Etching Silicon with Aqueous Acidic Ozone Solutions - ACS Publications

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Etching Silicon with Aqueous Acidic Ozone Solutions: Reactivity Studies and Surface Investigations Christoph Gondek, Ronny Hanich, Florian Honeit, Andreas Lißner, Andre Stapf, and Edwin Kroke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06332 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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The Journal of Physical Chemistry

Etching Silicon with Aqueous Acidic Ozone Solutions: Reactivity Studies and Surface Investigations Christoph Gondek,1 Ronny Hanich,1 Florian Honeit,2 Andreas Lißner,3 Andre Stapf,1and Edwin Kroke 1,*

1

Institute of Inorganic Chemistry, Technische Universität Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany

2

Institute of Applied Physics, Technische Universität Bergakademie Freiberg, Leipziger Str. 23, D-09596 Freiberg, Germany

3

Institute of Physical Chemistry, Technische Universität Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany

* Corresponding author: Prof. Dr. rer. nat. habil. E. Kroke, phone: + 49 3731 393174, e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Aqueous acidic ozone (O3) containing solutions are increasingly used for silicon treatment in photovoltaic and semiconductor industries. We studied the behavior of aqueous hydrofluoric acid (HF) containing solutions, i.e. HF-O3-, HF-H2SO4-O3- and HF-HCl-O3-mixtures, towards borondoped solar grade (100) silicon wafers. The solubility of O3 and etching rates at 20°C were investigated. The mixtures were analyzed for the potential oxidizing species by UV/Vis and Raman spectroscopy. Concentrations of O3 (aq), O3 (g), Cl2 (aq) were determined by titrimetric volumetric analysis. F-, Cl- and SO42- ion contents were determined by ion chromatography. Model experiments were performed to investigate the oxidation of H-terminated silicon surfaces by H2O-O2-, H2O-O3-, H2O-H2SO4-O3- and H2O-HCl-O3- mixtures. The oxidation was monitored by diffuse reflection infrared Fourier transformation spectroscopy (DRIFT). The resulting surfaces were examined by scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). HF-H2O-O3-mixtures show a polishing etching behavior whereas HF-HCl-H2O-O3-mixtures exhibit slight anisotropic etching. Formation of pyramidallike morphologies on (100) silicon surfaces was observed. In all cases cleaned and H-terminated silicon surfaces are obtained. The results were used to draw conclusions about the dissolution mechanism of silicon in the respective solutions. In HF-H2O-O3-mixtures silicon is dissolved by an O3(aq)-diffusion controlled tetravalent etching mechanism. Interestingly, in H2SO4-rich aqueous HF-H2SO4-O3-solutions only the native oxide is removed, silicon is not attacked and dissolved. In HCl-containing solutions Cl2 or Cl3- are responsible for silicon oxidation. HCl can be considered as a catalyst resulting in a divalent silicon dissolution mechanism similar to the etching in alkaline solutions.


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INTRODUCTION Ozone (O3) is increasingly used in photovoltaic and semiconductor industries, because it allows more cost efficient and environmentally friendly processes compared to other oxidation reagents. At the same time continuously more demanding limits for purity are set in both industry branches. Application of O3 in wet-chemical silicon treatment for microelectronics and photovoltaics offers ecological and economic advantages, especially less chemical consumption and easier waste management.1 O3 is generated from oxygen (O2) in an O3 generator and bubbled through the respective mixture. The lower explosion limit of O2/O3 mixtures begins between 9.5 % (v/v) and 15.0 % (v/v) O3 (room temperature, 1 bar) and depends on apparatus properties and energy input.2,3 Generation and handling of O3 is cheaper and safer than the usage of hydrogen peroxide, for example used for the well-known RCA clean.4,5 O2 is generally the only by-product of oxidation reactions. Excess O3 in the exhaust air is easily catalytically decomposed to O2 with activated charcoal. This requires no expensive waste management in contrast to the complex waste disposal of e.g. nitrate containing mixtures widely used for silicon cleaning and etching, in which dissolved NOx, NO2-, NO3-, NO2+ and/or NO+ act as oxidizing species.6,7,8,9,10 Various studies for cleaning silicon surfaces by hydrofluoric acid (HF) and O3 based mixtures were carried out.11-19 Some investigations also deal with formation of oxide layers in aqueous O3 or HF-H2O-O3-based mixtures.16-20 Less data is known about the etching behavior of these mixtures and additives like hydrochloric acid (HCl) as well as sulfuric acid (H2SO4) towards silicon surfaces.21,22 This paper presents investigations on the fundamental background of wetchemical silicon treatment with O3 containing aqueous HF solutions. Solubility of O3 in HF solutions and possible reactive species for aqueous HF-HCl-O3- and HF-H2SO4-O3-mixtures


