Etching silicon with HF-HNO3-H2SO4/H 2O mixtures- unprecedented formation of trifluorosilane, hexafluorodisiloxane, and Si-F surface groups

Article abstract of DOI:10.1002/ejic.201200674

The etching behaviour of sulfuric-acid-containing HF-HNO₃ solutions towards crystalline silicon surfaces has been studied over a wide range of H₂SO₄ concentrations. For mixtures with low sulfuric acid concentration, NO₂/N₂O₄, N ₂O₃, NO and N₂O have been detected by means of FTIR spectroscopy. Increasing concentrations of nitric acid lead to high etching rates and to an enhanced formation of NO₂/N₂O ₄. Different products were observed for the etching of silicon with sulfuric-acid-rich mixtures [c(H₂SO₄) > 13 mol L ^-1]. Trifluorosilane and hexafluorodisiloxane were identified by FTIR spectroscopy as additional reaction products. In contrast to the commonly accepted wet chemical etching mechanism, the formation of trifluorosilane is not accompanied by the formation of molecular hydrogen (according to Raman spectroscopy). Thermodynamic calculations and direct reactions of F ₃SiH with the etching solution support an intermediate oxidation of trifluorosilane and the formation of hexafluorodisiloxane. The etched silicon surfaces were investigated by diffuse reflection FTIR and X-ray photoelectron spectroscopy (XPS). Surprisingly, no SiH terminations were observed after etching in sulfuric-acid-rich mixtures. Instead, a fluorine-terminated surface was found.

Full text of DOI:10.1002/ejic.201200674

FULL PAPER
DOI: 10.1002/ejic.201200674
Etching Silicon with HF–HNO3–H2SO4/H2O Mixtures – Unprecedented
Formation of Trifluorosilane, Hexafluorodisiloxane, and Si–F Surface Groups
Marcus Lippold,[a] Uwe Böhme,[a] Christoph Gondek,[a] Martin Kronstein,[a]
Sebastian Patzig-Klein,[a] Martin Weser,[a] and Edwin Kroke*[a]
Keywords: Silicon / Silanes / Fluorine / Hydrofluoric acid / Wet chemical etching / Surface analysis
The etching behaviour of sulfuric-acid-containing HF–HNO3
solutions towards crystalline silicon surfaces has been
studied over a wide range of H2SO4 concentrations. For mix-
tures with low sulfuric acid concentration, NO2/N2O4, N2O3,
NO and N2O have been detected by means of FTIR spec-
troscopy. Increasing concentrations of nitric acid lead to high
etching rates and to an enhanced formation of NO2/N2O4.
Different products were observed for the etching of silicon
with sulfuric-acid-rich mixtures [c(H2SO4) Ͼ 13 molL–1]. Tri-
fluorosilane and hexafluorodisiloxane were identified by
FTIR spectroscopy as additional reaction products. In con-
trast to the commonly accepted wet chemical etching mecha-
nism, the formation of trifluorosilane is not accompanied by
the formation of molecular hydrogen (according to Raman
spectroscopy). Thermodynamic calculations and direct reac-
tions of F3SiH with the etching solution support an intermedi-
ate oxidation of trifluorosilane and the formation of hexa-
fluorodisiloxane. The etched silicon surfaces were investi-
gated by diffuse reflection FTIR and X-ray photoelectron
spectroscopy (XPS). Surprisingly, no SiH terminations were
observed after etching in sulfuric-acid-rich mixtures. Instead,
a fluorine-terminated surface was found.
