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Etching and chemical control of the silicon nitride surface Marine Brunet, Damien Aureau, Paul Chantraine, François Guillemot, Arnaud Etcheberry, Anne Chantal Gouget-Laemmel, and Francois Ozanam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12880 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on January 4, 2017

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ACS Applied Materials & Interfaces

Etching and Chemical Control of the Silicon Nitride Surface Marine Brunet†,‡, Damien Aureau§, Paul Chantraine†, François Guillemot‡, Arnaud Etcheberry§, Anne Chantal Gouget-Laemmel†,*, François Ozanam†,* †

Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique, CNRS, Université Paris-Saclay, 91128 Palaiseau, France ‡

Saint-Gobain Recherche, 39 quai Lucien Lefranc, 93303 Aubervilliers, France §

Institut Lavoisier, UVSQ-CNRS UMR 8180, 78035 Versailles, France

ABSTRACT :

Silicon nitride is used for many technological applications, but a quantitative knowledge of its surface chemistry is still lacking. Native oxynitride at the surface is generally removed using fluorinated etchants, but the chemical composition of surfaces still needs to be determined. In this work, the thinning (etching efficiency) of the layers after treatments in HF and NH4F solutions has been followed by using spectroscopic ellipsometry. A quantitative estimation of the chemical bonds found on the surface is obtained by a combination of infrared absorption spectroscopy in ATR-mode, X-ray photoelectron spectroscopy and colorimetry. Si-F bonds are the majority species present at the surface after silicon nitride etching; some Si-OH and a few SiNHx bonds are also present. No Si-H bonds are present, an unfavorable feature for surface

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functionalization in view of the interest of such mildly reactive groups for achieving stable covalent grafting. Mechanisms are described to support the experimental results and two methods are proposed for generating surface SiH species: enriching the material in silicon, or submitting the etched surface to a H2 plasma treatment.

KEYWORDS: silicon nitride; surface chemistry; etching; FTIR spectroscopy; XPS


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1. INTRODUCTION Silicon nitride (SixN4) is a material commonly used in microelectronic industry for its strong mechanical resistance and its insulator and chemical barrier properties.1, 2, 3, 4, 5 Because of its biocompability, SixN4 also finds application in biomedical and biosensor fields.6,

7, 8

In

comparison with silicon, it offers the extra advantage of a low optical refractive index,9, 10 which is favorable for several detection schemes in the biochip context.11 Such a material can be easily obtained as thin films by (plasma-enhanced) chemical vapor deposition ((PE)-CVD)12, 13, 14 or by reactive magnetron sputtering.15, 16, by a native oxynitride layer19,

17, 18

20, 21, 22

When exposed to atmosphere, silicon nitride is covered

which needs to be removed for some applications. One

method to remove the native oxynitride is to use wet etching solutions. Such procedures are widely used on crystalline silicon surface: the corresponding wet etching processes have been studied in depth to determine etching rates as a function of the etching conditions and to obtain a comprehensive overview of the underlying mechanisms.23,

24, 25, 26

In particular, the selective

removal of the silicon oxide layers is achieved with hydrogen fluoride (HF) or mixture of HF/ammonium fluoride (NH4F) solutions.27 Concerning silicon nitride, phosphoric acid (H3PO4) is well-known to selectively remove SixN4 on top of SiO2.27 On the opposite, there is no widely recognized process for selectively removing the native oxynitride without affecting SixN4.28 Whereas etching kinetics of the SixN4 material were largely studied using HF,29,

30, 31, 32, 33, 34

HF/NH4F35,36 or potassium hydroxide (KOH) solutions,37,38 few fundamental studies of the etching mechanisms are available.39, 28, 29,30,32,34 In addition to the selective removal of the native oxynitride on silicon nitride, the knowledge of the resulting chemical functions at the SixN4 surface after the wet etching represents another



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important

challenge.

