Fabrication of SU-8 multilayer microstructures based on successive CMOS compatible adhesive bonding and releasing steps

Abstract

This paper describes a novel fabrication process based on successive wafer-level bonding and releasing steps for stacking several patterned layers of the negative photoresist EPON SU-8. This work uses a polyimide film to enhance previous low temperature bonding technology. The film acts as a temporary substrate where the SU-8 is photopatterned. The poor adhesion between the polyimide film and SU-8 allows the film to be released after the bonding process, even though the film is still strong enough to carry out photolithography. Using this technique, successive adhesive bonding steps can be carried out to obtain complex 3-D multilayer structures. Interconnected channels with smooth vertical sidewalls and freestanding structures are fabricated. Unlike previous works, all the layers are photopatterned before the bonding process yielding sealed cavities and complex three-dimensional structures without using a sacrificial layer. Adding new SU-8 layers reduces the bonding quality because each additional layer decreases the thickness uniformity and increases the polymer crosslinking level. The effect of these parameters is quantified in this paper. This process guarantees compatibility with CMOS electronics and MEMS. Furthermore, the releasing step leaves the input and the output of the microchannels in contact with the outside world, avoiding the usual slow drilling process of a cover. Hence, in addition to the straightforward integration of electrodes on a chip, this fabrication method facilitates the packaging of these microfluidic devices.

Polymer SU-8-Based Microprobes for Neural Recording and Drug Delivery

“…Photolithography technique was used to define the pattern of the probe, and metallic layers were coated on top of a thick undeveloped SU-8 layer for the first time by a controlled sputtering process. In addition, SU-8 made microchannels were processed on top by thermocompression based on a recipe developed by Ikerlan/IK4 (Blanco et al, 2004;Agirregabiria et al, 2005;Arroyo et al, 2007). Preliminary electrical characterization verified the sensing capability of the probe and biological tests confirmed clean delivery into the brain (Fernández et al, 2009).…”

“…Recently, planar electrodes were integrated at probe surface and signal recording improved significantly (Altuna et al, 2012). Bonding at low temperature and pressure was first suggested as an optimal technique to create embedded microchannels (Blanco et al, 2004;Agirregabiria et al, 2005;Arroyo et al, 2007;Fernández et al, 2009). Now, even lower temperature combined with higher pressure has been used to avoid internal forces and to guarantee probe planarity.…”

Flexible free-standing SU-8 microfluidic impedance spectroscopy sensor for 3-D molded interconnect devices application

“…After the releasing step, the entire microfluidic sensor still remained on a second PMMA handling wafer; therefore, it was not applicable for the 3-D attachment. The same Kapton film releasing steps were used in Ezkerra et al (2007) where SU-8 free-standing cantilever structures were embedded in SU-8/Si-based structures and in Agirregabiria et al (2005) where SU-8 multilayer microstructures were adhesive bonded and released with the help of a Kapton film. In all referred contributions the final microfluidic structures remained on a handling wafer that makes them inapplicable for a threedimensional attachment.…”

Fabrication of 3D high aspect ratio PDMS microfluidic networks with a hybrid stamp

“…Photosensitive epoxy polymers such as SU-8 can be optically patterned and stacked through thermal bonding. [16][17][18] However, it is well known that during the spincoating process, uneven thickness around the structures can hinder uniform bonding across a large area, which potentially leads to fluid leakage.…”