Article pubs.acs.org/JPCC
Etching Silicon with HF‑H2O2‑Based Mixtures: Reactivity Studies and Surface Investigations Christoph Gondek,† Marcus Lippold,† Ingo Röver,§ Klaus Bohmhammel,‡ and Edwin Kroke*,† †
Institute of Inorganic Chemistry, TU Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg, Germany Institute of Physical Chemistry, TU Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg, Germany § Deutsche Solar GmbH, Berthelsdorfer Straße 111A, D-09599 Freiberg, Germany ‡
S Supporting Information *
ABSTRACT: In semiconductor and photovoltaic industries numerous process steps deal with etching and silicon surface modification. The present study focuses on the reactivity of HF-H2O2-based mixtures toward silicon surfaces in a wide range of concentrations. The generally very moderate reactivity is investigated regarding kinetic aspects and the silicon dissolution reactions. The activation energy of silicon dissolution in HF-H2O2 mixtures is determined to be ∼50 kJ/mol, which supports a surface reaction controlled mechanism. This interpretation is checked by oxidation experiments of Si surfaces with HF-free H2O2 solutions. Resulting silicon surfaces were characterized by means of diffuse reflection Fourier transform infrared spectroscopy and photoelectron spectroscopy. Surface properties give hints for an “electrochemical” silicon oxidation. Furthermore, the oxidation behavior of different H2O2 solutions is compared to that of HNO3 solutions. All results suggest kinetically limited silicon dissolution in HF-H2O2 mixtures and hole injection into the silicon surface to be the rate-determining part of the reaction process.
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+0.7 V.19 The standard redox potential of HNO3 and H2O2 in acidic media (pH 0) are +0.94 and +1.76 V, respectively (eqs 3 and 4; for thermodynamic data, see Table 1).20 Furthermore,
INTRODUCTION Numerous processing steps in microelectronics and photovoltaics are concerned with the purification and etching of different silicon materials. Examples are Si surface cleaning,1−3 chemical−mechanical polishing (CMP), 4−6 saw damage removal and texturizing,7−9 and formation of porous Si.10−12 Acidic etching of Si requires the stepwise oxidation of Si atoms by adequate oxidizing agents and the formation of water-soluble complexes (SiF62−, HSiF6−, and others) and/or fluorosilanes (SiF4, HSiF3, and others)13 by hydrofluoric acid. In most cases, HNO3 is used as an oxidizing agent.14 Regarding environmental and recycling aspects, etching with HF-HNO3-based mixtures possesses several disadvantages. Highly toxic and corrosive nitrous gases NO x are formed,14−17 and the disposal and recycling of exhausted etch baths requires removal of the nitrate ions.18 The replacement of HNO3 as oxidizing agent by H2O2 can prevent some of these disadvantages, as simply shown by eq 1. The reaction products are H2O and dissolved Si as well as possibly O2 formed by decomposition of hydrogen peroxide (eq 2).
Table 1. Standard Redox Potential (E0), Gibbs Energy (ΔRG0 (298 K)), and Standard Enthalpy of Reaction (ΔRH0 (298 K)) of Reactions in Equations 3 to 6
(1) (2)
An important point to be considered in this regard is the standard redox potential. Oxidizing Si surfaces in HFcontaining media requires a redox potential of at least © 2014 American Chemical Society
E0 (V)
ΔRG0 (298 K) (kJ mol−1)
ΔRH0 (298 K) (kJ mol−1)
3 4 5 6
0.9420 (0.9624) 1.7620 −0.1224 2.5921 (2.7324)
−276.924 −340.324 −11.1224 −262.924
−274.524 −380.524 −47.124 −280.824
decomposition of H2O2 forms OH-radicals, having an even higher redox potential (E0 = +2.59 V,21 eqs 5 and 6). Thus, with respect to thermodynamics, H2O2 should be an even better oxidant for Si than HNO3. However, in the sparse literature data a very moderate reactivity of HF-H2O2 mixtures toward boron-doped Si is reported, e.g., indicated by the low etch rates of
