Nanoscale structures with magnetic properties

Paul Boughton

Materials scientists at the National Institute of Standards and Technology (NIST) in the USA have developed a process to build complex, three-dimensional nanoscale structures of magnetic materials such as nickel or nickel-iron alloys using techniques compatible with standard semiconductor manufacturing.

The process, described in a recent paper,* could enable the development of whole new classes of sensors and micro-electromechanical system (MEMS) devices.

The NIST team also demonstrated that key process variables are linked to relatively quick and inexpensive electrochemical measurements, pointing the way to a fast and efficient way to optimize the process for new materials.

According to the researchers, the new process is a variation of a technique called Damascene metallisation that often is used to create complicated three-dimensional copper interconnections for linking circuit elements across multiple layers in advanced, large-scale integrated circuits.

Named after the ancient art of creating designs with metal-in-metal inlays, the process involves etching complex patterns of horizontal trenches and vertical 'vias' in the surface of the wafer and then uses an electroplating process to fill them with copper. The high aspect ratio features may range from tens of nanometres to hundreds of microns in width. Once filled, the surface of the disk is ground and polished to remove the excess copper, leaving behind the trench and via pattern.
The challenge in Damascene metallisation is to ensure that the deposited metal completely fills in the deep, narrow trenches without leaving voids. This can be done by adding a chemical to the electrodeposition solution to prevent the metal from building up too quickly on the sides of the trenches and by careful control of the deposition process, but both the chemistry and the process variables are significantly different for active ferromagnetic materials than for passive materials such as copper. In addition to devising a working combination of electrolytes and additives to perform Damascene metallisation with nickel and a nickel-iron alloy, the NIST team demonstrated straightforward measurements for identifying and optimising the feature-filling process, thereby providing an efficient path for the creation of good quality nanoscale ferromagnet structures.

It is believed that the new process makes it feasible to create complex three-dimensional MEMS devices such as inductors and actuators that combine magnetic alloys with non-magnetic metallisations such as copper interconnects. Moreover, this can be achieved using existing production systems.

* CH Lee, JE Bonevich, JE Davies and TP Moffat: 'Magnetic materials for three-dimensional Damascene metallization: void-free electrodeposition of Ni and Ni70Fe30 using 2-mercapto-5-benzimidazolesulfonic acid.' Journal of The Electrochemical Society, 155 (7) D499-D507 (2008).

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