Electrophoretic deposition of ferrite

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Title: Electrophoretic deposition of ferrite
Author: Washburn, Cody
Abstract: The ability to integrate a material with a high permeability on chip, allows for magnetically coupled circuits and structures to be designed and incorporated along side CMOS circuitry. Devices ranging from A.C. transformers to magnetically driven MEMS structures can be designed and fabricated. Desirable characteristics of magnetic cores for integrated inductors and transformers are first high saturation flux in order to obtain high saturation current; high permeability to obtain high inductance; high resistivity to reduce eddy current loss at high frequencies and compatible deposition and patterning processes. High frequency magnetic materials are oxide based ceramics and are therefore difficult to evaporate, sputter, plate and selectively etched. ElectroPhoretic Deposition (EPD) is a method where insulating particles are imparted charge in a suspension and are made to deposit on an electrode by applying electric field. EPD has been extensively employed in depositing oxide based phosphors for display applications. In this study, ferrite particles have been prepared by grinding sintered toroids and deposited by EPD. The electrophoretic solution bath is composed of isopropyl alcohol with traces of Mg(N03)2 and La(N03)3 salts. Glycerol is added to the solution bath as a surfactant to promote increased substrate adhesion. The dissociation of magnesium nitrate in the solution bath charges the ferrite particles. An electric field of ~ 50-160 V/cm is applied with negative terminal connected to the wafer to be plated and aluminum electrode is used as the anode. The deposition process is found to be self limiting with the initial high elerophoretic current declining to 10% of its value in 10 minutes. The deposition rate and zeta potential measurements indicate a high particle velocity on the order 5.7x10-3 cm/s with an electric field of 160V/cm generated across the 2 cm electrode spacing. Pattern filling and conformal coverage in copper damascene planar microinductors has been investigated. A method to extracted permeability from S11 impedance analysis has been employed. It has been found that grinding process deteriorates magnetic response. With recent advances in magnetic particle technology for high frequency materials, these results enable unique hard and soft powder ferrite material to be selectively deposited in wide variety of CMOS and MEM’s based applications.
Record URI: http://hdl.handle.net/1850/5196
Date: 2006-02

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