The

surface

chemistry

is

still

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controversial:

after

HF

etching, -NHx,40 -SiH,39,41 –SiOH42 or –SiF29,32 have been mentioned as resulting surface chemical species. Recently, Liu et al. reported that no -SiHx are found at the end of the etching step and that the surface is mainly covered by -SiF, -SiOH and a few -NHx.34 The absence of SiHx is in agreement with a previous report of Bermudez42 but in contradiction with other reports that claimed to make use of these species as reactive groups for further modification of the surface via hydrosilylation reaction of 1-alkene precusors.39,43, 44 Inducing surface SiH species as a results of silicon nitride wet etching therefore appears in some cases as an important target. Therefore, this work focuses on the issue of the surface composition of silicon nitride after various etching treatments aimed at stripping the native oxynitride, and determining conditions yielding surface SiH species. A special emphasis is put on the dependence of the surface composition on the stoichiometry of the starting material, especially on silicon enrichment in order to make a bridge with the well-known processes and etching mechanisms prevailing on silicon etching. Wet treatments in HF and in NH4F were investigated and surface composition was characterized by combining X-ray Photoelectron Spectroscopy (XPS) measurements and Attenuated Total Reflection Infra-Red (ATR-IR) spectroscopy. In parallel, the composition of the starting material was tuned by varying the silicon content of the SixN4 layer in the range 3.4 < x < 8. Finally, a complementary dry etching procedure is considered for inducing the presence of SiHx species after the wet etching.

2. MATERIALS AND EXPERIMENTAL SECTION 2.1. Materials. All cleaning (H2O2, 30%; H2SO4, 96%) and etching (HF, 50%; NH4F, 40%) reagents were of RSE grade and were supplied by Carlo Erba. The deuterated chloroform


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(99,80% D) was supplied by Euriso-Top. All the other chemicals, supplied by Sigma-Aldrich, were of the highest available grade and were used as received without further purification. Ultrapure water (Milli-Q, 18 MΩ cm) was used for the preparation of the solutions and for all rinses. The silicon samples were cut from double-side polished float-zone purified n-type (111) silicon (Siltronix, France). 2.2. SixN4 deposit. Silicon nitride SixN4 (20-40 nm) films were deposited on both sides of as received silicon wafers by magnetron sputtering from a silicon target containing 8% by weight of aluminium, under a pressure of 1.5 µbar in an argon/nitrogen atmosphere. The argon and nitrogen fluxes were adjusted to tune the Si/N ratio. The argon flow was 19 sccm for Si3.4N4 and Si5N4, 22 sccm for Si8N4; the nitrogen flow was 25 sccm for Si3.4N4, 15 sccm for Si5N4 and 10 sccm for Si8N4. The substrate carrier speed determines the deposit thickness; it was settled to achieve a 35 nm layer thickness (measured with a Dektak 8 Veeco profilometer). 2.3.

Surface modification. Safety considerations. The H2SO4/H2O2 (piranha) solution is a

strong oxidant which reacts violently with organic materials. Hydrogen fluoride (HF) is hazardous acid, which can result in serious tissue damage if burns are not appropriately treated. They must be handled with extreme care in a well-ventilated fume hood, while wearing appropriate chemical safety protection. 2.3.1. Wet etching. The SixN4 and Si(111) samples were cleaned in a 1/3 H2O2/H2SO4 piranha solution at 100°C for about 10 min and then copiously rinsed with ultrapure (Milli-Q) water. The bare silicon sample was chemically etched for 5 s in a 50% HF solution (29 M). The SixN4 coated silicon (prisms) samples were etched in different etching procedures: (a) in HF solution of three different concentrations 0.2% (0.12 M), 2% (1.2 M) and 5% (2.9 M) for 15 s or 30 s; or (b)



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in a 40% NH4F solution (12 M) for 30 s, 1, 5, 10, 15, 30 and 60 min. After etching, all surfaces were rinsed with ultrapure water and carefully dried under nitrogen. 2.3.2. Plasma etching. After a 30 s 0.2% HF wet etching, one side of the Si3.4N4 samples were exposed to a hydrogen plasma (H2), (radio-frequency excitation at 13.56 MHz, power density of ~100 mW.cm-2,~200 mbar H2 pressure) at room temperature, duration of 10 to 720 s. 2.3.3. NHx bonds dosing with Coomassie blue (CBB). After the oxynitride removal with a 30 s HF 0.2% or 2% solution or a 15 min NH4F 40%, the SixN4 surface was protonated in a first solution (S1) – 5% CH3COOH, 10% MeOH, 85% H2O (v/v/v) – during 10 min. The surface was then immersed 15 min in a second solution (S2) containing 0.1 g.L-1 of CBB in S1. The sample was then rinsed 3×10 min in S1, 10 min in ultrapure water and dried under nitrogen. The surface was finally immersed in a third solution (S3) – 1 M ammonia buffer/MeOH (1/1 v/v) – during 5 min. For the calibration curve, CBB solutions of several concentrations (1 mg/L to 20 mg/L) were prepared in a 1M ammonia buffer/MeOH (1/1 v/v). The absorbance of the resulting solution was measured by UV-Visible spectroscopy (610 nm). 2.3.4. SiOH bonds dosing. After the oxynitride removal with a HF 0.2% or 2% solution for 30 s or NH4F 40% solution for 15min, the SixN4 surface was immersed in hexamethyldisilazane (HMDS) for 1 min, followed by an efficient rinse in CH2Cl2 for 2 min under argon bubbling. The sample was then dried under a stream of nitrogen. For the IR calibration curve, hexamethyldisiloxane (HMDSO) solutions of several concentrations (0.0075 M to 0.06 M) were prepared in chloroform D and were measured in an in-situ IR cell, using an oxidized silicon prism as the IR window. 2.3.5. Si-H bonds dosing


