2 μm. Scattered inclusions of platelets (probably samarium mono- or hexaaluminate) can are found at grain boundaries as well as inside the grains.

magnifications). A-Ni shows mixed inter and transcrystalline fracture typical for fine grain alumina. In A-Cr there is a higher tendency to transcrystalline fracture. In A-Fe the broad distribution of grain sizes is visible. Fracture is intergranular for the fine grain matrix and transgranular for the large plate shape grains. In A-Sn the elongated grains form a very rough fracture surface with pores and microcracks. The change of microstructure by the dopant addition has a strong influence on the fracture behavior and mechanical properties as shown in the preceding paragraph.

Fig. 4 Microstructure on polished samples

A-Ni shows a homogenous, dense structure with isometric grains of a mean size of 2-3 μm similar to magnesia doped alumina of the same starting powder and sintering conditions (see Fig. 5).

Fig. 6 Microstructure of crack surface

3.2. Mechanical characterization Since metalization of the rare earth oxide and manganese oxide doped samples was not satisfactory (see 3.3), results of mechanical characterization are only shown for the rest of the samples (Table 1). Table 1: Mechanical properties

Fig. 5 Microstructure of magnesia doped (500ppm) alumina

XRD confirms the formation of nickel-spinel NiAl2O4. A-Cr microstructure is dense, grains are slightly elongated showing small, chipped edges at grain boundaries caused by sample preparation, due to the mutual solubility of alumina and chromia. A second phase is not detected. Grain size is about 1-2 μm. A-Fe forms large platelet structures with a length over 40-100 μm with fine grained, porous areas in between. XRD reveals the formation of FeO. A-Sn, in comparison to A-Mn, forms a finer platelet structure also showing hindered densification by a large amount of porosity in between the platelets. XRD analysis revealed traces of metallic Sn. SEM micrographs of fracture surfaces (same materials as shown in figure 4) are shown in figure 6 (different

Indentation modulus EI [GPa]

Sample

Vickers Hardness HV10

+/-

Microhardness HV0,1

A-Ni

1922

82

2114

334

4.2

A-Cr

1893

53

2091

328

4.3

A-Fe

1747

12

2081

323

-

A-Sn

1455

72

1566

274

4.6

Indentation Toughness KIC [MPa*m1/2]

Mechanical properties reflect the microstructural properties. Insufficient densification leads to pores residual stress of misoriented plate shape grains to weak grain boundaries. Consequently hardness and indentation modulus are low in cae of A-Fe and A-Sn. A-Sn with its very porous platelet structure shows lowest values of hardness and EI while toughness is improved by a self reinforcement by the platelets. HV10, HV0,1 and EI values for A-Ni, A-Cr and A-Fe are slightly below values for undoped alumina [10]. A-Fe showed extremely brittle behavior making crack length measurement impossible.

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3.3. Metallization The following images demonstrate the results of the first metallization experiments and allow for a first qualitative optical evaluation of the process efficiency. Fig. 7 to Fig. 10 show the laser structured surfaces after metallization. All rare earth oxide and MnO doped samples (Fig. 7, Fig. 8) show poor metallization. In case of MnO one laser parameter combination can be identified where a closed two-dimensional metallization was achieved (Fig. 7, A-Mn, C4).

Fig. 8 Metallized ceria doped sample

Fig. 7 Metallized rare earth doped samples (magnesia, samaria)

In all cases selective metallization happens at edges where molten material was deposited and high residual stress is likely as well as in areas structured with high laser power and low velocity (see A-Sm, A-Ce, A-Nd, B2). The same mechanisms are observed at the turning points of the laser at the upper and lower edges of the fields where the de- and accelerating laser has a prolonged impact time (see for example A-Sm, A-Ce, ANd, B4).

The other transition metal oxide and tin oxide doped samples shown in Fig. 9 and Fig. 10 show a much better metallization behavior. A-Ni showed the best results in terms of efficiency (Fig. 9). All activated areas as well as all single paths showed continuous metallisation. Even the parameter with the lowest laserpower, the highest frequency and speed showed metalization of single paths. Unfortunately metallisation also occurred on mechanically treated (scratched) areas of the surface starting from laser treated areas. Metalization of A-Cr was very efficinet showing only small, scattered, probably substrate induced undesired metallization (Fig. 10). Single paths were only metalized at specific parameters at have high laser power and lower speed and frequency.

