Humidity sensors for a pulmonary function diagnostic microsystem C. Laville * , J.Y. Dele Âtage, C. Pellet Laboratoire IXL, CNRS UMR 5818, Universite  Bordeaux I, 351 cours de la Libe Âration, 33405 Talence Cedex, France Abstract The work reported here is a part of an European research project. The aim of the project is the implementation of a multisensor microsystem for pulmonary diagnostic functions for chronic obstructive pulmonary diseases and asthma patients. The proposed microsystem is required to measure simultaneously peak expiratory ¯ow, temperature, relative humidity and pressure. We have in charge therealisationofthehumiditysensor.ThestructurerealisedwithcompatibleCMOStechnologyconsistsintwocoplanarelectrodescovered byahumiditysensingdielectric.Interdigitatedsensorshavebeenrealisedusingsiliconassubstrate,aluminiumasconductorandpolyimide Ultradel as dielectric. Sensors responses show that we have a quick absorption of humidity less than 1 s) and a longer desorption 15 s). The in¯uence of the geometry and the dielectric material on the performance of the sensor sensitivity, response time) will be reported. Specially, we will demonstrate that a plasma etching of the dielectric surface notably increases the sensitivity. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical sensors; Humidity sensors; Interdigitated electrodes; Polyimide 1. Introduction TheworkpresentedhereisapartofanEuropeanresearch project [1] conducted between companies Jaeger Ger- many),HymecandAnmarResearchLaboratoryTheNeth- erlands), ISA Portugal) and universities of Bordeaux France), Coimbra Portugal), Nijmegen The Netherlands) and Leuven Belgium). The aim of the project is the implementation of a multisensor microsystem for pulmon- ary function diagnostic for chronic obstructive pulmonary diseases and asthma patients. The proposed microsystem is required to measure simul- taneouslypeakexpiratory¯ow,temperature,relativehumid- ityandpressureFig.1).Ina®rsttime,thefoursensorsare developpedapartfromeachother.TheIXLlaboratoryhasin charge the realisation of the humidity sensor. It is speci®ed that this sensor should permit several measurementsduringonebreathingout.Hence,itsresponse time must be less than 1 s. According to these constraints, we have chosen to realise a capacitive structure with a dielectriclayersensitivetohumidity.Inliterature,humidity sensors are often encountered with polyimide [2±4]. In effect, polyimide relative permittivity is about three compared to 80 for water. The permittivity of sensitivity layer increases with humidity absorption and so does the capacitance of the structure. 2. Theory of operation TooptimisethehumiditysensorFig.2)designnatureof the sensitive layer, thickness...), we have studied diffusion of humidity in a plane sheet. First, assuming a Fickian diffusion of humidity, the humidity concentration variation versus time is given by the equation @ G @t  D @ 2 G @ x 2  @ 2 G @ y 2  @ 2 G @ z 2   1) where D is the diffusion coefficient, G the concentration of the diffusing specie and x, y , z the diffusion axes. To calculate the ideal thickness of polyimide layer, we havesolvedtheFicksecondlaw1)byusing®niteelements ANSYS 5.5 simulator. With this software, the humidity diffusion is solved using a thermal analogy as reported in Table 1 [5]. Theoutputnodalplotisdoneasafunctionoftemperature, so it corresponds here to the humidity concentration. The initial conditions are as follows:  G substrat  G electrodes  0;  G poly  theequivalentof35%RHabsorbedinpolyimide layer;  G air  95% RH;  D substrat  D electrodes  0;  D polyimide  5:10 13 m 2 /s. Sensors and Actuators B 76 2001) 304±309 * Correspondingauthor.Tel.: 33-556-8465-49; fax: 33-556-3715-45. E-mail address: laville@ixl.u-bordeaux.fr C. Laville). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00597-4