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were evaluated. We focus on the reactivity of these cleaning and etching mixtures towards solargrade boron-doped (100) silicon wafers and the resulting surface properties. From a chemical point of view, dissolving silicon formally requires a stepwise oxidation of the silicon surface atoms by suitable oxidants (e.g. O3). Four electrons must be transferred from silicon to the oxidizing agent and soluble complexes must be formed (e.g. fluoride containing silicon species such as SiF62-). Both processes proceed simultaneously at the silicon/etchant interface. Dissolving oxidized silicon atoms is a fast process, which is kinetically not hindered.23 Often the silicon oxidation/hole injection is crucially involved in the rate determining step. The chemical oxidation of the H-terminated silicon surface can start with Si—Si bonds (resulting in Si—O—Si) or Si—H bonds (resulting in Si—OH groups and partially in stretched surface Si— O—Si-bridges). Kurokawa and Ichimura have shown that a H-terminated silicon surface was more slowly oxidized by O3 than a “clean” silicon surface. Primarily, surface near Si—Si bonds were attacked by O3.24 Oxidation of silicon by O3 is independent on crystallographic orientation.25,26 Wet-chemical, pH-dependent silicon oxidation experiments by De Smedt et al. indicate that ozone decomposition products in the “bulk” solution (for example ionic radicals O3-, O2-, O- and HO, O22-, O) did not play a key role for silicon oxidation.20 Crucial is the concentration of reactive species at the silicon/electrolyte interface. De Smedt et al. propose the very reactive but also extremely short-living O- radical ions as oxidizing species.20 We propose that also the more “stable” radical ions O3-, O2- might be involved in the oxidation of Si—Si bonds. Formally, O3 decomposition at the silicon surface must take place (eq. 1) and O-insertion into the Si—Si bonds follows immediately (eq. 2).

Eq. 1

O3 → O2 + O


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Eq. 2

“Si—Si” + O → “Si—O—Si”

Electrochemical approaches claim a redox potential of above + 0.7 V to inject an electron hole into the silicon valance band, which can be accomplished by many oxidizing species including O3, HO radicals or O2 (E0(O3/O2) = 2.07 V; E0(HO/H2O) = 2.85 V; E0(O2/H2O) = 1.23 V).

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Moreover, cyclic voltammetry measurements show silicon dissolution under formation of porous silicon or surface structuring for applied bias between around + 0.3 V and + 0.7 V (see Figure S1 in Supporting Information). Mechanistic considerations corresponding to the number of exchanged charge carriers between silicon and the oxidizing agent distinguish between divalent (or current doubling) and tetravalent (or current quadrupling). The first step in both models is a hole injection into the silicon valence band. In the case of a divalent dissolution only one more electron is injected by fluoride species into the silicon conduction band. Three electrons are injected for the tetravalent dissolution in HF solutions, at least in the electrochemical model.23,29 In general: the pure chemical tetravalent dissolution of silicon leads to polished surfaces and the electrochemical divalent mechanism tends to formation of porous structures or surface textures.23,28 Electrochemical tetravalent silicon dissolution usually results in surface polishing.28 Furthermore, the resulting surface morphologies give hints to the rate limiting processes. Surface texturing indicates that a chemical surface reaction is rate limiting. Polishing hints to diffusion processes as rate limiting steps.30

EXPERIMENTAL Caution! Etching experiments with HF (hydrofluoric acid) must be performed in a HFapproved fume hood with HF-approved laboratory equipment.


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Preparation of HF-containing mixtures Mixtures of 48 % (w/w) hydrofluoric acid (analytical grade, Sigma-Aldrich) and deionized (DI) water were used for diluted HF solutions. 36 % (w/w) hydrochloric acid (analytical grade, VWR) was added for HF-HCl-mixtures. HF-H2SO4-mixtures were prepared by propounding 98 % (w/w) sulfuric acid (analytical grade, Merck) and adding HF with strong cooling.

Apparatus for O3 saturation Figure 1 shows a scheme of the apparatus. All components being in contact with HF are made from HF-resistant organic polymers (PTFE, PFA). Pure oxygen (pressurized gas cylinder, Praxair 99.5 % (v/v) O2) was used for generating an O2/O3 gas mixture with an ozonizer (ozone generator BMT 802 N). A flow rate of about 40 mL min-1 was adjusted, which results in 13 % (v/v) O3 in the O2/O3 gas mixture (Caution: near to the lower explosion limit). The gas flow was bubbled through the cleaning/etching mixture via a porous PTFE disc/membrane. Exhaust gas flow was passed through a HF scrubber (aqueous suspension of Ca(OH)2) and a catalytic O3 decomposition unit (activated charcoal contact).


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Figure 1: Scheme of the apparatus used for O3 saturation of HF-containing mixtures.

O3 quantification – analytics of mixtures The concentration of O3 in aqueous media (c(O3 (aq)) was determined by titrimetric volumetric analysis with potassium iodide buffer solution and sodium thiosulfate solution based on DIN 38408-3.31 The chemical background is expressed by eq. 3 and eq. 4 (further information to O3 quantification see Supporting Information). UV/Vis spectroscopy was used for qualitative evaluation (UV/Vis spectrometer: UV-1600PC (VWR), cuvette: Eppendorf UVette-cuvettes® (1 mm and 10 mm)). O3 has an absorption band at 260 nm (information to further UV/Vis active species see Supporting Information (figure S7)).32 The O3 concentration in the O2/O3 gaseous mixtures (c(O3 (g)) were determined by bubbling a well-defined gas volume through the potassium iodide buffer solution and subsequent titrimetric volumetric analysis.

