Sensors and Actuators B 163 (2012) 290–298 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical j o ur nal homep a ge: www.elsevier.com/locate/snb Titanium–aluminum–polytetrafluoroethylene coated stainless steel micromold via co-sputtering deposition: Replication performance and limitation in hot-embossing Biswajit Saha a , Shu B. Tor a,b,∗ , Erjia Liu b , David E. Hardt c , Jung H. Chun c a Singapore-MIT Alliance, Nanyang Technological University, 65 Nanyang Avenue, Singapore 637460, Singapore b School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore c Department of Mechanical Engineering, Singapore-MIT Alliance, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, USA a r t i c l e i n f o Article history: Received 17 September 2011 Received in revised form 5 December 2011 Accepted 28 December 2011 Available online 5 January 2012 Keywords: Stainless micromold Ti–Al–PTFE coating Magnetron sputtering Hot-embossing a b s t r a c t Stainless steel micromold is an alternative of silicon (Si) micromold in the fabrication of polymeric microfluidic devices because of the brittleness and short lifetime of Si mold. High adhesion and friction of stainless steel micromold can cause the distortion of the microstructures of polymeric products. In this work, titanium (Ti), aluminum (Al) and polytetrafluoroethylene (PTFE) were co-sputter deposited on stainless steel micromolds to improve their surface properties. The sputtering power applied to the PTFE target was varied to control the PTFE concentration in the Ti–Al–PTFE coatings, which affected the bond- ing structure, surface roughness, friction and contact angle of coatings characterized using micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), confocal microscopy, ball-on-disc tribometer and goniometer, respectively. It was observed that the Ti–Al–PTFE coatings were a mixture of carbide, PTFE- like material and amorphous carbon. The surface roughness of coated micromolds decreased with the increase in the PTFE concentration. The Ti–Al–PTFE coating deposited with 50 W sputtering power on the PTFE target showed the lowest friction coefficient and surface energy of about 0.17 and 13.1 × 10 -3 N/m, respectively. The coated stainless steel micromolds showed a better replication performance compared to the bare stainless steel micromolds in terms of the quality of polymeric microfluidic devices fabricated using hot embossing process. This work also investigated the coating properties at the sidewalls of the micromold channels and the limitations of the Ti–Al–PTFE coatings for application in hot-embossing. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Advanced coating materials with low friction and adhesion, which can be produced easily in a reproducible fashion, have an all-time demand. Co-deposition of metal and fluoropolymer by magnetron sputtering responding to the above mentioned needs is expected to provide good tribological coatings that could be coated on metal micromolds for hot-embossing process. Excellent tribological and surface properties of polytetrafluoroethylene (PTFE) have been well established by various researchers [1–3], such as high anti-corrosion resistance, ultra-hydrophobic property and low friction coefficient. Therefore, PTFE is promising to be used as an anti-sticking layer or self-lubrication coating in various appli- cations like micro/nanoelectromechanical systems (MEMS/NEMS). Main challenges of PTFE coatings are high wear rate and poor ∗ Corresponding author at: Singapore-MIT Alliance, Nanyang Technological Uni- versity, 65 Nanyang Avenue, Singapore 637460, Singapore. Tel.: +65 67904725; fax: +65 67922619. E-mail address: msbtor@ntu.edu.sg (S.B. Tor). adhesion with metal substrates [4]. Many researchers have tried to overcome these limitations by using various methods [5–7]. Though an injection molded stainless steel micromold can be used for a large number of replication operations in hot emboss- ing without damage but high friction and adhesion of stainless steel micromold with polymeric products can cause distortion and improper replication of polymeric micro-structures. Because of that an attempt is needed to improve replication quality and effi- ciency of stainless steel micromolds by titanium–aluminum–PTFE (Ti–Al–PTFE) coating. Extensive studies on the structural, mechan- ical and tribological properties of Ti–Al coatings and PTFE based materials have been reported by many researchers [1–3,8,9], but studies of Ti–Al–PTFE coatings have seldom been reported. Other researchers have also tried to improve the lifetime of micromolds by using different methods [10] and further research works are necessary. This work aims to exploit the superior properties of PTFE coat- ings to be coated on stainless steel micromolds through modifying their properties by Ti–Al–PTFE co-deposition via magnetron sput- tering. PTFE coatings can achieve very low friction, low surface energy and high corrosion resistance but they also have poor wear 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.096