Transformers

In the recent year, the small dimension and high-efficiency transformers/inductors are greatly needed due to the rapid increase of portable and miniaturized electronic products such as cellphone, notebook, digital camera, etc. In order to reach these demands, micro transformers/inductors fabricated by MEMS process have been developed. Up to now, MEMS transformers/inductors which have been proposed for the use of switching power converters or signal isolators can be divided into two types:

planar spiral type and 3D solenoid type. Both two types have their own advantages and disadvantages.

For the planar spiral type, a transformer/inductor is commonly composed of planar

spiral coils, isolating layers and magnetic films and fabricated on silicon substrate using MEMS techniques. Kim’s group [60-62] presents several planar spiral type inductors using magnetic thin film core of Fe-Zr-C-N nanocrystalline, Ti/FeTaN and FeBN.

However, the inductance of inductor is dropped slightly due to eddy current. Wang et al.

[63] present gapped and ungapped micro-machined inductors realized on a silicon wafer using low temperature IC compatible electrochemical processes. Two layers of bottom FeCoBN and single layer of top FeCoBN are electroplated as magnetic thin film core.

The ungapped inductor has a higher inductance value compared to the gapped inductor.

However, the inductance of the ungapped inductor drops much more rapidly with bias current. The planar spiral type transformer/inductor usually has larger size, and the magnetic flux is perpendicular to the plane of substrate and magnetic core, which causes the eddy current loss in the substrate and magnetic core. So, some solutions have been developed to reduce the eddy current loss. Yoon’s group [64-67] presents a CMOS-compatible versatile thick-metal surface micromachining technology which enables to build 3D metal microstructures on standard silicon substrate as post-IC processes at low temperature below 120 C. Spiral inductors suspended 100 μm over the substrate, coplanar waveguides suspended 50 μm over the substrate, and complicated micro-coaxial lines, which have 50 μm suspended center signal lines surrounded by inclined ground shields of 100 μm in height are demonstrated. Chong et al. [68] present

the performance of spiral transformers on silicon substrate with micro-porous silicon (PS) region. The use of PS significantly reduces the substrate effects including eddy current and capacitive coupling between spirals and the substrate and lead to higher quality factor and resonant frequency, mutual reactive coupling coefficients with larger useable band width and higher available gain mainly because of the reduction in power loss to the substrate. Zhao et al. [69] present a planar inductor with the magnetic core of permalloy-SiO2 granular film. By controlling the composition and microstructure, a permalloy-SiO2 granular film with excellent soft magnetic properties and high electrical resistivity is produced. Yunas et al. [70, 71] present planar transformers with stacked double coil structure on high resistive glass substrate, which introduces the simple micromachining fabrication process with a bonding step. The planar transformers on glass substrate is flipped and bonded on the etched silicon substrate. The eddy current is reduced due to the air gap between coils and silicon substrate.

For the 3D solenoid type, the key challenge of miniaturized transformers/inductors is to construct the 3D structure and magnetic core which are more complex than that of planar spiral type. Laney et al. [72] present a set of microwave inductors and transformers fabricated in a solenoid design utilizing two metal layers rather than a single metal layer as used in conventional planar magnetic devices. The fabrication process utilized a production Si/SiGe HBT technology with standard metallization and a

thick polyimide dielectric. Yoon’s group [73] presents a fabrication process for monolithic integration of solenoid inductors. The fabrication of air bridges of inductor is possible by forming a three-dimensional photoresist mold using multi exposures with varying exposure depths, following by a single development step, which realizes the 3D latent image of the unexposed volume in the photoresist. Yoon’s group [74, 75] also presents a sacrificial metallic mold (SMM) method to fabricate a solenoid-type microwave transformer on a glass wafer. The SMM method requires a single seed metal layer and provides a thermally-stable metal mold for successive thick photoresist patterning and electroplating. Xu et al. [76] present a solenoid-type micro transformer with a laminated core structure for high frequency power or signal conversion. The laminated core structure has been adopted and implemented by using micromachining techniques to reduce the eddy current loss. Zhuang et al. [77] present a solenoid type inductor with the magnetic core of Cr/Fe10Co90 /Cr films which is performed by magnetron sputtering at room temperature under a dc magnetic field. Park et al. [78]

present a solenoid-type inductor with highly laminated magnetic cores for low-megahertz power applications. The magnetic core of inductor has 72 laminations of 1 μm thick Ni/Fe films. A laminated core is used for reducing the eddy current in the electroplated Ni/Fe core. Gao et al. [79] present a solenoid-type micro inductor fabricated by MEMS technique, and the NiFe film is electroplated as the magnetic core,

and the polyimide which has low permittivity is used as the isolation material. Lei et al.

[80] present a solenoid-type micro inductor with the Fe-based magnetic core. The magnetic core of FeCuNbCrSiB soft magnetic thin film is deposited by magnetron sputtering and patterned by UV-photolithography.

Although many manufacturing processes and materials have been proposed, in order to meet the requirement of increasingly miniaturized electronic component, further investigations are needed to develop high performance transformers/inductors.