High Current Integrated Microinductors and Microtransformers using Low Temperature Fabrication Processes
In many systems which utilise magnetic components, e.g., miniaturised DC‐DC converters, PCMCIA cards, and modem stand‐off transformers, the magnetic device is the largest single component in the package. Surface‐mount magnetic devices may be unacceptably thick where low profiles are required. The authors's approach to this problem is to use micromachining techniques to realise inductors and transformers built into the multilayer structure of a multi‐chip package, allowing compact integration with chips, sensors and other components. Microinductors and micro‐transformers composed of thick cores and multiwinding conductors have high inductance, high saturation current, and low resistance compared with previous integrated inductors. The total size of the microinductive device is 4 mm ×4 mm× 0.145 mm, having 156 turns of multilevel electroplated copper coils (40 μm thick) and electroplated permalloy magnetic core (35 μm thick). These devices have inductances up to 1.5 μH and current‐carrying capability of up to 3A steady DC current, making them applicable to power converters. The processing steps chosen are all low‐temperature, which allow the use of low‐cost substrates such as MCM‐L compatible materials.