model, the injection of holes (h+) in the valence band by
nitric acid at cathodic sites is followed by the formation of
an intermediate SiO2 layer, which is dissolved by hydroflu-
oric acid, at local anodic sites.[5] However, X-ray photoelec-
tron spectroscopy (XPS) measurements of etched silicon
surfaces exclude the formation of intermediate SiO2 lay-
ers.[6,7] Silicon surfaces etched with HF–HNO3–H2O mix-
tures are covered by hydrogen-containing –SiHx (x = 1–3)
groups. In a few cases partial fluorine coverage has been
reported. F-containing groups such as –SiH2F and –SiHF2
were identified by means of real-time, in-situ infrared stud-
ies.[9] SiH-terminated surfaces are inert against hydrofluoric
acid solutions. Only the injection of holes in the valence
band by oxidants and their existence on the silicon surface
enable an attack by fluoride-containing species.[10–12] There-
fore, etching rates of hydrogen-terminated silicon bulk ma-
terial are quite low in hydrofluoric acid without further oxi-
dants (about 0.5 Åmin–1).[14] Recent investigations of acidic
etching mixtures support the concept of nitrogen–oxygen
cations, as crucial oxidants in the silicon dissolution pro-
cess.[1a,15–19] Nitrous acid HNO2 and nitrosyl ions NO+, in-
termediates formed by reduction of nitric acid, are strong
oxidants of hydrogen-terminated silicon surfaces.[15] During
the etching of silicon in mixtures with 6 molL–1 HF and
6 molL–1 HNO3, Kooij et al. determined H2 (80.0%), N2O
(18.3%), NO (1.0%) and NO2 (0.7%) as gaseous reaction
products.[20] The amount of H2 strongly depends on the
HF/HNO3 ratio. Especially in mixtures with low amounts
of nitric acid, hydrogen is generated.[21] For etching n-type
silicon under strong illumination, the intermediate forma-
Introduction
For microelectronic and photovoltaic applications, the
etching of silicon surfaces is of enormous technical and
economic relevance. The removal of metal impurities from
silicon materials, texturing of silicon wafers and recycling
of solar cells are realized by wet chemical etching pro-
cesses.[1] In conventional HF–HNO3-based mixtures, dissol-
ution of silicon requires the stepwise oxidation of silicon
atoms and the formation of water-soluble complexes. Ac-
cording to Robbins and Schwartz, nitric acid produces an
intermediate SiO2 layer [Equation (1)], which is dissolved
by hydrofluoric acid [Equation (2)].[2–4] The overall reaction
is written in Equation (3).
3Si + 4HNO3 Ǟ 3SiO2 + 4NO + 2H2O
(1)
(2)
(3)
SiO2 + 6HF Ǟ H2SiF6 + 2H2O
3Si + 4HNO3 + 18HF Ǟ 3H2SiF6 + 4NO + 2H2O
Turner described the mechanism of silicon dissolution as
an electrochemical process with local anodic and local cath-
odic areas on the silicon surface. Corresponding to this
[a] Department of Inorganic Chemistry, TU Bergakademie
Freiberg,
Leipziger Str. 29, 09596 Freiberg, Germany
Fax: +49-3731-394058
E-mail: edwin.kroke@chemie.tu-freiberg.de
Homepage: http://tu-freiberg.de/fakult2/aoch/agsi/index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200674.
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Etching Silicon with HF–HNO3–H2SO4/H2O Mixtures
tion of trifluorosilane HSiF3 was proposed.[11] In aqueous
solution, the hydrolysis of HSiF3 should lead to the forma-
tion of hydrogen according to Equation (4).[12,13,22]
+ HF
HSiF3 + H2O Ǟ HOSiF3 + H2 Ǟ SiF4 + H2O + H2
(4)
In the present study the reaction behaviour of crystalline
silicon in HF–HNO3–H2SO4/H2O etching mixtures is
studied. Our investigations are focused on fundamental ef-
fects on the silicon dissolution process by the addition of
sulfuric acid to conventional HF–HNO3-based mixtures.
The formation of trifluorosilane, hexafluorodisiloxane and
fluorine-terminated silicon surfaces are reported, analyzed
and discussed with respect to the solution. A new silicon
etching reaction sequence is proposed.
Results and Discussion
Figure 1. FTIR spectra of gaseous reaction products after dissolv-
ing silicon in HF–HNO3–H2SO4/H2O etching mixtures. The con-
centrations of hydrofluoric acid and sulfuric acid were kept con-
stant [c(HF) = 5.8 molL–1, c(H2SO4) = 1.1 molL–1].
