Nanogrids and Beehive-Like Nanostructures Formed by Plasma Etching the Self-Organized SiGe Islands
© The Author(s) 2010
Received: 19 April 2010
Accepted: 25 May 2010
Published: 8 June 2010
A lithography-free method for fabricating the nanogrids and quasi-beehive nanostructures on Si substrates is developed. It combines sequential treatments of thermal annealing with reactive ion etching (RIE) on SiGe thin films grown on (100)-Si substrates. The SiGe thin films deposited by ultrahigh vacuum chemical vapor deposition form self-assembled nanoislands via the strain-induced surface roughening (Asaro-Tiller-Grinfeld instability) during thermal annealing, which, in turn, serve as patterned sacrifice regions for subsequent RIE process carried out for fabricating nanogrids and beehive-like nanostructures on Si substrates. The scanning electron microscopy and atomic force microscopy observations confirmed that the resultant pattern of the obtained structures can be manipulated by tuning the treatment conditions, suggesting an interesting alternative route of producing self-organized nanostructures.
KeywordsSiGe High-resolution reciprocal space mapping SEM AFM TEM
Periodical nanostructures are of great research interest because of their potential applications in data storage [1–3] as well as in preparing photonic crystals [4, 5]. In order to realize such opportunities, the development of lithography techniques that are capable of fabricating large area periodical nanostructures with reasonable control over their size and periodicity is required. In general, two approaches, namely the top–down and the bottom–up, have been coined to label the techniques used to generate nanometer-sized structures. The conventional lithographical methods, including electron-beam lithography , photolithography  and focused ion beam lithography , are the representative top–down approaches widely implemented in manufacturing nano-scale semiconductor devices as well as nanostructures for various materials. However, these techniques often require very high capital investment and involve multiple-step processes, which not only limits the facility accessibility but also results in relatively high operation cost.
On the other hand, self-organized growth [9–11] has been demonstrated to be a viable bottom–up method for fabricating large area nanostructures with reasonable control of size and shape distributions. These structures can be, in turn, used as templates for building nanometer-scale structures. Here, we report a simple fabrication technique capable of producing large area, well-ordered, periodic nanogrids with sufficient size control in the sub-500-nm region. The present method consists of two major steps. First, the SiGe films deposited on Si substrates by ultrahigh vacuum chemical vapor deposition (UHVCVD) were transformed into self-assembled SiGe nanoisland arrays by thermal annealing. Second, the resultant SiGe nano-island arrays after subjected to subsequent reactive ion etching (RIE) treatments were found to result in either the quasi-beehive nanostructures or the self-organized nano-grids (SONGs) on Si substrates, depending on the conditions of RIE processes. It is noted that the current fabrication method is advantageous in several respects. First, since the SiGe thin films were deposited in the UHVCVD system, hence the issue of contamination during the annealing process can be largely minimized. Moreover, owing to the fact that no aqueous chemical solution and metallic material were used in the manufacturing procedures, protection of the RIE system from major pollution sources is guaranteed. Finally, the lithography-less anisotropic etching process can reduce the fabrication cost significantly.
Following the film growth, in situ thermal annealing was carried out at 900°C for 30 min in the UHVCVD chamber to form the well-ordered SiGe nanoislands, as illustrated schematically in Fig. 1b. The annealed SiGe/Si assembles were then placed into the reactive ion etching (RIE, TEL TE5000 Japan) chamber and, subjected to RIE using CF4 (40 sccm) and argon (200 sccm) at an RF power of 200 W for 3, 5, or 10 min, respectively, as depicted in Fig. 1c. The effect of ion bombardment was primarily determined by the ion energy, which, in turn, was dependent on the RF power and the self-bias. During the RIE process, when the reactive ions passed through the sheath region, the positive ions were accelerated under the inserted electric field to produce the ion bombardment effect that, in turn, etches the target materials to form the quasi-beehive Si nanostructures and the SONGs, as shown in Fig. 1d.
The high-resolution cross-sectional transmission electron microscopy (XTEM) image of thin film was analyzed with an operating voltage of 200 kV. The composition of the films was analyzed by Auger electron spectroscopy (AES, VG Scientific Microlab 310F). The Auger analyses were performed in a Physical Electronics-650 scanning Auger microprobe with a background pressure of 1.0 × 10−9 Torr. High-resolution X-ray diffraction was used to determine the phase formation and crystallographic structure of all samples. High-resolution reciprocal space mapping (HRRSM) was applied to observe the structural features of SiGe thin films. The characteristics of the surface morphology of the Si substrate as well as that of the SiGe films were observed by field-emission scanning electron microscopy (FESEM). Atomic force microscopy (AFM) was also used to image the surface morphologies of the fabricated samples.
Results and Discussion
In any case, the present study has not only presented a detailed account for the formation of the self-organized nanoislands arrays by thermal annealing, but also has indicated a very efficient method of producing the much desired self-organized nano-grids (SONGs) on Si substrate by using the self-organized nanoisland arrays as the “sacrificing” mask. We emphasize that the present process has completely avoided the usage of any lithographic process, which should be of significant practical importance in future applications. Experiments using these SONGs nanostructure as the template substrate to fabricate nanostructures of various interesting materials are underway.
In summary, we have shown that it is possible to fabricate self-organized nanogrids arrays on Si substrate by simply combining the thermal annealing and RIE (or Ar plasma) processes on the SiGe layers grown on Si substrate. The compositions and structures of SiGe thin film are characterized by using Auger and XTEM techniques to reveal island formation mechanism. The results indicate that the self-organized SiGe islands were formed via the Asaro-Tiller-Grinfeld instability-induced surface roughening driven by the strain established between the heteroepitaxy SiGe film and the Si substrate. A well-ordered self-organized nanogrids structure formed on the Si substrate was successfully demonstrated by treating the annealed SiGe film in Ar plasma for as short as only 1 min and without resorting to any lithographical means.
1 Various values (ranging from 0.22  to 0.28 ) of the Poisson’s ratio for Si x Ge1−x films have been reported. Here, we take an average value for estimation only.
This work was partially supported by the National Science Council of Taiwan, under Grant No.: NSC 97-2112-M-214-002-MY2. JYJ is supported in part by the National Science Council of Taiwan and the MOE-ATP program operated at NCTU. The authors would like to thank Prof. Ching-Liang Dai (Department of Mechanical Engineering, National Chung Hsing University, Taiwan) and Dr. Jiann Shieh and Hung-Min Chen (National Nano Device Laboratories, Taiwan) for their useful discussions. Assistances from Fu-Kuo Hsueh for UHVCVD, Chiung-Chih Hsu for TEM, Jie-Yi Yao for XRD and Chih-Ming Wu for RIE technical supports in National Nano Device Laboratories are also gratefully acknowledged.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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