MEMS Micro-coils for Magnetic Stimulation of Brain Tissue

Implantable micromagnetic stimulation (μMS) is an emerging method of brain stimulation. Unlike traditional electrodes, the conductive metallic materials in the μMS device avoid direct contact with neural tissues. Furthermore, in contrast to
transcranial magnetic stimulation (TMS), μMS employs compact implantable coils, achieving superior spatial and temporal precision. Such contact-free neural stimulation techniques, utilizing microelectromechanical systems (MEMS) coil
technology, hold the promise to enhance the longevity and efficiency of neural interface devices. However, duo to its nascent stage in neuroscience, there are certain unresolved issues that the technology still grapples with. This research addresses several of these challenges

This doctoral research investigates the design and fabrication methodologies for micro-coils. This thesis introduces a MEMS fabrication technique distinguished by three main attributes: (i) a multilayer resist lift-off procedure for patterning metal films up to 1800 nm in thickness, with a rigorous approach to ensuring highconductivity thin films via physical vapor deposition, (ii) the encapsulation of all micro-coil Al wires within a minimum of 200 nm of atomic layer deposition (ALD) alumina and 6 μm of parylene C, ensuring a leak resistance exceeding 210 GΩ, and (iii) the integration of a multi-step deep reactive ion etching (DRIE) method with maskless photolithography, facilitating adaptive design and fabrication. By circumventing the use of silicon on insulator (SOI) wafers and lithography mask fabrication, the duration from design to device has been notably reduced. The resultant probes exhibit a resistance under 5 Ω, markedly lower than previous
reported micro-coils.

Furthermore, this novel methodology can be employed to produce micro-coil probes compatible with two-photon microscopy. In vivo tests on anesthetized mice demonstrate the micro-coils' capability in modulating neuronal activity in specific regions, with neuronal reactions detected through fluorescence fluctuations in proximity to the micro-coil. Such observations underscore the potential of micro-coils in neural prosthetics, particularly in artificial vision.