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For the IR calibration curve, tetramethyldisilazane solutions of several concentrations (0.015 M to 0.06 M) in ethanol were measured in an in-situ IR cell, using an oxidized silicon prism as the IR window. 2.3.6. Statistical analysis. All quantitative experiments were repeated at least twice and the results were expressed as average ± standard deviation. For the XPS measurement, an error of 7% for the F1s peak is found and is due to differences between SixN4 samples deposited in the same conditions. For the IR-ATR measurement, systematic and statistical errors have been carefully estimated; they do not exceed 20% in the most unfavorable cases. 2.4. Surface Characterizations. 2.4.1. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR). ATR-FTIR spectra were recorded using a Bomem MB100 FTIR spectrometer or a Bruker Equinox spectrometer coupled to a home-made, nitrogen-gas purged external ATR compartment, equipped with a liquid-nitrogen-cooled MCT photovoltaic detector. The spectra were collected with p and s polarization over the 950 – 4000 cm-1 spectral range (4 cm-1 resolution). The ATR elements were home made by mechanical polishing from a silicon wafer on which similar silicon nitride layers have been deposited on both faces. The dimensions of the prism (typically 15 × 15 × 0.5 mm3) limited the infrared path length in silicon providing access to observable vibrations at wavenumbers as low as 1000 cm-1. They were displayed as absorbance per reflection (computed using natural logarithm) by using a reference spectrum recorded prior to surface modification. The displayed spectra were normalized to the actual number of reflections N. For the results presented here, N ≈ 27 with a bevel angle of 45°. The calibration was performed in a homemade PTFCE IR cell of ∼0.5 mL volume. The top and the bottom of the cell are connected by a 0.8 mm diameter PTFE tube for the addition of the various



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solutions without breaking the spectrometer purge. On the side, there is an opening of 10 mm diameter against which the prism is pressed via a nitrile O-ring seal, resulting in 9.4 reflections. 2.4.2. FTIR Spectroscopy in transmission mode. Transmission FTIR spectra were recorded with a N2-purged Bruker Equinox spectrometer, at an incidence angle of 45° over the 500-4000 cm-1 spectral range (4 cm-1 resolution). Spectra were presented in absorbance, using as a reference a spectra recorded before the surface modification. 2.4.3. X-ray Photoelectron Spectroscopy (XPS). The XPS spectrometer was a Thermo K-Alpha or a Thermo-VG Escalab 250Xi model. Data were acquired using an Al Kα1 monochromatic X-ray excitation. The photoelectrons were collected perpendicularly to the sample surface. The detection was performed in a constant analyzer energy mode (CAE), using a pass energy of 20 eV. For the composition profiles, the etching came from Ar+ or Arn+ ion sputtering. Due to the low conductivity of the silicon nitride layer, slight charge effects could appear during XPS measurements. Therefore, the energy scale in the XPS spectra was translated to set the energy of the C1s peak at 285.0 eV. The data were processed using the Thermo Electron “Avantage XPS software”. 2.4.4. Ellipsometry. Spectroscopic ellipsometry data in the visible range were obtained using a J.A. Woolam VASE M-2000 Xi Spectroscopic Ellipsometer equipped with Complete Ease data analysis software. The system acquired a spectrum ranging from 200 to 1000 nm by step of 2 nm. The spectrometer was calibrated with a reference silicon wafer. Data were taken using two angles of incidence of 50° and 75°. Data were fitted by regression analysis with a Tauc-Lorentz model for the silicon nitride layer. 2.4.5. UV-Vis spectrometer


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Absorption spectra were recorded with a Cary 300 UV/vis Spectrophotometer in Helma 6040-UV quartz cuvettes of 10 mm length. The wavelength range was 400–800 nm.