Gaseous Reaction Products of HF–HNO3–H2SO4/H2O
Etching Solutions
In HF–HNO3-based solutions, silicon etching rates
strongly depend on the concentrations of hydrofluoric acid
and nitric acid.[2,23] Additional components, for example,
CH3COOH and H2SO4, influence the reactivity and deter-
mine the surface structures of the etched silicon.[3,19,24,25]
H2SO4-containing mixtures are highly reactive towards
crystalline silicon. Depending on the mixture composition,
etching rates up to 4000–5000 nms–1 were obtained.[19]
During silicon etching, nitric acid molecules and several in-
termediates are reduced at the silicon/electrolyte interface
by transfer of electrons (e–) from the silicon surface to the
oxidation agent or by injection of h+ into the valence band
of the semiconductor. FTIR spectroscopy enables the
analysis of gaseous reaction products (Figure 1). De-
pending on the concentration of nitric acid, different nitro-
gen oxides are formed. HF–HNO3–H2SO4/H2O etching
mixtures with constant concentrations of hydrofluoric and
sulfuric acid and HNO3 concentrations of 4.3 molL–1 lead
to the formation of NO2, NO and N2O. Increasing the ni-
tric acid concentration accelerates the silicon oxidation
steps, and the silicon etching rates increase from 30.6 nms–1
for 4.3 molL–1 HNO3 to 225.1 nms–1 for 7.5 molL–1 HNO3
(Figure 1). For nitric acid concentrations higher than
5.3 molL–1, N2O4 and N2O3 were detected in addition to
NO2, NO and N2O. The assignment of the vibrational
bands of the NxOy species is based on literature data
(Table 1).
Table 1. Selected IR vibrations of gaseous NxOy species [cm–1].
Species
HNO3
Observed
Literature
3550
1710
1311
3441
3116
2972
1748
1260
2905
2628
1612
1324
1827
1595
1293
1875
2224
1288
3550[26,27]
1708[28]
1325[28]
3442[26]
3117[26]
2973[26]
1749[26]
1261[26]
2906[26]
2627[26]
1617[26]
1320[26]
1830[26]
1594[26]
1296[26]
1876[26]
2224[26]
1284[26]
N2O4
NO2
N2O3
NO
N2O
nitrous acid by a further single electron transfer [Equa-
tion (8)]. The simultaneous transfer of two electrons to ni-
tric acid molecules or to nitronium ions should reduce them
to N(+3) intermediates according to Equations (9) and (10).
+
–
+
–
HNO3 + H2SO4 h H2NO3 + HSO4 h NO2 + HSO4 + H2O
(5)
HNO3 + H3O+ + e– Ǟ NO2 + 2H2O
(6)
(7)
In HF–HNO3–H2SO4/H2O mixtures, increased sulfuric
acid concentrations and low water content support the for-
mation of nitronium ions NO2+. According to Equation (5),
the equilibrium of undissociated HNO3 and NO2+ is shifted
because of the high sulfuric acid concentration.[19] Accord-
ing to Equations (6) and (7), the transfer of an electron
from the valence band of silicon to a nitric acid molecule or
to a nitronium ion reduces these substrates to NO2, which
partially dimerizes at room temperature to dinitrogen te-
+
NO2 + e– Ǟ NO2
NO2 + H3O+ + e– Ǟ HNO2 + H2O
(8)
HNO3 + 2H3O+ + 2e– Ǟ HNO2 + 3H2O
(9)
+
NO2 + 2H3O+ + 2e– Ǟ NO+ + 3H2O
(10)
Owing to their high oxidation potentials, intermediate
troxide N2O4. Dissolved nitrogen dioxide can be reduced to N(+3) species act as additional oxidants of the silicon sur-
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E. Kroke, et al.
FULL PAPER
face.[15,16,18] Thus, nitrous acid is reduced to nitrogen mon-
oxide by a single electron transfer [Equation (11)]. Accord-
ing to Equation (12), the simultaneous transfer of two elec-
trons might reduce N(+3) species to nitroso acid HNO,
which can dimerize to hyponitrous acid H2N2O2 and finally
decompose to nitrous oxide and water [Equation (13)].
HNO2 + H3O+ + e– Ǟ NO + 2H2O
HNO2 + 2H3O+ + 2e– Ǟ HNO + 3H2O
2HNO Ǟ H2N2O2 Ǟ N2O + 2H2O
(11)
(12)
(13)
Figure 2a shows the FTIR spectrum of the gaseous reac-
tion products after etching silicon in an HF–HNO3–H2SO4
mixture with high concentrations of sulfuric acid and no
additional water. NO2/N2O4 and N2O were identified as re-
duced nitrogen-containing species. The absence of the
prominent ν(NO) band of nitrogen monoxide at 1875 cm–1
indicates that the reduction of N(+3) species in H2SO4-rich
mixtures preferentially proceeds by 2e– steps. The vibration
band at 2316 cm–1 can be assigned to the prominent SiH
band of trifluorosilane (HSiF3).[28,29] As mentioned in the
Introduction, trifluorosilane was postulated as an interme-
diate in the electrochemical etching process of n-type silicon
under illumination. Until now, the generation of trifluorosi-
lane could not be verified. Even in-situ FTIR spectroscopic
investigations provided no evidence for the formation of tri-
fluorosilane.[30] In addition to the identification of trifluoro-
silane, a further silicon-containing gaseous compound was
Figure 2. (a) FTIR spectrum of gaseous reaction products after
etching silicon in an HF–HNO3–H2SO4 mixture. (b) At-line FTIR
spectra in the range of the ν(SiH) band of trifluorosilane for dif-
ferent reaction times. Mixture composition: c(HF) = 2.3 molL–1,
c(HNO3) = 1.7 molL–1, c(H2SO4) = 15.0 molL–1 (see also Fig-
ure S2).
