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Electrochemically deposited nickel membranes; process–microstructure–property relationships
Surface and Coatings Technology 172 (2003) 79–89
Electrochemically deposited nickel membranes; process–microstructure– property relationships ˚ Perssonb, Karen Pantleona, Magnus Oden ´ c, Lars Hultmanb, Jens A.D. Jensena,b,*, Per O.A. Marcel A.J. Somersa a
The Technical University of Denmark, Institute of Manufacturing Engineering and Management, b. 204, DK-2800 Lyngby, Denmark b ¨ ¨ Thin Film Physics Division, IFM, Linkoping University, S-581 83 Linkoping, Sweden c ¨ ¨ Linkoping University, Department of Mechanical Engineering, Linkoping, Sweden Received 20 September 2002; accepted in revised form 22 January 2003
Abstract This paper reports on the manufacturing, surface morphology, internal structure and mechanical properties of Ni-foils used as membranes in reference-microphones. Two types of foils, referred to as S-type and 0-type foils, were electrochemically deposited from a Watts-type electrolyte, with (S-type) or without (0-type) the use of the sulfur-containing additive sodium saccharin. Both types of Ni-foils appeared perfectly smooth when investigated with scanning electron microscopy (SEM), while atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed differences in the surface morphologies and a smaller grain-size in the S-type foils. X-Ray diffraction showed a N311M texture component in both types of Ni-foils, most pronounced for 0-type foils. A minor N111M texture component observed in both foil types was strongest in the S-type foils. Mechanically 0type foils proved more ductile than S-type foils during thin film tensile testing, due to microstructural defects caused by sodium saccharin during deposition. Tensile strengths in the order of 700–1000 MPa were observed—highest for the more ductile 0-type foils. A hardness in the order of 6 GPa (590 HV) was found by nanoindentation. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nickel; Electrodeposition; Microstructure; Mechanical testing; Thin films
1. Introduction Electrochemical deposition has become a key technology in the development of metallic components for microsystems. The manufacturing of 3D micro-components and structures such as interconnects, CDyDVDstamping tools, cantilevers for scanning probe microscopy, membranes for microphones, microfluidic systems and optical waveguides has been made possible by the combination of Si-wafer processing and advanced electrochemical deposition techniques w1–7x. The ability to tailor and reproduce Ni-electrodeposits with specific microstructures and well-defined mechanical properties is essential in micro-electro-mechanical systems (MEMS) w2x. For this reason, fine-grained (preferably nano-crystal line), smooth, mechanically *Corresponding author. Tel.: q45-45-25-2212; fax: q45-45-936213. E-mail address: [email protected] (J.A.D. Jensen).
isotropic, reproducible and thermodynamically stable deposits are desired, which requires controlled, robust electrodeposition processes. Ni-membranes, used e.g. in microphones, manufactured by electrochemical deposition are a good example of an application, where an electrodeposited Ni-film has to meet very high demands. In the present study, the manufacturing of Ni microphone membranes, and the influence on microstructure and mechanical properties are investigated. Both microstructure and mechanical properties are related to the level and type of organic additives used in the electrochemical deposition process. Na-saccharin is used as an additive in Ni-electrolytes due to its very strong levelling abilities. During electrodeposition the saccharin decomposes, thereby incorporating an amount of S into the Ni-deposit w8–11x. Kendrick w11x showed how the incorporation of S affects the internal stress state in the nickel deposit: a low concentration (0.02–0.04%) of S in the deposit causes
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00253-6
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Table 1 Electrolyte and additive chemistry Basic Ni-electrolyte composition 46 gyl Nickel Chloride NiCl26H2O, technical grade 367.5 gyl Nickel Sulfate, NiSO4 5H2O, technical grade 44 gyl Boric Acid, H3BO3, technical grade 0.1 gy1 Sodium Lauryl Sulfate, NaC12H25SO4, analytical grade Additives Electrolyte no. 1, 0-type Ni-foils 3 gy1 Naphthalene-1,3,6-trisulfonic acid trisodium salt C10H5Na3O9S3 analytical grade 0.3 gyl 2-butyn-1,4-diol HOCH2C'CCH2OH analytical grade
Electrolyte no. 2, S-type Ni-foils 1 gy1 Naphthalene-1,3,6-trisulfonic acid trisodium salt C10H5Na3O9S3 analytical grade 0.2 gyl 2-butyn-1,4-diol HOCH2C'CCH2OH analytical grade 1.0 gy1 Saccharine Sodium Salt C7H4NO3SNa analytical grade
Operating conditions for Ni-deposition pH 4.0–4.2 Temp.s50 8C Is50 A is4.8 Aydm2 Continuous filtration (1 mm filter)
a compressive stress state, whereas a higher S-content (0.04–0.06, possibly much higher) leads to a tensile stress state in the deposit. ‘Alloying’ with S may cause embrittlement of the Ni-material, especially during prolonged heating of the coating, allowing S to migrate to grain-boundaries and forming brittle nickel-sulfides leading to grain boundary decohesion w12x. In spite of this drawback, Na-saccharin is used extensively in several applications of Ni electrodeposition, both because of the unrivalled levelling characteristics, and especially because of the possibility to obtain a compressive stress state, facilitating stress control in Ni-deposits. The present study is devoted to the effect of Nasaccharin on the microstructure and the mechanical properties of Ni deposited from a Watts electrolyte. To this end, foils deposited from a Watts-type electrolyte with and without Na-saccharin addition are compared. 2. Experimental 2.1. Electrochemical deposition Ni-foils were electrodeposited from a Watts-type electrolyte with and without the S-containing additive Nasaccharin (Table 1). The two types of foils are referred to as 0-type (zero-type, no S in electrolyte nor deposit) and S-type (S-containing electrolyte and electrodeposit), respectively. In order to obtain high quality Ni-foils with no defects, great care was taken to prepare the substrate surface onto which the foil was deposited. The substrate consisted of a brass-plate measuring 30=35 cm, coated with a 100–200 mm layer of S-type Ni (in order to get the highest possible levelling). The Ni-surface was
carefully polished with 6-, 3- and 1 mm diamond-slurry successively to a microscopically mirror-bright finish. Prior to electrochemical deposition of the Ni-foil, a thin oxide-layer was formed on the polished Ni-substrate by anodic passivation in an alkaline solution (Table 2). As a consequence of this processing step, a controlled, limited adhesion was obtained between substrate and Ni-foil deposit. This facilitated the separation of the foil from the passivated substrate after finishing the deposition. Polypropylene containers, 200 l in volume, with constant filtration (0.1 mm filter) were used as plating cells. A Ti-basket, approximately 30=50=8 cm, with S-alloyed Ni-rounds, dressed in woven polypropylene bags, served as anode. The substrate was cyclically moved parallel to its surface plane in the container at a linear velocity of approximately 10 cmys during electrodeposition to ensure sufficient exchange of electrolyte at the cathode surface. The procedure for the manufacturing of Ni-foils is given in Table 3.
Table 2 Electrolyte and operating conditions for cathodic rinseyanodic passivation 89 gy1 Sodium Hydroxide, NaOH, technical grade 3.5 gy1 Sodium Cyanide, NaCN, technical grade pH)12 Temp.sR.T. Is50 A is4.8 Aydm2 Continuous filtration (1 mm filter)
J.A.D. Jensen et al. / Surface and Coatings Technology 172 (2003) 79–89 Table 3 Manufacturing procedure for electrodeposited Ni-foils Process step
Process parameters
Cathodic rinse
Alkaline electrolyte (Table 1), is4.8 Aydm2, ts3 min, TsR.T. 8C, continuous filtration (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Dilute H2SO4 (5 1 H2SO4 to 90 l), ts10 s Counterflow tap-water, tank 2, ts20 s Alkaline electrolyte (Table 1), is4.8 Aydm2, ts3 min, TsR.T., continuous filtratrion (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Distilled H2O, ts20 s Watts electrolyte (Table 1), is4.8 Aydm2, ts10 min, continuous filtration (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Dilute H2SO4 (5 l H2SO4 to 90 1), ts3 s Counterflow tap-water, tank 2, ts20 s Distilled H2O, ts20 s Drying cupboard, Ts40–50 8C, ts2–5 min Opening with grinding-machine around edge of substrate, manual removal of Ni-foil.
Water rinse 1 Water rinse 2 Pickling Water rinse 2 Anodic passivation Water rinse 1 Water rinse 2 Water rinse 3 Ni electrodeposition Water rinse Water rinse Acid rinse Water rinse Water rinse Hot air dry Mechanical of Ni-foil
1 2 2 3 removal
2.2. Light optical microscopy The appearance of the foil and associated defects caused by impurities, either from the pretreatment or the electrodeposition process, were studied in planar view using a Zeiss stereo microscope and an Olympus BH2 upright microscope, using differential interference contrast imaging. 2.3. Scanning electron microscopy (SEM) SEM-micrographs were obtained from the Ni-foil surfaces in a LEO 1550 Gemini Field Emission Gun (FEG) SEM. The back-scattered electron signal was used for imaging with an electron acceleration voltage in the range 5–20 kV and a working distance of 4 mm. 2.4. Atomic force microscopy (AFM) AFM-analysis was conducted with a prototype DUALSCOPE娃 scanning probe microscope from Danish Micro Engineering (DME), combining an AFMmicroscope with a Zeiss optical microscope. The microscope has been constructed for maximum rigidity (solid steel construction) and is separated from the surroundings by a spring system to reduce noise. The AFM probe tip oscillates at its resonance frequency (e.g. 20 kHz), while a deflection sensor with a laserbeam focused on the back of the probe constantly measures the amplitude and frequency of the tip. Changes of amplitude are transformed into a 3-D picture by means of a specific imaging software. During the scanning process, the probe tip does not touch the sample,
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because the atomic forces reflect the probe tip before a collision occurs. 2.5. Transmission electron microscopy (TEM) Cross-sectional and plan-view samples for TEM were prepared by sandwiching a foil in between two dummy Si pieces, followed by mechanical thinning and polishing. Low-angle (48) ion milling in a BalTec RES 010 rapid ion etch operated at 8 kV, was used to make the samples electron transparent. A final polishing stage using low-energy ions at 2 kV was applied to remove the amorphous surface layer formed by the previous ion etch. The TEM investigations were carried out using a Philips CM 20 UT microscope, equipped with a LaB6 filament, operated at 200 kV. 2.6. X-Ray diffraction (XRD) X-Ray diffraction analysis was performed using a powder diffractometer D8 Discover from Bruker AXS, equipped with a 1y4-circle Eulerian cradle for texture analysis. Measurements were carried out using Cu-Ka radiation, giving an information depth of approximately 5 mm wfor the (200)-Bragg peak 63% of the diffracted intensity originates from the top 5 mm of the Nix. All measured intensities were background corrected, and correction for defocusing was performed using a textureless Ni-powder sample. Pole figures of (111), (200) and (220) were measured up to 758 tilt angles and orientation distribution functions (ODF) w13,14x were calculated. Complete pole figures of (111), (200) and (220), supplementary pole figures of (311) and (331), as well as inverse pole figures were derived from the ODF. 2.7. Thin film tensile testing The tensile testing apparatus used was developed and manufactured with the specific aim of testing thin metal foils according to ASTM standard E 345-87 w15x. The apparatus is sensitive to the specific sample geometry and pulling velocity and is calibrated according to ASTM standard E 74-91 w16x. Rectangular specimens measuring 12=140 mm in accordance with Danish standard DS 10 110 w17x were used in the present study. The tested length of each specimen was approximately 100 mm. The ASTM standard E 345-87 does not prescribe the pulling velocity. Considering the pulling distance of approximately 1 mm to fracture however, a velocity of 1 mm sy1 was used in this study. Young’s modulus and the stress-state in the foils were calculated, knowing the average thickness of each specimen as determined with a calibrated Fisherscope X-ray fluorescence thickness measurer.
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Fig. 1. Typical defects in Ni-foils manufactured by electrochemical deposition.
2.8. Nanoindentation Nano indentation was carried out using a NanoIndenter娃 II, according to the method described by Oliver et al. w18x. Two samples were studied: a 6 mm, 0-type Ni-foil, attached to its substrate and a polished S-type Ni-substrate. For each sample, a series of 20 indentations was made. A spacing of at least 16.5 mm existed between two successive measurement locations. Stiffnesses were calculated from unloading curves. A power-law was fitted over 90% of the unloading curve to find the best correlation of the slope. The hardness, defined as the mean pressure the material will support under load, was computed from: Hs
Pmax A
(1)
where A is the projected area of contact at peak load. 3. Results In order to obtain similar brightness of the Ni-deposits in the Na-saccharin-free electrolyte to that obtainable in
the traditional electrolyte, a series of additive-combinations were tested in a Hull-Cell w19,20x. The highest brightness in the Hull-Cell test was found for the additive combination given in Table 1 for electrolyte 1. 3.1. Light optical microscopy Inspection with light optical microscopy of the manufactured Ni-foils showed few defects, even at high magnification and with the use of differential interference contrast. Generally, defects could be attributed to insufficient pre-treatment cleanliness. The most frequently encountered defects and their proposed origin are shown in Fig. 1. 3.2. Scanning electron microscopy (SEM) Apart from areas where the foils suffered from obvious defects caused by impurities or faults during the electrodeposition, the overall morphology of both types of foils investigated was smooth and contourless. Even in the FEG-SEM used at over 100.000= magnification,
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Fig. 2. AFM-micrographs of 0-type (aqb) and S-type (cqd) Ni-foils.
the surfaces of the foils did not show any observable features.
growth-behaviour perpendicular to the film-growth direction (Fig. 2c,d).
3.3. Atomic force microscopy (AFM)
3.4. Transmission electron microscopy (TEM)
AFM did reveal differences in surface topography between the S-type and the 0-type Ni-foils. Typical values of maximum peak-to-bottom distances as measured with AFM over an area of 43 by 43 mm2 were 100–140 nanometers for both types of Ni-foils. In the electrolyte without Na-saccharin addition (Table 1, electrolyte no. 1), the individual grains on the surface of the Ni-foil appeared sharper and with a clear indication of crystal growth perpendicular to the substrate surface (Fig. 2a,b). In the electrolyte containing the Na-saccharin additive (Table 1, electrolyte no. 2) a smoother surface was obtained with more rounded grains and a more laminar
A TEM-micrograph of a plan-view section of a 0type Ni-foil is shown in Fig. 3a. The micrograph suggests that the film is composed of equiaxed grains, ranging in size from 20 to 40 nm. As follows from the accompanying electron diffraction pattern, the distribution of grain-orientation within the plane of the layer appears random. A cross-sectional view of the same sample (Fig. 3b) confirms equiaxed grains at a distance of approximately 250 nm from the substrate. In the vicinity of the substrate, relatively large grains are present, upon which smaller grains appear to have nucleated; possibly the first nuclei developed by epitaxial growth on the substrate crystals, in spite of the thin
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Fig. 3. TEM micrographs of 0-type Ni-foil (aqb) and S-type Si foils (cqd).
passive layer between substrate and deposit. The assumed epitaxial layer in Fig. 3b was observed over an extensive area of the investigated sample and is believed to be a general characteristic of the 0-type Nifoils. TEM-micrographs of the S-type Ni-foil (Fig. 3c,d) showed an average grain size of 10–20 nm, and no indications of epitaxial growth onto the substrate grains. Smoother and more continuous rings in the accompanying electron diffraction patterns as compared to those in Fig. 3a are consistent with a finer grain size in the S-type Ni-foil as compared to the 0-type. Comparing Fig. 3a and Fig. 3c, the grain boundaries in the S-type Ni-foil (Fig. 3c) are not only more abundant, but also seem more pronounced than in the 0-type Ni-foil (Fig. 3a). Although no inclusions or impurities are readily observable in the grain boundaries, this difference in grain coherence may explain the difference in mechanical properties of the two foil types (see later Section 3.6).
