The design, fabrication, and testing of a sorbent-coated microfabricated preconcentrator device in complementary metal-oxide-semiconductor is presented. As a sorbent-coated device, the preconcentrator is used to collect, concentrate, and deliver analyte sampled from air for analysis with a detector. The preconcentrator in this paper is based on a perforated flowthrough microhotplate structure that is coated with a sorbent layer to maximize vapor trapping efficiency. The coating sorbs the analytes of interest during the collection phase at ambient temperatures. A thermal desorption cycle is then used to rapidly heat the preconcentrator to 180 degC in 40 ms to release a concentrated wave of analyte. A finite-volume method was used to simulate the temperature distribution on a microhotplate and to model the time to reach the steady-state temperature. The experimental electrical measurements of the device were found to be in good agreement with the predicted values obtained using the finite-volume method. The preconcentrator device was demonstrated by interfacing to the front end of a handheld chemical agent detector and a handheld trace explosives detector. The preliminary results showed signal enhancement for the detection of the nerve agent simulant dimethylmethylphosphonate and the explosive 2,4,6-trinitrotoluene

Micropreconcentrator for Enhanced Trace Detection of Explosives and Chemical Agents

Micro-hotplates consisting of a platinum-based heating element embedded in two low-stress silicon nitride layers were fabricated. In this communication, we report on the composition of the heater and its influence on both the micro-hotplate's life expectancy and the highest reachable operating power. These factors were characterized by measuring the deformation of the membrane, by ramping up the power until breakdown and by performing accelerated aging tests. Finally, the failure mechanisms were investigated using optical and SEM observations. The type of material used as adhesion layer has a strong influence on the heating element's performances. Addition of iridium significantly improved the lifetime of the device.

https://doc.rero.ch/record/17013/files/Courbat_J._-_Reliability_improvement_of_suspended_platinum-based_20100115.pdf

Reliability improvement of suspended platinum-based micro-heating elements

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.

/pdf/microhotplates-with-tin-heaters-27ul5xu5ye.pdf

Microhotplates with TiN heaters

This paper reports a method of fabricating very thick (10–100 µm) silicon dioxide layers for thermal isolation without the need for very long deposition or oxidation. Deep reactive ion etching (DRIE) is used to create high-aspect-ratio trenches and silicon pillars, which are then oxidized and/or refilled with LPCVD oxide to create oxide layers as thick as the DRIE allows. Stiffeners are used to provide support for the pillars during oxidation. Thermal tests show that such thick silicon dioxide layers can effectively thermally isolate heated structures from neighboring structures within a distance of hundreds of microns. The thermal conductivity of the thick SiO2 is measured to be ~1.1 W (m K)−1. Such SiO2 diaphragms of thickness 50–60 µm can sustain an extrinsic shear stress up to 3–5 MPa.

/pdf/fabrication-of-thick-silicon-dioxide-layers-for-thermal-4rbhyy4p7n.pdf

Fabrication of thick silicon dioxide layers for thermal isolation

In recent years, engineered nanostructure assemblies such as nanowire arrays have attracted much research attention due to their unique chemical and functional characteristics collectively. The engineered nano-assemblies usually carry the characteristics distinct from bulk as a result of a size effect in their comprised elemental building blocks. The nanoscale size induced high surface-to-volume ratio is a fundamental attribute responsible for various chemical and physical properties required in various technologically important applications such as catalysts and sensors. This review article surveys the latest progress in engineered metal oxide nanostructure arrays, i.e., nanoarrays, for advanced chemical sensors’ design and application. It starts with an overview of gaseous chemical sensors followed by surveys of various fabrication methods and routes for metal oxide nanoarrays. Different sensing modes and corresponding applications have been highlighted in the mixed gaseous chemical sensing, which provides new approaches and perspectives to meet the challenges of selective gas sensing, such as the cross-sensitivity and inter-correlation of multiple sensing signals.

/pdf/metal-oxide-nanoarrays-for-chemical-sensing-a-review-of-1p5xx09hfa.pdf

Metal Oxide Nanoarrays for Chemical Sensing: A Review of Fabrication Methods, Sensing Modes, and Their Inter-correlations

We report several practical issues in the fabrication and high-temperature operation of micro suspended heating structures compatible with standard CMOS technology. Suspended microstructures are fabricated in a standard CMOS process and are released by post-process silicon etching. TMAH at 25 wt% with 15 vol% of IPA is found to greatly increase yield by reducing mechanical disturbances during etching. Electro-thermal properties of the polysilicon are investigated during high-temperature operation. Significant thermally-induced negative drift in resistance at high temperatures is found, and the impact on temperature control is discussed. Thermal isolation is found to be about 50 K/mW, and reliable operation is observed near 1000/spl deg/C.

Microstructure release and test techniques for high-temperature micro hotplate

Suspended microstructures consisting of a thin oxide/nitride diaphragm with embedded polysilicon heaters were designed and fabricated using a standard complementary metal–oxide–semiconductor process and simple postprocessing. Thin films of gas sensitive materials based on the SrFeO2.5+x nonstoichiometric perovskite family were deposited onto the diaphragms by room-temperature pulsed excimer laser deposition. Successful chemical sensor functionality was demonstrated. With applied power up to 30 mW, estimated temperatures of the gas sensor film up to 900 °C were reached. When the device was exposed to volatile organic compounds (VOCs) such as acetone and methanol, a reversible ten to 100-fold increase in sensor film resistance was observed, with response times from less than 1 s to a few minutes. Sensor response sensitivity depended on applied power and on the nature of the VOC analyte. This sensor device has the potential for use in multiarray configurations such as in an electronic nose.

/pdf/high-temperature-gas-sensor-using-perovskite-thin-films-on-a-49v08170yd.pdf

M. Kahrizi

Papers

Microstructure release and test techniques for high-temperature micro hotplate

High-temperature gas sensor using perovskite thin films on a suspended microheater