Patterning thick SU-8 with conventional photolithography facilities is important for fabricating various MEMS structures. However, the fabrication of thick SU-8 MEMS has experienced severe problems such as cracks, distortions, or delaminations during the fabrication process and/or postservices, due to the large internal stress generated during the photolithography process. In this work, an in-depth finite element analysis (FEA) is performed to investigate the causes and effects of the internal stresses. Analytical results show that the post-exposure bake (PEB) temperature is the main factor in developing the resulted internal stress. Under the guidance of analytical results, an optimized UV photolithography process for the fabrication of ultra-thick low-stress SU-8 patterns is developed with conventional (simple) equipment. A low PEB temperature of 55°C reduces the internal stresses by more than 70% compared to those fabricated with the recommended procedure. Experimental results indicate that cracks, distortions, and delaminations are eliminated from the fabricated SU-8 structures using the newly developed procedure. In addition, the patterned SU-8 has a Young's modulus of 2.5 GPa and an ultimate strength of 50 MPa, which is about 50% higher than previous reported values.
Compact robust hydraulic actuators are very important for space related applications because of their capability of producing much larger forces per unite volume/mass than existing technologies. The major components of these actuators are PZT stacks (pusher) and microvalves. The PZT pusher works at high frequencies to produce large flow rates (proportional to displacement traveled) and high pressures. As a component of the hydraulic actuator, the microvalves are challenged in matching the requirements of the PZT in terms of high operational frequencies, large flow rates and high-pressure support capabilities. In order to fulfill these requirements, the authors have developed robust self-assembled solid nickel micro valve arrays consisting of 80 single micro check valves, to achieve the required flow rate (>10 cc/second). A single micro check valve consists of an inlet channel (200 μm in diameter), a specially designed valve flap held by four identical micro beams, and outlet channels. All these structures are made from electroformed nickel and are self-assembled during a novel in situ UV-LIGA fabrication process. Finite element simulation results show that the micro check valve has a 1st resonant frequency of 16 kHz and is able to support pressures greater than 10 MPa. Test results show the flow rate is 19 cc/s at a pressure difference of 100 psi, and is roughly proportional to the pressure applied. Based on Poiseuille's law, it is reasonable to predict larger flow rates if higher-pressure differences are applied.
SU-8 is an ultra-thick negative photoresist with low optical absorption in the near UV range, which makes it an ideal material
for generating thick molds for electroforming as well as a structural material for MEMS devices. However, the MEMS fabrication of using SU-8 is largely limited by its well-known poor adhesion to metallic layers as well as the high internal stress induced after baking. In this paper, an optimized process for fabricating ultra-thick low stressed SU-8 mold is developed and good adhesion between SU-8 and metals is obtained by applying a newly developed material, Omnicoat from Microchem Inc. A laminated (sandwiched) micro heat exchanger has been fabricated using the developed process in which sandwiched microchannels (one layer of Ni and one layer of SU-8) has been fabricated using the patterned SU-8 and nickel electroforming process. Test results show that the micro structure fabricated can stand at cryo temperature (77K) without damages such as cracks or delamination.
Robust compact hydraulic actuators are extremely needed in space industry where payload is critical. Microvalves are key component for compact hydraulic actuators. Robust microvalves with large load bearing ability, large flow rate, and high operational frequency are objectives of this research. A FEM analytical approach was used to optimize the valve design. The microvalves were fabricated by novel microfabrication process and scaling laws. Electroformed nickel on silicon substrate was used to make the valve flap and deep RIE etching was adopted to make the valve channels while the metallic valve flap as the etching stop. Test results shown that the flow rate is proportional to the pressure applied. The flow rate is larger than 10 cc/sec at pressure or 40 psi. These microvalves can be used to solve engineering problems where both load bearing and flow rate are major concerns.