´ Moire-fringes are observable in both types of foils investigated in the TEM as a result of overlapping grains systematically scattering the incoming electron beam. 3.5. X-Ray diffraction Both types of Ni-foils investigated show ideal fibre textures, i.e. the fibre axis is parallel to the growth direction of the foil. The results of the texture analysis are represented in the calculated inverse pole figures given in Fig. 4. The major texture component in both 0-type and S-type deposits is N311M with a calculated maximum density of 2.75 for 0-type Ni and 2.20 for S-type Ni. A minor N111M texture component was observed in both the 0type and S-type foil investigated. Calculated densities for the N111M poles were 1.2 and 1.9 for the 0- and Stype Ni-foils, respectively.
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Fig. 4. Inverse pole figures for: (a) 0-type Ni (6 mm foil); and (b) S-type Ni (6 mm foil).
3.6. Tensile testing Tensile testing was carried out on 2 mm thick Ni-foils of both 0- and S-type (Fig. 5), and on a series of 0type Ni-foils with various thicknesses (Fig. 6). The two types of foils showed similar behaviour in the elastic region. In the plastic region, the 0-type Nifoils behaved in a much more ductile way than the two S-type foils tested (Fig. 5) as reflected by the higher
ultimate tensile strength and the larger plastic strain. Increased foil thickness resulted in an increase of the ‘apparent’ Young’s modulus and on extension of the plastic region (Fig. 6). This result is explained as follows. With increased thickness, the tensile test becomes less sensitive to handling and clamping of the foils. In addition, the effect of surface roughness on the calculated E modulus becomes less pronounced. Moreover, the role of the surface roughness in fracture
Fig. 5. Stress-strain diagram from tensile testing of two 2.1 mm thick S-type Ni-foils and one 2.5 mm thick 0-type Ni-foil. Please observe that the stress-values for the two S-type foils have been added 100 and 200 MPa, respectively, in order to improve visibility of the individual datapoints.
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Fig. 6. Stress-strain diagram from the tensile testing of four 0-type Ni-foils, 2.5, 4.0, 5.0 and 6.0 mm thick. Please observe that the stress-values for the 5.0 and 6.0 mm foils have been added 100 and 200 MPa, respectively, in order to improve visibility of the individual data-points.
initiation is reduced. The tested 6 mm 0-type foil was in fact so strong, that the strain-measuring device went out of range at approximately 1000 MPa before failure. The average value for the elastic modulus is approximately 157 GPa. 3.7. Nanoindentation The nano-indentation behaviour of a polished S-type Ni-substrate and a 0-type foil is illustrated by the load– displacement curve in Fig. 7. Young’s modulus for the Ni-substrate was calculated from the unloading curve, by power law fitting w18x to range between 230 and 235 GPa for all of the 20 indents made. The hardness for the Ni-substrate, calculated by Eq. (1), ranged from 6 to 7 GPa. For the 0-type Ni-foil attached to its substrate, values for the Young’s modulus between 180 and 205 GPa were found for the 20 indents made. Hardness values were in the interval 5–6 GPa. 4. Discussion 4.1. Topology, morphology and crystallography The electrochemical deposition of Ni-foils, to be applied as microphone membranes, was optimised in
both of the studied electrolytes. This was evidenced by the seemingly mirror-bright appearance of the foils in both the light optical microscope (LOM) and the scanning electron microscope (SEM). Atomic force microscopy (AFM) revealed that the addition of Na-saccharin changes the surface topography of the Ni-foil at a nanometer-scale. Instead of nodular dendritic crystallite growth (Fig. 2a,b), Na-saccharin affects the crystallisation mechanism, such that levelling is achieved (Fig. 2d). Without the use of Na-saccharin, the topology observed with the AFM indicates nodular dendritic growth, very similar to recently published AFM-morphologies of Ni deposits from a Ni-sulfamate electrolyte w21x. The pronounced growth control of the Na-saccharin additive was further evidenced by observations made with transmission electron microscopy (TEM). Not only is the grain size of the individual grains reduced by the use of Na-saccharin, but epitaxial growth governed by the passivated Ni-substrate surface seems to be efficiently hindered. In 0-type foils deposited without the use of Na-saccharin, epitaxial growth on the passivated Ni-substrate was observed. The fact that epitaxy occurs in this case indicates that: (a) the oxide-film forming on the polished Ni-substrate during passivation must be very thin, presumably less than 1 nm; and (b) the additives used in electrolyte 1 (Table
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Fig. 7. Load-displacement curves obtained by nanoindentation of a 0-type foil and a polished S-type Ni-substrate.
1) have a moderate effect on the initial crystallisation mechanism during electrochemical Ni-deposition. An essential observation made with TEM, which helps explain the mechanical properties of the Ni-foils (see Section 4.2) was the abundance of more micro structural defects in the S-type foils deposited with sodium saccharin in the electrolyte. All of the investigated Ni-foils were ‘as deposited’ and were not subjected to additional heat-treatment. Any structural defects in the S-type Nifoils are therefore regarded as governed by local growth phenomena controlled by Na-saccharin during electrodeposition. Texture analysis with X-ray diffraction agreed well with TEM-micrographs and electron diffraction (Fig. 3), indicating that the deposited Ni consisted of fine grains which are more or less randomly oriented. The degree of preferred crystal orientation seemed to vary with the use of Na-saccharin as seen by the differences in calculated densities in the inverse pole figures (Fig. 4). 0-type Ni foils showed most pronouncedly a N311Mtexture, while an additional weak N111M-component occurs in both types of Ni-foils, but most evident in the S-type. This could indicate that Na-saccharin promotes the growth of close-packed crystal planes exposed to the electrolyte, where the number of nearest neighbour atoms is largest. Alternatively, the above observations could be the outcome of changes in the nucleation stage,
caused by the presence of Na-saccharin during deposition. 4.2. Mechanical properties The additive Na-saccharin was specifically chosen for these studies because of its widespread use as a stressrelieving agent in Ni-electrodeposition w11x, The ability
Fig. 8. Apparent Young’s modulus vs. foil thickness for 0-type Nifoils (data from Fig. 6).