detected. The vibration bands at ν = 1190, 1826 and
˜
2054 cm–1 correspond to hexafluorodisiloxane.[31] At-line
FTIR spectroscopic measurements clearly indicate the for-
mation of trifluorosilane during the etching of silicon in
HF–HNO3–H2SO4 mixtures. With increasing reaction
times, the intensities of the SiH band decreases (Figure 2b).
After 20 min, no trifluorosilane was detected by FTIR spec-
troscopy. During the etching process, the accumulation of NO2 and NO+, might lead to the oxidation of trifluorosil-
species that react with trifluorosilane or a decrease in spe- ane in HF–HNO3–H2SO4 etching mixtures. To identify
cies that are essential for the formation of trifluorosilane possible reactions of trifluorosilane and to clarify the for-
might be the cause for the time-dependent reaction behav- mation of hexafluorodisiloxane, DFT calculations were car-
iour. The intensities of the bands of other gaseous species ried out to determine the Gibbs free energies of Equa-
increase until a reaction time of 20 min. The consumption tions (14), (15) and (16). The negative Gibbs free energy
of bath components and the evaporation of hydrofluoric ΔRG = –265.4 kJmol–1 for the reaction of trifluorosilane
acid as well as nitric acid lead to decreasing reactivities for and nitronium ions [Equation (14)] shows that the forma-
reaction times longer than 20 min (spectra not shown). tion of trifluorosilanol (HOSiF3) under thermodynamic
Higher HNO3 concentrations result in lower F3SiH considerations is possible. However, there is no spectro-
amounts and much larger hexafluorodisiloxane concentra- scopic evidence for the formation of trifluorosilanol to date.
tions in the gas phase (Figure S1).
The weak exergonic condensation of two trifluorosilanol
According to Equation (4), the hydrolysis of trifluorosil- molecules according to Equation (15) might be responsible
ane in aqueous hydrofluoric acid solutions should generate for this and should generate hexafluorodisiloxane (ΔRG =
silicon tetrafluoride and molecular hydrogen. In spite of the –0.6 kJmol–1). Additionally, large amounts of sulfuric acid
formation of trifluorosilane, no hydrogen was detectable by bind the water molecules formed and shift the equilibrium
means of Raman spectroscopy after etching silicon in in Equation (15) towards the products. For sulfuric-acid-
H2SO4-rich etching mixtures (Figure S6), i.e., trifluorosil- rich mixtures with HNO3 concentrations of 8.9 molL–1, in-
ane is not hydrolyzed according to Equation (4) in the etch- tense hexafluorodisiloxane vibration bands indicate a nearly
ing mixtures investigated. Low concentrations of free water complete oxidation of trifluorosilane to trifluorosilanol fol-
and hydrofluoric acid should hamper the hydrolysis of tri- lowed by condensation according to Equation (15). Increas-
+
fluorosilane. Several oxidizing agents, e.g., HNO3, NO2
,
ing concentrations of nitronium ions, as detected in these
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Etching Silicon with HF–HNO3–H2SO4/H2O Mixtures
solutions[19] might be responsible for this observation. The Table 2. Selected IR vibrations of gaseous Si-containing species
generated during silicon etching in HF–HNO3–H2SO4 mixtures in
formation of hexafluorodisiloxane is described in the litera-
ture to occur by partial hydrolysis of silicon tetrafluoride,
cm–1.
according to Equation (16), only at high temperatures.[32,33]
At room temperature, the calculated positive Gibbs free en-
ergy ΔRG = 54.3 kJmol–1 does not support the hydrolysis
of silicon tetrafluoride in the HF–HNO3–H2SO4 etching
system.
