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Journal of Vacuum Science & Technology A / Volume 30 / Issue 2 / Plasma Science and Technology

J. Vac. Sci. Technol. A 30, 021306 (2012); http://dx.doi.org/10.1116/1.3681285 (10 pages)

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

Hyungjoo Shin, Weiye Zhu, Vincent M. Donnelly, and Demetre J. Economou

Plasma Processing Laboratory, Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun Road, Houston, Texas 77204

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(Published online 6 February 2012)

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The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only likely explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths < 1700 Å. The photo-assisted etching rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, photo-assisted etching likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, contributing to the commonly observed sloped sidewalls and microtrenches.

© 2012 American Vacuum Society

ACKNOWLEDGMENTS

This work was supported by the Department of Energy, Office of Fusion Energy Science, Contract No. DE-SC0001939, the National Science Foundation Grant No. CBET 0903426, the Department of Energy Grant No. DE-SC0000881, and Varian Semiconductor Equipment Associates (VSEA). Many thanks to Ludovic Godet of Varian Semiconductor Equipment Associates, for SEMs and for providing patterned Si samples.

Article Outline

  1. INTRODUCTION
  2. EXPERIMENT
  3. RESULTS AND DISCUSSION
    1. Ion-assisted etching rates, thresholds, and yields
    2. Sub-threshold etching
    3. Spontaneous chemical etching by Cl atoms
    4. Ar metastable-assisted etching
    5. Grid experiments that rule out very low energy ( E  <  E th ) ion-assisted etching, and provide evidence for photo-assisted etching
    6. Wavelength dependence for photo-assisted etching
  4. SUMMARY AND CONCLUSIONS

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KEYWORDS and PACS

Keywords

Auger effect, elemental semiconductors, Faraday effect, silicon, sputter etching

PACS

History
Received 29 November 2011
Accepted 11 January 2012
Published online 6 February 2012
Digital Object Identifier
http://dx.doi.org/10.1116/1.3681285

PUBLICATION DATA

ISSN

0734-2101 (print)  
1520-8559 (online)

Publisher

American Vacuum Society

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