Electrochemical Process for Fabrication of Laminated Micro

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Electrochemical Process for Fabrication of Laminated Micro

Transcript Of Electrochemical Process for Fabrication of Laminated Micro

Electrochemical Process for Fabrication of Laminated Micro-Inductors on Silicon
Ricky Anthony, Cian Ó Mathúna, James Rohan 1
Micro-Nano Systems Center, Tyndall National Institute, University College Cork, Lee Maltings, Ireland 1e-mail: [email protected]
Keywords: DC-DC converters, eddy current loss, electroless deposition, magnetics on silicon, Micro-
inductor.
Abstract: Soft metallic magnetic alloys (such as Co, Ni and Fe based alloys) have been extensively used as core material in integrated inductors and transformers for DC-DC conversion. Compared to ferrite materials, these materials have excellent soft magnetic properties such as high saturation flux density, low coercivity and high permeability. But low resistivity values make it susceptible to eddy current losses [1]. As a result, core material (e.g. Ni45Fe55) thickness is limited by the skin depth depending on operational frequency (10 MHz-100 MHz). This however reduces the inductance density substantially. Subsequent laminated core structures where magnetic layer is separated by a thin dielectric film can increase the overall core thickness without compromising on its power handling capability. In this work, a CMOS compatible electrochemicalresist-electrochemical based lamination technique has been described whereby magnetic core in racetrack micro-inductor can be laminated. Previous lamination processes involve depositing electrophoretic resist and nickel as seed layer [2]. This needs multiple step seed deposition process prior to metal deposition. More recently, depositing copper between magnetic films successively followed by selective Cu etching has been reported [3-4]. These processes are either time consuming, involve additional process steps or require dedicated equipment. SU-8 is widely used permanent photoresist in MEMS based device fabrication. We report a new metal deposition process on dielectrics such as SU-8. This induces no change in SU-8 electrical properties as suggested previously [5]. The SU-8 film is chemically activated and thin permalloy films are deposited electrochemically in an in-house developed bath [6]. Figure 1 depicts the scanning electron micrograph (SEM) of 210 nm thin uniform permalloy deposited on planar SU-8 electrochemically without any seed layer. This process is repeated on patterned SU-8 tests structures and selective deposits (~ 1.4 µm) on SU-8 could be attained in silicon substrate (figure 2). The process is integrated in a CMOS fabricated micro-inductor. A conformal ~1 µm thick deposition on 90 µm thick SU-8 pattern is achieved (figure 3). Interestingly, device under-cuts are also well covered by permalloy. This indicates the accessibility of electrochemical deposition to regions which is not possible by line-of-sight deposition processes (as observed from figure 4). The next process would be to deposit > 2 µm cores on thinner dielectrics such as parylene-C and SU-8. This would allow more flux to confine in core material, thereby increasing the inductance density. Nevertheless, this new reported lamination technology could be extended in batch fabrication process of micro-inductors/transformers with thinner dielectric separation (like SU-8) for eddy-current loss suppression for DC-DC converter circuitry.
Abstract: The authors wish to acknowledge Central Fabrication Facilities (CFF) at Tyndall National Institute for the support and European Union for funding the work through FP7 (Project: PowerSwipe) under grant no.: 318529.
References
[1] R. Meere, T. O’ Donnell, N. Wang, N. Achotte, S. Kulkarni and S.C. Ó Mathúna, IEEE T. Magn. 45 (2009), pp.4234-4237.
[2] M. Brunet, T. O’ Donnell, A.M. Connell, P. McCloskey and S.C. Ó Mathúna, IEEE Microelectromech. Syst., 15 (2006), 94-100.

[3] M. Kim, J. Kim, F. Herrault, R. Schafer and M.G. Allen, J. Micromech. Microeng., vol. 23 (2013), pp.1-9.
[4] J.W. Park, F. Cros and M.G. Allen, IEEE T. Magn., vol. 39 (2003), pp. 3184-3186. [5] S. Jiguet, A. Bertsch, H. Hofmann and P. Renaud, Adv. Func. Mater., vol. 15 (2005), pp.1511-1516. [6] R. Anthony, B. J. Shanahan, F. Waldron, C. Ó Mathúna and J.F. Rohan, Anisotropic Ni-Fe-B Films
with Varying Alloy Composition for High Frequency Magnetics on Silicon (under review).

Figure 1. Cross-section of test structure with 210 nm permalloy.

Figure 2. Cross-section of patterned SU-8 test structure with insets depicting the uniform permalloy deposits on SU-8 side-wall.

Figure 3. Complete inductor structure with electrochemically laminated core. Deposits observed in inaccessible regions.

Figure 4. Cross-section of a micro-inductor with top core laminated with permalloy. The over-etch regions are well covered with permalloy.
PermalloyCore MaterialProcessCoreElectrochemical Process