Analysis of Optical Characteristics of Transparent Electrode ITO Material in FPD Routing Process

Analysis of Optical Characteristics of Transparent Electrode ITO Material in FPD Routing Process
Analysis of Optical Characteristics of Transparent Electrode ITO Material in FPD Routing Process

Analysis of Optical Characteristics of Transparent Electrode ITO Material

in FPD Routing Process

Longlong Chen1,a, Xifeng Li1,b, Jifeng Shi1, Chang Zhou2, Jianhua Zhang1

1 Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai

University, Shanghai 200072, P. R. China

2Shanghai Micro Electronics Equipment Co. LTD., Shanghai 201203, P. R. China

a llchen@https://www.360docs.net/doc/dd17814630.html,,

b xifeng.li@https://www.360docs.net/doc/dd17814630.html,

Keywords: ITO, exposure, photolithography, transparent, routing

Abstract.ITO transparent film was deposited on glass substrates by RF-magnetron sputtering equipment as electrode, routing material and the photolithography process was studied. The results shown that the ITO based transparent electrode routing needs larger dose energy than that Mo based substrate when the PR processes were all identical. The photo resist cannot be exposed thoroughly where the length/width ratio of gap between routing serried lines lager than 80 as the typical Mo based exposure process. The width of gap reached to 8.7um which was the limit of exposure machine of transparent film after the 2000um-length, 4um-width gap was exposed thoroughly. It should be attention of the routing gap design for electrical characters in transparent flat panel display.

Introduction

Transparent conductive oxides have been widely used as passive applications like transparent electrodes for liquid crystal display (LCD), organic light-emitting diodes (OLED), solar cells and other flat panel display (FPD) areas [1]. The study of active layer material has attracted considerable attention of thin-film transistors (TFTs) [2, 3]. Variety of oxide-based TFT have been proposed such as zinc oxide (ZnO), indium zinc oxide (IZO), zinc tin oxide (ZTO), and indium gallium zinc oxide (IGZO) as channel layers of transparent TFTs [4-7]. The a-IGZO based TFTs are employed for the study of mobility, on-off current ratio and so on. ITO was used as conductive layer. ITO is a suitable material can be used in the transparent display for its characters of transparent and conductive. However, the glass substrate with the ITO deposited latter is transparent which is totally different from the glass substrate with metal deposited on it in the photolithography process. Arun et. al used IGZO based all-transparent integrated circuits to research ring oscillators characters [8]. But the process to make several sparse lines was totally different from which of serried lines with narrow gaps between them. The gaps between serried lines couldn’t be developed thoroughly as the metal based substrate which the ratio of gap’s length/width exceeded 80 when the width was at 4 um. It was a crucial issue which may impact the routing in the display panel. Therefore, it is very interesting in study of the ITO based photolithography process for critical dimension (CD) loss, resolution of serried lines and so on. In this paper, the ITO films production is focused which is as transparent routing lines. ITO film is deposited on the glass substrate by radio-frequency sputtering and resolution of serried lines is studied.

Experiments

ITO films were deposited on unheated glass substrates (Corning EXG 200mmx200 mm) using ULVAC SME-200E rf-magnetron sputtering equipment with a ITO sputtering target. The deposition condition employed in the sputtering deposition was the gas mixing ratio of Ar:O2=65:0.5, the growth temperature is 150℃, and the RF power of 1 KW. The thickness of as-deposited ITO film was about 100 nm. The transmission of ITO film was measured by Hitachi

U-3010 spectrophotometer, and the transmission is over 80% when the spectra in the visible light

which wavelengths between 400nm to 800nm (see Fig. 1). The ITO film transmission is fit for the

usage in flat panel display. The square resistance of ITO film was tested use four-probe meter method by QUATEK QT-50, which the square resistance was around 24?/□. The cross-section of the ITO film was observed by field emission scanning electron microscopy (SEM) using Hitachi S4800, operating at the electron acceleration voltage of 10 kV. Then, the as-deposited ITO films were spin with photo resist (PR) which the thickness was 1.1 μm. The glass substrate deposited with Molybdenum (Mo) which was spun with PR and was exposed in the contrast.

