et forces are driving power systems to further levels of integration and miniaturisation — from the power system in package (PSiP) to the power system-on-chip (P-SoC). The successful integration of passive components required by power systems, such as the power inductor, has so far been a barrier to further miniaturisation of power systems. A new development from Enpirion promises to break down this barrier and open the door to high volume, low cost P-SoCs with integrated inductors.
In November Enpirion announced that it was commercialising its magnetic alloy material, which enables the construction of passive magnetic power components directly onto a silicon wafer. Another announcement quickly followed, announcing a global deal with distributor Future Electronics for the company’s first products; a family of low power power-system-on-chip PoL DC-DC converters, which they claim are the world’s lowest cost.
The EL700 power system-on-chips are based on Enpirion’s technology which allows a power inductor to be built directly on top of a 6 inch or 8 inch wafer, with potential for achieving a monolithic power system-on-chip (P-SoC) in the near future.
Wafer Level Magnetics
The company’s wafer level magnetics (WLM) technology is based on its proprietary Iron-Cobalt alloy which it calls FCA. Unlike existing MEMS materials, FCA keeps its magnetic properties up to and beyond 20MHz, including high resistivity, low coercivity and high effective permeability. This means FCA can be used for power inductor cores which operate at extremely high frequencies — the switching frequency of the EL711 is 18MHz. The ability to increase the frequency to these never before seen levels means the devices can be extremely small, which is the first step towards integrating them onto a chip.
“Simply put, high operating frequency requires less energy to be stored in each cycle in the inductor thus we can use smaller inductors with thin magnetic cores,” Dr. Liakopoulos explained. “Additional to the wafer level magnetics and careful inductor design, our advanced LDMOS processing and physical device design enable high efficiency power switching. Finally, packaging and overall product layout design with emphasis on minimising parasitic related losses also contributes to the overall product efficiency at high switching frequencies.”
In the EL711, the power inductor has been turned from a three-dimensional coil around a core into a two-dimensional thin film device. The magnetic core, essentially a pattern of FCA, is electroplated onto a blank silicon wafer (see Figure 1). Wafer die are then inverted and assembled over a coil on the substrate, making a type of magnetic MEMS (MMEMS) device with no moving parts.
“The FCA deposition process is designed to be fully CMOS compatible and can be post processed on a 6 or 8 inch CMOS wafer. This is a key enabling property, allowing us to execute our roadmap towards the complete monolithic inductor,” Dr. Liakopoulos said.
Figure 1. The construction steps for Enpirion’s EL711 PoL converter. The FCA core is electroplated to a blank silicon wafer, then assembled above a copper coil to form the power inductor
The development of the FCA material has been instrumental in the progress of wafer level magnetics, but transferring the electroplating of this new material to volume production was also a major challenge. As a brand new material, there weren’t any foundries that had any experience with FCA, so Enpirion faced a large cost barrier to move the technology to production.
“In order to lower this barrier, we had to design a simple but efficient silicon-based inductor that would allow an attractive product introduction at mature process costs — we achieved that by separating the FCA core deposited on silicon and the coils built on the substrate,” Dr. Liakopoulos explained.
The company developed and qualified a pilot FCA process module in-house before transferring and embedding it as a complete module in a wafer manufacturing facility in Asia.
“That accelerated the FCA volume manufacturing dramatically, mitigating process development risk in a manufacturing environment and reducing the process to production cost development,” he added.
The construction of the wafer level magnetics will likely be done in post-CMOS back-end of line (BEOL) fabs, since their less demanding process constraints can offer better cost structure compared to high-end CMOS processing facilities. Dr. Liakopoulos said that wafer processing fabs with experience in electroplating thick copper for redistribution layers and electroplating solder bumps could be enabled to build wafer level magnetics.
It is, of course, the economics of silicon wafers over traditional discrete bulk magnetics that holds the key to the technology’s low cost; the EL711 is priced at US $0.57 in volumes of 1k units, which Enpirion claims makes it the world’s lowest cost P-SoC DC-DC converter. Since the products use just two silicon die, one for the inductor and another for both power FETs and the controller, and no discrete passives, the product can take advantage of the cost benefits of wafer batch processes.
The company says it is already looking towards a future when it will be able to produce more complicated MEMS structures, with a view to monolithically integrating power inductors. The company has already produced prototypes of more advanced structures, such as copper windings plated directly onto an FCA core. According to Dr. Liakopoulos, once the FCA process becomes more mature, these complex structures can be introduced in a more efficient and cost effective way than is currently possible. The company plans to create a completely monolithic power SoC by 2016 (see Enpirion’s roadmap in Figure 2).
Figure 2. Enpirion’s technology roadmap promises a monolithic power SoC by 2016
“Enpirion’s core business is integrated power management and we are focused on producing the most cost effective magnetic devices... In this aspect, we are committed to a very clear roadmap,” said Dr. Liakopoulos. “On the other hand, the ability to combine a high performance magnetic alloy, like the FCA, with advanced MEMS processing techniques (multilayers of dielectrics and thick copper, for example) will enable all kinds of wafer level magnetic devices for a variety of products.”
Some of the possible products include micro transformers for power management or signal isolation, magnetic sensors such as fluxgate sensors and even more complicated micro-electromagnetic actuators, for applications in the portable navigation, bio-medical and aerospace industries. Dr. Liakopoulos said that the company would also consider discussing potential licensing of FCA and its WLM technology to non-competing fields of use, if there is an interest.