Possibility to form an ultrahigh packed fine pit and dot arrays for future storage using EB writing
Introduction
Areal recording densities of magnetic and optical storages have increased at a rate of 60% per year. A high-end magnetic storage with over 100 Gb/in.2 has already been developed. For further high packed recording, however, we have to research many technical issues such as super-paramagnetics, signal noise ratio, perpendicular recording, etc. In optical recording, blu-ray disc and high definition digital versatile disk (HD-DVD) with 25-GB capacity have also been developed. Then, there remain significant technical issues such as an optical diffraction limit to increase areal recording density in the next generation. We have to find some break through for further high packed storage. Now, we have some technical proposals such as patterned media [1] and near field optical recording [2] to solve them. For these techniques, it is very important for over 500 Gb/in.2 to develop very fine pattern fabrication at first.
EB writing has been expected as a candidate to form very fine pit or dot array for patterned media and next DVD read only memory (ROM). In EB writing system, it has mainly been developed to apply to semiconductor device fabrication. Since Hosaka et al., have introduced an EB writing system with a field emission (FE) electron gun for very fine Gaussian beam [3], nanometer sized pattern fabrications have almost been used by this type system, and have been applied to quantum device, optical and magnetic recording media [4], [5], [6]. The results were, however, very far from 1 trillion bits (Tb)/in.2 recording. Therefore, in order to achieve 1 Tb/in.2 storage media, we have to research whether EB writing can form very fine pit or dot array with both bit pitch (BP) and track pitch (TP) of <30 nm.
In this paper, we describe the EB writing using the resists, ZEP520 and calixarene, and its possibility to form such an ultrahigh packed bit array.
Section snippets
Experimental approach for ultrahigh packed pattern
For our goal, we have to research the EB resists and demonstrate such a pit or dot array. Before the research, we performed following approaches: (1) to thin the resist film to protect from spreading an incident electron scattering range, (2) to design highly packed pattern to avoid a proximity effect and (3) to write the pattern by very fine probe with high probe current. Then, we investigate whether the advanced EB resists of ZEP520 and calixarene in positive and negative resists,
EB writing
At first, we coated the resists on a piece of Si substrate with a thickness of 70 nm and 15 nm in ZEP520 and calixarene, respectively. After pre-baking of them, we performed to write the ultrahigh packed pit and dot patterns with a BP of 100–20 nm and a TP of 100–20 nm. Then, after developing and rinsing the resists, we checked the written patterns using the same high resolution SEM. The experimental condition is represented in Table 1. In particular, we paid attention to adjust the focus to get
Using ZEP520 EB resist
Fig. 4 shows SEM images of ZEP520 pit patterns written in an exposure dosage of around 180 μC/cm2. The figure shows minimum pit array were written with a minimum pit diameter of <20 nm in a BP of 60 nm and a TP of 40 nm. We could not write the higher packed pit pattern than Fig. 4(b) in a case of ZEP520. The pit size changes with a fluctuation of about 18 nm. While, it was about 11 nm in a case with 100 nm (BP) and 60 nm (TP) (Fig. 4(a)). The fact indicates that the pit arrays pattern with 60 nm (BP)
Conclusion
We study the possibility to form very fine pit and dot arrays with very small pitches using EB writing. We have obtained following experimental results.
- (1)
We demonstrate to form an ultrahigh packed dot array with a diameter of around 13 nm, and BP of 30 nm and TP of 25 nm using calixarene.
- (2)
The EB writing using calixarene resist has a potential to achieve an ultrahigh recording density with both BP and PT of <25 nm, which corresponds to over 2 Tb/in.2 in EMR.
- (3)
Using ZEP520 resist, the limitation of packing
Acknowledgements
We thank Mr. S. Watanabe and Mr. M. Noguchi in Dept. of Electronic Eng., Gunma Univ. for technical supports in EB writing. This research was performed in Kiryu Ohta Area project supported by the Ministry of Monbukagaku-sho of Japan.
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