S SB200/10A stepper exposure machine was employed in the whole exposure processes include the Mo metal based glass and ITO based glass, respectively. And the exposed PR patterns after development were inspected with Nikon L200ND microscope in the lab. The characters of gaps between every two ITO serried lines were studied which the gap width was expected at 4um

ITO upper 80% when the wavelength between 400nm to 800nm.

Results and discussion

The conductive character of ITO film is a key point which as electrode and routing material used in display panels. The conductivity is depended on the deposition condition, thickness of film and so on. The deposition condition of ITO film has been optimized which made compromise with transmission ability. So thickness was inspected by SEM after the ITO film was deposited. The thickness of ITO film is controlled at 1um which the cross-section picture is seen in Fig. 2.

Fig. 2 Cross-section of ITO film inspected by SEM

Exposure process in Mo based substrates

PR exposure on Mo metal substrate is a typical process in display panel TFT produces. The characters are studied to verify the correctness of patterns on the mask. Patterns on Mo metal after exposure and development are shown in Fig. 3. The length of gap between ITO serried lines was at 2000um and the width was at 4um. The process of the exposure on Mo based metal was easy to accomplish. The width of gap was nearly at 4.7um and CD loss was 0.7um. Middle of the gap was inspected too and it was exposed thoroughly. The widths changes of the whole gap no more than 0.2um were unconspicuous.

Applied Mechanics and Materials Vols. 198-19997

PR

Mo

Fig. 3 Patterns on Mo metal based substrate which the gap width was 4.7 um Exposure process in ITO based substrates

The same photolithography processes were carried out in ITO based substrate too including thickness of PR, exposure time or dose energy, development time, bake times and so on. Results of this process were confusing which the PR of the gap couldn’t be exposed thoroughly, and PR of the gap was remained on although the develop time was extended to 120 seconds compared to 60 seconds normally, which was seen in Fig. 4 (a).

(a) (b)

PR

ITO

(c) (d)

Fig. 4 Patterns exposed with different dose energy in the ITO based glass substrate. (a) dose energy was seem as the Mo based substrate mentioned above. (b) dose energy was increased first time and the end of gap was exposed totally. (c) PR in the middle of the gap was half exposed. (d) dose energy was increased once again and the PR was all exposed.

So we noticed that the exposure process should be modified in the long-thin-gap condition. In order to make the PR developed thoroughly which the exposure time was increased. The patterns were inspected and we could see them on Fig. 4(b) and (c). PR in the end of gap was developed totally, but only half of the PR in middle of gap was exposed as shown in Fig.4 (c). If we wanted the PR in the middle of gap was exposed totally (seen Fig.4 (d)), and the dose energy was nearly twice of that of the beginning exposure energy. The gap width reached 8.7um when it was exposed thoroughly. It reached to the limit of exposure machine during photolithography process to make long-thin kinds of serried lines.

The phenomenon could boil down to the difference of materials deposited on glass substrate. The glass substrate with metal can reflect most of the UV light when it reached to metal surface, and photo resist (PR) could be re-exposed by the reflected light. But a great of UV lights were pass through the ITO based glass substrate in the expose process which the dose energy was decreased relatively. The width of gap reached to 8.7 um when the PR could be exposed totally. It was a critical width where the gaps among serried lines used in electrode and routing in the flat panel display. The trial-manufacture of transparent TFT will be carried through and discussed in the subsequently research later.

Summary

ITO is a suitable material used as TFT electrodes and routing which used in transparent flat panel display in the future. The ITO based glass substrate exposure mechanism is quite different from the typical Mo based substrate which used in the flat panel production nowadays. The Mo based substrate can reflect the UV light during the exposure process. But the UV lights are mostly passed through the ITO based transparent substrate which the dose energy has to be increased in the exposure process. The limit of exposure ability is exist of serried lines when the ratio of length/width lager than 80.

Acknowledgements

This work was financially supported by the Science and Technology Commission of Shanghai Municipality (Grant No. 10DZ1100102), by national natural science foundation of china (Grant No. 61006005) and by excellent youth project funding of Shanghai Municipal Education Commission. References

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