Characterization of Ti/TiN and TiN Conductive Layer for High Temperature MEMS Devices
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Characterization of Ti/TiN and TiN Conductive Layer for High Temperature MEMS Devices Peter Lange1, Birger Ohlsen2, Sebastian Puls1 and Joerg Syre1 1 Fraunhofer Institut fuer Siliziumtechnologie, Fraunhofer Str.1, 25524 Itzehoe, Germany 2 FH Westkueste, Fritz-Thiedemann-Ring 20, 25746 Heide, Germany ABSTRACT The effects of temperature on micro heaters made of Ti/TiN stacks and pure TiN layers on bulk micromachined membranes have been studied. Ti/TiN stacks show a thermal stability up to 380°C, beyond that temperature an enhanced interaction within the stack and/or with adjacend layers leads to a degradation of the resistance. The pure TiN layers withstand temperatures up to 600 °C, this limitation is only given by the mechanical stability of the membran stack, which is destroyed beyond this temperature. These layers are suitable for sensors in which an elevated temperature provided by heating lines on a membran for thermal isolation and fast response is necessary for functionality. INTRODUCTION New conductive layer for heating and sensing in MEMS (Microelectromechanical System) devices operated at elevated temperatures are reported. These micro heaters are formed on membranes for fast response and thermal isolation to the chip frame. In applications such as calorimetric and anemometric flow sensors, gas sensors, thermopiles and IR sources the resistors are heated by a current flow. Thereby they have to withstand temperatures of several hundreds degree of Celsius. Degradation effects in the resistors have to be prevented, otherwise the external/internal signal conditioning circuit could not work properly. The fabrication of such devices in conjunction with standard CMOS processes shows considerable advantages as monolithic integration, low cost production and short cycle times. A variety of metals like Cr, Au or Pt are applied up to now in MEMS devices for this purpose. However, they are exhibiting the disadvantage of being mostly not compatible to IC processing [1]. In this respect polysilicon is the most applied material [2,3], but shows some altering behaviour at elevated temperatures. The refractory metals, for example Ta, W, Ti, are also providing candidates for this purpose. But many of these materials are highly reactive at elevated temperatures or show other drawbacks, for example a complicated process management, so the possibilities of choice are rather limited. Ti and TiN thin films are state of the art technology in MOS devices as diffusion barrier between Si and Al or Cu [4]. Because of this proven compatibility with front end MOS technology we have investigated Ti/TiN layers and pure TiN layers for the application as heating and sensing elements. For the protection against interdiffusion/oxidation effects the Ti thin films were covered with thin TiN layers. EXPERIMENTAL All experiments were carried out in a conventional MOS line on Si-wafers with orientation and 150 mm in size. The Ti /TiN thin film consists of a sandwich of 25 nm TiN, 650 nm Ti and 50 nm TiN. These layers and the pure TiN layer
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