Morphology control of silicon nanotips fabricated by electron cyclotron resonance
plasma etching
C. H. Hsu, Y. F. Huang, L. C. Chen, S. Chattopadhyay, K. H. Chen, H. C. Lo, and C. F. Chen
Citation: Journal of Vacuum Science & Technology B 24, 308 (2006); doi: 10.1116/1.2163894
View online: http://dx.doi.org/10.1116/1.2163894
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/24/1?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
Articles you may be interested in
Silicon nitride nanotemplate fabrication using inductively coupled plasma etching process
J. Vac. Sci. Technol. B 29, 051802 (2011); 10.1116/1.3628593
Trap density of GeNx/Ge interface fabricated by electron-cyclotron-resonance plasma nitridation
Appl. Phys. Lett. 99, 022902 (2011); 10.1063/1.3611581
Self-organized vertically aligned single-crystal silicon nanostructures with controlled shape and aspect ratio by reactive plasma etching
Appl. Phys. Lett. 95, 111505 (2009); 10.1063/1.3232210
In vacuo substrate pretreatments for enhancing nanodiamond formation in electron cyclotron resonance plasma
J. Vac. Sci. Technol. A 24, 1802 (2006); 10.1116/1.2221322
Effects of bias frequency on reactive ion etching lag in an electron cyclotron resonance plasma etching system
resonance plasma etching
C. H. Hsu, Y. F. Huang, and L. C. Chen
Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan
S. Chattopadhyaya兲and K. H. Chen
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
H. C. Lo and C. F. Chen
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
共Received 9 May 2005; accepted 12 December 2005; published 20 January 2006兲
Formation of well-aligned silicon nanotips etched monolithically from a silicon substrate has been demonstrated. The effect of the process temperature on the physicochemical etching of silicon and subsequent fabrication of these nanotips has been investigated. 2.2-m-long nanotips were formed at the process temperature of 250 ° C and then decreased in length with increasing process temperature. Above 800 ° C, the formation of the silicon nanotips was inhibited. Spectroscopic evidence attributes this fact to the efficient formation of silicon carbide thin film at higher process temperatures, instead of discontinuous nanomasks at lower process temperatures that prevent etching of the substrate. Another reason for this inhibited formation of nanotips is the reduced etching rate of the silicon by agents such as atomic H at higher process temperatures. © 2006
American Vacuum Society. 关DOI: 10.1116/1.2163894兴
I. INTRODUCTION
Among the widely published nanotubes, nanorods, and nanowires there have also been several reports on one di-mensional 共1D兲 conical structures. Starting from metallic,1 silicon,2 and diamond3 nanotips, a wide variety of conical, needlelike structures has been reported in recent years. These include silicon nanotip,2nanocone,4and nanoneedle,5carbon nanotips,6 ZnO nanoneedle arrays,7 AIN nanotips,8 and so forth. The sharp apex angle and relatively high aspect ratio make the nanotips promising for many applications. The fas-cinating structures of 1D nanomaterials and their unique properties are opening up applications in nanodevices,9such as field-emission displays,10 laser diodes,11 scanning probe microscopy,12solar cells,13 biosensors,14and so on.
In our previous work,15 we have demonstrated a self-masked dry-etching technique for fabricating silicon 共Si兲 nanotips in one step. These Si nanotips exhibited a low turn-on field during field-emission measurements and inher-ent Si process compatibility.16Such tips have also been used in solar cells,17 imprint lithography,18 and molecular sensing.19,20However, the control of tip morphology is cru-cial for such wide-ranging applications. This letter reports the influence of process temperature on the mechanism of a self-masked dry-etching technique of Si, giving rise to the nanotip morphology.
II. EXPERIMENT
The processes were carried out in an electron cyclotron resonance 共ECR兲 plasma reactor using a gas mixture of si-lane共SiH4兲, methane 共CH4兲, argon 共Ar兲, and hydrogen 共H2兲
as dry-etching gases. The details of the process can be found elsewhere.15,16The starting substrates were boron共B兲-doped
p-type silicon共100兲 wafers with a resistivity of 1–10 ⍀ cm.