Species
HSiF3
Observed
Literature
2316, 2316[a]
992,[b,c] 999[a]
858,[b] 858[a]
844,[b] 846[a]
424,[b] 424[a]
2054
2316[28,29]
998[28,29]
858[28,29]
844[28,29]
425[28,29]
2040[31]
1825[31]
1200[31]
1028[31]
1031[31]
+
F3SiOSiF3
SiF4
HSiF3 + NO2 Ǟ HOSiF3 + NO+
(14)
1826
1190
1021[c]
2HOSiF3 h F3Si–O–SiF3 + H2O
(15)
1021[c]
2SiF4 + H2O Ǟ F3Si–O–SiF3 + 2HF
(16) [a] Vibration bands of synthesized HSiF3. [b] Vibration bands ap-
pear as shoulders. [c] The ν(SiF) vibration bands of HSiF3,
To confirm the HSiF3 formation during wet chemical
etching and the subsequent generation of F3SiOSiF3, tri-
fluorosilane was prepared according to Equation (17) by
the addition of trichlorosilane to a zinc fluoride/tetra-
hydrofuran suspension.[34] The spectra of the synthesized
trifluorosilane and of the gaseous reaction products are
shown in Figure 3. The ν(SiH) bands at 2316 cm–1 corre-
spond in both spectra. Because of the intensive bands of
further gaseous species (SiF4, F3SiOSiF3, NxOy), the ad-
ditional bands of trifluorosilane [ν(SiF3) = 992, ν(SiF) =
858, δ(SiH) = 844 and δ(SiF3) = 424 cm–1] appear as shoul-
ders in the spectrum of the gaseous reaction products. The
vibration bands were assigned according to Table 2.
F3SiOSiF3 and SiF4 interfere.
reaction of trifluorosilane with the sulfuric-acid-rich mix-
ture, the 19F NMR spectrum of the HF–HNO3–H2SO4
solution indicated two intensive signals, which could be as-
signed to hydrofluoric acid (δ = –147.1 ppm) and fluorosul-
furic acid (δ = 37.6 ppm) (Figure S4). Additionally, a small
peak at δ = –147.2 ppm was observed in the range of the
chemical shifts of trifluorosilane (δ = –159.0 ppm) and
hexafluorodisiloxane (δ = –161.7 ppm) (Figure S5).[35,36]
However, 29Si NMR spectra of etching solutions, through
which trifluorosilane was passed, did not show any signals;
this was attributed to the relatively low solubility of the
silane and the low sensitivity of the 29Si nucleus.
2HSiCl3 + 3ZnF2 Ǟ 2HSiF3 + 3ZnCl2
(17)
Gaseous Reaction Products for HF–NOHSO4–H2SO4
Etching Solutions
HF–NOHSO4–H2SO4 solutions serve as model etching
systems for HF–HNO3-based mixtures. Both etching sys-
tems are characterized by low water content, relatively high
mixture viscosities and high oxidation potentials. Therefore,
a similar silicon dissolution process is expected. The ad-
dition of sulfuric acid leads to the stabilization of nitrosyl
ions NO+ according to Equation (18). The use of nitrosyl
hydrogensulfate as the oxidizing agent enables systematic
reactivity studies of N(+3) species.[16,18]
HNO2 + H2SO4 h H2NO2 + HSO4 h NO+ + HSO4 + H2O
(18)
+
–
–
Figure 4 shows the FTIR spectrum of the gaseous reac-
tion products after etching silicon in HF–NOHSO4–H2SO4
mixtures with low HF concentration (1.7 molL–1). Nitrogen
monoxide (ν = 1875 cm–1) and nitrous oxide (ν =
Figure 3. Characteristic vibration bands of trifluorosilane. FTIR
spectra of synthesized trifluorosilane (dashed line) and a spectrum
of the gaseous reaction products (solid line) after etching silicon
in HF–HNO3–H2SO4 mixtures [c(HF) = 2.6 molL–1, c(HNO3) =
3.2 molL–1, c(H2SO4) = 13.5 molL–1, t = 120 s].
