Microhotplates with TiN heaters

Titanium nitride (TiN) has been investigated as a heater material for microhotplates and microreactors. TiN is available in many CMOS processes, unlike many other microheater materials. In addition, TiN has a very high melting point (2950 °C) meaning that it is stable up to higher temperatures than platinum (Pt) and polysilicon. For the first time, TiN is tested inside a conventional membrane of LPCVD silicon nitride (SiN). Two types of sputtered TiN are considered: high stress and low stress. Their performance is compared with that of e-beam evaporated Pt. The maximum average temperature of TiN heaters is 11% higher than those of Pt, and reaches over 700 °C. Failure of the TiN heaters is due to rupture of the membrane. Failure of the Pt heater is due to electro-stress migration. For high-stress TiN, the temperature coefficient of resistance is almost constant and close to that of Pt, making the material very suitable for temperature sensing. In the case of low-stress TiN the temperature coefficient of resistance (TCR) becomes nonlinear and changes sign. The large differences between the materials are explained by the grain structure. The different grain structures are related to the sputtering parameters according to the Thornton model.