Nickel electrodeposition is a very common technique used to produce metallic thick layers applied mainly as corrosion protective layers or for decorative purpose [1]. With the MEMS booming, Ni electrodeposition attracted researchers’ attention since it allows to create thick patterns when it is combined with lithography (micromolding process). Moreover, nickel mechanical properties make this material ideal for inertial MEMS. However, it presents ferromagnetic properties which are not compatible with Magnetic Resonance Imaging (MRI), thus preventing its use in implantable medical devices.
By doping nickel with at least 10 wt % phosphorous, the material becomes amorphous [2] and loses its magnetic properties whereas maintaining pretty good mechanical properties for MEMS application. We present here a study of micromolded Ni-P deposits from a sulfate bath including characterizations of its mechanical and magnetic properties. Our aim is to use this material to fabricate a tri-dimensional electrostatic energy harvesting MEMS designed to power a leadless pacemaker (Fig.1) [3]. The MEMS fabrication method is based on layer-by-layer electrodeposition of structural (Ni-P alloy) and sacrificial metals (copper).
Electrodeposits of Ni and Ni-P patterns of different dimensions were micromolded on a Ti/Cu seed layer sputtered on SiO2/Si or glass substrates. Ni and Ni-P deposits were obtained by direct current electrodeposition technique, using a sulfate bath solution consisting of nickel sulfate (197g/l), nickel chloride (5g/l), boric acid (25g/l) and phosphorous acid (0 – 20g/l). A nickel plate was used as soluble anode. Ni-P alloys were elaborated in different conditions by varying the concentration of phosphorous acid between 5g/L and 20g/L or the current density (10 – 100mA/cm²).
The deposits thickness was measured by optical profilometry in order to deduce the deposition rate. The deposits were observed by Scanning Electron Microscope (SEM) and their composition were analyzed by Energy-Dispersive Spectroscopy (EDS).
As seen for other types of sulfate bath, it was possible to dope nickel up to 20% wt P. For small phosphorous acid bath concentrations, the current density had a significant effect on the P content. (Fig.2). It was also observed that adding phosphorous acid reduced the deposition rate.
Young's Modulus and Hardness of nickel films were measured conventionally by micro-indentation. Young's Modulus was also evaluated by vibrometry, by measuring the resonant frequency of cantilever beams and microbridges [4]. Young’s modulus was found to be between 130 and 160GPa, which correspond to usual values for electroplated nickel according to literature. We are currently performing the same tests for Ni-P deposits.
Magnetic properties of Ni-P alloys were measured with an Alternating Gradient Force Magnetometer (AGFM) for Ni and Ni-P. For a 20 wt% P Ni-P alloy, the saturation magnetization is 1.5 mT (0.1 emu/g) corresponding to 400 times less the value of the nickel one (0.6T; 54 emu/g). This result is very promising for MRI compatibility of nickel-based MEMS.
[1] Modern Electroplating, M. Schlesinger and M. Paunovic, Wiley (2010) 736 pages, ISBN: 978-0-470-16778-6
[2] A. M. Pillai, A. Rajendra, A. K. Sharma, Electrodeposited nickel–phosphorous (Ni–P) alloy coating: an in-depth study of its preparation, properties, and structural transitions; J Coat. Technol. Res. 9(6) 785-707 (2012)
[3] S. Risquez, M. Woytasik, J. Wei, F. Parrain E. Lefeuvre, Design of a 3D Multilayer Out-of-plane Overlap Electrostatic Energy Harvesting MEMS for medical implant applications; Symposium on Design, Test, Integration & Packaging of MEMS/MOEMS (DTIP’2015), 27-30 avril 2015, Montpellier (France), pp 5-9
[4] T. Fritz, M. Griepentrog, W. Mokwa, U. Schnakenberg, Determination of Young's modulus of electroplated nickel, Electrochimica Acta 48(20–22) 3029-3035 (2003)