Thin-film Inductors

What Are Thin Film Inductors?

Thin film inductors are passive inductive components fabricated by depositing and patterning conductive and, in some designs, magnetic material layers onto a substrate using thin film processing techniques. Unlike wound or molded inductors assembled from discrete wire coils, thin film inductors are formed through the same photolithographic and vacuum deposition processes used to build integrated circuits, making them compatible with monolithic fabrication on silicon, ceramic, or glass substrates. The resulting structures are planar or quasi-planar spiral coils, typically a few micrometers thick, capable of operating at radio frequencies well above the range accessible to conventional wound components.

The field draws on microelectronics fabrication, electromagnetic theory, and materials science. Thin film inductors occupy a central role in RF integrated circuits (RFICs), on-chip power management, and impedance matching networks, where the physical size and parasitic behavior of conventional wound inductors become unacceptable.

Fabrication and Conductor Materials

Thin film inductor coils are most commonly formed from electroplated or sputtered copper or gold, chosen for their low resistivity and compatibility with standard photolithographic patterning. The spiral geometry is defined by a lithographic mask, and the conductor is built up to thicknesses of several micrometers to reduce ohmic losses. Substrate selection strongly influences parasitic capacitance and substrate eddy current losses: high-resistivity silicon and glass substrates are preferred over standard silicon because standard silicon introduces significant eddy current damping at gigahertz frequencies. Research on on-chip integrated inductors with ferrite thin films, published through IEEE, established that integrating a magnetic core layer over the spiral can increase inductance density substantially compared with air-core designs. Ion beam etching and precision deposition techniques described by specialty thin film foundries allow inductor linewidths to be held to tight dimensional tolerances, directly controlling series resistance and self-resonant frequency.

Magnetic Core Integration

Air-core thin film inductors are limited in inductance density because the permeability of free space constrains how much flux a given geometry can store. Deposited magnetic core layers, typically ferrite compositions such as nickel-zinc or cobalt-zirconium, surround or overlay the spiral conductor to increase the effective permeability and raise inductance values without enlarging the footprint. The challenge is frequency-dependent loss: magnetic core materials exhibit increasing eddy current and ferromagnetic resonance losses as frequency rises above a few hundred megahertz. Selecting materials with high resistivity and low coercivity, as detailed in work on magnetic thin-film inductors for RF integrated circuits published in the Journal of Magnetism and Magnetic Materials, is central to pushing useful operating frequencies higher. Laminating the core into thin alternating layers of magnetic and insulating material further suppresses eddy currents.

Performance Characteristics

The key figures of merit for a thin film inductor are inductance value, quality factor (Q), and self-resonant frequency. Q, the ratio of inductive reactance to series resistance, determines how much energy the component stores versus dissipates per cycle. Thin film copper spirals on optimized substrates achieve Q factors in the range of 10 to 30 at frequencies between 1 and 5 GHz, which is adequate for many filter and matching applications. The PSMA overview of thin film inductors for integrated power conversion documents how Q and inductance density trade against each other as switching frequencies in power converters push above 100 MHz.

Applications

Thin film inductors have applications across a range of electronic systems, including: