Fabrication Of High Aspect Ratio Microstructures Research Articles

Challenges in fabrication of high aspect ratio electrostatic comb-drive microactuator using one-step X-ray lithography

A High Aspect Ratio (HAR) electrostatic comb-drive microactuator of polymethyl-methacrylate (PMMA) is designed to deliver nearly 40 µm uniaxial displacement at 25 V DC. The HAR comb-drive microstructures are fabricated via one-step X-ray lithography (OXL). Polyimide-Au X-ray mask is fabricated and used to pattern 200 μm, 500 μm and 800 μm thick PMMA by deep X-ray lithography (DXRL), followed by drying/release and selective metallization for the development of HAR electrostatic microactuator. The release was optimized with the help of various low-surface-tension liquids. 200 μm and 500 μm thick microstructures were successfully released whereas, 800 μm structures could not be released due to the higher depth and associated capillary force. In addition, non-uniform distribution of Au on the side walls of overlapped region of comb fingers, during metallization, resulted in uneven distribution of electrostatic force followed by short-circuiting of HAR (i.e. 40) microactuator. These are the potential issues in the fabrication of HAR microstructures and devices by OXL and discussed in details in this paper.

Fabrication of high-aspect-ratio micropatterns in soluble block-copolymer polyimides by a UV-assisted thermal imprint process

High-aspect-ratio microstructures (HARMS) of polyimides have many applications, such as components of microelectromechanical systems and X-ray gratings. This study demonstrates the fabrication of HARMS with vertical and smooth sidewalls in a thick, soluble block-copolymer polyimide (SBCP) film by an UV-assisted thermal imprint process. A preheating condition was controlled as a parameter that dominated the imprint pattern fidelity. After investigating a proper prebaking condition, patterns with a width of 3.4 μm and a height of 35.1 μm with a high-aspect ratio of 10 were successfully structured in an SBCP film that has a thickness of approximately a hundred micrometers. The process has potential as a low-cost fine pattern fabrication process for polyimide-based polymers.

Wire electrochemical micromachining of high-aspect ratio microstructures on stainless steel 304 with 270-μm thickness

High-aspect ratio microstructures made of stainless steel 304 have found its extensive applications in the microsystem industry. Using a nanosecond pulse generator, high-aspect ratio components can be obtained by wire electrochemical micromachining (WEMM). Mass transport is substantially vital in WEMM to achieve high aspect ratio. Combining traveling and vibrating anode can effectively improve mass transport and consequently enhances machining result. When workpieces become thicker, mass transport would get worse. Many researchers have investigated the fabrication of microstructures on 304SS plate with thickness of 100 μm using WEMM. However, not enough attention has been paid to the machining of microstructures on 304SS plates with thickness of more than 100 μm which is an alternative to obtain high aspect ratio. Moreover, the parameters used for 100-μm-thick 304SS are not suitable for thicker 304SS. Therefore, it is necessary to investigate the fabrication of high-aspect ratio microstructures on 304SS plates with thickness of more than 100 μm. In this work, the machining of micro slits on 270-μm-thick 304SS plates by WEMM was investigated. A combination of traveling wire and vibrating anode was also employed to enhance mass transfer. The effects of traveling wire parameters and vibrating anode parameters on the slit width and slit profile were researched. Experimental results show that the stroke and traveling speed of the traveling wire together with frequency and amplitude of the vibrating anode for 270-μm-thick 304SS plates are higher than those for 100-μm-thick 304SS plates. The average slit width machined with optimized parameters using WEMM is 26 μm and the standard deviation is about 1.3. In the meantime, the slit profile is smooth, and the average aspect ratio is about 10.

Scum-free patterning of SU-8 resist for electroforming applications

A simple approach is developed to obtain scum-free pattern transfer in SU-8 resist by UV-lithography for electroforming applications. SU-8 is an epoxy-based negative resist used in the fabrication of high-aspect ratio microstructures. SU-8 resist poses considerable processing difficulties and tends to leave organic residues of undeveloped resist on the surface of the exposed metallic seed layer. Scum-free pattern transfer in SU-8 resist is important for the subsequent electroplating process used to fabricate the high-aspect ratio metallic microstructures. Dry plasma etching processes are commonly used to remove the SU-8 residue prior to electroplating. However, these processes are complex and not cost-effective. In this work, it is shown that scum-free SU-8 patterns could be achieved by proper selection of process parameters such as soft-bake temperature, exposure time, post-exposure-bake temperature, developing time and rinsing in solvents. The success of this particular process is due to controlled rinsing of the patterned sample in acetone to remove the resist scum. The results of our method for patterning the SU-8 resist to fabricate nickel microstructures using this mold are reported.

On the radiation tolerance of SU-8, a new material for gaseous microstructure radiation detector fabrication

SU-8 photosensitive epoxy resin was developed for the fabrication of high-aspect ratio microstructures in MEMS and microengineering applications, and has potential for use in the construction of novel gaseous micropattern radiation detectors. However, little is known of the behaviour of the cured material under irradiation. Mechanical properties of SU-8 film have been measured as a function of neutron exposure and compared with Kapton ® polyimide and Mylar ® PET polyester films, materials routinely used in gaseous radiation detectors, to asses the suitability of SU-8 based microstructures for gaseous detector applications. After exposure to a reactor core neutron fluence of 7.5×10 18 n cm −2 , the new material showed a high level of resistance to radiation damage, comparable to Kapton film.

Effects of rotor electrode in the fabrication of high aspect ratiomicrostructures by localized electrochemical deposition

This work reports on the effect of electrode rotation on thegrowth of nickel micro-columns fabricated by the localized electrochemicaldeposition (LECD) process - a new truly three-dimensional micro-rapidprototyping and direct-fabrication process capable of producing extremelyhigh aspect ratio microstructures. The results shown here present a noveldevelopment in the area of LECD and a potential capability of the processfor the fabrication of micro-pipes for possible applications such asmedical drug delivery. Experiments were conducted on a specially assembledapparatus where nickel structures were electrodeposited onto Cu cathodesfrom a nickel sulfamate plating solution, using a non-soluble anodeelectrode. Deposition with electrode rotation produced columns with annuluscross sections, which indicate the existence of a uniform hollow core inthe columns. This feature and dimensional control in the growth of thecolumns were not observed when using a stationary electrode.