˜
˜
2224 cm–1) were identified as the nitrogen-containing prod-
Synthesized trifluorosilane was introduced into a sulfu- ucts. The transfer of an electron from the silicon surface to
ric-acid-rich etching mixture. After the reaction, the SiH a nitrosyl ion reduces this species to NO. A simultaneous
vibration band of trifluorosilane could not be observed in transfer of two electrons or the consecutive transfer of two
the FTIR spectrum (Figure S3). Instead, bands of hexa- single electrons to NO+, protonation, dimerization and
fluorodisiloxane were detected. Therefore, the calculated dissociation according to Equation (13) should lead to the
Gibbs free energies (vide supra) and experimental data con- formation of N2O. Similar to that discussed above, the
firm the subsequent oxidation of trifluorosilane, most likely evaporation of hydrofluoric acid (ν = 3693–3919 cm–1) is
˜
via trifluorosilanol, to form hexafluorodisiloxane. After the favoured by high sulfuric acid concentrations. Trifluorosil-
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E. Kroke, et al.
FULL PAPER
ane (ν = 2316 cm–1) is also generated during silicon etching
˜
in HF–NOHSO4–H2SO4 solutions. In addition, no molecu-
lar hydrogen was detected by means of Raman spectroscopy
(Figure S7). In both etching systems, similar reaction condi-
tions (low water and hydrofluoric acid concentrations) lead
to the formation of trifluorosilane and inhibit hydrolysis
according to Equation (4). However, the vibration bands of
hexafluorodisiloxane were not detectable in this case.
Therefore, the described oxidation of trifluorosilane, fol-
lowed by the condensation of trifluorosilanol, appears to
be inhibited in the HF–NOHSO4–H2SO4 mixtures investi-
gated. This may be, because N(+4) and N(+5) species such
+
as NO2 or NO2 are unlikely to form in these HF–
NOHSO4–H2SO4 etching solutions.
Figure 5. DR/FTIR spectra of etched silicon surfaces. (a) Hydro-
gen-terminated surface after etching in a HF-rich mixture [c(HF)
= 9.0 molL–1, c(HNO3) = 1.6 molL–1, c(H2SO4) = 8.9 molL–1, t =
300 s]. (b) Silicon surface after etching in a H2SO4-rich mixture
[c(HF)
= = =
2.6 molL–1, c(HNO3) 3.2 molL–1, c(H2SO4)
13.5 molL–1, t = 600 s].
polished surfaces. In both spectra, the Si–OH vibrations
bands at ν ≈ 3700 cm–1 support the intermediate existence
˜
of Si–F surface species. Rinsing of the etched silicon sur-
faces with deionized water leads to hydrolysis of the SiF
groups [Equation (19)]. The OH-group surface concentra-
tion increases with the duration of water rinsing.[38]
Figure 4. FTIR spectrum of gaseous products after the reaction of
silicon with HF–NOHSO4–H2SO4. Mixture composition: [c(HF) =
1.7 molL–1, c(NOHSO4) = 1.4 molL–1, c(H2SO4) = 16.8 molL–1,
t = 300 s].
ϵSi–F + H2O Ǟ ϵSi–OH + HF
(19)
The XPS spectrum of an etched silicon surface is de-
picted in Figure 6. The H2SO4-rich etching solution con-
Formation of Si–F Surface Groups
Silicon surfaces etched with HF–HNO3–H2O mixtures tains relatively small amounts of hydrofluoric acid [c(HF)
are hydrophobic and hydrogen-terminated. IR spectro- = 2.6 molL–1]. In the F 1s region, two peaks were observed.