Eq. 3

O3 (aq) + 2 I-(aq) + 2 H3O+(aq) → 3 H2O(l) + I2 (aq) + O2 (g)

Eq. 4

I2 (aq) + 2 S2O32-(aq) → S4O62-(aq) + 2 I-(aq)

The concentrations of fluoride, chloride and sulfate ions were monitored by ion chromatography (Dionex ICS 2000). Raman spectroscopy measurements of HF-H2SO4-O3mixtures were performed with a Bruker RFS 100/S spectrometer in 10 mm plastic UV-cuvette micro (BRAND) without diluting the etching mixtures.

Etching experiments – oxidation experiments


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Etching experiments were performed in the sample compartment of the mentioned apparatus (see figure 1). Before etching, the solutions were saturated for 20 minutes with the O2/O3 gas mixture. Then the silicon wafer (Si (100), boron-doped, diamond wire sawn, as-cut, thickness: 190 µm, resistivity 0.5 to 2.0 Ω cm, 1 x 2.5 cm, Deutsche Solar GmbH) was placed into the etching mixture. During etching O2/O3 gas was passed through the etching mixture. The gas bubbles cause a stirring effect. Direct gas flow with contact to the silicon wafer was avoided. At the end of each experiment the wafers were rinsed with DI water and dried in air. Etching rates (in nm s-1) per side were calculated by the mass loss after differential weighing before and after each experiment. For evaluating the impact of saw damage, SiC-slurry sawn silicon wafers were used as well (Si (100), boron-doped, as-cut, thickness: 190 µm, resistivity: 2.0 Ω cm, 1 x 2.5 cm, PV Crystallox Solar). For studying the oxidation behavior of H2O-O2-, H2O-O3-, HCl-H2O-O3- and H2SO4-H2O-O3mixtures towards H-terminated silicon surfaces, the silicon wafer (Si (100), boron-doped, diamond wire sawn, as-cut, thickness: 190 µm, resistivity 0.5 to 2.0 Ω cm, 1.0 x 1.0 cm, Deutsche Solar GmbH) is pre-etched for one minute in a HF-HNO3-mixture (c(HF) = 10.8 mol L-1; c(HNO3) = 2.8 mol L-1) for H-termination. Then the wafer was placed for the selected period of time into the oxidizing mixtures.

Characterization of the resulting silicon surfaces Resulting silicon surfaces were characterized by scanning electron microscopy (SEM). A Vega Tescan TS 5130 SB microscope was used. For diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements a Nicolet 380 FT-IR-spectrometer (Thermo Electron Corporation) with Smart Collector Avatar accessory (Thermo Fisher Scientific) was utilized to


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identify silicon surface species. XPS measurements were carried out on a Specs Phoibos 150 MCD-9 spectrometer with an Al Kα source (1486.6 eV) in a vacuum of 5 · 10-9 mbar. Binding energies for all spectra were referenced to the C1s photoelectron peak for adventitious surface carbon at 284.8 eV (further information see Supporting Information).

RESULTS AND DISCUSSIONS Solubility of O3 in HF-H2O-solutions The solubility of gases in solutions is described by the Henry law (eq. 5 for O3 (aq)).33 The solubility of O3 and O2 in water and different acidic, neutral and alkaline salt solutions has been investigated.34-38 O3 (Hecp298 K(O3) = 1.26 · 10-7 mol Pa-1 L-1

35

) is around ten-times better

dissolved in DI water than O2 (Hecp298 K(O2) = 1.29 · 10-8 mol Pa-1 L-1

37

). To the best of our

knowledge there is no data available for O3 solubility in aqueous HF-containing solutions.

Eq. 5

c(O3 (aq)) = Hecp(O3) · p(O3 (g))

The O3 solubility in aqueous HF solutions is described by the Henry coefficient (Hecp(O3)) for different HF concentrations (figure 2). The p(O3 (g)) was kept constant at 0.13 bar. Concentration of dissolved O3 (O3 (aq)) was measured at steady state conditions, not at equilibrium. Therefore, the determined values are pseudo Henry coefficients. In our experiments less O3 was dissolved in DI water compared to data reported on DI water, diluted nitric and hydrochloric acid.35,38 Figure 2 illustrates increasing amounts of dissolved O3 with increasing HF concentration. In concentrated HF solution (c(HF) = 27 mol L-1, figure 2) there was about 45 % more O3 dissolved than in DI water. There might be interactions between HF and O3 molecules, allowing higher O3 solubility.


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Hecp(O3) in 10-7 mol Pa-1 L-1

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1.2

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c(HF) in mol L-1

Figure 2: Solubility of O3 in aqueous HF solutions represented by Hecp(O3) at 20 °C ( this work); reference data: ▲ at 20 °C in DI H2O;35 at 26 °C in 0.04 mol L-1 HNO3;38 at 21.2 °C in 0.004 mol L-1 HCl.38

Reactions of silicon with HF-H2O-O3-mixtures Reactivity studies Figure 3 presents measured concentrations of O3 (aq) in different aqueous HF mixtures and the resulting etching rates towards silicon. The higher concentration of O3 (aq) in higher concentrated HF solutions results in higher etching rates. Etching rates correlate with the amount of O3 (aq) (see also Supporting Information figure S1). These facts point to a etch rate limitation by the oxidation of silicon. A similar trend was already observed by Chen et al. on polycrystalline silicon, although the reactivity was reported to be twice as high as our results.21


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r in nm s-1

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0.0 0

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c(HF) in mol L-1

Figure 3: O3 concentration in aqueous HF solutions () and reactivity of these mixtures towards boron-doped (100) silicon at 20 °C ().