In this study, a set of experiments were performed with vari-ous process temperatures ranging from 250 to 850 ° C by keeping a constant flow rate ratio of SiH4/ CH4/ Ar/ H2
= 0.2: 2 : 5 : 8, a total pressure of 5.8 mTorr, and a power of 1200 W. The morphology of the silicon nanotips has been imaged with a JEOL 6700 field-emission scanning electron microscope 共SEM兲. The x-ray photoelectron spectroscopy 共XPS兲 was carried out in a PHI 1600 system. The Fourier transform infrared 共FTIR兲 spectra were obtained with a Bomem 共Hartmann-Braun兲 MB series FTIR spectrometer in the reflection mode. The resolution of the spectrum was 4 cm−1.
III. RESULTS AND DISCUSSION
Figure 1共a兲 shows a typical cross-sectional SEM image of a well-aligned Si nanotip array fabricated at 300 ° C with a uniform distribution and a high density of ⬃3⫻1010/ cm2.
Figures 1共b兲–1共d兲 show cross-sectional SEM images of nanotip arrays grown at 400, 600, and 800 ° C, respectively. A typical high-resolution transmission electron microscopy 共HRTEM兲 image of a Si nanotip indicates that all of these nanotips were capped at the apex with SiC nanomasks.16We observed that the length and density of the nanotips de-creased with increasing process temperature. At process tem-peratures exceeding 800 ° C, for example, 850 ° C, there was hardly any nanotip visible. Instead a thick-film-like morphol-ogy was observed. Figure 2 shows a plot of the nanotip length as a function of the process temperature. From 2200 nm at a process temperature of 250 ° C, the length of these nanotips decreased to less than 300 nm at a process
a兲Author to whom correspondence should be addressed; electronic mail:
temperature of 600 ° C and the formation of the nanotips were totally inhibited at temperatures above 800 ° C.
FTIR spectroscopy was done to determine the bonding characterization of these nanotips as the process temperature was varied. As shown in the FTIR spectra 共Fig. 3兲, the samples fabricated at low process temperatures did not ex-hibit any significant SiC signal until the process temperature exceeded 700 ° C. A peak evolved at 800 cm−1
correspond-ing to the 3C–SiC共TO兲 mode.21
The weak peak located at approximately 600 cm−1 was
due to the diffusion of excess carbon into the Si substrate during process.22 XPS results corroborated the FTIR find-ings, qualitatively, by showing that the carbon content of the surface of the samples produced at 800 ° C was much higher than those produced at 250 ° C关Fig. 4共a兲兴, whereas the sili-con sili-content at the surface was much lower for the sample fabricated at 800 ° C 关Fig. 4共b兲兴.
At lower process temperatures, well-separated nanomasks of SiC may have existed on the Si substrate but remained undetectable due to minimal volume. Such SiC nanomasks, being harder to etch than silicon, however, still prevented
uniform etching of the silicon substrate and gave rise to the nanotips. As the process temperature was increased, methane dissociation became more efficient in the vicinity of the sili-con substrate and, instead of well-separated SiC nanomasks, a continuous layer of SiC or a carbon-rich SiC layer was formed. The formation of a SiC layer decreased the etching rate of silicon and finally inhibited further lengthening of nanotips. We estimated that the rate constants for
dissocia-FIG. 1. Cross-sectional SEM images of Si nanotips prepared under process temperatures of共a兲 300, 共b兲 400, 共c兲 600, and 共d兲 800 °C.
FIG. 2. Length of Si nanotips as a function of process temperature.
FIG. 3. FTIR spectra of the Si nanotips grown at different process
tempera-tures共marked on each plot兲.
FIG. 4. 共a兲 XPS C 共1s兲 spectra of Si nanotip samples prepared at 共—兲 250 and共-·-·-兲 800 °C. 共b兲 XPS Si 共2p兲 spectra of Si nanotip samples prepared at 共—兲 250 and 共-·-·-兲 800 °C.
