Study On Characteristics Of Rapid Response Humidity Sensor And Its Test Mothed

Humidity measurement and control technology is more and more widely used in livingenvironment, health care, transportation, warehousing logistics, production processing andother fields. The fields, such as Fuel Cell vehicles, breath disese care, etc. have an urgent needfor the fast response humidity sensors.Because of narrow application range and complex mechanism, the research on the dynamicresponse characteristics of the rapid response humidity sensor is relatively lag, mainlyincluding three aspects:1) The fruits on the dynamic response characteristics of humiditysensor still can not meet the requirements.2) The need for the calibration and test facilityused for the dynamic characteristics of humidity sensors with the characteristics of simpleness,feasiblility, accuration, rapid response time and cheap is urgent.3) Results obtained fromexperiment method can not fully reveal the mechanism of humidity sensor and the numericalresults existed are not very perfect because of lacking of the perfect, reasonable and effectivenumerical simulation models and methods.In order to solve these problems, the purpose of this paper is to improve the existednumerical model more perfect based on considering the porosity of membrane of humiditysensors, to study the dynamic characteristics of humidity sensors by numerical method, toobtain a humidity sensor with rapid dynamic characteristics and simple technology byreasonable design and fabrication based on imformation from numerical study, and tofabricate a simple, feasible, accurate, rapid response and cheap facility used for calibratingdynamic characteristics of fast response humidity sensor.Because there are different dynamic characteristics among various types of humiditysensors, first of all, the type of parallel plate capacitor with grid upper electrode is determinedby analyzing in the second chapter of this paper.Humidity sensing material is the key factor on the characteristics of humidity sensor. Inview of the excellent physical and chemical properties, especially the potential ability inimproving of humidity-sensitive characteristics, polyimide (PI) is preferred. The third chapterof this paper, both films of insoluble polyimide and soluble polyimide are synthesized withdifferent materials and their porosities are tested. The porosity of the insoluble polyimide isgreater than that of the soluble polyimide, but the aperture diameter of the insolublepolyimide is smaller than that of the soluble polyimide.Numerical simulation method needs shorter time and less effort and can effectively makeup the inadequacy of other research methods. Based on the second diffusion law of Fick,Looyenga equation and Shibata equation, and giving eyes on the effects of the porosity onhumidity sensor dynamic characteristics, the fourth chapter of this paper set a numerical model and method, and its validation was tested. The effects of factors on the dynamiccharacteristics of humidity sensors were studied by numerical method. The results show:1)The fact of no difference of dynamic characteristics between the absorption and adsorptionprocess shows that the model in this paper is not suitable for the problem with obviouscondensation of water molecules and chemical absorption in membrane.2) The effects of thefilm porosity, the diffusion coefficient, the size of upper electrode and the thickness of filmlayer on dynamic characteristics of humidity sensors are remarkable. The dynamiccharacteristics of humidity sensors will be improved effectively by enhancing the diffusioncoefficient and the porosity, controlling the width of upper electrode and the thickness of film.If raising the environmental temperature, the dynamic characteristics will be improved atsome extent.3) Under the same conditions, the same humidity sensor tested in differentchange ranges of environmental humidity shows a very little difference of the dynamiccharacteristics.Structure parameters are the important factors on the characteristics of the parallel platecapacitive humidity sensor, and manufacturing technologies are the guarantee to realize theseparameters. The fifth chapter of this paper made two batches of humidity sensors based on thedesign parameters and the manufacturing process, of which, one batch is used to indicate thesize effects of upper electrodes on the dynamic characteristics of humidity sensors, includingthree sensors with the bar/gap widths of2m/2m,4m/4m and6m/6m respectively,the other batch is used to indicate the effects of different PI film layer on the dynamiccharacteristics of humidity sensors, including two sensors with the film layer of the insolublepolyimide and the soluble polyimide respectively. The humidity sensors all are based on thep-type Si substrate with a SiO2insulation layer of7600thickness, the Mo-Al lowerelectrode of10500thickness and44-45square resistance, the polyimide layer of5400thickness and the Mo-Al upper electrode of11500thickness and17-19square resistance.The film layer is non-conductive by the probe test, its breakdown voltage is higher than150V,and insulation performance is good.The calibration technology and facility with fast response characteristics is a premisecondition to develop a rapid response humidity sensor. Firstly, the sixth chapter of this paperintroduced a calibration principle and method used for the dynamic characteristics of the fastresponse humidity sensor. Secondly, a calibration test system based on the principle andmethod was set up. The response time of the test system is less than250ms, according to thecalculation results based on the structure size and the analysis results based on test data. Theprototype of calibration system is with a test error less than0.69%, and a repeatability errorless than13.9%. The results show that the calibration prototype system and the method cansatisfy the test requirements of the fast response humidity sensor. The characteristics of the prototype system are simple, accurate, cheap and stable. The response time of the system canbe further improved by using the parts with batter characteristics, such as faster response timeof solenoid valves and capacitance measuring meter, etc. Due to the defects of the prototype,such as long balance time and inconvenient exchange process between absorption andadsorption during test, the paper has put forward a new ideas and principles of calibrationsystem.The characteristics of the humidity sensors of two batches were indicated by experiments inthe seventh chapter of this paper. The test results show that the dynamic characteristics basedon the soluble PI is better than that of the insoluble PI, but the comparison result of sensitivitybetween two sensors is contrary. The humidity sensor developed in this paper shows excellentperformance and can satisfy the requirements for humidity measurement in the fields of fuelcars and medical care.Finally, the conclusion and innovation was given:1) Based on reasonable design andfabricated technology, a humidity sensor with the characteristics of good linearity ofhumidity-capacitance line, high sensitivity, small hysteresis, stablility and rapid response timewas obtained.2) Considering the effect of membrane porosity on the characteristics of sensors,the new numerical model based on existed models is more reasonable and effective.3) Themethod and facility used for calibrating sensors were provided and the facility is able tosatisfy the requirement for calibrating dynamic characteristics of sensors and with thecharacteristics of simplity, accuration and rapid response time.