Fig. 9 Metallized nickel oxide doped sample

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narrow conductor paths at low laser power. Undesired metallization in areas which were mechanically activated however suggest that these dopants in the chosen concentrations are actually a bit too active. As the final objective is to produce complex shape ceramic MID components from as fired injection molded ceramics this high activity may be tolerable. A-Fe and A-Cr are less active but show only selective metallization, these dopants require stronger laser activation. Rare earth oxides as well as manganese oxide doped samples show no or little improvement compared to plain alumina and can be ruled out. Nonetheless, those samples show that activation can occur at strongly molten areas at the edges of the patterns but the mechanism is not sufficient to produce fully metalized surfaces or .single paths. As the ceramic parts must provide a certain minimum level of strength and toughness in structural applications mechanical properties are equally important. Considering this second aspect only A-Ni and A-Cr seem suitable. All other dopants cause too irregular microstructure and have a negative impact on mechanical properties. For these two promising candidates a more detailed study on dopant concentration has to follow to provide ceramic substrates with both sufficient selectivity and activity. Acknowledgements Research funding by German Federal Ministry of Economics and Technology (BMWi) and the German Federation of Industrial Research Associations (AiF), grant number IGF 18967 N/2, is gratefully acknowledged. The authors would like to thank Mrs A. Gommeringer (IFKB) for the XRD analysis and Mrs F. Predel (MPI-Stuttgart-Germany) for beautiful SEM images. References [1]

Fig. 10 Metallized metal oxide doped samples

A-Sn showed very good metalization of single paths and larger areas. As A-Ni , A-Sn also showed undesired metallization of mechanically treated areas. Metalization effectivity for A-Fe is relatively low for the majority of the laser parameters, undesired metalization is not observed. 4. Summary and Conclusion The results gathered in this first screening study may be interpreted from two different points of view. As far as the metallization is concerned the study was successful as four of the eight dopants tried are suitable additives to trigger electroless plating subsequent to a laser treatment. A-Sn and ANi were identified as the most active dopants followed by ACr and A-Fe. The two most active dopants allow writing single

Franke, J.: Three-Dimensional Molded Interconnected Devices (3D-MID); Carl Hanser Verlag München 2014 [2] Hofmann, H. Spindler, J.: Verfahren der Oberflächentechnik; Carl Hanser Verlag München, 2014 [3] H. Wißbrock: Laser-Direkt-Strukturieren von Kunststoffen - ein neuartiges Verfahren im Spiegel eingeführter MID-Technologien. In: Kunststoffe. 11, (2002), Vol. 92, S. 101–105 [4] Shafeev, G.A.: Laser activation and metallisation of insulators. Quantum Electronics 1997; 27(12): 1104-1110 [5] Shafeev, G.A.: Laser activation and metallisation of oxide ceramics; Adv. Mater. Opt. Electron. 1993; 2: 183 [6] Shafeev, G.A.: Laser assisted activation of dielectrics for electroless metal plating; Appl. Phys. A 67,303-311, 1998 [7] Ermantraut E, Müller H, Sommer F, Landfried R, Kern F, Eberhardt W, Kück H, Gadow R. Finest Conductor Paths on Injection Moulded Ceramic Substrates. Proceedings of the Interregional Conference on Ceramics CIEC14, 2014 [8] Markl, G.: Minerale und Gesteine, Mineralogie – Petrologie – Geochemie, Springer, Berlin 2015. [9] Niihara K.. A fracture mechanics analysis of indentation-induced Palmqvist crack in ceramics. Mater. Sci. Lett., 2, pp.221 1983 [10] Sommer F, Kern F, Gadow,R. Injection Molding of AluminaChromia-Yttria Composites. J.Ceram.Sci.Tech. 2011;02: 211-216