The reactivity behavior of HF-H2O-O3-mixtures towards silicon indicates that the concentration of O3 (aq) is the reactivity limiting factor. Silicon oxidation/hole injection by O3 or resulting reactive species such as radical ions like O-, O3- or O2- at the silicon/electrolyte interface is the rate limiting process. Higher reactivity towards silicon will result for technical procedures, which enable higher O3 concentrations in solution and hence at the silicon surface (e. g. caused by higher O3 partial pressure or microbubbles).39

Surface properties As-cut silicon wafer surfaces treated in HF-H2O-O3-mixtures show a polished surface. This is indicated in figure 4 for as-cut diamond wire sawn and SiC-slurry sawn silicon wafers. Polishing occurs regardless of surface damage. Polishing quality depends on the etch depth. This is in


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accordance with the direction independent oxidation of silicon by O3 (g) reported by Kameda et al.25 The observed polishing indicates a diffusion-controlled silicon dissolution mechanism.30

Figure 4: Surface morphologies (SEM images) of diamond wire sawn (A-C) and SiC-slurry sawn (D-F) as-cut (100) Si surfaces before (A and D) and after etching in HF-O3-mixtures; mixture 1 (B and E): c(HF) = 2.0 mol L-1, c(O3 (aq)) ≈ 1.29 mmol L-1; mixture 2 (C and F): c(HF) = 4.1 mol L-1, c(O3 (aq)) ≈ 1.35 mmol L-1; 20 °C. The oxidation behavior of H2O-O2- and H2O-O3-mixtures towards silicon wafer surfaces was studied by DRIFT spectroscopy measurements (figure 5). Vibrational band analysis of the ν(SiH) region between 2070 cm-1 and 2260 cm-1 enable conclusions on the oxidation of Si—Si bonds at the wafer surface (see Supporting Information Table S1 with references).38-43 Decreasing intensity of ν(Si—H) and increasing intensity of the broad band of different ν(O—H) (3000 cm-1 to 3700 cm-1) hint to the oxidation of surface Si—H groups. The H-terminated silicon surface was slowly oxidized by dissolved oxygen (O2 (aq)). Even after 25 minutes of treatment in water


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bubbled with O2 only small amounts of oxidized surface Si—Si bonds were observed. This is in accordance with results of solid-gas-phase and oxidation experiments with oxygen-saturated aqueous solutions.24,44 The non-polar O2 molecule reacts very slowly with the hydrophobic Hterminated silicon surface. Almost no oxidation occurs.

(2185 cm-1) (O2)SiH2

SiH2 (2108 cm-1) H2O-O2

(2252 cm-1) (O3)SiH

15 min 25 min

Kubelka-Munk

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H-terminated Si surface

H2O-O2/O3 1 min 5 min 15 min 25 min

3600 3100 2600 2100 1600 1100 wavenumber in cm-1

600

Figure 5: DRIFT spectra of time dependent model oxidation experiments of H-terminated (100) Si surfaces; black (middle): H-terminated Si surface; light grey (top): oxidation in H2O-O2mixtures; dark grey (bottom): oxidation in H2O-O3-mixtures; 20°C. O3 (aq) is a stronger oxidizing agent than O2 (aq) and has a dipolar character (dipole moment: p = 1.7803 · 10-30 C m).45 Furthermore radical ions (O3-, O2- O-) are probably formed by O3 decomposition at the silicon surface.20 Already after a one minute dip of the silicon wafer in ozonized water, the Si—Si bonds at the surface are markedly oxidized shown by the vibrational bands for (O)SiH2, (O,O)SiH2 and (O,O,O)SiH surface groups. This is in good agreement with the observations of De Smedt et al.20 Longer oxidation times lead to nearly complete oxidation of


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surface near Si—Si bonds and also surface Si—H bonds are more and more oxidized. If sterically possible, the unstable Si—OH groups condensate under siloxane bridge and water formation.46 Oxidized silicon surface atoms are immediately dissolved in HF containing solutions. This is shown by XPS measurements of a silicon wafer etched in a HF-H2O-O3-mixture. As presented in figure 6 there are no signals at binding energies of oxidized silicon species. Hence, no significant amounts of chemically bound oxygen or fluorine exist at the silicon surface. The spectrum was well represented by the fit of two silicon species – Si(0) and Si(SiH2) (see Supporting Information Table S2). Silicon etching in HF-H2O-O3-mixtures results in clean H-terminated silicon surfaces. Si(0) Si2p spektrum Si2p 3/2 Si(0) Si2p 1/2 Si(0) Si(SiH2) Si2p 3/2 Si(SiH2) Si2p 1/2 Si(SiH2) background

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1.5 Si(1) 1.0

Si(2) Si(3)

0.5

Si(4)

0.0 105

104

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binding energy in eV

Figure 6: Si 2p XPS spectrum of a (100) Si surface, etched in HF-H2O-O3-mixture (c(HF) = 6.9 mol L-1), marked comparing binding energies of Si species with oxidation number in brackets based on Cerofolini et al.,47 further information see Supplementary Information.