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to tailor Ni-deposits, especially for micro electro mechanical systems (MEMS) also includes the ability to control residual stresses in the deposited Ni. Therefore, the saccharin-type additives play an important role in many recent applications of electrodeposited Ni. For the Ni-membranes studied, Young’s modulus derived from the tensile testing results ranged from approximately 130 GPa for 2.1 mm thick S-type foils up to approximately 170 GPa for a 6.0 mm thick 0-type foil. These values are systematically lower than those tabulated for Ni in its polycrystalline form. Ni is strongly elastically anisotropic: Young’s modulus for single crystal Ni in its unmagnetised state can vary from 130 GPa in the N100M direction to 297 GPa in the N111M direction w22x. For polycrystalline Ni, Young’s modulus lies in the range 197–225 GPa, depending on the state of magnetisation w23x. Since the present foils consist of near randomly distributed Ni crystals within the plane of the foil, a value for Young’s modulus close to that of polycrystalline Ni should be expected. The discrepancy can be explained from sliding of the foils in the jaws of the testing equipment. Another factor, which needs to be taken into account, is the surface roughness, which makes the measured thickness value of the film larger than the actual film thickness contributing to the tensile strength of the deposit. The clamping and roughness problems encountered in the tensile test were estimated to cause an error of 20–25% in the determination of Young’s modulus. Values for Young’s modulus found by nano-indentation ranged from 180 to 230 GPa, which support the above explanation that the low values determined with the uniaxial tensile testing are associated with the testing device rather than the material. A further indication of the problems with clamping and surface roughness in the tensile test was given by the observed increase in Young’s modulus with increasing foil thickness for 0-type Ni-foils (Fig. 8). The 0-type foils are more ductile and stronger than the S-type foils. The presence of S or NiS in the grain-boundaries of S-containing Ni has previously been reported as the main cause for de-cohesion failure in the metal lattice w12x. However, the formation of nickel sulfides at the grain boundaries normally requires heat-treatment, and none of the Ni-deposits investigated during the present study were heat-treated. No nickel sulfide particles were observed in the TEM-studies performed, rather an increased number of defects (particularly grain boundaries) in the S-type Ni-deposit as compared to the 0-type deposit was found (Fig. 3). It is therefore believed that in the present case, growth-defects, caused by the Nisaccharin additive during electrodeposition, contribute to the lower ductility and tensile strength observed for the S-type Ni-foils. Mockute et al. w24x described the action of saccharin during nickel electrodeposition as a combination of cathodic reactions of saccharin and its
transformation product, benzamide, as well as the incorporation of both sulfur and carbon. The incorporation of small amounts of carbon and other breakdown products from the Na-saccharin (15 cathodic reactions were identified by Mockute et al.), leading to dispersion hardening effects could explain the difference in mechanical properties observed between the S-type and 0-type Ni-foils. Compared to reported tensile strengths of 520–559 MPa for annealed, hot-rolled and cold-stretched Ni w23x, measured values of up to 1000 MPa for the 0-type Nifoils are relatively high, but in good agreement with the general strengthening by grain size reduction as described by the Hall–Petch equation w25x: syss0qkyØdy1y2 where d is the average grain diameter and s0 and ky are material constants. Using the values KNis158, sNis110 MPa for Ni reported by Hughes and Hansen w26x one obtains a calculated yield stress in the order of 1000 MPa for Ni with an average grain size of 25 nm, which is similar to the grain sizes observed by TEM in this study. Observing the roughness of 0-type Ni foils in the AFM (Fig. 2a), it could be argued that crack-initiators in the surface of the 0-type foils might cause early failure. The rounded geometry of each dendrite (Fig. 2b) on the 0type foil, however, explains, why the 0-foils do not show any signs of early fracture during the tensile test. It should also be noted that all the AFM micrographs shown in Fig. 2 were scaled by a factor of 4 in the growth-direction to enhance the appearance of the surface topography. Accordingly, surface-phenomena are not expected to play a dominant role in foil-failure. Nano-indentation on a 0-type Ni-foil with a polished S-type Ni-substrate clearly showed the substrate to be the harder of the two. The smaller grain size in the S-type Ni substrate as compared to the 0-type Ni probably only partly explains this observed difference. Another contribution to the higher hardness in the substrate is mechanical polishing of the S-Ni substrate prior to nano-indentation. 4.3. Application of Ni-foils The difference in ductility observed for the two types of Ni-foils could prove decisive for the application of the foils as microphone membranes, where pre-straining and tuning of the membrane will leave the Ni-foil in a stressed state. A critical point in this context is how close to the onset of plastic deformation the foils are tuned in the microphone. A relatively brittle film, which can hardly accommodate plastic deformation, will be difficult to tune and handle without breaking it. For a ductile foil, plastic deformation may cause the foil to
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lose its tuned stress-state, if tuned beyond its yield strength. Further studies should, therefore, include stress-measurements on tuned microphone membranes by e.g. Xray diffraction measurements. 5. Conclusion Ni-foils with an optically mirror bright finish can be synthesised from a Watts-type electrolyte both with and without the use of the S-containing additive Na-saccharin. Ni-foils deposited with Na-saccharin are smooth and fine-grained with a nearly random grain-orientation and a typical grain-size of 10–20 nm. Weak N311M and N111M texture components were found with maximum pole densities of 2.2 and 1.9, respectively. Ni-foils deposited with no Na-saccharin addition show more nodular topography when studied by AFM and a grain size of 20–40 nm. Again weak N311M and N111M texture components were observed with maximum pole densities of 2.75 and 1.2, respectively. Both in terms of numbers and appearance, grain boundaries become more dominant in the internal microstructure of the Ni-deposit with the use of the Na-saccharin additive. This was observed by transmission electron microscopy in both planar- and cross-sectional studies. Mechanically, Ni-foils deposited with Na-saccharin are harder and more brittle than Nifoils deposited without Na-saccharin addition. The reduction in ductility, and hereby also in ultimate tensile strength, is believed to be caused by strain hardening caused by impurities of both sulfur and carbon codeposited into the Ni with the use of Na-saccharin as reported in previous studies. Na-saccharin, as used here, causes grain refinement and promotion of planar crystal growth, but as mentioned above the mechanical properties are deteriorated by growth-defects or foreign inclusions. S-type Ni-foils are more brittle and have lower tensile strength. This deterioration is attributed to segregation of sulfur and carbon in grain boundaries (due to the high grain boundary density) and, possibly, higher concentration of intrinsic defects. The fact that even in the ‘as-deposited’ state, S-type Ni-foils show reduced ductility as compared to the more pure 0-type Ni-foils, has not been reported previously and should be noted for all the many applications, where Na-saccharin is used primarily as a stressrelieving additive in Ni-electrodeposition. Acknowledgments The authors would like to thank Danish Micro Engineering AyS for giving the authors access to the Dualscope娃 AFM apparatus. Technician Flemming Bjerg Grumsen at DTU is kindly acknowledged for his assistance in obtaining the X-ray data presented here, as are
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M.Sc. Michael Eis Benzon and Fin Mertins for their help with the tensile testing of the Ni-foils. Prof. Sten ¨ Johansson at IKP, Linkoping University, is kindly acknowledged for his comments regarding the good match between measured mechanical data and the Hall– Petch equation. Financial support from the Danish Research Council (STVF) is gratefully acknowledged. References w1x J.P. Rasmussen, P.T. Tang, C. Sander, O. Hansen, P. Møller, Fabrication of an All-Metal Atomic Force Microscope Probe, Transducers ‘97, Chicago, June 16–19, 1997, pp. 463–466. w2x Tang, P.T., Ph.D thesis, Fabrication of micro components by electrochemical deposition, DTU, March 1998. w3x W. Ruythooren, K. Attenborough, S. Beerten, et al., J. Micromech. Microeng. 10 (2000) 101–107. w4x S. Abel, H. Freimuth, H. Lehr, H. Mensinger, J. Micromech. Microeng. 4 (1994) 47–54. w5x J. Gobet, F. Cardot, J. Bergqvist, F. Rudolf, J. Micromech. Microeng. 3 (1993) 123–130.. w6x W. Qu, C. Wenzel, A. Jahn, J. Micromech. Microeng. 8 (1998) 279–283. w7x V. Sommer, Galvanotechnik 91 (3) (2000) 791–797. w8x S.A. Watson, J. Edwards, Tram. IMF 34 (1957) 167. w9x D. Mockute, G. Bernotiene, Surf. Coat. Technol. 135 (2000) 42–47. w10x J. Edwards, Trans. IMF 41 (1964) 169. w11x R.J. Kendrick, Trans. IMF 40 (1963) 19. w12x ‘Die Warmebehandlung ¨ ¨ von Nickel’, Nickel Informationsburo ¨ GmbH Dusseldorf, 10–15 (1964). w13x H.J. Bunge, Texture Analysis in Materials Science: Mathemat¨ ical Methods, Cuvillier Verlag, Gottingen, 1993. w14x K. Pantleon, J.D. Jensen, M.A.J. Somers, X-Ray diffraction investigation of electrochemically deposited copper. In: Proceedings EPDIC-8 Conference, Uppsala Sweden, 23–26th May 2002, Materials Sci. Forum, in print. w15x American National Standards Institute: ASTM E 345-87, Standard Test Methods Of Tension Testing Of Metallic Foil, New York, 1987.. w16x American National Standards Institute: ASTM E 74-91, Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines, New York, 1991. w17x Danish Standardisation Council (Dansk Standardiseringsrad): ˚ DS 10 110, Metal Testing, Tensile Testing, Copenhagen, 1968. w18x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564–1582. w19x R.O. Hull, Proc. Am. Electroplater’s Soc. 27 (1939) 52–60. w20x DIN 50 957 (1978). w21x M. Saitou, W. Oshikawa, A. Makabe, J. Phys. Chem. Solids 63 (2002) 1685–1689. w22x E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, 7th edition, Butterworth & Heinemann Ltd, Oxford, 1992, ISBN 0 7506 10204. w23x K.E. Voelk, Nickel und Nickellegierungen, Eigenschaften und Verhalten, Springer–Verlag, Berlin, 1970. w24x D. Mockute, G. Bernotiene, R. Vilkaite, Surf. Coat. Technol. 160 (2002) 152–157. w25x W.D. Callister Jr., Materials Science and Engineering, an Introduction, 6th edition, John Wiley & Sons Publ, 2003. w26x D.A. Hughes, N. Hansen, Acta Mater. 48 (11) (2000) 2985–3004.
