Composite ferromagnetic photoresist for the fabrication of microelectromechanical systems

Abstract

This paper describes a simple method for the microfabrication of mechanically compliant, magnetically-responsive microstructures. These microstructures were fabricated in one step by using a ferromagnetic photoresist, which, in turn, was prepared by suspending nickel nanospheres in a negative photosensitive epoxy (SU8). The nominal diameter of the nickel nanospheres was 80–150 nm, that is, much smaller than the wavelength of the UV light (365 and 405 nm) used to expose the photoresist. Diffraction and scattering of light from the nanospheres allowed for full exposure of the photoresist, even after the incorporation of nanospheres at levels at which it became opaque. The ferromagnetic photoresist was cross-linked after exposure and development, and yielded a stable, compliant, ferromagnetic pattern. The paper characterizes the effect of the weight density of the nickel nanospheres on the transmittance of films made by this technique at wavelengths from 330 nm to 610 nm. It also describes a number of microstructures made with the photoresist: examples include lines, posts and meshes. As a demonstration, the procedure was applied to the microfabrication of a set of magnetically-actuated micromirrors. These micromirrors achieved large deflections: deflection at the tip of a 12 mm long, 250 µm wide and 70 µm thick cantilever of the ferromagnetic photoresist exceeded 1.4 mm, when actuated by a NbFeB permanent magnet with field strength ∼120 mT. The cantilever maintained its mechanical properties after cycling ∼106 times.

A photopatternable superparamagnetic nanocomposite: Material characterization and fabrication of microstructures

“…Using a photosensitive polymer in combination with magnetic particles has been reported for the fabrication of microstructures with feature widths larger than 5 m by conventional photolithography using ferromagnetic particles [8,9]. However, when further device miniaturization is desired the processing of composite materials that contain particles dispersed in a matrix becomes a challenge.…”

Characterisation of the fabrication process of freestanding SU-8 microstructures integrated in printing circuit board in microelectromechanical systems

“…Several authors have proposed different contributions to this area, coating SU-8 over PCBs for encapsulating microchips [3], as planarization layers [4] and also for microchannel manufacturing [5]. Another remarkable application is the use of conductive SU-8 [6] or ferromagnetic SU-8 [7], making possible an electronic integration of SU-8 and…”

Single cell manipulation using ferromagnetic composite microtransporters

“…Using a similar composition, magnetically actuated micromirrors have been fabricated with nickel nanospheres and SU8 photoresist. 20 In contrast to nickel or neodymium particles, magnetite particles are biocompatible which is an essential property needed for working with living cells. There is also a parallel line of research where magnetite particles are mixed with polydimethylsiloxane to fabricate flexible, ferromagnetic polymer structures such as millimeter-sized swimmers 21 and tools.…”

“…Although magnetite nanoparticles are opaque, standard lithography still works as reflection, scattering and diffraction of light from the particles assist in the proper exposure of the photoresist. 20 The ferromagnetic photoresist was prepared by mixing iron oxide powder ͑spherical, 50 nm in diameter, Alfa Aesar, IL, USA͒ with SU8-25 ͑Micro-Chem, MA, USA͒ in a glass Petri dish until it yielded a homogenous suspension. We waited one hour before using the photoresist to eliminate entrapped air bubbles.…”

“…The microfabrication process can also be improved by measuring the optical properties of the ferromagnetic photoresist and accordingly optimizing the exposure time. 20 In this letter, a simple method for the fabrication of truly micron-sized, ferromagnetic structures was implemented to fabricate microtransporters. We also demonstrated that single cells can be separated based on their shape without disturbing the local environment.…”