MULTI-LAYER MICRO COILS IN PARYLENE FOR WIRELESS BIOMEDICAL SENSING

The goal of this project was to design, fabricate and test a multi-level micro coil, encased
in parylene, to serve as part of a wireless resonant sensing circuit for a biomedical sensor.
The goal was motivated by researchers at The Jackson Laboratory, a biological testing
facility on Mount Desert Island, who are seeking a means to monitor, wirelessly the
intraocular pressure in mouse eyes to study the genetics and progression of glaucoma. A
mouse’s eye is approximately 1 mm in diameter, so any sensor that is to be physically
attached or implanted has to be very small. Additionally, in order to perform long term
monitoring (days) on an unanesthetized, freely moving mouse requires wireless
interrogation. To address this problem, a microfabricated resonating bio-implantable
pressure sensor no greater than 100 μm in all three dimensions is proposed.
For this purpose multilayer micro coils with outer diameters from 100 μm to 1000 μm in
size were designed and fabricated using microelectronic device fabrication techniques.
Theoretical equations were applied to an equivalent circuit model of a multilayer micro
coil with which the equivalent circuit component values were determined based on coil
geometry. The coils’ S11 parameters were measured using a vector network analyzer, and
non linear regression was used to determine the equivalent circuit component values from
the S11 data. Coils bound to the substrate on which they were fabricated and released
coils were both tested. Also, the temperature response of the coils was examined.
The experimental measurement results are compared with the theoretical values for the
electrical characteristics of the micro coils. Poor correlation was obtained for the values
of intermetal insulator resistance which is attributed to a failure in fabrication. During
fabrication pin holes formed along the edge of the lower metal layer when the via holes
were etched, resulting in shorting of the top and lower metal layers. Based on the
theoretical results, a device with parameters that satisfy the estimated requirements for
mouse intraocular pressure sensing can be fabricated, once the process is optimized to
eliminate the pinholes.