Electrochemically deposited nickel membranes; process–microstructure– property relationships ˚ Perssonb, Karen Pantleona, Magnus Oden ´ c, Lars Hultmanb, Jens A.D. Jensena,b,*, Per O.A. Marcel A.J. Somersa a
The Technical University of Denmark, Institute of Manufacturing Engineering and Management, b. 204, DK-2800 Lyngby, Denmark b ¨ ¨ Thin Film Physics Division, IFM, Linkoping University, S-581 83 Linkoping, Sweden c ¨ ¨ Linkoping University, Department of Mechanical Engineering, Linkoping, Sweden Received 20 September 2002; accepted in revised form 22 January 2003
Abstract This paper reports on the manufacturing, surface morphology, internal structure and mechanical properties of Ni-foils used as membranes in reference-microphones. Two types of foils, referred to as S-type and 0-type foils, were electrochemically deposited from a Watts-type electrolyte, with (S-type) or without (0-type) the use of the sulfur-containing additive sodium saccharin. Both types of Ni-foils appeared perfectly smooth when investigated with scanning electron microscopy (SEM), while atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed differences in the surface morphologies and a smaller grain-size in the S-type foils. X-Ray diffraction showed a N311M texture component in both types of Ni-foils, most pronounced for 0-type foils. A minor N111M texture component observed in both foil types was strongest in the S-type foils. Mechanically 0type foils proved more ductile than S-type foils during thin film tensile testing, due to microstructural defects caused by sodium saccharin during deposition. Tensile strengths in the order of 700–1000 MPa were observed—highest for the more ductile 0-type foils. A hardness in the order of 6 GPa (590 HV) was found by nanoindentation. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nickel; Electrodeposition; Microstructure; Mechanical testing; Thin films
1. Introduction Electrochemical deposition has become a key technology in the development of metallic components for microsystems. The manufacturing of 3D micro-components and structures such as interconnects, CDyDVDstamping tools, cantilevers for scanning probe microscopy, membranes for microphones, microfluidic systems and optical waveguides has been made possible by the combination of Si-wafer processing and advanced electrochemical deposition techniques w1–7x. The ability to tailor and reproduce Ni-electrodeposits with specific microstructures and well-defined mechanical properties is essential in micro-electro-mechanical systems (MEMS) w2x. For this reason, fine-grained (preferably nano-crystal line), smooth, mechanically *Corresponding author. Tel.: q45-45-25-2212; fax: q45-45-936213. E-mail address: [email protected] (J.A.D. Jensen).
isotropic, reproducible and thermodynamically stable deposits are desired, which requires controlled, robust electrodeposition processes. Ni-membranes, used e.g. in microphones, manufactured by electrochemical deposition are a good example of an application, where an electrodeposited Ni-film has to meet very high demands. In the present study, the manufacturing of Ni microphone membranes, and the influence on microstructure and mechanical properties are investigated. Both microstructure and mechanical properties are related to the level and type of organic additives used in the electrochemical deposition process. Na-saccharin is used as an additive in Ni-electrolytes due to its very strong levelling abilities. During electrodeposition the saccharin decomposes, thereby incorporating an amount of S into the Ni-deposit w8–11x. Kendrick w11x showed how the incorporation of S affects the internal stress state in the nickel deposit: a low concentration (0.02–0.04%) of S in the deposit causes
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00253-6
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Table 1 Electrolyte and additive chemistry Basic Ni-electrolyte composition 46 gyl Nickel Chloride NiCl26H2O, technical grade 367.5 gyl Nickel Sulfate, NiSO4 5H2O, technical grade 44 gyl Boric Acid, H3BO3, technical grade 0.1 gy1 Sodium Lauryl Sulfate, NaC12H25SO4, analytical grade Additives Electrolyte no. 1, 0-type Ni-foils 3 gy1 Naphthalene-1,3,6-trisulfonic acid trisodium salt C10H5Na3O9S3 analytical grade 0.3 gyl 2-butyn-1,4-diol HOCH2C'CCH2OH analytical grade
Electrolyte no. 2, S-type Ni-foils 1 gy1 Naphthalene-1,3,6-trisulfonic acid trisodium salt C10H5Na3O9S3 analytical grade 0.2 gyl 2-butyn-1,4-diol HOCH2C'CCH2OH analytical grade 1.0 gy1 Saccharine Sodium Salt C7H4NO3SNa analytical grade
Operating conditions for Ni-deposition pH 4.0–4.2 Temp.s50 8C Is50 A is4.8 Aydm2 Continuous filtration (1 mm filter)
a compressive stress state, whereas a higher S-content (0.04–0.06, possibly much higher) leads to a tensile stress state in the deposit. ‘Alloying’ with S may cause embrittlement of the Ni-material, especially during prolonged heating of the coating, allowing S to migrate to grain-boundaries and forming brittle nickel-sulfides leading to grain boundary decohesion w12x. In spite of this drawback, Na-saccharin is used extensively in several applications of Ni electrodeposition, both because of the unrivalled levelling characteristics, and especially because of the possibility to obtain a compressive stress state, facilitating stress control in Ni-deposits. The present study is devoted to the effect of Nasaccharin on the microstructure and the mechanical properties of Ni deposited from a Watts electrolyte. To this end, foils deposited from a Watts-type electrolyte with and without Na-saccharin addition are compared. 2. Experimental 2.1. Electrochemical deposition Ni-foils were electrodeposited from a Watts-type electrolyte with and without the S-containing additive Nasaccharin (Table 1). The two types of foils are referred to as 0-type (zero-type, no S in electrolyte nor deposit) and S-type (S-containing electrolyte and electrodeposit), respectively. In order to obtain high quality Ni-foils with no defects, great care was taken to prepare the substrate surface onto which the foil was deposited. The substrate consisted of a brass-plate measuring 30=35 cm, coated with a 100–200 mm layer of S-type Ni (in order to get the highest possible levelling). The Ni-surface was
carefully polished with 6-, 3- and 1 mm diamond-slurry successively to a microscopically mirror-bright finish. Prior to electrochemical deposition of the Ni-foil, a thin oxide-layer was formed on the polished Ni-substrate by anodic passivation in an alkaline solution (Table 2). As a consequence of this processing step, a controlled, limited adhesion was obtained between substrate and Ni-foil deposit. This facilitated the separation of the foil from the passivated substrate after finishing the deposition. Polypropylene containers, 200 l in volume, with constant filtration (0.1 mm filter) were used as plating cells. A Ti-basket, approximately 30=50=8 cm, with S-alloyed Ni-rounds, dressed in woven polypropylene bags, served as anode. The substrate was cyclically moved parallel to its surface plane in the container at a linear velocity of approximately 10 cmys during electrodeposition to ensure sufficient exchange of electrolyte at the cathode surface. The procedure for the manufacturing of Ni-foils is given in Table 3.
Table 2 Electrolyte and operating conditions for cathodic rinseyanodic passivation 89 gy1 Sodium Hydroxide, NaOH, technical grade 3.5 gy1 Sodium Cyanide, NaCN, technical grade pH)12 Temp.sR.T. Is50 A is4.8 Aydm2 Continuous filtration (1 mm filter)
J.A.D. Jensen et al. / Surface and Coatings Technology 172 (2003) 79–89 Table 3 Manufacturing procedure for electrodeposited Ni-foils Process step
Process parameters
Cathodic rinse
Alkaline electrolyte (Table 1), is4.8 Aydm2, ts3 min, TsR.T. 8C, continuous filtration (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Dilute H2SO4 (5 1 H2SO4 to 90 l), ts10 s Counterflow tap-water, tank 2, ts20 s Alkaline electrolyte (Table 1), is4.8 Aydm2, ts3 min, TsR.T., continuous filtratrion (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Distilled H2O, ts20 s Watts electrolyte (Table 1), is4.8 Aydm2, ts10 min, continuous filtration (1 mm) Counterflow tap-water, tank 1, ts20 s Counterflow tap-water, tank 2, ts20 s Dilute H2SO4 (5 l H2SO4 to 90 1), ts3 s Counterflow tap-water, tank 2, ts20 s Distilled H2O, ts20 s Drying cupboard, Ts40–50 8C, ts2–5 min Opening with grinding-machine around edge of substrate, manual removal of Ni-foil.
