Fabricating micromachines from photoresist

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge April 2013

A common hard-disk lubricant and nanoparticle fillers open new tribological applications for microfabricated devices made from photoresist.
 

A 200-μm thick gear made of SU-8 + PFPE composite using a UV lithographic process.
IN THIS COLUMN WE’VE DISCUSSED MICROELECTROMECHANICAL DEVICES (MEMS) and the trials and tribulations involved in reducing friction and wear in sliding micromachines constructed from silicon (1). Because of its use in electronic devices, silicon is, of course, a highly versatile material for microfabrication by means of photolithography and etching processes.

However, there is increasing interest in using a photoresist material, known as SU-8, itself to fabricate micromachines. SU-8 is a negative, thick-film UV photoresist based on epoxy resin, a solvent and a photoacid generator, which is what imparts the photosensitivity.

Fabricating microstructures from SU-8 is much simpler than using silicon, since no etching is necessary. Making MEMS devices requires SU-8 to be (1.) spin-coated on a substrate, (2.) exposed to UV light through a mask to crosslink it and (3.) the non-cross-linked photoresist to be removed, followed by some heat-treatment steps.

There is, of course, a catch. While SU-8 is convenient to use, it has poor mechanical and tribological properties (low modulus, low hardness, high friction and poor wear resistance). However, in a recent paper published in Tribology Letters, Prabakaran Saravanan, Nalam Satyanarayana and Sujeet K. Sinha of the National University of Singapore, described a novel approach to improving these properties significantly (2). The approach was two-pronged: a well-known lubricant used by the hard-disk industry, hydroxyl-terminated perfluoropolyether (PFPE), was mixed with the SU-8 prior to spin-coating.

Thanks to the presence of hydroxyl groups in the PFPE, these lubricant molecules reacted with the epoxy groups of the SU-8 during curing, becoming incorporated throughout the crosslinked structure. To further boost the mechanical properties, nanoparticles are also mixed into the SU-8.

The effects of PFPE incorporation on the tribological properties of the SU-8 were dramatic. The friction coefficient of SU-8 measured against Si3N4 in air was a hefty 0.82, while SU-8 with PFPE incorporation displayed a value of 0.09. The wear resistance of the material was also increased by a factor of 104.

Experiments with nanoparticles alone (silica, carbon nanotubes or graphite) incorporated into the SU-8 showed negligible improvement of nanotribological properties. Moreover, the nanoparticles showed no improvement in modulus and actually reduced the hardness, compared to pure SU-8. Interestingly, a combination of PFPE and nanoparticles showed similar improvement in friction and wear behavior to the SU-8 + PFPE composites, with enhancements in modulus (x1.4) and hardness (x1.4).

The authors explain these interesting results in terms of the distribution of PFPE throughout the matrix, rendering it available to the sliding contact, even if wear should occur. The presence of PFPE on the surface, even after 106 sliding cycles, was confirmed by both x-ray photoelectron spectroscopy and water-contact-angle measurements.

The ease with which this tribologically enhanced SU-8 can be fabricated (3) suggests that it might have a bright future in applications such as bearings, raceways, gears, bio-devices, precision-positioning stages and components in consumer electronics such as cameras and printers.

FOR FURTHER READING
1. Tysoe, W. and Spencer, N.D. (2008), “Alcohol Gets You No Wear,” TLT, 64(4), p. 64.
2. Saravanan, P., Satyanarayana, N. and Sinha, S.K. (2013), “Self-Lubricating SU-8 Nanocomposites for Microelectromechanical Systems Applications,” Tribology Letters, 49 (1), pp. 169-178.
3. “SU-8 Nano-Composites with Improved Tribological and Mechanical Properties,” U.S. Patent Application No. 13/683,467, Filing date: Nov. 21, 2012 (Inventors: S.K. Sinha, N. Satyanarayana and P. Saravanan).


Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.


Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.