A Top-down Method to Fabricate Silicon Nanopillars

by Joshua B. Ratchford, Nelson Y. Mark, Madhu Roy, William L. Benard, 

and Cynthia A. Lundgren

ARL-TR-5500 March 2011

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Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TR-5500 March 2011

A Top-down Method to Fabricate Silicon Nanopillars

Joshua B. Ratchford, Nelson Y. Mark, Madhu Roy, William L. Benard, 

and Cynthia A. Lundgren

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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A Top-down Method to Fabricate Silicon Nanopillars

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Joshua B. Ratchford, Nelson Y. Mark, Madhu Roy, William L. Benard, and 

Cynthia A. Lundgren

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14. ABSTRACT

We report a method to fabricate silicon nanopillars with high-aspect ratios. Electron-beam lithography and a negative resist 

were used to pattern nanometer scale etch masks onto silicon wafers. Inductively coupled plasma-reactive ion etching (ICPRIE) with mixtures of Cl2 and HBr was used to etch nanostructures into the wafers. Silicon nanopillars with tip diameters of 

80 nm and aspect ratios of 10 were readily obtained.

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Top-down, silicon nanopillars inductively coupled reactive ion etching

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Joshua B. Ratchford

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iii

Contents

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental 1

2.1 Nanopattern Fabrication ..................................................................................................1

2.2 Inductively Coupled Reactive Ion Etching (ICP-RIE)....................................................1

3. Results and Discussion 3

4. Conclusion 4

5. References 5

Distribution List 6
iv

List of Figures

Figure 1. Panel of SEM images taken from Si nanostructures. The conditions used to 

prepare samples A–C are shown in table 1................................................................................3

List of Tables

Table 1. Composition of gas mixtures, bias powers, and aspect ratios. .........................................2
v

Acknowledgments

This research was performed while JBR held a National Research Council Research 

Associateship Award at the U.S. Army Research Laboratory (ARL).
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction

Several researchers have shown substantial improvements in the cycle lifetime of silicon (Si)

anodes in lithium (Li)-ion batteries by nanostructuring Si (1–3). These reports suggested that 

damage to the Si was reduced because the ratio of surface area-to-volume of these nanostructures 

was larger than that of bulk Si. Specific explanations as to why nanostructures substantially

improved the cycle lifetime of Si anodes in Li-ion batteries are lacking. 

The objective of this work was to develop a method for fabricating nanostructures with userdefined profiles and separations. We report a method to form Si nanostructures that takes 

advantage of techniques used to fabricate electronic elements in semiconductor processing. Si

nanopillars with tip diameters of 80 nm and aspect ratios of 10 were readily obtained. This 

method will enable basic research on the mechanical properties of Si nanostructures in Li-ion 

batteries.

2. Experimental

2.1 Nanopattern Fabrication

Nanopillars were fabricated as follows: a Si wafer with 100 nm of thermal oxide was pretreated 

with 10 ml of liquid hexamethyldisilazane (HMDS) for 40 s. The HMDS was removed by 

rotating the wafer at 500 rpm for 5 s. 3 ml of a negative electron-beam resist (NEB31A) were 

then added to the surface. The wafer was then rotated at 4000 rpm for 55 s, and then heated to 

115 °C for 2 min on a hotplate. The resulting thickness of the resist layer was 300±30 nm, as 

measured by spectroscopic reflectometry and confirmed by profilometry. 

Electron-beam lithography (Vistec EBPG5000+HR) was used to expose circular etch masks into 

the layer of resist. The diameter of each mask was 80 nm. The centers of the masks were 

separated by 230 nm. A 100 kV acceleration voltage, 600 pA beam current, and 80 μCcm–2dose 

were used. Each cell contained 1x104 masks. The writing time of each cell was less than 1 min. 

A post-exposure bake at 95 °C on a hotplate for 4 min was used. The resist was developed in 

Shipley Microposit MF-321 for approximately 10 s.

2.2 Inductively Coupled Reactive Ion Etching (ICP-RIE)

An ICP-RIE system (Unaxis, VLR 700) was used to remove the unpatterned thermal oxide. 

Plasma made with 1 mTorr CHF3 and 4 mTorr CF4, and a coil power of 500 W, and a bias power 

of 50 W were used. ICP-RIE with various mixtures of Cl2 and HBr were used to etch 

nanostructures into the patterned wafers. The total pressure of each mixture was 5 mTorr. The 

flow rate of the mixture through the chamber was 40 sccm. The coil power used to generate 
2

plasma was 300 W. The etch time was 360 s. Table 1 lists the various compositions of the 

mixtures and the bias powers used to etch the patterned wafers. Samples were treated with 

plasma made with 1 Torr O2 and RF power 500 W for 60 s after etching.

