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Diamond Tip Arrays for Parallel Processing of Microelectromechanical Systems

Fumiya Watanabe*, a, Makoto Aritaa, Teruaki Motookaa, Ken Okanob, Takatoshi Yamadac

aDepartment of Materials Science and Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-81, Japan
bDepartment of Electronic & Photonics System Engineering, Kochi University of Technology, Tosa-Yamada, Kochi 782, Japan
cDepartment of Electrical Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-12, Japan

This is an abstract for a presentation given at the
Sixth Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is available on the web.

 

      We have built a prototype system capable of scanning probe microscope tip based parallel processing for microelectromechanical fabrication. It utilizes uniform array of diamond tips originally intended for field emission sources. Parallel patterns are simultaneously fabricated on polymeric thin films spin coated on silicon wafers. The processing line width of 100 nm is demonstrated. In addition, a regularly spaced carbon islands have been deposited onto silicon surfaces by simultaneous field evaporation. The diamond tip array method has potential to greatly enhance nanoscale lithography and data storage derived from proximal probe techniques whose shortcoming is the processing speed.

I. Introduction

      Microelectromechanical systems have great potentials to revolutionize all areas of science and technology by creating sophisticated miniature devices. The basis of the microelectromechanical systems fabrication is today's silicon processing technology mostly based on optical lithography and wet or dry etching processes. The optical limitation of such lithographic techniques will eventually force the semiconductor industry to adapt new ways of processing ever smaller structures. Since Eigler and coworkers have shown that a scanning microscope tip can be used to manipulate single atoms, creation of nanoscale structures with scanning probe microscopes (SPM) has been a popular subject of study. However, compared to the optical lithography, the processing speed of SPM is too slow for implementation in actual device construction. There have been some efforts to construct parallel processing SPM devices with multi-tip cantilevers. Most of these are delicate devices based on atomic force microscopes (AFMs).

We have proposed a apparatus with diamond tip arrays, called scanning field emitter arrays (SFEAs), for parallel tip processing[1]. Simpler and sturdier than the cantilever based AFMs, structures ranging from 100 nm ~ 300 µm have been fabricated on silicon surfaces by direct ablation with the diamond scanning field emitter arrays. In addition, 1~25 µm carbon islands have been deposited by field evaporation from the diamond tips in air as shown in Fig. 1 below. We also present recent results on patterning of polymer films spin coated on Si wafers with the diamond SFEA.

Fig. 1. Carbon islands simultaneously field evaporated from diamond tips (from ref. [1]).

II. Device Description

Diamond thin films are known to field emit at very low voltages in some cases due to its negative electron affinity. And since diamond is very stable both chemically and physically, it has become very popular material to investigate for possible application in flat panel displays. Okano et al. has developed an array of pyramidal shaped diamond tips utilizing pitted silicon wafers as molds. The boron doped tips shown in Fig. 2 are approximately 50 µm in height and 70 µm at their bases, each separated by the 7 µm wide channels. The approximate apex radius is 500 nm. The size and the spacing are easily controlled by varying the silicon wafer molds.

Fig. 2. SEM micrograph of diamond tip array with 7 µm spacings. The base of the tips is 70 µm long and their height is 35 µm for these tips.

Device used in this study is constructed from a hardness tester with a diamond FEA mounted instead of a diamond indenter. The lateral (X and Y) movements are by micrometers with better than 10 µm resolution, and the vertical (Z) movements can be controlled down to 0.1 µm. The array can be constructed with varying tip sizes and spacings. The ones used in this study are approximately 2 mm x 2 mm, containing 40~600 tips depending on the tip size and the spacing.

III. Experimental Results

The samples used in our trials are 20 mm x 20 mm chips cut out of a boron doped Si(100) wafer. Si chip is chemically cleaned prior to the spin coating by PVC solved in cyclohexanone (1 wt. %) at 2000 rpm for 50 sec. The approximate thickness of the PVC film ranges from 100 to 200 nm.

The diamond tips were brought into contact with the PVC thin film coatings and various patterns were drawn. Eleven pattern all ablated simultaneously are recognizable in Fig. 3 below. The silicon wafer was later dipped in a dilute HF solution. In Fig. 4, 150 nm wide line, left on the wafer after the PVC film was removed, is given. In Fig. 3, some of the patterns are missing indicating that there was some non-uniformness in the tip height. The reason for this may be due to the deformation of substrate. In addition, these patterns were drawn with the same tip array used in the carbon island deposition and had some damages.

Fig. 3. Parallel patterns drawn on a PVC coated Si wafer before the etching in HF.

Fig. 4. SEM micrograph of a fine line feature created after HF dip of the diamond tip processed Si with a PVC coating.

IV. Conclusion

We have demonstrated a way for greatly enhancing the speed of SPM based tip processing. Instead of relying on the fine control of tips, this device utilizes the toughness of the diamond tips. To improve the performance, we must rectify such factors as vibrational dumping, thermal stability, and accuracy of scanning mechanism. The device should be equipped with standoff columns which enable large size array to be placed on wafers directly. Ultimately, these columns should contain piezoelectric material for scanning, making the device a self sustained entity placed on a wafer. In addition to semiconductor device processing such as fabrication of quantum dots, SFEA may be a way of extending the abilities of other single tip techniques. Since diamond has large band gap of ~5 eV, a properly formed tip can focus light being illuminated from the back onto the apex of the tip with wavelength down to the ultraviolet range. The diamond tip array may be suitable for direct near field optical patterning of lithographic masks and thermomechanical data storage.

Acknowledgement

This work was partially supported by a Grant-in-Aid from the Ministry of Education, Science, Sports, and Culture, Japan and by JSPS Research for the Future Program in the Area of Atomic-Scale Surface and Interface Dynamics under the project of Dynamic Behavior of Silicon Atoms, Lattice Defects and Impurities near Silicon Melt-Crystal Interface.

References

[1] F. Watanabe, M. Arita, T. Motooka, K. Okano, and T. Yamada, Jpn. J. Appl. Phys. Part 2, Vol. 37, pp. L562-564, (1998).


*Corresponding Address:
Dr. Fumiya Watanabe
Department of Materials Science & Engineering, Kyushu University
6-10-1 Hakozaki, Higashai-ku, Fukuoka, 812-81 JAPAN
Phone: 81-92-642-3676; Fax: 81-92-632-0434
E-mail: watanabe@zaiko.kyushu-u.ac.jp



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