Silicon dissolution in HF-H2O-O3-mixtures


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Based on our results, silicon dissolution in aqueous HF-H2O-O3-mixtures can be described by two processes immediately following each other. Formally, starting with the chemical oxidation of silicon by O3 (eq. 6) followed by complexation of oxidized silicon by HF or similar species (HF2-, F-) (eq. 7). Eq. 8 represents the overall reaction for the whole process of Si dissolution. “SiO2”(surface) represents a completely oxidized silicon surface atom, not bulk silicon dioxide.

Eq. 6

Si(surface) + 2 O3 (aq) → “SiO2”(surface) + 2 O2 (g)

Eq. 7

a) “SiO2”(surface) + 6 HF(aq) → H2SiF6 (aq) + 2 H2O(l) b) “SiO2”(surface) + 6 HF2-(aq) → H2SiF6 (aq) + 2 H2O(l) + 6 F-(aq)

Eq. 8

Si(surface) + 2 O3 (aq) + 6 HF(aq) → H2SiF6 (aq) + 2 O2 (g) + 2 H2O(l)

The primary chemical oxidation of surface near Si—Si bonds is caused by reactive species (such as O3-, O2-, and/or O-), which are formed at the silicon/electrolyte interface. The concentration of these reactive species correlates with the concentration of O3 (aq). This enables a tetravalent silicon dissolution mechanism in HF-H2O-O3-mixtures. Reactivity studies (figure 3 and figure S2) indicate that overall silicon oxidation by O3 is the rate determining process. Furthermore, the observed polishing etching regime indicates the diffusion of O3 (aq) to the silicon surface as rate determining step. The other processes, especially the silicon oxidation surface reactions (see figure 5) as well as complexation by HF (and/or HF2-, F-) are fast. There seems to be no reaction control. Dissolution of silicon in aqueous HF-O3-mixtures can be described as O3 (aq) diffusion-controlled, tetravalent mechanism.

Reactions of silicon with aqueous HF-H2SO4-O3-mixtures


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Adding H2SO4 to HF-H2O-O3-mixtures decreases the reactivity towards silicon as illustrated in figure 7A. At H2SO4 concentrations lower than 7 mol L-1 this might be due to decreasing O3 solubility (figure 7B). For H2SO4 concentrations higher than 7 mol L-1 the amount of oxidizing agents in solution increases again, even higher than in HF-H2O-O3-mixtures (figure 3). Similar behavior was reported for H2SO4-O3-mxitures by Levanov et al. An increasing O3 solubility for H2SO4 concentrations higher than 12 mol L-1 was reported.36

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L-1

Figure 7: A: Reactivity of HF-H2SO4-O3-mixtures towards silicon at 20°C, diamond wire sawn wafer: (c(HF) = 1.8 mol L-1), ▲ (c(HF) = 4.0 mol L-1), SiC-slurry sawn wafer: (c(HF) = 1.8 mol L-1); B: O3 concentration in O2/O3 bubbled HF-H2SO4-mixture ( c(HF) = 1.8 mol L-1).


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Raman measurements were used to identify the formation of further potentially oxidizing agents (such as HSO5- or S2O82-) in concentrated HF-H2SO4-O3-mixtures (Raman spectra see Supporting Information figure S3). The spectra show no differences between mixtures with and without O3 bubbling. Especially, no vibrational band around 780 cm-1 (representing S-O2-H fragments) was noticeable.48 Obviously, S2O82- did not oxidize the silicon surface in spite of the high standard redox potential of 2.01 V (pH = 0), as presented by Stapf et al.49 We conclude that the titrimetrically determined amount of oxidizing agent is traced back to O3 (aq). The high O3 (aq) concentration caused a slight blue coloration of the mixture (photo see Supporting Information figure S2). Quite low reactivity of highly concentrated HF-H2SO4-O3-mixtures towards silicon was observed despite higher O3 (aq) concentrations (figure 7) and presumably formation of highly reactive HO3+ ions.50 The low reactivity was also indicated by the obtained silicon surface morphologies. SEM measurements show a cleaned, but morphologically unchanged surface (see Supporting Information figure S4). Slight crack widening is visible. XPS measurements (see Supporting Information figure S20) show an oxide-free H-terminated silicon surface, comparable to HFH2O-O3-mixtures. This was also supported by DRIFT measurement of SiC-slurry sawn silicon wafers treated with concentrated HF-H2SO4-O3-mixtures (see Supporting Information figure S5). The H-terminated surface suggests that HF is not the rate determining component. Model oxidizing experiments using aqueous H2SO4-O3-mixtures to treat H-terminated silicon surfaces provide information on the silicon oxidation (figure 8). For low H2SO4 concentration a fast oxidation of surface near Si—Si bonds occurs, shown by vibrational bands for (O)SiH2, (O,O)SiH2 and (O,O,O)SiH surface groups (figure 8, 1.8 mol L-1 H2SO4) – comparable to