Water rinse 1 Water rinse 2 Pickling Water rinse 2 Anodic passivation Water rinse 1 Water rinse 2 Water rinse 3 Ni electrodeposition Water rinse Water rinse Acid rinse Water rinse Water rinse Hot air dry Mechanical of Ni-foil
1 2 2 3 removal
2.2. Light optical microscopy The appearance of the foil and associated defects caused by impurities, either from the pretreatment or the electrodeposition process, were studied in planar view using a Zeiss stereo microscope and an Olympus BH2 upright microscope, using differential interference contrast imaging. 2.3. Scanning electron microscopy (SEM) SEM-micrographs were obtained from the Ni-foil surfaces in a LEO 1550 Gemini Field Emission Gun (FEG) SEM. The back-scattered electron signal was used for imaging with an electron acceleration voltage in the range 5–20 kV and a working distance of 4 mm. 2.4. Atomic force microscopy (AFM) AFM-analysis was conducted with a prototype DUALSCOPE娃 scanning probe microscope from Danish Micro Engineering (DME), combining an AFMmicroscope with a Zeiss optical microscope. The microscope has been constructed for maximum rigidity (solid steel construction) and is separated from the surroundings by a spring system to reduce noise. The AFM probe tip oscillates at its resonance frequency (e.g. 20 kHz), while a deflection sensor with a laserbeam focused on the back of the probe constantly measures the amplitude and frequency of the tip. Changes of amplitude are transformed into a 3-D picture by means of a specific imaging software. During the scanning process, the probe tip does not touch the sample,
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because the atomic forces reflect the probe tip before a collision occurs. 2.5. Transmission electron microscopy (TEM) Cross-sectional and plan-view samples for TEM were prepared by sandwiching a foil in between two dummy Si pieces, followed by mechanical thinning and polishing. Low-angle (48) ion milling in a BalTec RES 010 rapid ion etch operated at 8 kV, was used to make the samples electron transparent. A final polishing stage using low-energy ions at 2 kV was applied to remove the amorphous surface layer formed by the previous ion etch. The TEM investigations were carried out using a Philips CM 20 UT microscope, equipped with a LaB6 filament, operated at 200 kV. 2.6. X-Ray diffraction (XRD) X-Ray diffraction analysis was performed using a powder diffractometer D8 Discover from Bruker AXS, equipped with a 1y4-circle Eulerian cradle for texture analysis. Measurements were carried out using Cu-Ka radiation, giving an information depth of approximately 5 mm wfor the (200)-Bragg peak 63% of the diffracted intensity originates from the top 5 mm of the Nix. All measured intensities were background corrected, and correction for defocusing was performed using a textureless Ni-powder sample. Pole figures of (111), (200) and (220) were measured up to 758 tilt angles and orientation distribution functions (ODF) w13,14x were calculated. Complete pole figures of (111), (200) and (220), supplementary pole figures of (311) and (331), as well as inverse pole figures were derived from the ODF. 2.7. Thin film tensile testing The tensile testing apparatus used was developed and manufactured with the specific aim of testing thin metal foils according to ASTM standard E 345-87 w15x. The apparatus is sensitive to the specific sample geometry and pulling velocity and is calibrated according to ASTM standard E 74-91 w16x. Rectangular specimens measuring 12=140 mm in accordance with Danish standard DS 10 110 w17x were used in the present study. The tested length of each specimen was approximately 100 mm. The ASTM standard E 345-87 does not prescribe the pulling velocity. Considering the pulling distance of approximately 1 mm to fracture however, a velocity of 1 mm sy1 was used in this study. Young’s modulus and the stress-state in the foils were calculated, knowing the average thickness of each specimen as determined with a calibrated Fisherscope X-ray fluorescence thickness measurer.
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Fig. 1. Typical defects in Ni-foils manufactured by electrochemical deposition.
2.8. Nanoindentation Nano indentation was carried out using a NanoIndenter娃 II, according to the method described by Oliver et al. w18x. Two samples were studied: a 6 mm, 0-type Ni-foil, attached to its substrate and a polished S-type Ni-substrate. For each sample, a series of 20 indentations was made. A spacing of at least 16.5 mm existed between two successive measurement locations. Stiffnesses were calculated from unloading curves. A power-law was fitted over 90% of the unloading curve to find the best correlation of the slope. The hardness, defined as the mean pressure the material will support under load, was computed from: Hs
Pmax A
(1)
where A is the projected area of contact at peak load. 3. Results In order to obtain similar brightness of the Ni-deposits in the Na-saccharin-free electrolyte to that obtainable in
the traditional electrolyte, a series of additive-combinations were tested in a Hull-Cell w19,20x. The highest brightness in the Hull-Cell test was found for the additive combination given in Table 1 for electrolyte 1. 3.1. Light optical microscopy Inspection with light optical microscopy of the manufactured Ni-foils showed few defects, even at high magnification and with the use of differential interference contrast. Generally, defects could be attributed to insufficient pre-treatment cleanliness. The most frequently encountered defects and their proposed origin are shown in Fig. 1. 3.2. Scanning electron microscopy (SEM) Apart from areas where the foils suffered from obvious defects caused by impurities or faults during the electrodeposition, the overall morphology of both types of foils investigated was smooth and contourless. Even in the FEG-SEM used at over 100.000= magnification,
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Fig. 2. AFM-micrographs of 0-type (aqb) and S-type (cqd) Ni-foils.
the surfaces of the foils did not show any observable features.
growth-behaviour perpendicular to the film-growth direction (Fig. 2c,d).
3.3. Atomic force microscopy (AFM)
3.4. Transmission electron microscopy (TEM)
AFM did reveal differences in surface topography between the S-type and the 0-type Ni-foils. Typical values of maximum peak-to-bottom distances as measured with AFM over an area of 43 by 43 mm2 were 100–140 nanometers for both types of Ni-foils. In the electrolyte without Na-saccharin addition (Table 1, electrolyte no. 1), the individual grains on the surface of the Ni-foil appeared sharper and with a clear indication of crystal growth perpendicular to the substrate surface (Fig. 2a,b). In the electrolyte containing the Na-saccharin additive (Table 1, electrolyte no. 2) a smoother surface was obtained with more rounded grains and a more laminar
A TEM-micrograph of a plan-view section of a 0type Ni-foil is shown in Fig. 3a. The micrograph suggests that the film is composed of equiaxed grains, ranging in size from 20 to 40 nm. As follows from the accompanying electron diffraction pattern, the distribution of grain-orientation within the plane of the layer appears random. A cross-sectional view of the same sample (Fig. 3b) confirms equiaxed grains at a distance of approximately 250 nm from the substrate. In the vicinity of the substrate, relatively large grains are present, upon which smaller grains appear to have nucleated; possibly the first nuclei developed by epitaxial growth on the substrate crystals, in spite of the thin
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Fig. 3. TEM micrographs of 0-type Ni-foil (aqb) and S-type Si foils (cqd).
passive layer between substrate and deposit. The assumed epitaxial layer in Fig. 3b was observed over an extensive area of the investigated sample and is believed to be a general characteristic of the 0-type Nifoils. TEM-micrographs of the S-type Ni-foil (Fig. 3c,d) showed an average grain size of 10–20 nm, and no indications of epitaxial growth onto the substrate grains. Smoother and more continuous rings in the accompanying electron diffraction patterns as compared to those in Fig. 3a are consistent with a finer grain size in the S-type Ni-foil as compared to the 0-type. Comparing Fig. 3a and Fig. 3c, the grain boundaries in the S-type Ni-foil (Fig. 3c) are not only more abundant, but also seem more pronounced than in the 0-type Ni-foil (Fig. 3a). Although no inclusions or impurities are readily observable in the grain boundaries, this difference in grain coherence may explain the difference in mechanical properties of the two foil types (see later Section 3.6).
