Controllable Transfer of MOx Nanowires on Microhotplate-Based Gas Sensing Devices

Abstract

Introduction SnO2 nanowires (NWs) have been synthesized and implemented on CMOS-based gas sensors devices with gold inter-digital electrode structures (IDES). For the implementation process of NWs a polydimethylsiloxane-based stamping transfer was employed with help of a Condor Sigma bond tester. The multi-nanowire gas sensor has demonstrated sensitivity for toluene concentrations as low as 100 ppb. The transfer method applied in this work is one step further from previous manual transfer [1] to the industrial adaptation of the process. Materials and substrates A two-step synthesis was applied to prepare SnO2 nanowires, as described elsewhere [2]. The gas sensing substrate was a commercial chemical sensor platform provided by ams AG, Austria - a 2x2 mm micro-hotplate chip with a Pt-based microheater and Pt-IDES with an electrode width and the distance between the electrode fingers both 5 µm. SnO2 NWs were transferred on the IDES-structure with a PDMS stamp, prepared by 1:10 standard recipe and cut into 3x3mm pieces. Transfer of SnO2 NWs A Condor Sigma bond tester tool was employed to transfer SnO2 NWs. The PDMS-stamp was glued on a specific tool of the bond tester, which is usually employed for testing wire bond connections. The PDMS stamp was pressed with well controlled parameters (force, distance and position) onto the substrate with MOx NWs in order to collect the NWs with the stamp. Afterwards the NWs were transferred onto the gas sensing substrate with maximal force of 0.7N and 10s time of contact between the stamp and the micro-hotplate substrate, which resulted in NW transfer and interconnection to the electrodes. Gas measurement Gas measurements were performed by an automatized setup, with synthetic air (80% N2, 20% O2) as a background gas and a constant flow rate of 1000 sccm. Three different humidity levels of 25, 50 and 75% were investigated in the presence of 5 different toluene concentrations – 0.05, 0.1, 0.5, 1 and 5 ppm. The temperature of the microhotplate was kept constant at 300°C. Response was calculated as follows:R = (Rair -Rgas)/Rair *100%where Rair is a resistance of the sensor in synthetic air before the gas pulse and Rgas is a resistance of the sensor in the presence of a test gas at the end of a gas pulse. Two independent sensors were measured, prepared by the same technique. Results and Conclusions The Condor Sigma bond tester enabled precise manipulation of the PDMS NW transfer stamp not only in all 3 axis but also by using force sensor. Due to the high precision of the machine the transfer process could be performed with high precision and control without damage to the micro-hotplate. This demonstrates that such a NW transfer process is suited for realization of a NW-based chemical sensor devices. The response towards toluene is shown in Fig.1. The multi-nanowire based devices are able to sense a gas concentration as low as 100 ppb (where the threshold limit value for Switzerland is 50 ppm [3]). There is also visible dependence towards humidity – but it is not as high as for toluene – i.e. for 5 ppm of toluene the mean response for both sensors is 9%, 8% and 8% for 25, 50 and 75% rH levels, respectively. Presently the work is focused on the NWs transfer on CMOS-based microhotplate array chips. Acknowledgements This work was partly performed within the project “FunkyNano – Optimized Functionalization of Nanosensors for Gas Detection by Screening of Hybrid Nanoparticles” funded by the FFG - Austrian Research Promotion Agency (Project No. 858637).