Deposition of thick Co-rich CoPtP films with high energy product for magnetic microelectromechanical applications

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Abstract

We report the development of Co-rich CoPtP films of thicknesses up to 82 μm for use in magnetic MEMS applications. These films have been deposited using a combination of pulse-reverse plating with the addition of stress-relieving additives to the bath. The films were electroplated on sputtered Cu/Ti seed layer on silicon with an optimized thickness of 100/20 nm. The composition, crystalline structure, grain size and magnetic properties of the CoPtP films of varying thickness are compared and analyzed. The 3-μm-thick CoPtP film showed a columnar structure and strong perpendicular anisotropy. This film shows a perpendicular coercivity of 2150 Oe, a remanence of 0.564 T and a maximum energy product of 20 kJ/m3. As the thickness of the plated film is increased, there is a gradual decrease in the coercivity and anisotropy. The 82-μm-thick film had a perpendicular coercivity of 1150 Oe and a remanence of 0.35 T. While there is a drop in coercivity and anisotropy, the remanence and maximum energy product remain constant for film thicknesses greater than 13 μm. The reason for the drop in coercivity and the near-constant remanence for thicker CoPtP films is discussed here. The coercivity of the thick Co-rich CoPtP film reported in this work is similar to those reported in the literature; the values of remanence, maximum energy product and saturation magnetization are the highest of all the thick (>50 μm) electroplated films in the literature.

Introduction

The rapid advances made in the field of magnetic MEMS [1], [2], [3], [4], [5], [6] have highlighted the need to develop new hard magnetic materials capable of retaining their magnetic properties up to thicknesses of tens of microns using silicon-compatible deposition processes. To date, Fe50Pt50 (FCT phase) [7], [8], Co50Pt50 (FCT phase) [9], Co-rich CoPtP (hcp phase) [10], [11], [12], [13], [14], CoNiP [15] and CoMnNiP [15] hard magnetic materials have been successfully electrodeposited from aqueous electrolytes. CoNiP and CoNiMnP have previously been reported to maintain perpendicular anisotropy at thickness up to 45 μm, but their energy product, (BH)max, is small compared to the electrodeposited CoPt and FePt hard materials [16]. Although Co50Pt50 and Fe50Pt50 (FCT phase) have higher (BH)max compared to Co-rich CoPtP (hcp phase) [17], the post-deposit annealing required to synthesize the hard magnetic fct phase makes it unsuitable for batch-fabrication in any CMOS compatible process [18]. Fujita et al. [19] have developed up to 40-μm-thick Co50Pt50 films using the pulse plating technique; however, they were unable to crystallize the as-deposited FCC structure to FCT phase even after annealing at 973 K. An alternative to the above materials is to use Co-rich CoPtP magnets which can exhibit large perpendicular magnetic anisotropy in their as-deposited state, due to the presence of a Co-hcp hard magnetic phase [13]. Previous works [12], [13] have demonstrated the use of the electroplating technique for deposition of Co-rich CoPtP films, having a single hcp phase. They reported electroplated films with large coercivities of 6 kOe for thickness up to 1 μm. As most of the existing electrodeposition techniques for Co-rich CoPtP are employed for magnetic recording media, the investigated film thickness has been limited to less than a few microns. To date, Cavallotti et al. [20] have reported on thick Co-rich CoPtP with a maximum coercivity of 2 kOe for a maximum film thickness of 10 μm.

Although the electroplated Co-rich CoPtP films reported previously have higher values of coercivity, the residual stress in these films is generally too high to satisfy MEMS processing requirements. It is particularly difficult to electroplate films thicker than a few microns with good magnetic properties because electrodeposition tends to induce high residual stress [16]. To overcome stress in the plated films, in our recent work [21], we use a novel method [22] of deposition that involves a combination of pulse-reverse (PR) plating and the addition of stress-relieving additives such as saccaharin, to plate thick (3 μm), stress-free, and nano-structured Co-rich CoPtP films with an improvement in maximum coercivity over direct current plating. In this paper, we are the first to report the deposition of CoPtP films with a thickness of 82 μm with relatively high remanence and high (BH)max, using similar plating conditions, with optimized plating parameters. Further, we study and analyze the effect of increasing thickness on the crystalline and magnetic properties of the different plated films. We analyze the effect of change in Pt% and P% on the grain size and magnetic properties of thick films.

Section snippets

Experiment

Co-rich CoPtP films were galvanostatically deposited using a bath [23] consisting of 0.1 M of cobalt sulfamate, 10 mM of Pt P-salt, 0.1 M of dibasic ammonium citrate, 0.1 M of glycine, and 0.1 M of sodium hypophosphate. Along with this 4.5 g/l of saccharin (stress-relieving additive) was added to the bath. The pH of the solution was adjusted to 8 by adding sodium hydroxide. Co pieces were used as anode and silicon pieces (∼2 cm2 area) with sputtered (100/20 nm) Cu/Ti seed layer were used as cathode. In

Results and discussion

From the seed layer optimization experiment, 100/20-nm-thick Cu/Ti seed layer on silicon gave the maximum coercivity as compared to other thicknesses of Cu/Ti seed layers. Hence, 100/20-nm-thick Cu/Ti seed layer was used as a cathode for depositing the films. As mentioned previously, all the films showed a columnar structure. The 3- and 13-μm-thick films were stress free, but the remaining films appeared to be stressed with the presence of micro (film thicknesses<40 μm) to large cracks (film

Conclusion

In this work, we report the use of a combination of the pulse-reverse (PR) plating technique and stress-relieving additives such as saccharin, to plate thick stress-free Co-rich CoPtP films. We use the optimized plating conditions for pulse-reverse plating to deposit Co-rich CoPtP films with 3, 13, 28, 40, 50 and 82 μm thickness on 100/20-nm-thick sputtered Cu/Ti seed layer on silicon. We study the effects of varying film thickness on the surface morphology, composition, grain size and magnetic

Acknowledgement

This work is supported by the Science Foundation of Ireland (SFI) Principal Investigator (PI) Grant no. 06/IN.1/I98 and by the European Union Framework 6 STREP project VIBES _No. 507911. One of the authors acknowledges Dr. F.M.F. Rhen for useful discussions.

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