´ Moire-fringes are observable in both types of foils investigated in the TEM as a result of overlapping grains systematically scattering the incoming electron beam. 3.5. X-Ray diffraction Both types of Ni-foils investigated show ideal fibre textures, i.e. the fibre axis is parallel to the growth direction of the foil. The results of the texture analysis are represented in the calculated inverse pole figures given in Fig. 4. The major texture component in both 0-type and S-type deposits is N311M with a calculated maximum density of 2.75 for 0-type Ni and 2.20 for S-type Ni. A minor N111M texture component was observed in both the 0type and S-type foil investigated. Calculated densities for the N111M poles were 1.2 and 1.9 for the 0- and Stype Ni-foils, respectively.
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Fig. 4. Inverse pole figures for: (a) 0-type Ni (6 mm foil); and (b) S-type Ni (6 mm foil).
3.6. Tensile testing Tensile testing was carried out on 2 mm thick Ni-foils of both 0- and S-type (Fig. 5), and on a series of 0type Ni-foils with various thicknesses (Fig. 6). The two types of foils showed similar behaviour in the elastic region. In the plastic region, the 0-type Nifoils behaved in a much more ductile way than the two S-type foils tested (Fig. 5) as reflected by the higher
ultimate tensile strength and the larger plastic strain. Increased foil thickness resulted in an increase of the ‘apparent’ Young’s modulus and on extension of the plastic region (Fig. 6). This result is explained as follows. With increased thickness, the tensile test becomes less sensitive to handling and clamping of the foils. In addition, the effect of surface roughness on the calculated E modulus becomes less pronounced. Moreover, the role of the surface roughness in fracture
Fig. 5. Stress-strain diagram from tensile testing of two 2.1 mm thick S-type Ni-foils and one 2.5 mm thick 0-type Ni-foil. Please observe that the stress-values for the two S-type foils have been added 100 and 200 MPa, respectively, in order to improve visibility of the individual datapoints.
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Fig. 6. Stress-strain diagram from the tensile testing of four 0-type Ni-foils, 2.5, 4.0, 5.0 and 6.0 mm thick. Please observe that the stress-values for the 5.0 and 6.0 mm foils have been added 100 and 200 MPa, respectively, in order to improve visibility of the individual data-points.
initiation is reduced. The tested 6 mm 0-type foil was in fact so strong, that the strain-measuring device went out of range at approximately 1000 MPa before failure. The average value for the elastic modulus is approximately 157 GPa. 3.7. Nanoindentation The nano-indentation behaviour of a polished S-type Ni-substrate and a 0-type foil is illustrated by the load– displacement curve in Fig. 7. Young’s modulus for the Ni-substrate was calculated from the unloading curve, by power law fitting w18x to range between 230 and 235 GPa for all of the 20 indents made. The hardness for the Ni-substrate, calculated by Eq. (1), ranged from 6 to 7 GPa. For the 0-type Ni-foil attached to its substrate, values for the Young’s modulus between 180 and 205 GPa were found for the 20 indents made. Hardness values were in the interval 5–6 GPa. 4. Discussion 4.1. Topology, morphology and crystallography The electrochemical deposition of Ni-foils, to be applied as microphone membranes, was optimised in
both of the studied electrolytes. This was evidenced by the seemingly mirror-bright appearance of the foils in both the light optical microscope (LOM) and the scanning electron microscope (SEM). Atomic force microscopy (AFM) revealed that the addition of Na-saccharin changes the surface topography of the Ni-foil at a nanometer-scale. Instead of nodular dendritic crystallite growth (Fig. 2a,b), Na-saccharin affects the crystallisation mechanism, such that levelling is achieved (Fig. 2d). Without the use of Na-saccharin, the topology observed with the AFM indicates nodular dendritic growth, very similar to recently published AFM-morphologies of Ni deposits from a Ni-sulfamate electrolyte w21x. The pronounced growth control of the Na-saccharin additive was further evidenced by observations made with transmission electron microscopy (TEM). Not only is the grain size of the individual grains reduced by the use of Na-saccharin, but epitaxial growth governed by the passivated Ni-substrate surface seems to be efficiently hindered. In 0-type foils deposited without the use of Na-saccharin, epitaxial growth on the passivated Ni-substrate was observed. The fact that epitaxy occurs in this case indicates that: (a) the oxide-film forming on the polished Ni-substrate during passivation must be very thin, presumably less than 1 nm; and (b) the additives used in electrolyte 1 (Table
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Fig. 7. Load-displacement curves obtained by nanoindentation of a 0-type foil and a polished S-type Ni-substrate.
1) have a moderate effect on the initial crystallisation mechanism during electrochemical Ni-deposition. An essential observation made with TEM, which helps explain the mechanical properties of the Ni-foils (see Section 4.2) was the abundance of more micro structural defects in the S-type foils deposited with sodium saccharin in the electrolyte. All of the investigated Ni-foils were ‘as deposited’ and were not subjected to additional heat-treatment. Any structural defects in the S-type Nifoils are therefore regarded as governed by local growth phenomena controlled by Na-saccharin during electrodeposition. Texture analysis with X-ray diffraction agreed well with TEM-micrographs and electron diffraction (Fig. 3), indicating that the deposited Ni consisted of fine grains which are more or less randomly oriented. The degree of preferred crystal orientation seemed to vary with the use of Na-saccharin as seen by the differences in calculated densities in the inverse pole figures (Fig. 4). 0-type Ni foils showed most pronouncedly a N311Mtexture, while an additional weak N111M-component occurs in both types of Ni-foils, but most evident in the S-type. This could indicate that Na-saccharin promotes the growth of close-packed crystal planes exposed to the electrolyte, where the number of nearest neighbour atoms is largest. Alternatively, the above observations could be the outcome of changes in the nucleation stage,
caused by the presence of Na-saccharin during deposition. 4.2. Mechanical properties The additive Na-saccharin was specifically chosen for these studies because of its widespread use as a stressrelieving agent in Ni-electrodeposition w11x, The ability
Fig. 8. Apparent Young’s modulus vs. foil thickness for 0-type Nifoils (data from Fig. 6).