3. RESULTS The composition of the SixN4 layer was determined by XPS depth profile coupled with Ar+ abrasion. This strategy allows for getting rid of possible chemical changes at the uppermost surface in contact with air. Several points were taken on each sample to ensure the reproducibility of the analysis. The abrasion was performed thanks to monoatomic Ar+ beam. Figure 1 shows the atomic percentage of the different elements detected in the XPS spectra determined from the area of the Si2p, N1s, O1s, C1s and Al2p peaks. The composition strongly changes in the first seconds of the abrasion, which is likely associated to the native oxynitride film at the surface in contact with air. A bump in the oxygen profile, located at the position where simultaneously the nitrogen signal drops and the silicon signal increases (abrasion time between 360 and 480 s), reveals the presence of a native oxide on the silicon wafer prior to silicon nitride deposition. Except at the extreme surface (first 10 s of abrasion) where it arises from adventitious contamination, the carbon contribution is hardly detectable on figure 1, indicating that no organic contaminants are detected in the layer. Both silicon and nitrogen peak intensities are stable between 50 and 300 s, when the superficial oxynitride surface layer has been removed. This observation demonstrates the in-depth homogeneity of the deposited silicon nitride layer. The Al2p peaks follow the evolution of the Si2p ones in the SiN layer. About 5% of aluminum is found in the global composition, which arises from the aluminum present in the sputtering target



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used during the deposition process. For the sample shown on Figure 1, the Si/N ratio measured between 50 and 300 s amounts to 0.85, corresponding to the composition Si3.4N4 for the deposited silicon nitride layer. Owing to the presence of a small amount of oxygen and aluminum in the material, this composition can be considered as quasi-stoichiometric (Si3N4-like). This is confirmed by a close examination of the Si2p XPS peak, which exhibits a single compound associated to a silicon atom bonded to nitrogen atoms, and no additional compound revealing the presence of Si-Si bonds.

Figure 1. Atomic contributions derived from an XPS profile performed using Ar+ abrasion. The depth scale is deduced from ellipsometry measurements on the deposited film, under the assumption of a constant abrasion rate. The material stoichiometry deduced from the Si:N ratio is Si3.4N4.

3.1. Wet etching


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Two chemical etching treatments (either in HF or NH4F) were performed on Si3.4N4 layers to remove the native oxynitride. First, acidic HF solutions were used at different concentrations and reaction times. As displayed in Figure 2A, an efficient removal of the native oxynitride is obtained after 30 s for HF concentrations ranging from 0.2 to 5%: the O1s signal at 532 eV exhibits a strong decrease after etching and reaches the magnitude corresponding to the signal found while profiling the bulk of the silicon nitride layer. The Si2p XPS spectra show that the removal of the native oxynitride layer is efficient (disappearance in the Si2p peak of the highenergy tail at ~104 eV ascribed to oxidized silicon, see Supporting Information, Fig. S1). The etched thickness as a function of HF concentration and reaction time was followed by using ellipsometry; it has also been monitored using FTIR spectroscopy by measuring the loss of the integrated IR peak associated to the vibration modes of Si-N-Si and Si-O-Si between 680 and 1350 cm-1 (Figure 2B). We found a good correlation between both measurements demonstrating that a higher HF concentration and/or a longer etching time give a higher loss of thickness. The best results were obtained for an etching in HF 0.2% for 30s leading to the complete removal of the oxynitride layer with a partial loss of 3-4 nm of the starting Si3.4N4 material. On surfaces etched at different concentrations, fluorine was detected from the presence of a F1s peak in the XPS spectra (Figure 2C) at 686.4 eV. The magnitude of this signal amounts to an atomic percentage of 2.8 ± 0.2%, suggesting the formation of SiF bonds at the surface. As a matter of fact, the slight asymmetry of the F1s line reveals the presence of a small contribution of fluorinated physisorbed residues at 684.8 eV (accounting for ~ 4 ± 2% of the total peak intensity). Abrasion using argon cluster ions confirms the attribution of the 684.8 eV peak to physisorbed residues and shows that the F atoms are located at the surface only. The values of Si and F atomic percentages and of the Si2p photoelectron escape depth (3.14 nm, estimated from



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the silicon nitride density) yield a surface concentration of 8.1 ± 0.6 nm-2 for fluorine atoms bound to silicon (see Supporting Information, Fig. S2). The surface chemical composition of the optimized etched Si3.4N4 layer was also analyzed by ATR-FTIR (Figure 2D). Whereas the Si-Hx bonds on crystalline (111) silicon surfaces are easily detected by a characteristic IR signature related to the stretching modes νSiHx around 2100 cm-1, no Si-Hx bonds are detected in this region on the etched Si3.4N4. At this stage, no conclusion may be drawn by ATR-IR concerning the eventual presence of NHx or SiOH bonds.

