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Evaluation of ALD Oxides as the Sensing Layer for Ion Sensors
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2016 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In this thesis thin films of aluminium(III) oxide (Al2O3), titanium dioxide (TiO2), and tantalum(V) oxide (Ta2O5) were evaluated as the sensing layer for ion sensors.

The oxide films were deposited using atomic layer deposition (ALD) and characterized with ellipsometry. The oxide-electrolyte interface was then evaluated with electro-chemical measurements. First, the open-circuit potential was measured at different pH values to extract the pH sensitivity of the oxides, and the potential drift with time. Second, the impedance was measured at different pH values to determine the thermal noise level of the different oxides and its dependence on pH. Finally, cyclic voltammetry measurements were done to check the surface coverage of the oxides.

Both thin titanium dioxde and thin tantalum(V) oxide show sensitivities around 55 mV/pH. Aluminium(III) oxide shows significantly lower sensitivity. A thinner oxide reduces the impedance of the solid liquid interface and thus gives lower thermal noise. Also, a thinner oxide reduces the potential drift. For these reasons, thin titanium dioxide and tantalum(V) oxide are both good candidates for the sensing layer. Increasing the thickness of the oxide from 3 nm to 10 nm increases the noise and drift, and also reduces the sensitivity in the case of titanium dioxide. We also find that the impedance and thermal noise varies with the pH of the electrolyte.

Abstract [sv]

pH-sensorer som tillverkas med samma processer som används inom halvledarindustrin kan göras små och billiga, och dessutom enkelt integreras på ett chip med annan elektronik. I detta exjobb har tunna filmer av aluminiumoxid (Al2O3), titandioxid (TiO2) och tantaloxid (Ta2O5) utvärderats för användning som sensorlagret i pH-sensorer.

Oxiderna deponerades med ALD (Atomic Layer Deposition) och karaktäriserades med ellipsometriska mätningar. Gränssnittet mellan oxiden och elektrolyten undersöktes sedan med elektrokemiska mätningar. Först mättes spänningen över gränsnittet för olika pH-värden. På detta sätt kunde spänningens känslighet för pH-förändringar bestämmas. Det var också möjligt att uppskatta hur stor driften av spänningen var. Vidare mättes gränsnittets småsignalimpedans. Det förväntade termiska bruset kan beräknas från realdelen av impedansen. Till sist gjordes CV-mätningar (Cyclic Voltammetry) för att verifiera att oxiderna var heltäckande.

Både tunn titanoxid och tantaloxid hade känsligheter nära 55 mV/pH, medan aluminiumoxid uppvisade betydligt lägre känslighet. En tunnare oxid reducerar gränssnittets impedans, vilket leder till lägre termiskt brus. En tunnare oxid reducerade även drift i spänningen för titandioxid. Därför kan både tunn titandioxid och tunn tantaloxid anses vara bra kandidater för sensorlagret i pH-sensorer.

Place, publisher, year, edition, pages
2016. , 37 p.
Series
UPTEC F, ISSN 1401-5757 ; 16050
National Category
Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:uu:diva-302507OAI: oai:DiVA.org:uu-302507DiVA: diva2:957954
Educational program
Master Programme in Engineering Physics
Supervisors
Examiners
Available from: 2016-09-06 Created: 2016-09-05 Last updated: 2016-09-06Bibliographically approved

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