Application of Ferrite Nanomaterial in RF On-Chip Inductors
Application of Ferrite Nanomaterial in RF On-Chip Inductors
Hua-Lin Cai,1,2 Jing Zhan,1,2 Chen Yang,1,2 Xiao Chen,1,2 Yi Yang,1,2 Bao-Yong Chi,1,2 Albert Wang,3 and Tian-Ling Ren1,2
1Institute of Microelectronics, Tsinghua University, Beijing 100084, China
2National Laboratory for Information and Science Technology, Tsinghua University, Beijing 100084, China
3Department of Electrical Engineering, University of California, Riverside, CA 92521, USA
Received 26 April 2013; Accepted 23 May 2013
Academic Editor: Yang Chai
Copyright © 2013 Hua-Lin Cai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Several kinds of ferrite-integrated on-chip inductors are presented. Ferrite nanomaterial applied in RF on-chip inductors is prepared and analyzed to show the properties of high permeability, high ferromagnetic resonance frequency, high resistivity, and low loss, which has the potential that will improve the performance of RF on-chip inductors. Simulations of different coil and ferrite nanomaterial parameters, inductor structures, and surrounding structures are also conducted to achieve the trend of gains of inductance and quality factor of on-chip inductors. By integrating the prepared ferrite magnetic nanomaterial to the on-chip inductors with different structures, the measurement performances show an obvious improvement even in GHz frequency range. In addition, the studies of CMOS compatible process to integrate the nanomaterial promote the widespread application of magnetic nanomaterial in RF on-chip inductors.
1. Introduction
For decades the RF on-chip inductors have a large occupation of chip area and low factor [1–4], it hindered the realization of high-performance RF circuits and systems. Nowadays, the situation becomes more and more serious, as the development of on-chip RF inductor cannot keep up with the pace of miniaturization of integrated circuit system [5]. There is an increasing demand for small-sized and high-performance RF on-chip inductors in CMOS circuits and systems. Many efforts are made to solve the problem, and the works about these are impressive. To improve the -factor, suspension and solenoid structure [6–9], thick insulating layer [10], grounded shield [11, 12], and high resistivity substrate structure [2, 13] are applied to reduce the substrate loss; coils made by high conductivity metal [14, 15] and low k interlayer dielectric [16] are applied to reduce the resistive loss; multilayer inductors are applied to reduce the area occupation [17, 18]. However, all the previosely mentioned works cannot both satisfy the demands of small size and high factor at the same time. Meanwhile most of them are not CMOS compatible.
Recently more and more attention is being paid to the integration of magnetic nanomaterial to the on-chip inductors, which can enhance the storage of magnetic energy and thus increase the inductance and reduce the area of on-chip inductors [19–21]. However, most of the adopted magnetic materials are ferromagnetic alloy [22–25], which lead to large magnetic loss and reduced in high frequency due to its low ferromagnetic resonance . On the other hand, ferromagnetic alloy film must be isolated from the coils due to its low resistivity, which limits its magnetic flux enhancing effect. Other methods to improve the inductors’ performance are also discussed in [26–31]. But most of these magnetic materials have their limitations. Compared with other materials, ferrite nanomaterial featured high , and low imaginary part of permeability can improve the factor of inductors in GHz range [32–36], which is expected to provide a perfect solution to this situation. Furthermore, nanostructure magnetic material also has high resistivity and hence is more likely to contact the coils directly and forms a fully filled magnetic structure [37–39], which will take advantage of magnetic flux enhancement effect in the maximum degree. Application of the ferrite nanomaterial shows a great prospect in RF on-chip inductors [38–41].
2. Materials
Magnetic nanomaterial will exhibit superparamagnetic property when the diameter of the particle is small enough. This phenomenon weakens the hysteresis of the magnetic material, which is beneficial to the property of low loss that RF on-chip inductors require. Ferrite has high relative permeability, low magnetic loss, and high resistivity and therefore is suitable to be used for RF on-chip inductors in GHz working frequency. Among all the ferrite material, spinel and magnetoplumbite ferrite stand out. Nanomaterials of Ni-Zn-Cu spinel and magnetoplumbite ferrite are mainly introduced with nanopowder-mixed-photoresist spin-coating and ink-jetting processes which are CMOS-compatible [42–45].
Ni-Zn-Cu and nanopowder are prepared through sol-gel material and self-progating method. The redox between nitrates and organic acids triggers self-propagating combustion when the sol-gel is heated to a certain temperature. The material preparation flow is shown below.(1)Dissolve the compound in a citric acid solution, and keep the solution density at 0.2 mol/L.(2)Heat the solution in an oven under a low temperature of 60°C for 48 h until the solution is transformed into a brown viscous gel.(3)Heat the viscous on hot plate at 200°C to dry; then ignite it to burn completely.(4)Ball-mill the remaining powders for over 72 h to get Ni-Zn-Cu nanoparticles of less than 100 nm diameter.
is prepared through solid reaction method, and the flow is shown below.(1)Mix all the ingredients evenly in alcohol, and then put in crucible.(2)Heat the mixture in 1270°C for 4 h during which the crystallization reaction proceed.(3)Ball-mill the powders for over 72 h to get nanoparticles of less than 100 nm diameter.
After the ferrite nanoparticles are prepared, mix them in photoresist with a proper percentage to get the composite ferrite nanomaterial. Then two methods are developed to apply the as-fabricated ferrite nanomaterial on the inductors. The first one is spin-coating, followed by heating and solidifying at 120°C to obtain the film with certain thickness. Then the film is patterned through an extra photolithography and etching processes. The SEM photo of ferrite film spin-coated on the substrate is shown in Figure 1. The second method is inkjetting. It is carried out by a microprobe dropping nanomaterial at the place of inductor coil area.
Figure 1: Ferrite nanomaterial film spin-coated on Si substrate.
The hysteresis loop of nanomaterial is shown in Figure 2. The prepared ferrite (mixed in photoresist) has a high saturation magnetization compared with other ferrite materials, low remanent magnetization , and low coercivity .
Figure 2: Hysteresis loops of Ni-Zn-Cu and .
Ring-shaped samples are pre (...truncated)