309 Hsu et al.: Morphology control of silicon nanotips 309
tion of CH4were enhanced by two orders of magnitude from
1⫻109 at 250 ° C to 1⫻1011 at 700 ° C,23
whereas the rate constants for the dissociation of SiH4 increased from 1
⫻1012at 250 ° C to 3⫻1012at 700 ° C. Moreover, at a
pro-cess temperature of 700 ° C, the dissociation rate constant for CH4 is only one order smaller than that of SiH4, which would predict an efficient pathway for the formation of SiC. In a plasma system the addition of SiH4can greatly enhance
the deposition of SiC.24As SiH4can be dissociated into SiH2
and SiH3radicals, CH4 can similarly be dissociated to CH3 or CH2radicals with the release of H through thermal
disso-ciation or electron or Ar*共excited Ar兲 impact dissociation in
the ECR plasma. These radicals can react via a series of neutral-neutral reactions to result in radicals such as C2H2.25
The silyl radicals can then react with C2H2or CH3following
a series of H abstraction to form SiC.26
For the surface-etching process, we apply the ion-assisted dry-etching theory. From our previous investigation, only physicochemical sputtering, instead of pure physical sputtering,27 can contribute to a significant change in mor-phology when applied to different materials as substrates.15 Therefore, ion-assisted chemical etching must occur through hydrogen atoms as etchant, for instance. Several other spe-cies are also involved in this etching, such as H, H+, H
2 +, H 3 +, Ar+, Ar 2
+, and ArH+. It is known that ion bombardment can
activate the surface, changing the surface reaction rates and mobility of reactive species. In the ECR plasma, the surface is bombarded by heavy ions causing a sputtering effect or molecular desorption from the Si substrate.
One can visualize the process as follows: First, H radicals are absorbed on the Si surface to form a layer of SiH. The formation of SiH2, SiH3, and subsequent gas-phase SiH4
fol-lows through reactions with H3+and ArH+ions and reduction
by H atoms. This mechanism becomes less probable due to the low sticking coefficients of these radicals on the Si sub-strates at higher temperatures, leading to a decrease of etch-ing rate which is in accordance with our observation. Fur-thermore, Strass et al.28 reported that there are two temperature regimes of Arrhenius dependence with different activation energies occurring in the hydrogen etching pro-cess. For temperatures above 300 ° C the activation energy is −21 kcal/ mol and that below 300 ° C is only −1.7 kcal/ mol. This leads to a decrease in etching rate when temperature is beyond 300 ° C. Assuming the growth rate共R兲, in nm/h, of the nanotips to be proportional to the rate of any chemical reaction, including etching, taking place, we can plot the ln共R兲 as a function of inverse process temperature. This plot 共Fig. 5兲 also reveals two activation energies of −134.2 and −19.9 kJ/ mol above and below 600 ° C, respectively. The difference in the activation energies in this case as against the values reported in Ref. 28 may have been caused by the fact that we are etching a partially masked共with SiC兲 silicon substrate, whereas Strass et al.28were etching silicon or sili-con dioxide samples. In other words, the presence of both silane and methane will increase and decrease, respectively, the activation energies by forming the nanomasks and pro-ducing more etchants in the form of reactive H species.
From Fig. 5, it is clear that our samples with process temperatures above 600 ° C were much shorter than those produced below 600 ° C. The experimental evidence of re-duced nanotip length with increasing process temperature, along with the FTIR and XPS data put forward to support the formation of a continuous SiC film at higher process tem-peratures, clearly establishes the role of the SiC nanomasks in the formation of these Si nanotips and elucidates the dry-etching mechanism in Si.
IV. CONCLUSIONS
In summary, a temperature-controlled plasma etching of silicon substrates producing monolithic silicon nanotips has been described. The silicon nanotip originates from the non-uniform etching of the silicon substrate protected by SiC nanomasks in the reactive microwave plasma. The length of the Si nanotips decreased with increasing process tempera-ture, and beyond 800 ° C, the nanotip formation was inhib-ited. The observed length reduction and ultimate extinction of the nanotips with increasing process temperature are due to the growth of a continuous SiC film at high temperatures preventing effective etching. On the other hand, a low stick-ing coefficient of the hydrogen-related species reduced the effective chemical etching rate at higher process temperatures.
ACKNOWLEDGMENTS
The work was carried out with financial assistance from National Science Council, Ministry of Education, Taiwan, Air Force Office of Scientific Research, USA, and Asian Of-fice of Aerospace Research and Development. One of the authors共S.C.兲 would like to acknowledge a fellowship from the Institute of Atomic and Molecular Science, Academia Sinica, Taiwan.
This paper was presented at the First International Workshop on One-Dimensional Materials, January 10–14, 2005, National Taiwan University, Taipei, Taiwan.
FIG. 5. Arrhenius plot of the etching rate as a function of inverse process
1C. H. Oon, J. T. L. Thong, Y. Lei, W. Lei, and W. K. Chim, Appl. Phys.
Lett. 81, 3037共2002兲.
2M. H. Yun, V. A. Burrows, and M. N. Kozicki, J. Vac. Sci. Technol. B 16,
2844共1998兲.