scopic investigations confirm the SiH termination. The The peak at 686.0 eV corresponds to Si3–xSiFx surface
ν(SiH) stretching bands of the relevant SiH and SiH2 groups and the second peak at 689.5 eV can be assigned to
groups are located in the range 2085–2115 cm–1.[37] Fig- silicon oxyfluoride species such as (SiO)3–xSiFx.[39,40] Ad-
ure 5a shows the diffuse reflectance infrared (DR/FTIR) ditional peaks for oxygen-containing surface species occur
spectrum of an etched multicrystalline silicon surface. The in the O 1s region at 532.3 eV and in the Si 2p region at
intense band at ν = 2100 cm–1 indicates SiH surface groups. 102.5 eV. For SiOF suboxides, a binding energy of 102 eV
˜
In this sulfuric acid containing etching solution, the con- was determined.[41] Intensive oxygen coverage should result
centration of hydrofluoric acid is much higher than the con- from the hydrolysis of SiF groups. Therefore, Si–F bonds
centration of nitric acid. Etching silicon surfaces in mix- were formed during the etching of silicon in H2SO4-rich
tures with low hydrofluoric acid concentrations leads to etching mixtures. In contrast, no oxygen-containing surface
completely different surface chemistry. Etched silicon sur- species were observed by XPS analysis after etching silicon
faces are hydrophilic. On silicon wafers etched in H2SO4- wafers in HF–HNO3–H2O mixtures.[7,8]
rich mixtures, no SiH stretching vibrations were detected
The existence of an SiF-terminated silicon surface and
by DR/FT-IR spectroscopy (Figure 5b). In addition to this, the formation of trifluorosilane during etching in H2SO4-
porelike surface textures were observed by means of scan- rich HF–HNO3-based mixtures indicate different silicon
ning electron microscopy.[19] This disagrees with the prin- dissolution steps. Scheme 1 illustrates possible reaction
ciples of silicon dissolution, in which the transport of hy- steps on the silicon/electrolyte interface for etching in sulf-
drofluoric acid to the silicon surface is expected to be the uric-acid-rich HF–HNO3–H2SO4 mixtures. During step (1)
rate-limiting process and should lead to the formation of of silicon dissolution, the native oxide is etched by hydro-
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Etching Silicon with HF–HNO3–H2SO4/H2O Mixtures
saturate the oxidized silicon atoms. The simultaneous trans-
fer of two electrons (injection of two holes) followed by the
reaction with fluoride ions forms an –SiF2H group and a
fluorine-covered silicon surface atom, see steps (3) and (4).
In step (5), the further injection of holes and the reaction
with two fluoride species release trifluorosilane and result
in a fluoride-covered silicon surface. The fluorine termina-
tion of the surface might result from the high oxidation
potential and the low concentration of hydrofluoric acid in
the etching mixture, i.e., the reaction of fluoride ions with
silicon surface atoms is the rate-determining step. There-
fore, the reaction behaviour of sulfuric-acid-rich HF–
HNO3–H2SO4 mixtures differs from the conventional con-
ception of silicon dissolution. For dissolving silicon in
aqueous hydrofluoric acid solutions, the injection of holes
into the valence band is described as the rate-determining
step.[13]
Figure 6. XPS spectrum of a silicon surface etched by a H2SO4-rich
mixture [c(HF) = 2.6 molL–1, c(HNO3) = 3.2 molL–1, c(H2SO4) =
13.5 molL–1, t = 600 s].
Conclusions
fluoric acid. The removal of the native oxide generates an
intermediate hydrogen coverage. The oxidation of the sili-
con surface proceeds through reduction of nitrogen-con-
In contrast to the conventional HF–HNO3–H2O etching
system, sulfuric-acid-containing mixtures exhibit a varying
etching behaviour. This is clearly indicated by thorough
analysis of the gaseous reaction products by FTIR and Ra-
man spectroscopy as well as investigation of the etched sili-
con surfaces with DR/FTIR and XPS. During the etching
of silicon in H2SO4-rich mixtures, trifluorosilane, hexa-
fluorodisiloxane and fluorine-terminated surfaces are
formed in contrast to SiF4 (and H2SiF6) and hydrogen-ter-
minated surfaces, as usually found. Fundamental studies of
the HF–HNO3–H2SO4 etching system indicate an interme-
diate oxidation of trifluorosilane most likely to trifluorosil-
anol by nitric acid or nitronium ions, followed by condensa-
tion of HOSiF3 to form hexafluorodisiloxane. Silicon
wafers etched by H2SO4-rich mixtures exhibit a completely
different surface situation as shown by the absence of the
prominent SiH band. XPS analysis of a freshly etched sili-
con surface indicates peaks that prove the existence of Si–
F surface groups. However, the basic oxidation step corre-
sponds to the well-established reaction mechanism of sili-
con dissolution in HF-containing mixtures, which is based
on the removal of electrons from the Si–Si bonds instead of
the Si–H surface moieties. This is supported by the absence
of molecular hydrogen and also the formation of trifluoro-
silane. The unprecedented surface chemistry results from
different silicon dissolution steps.
