The Effect of Anodization Parameters on the Aluminum Oxide Dielectric Layer of Thin-Film Transistors

Summary

Anodization parameters for growth of the aluminum-oxide dielectric layer of zinc-oxide thin-film transistors (TFTs) are varied to determine the effects on the electrical parameter responses. Analysis of variance (ANOVA) is applied to a Plackett-Burman design of experiments (DOE) to determine the manufacturing conditions that result in optimized device performance.

Abstract

Aluminum-oxide (Al₂O₃) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the dielectric layer of thin-film transistors (TFTs). Growth of aluminum-oxide layers from anodization of metallic aluminum films is greatly advantageous when compared to sophisticated processes such as atomic layer deposition (ALD) or deposition methods that demand relatively high temperatures (above 300 °C) such as aqueous combustion or spray-pyrolysis. However, the electrical properties of the transistors are highly dependent on the presence of defects and localized states at the semiconductor/dielectric interface, which are strongly affected by the manufacturing parameters of the anodized dielectric layer. To determine how several fabrication parameters influence the device performance without performing all possible combination of factors, we used a reduced factorial analysis based on a Plackett-Burman design of experiments (DOE). The choice of this DOE permits the use of only 12 experimental runs of combinations of factors (instead of all 256 possibilities) to obtain the optimized device performance. The ranking of the factors by the effect on device responses such as the TFT mobility is possible by applying analysis of variance (ANOVA) to the obtained results.

Introduction

Flexible, printed and large area electronics represent an emerging market that is expected to attract billions of dollars in investments in upcoming years. To achieve the hardware requirements for the new generation of smartphones, flat panel displays and internet-of-things (IoT) devices, there is a huge demand for materials that are lightweight, flexible and with optical transmittance in the visible spectrum without sacrificing speed and high performance. A key point is to find alternatives to amorphous silicon (a-Si) as the active material of the thin-film transistors (TFTs) used in the drive circuits of most of the current active-matrix displays (AMDs). a-Si has lo....

Protocol

The protocol described in the present work is separated into: i) preparation of the electrolytic solution for anodization; ii) substrate cleaning and preparation; iii) anodization process; iv) deposition of the TFT active layer and drain/source electrodes; v) TFT electrical characterization and analysis and vi) application of ANOVA to determine the significance of the manufacturing factors in the TFT mobility.

1. Preparation of the electrolytic solution for anodization

1. Perform all the procedures of the protocol inside a cleanroom or a laminar flow cabinet, to avoid dust or contaminants during the sample preparation.
2. Pr....

Results

Eight different aluminum oxide layer manufacture parameters were used as the fabrication factors which we used to analyze the influence on the TFT performance. These factors are enumerated in Table 1, where the corresponding “low” (-1) and “high” (+1) values for the two-level factorial DOE are presented.

For simplicity, each manufacturing factor was named by a capital letter (A, B, C, etc.) and the corresponding “low” or “high” level represented by -1 and +1, respectively. The.......

Discussion

The anodization process used to obtain the dielectric has a strong influence on the performance of the TFTs fabricated, keeping constant all geometrical parameters and the fabrication parameters of the active. For the TFT mobility, which is one of the most important performance parameters for TFTs, it can vary more than 2 orders of magnitude by changing the manufacturing factors in the range given by Table I. Therefore, the careful control of the anodization parameters is of great importance when fabricating devices comp.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetone	LabSynth	A1017	ACS reagent grade
Aluminum (Al) Wire Evaporation	Kurt J. Lesker Company	EVMAL40060	1.5 mm (0.060") Dia.; 1lb; 99.99%
Ammonium hydroxide solution	Sigma Aldrich	338818	ACS reagent, 28.0-30.0% NH3 basis
Chemoface - Software to set a design of experiment (DOE)	Federal University of Lavras (UFLA), Brazil		Free software developed by Federal University of Lavras (UFLA), Brazil - http://www.ufla.br/chemoface/
Cleaning detergent	Sigma Aldrich	Alconox	Alkaline detergent for substrate cleaning
Ethylene glycol	Sigma Aldrich	102466	ReagentPlus, ≥99%
Isopropanol	LabSynth	A1078	ACS reagent grade
Glass substrates	Sigma Aldrich	CLS294775X50	Corning microscope slides, plain
L-(+)-Tartaric acid	Sigma Aldrich	T109	≥99.5%
Mechanical shadow mask for deposition of the sputtered ZnO active layer	Lasertools, Brazil	custom mask	10 mm x 10 mm square.
Mechanical shadow mask for TFT gate electrode	Lasertools, Brazil	custom mask	25 mm long stripe, 3 mm wide.
Mechanical shadow mask for TFT source/drain electrodes	Lasertools, Brazil	custom mask	100 µm stripes, separated by 100 µm gap, overlapping of 5 mm
Plasma cleaner	MTI	PDC-32G	Campact plasma cleaner with vacuum pump
Sputter coating system	HHV	Auto 500	RF sputtering system with thickness and deposition rate control
Stiring plate	Sun Valley	MS300	Stiring plate with heating control
Thermal evaporator	HHV	Auto 306	it has a high precision sensor for measure the thickness and rate of deposition of thin films
Two-channel source-measuring unit	Keithley	2410	Keithley model 2410 or similar/for anodization process
Two-channel source-measuring unit	Keithley	2612B	Dual channel source-measure unit (SMU) for TFT measurements
Ultrasonic bath	Soni-tech	Soni-top 402A	Ultrasonic bath with heating control
Zinc Oxide (ZnO) Sputtering Targets	Kurt J. Lesker Company	EJTZNOX304A3	3.0" Dia. x 0.250" Thick; 99.9%