Table 1. Composition of gas mixtures, bias powers, and aspect ratios.

Sample Cl2/mTorr HBr/mTorr Bias Power/W Aspect Ratio

A 2.5 2.5 20 11

B 2.5 2.5 90 10

C 0.4 4.6 40 10

The profiles of the resulting nanostructures were observed by scanning electron microscopy 

(SEM). Images were captured 45° from plan view, with samples rotated ~15° around their 

vertical axis. Aspect ratios, A, were calculated from the SEM images using equation 1:

T

S

A





2

, (1)

where S is the stem length and T is the tip width. Examples of these values are marked in figure

1a. Profilometry was used to confirm the etch rates that were observed by SEM.
3

Figure 1. Panel of SEM images taken from Si nanostructures. The conditions used to prepare samples 

A–C are shown in table 1.

3. Results and Discussion

Low-magnification, plan-view SEM images of cells after etching showed that all patterns were 

transferred into the Si surface with excellent fidelity; no stitching errors or aberrations of the 

original image were observed. Figure 1 shows high-magnification SEM images of nanopillars 

formed from the circular etch masks using the corresponding conditions listed in table 1. 

As the values in table 1 show, nanopillars with aspect ratios of 10 were readily fabricated. The 

etch rate of Si using these conditions was 160±20 nm/min. We presume that the particles, like 

that marked with the white arrow in figure 1c, are residues from the etch process.

Figure 1d shows a representative image of a nanopillar formed from a 1300 nm diameter etch 

mask. Because the anisotropy of etched nanostructures decreases as the diameter of the etch 

B

C

A

D

200 nm 200 nm

200 nm 800 nm

S

T
4

mask increases (4–6), this structure was used for comparison with those made from the 80 nm 

diameter etch masks. Indeed, SEM images of nanopillars formed from the 1300 nm etch masks 

showed that their anisotropy was not markedly affected by any of the conditions used. However, 

as figure 1 shows, the anisotropy and profile of nanopillars made from 80 nm diameter etch 

masks were strongly dependent on the conditions of the plasma. 

Further inspection of figure 1 reveals the effects that the bias power and the composition of the 

gas mixture had on the profile of the nanopillars. For example, comparison of figure 1a to 1b

clearly shows that significantly increasing the bias power from 20 to 90 W enhanced the 

anisotropy of the nanopillars—presumably because the larger power increased the bombardment 

energy and amount of reactive ions that form volatile Si chlorides and Si bromides on the 

unpatterned surface (5). However, figure 1b also shows that increasing the bias power caused 

substantially more microtrenching (7). 

Other reports (4, 6, 8) have shown that perfectly anisotropic structures can be made in neat HBr 

plasmas. We were unable to reproduce these process conditions in our instrument with neat HBr. 

However, anisotropy was improved by decreasing the partial pressure of Cl2. For example, 

figure 1c shows nanopillars that are more anisotropic than those of figure 1a. Further inspection 

of figure 1c also shows that microtrenching of this sample was reduced as compared to the 

microtrenching of the sample in figure 1b. This result is attributed to the significantly reduced 

bias power used.

4. Conclusion

A method to fabricate Si nanopillars with aspect ratios greater than 10 was developed. The 

anisotropy of these Si nanopillars was affected by the partial pressures of Cl2 and HBr used 

during ICP-RIE. The amount of microtrenching around the nanopillars was controlled by the 

bias power used.
5

5. References

1. Chan, C. K.; Peng, H. L.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. 

Nanotechnol. 2008, 3, 31–35.

2. Kasavajjula, U.; Wang, C. S.; Appleby, A. J. J. Power Sources 2007, 163, 1003–1039.

3. Tarascon, J. M.; Armand, M. Nature 2001, 414, 359–367.

4. Bestwick, T. D.; Oehrlein, G. S. Journal of Vacuum Science & Technology A: Vacuum, 

Surfaces, and Films 1990, 8, 1696–1701.

5. Schwartz, G. C.; Schaible, P. M. J Vac Sci Technol 1979, 16, 410–413.

6. Tsou, L. Y. J. Electrochem. Soc. 1989, 136, 3003–3006.

7. Abdollahi-Alibeik, S.; McVittie, J. P.; Saraswat, K. C.; Sukharev, V.; Schoenborn, P. J Vac 

Sci Technol A 1999, 17, 2485–2491.

8. Cheng, C. C.; Guinn, K. V.; Herman, I. P.; Donnelly, V. M. J Vac Sci Technol A 1995, 13, 

1970–1976.
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