3C. L. Tsai, C. F. Chen, and C. L. Lin, J. Appl. Phys. 90, 4847共2001兲. 4N. G. Shang, F. Y. Meng, F. C. K. Au, Q. Li, C. S. Lee, I. Bello, and S.
T. Lee, Adv. Mater.共Weinheim, Ger.兲 14, 1308 共2002兲.
5M. Kanechika, N. Sugimoto, and Y. Mitsushima, J. Vac. Sci. Technol. B 20, 1843共2002兲.
6C. L. Tsai, C. F. Chen, and C. L. Lin, Appl. Phys. Lett. 80, 1821共2002兲. 7W. I. Park, G. C. Yi, M. Kim, and S. J. Pennycook, Adv. Mater.
共Wein-heim, Ger.兲 14, 1841 共2002兲.
8S. C. Shi, C. F. Chen, S. Chattopadhyay, Z. H. Lan, K. H. Chen, and L. C.
Chen, Adv. Funct. Mater. 15, 781共2005兲.
9Z. L. Wang, Adv. Mater.共Weinheim, Ger.兲 12, 1295 共2000兲.
10W. B. Chou, D. S. Chung, J. H. Kang, H. Y. Kim, Y. W. Jin, I. T. Han, Y.
H. Lee, J. E. Jung, N. S. Lee, G. S. Park, and J. M. Kim, Appl. Phys. Lett.
75, 3129共1999兲.
11M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber,
R. Russo, and P. Yang, Science 292, 1897共2001兲.
12H. Dai, J. H. Hafner, A. G. Rinzler, A. G. Colbert, and R. E. Smalley,
Nature共London兲 384, 147 共1996兲.
13W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 2425
共2002兲.
14J. N. Wohlstadter, J. L. Wilbur, G. B. Sigal, H. A. Biebuyck, M. A.
Billadeau, L. Dong, A. B. Fischer, S. R. Gudibande, S. H. Jameison, J. H. Kenten, J. Leginus, J. K. Leland, R. J. Massey, and S. J. Wohlstadter,
Adv. Mater.共Weinheim, Ger.兲 15, 1184 共2003兲.
15C. H. Hsu, H. C. Lo, C. F. Chen, C. T. Wu, J. S. Hwang, D. Das, J. Tsai,
L. C. Chen, and K. H. Chen, Nano Lett. 4, 471共2004兲.
16H. C. Lo, D. Das, J. S. Hwang, K. H. Chen, C. H. Hsu, C. F. Chen, and
L. C. Chen, Appl. Phys. Lett. 83, 1420共2003兲.
17H. Yoshida, Y. Terada, H. Miyake, and K. Hiramatsu, Jpn. J. Appl. Phys.,
Part 2 41, L1134共2002兲.
18S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl. Phys. Lett. 67, 3114
共1995兲.
19S. Chattopadhyay, H. C. Lo, C. H. Hsu, L. C. Chen, and K. H. Chen,
Chem. Mater. 17, 553共2005兲.
20S. Chattopadhyay, S. C. Shi, C. F. Chen, L. C. Chen, and K. H. Chen, J.
Am. Chem. Soc. 127, 2820共2005兲.
21S. Liedtke, K. Jahn, F. Finger, and W. Fuhs, J. Non-Cryst. Solids 77/78,
849共1985兲.
22Y. Sun, T. Miyasato, and N. Sonoda, J. Appl. Phys. 84, 6451共1998兲. 23M. J. Rabinowitz, J. W. Sutherland, P. M. Patterson, and R. B. Klemm, J.
Phys. Chem. 95, 674共1991兲.
24I. Solomon, M. P. Schmidt, and H. Tran-Quoa, Phys. Rev. B 38, 9895
共1988兲.
25W. L. Hsu, J. Appl. Phys. 72, 3102共1992兲.
26W. H. Lee, J. C. Lin, C. Lee, H. C. Cheng, and T. R. Yew, Diamond Relat.
Mater. 10, 2075共2001兲.
27P. Cuerno and A. L. Barabasi, Phys. Rev. Lett. 74, 4746共1995兲. 28A. Strass, W. Hansch, P. Bieringer, A. Neubecker, F. Kaesen, A. Fischer,
and I. Eisele, Surf. Coat. Technol. 97, 158共1997兲.
311 Hsu et al.: Morphology control of silicon nanotips 311
