The effect of heat treatments on the properties of Ti/Pt heating elements for gas sensor applications N.H. Al-Hardan n , M.J. Abdullah, A. Abdul Aziz, Z. Hassan School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia article info Available online 20 November 2010 Keywords: Gas sensors Heating element Thermal treatment abstract Ti/Pt as heating element for gas sensor applications was fabricated on silicon (Si) wafer substrate. The fabricated device was subjected to heat treatment at different prescribed time periods for thermal stability. The energy dispersion spectroscopy (EDS) results of the device indicated that there were no Ti traces on the Pt surface after heat treatment at 450 1C for 3 and 4 h in an argon (Ar) atmosphere. A maximum temperature coefficient of resistance (TCR) with a value of 2.88 Â 10 À3 K À1 was obtained for the device with 3 h heat treatment. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Platinum (Pt) is a well-known element for micro heater applications, particularly self-heating gas sensors [1,2]. Pt is used for its high melting point, resistance to oxidation, good linear thermal coefficient, and ability to be patterned by microelectronic fabrication techniques. Silicon (Si) based sensors are widely used for their compatibility with the integrated circuits (IC) process [3]. However, Pt layers are known to have adhesion problems with SiO 2 insulation layers [4]. Hence, elements and compounds, such as tantalum (Ta), titanium (Ti), ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ), and platinum dioxide (PtO 2 ) [4–8] have been used to improve adhesion and prevent the diffusion of Si atoms in the Pt layer. Puigcorde et al. [6] studied the effect of high tempera- ture degradation on the Ti/Pt element and found that the main mechanical failures were due to the hillocks and delaminations caused by the high thermo-mechanical stresses induced during the operation. Millon et al.[9] studied the effect of underlying layer of Ti and TiO x on the diffusion of Ti through Pt layer, and they showed that the TiO x film, has drastically reduced the diffusion of Ti in both Pt and Si. Nam et al. [7] studied the formation of hillocks in Pt/Ti electrodes and its effect on the electrical short of the PZT ferroelectric film capacitor. They showed that the formation of the hillocks was due to the large volume expansion of the Pt layer after the diffusion of Ti. It has also been observed that increasing the thickness of the Ti layer caused the decrease in the formation of the hillocks. Pt–Ti phase diagram has been studied extensively by many groups [10,11]. It was shown that the Ti remains in its metal state at temperature approximately 800 1C with composition of Ti (at%) approximately ranging from 92% to 98% [11]. Furthermore, it was observed previously by several authors that Ti may react with oxygen at relatively low temperatures (from 200 1C onwards) [12,13]. For a stable performance of the Ti/Pt heating element and electrodes, heat treatment is required. The goal of this study is to investigate the effect of heat treatment on the microstructure and electrical properties of Ti/Pt heating element. 2. Experimental details N-type silicon wafers with (1 0 0) orientation and resistivity of 0.75 O cm were used. The wafers were initially cleaned according to the RCA cleaning procedures Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing 1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.10.013 n Corresponding author. Tel.: + 6017 4961778; fax: + 604 6579150. E-mail address: naif.zd06@student.usm.my (N.H. Al-Hardan). Materials Science in Semiconductor Processing 13 (2010) 199–204