+
taining species, e.g., HNO3, NO2 and NO+. Thereby, elec-
trons are transferred from silicon to oxidizing agents by the
injection of holes hVB into the valence band of the semi-
+
conductor. In step (2), the injection of holes and their exis-
tence near the silicon surface weaken the Si–H bonds and
enable the replacement of H atoms by F atoms. The formed
Si–F bonds polarize the Si–Si backbonds. This facilitates a
subsequent oxidation and the attack of fluoride species to
Further spectroscopic investigations of intermediates in
HF–HNO3–H2SO4 etching solutions should clarify single
reaction steps, especially the oxidation of Si–H surface
groups by NOx+ at the silicon/electrolyte interface. Reactiv-
ity studies of trifluorosilane towards several bath compo-
nents should provide an indication of the formation of mo-
lecular hydrogen during etching silicon in conventional
HF–HNO3–H2O mixtures.
Experimental Section
Etching Mixtures: Caution: Suitable safety precautions have to be
Scheme 1. Proposed reaction steps for the formation of trifluorosil-
ane and fluorine-covered silicon surfaces during etching in sulfuric-
acid-rich HF–HNO3–H2SO4 mixtures.
taken into account when performing etching experiments with hy-
Eur. J. Inorg. Chem. 2012, 5714–5721
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5719

E. Kroke, et al.
FULL PAPER
drofluoric acid. All experiments were performed in an HF-ap-
proved fume hood and with HF-approved laboratory equipment.
The obtained harmonic frequencies were combined with standard
statistical thermodynamics to calculate Gibbs free energies at
298.15 K and 1 atm, which allow better comparison with experi-
mental data. Values of Gibbs free energies (G), sum of electronic
and thermal enthalpies (H) and sum of electronic and zero-point
energies (E + ZPE) are listed in the Supporting Information.
HF–HNO3–H2SO4 etching solution (25 mL) was prepared by add-
ing freshly distilled nitric acid (100 wt.-%) and analytical-grade hy-
drofluoric acid (40 wt.-%) dropwise to analytical-grade sulfuric
acid (97 wt.-%) in a PP beaker with ice/NaCl cooling. To distil
nitric acid, sulfuric acid (200 mL, 97 wt.-%) was added to analyti-
cal-grade nitric acid (100 mL, 65 wt.-%) with cooling. The mixture
was heated slowly and distilled at 2 kPa. After adding double the
volume of sulfuric acid to the distillate, the mixture was distilled
again.
Supporting Information (see footnote on the first page of this arti-
cle): FTIR, 19F NMR and Raman spectra, list of calculated ener-
gies, SEM images.
Acknowledgments
HF–NOHSO4–H2SO4 etching mixtures were prepared by dissolv-
ing nitrosyl hydrogensulfate in analytical-grade sulfuric acid
(97 wt.-%) and adding hydrofluoric acid (40 wt.-%). Nitrosyl hydro-
gensulfate was synthesized by bubbling dried SO2 into fuming nitric
acid (100 mL, 2.4 mol) for several hours, followed by filtration and
rinsing with glacial acetic acid and tetrachloromethane under ar-
The authors gratefully acknowledge Prof. Dr. Gerhard Roewer for
his useful discussions. Dipl.-Chem. Tobias Weling is acknowledged
for collecting the XPS spectra. This work was performed within
the Cluster of Excellence “Structure Design of Novel High-Per-
formance Materials via Atomic Design and Defect Engineering
(ADDE)” that is financially supported by the European Union
(European regional development fund) and by the Ministry of Sci-
ence and Art of Saxony.
gon. 14N NMR (H SO ): δ = 11.4 ppm. Raman (solid): ν = 414
˜
2
4
[ρ(SO3)], 571 [δs(SO3)], 597 [δas(SO3)], 871 [ν(SOH)], 1031 [νs(SO3)],
1170 [νas(SO3)], 2275 [ν(NO)] cm–1.
Trifluorosilane: Because of the rapid hydrolysis of HSiF3, the fol-
lowing manipulations were carried out under argon. In a three-
necked flask, trichlorosilane (4.74 g, 35 mmol) was added dropwise
to a suspension of zinc fluoride (10.3 g, 0.10 mol) and tetra-
hydrofuran (50 mL) with ice/NaCl cooling. To ensure complete re-
action of the trichlorosilane, the evolved gas was bubbled through a
second three-necked-flask with a suspension of zinc fluoride (5.0 g,
48 mmol) and tetrahydrofuran (40 mL) at room temperature. The
trifluorosilane was condensed in a 200 mL Schlenk flask. IR (gas
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Received: June 20, 2012
Published Online: October 8, 2012
Eur. J. Inorg. Chem. 2012, 5714–5721
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5721