Figure 2. (A) XPS spectra of the O1s region for a Si3.4N4 surface as received (orange), after piranha cleaning (green) and after a 30 s HF etching at 5% (black), 2% (red), 0.2% (blue). (B) Etched thickness derived from spectroscopic ellipsometry (open boxes) and integrated FTIR


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absorption in the 680-1350 cm-1 range (solid boxes), after a 15 s (circles) or a 30 s (squares) HF etching of a Si3.4N4 surface. Notice the proportionality of the two quantities. (C) XPS spectra of the F1s region for a Si3.4N4 surface after a 30s etching in 5% (black), 2% (red), 0.2% (blue) HF. (D) ATR-IR spectra in p-polarization of a Si3.4N4 surface after 30 s etching in 0.2% HF (light grey) and of a Si(111) surface after 5 s etching in 50% HF (black), the reference being the oxidized surface. The inset represents the SiH stretching vibration range.

A colorimetric assay based on Coomassie Brilliant Blue (CBB) was first performed to determine the density of surface NHx bonds.45, 46 Using a calibration curve (see Supporting Information, Figure S3), a surface concentration of 0.5 ± 0.1–NHx per nm2 is found after a 2% or 0.2% HF treatment. Furthermore, a chemical dosing consisting in reacting the etched surface with hexamethyldisilazane (HMDS) was achieved to determine the surface concentration in silanol groups.47,48 The amount of trimethylsilyloxy groups resulting from the reaction of surface SiOH with HMDS molecules is determined by measuring the absorbance of the δCH3 bending mode at 1260 cm-1 characteristic of the methyl-silyl moieties and using a calibrate curve of the hexamethyldisiloxane IR absorption (see Supporting Information, Fig. S4). A SiOH surface concentration of 1.4 ± 0.3 nm-2 is found after a 2% HF treatment and of 1.2 ± 0.3 nm-2 after a 0.2% HF treatment. The etching of Si3.4N4 in concentrated NH4F (12 M) was also investigated. In comparison with HF etching, the dissolution rate is therefore much slower (see Supplementary Information, Figure S5). In XPS spectra, the Si2p peak confirms that the etching efficiently removes the native oxynitride layer and the F1s peak shows that a large amount of fluorine is also left on the surface (see Supporting Information, Fig. S1B). Using the same techniques as above for HF



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etching, the concentrations of the various surface species are determined: 2.3 ± 0.4 nm-2 for SiOH species (slightly higher than after a HF treatment), 5.3 ± 0.3 nm-2 for F atoms (~ 70% of that found after a HF treatment), 0.9 ± 0.1 nm-2 for –NHx species (a somewhat larger amount than after HF etching). Again, no formation of SiHx bonds is detectable by ATR-IR spectroscopy. In summary, etching in fluorinated solutions efficiently removes the native oxynitride present at the Si3.4N4 surface, while keeping weak the silicon nitride etching if the etching time is properly controlled. The surface is mostly covered by fluorine atoms, and no SiH species is formed.

3.2 Change of the surface chemistry by tuning the layer composition In order to obtain surface SiH species after wet etching, a possible strategy could consist in making the silicon nitride layer silicon-rich. This would make the material composition closer to silicon, whose surface is covered by SiH species after a fluorinated wet etching.24,49,50 Silicon nitride can be easily made silicon-rich by tuning the gas ratio during the deposition (cf. 2.2.). We prepared two SixN4 layers with x = 5 and x = 8, as determined by XPS measurement after chemical etching in the optimized conditions (2% HF for 30 s). In the XPS spectra displayed in Fig. 3A, the lineshape of the Si2p peaks of the Si-rich silicon nitride layer after HF etching appears complex. The Si-N peak detected for Si3.4N4 is now shifted to low energy and a new component around 99 eV appears. This evolution is a clear indication of the formation of Si-Si bonds inside the material, an expected consequence of the Si enrichment.