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to tailor Ni-deposits, especially for micro electro mechanical systems (MEMS) also includes the ability to control residual stresses in the deposited Ni. Therefore, the saccharin-type additives play an important role in many recent applications of electrodeposited Ni. For the Ni-membranes studied, Young’s modulus derived from the tensile testing results ranged from approximately 130 GPa for 2.1 mm thick S-type foils up to approximately 170 GPa for a 6.0 mm thick 0-type foil. These values are systematically lower than those tabulated for Ni in its polycrystalline form. Ni is strongly elastically anisotropic: Young’s modulus for single crystal Ni in its unmagnetised state can vary from 130 GPa in the N100M direction to 297 GPa in the N111M direction w22x. For polycrystalline Ni, Young’s modulus lies in the range 197–225 GPa, depending on the state of magnetisation w23x. Since the present foils consist of near randomly distributed Ni crystals within the plane of the foil, a value for Young’s modulus close to that of polycrystalline Ni should be expected. The discrepancy can be explained from sliding of the foils in the jaws of the testing equipment. Another factor, which needs to be taken into account, is the surface roughness, which makes the measured thickness value of the film larger than the actual film thickness contributing to the tensile strength of the deposit. The clamping and roughness problems encountered in the tensile test were estimated to cause an error of 20–25% in the determination of Young’s modulus. Values for Young’s modulus found by nano-indentation ranged from 180 to 230 GPa, which support the above explanation that the low values determined with the uniaxial tensile testing are associated with the testing device rather than the material. A further indication of the problems with clamping and surface roughness in the tensile test was given by the observed increase in Young’s modulus with increasing foil thickness for 0-type Ni-foils (Fig. 8). The 0-type foils are more ductile and stronger than the S-type foils. The presence of S or NiS in the grain-boundaries of S-containing Ni has previously been reported as the main cause for de-cohesion failure in the metal lattice w12x. However, the formation of nickel sulfides at the grain boundaries normally requires heat-treatment, and none of the Ni-deposits investigated during the present study were heat-treated. No nickel sulfide particles were observed in the TEM-studies performed, rather an increased number of defects (particularly grain boundaries) in the S-type Ni-deposit as compared to the 0-type deposit was found (Fig. 3). It is therefore believed that in the present case, growth-defects, caused by the Nisaccharin additive during electrodeposition, contribute to the lower ductility and tensile strength observed for the S-type Ni-foils. Mockute et al. w24x described the action of saccharin during nickel electrodeposition as a combination of cathodic reactions of saccharin and its
transformation product, benzamide, as well as the incorporation of both sulfur and carbon. The incorporation of small amounts of carbon and other breakdown products from the Na-saccharin (15 cathodic reactions were identified by Mockute et al.), leading to dispersion hardening effects could explain the difference in mechanical properties observed between the S-type and 0-type Ni-foils. Compared to reported tensile strengths of 520–559 MPa for annealed, hot-rolled and cold-stretched Ni w23x, measured values of up to 1000 MPa for the 0-type Nifoils are relatively high, but in good agreement with the general strengthening by grain size reduction as described by the Hall–Petch equation w25x: syss0qkyØdy1y2 where d is the average grain diameter and s0 and ky are material constants. Using the values KNis158, sNis110 MPa for Ni reported by Hughes and Hansen w26x one obtains a calculated yield stress in the order of 1000 MPa for Ni with an average grain size of 25 nm, which is similar to the grain sizes observed by TEM in this study. Observing the roughness of 0-type Ni foils in the AFM (Fig. 2a), it could be argued that crack-initiators in the surface of the 0-type foils might cause early failure. The rounded geometry of each dendrite (Fig. 2b) on the 0type foil, however, explains, why the 0-foils do not show any signs of early fracture during the tensile test. It should also be noted that all the AFM micrographs shown in Fig. 2 were scaled by a factor of 4 in the growth-direction to enhance the appearance of the surface topography. Accordingly, surface-phenomena are not expected to play a dominant role in foil-failure. Nano-indentation on a 0-type Ni-foil with a polished S-type Ni-substrate clearly showed the substrate to be the harder of the two. The smaller grain size in the S-type Ni substrate as compared to the 0-type Ni probably only partly explains this observed difference. Another contribution to the higher hardness in the substrate is mechanical polishing of the S-Ni substrate prior to nano-indentation. 4.3. Application of Ni-foils The difference in ductility observed for the two types of Ni-foils could prove decisive for the application of the foils as microphone membranes, where pre-straining and tuning of the membrane will leave the Ni-foil in a stressed state. A critical point in this context is how close to the onset of plastic deformation the foils are tuned in the microphone. A relatively brittle film, which can hardly accommodate plastic deformation, will be difficult to tune and handle without breaking it. For a ductile foil, plastic deformation may cause the foil to
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lose its tuned stress-state, if tuned beyond its yield strength. Further studies should, therefore, include stress-measurements on tuned microphone membranes by e.g. Xray diffraction measurements. 5. Conclusion Ni-foils with an optically mirror bright finish can be synthesised from a Watts-type electrolyte both with and without the use of the S-containing additive Na-saccharin. Ni-foils deposited with Na-saccharin are smooth and fine-grained with a nearly random grain-orientation and a typical grain-size of 10–20 nm. Weak N311M and N111M texture components were found with maximum pole densities of 2.2 and 1.9, respectively. Ni-foils deposited with no Na-saccharin addition show more nodular topography when studied by AFM and a grain size of 20–40 nm. Again weak N311M and N111M texture components were observed with maximum pole densities of 2.75 and 1.2, respectively. Both in terms of numbers and appearance, grain boundaries become more dominant in the internal microstructure of the Ni-deposit with the use of the Na-saccharin additive. This was observed by transmission electron microscopy in both planar- and cross-sectional studies. Mechanically, Ni-foils deposited with Na-saccharin are harder and more brittle than Nifoils deposited without Na-saccharin addition. The reduction in ductility, and hereby also in ultimate tensile strength, is believed to be caused by strain hardening caused by impurities of both sulfur and carbon codeposited into the Ni with the use of Na-saccharin as reported in previous studies. Na-saccharin, as used here, causes grain refinement and promotion of planar crystal growth, but as mentioned above the mechanical properties are deteriorated by growth-defects or foreign inclusions. S-type Ni-foils are more brittle and have lower tensile strength. This deterioration is attributed to segregation of sulfur and carbon in grain boundaries (due to the high grain boundary density) and, possibly, higher concentration of intrinsic defects. The fact that even in the ‘as-deposited’ state, S-type Ni-foils show reduced ductility as compared to the more pure 0-type Ni-foils, has not been reported previously and should be noted for all the many applications, where Na-saccharin is used primarily as a stressrelieving additive in Ni-electrodeposition. Acknowledgments The authors would like to thank Danish Micro Engineering AyS for giving the authors access to the Dualscope娃 AFM apparatus. Technician Flemming Bjerg Grumsen at DTU is kindly acknowledged for his assistance in obtaining the X-ray data presented here, as are
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M.Sc. Michael Eis Benzon and Fin Mertins for their help with the tensile testing of the Ni-foils. Prof. Sten ¨ Johansson at IKP, Linkoping University, is kindly acknowledged for his comments regarding the good match between measured mechanical data and the Hall– Petch equation. Financial support from the Danish Research Council (STVF) is gratefully acknowledged. References w1x J.P. Rasmussen, P.T. Tang, C. Sander, O. Hansen, P. Møller, Fabrication of an All-Metal Atomic Force Microscope Probe, Transducers ‘97, Chicago, June 16–19, 1997, pp. 463–466. w2x Tang, P.T., Ph.D thesis, Fabrication of micro components by electrochemical deposition, DTU, March 1998. w3x W. Ruythooren, K. Attenborough, S. Beerten, et al., J. Micromech. Microeng. 10 (2000) 101–107. w4x S. Abel, H. Freimuth, H. Lehr, H. Mensinger, J. Micromech. Microeng. 4 (1994) 47–54. w5x J. Gobet, F. Cardot, J. Bergqvist, F. Rudolf, J. Micromech. Microeng. 3 (1993) 123–130.. w6x W. Qu, C. Wenzel, A. Jahn, J. Micromech. Microeng. 8 (1998) 279–283. w7x V. Sommer, Galvanotechnik 91 (3) (2000) 791–797. w8x S.A. Watson, J. Edwards, Tram. IMF 34 (1957) 167. w9x D. Mockute, G. Bernotiene, Surf. Coat. Technol. 135 (2000) 42–47. w10x J. Edwards, Trans. IMF 41 (1964) 169. w11x R.J. Kendrick, Trans. IMF 40 (1963) 19. w12x ‘Die Warmebehandlung ¨ ¨ von Nickel’, Nickel Informationsburo ¨ GmbH Dusseldorf, 10–15 (1964). w13x H.J. Bunge, Texture Analysis in Materials Science: Mathemat¨ ical Methods, Cuvillier Verlag, Gottingen, 1993. w14x K. Pantleon, J.D. Jensen, M.A.J. Somers, X-Ray diffraction investigation of electrochemically deposited copper. In: Proceedings EPDIC-8 Conference, Uppsala Sweden, 23–26th May 2002, Materials Sci. Forum, in print. w15x American National Standards Institute: ASTM E 345-87, Standard Test Methods Of Tension Testing Of Metallic Foil, New York, 1987.. w16x American National Standards Institute: ASTM E 74-91, Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines, New York, 1991. w17x Danish Standardisation Council (Dansk Standardiseringsrad): ˚ DS 10 110, Metal Testing, Tensile Testing, Copenhagen, 1968. w18x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564–1582. w19x R.O. Hull, Proc. Am. Electroplater’s Soc. 27 (1939) 52–60. w20x DIN 50 957 (1978). w21x M. Saitou, W. Oshikawa, A. Makabe, J. Phys. Chem. Solids 63 (2002) 1685–1689. w22x E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, 7th edition, Butterworth & Heinemann Ltd, Oxford, 1992, ISBN 0 7506 10204. w23x K.E. Voelk, Nickel und Nickellegierungen, Eigenschaften und Verhalten, Springer–Verlag, Berlin, 1970. w24x D. Mockute, G. Bernotiene, R. Vilkaite, Surf. Coat. Technol. 160 (2002) 152–157. w25x W.D. Callister Jr., Materials Science and Engineering, an Introduction, 6th edition, John Wiley & Sons Publ, 2003. w26x D.A. Hughes, N. Hansen, Acta Mater. 48 (11) (2000) 2985–3004.