17


oxidation in ozonized water (figure 5). Surprisingly, in highly concentrated aqueous H2SO4-O3mixtures the Si—H vibrational band remains intact for longer times. Only a slight silicon surface oxidation was observed (figure 8, 17.0 mol L-1 H2SO4). This supports an improved stability of Hterminated silicon surfaces in highly acidic media, which has been indicated already by Higashi et al.51 and Pietsch et al.52 Moreover, the decomposition of protonated O3 (HO3+ formed in strongly acidic media) at the silicon surface is probably inhibited compared to O3 (Eq. 1). However, HO3+ is generally considered to be more reactive than O3.50

(2185 cm-1) (O2)SiH2

SiH2 (2108 cm-1) 17 mol L-1 H2SO4

(2252 cm-1) (O3)SiH

5 min 15 min 25 min

Kubelka-Munk

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H-terminated Si surface

1.8 mol L-1 H2SO4 1 min 5 min 15 min 28 min 3600

3100

2600

2100

wavenumber in

1600

1100

600

cm-1

Figure 8: DRIFT spectra of time dependent model oxidation experiments of H-terminated (100) Si surfaces; black (middle): H-terminated Si surface; light grey (top) oxidation in highly concentrated aqueous H2SO4-O3-mixtures (c(H2SO4) = 17.0 mol L-1); dark grey (bottom): oxidation in diluted aqueous H2SO4-O3-mixtures (c(H2SO4) = 1.8 mol L-1); 20 °C.


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Silicon treatment in aqueous HF-H2SO4-O3-solutions offers slight etching for low H2SO4 concentrations and surface cleaning for higher H2SO4 concentrations. This is due to an enhanced silicon surface passivation by SiH groups in strongly acidic milieu, preventing oxidation of silicon by O3 (aq) or other reactive species. In H2SO4-rich aqueous HF-H2SO4-O3-mixtures primarily, native oxide is removed by HF, resulting in H-terminated silicon surfaces.

Reactions of silicon with aqueous HF-HCl-O3-mixtures Reactivity studies Dissolution of silicon was generally observed in most ranges of concentrations for HF-HClO3-mixtures. At low HCl concentrations the reactivity towards silicon decreases (figure 9). This is due to salting out effects (lower solubility of O3 (aq)) and the reaction of O3 (aq) with chloride (O3 decomposition eq. 9 - 11).36,53,54

Eq. 9

O3 (aq/g) + Cl-(aq) → O2 (aq/g) + ClO-(aq)

53

Eq. 10

2 H+(aq) + Cl-(aq) + ClO-(aq) ⇄ Cl2 (aq) + H2O(l)

Eq. 11

O3 (aq) + 2 H+(aq) + 2 Cl-(aq) ⇄ O2 (aq/g) + H2O(l) + Cl2 (aq)

53

53

For HCl concentrations between 0.2 and 2.0 mol L-1 the amount of oxidizing agents is comparatively low. Hence, there is low reactivity towards silicon. For HCl concentrations above 2.0 mol L-1 the titrimetrically determined concentration of oxidizing agents increases, so that the etching rates increase, too (figure 9). In these mixtures the equilibrium in eq. 10 is shifted towards the formation of chlorine (Cl2), which is better dissolved in aqueous solutions than O3.


19


This is additionally related to the formation of trichloride (Cl3-, eq. 12) in HCl-rich Cl2 solutions.55

Eq. 12

Cl2 (aq) + Cl-(aq) ⇄ Cl3-(aq)

(K = 0.191) 55

The H+-catalyzed formation of Cl2 in O3-Cl--mixtures was already investigated (eq. 11).50,56 It was shown that the chlorine species are Cl2 and Cl3- are present in these mixtures with pH < 3. Concentrations of HOCl/OCl- or other oxidizing Cl-species were below the detection limit and need not be considered for mechanistic discussions.50,56 UV/Vis measurements show the presence of Cl2/Cl3-.49,56 Surprisingly, no O3 (aq) was detected in HF-HCl-O3-mixtures (see Supporting Information figure S7). Therefore, and because of the above mentioned observations concerning other potential oxidizing species, the oxidation of silicon in these mixtures is caused by Cl2 or Cl3- (E0(Cl2/Cl-) = 1.36 V).

0.40 0.35 0.30

r in nm s-1

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0.25 0.20 0.15 0.10 0.05 0.00 0.0

3.0 6.0 c(HCl) in mol L-1

9.0

Figure 9: Reactivity of HF-HCl-O3-mixtures towards silicon at 20°C; c(HF) = 0,5 mol L-1, diamond wire sawn wafer; c(HF) = 0,5 mol L-1, SiC-slurry sawn wafer; c(HF) =


20

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2.0 mol L-1, diamond wire sawn wafer; c(HF) = 2.0 mol L-1, SiC-slurry sawn wafer; broad light grey line: general trend.

