MEMS Image
Dean Stephens, Senior Applications Engineer, Oxford Instruments Plasma Technology
MEMS – Fabrication of silicon micro-needles using isotropic and anisotropic plasma etching techniques for biomedical applications
When people think of the term MEMS (Micro-electro-mechanical-systems) it generally conjures up images of micro sized turbines, motors and accelerometers. However, when applied to the field of BioMEMS (MEMS for use in biomedical situations), most individuals would struggle to imagine many current applications.
One exciting use of this technology is the fabrication of arrays of silicon micro-needles, for the transdermal delivery of drugs or, conversely, blood sampling.
This technique is currently at the forefront of “pain free” delivery of drugs, as it does not penetrate deeply into subcutaneous tissue that is full of nerve endings, merely piercing the epidermis, and also has the benefit of greatly reducing the possibility of infection of the injection site itself. Likewise, in blood sampling for making blood-glucose measurements, the use of an array of micro-needles increases the permeability of the skin by many orders of magnitude, enabling larger sample volumes without the pain normally associated with this procedure.
The fabrication of such micro-needles, in silicon, has long been an aim of the biomedical industry due to the high strength of the material and the potential to create huge numbers of identical devices simultaneously in plasma etching equipment.
A technique for fabricating these devices repeatibly, and without having to go to the lengths of using many “dry” and “wet” methods, has been demonstrated at OIPT, using a System 100 ICP380 etch tool.
A photoresist masked silicon wafer, with features the size of the desired width of the needle shaft, is first etched isotropically to create the “point” of the needle, beneath the mask. This feature is not initially etched to a complete needle-tip, as the mask would then detach, but stops short of this at a few microns width. Then, using a deep Bosch™ etch to the targeted depth of the micro-needle array, the needle is anisotropically etched beneath the tip.