A study of process optimization and inhomogeneous electro-optical properties for ITO thin films prepared by DC magnetron sputtering

Abstract

Tin doped indium oxide (ITO) is a kind of transparent conductive oxide (TCO) which has high transparency in the visible spectrum combined with useful electrical conductivity Selection of TCO film for application depends on device performance requirement and reproducibility using commercially feasible large area processing techniques ITO was chosen for this study for the reason that among different flavors of TCOs actively applied in recent year ITO is still the most popular one for high quality application such as smart phone and liquid crystal displayer (LCD) In this dissertation we present fabrication and characterization of ITO thin film on various types of substrate material including polyethylene terephthalate (PET) silicon and glass The DC magnetic sputtering technique was selected to prepare all of our ITO samples because among different available techniques for ITO deposition including sol-gel deposition pulsed laser deposition (PLD) and so forth DC magnetron sputtering is commonly preferred due to its large area producible good reproducibility and relatively low cost Moreover the plastic substrates used in flexible electronics applications have a low thermal resistance and thus sputtering should be performed at a low temperature in order to avoid thermal damage to the substrate surface DC magnetic sputtering has been approved to deposit high quality ITO films on plastic substrate at room temperature There are two subjects consisted in the present work; the first: process analysis and optimization for ITO films deposited on the PET substrate using DC magnetic sputtering technique The second: investigation into inhomogeneous optical and electrical properties of ITO thin film using spectroscopic ellipsometry In the first subject an experiment of deposition process for sputtered ITO thin film on the PET substrate was first performed to recognize the effects of deposition parameters on the characters of ITO film The changes induced by changing of the deposition time the argon flow rate and the target-substrate distance on the optical and electrical properties were identified in this section work Consequently for the purpose of process optimization the electrical and optical properties of the ITO films on the PET substrate were then investigated as a function of the cumulative sputtering gas (CSG); a parameter newly proposed in this study and defined as the product of the gas flow rate and the deposition time Meanwhile a silicon based ITO thin film was prepared simultaneously with ITO film on the PET substrate in order to investigate the different ion bombardment condition between soft and hard substrates In this section work a critical deposition guideline was proposed and verified to obtain minimized resistivity without any significant reduction in the transmittance of ITO films Finally this work investigated the thickness dependent inhomogeneous electrical and optical properties of ITO thin films deposited on glass substrates using spectroscopic ellipsometry (SE) Given multi-layer optical models and specific dielectric function models the inhomogeneous optical function inside of ITO film can be measured by SE Such multi-layer optical properties revealed an apparently different optical absorption between the bulk and transition area of ITO thin films By the selected dielectric function models and effective mass models the inhomogeneous electrical properties inside of ITO film can be measured by SE either A comparison was performed between estimated electrical properties by SE and the values measured by Hall measurement for accuracy checking The electrical resistivity estimated by the multi-layer model unveils apparent difference between the bulk and transition layer of ITO thin film The difference on free carrier density and carrier mobility within the multi-layer ITO film can be estimated by modeling the effective mass of a free carrier in the Drude model Accordingly we had a feasibility study on effective mass models for ITO films with different thickness In summary this report offers some remarkable contributions to the research on sputtered ITO thin film The first contribution is that we proposed a method to locate the principle optical axis of anisotropic The PET substrates so that the dielectric function and optical gap obtained by SE measurement is more accurate for ITO films on such PET substrate The obtained SE measurement results give some reasonable description of the changing of transmittance for our ITO samples The second one is: a new concept about “the cumulative ion bombardment” for sputtering technique was proposed and used to observe the variation of resistivity of ITO film The cumulative sputtering gas defined by the product of the gas flow rate and the deposition time was used to approach the cumulative ion bombardment in practice and adopted as a control parameter in the optimal experiment for resistivity A critical cumulative sputtering gas condition was discovered and verified by a repeatability experiment The resistivity of the sputtered ITO/PET thin films can be minimized without any significant reduction in the transmittance following the guideline of critical cumulative sputtering gas condition The third one is: a novel method is developed in this study to estimate inhomogeneous electrical properties of ITO films Such new method enables us to estimate the total resistivity whenever the Hall measurement is not accessible for example in-line measurement Meanwhile the method is available to obtain the resistivity carrier density and carrier mobility of every sub-layer within an ITO film

Date of Award	2014 May 6
Original language	English
Supervisor	Yu-Lung Lo (Supervisor)