Highest reactivities were observed for HCl-rich HF-HCl-O3-mixtures (see figure 9 and Supporting Information figure S8). The graph also shows the dependence between concentration of oxidizing agents – mainly Cl2/Cl3- – and etching rates. The HCl-rich mixtures exhibit the highest Cl2/Cl3- concentrations. The reached Cl2/Cl3- concentrations are steady state concentrations, which are relatively low compared to Cl2 saturated solutions. This is due to the Cl2 discharge by the O2/O3 gas flow and the nearly Cl2 free gas phase.

Surface properties The higher etching rates result in changed surface morphologies as presented in figure 10. Anisotropic silicon dissolution behavior was observed regardless of the saw damage. This was already reported for other acidic Cl2-containing HF-mixtures.49 The anisotropy suggests a reaction-controlled mechanism of silicon dissolution.


21


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Figure 10: Surface morphologies (SEM images) of diamond wire sawn (A and B) and SiCslurry sawn (C and D) as-cut (100) Si surfaces before (A and C) and after treatment in HF-HClO3-mixture (c(HF) = 2.0 mol L-1, c(HCl) = 9.2 mol L-1, c(Cl2

(aq))

≈ 7.0 mmol L-1); silicon

removal B: 2.6 µm; silicon removal D: 3.8 µm; 20°C.

As found for aqueous HF-O3- and HF-H2SO4-O3-mixtures (vide supra), XPS measurements (see Supporting Information figure S25) indicate an oxygen-free H-terminated silicon surface. There were no hints to covalently attached O-, F- or Cl-atoms at the silicon surface. The oxidation behavior of HF-free aqueous HCl-O3- or HCl-Cl2-mixtures towards Hterminated silicon surfaces was investigated by DRIFT spectroscopy (see Supporting Information figure S9). Surface near Si—Si bonds were oxidized resulting in (O)SiH2, (O,O)SiH2 and (O,O,O)SiH surface groups. There were also non-oxidized silicon surface groups (SiH2) detectable. The broad vibrational band between 3000 cm-1 and 3700 cm-1 indicates a hydrophilic character of the oxidized surface. This also indicates an oxidation of SiH surface groups. There are no indications for Si—Cl moieties at the silicon surface. Such potentially intermediately formed units would be hydrolyzed in aqueous media. Cl2/Cl3- in aqueous solutions react in two ways with H-terminated silicon surfaces: For some surface regions most Si—Si bonds are oxidized. At other silicon surface atoms the Si-H bond is oxidized. Some SiH2 surface groups remain unchanged, which might be due to the passivation of SiH2 groups in strongly acidic media. The different oxidation behavior of Cl2 (E0(Cl2/Cl-) = + 1.36 V) and O2 (E0(O2/H2O) = + 1.23 V) towards H-terminated silicon surfaces might be caused by the presence of polar Cl3- ions or Cl/Cl2- radicals in contrast to the non-polar Cl2 and O2 molecules. Furthermore, double bond cleavage in O2 (binding energy 498 kJ mol-1) is twice energy intensive than in Cl2 (binding energy 242 kJ mol-1). Thus, we assume Cl3- or Cl/Cl2- to be


22

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the most relevant species especially in HCl-rich aqueous HCl-O3-mixtures for oxidation of silicon.

Silicon dissolution in aqueous HF-HCl-O3-mixtures Similar to dissolution of silicon in aqueous HF-O3-mixtures, in HF-HCl-O3-mixtures there is also a formal oxidation of silicon surface atoms, immediately followed by complexation of the oxidized Si atoms by fluoride-containing species. O3 (aq) is acting as oxidizing agent besides Cl2 (aq) only for HCl concentrations smaller than 2.0 mol L-1 (see eq. 6). At higher HCl concentration the formed Cl2/Cl3-/Cl oxidize the silicon surface (eq. 13). Considering that Cl2/Cl3- is formed by O3 (eq. 11), the overall reaction is the same as in aqueous HF-O3-mixtures (eq. 8) with HCl acting as a homogeneous catalyst.

Eq. 11

O3 (aq) + 2 H+(aq) + 2 Cl-(aq) → O2 (g) + H2O(l) + Cl2 (aq)

Eq. 13

a) Si(surface) + 2 Cl2 (aq) + 2 H2O(l) → “SiO2”(surface) + 4 HCl (aq) b) Si(surface) + 2 Cl3-(aq) + 2 H2O(l) → “SiO2”(surface) + 4 HCl (aq) + 2 Cl-(aq)

Eq. 7

“SiO2”(surface) + 6 HF(aq) → H2SiF6 (aq) + 2 H2O(l)

Eq. 8

Si(surface) + 2 O3 (aq) + 6 HF(aq) → H2SiF6 (aq) + 2 O2 (g) + 2 H2O(l)

The observed anisotropic surface morphology indicates a reaction-controlled dissolution of silicon. Because of the oxide-free surface and the rate dependency on the oxidizing agent concentration, the silicon oxidation/e--hole injection seems to be the rate determining process. Besides, there might be a rate determining formation of intermediate species and surface states. Further reaction steps – as mechanistically discussed by Stapf et al.49 and in the Supporting


23


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Page 24 of 34

Information of this work – are fast and cause the formation of H-terminated silicon surfaces.23 The anisotropic dissolution behavior suggests a divalent dissolution mechanism of silicon – similar to etching in alkaline solutions (proposed mechanism for dissolution of silicon in HClrich aqueous HF-HCl-O- mixtures see Supporting Information figure S10).49,57

