high resolution nano gap pirani sensor for pressure measurement in wide dynamic range operation around atmospheric pressure

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi: 10.1016/j.proeng.2016.11.271 ScienceDirect 30th Eurosensors Conference, EUROSENSORS

Procedia Engineering 168 ( 2016 ) 798 – 801

1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi: 10.1016/j.proeng.2016.11.271

ScienceDirect

30th Eurosensors Conference, EUROSENSORS 2016

High resolution nano-gap Pirani sensor for pressure measurement in wide dynamic range operation around atmospheric pressure

Julien CLAUDELa, Cecile GHOUILA-HOURIa,b, Jean-Claude GERBEDOENa, Quentin GALLAS b, Eric GARNIER b, Alain MERLEN a,b, Omar Elmazria c , Romain VIARD d,

Abdelkrim TALBIa*, Philippe PERNOD a

a Univ Lille, Centrale Lille, LIA LICS/LEMAC - IEMN UMR CNRS 8520, 59000 Lille, France

b ONERA, Chemin de la Hunière 91123 Palaiseau, France

c Institut Jean Lamour, UMR 7198, Université de Lorraine-CNRS, Vandoeuvre les Nancy, France

d Fluiditech, Thurmelec, 68840 Pulversheim, France

Abstract

We present an innovative and practical pressure sensor based on Pirani effect enabling high sensitivity, high resolution, and high dynamic range around atmospheric pressure The structure is composed of 3µm width and 1mm long metallic resistors suspended

by periodic SiO2 micro-bridges to improve structure toughness and temperature uniformity One resistor is acting as a heater and another one is the sensing element A constant 100 nm nano-gap separates the wires from the substrate The sensor is especially designed to obtain the greatest sensitivity around atmospheric pressure The pressure experimentally measured ranges from 1 kPa

to 150 kPa without reaching the saturation limit The sensor design and experimental characteristics are presented

© 2016 The Authors Published by Elsevier Ltd

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

Keywords: MEMS Pirani sensor, pressure sensing

1 Introduction

Flow control on an airplane, intravascular pressure control or vacuum quality check are examples of applications for pressure sensors With Micro-Electro-Mechanicals-Systems (MEMS) techniques, pressure sensors have been developed with objectives of miniaturization, for less impact on the measured system, and efficiency, depending on the pressure range considered To match the requirements in terms of size and sensitivity, several concepts have been used to develop MEMS pressure sensors: piezo-resistance effect [1], capacitive effect [2], optical

* Corresponding author Tel.: +33 3 20

E-mail address: abdelkrim.talbi@iemn.univ-lille1.fr

© 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

interferometry [3] or thermal transfer ([4], [5], [6], [7]) This paper presents a MEMS pressure sensor based on the latter concept

To measure the pressure, thermal sensors use the Pirani effect These sensors are suspended resistors in which the heat loss is due to the gaseous conduction heat transfer where the gas is in molecular flow regime When the gap between the resistor and the support is near to the gas mean free path, the gas thermal conductivity decreases with pressure First Pirani pressure sensors have thus been developed for vacuum pressure measurement as the gap was macro-sized The development of MEMS and NEMS (Nano-Electro-Mechanical-Systems) allowed then to reduce the gap height shifting the thermal conductivity sensitivity to pressure towards higher levels as shown on Fig.1 For nano-gap Pirani sensors, the maximum sensitivity is reached for pressure values around the atmospheric one

The main difficulty with nano Pirani devices deals with the necessity of having a constant nano-gap and a precise heating uniformity along the resistor for sensitivity A small wire length allows to reduce the nano-gap size but decreases the heating uniformity along the wire On the contrary, a long wire allows to maintain proper heating uniformity but not a constant thin nano-gap as it increases mechanical stresses and the risk of wire collapse

The sensor presented in this work leverages these advantages and inconvenients by proposing a design that consists in a long metallic wire suspended by periodic silicon oxide micro-bridges This design, patented by IEMN LIA LICS/LEMAC [8,9], provides efficient thermal insulation, fast response time, good sensitivity and mechanical toughness as the structure allows precise heating uniformity and constant 100 nm nano-gap

The first part of this paper presents the sensor design Experimental results are then presented and discussed in the second part

2 Structure of thermal sensors prototype

The geometry sensor consists in two 1mm-long and 3µm-wide suspended wires on periodic 7µm-wide and 600nm-high silicon oxide micro-bridges One wire is the heater and the other one is the sensitive Pirani element

Fig 1: Calculated thermal conductivity response for different gap heights

They form a multilayer placed at the center of the sensor with a layer of silicon oxide that ensures electrical insulation between them The wires are separated from the substrate by a constant 100 nm nano-gap Fig 2 (a) and (b) are Scanning Electron Microscope (SEM) pictures of the sensor structure (a) and a zoom on the gap (b) The measure and heating wires have been uncoupled to improve the signal to noise ratio It also allows choosing different metallic materials for heating and measurement Gold is thus chosen for the heater wire and the Pirani element is a Ni/Pt multi-layer

3 Experimental results

3.1 Electrical characterization

The first set of experiments were devoted to electrical and thermal characterizations at atmospheric pressure The temperature coefficient of resistance (TCR), defined by ܶܥܴ ൌ οܴȀሺοܶǤ ܴ଴ሻwhere R is the resistance, R0 is resistance of reference and T the temperature, was first determined for the self-compensating stress Ni/Pt multilayer

We used a hot plate to heat the whole sensor structure The TCR is about 2300 ppm/K The coefficient of temperature rise was measured using a Keithley 2400 source-meter and expresses the temperature increase according to the heating power This coefficient, estimated at 1.15 °C/mW from Fig 3, confirms the quality of the thermal insulation from the substrate

3.2 Characterization in pressurized environment

Measurements were next performed in a pressurized chamber connected to a Fluke PPC4 pressure calibrator (range 1 kPa to 150 kPa) The sensor operates in a constant current mode of 7 mA, and the pressure response is shown in Fig 4 These results prove the sensor ability to accurately measure the pressure in a large bandwidth with a maximum sensitivity starting at 10 kPa and around atmospheric pressure The sensitivity starts to decrease for pressure under 10 kPa, without reaching the low pressure saturation limit even at 1 kPa The high pressure saturation limit is not reached either for 150 kPa

Fig 3: Variation of resistance (in %) and equivalent temperature variation (in °C) as function of injected heating current at atmospheric pressure

4 Conclusion

A nano-gap Pirani pressure sensor with high resolution in a wide pressure range operation have been successfully developed The sensor structure has been precisely designed to reach maximum sensitivity around atmospheric pressure for flow control applications Experimental results demonstrate the sensor capability to achieve pressure measurement in a large bandwidth, ranging from 1 kPa to pressure higher than 150 kPa

Acknowledgments

This work has been financially supported by the French National Research Agency (ANR) in the frame of the ANR ASTRID “CAMELOTT” project The authors also thank RENATECH the French national nanofabrication network

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Fig 4: Resistance variations obtained in 7 mA constant current mode as function of absolute pressure Values in percent with reference at 100kPa