A modular system of SiC-based microhotplates for the application in metal oxide gas sensors

doi:10.1016/S0925-4005(99)00490-6

Sensors and Actuators B: Chemical

Volume 64, Issues 1–3, 10 June 2000, Pages 95-101

https://doi.org/10.1016/S0925-4005(99)00490-6 Get rights and content

Abstract

A modular system of SiC-based microhotplates with integrated IDC for the use in metal oxide gas sensors is presented. Basic building block of the sensors is a 1 μm thick polycrystalline 3C-SiC membrane, which acts as supporting structure. A 200 nm thick HfB₂ thin film heater with a dc-resistance of about 12 Ω (1–2 V operating voltage) or alternatively an N-doped area in the SiC-membrane (12 V operating voltage) is used as hotplate. The active part of the membrane is separated from the surrounding membrane by six SiC-microbridges of 150 μm length and widths of 10, 20 and 40 μm. The heater is designed for operating temperatures up to 700°C and can be operated at about 400°C with a power of 35 mW.

Introduction

Recent market analysis expect gas sensors to experience the biggest market growth of all sensor systems in the next decade [1], [2]. A number of authors have proposed and demonstrated microhotplates based on Si₃N₄- or SiO₂-membranes with low power consumption [3], [4], [5]. The long-term stability of these devices was rarely studied. We have developed a modular system of microhotplates based on the combination of SiC and HfB₂ for low voltage (battery powered) operation or alternatively higher voltages for automotive applications. Both, SiC and HfB₂ are subject to extensive studies on high temperature materials in electronics and thermal shielding and thus constitute the ideal material combination for very durable high-temperature microhotplates.

Section snippets

Device design

Since the development of the new basic gas sensor module (i.e., the microhotplate) had to account for two different application fields (battery operation at 1–2 V and automotive applications at 12 V) a modular design with two different heaters but identical supporting structure was chosen. Further requirements were: fast response time, good high temperature and long-term stability and the possibility to integrate several hotplates into an array with little heat crosstalk.

FEM simulations

Thermo-mechanical and transient thermal FEM simulations were performed to achieve an optimised sensor design to reduce power losses, homogenise the temperature distribution, reduce the response times and prevent stress singularities, which can lead to mechanical failure of the structure.

The thermal behaviour of the microhotplate is primarily controlled by thermal conduction through the supporting bridges and the surrounding air. Despite the high temperatures, radiation losses can be neglected

Device fabrication

A standard 390 μm thick double-sided polished Si-wafer with 500 nm thermally grown SiO₂ was used as substrate. A 1 μm thick polycrystalline 3C SiC layer was deposited by CVD at 1200°C deposition temperature. 100 nm thermal SiO₂ was grown on top of the SiC layer and reinforced by 200 nm PECVD SiO₂. RIE with an O₂/SF₆ (16 sccm/4 sccm) plasma at 150 W and a graphite electrode was used to structure the SiO₂/SiC/SiO₂ sandwich on the front side (microbridges) and backside (etch mask for anisotropic

Device characterization

The fabricated devices were characterised using a manual needle-prober with integrated hotplate or were assembled and wire bonded on a TO16 package. The RTD's were calibrated using the hotplate with a Pt-100 reference sensor. This procedure is justifiable due to the high thermal conductivity of the substrate. The TCR α of the RTD, and the HfB₂-heater and a calibration curve of the SiC-heater were determined using:

R=R₂₀(1+α(υ−20°C)
R₂₀=9±0.78 Ω (thermistor)
α_therm=3.6×10⁻³ K⁻¹
R₂₀=19±1 Ω (HfB₂

Conclusions

A new modular system of microhotplates based on the promising new materials SiC and HfB₂ was presented. The heater consists of a basic SiC membrane structure, which can conveniently be adjusted to battery powered and automotive applications by either using an HfB₂ thin film heater or doping and directly contacting the SiC in the heater area. The fabrication processes are compatible for both versions, such that no further changes in the processing steps have to be made. The choice of materials

Acknowledgements

The authors would like to thank Dr. Skorupa at Forschungszentrum Rossendorf for performing the high temperature ion implantations. Financial support through the German Ministry of Education, Research and Technology BMBF is gratefully acknowledged.

Florian Solzbacher studied in the UK, Germany and Utah, USA. He received his M.Sc. degree in Electrical Engineering from the Technical University Berlin in 1997. Since August 1997 he has been studying as a PhD student at the Microsensor and Actuator Technology Center at the Technical University Berlin. His field of interest covers chemical sensors, biomedical engineering and III–V semiconductors.

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Cuk Imawan received his BSc degree in physics from Gadjah Nada University in 1991 and his MSc degree in physics in 1995 from the University of Indonesia. In 1991 he joined the department of physics of the University of Indonesia. He is now studying towards his PhD in Electrical Engineering on a DAAD scholarship at the Microsensor and Actuator Technology Center of the Technical University Berlin.

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