CONCLUSIONS A summary of the investigated oxidizing species in the different aqueous HF-O3-based mixtures is shown in table 1. Noticeable silicon etching appears in aqueous HF-O3-, H2SO4-poor HF-H2SO4-O3- and HF-HCl-O3-mixtures. The resulting silicon surfaces are oxygen-free and Hterminated. In aqueous mixtures with dissolved O3 as an oxidizing agent (HF-H2O-O3-, H2SO4-poor HFH2SO4-O3-mixtures) the surface near Si—Si bonds are oxidized first. Later also Si—H bonds are attacked. The chemical oxidation of silicon by O3 enables a tetravalent silicon dissolution mechanism in HF containing mixtures. In HF-H2O-O3-mixtures, silicon oxidation and dissolution seems to be fast, there was no obvious reaction rate control. Etching rate dependence on O3 concentration indicates a diffusion-controlled silicon oxidation, because of slow diffusion of O3 (aq) to the silicon surface and the comparatively low concentrations of O3. This results in a polishing etch regime for HF-H2O-O3-mixtures. Aqueous HF-H2SO4-O3-mixtures with higher H2SO4 concentrations exhibit a decreased reactivity, only silicon surface cleaning and no significant etching is observed. Obviously, increasing H-passivation in the strongly acidic mixtures prevents a silicon oxidation and in turn silicon dissolution.


24

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In aqueous solutions containing HCl and O3 the oxidizing species Cl2 and Cl3- are formed. Especially HCl-rich mixtures contain larger amounts of Cl3-. Most likely, these chlorine species act as oxidizing agents towards the silicon surface. Model oxidation experiments without HF show all possible silicon surface species: (O,O)SiH2, (O,O,O)SiH, Si-OH/Si-O-Si and SiH2 groups. The observed anisotropic surface morphology indicates a divalent, reaction-controlled dissolution mechanism of silicon. The rate determining process seems to be the silicon oxidation/e--hole injection.

Table 1: Summary of oxidizing species present in HF-H2O-O3-, aqueous HF-H2SO4-O3- and aqueous HF-HCl-O3-mixtures. oxidizing species

standard redox potential relevant for these mixtures (E0, pH = 0, c(Ox.) ≈ 1 mol L-1)

O3 (aq)

+ 2,07 V

- HF-H2O-O3-mixtures (present in the whole concentration area, increasing with c(HF)) - HF-H2SO4-O3-mixtures (low amounts for c(H2SO4) < 7 mol L-1, increasing amounts for c(H2SO4) > 7 mol L-1) - not present in HF-HCl-O3-mxitures

HO3+(aq)

> + 2,07 V because of - as shown by Levanov et al.36 in H2SO4-rich pH < 0

HF-H2SO4-O3-mixtures

HSO5 (aq)

+ 1,81 V

- not detectable in HF-H2SO4-O3-mixtures

Cl2 (aq)

+ 1,36 V

- HF-HCl-O3-mixtures (present in the whole


25


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Page 26 of 34

concentration area, increasing with c(HCl)) Cl3-(aq)

+ 1,36 V

- as shown by Zimmerman and Strong55 in HCl-rich HF-HCl-O3-mixtures

O2 (aq)

+ 1,23 V

- present in all mixtures, no significant effect on the etching rate of silicon

ASSOCIATED CONTENT SUPPORTING INFORMATION - Cyclic voltammetry measurement of silicon electrode dissolution in aqueous HF (Figure S1) - Further information about O3 (aq) quantification - Selected IR vibrations and assigned Si surface groups (Table S1) - XPS signals of Si 2p region – reference and measured data (Table S2) - Further reactivity data (Figure S2, Figure S9) - Photographs of the etching mixtures (Figure S3, Figure S7) - Raman spectrum of aqueous HF-H2SO4-O3-mixtures (Figure S4) - SEM images Si surfaces before and after treatment in aqueous HF-H2SO4-O3-mixtures (Figure S5) - Further DRIFT spectra of treated Si surfaces (Figure S6, Figure S10) - UV/Vis spectra presenting the Cl2 formation in HF-HCl-O3-mixtures (Figure S8) - Proposed dissolution mechanism of (100) silicon surface atoms in aqueous HCl-rich HFHCl-O3/Cl2-mixtures (Figure S11)


26

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- Further information to XPS measurements and XPS spectra (C 1s, F 1s, O 1s, Si 2p, Cl 2s; Figure S12 – Figure S27)

AUTHOR INFORMATION Corresponding Author * Prof. Dr. rer. nat. habil. E. Kroke, Institute of Inorganic Chemistry, Technische Universität Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany, e-mail: [email protected]

ACKNOWLEDGMENT The authors gratefully acknowledge Dipl.-Ing. Beate Kutzner for performing IC analyses and Dipl.-Ing. Regina Moßig for collecting Raman spectra. C.G. was financially supported by the German Federal Ministry of Education and Research for funding within the project TEMPO. The authors also thank the German Research Foundation (DFG, Bonn) for financial support.

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Etching Silicon with Aqueous Acidic Ozone Solutions - ACS Publications

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