Development of electroplated magnetic materials for MEMS

More documents

Recommendations

Info

ARTICLE IN PRESS 196 N.V. Myung et al. / Journal of Magnetism and Magnetic Materials 265 (2003) 189–198 Fig. 7. Dependence of magnetic saturation with deposit P content for two different CoNiP electrodeposited thin films. Fig. 9. Hysteresis loop (a) and 2nd quadrant (b) B2H curves for various electrodeposited magnetic alloys. Fig. 8. Coercivity of electrodeposited NiP 40Co60NiP, 85Co15NiP and CoP films: All films were electrodeposited from chloride baths. also seen that the alloy that is richer in Co can incorporate more phosphorus. Fig. 7 shows how the magnetic saturation of CoNiP alloys varies with deposit P content. It shows that B S varies linearly with P content and that Co-based alloys have higher magnetic saturation, as cobalt has a higher B S value. The variation of coercivity of different phosphorus containing alloys with the P content is shown in Fig. 8. Non- Ni containing alloys, such as CoP, exhibit increasing coercivity with increasing P incorporation into the alloy. On the other hand, Ni containing alloys show this increase in coercivity with increasing amount of P in the alloy, only for low P concentrations. As more phosphorus is added to these alloys, the coercivity reaches a maximum, followed by a significant reduction at high P concentrations. The highest coercivities were observed for the (85Co15Ni) 100 X P X films. Fig. 9 shows the hysteresis loops (Fig. 9a) and 2nd
ARTICLE IN PRESS N.V. Myung et al. / Journal of Magnetism and Magnetic Materials 265 (2003) 189–198 197 Table 1 Stress and magnetic properties of electrodeposited Ni, (Co 85 Ni 15 ) 100 X P X , and Ni/(Co 85 Ni 15 ) 100 X P X /Ni composite films Electrodeposited film Thickness (mm) Stress (MPa) H c;parallel (Oe) B S =T S parallel Ni a 5 100 21 0.6 0.02 (Co 85 Ni 15 ) 100 b X P X 5 200 324 1.3 0.40 Ni/(Co 85 Ni 15 ) 100 X P X /Ni 0.8/3.3/0.8 total of 5 66 294 1.1 0.42 a Ni bath composition: 1 M nickel sulfamate+0.5 M boric acid+10 mM saccharin; pH=4, current density=10 mA cm 2 . b CoNiP bath composition: 0.2 M NiCl 2 +0.206 M CoCl 2 +0.7 M NaCl+0.4 M boric acid+0.009 M NaH 2 PO 2 +75 mM saccharin, pH=4, current density=10 mA cm 2 . quadrant B2H curves (Fig. 9b) for these various electrodeposited alloys. 2.2.2. Corrosion resistance of hard magnetic materials As shown in Fig. 2, electrodeposited cobalt is much more susceptible to corrosion attack than either nickel or permalloy. Non-magnetic alloying elements (e.g. phosphorus, tungsten, manganese) are not beneficial in this respect and can significantly lower the corrosion resistance of these alloys. Furthermore, it has been observed that some of these hard magnetic materials are etched away during the photoresist removal step during lithographic patterning. It is therefore recommended that these magnetic materials have an overcoat of high corrosion resistance materials (e.g. Au, Pt, Ni) in order to prevent their dissolution during MEMS processing. 2.2.3. Residual stress of hard magnetic materials In general, hard magnetic alloys become mechanically brittle and hard as their magnetic hardness increases. Therefore, this reduction in mechanical ductility results also in hard magnetic materials having high residual stress. Fig. 8 shows the residual stress of electrodeposited CoNiP and CoP films plotted as a function of the P content. Electrodeposited CoNiP and CoP films have a much higher (above 200 MPa) tensile residual stress than pure nickel or cobalt films and show a maximum in the residual stress when the electrolyte baths concentrations of NaH 2 PO 2 are in the range of 1–2 g/l. In order to lower the residual stress while yet maintaining high coercivity, composite films consisting of these highly tensile magnetic films sandwiched between compressive films can be electrodeposited. Table 1 shows plating conditions and film characteristics of a multilayered composite consisting of electrodeposited alternating layers of Ni and (85Co- 15Ni) 100 X P X films. It shows that in fact, residual stress can be reduced by producing such composite materials, in the case, from a residual stress of around 200 MPa for the alloy film down to 66 MPa to a multilayered composite. Ni/CoNiP composite films can maintain their hard magnetic properties while reducing the film stress. 3. Conclusion Magnetic materials, particularly ferromagnetic materials, have many uses in MEMS. Although soft magnetic materials have found the greatest utility in MEMS, improved processes for integrating hard magnetic materials with MEMS are being developed. These electrochemical processes and resulting properties of the deposited magnetic films must be further optimized for use in high-force microactuators and low-power microsensors. Acknowledgements The authors wish to acknowledge thier collaborators, including T. George, D. Wiberg, K.-ah Son, B. Eyre, O. Orient, D. Miller from JPL and K. Nobe, M. Schwartz and J.W. Judy from UCLA. This work was supported by the NASA Code-R and DARPA MEMS Program DABT63- 99-1-0020.
Page 1 and 2: ARTICLE IN PRESS Journal of Magneti
Page 3 and 4: ARTICLE IN PRESS N.V. Myung et al.
Page 5 and 6: ARTICLE IN PRESS N.V. Myung et al.
Page 7: ARTICLE IN PRESS N.V. Myung et al.

Development of electroplated magnetic materials for MEMS

Create successful ePaper yourself

Delete template?

Save as template?