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Figure 3. (A) XPS spectra of the Si2p region and (B) ATR-IR spectra (p polarization) of a freshly HF etched surface of Si(111), Si8N4, Si5N4 and Si3.4N4. The silicon nitride samples are etched in a 2% HF solution during 30s. The silicon sample is etched in a 50% HF solution during 5s. For the IR spectra, the reference is the same surface before etching.

The chemistry of the etched surface is also affected by the Si/N ratio. As shown in figure 3B, HF etching of a silicon-rich silicon nitride layer leads to the formation of surface Si-H bonds. In comparison with the IR spectrum of the hydrogenated crystalline silicon surface, the band related to the stretching modes of the SiHx is larger and shifted to higher energy due to the fact that the hydrogenated silicon atoms are also linked to nitrogen atoms. At least two contributions are present in the broad signal from Si5N4 and Si8N4 layers: one is peaked at ∼2230 cm-1, and the other one is peaked at ∼2130 cm-1. The latter is dominant in Si8N4 layers. The frequency of SiH vibrations are known to be much affected by the electronegativity of the atoms linked to silicon.51,52,53 In the following, we note (R1 R2 R3)SiH a SiH group where the 15 Environment ACS Paragon Plus


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silicon atom is bonded to R1, R2 and R3 and (R1 R2)SiH2 a SiH2 group where the silicon atom is bonded to R1 and R2. SiH in Si3N4, i.e., a (N N N)SiH species, exhibits a vibration at 2200 cm-1.42 The presence of sizeable contributions at much higher wavenumbers (at 2230 cm-1) in the IR spectrum shows that some of the contributing species are SiH species linked to atoms more electronegative than nitrogen. In the present context, fluorine is the likely candidate, and possible species could be (N N F)SiH. Conversely, correlation charts from the literature indicate that for contributing to vibrations around 2130 cm-1, SiH groups must be bonded to atoms of low electronegativity. In the present context, it comes that at least one of the silicon neighbor atoms must be a silicon atom. Possible candidates are (Si N N)SiH, (Si Si F)SiH or (Si N)SiH2. In view of the broadness of the IR SiH band, contributions from species exhibiting vibrations located between the two main peaks can also be considered. In addition to (N N N)SiH, possible candidates are (N N)SiH2 or (Si N F)SiH. The emerging picture is therefore that after etching in HF, the surface of silicon-rich silicon nitride is covered with atomic Si-based units decorated with F and/or H atoms, resulting in a variety of surface groups. An estimate of the surface concentration SiH groups was worked out by assuming a single infrared cross-section for all the SiH vibrators at the surface. This cross section was derived from the absorption of tetramethyldisilazane, a compound in which the SiH vibrators are linked to one nitrogen atom. This crude procedure yields an estimate of 2.1± 0.4 SiH groups per nm2 (See Supporting Information, Fig. S6). The concentrations of the other surface species have been determined according to the previously described procedure. It comes 8.6 nm-2 F atoms bonded to Si (from F1s and Si2p XPS signals), 1.5 ± 0.3 nm-2 for SiOH species (from the reaction with HMDS), and less than 0.1 ± 0.02 nm-2 NHx groups (from the CBB dosing).


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The etching of silicon-rich silicon nitride in fluorinated solutions yields surface with as much fluorine atoms as quasi-stoichiometric silicon nitride, but with a significant amount of surface SiH species. These SiH species are in a variety of local environment, the silicon atom being linked to nitrogen, fluorine or silicon neighbors and combination thereof.

3.3 Hydrogen plasma treatment Another strategy to obtain SiH-terminated surface is to perform a post-etching hydrogenation treatment. Such a treatment could be a H2 plasma treatment which can plausibly induce the formation of Si-Hx or N-Hx bonds when performed after the wet native oxynitride stripping. Figure 4A represents the ATR-IR spectra of the etched Si3.4N4 surface before and after exposure to H2 plasma during different times. After plasma, a broad band appears in the 2200 cm-1 wavenumber range, corresponding to the stretching modes of Si-Hx bonds. This band increases with the plasma duration. When comparing the plasma-treated Si3.4N4 surface to a hydrogenated silicon surface, the shift of the SiH peak at higher energy is due to the fact that in Si3.4N4, the hydrogenated Si atoms are chemically bonded to nitrogen atoms. However, the band appears here much broader, suggesting the contribution of various SiH species. After a 10 s plasma exposure, the IR band is weak and centered at 2240 cm-1. As mentioned previously (in 3.2.), this observation strongly suggests that it mostly comes from SiH groups back bonded to fluorine atoms, like (N N F)SiH species. For exposure times from 30 to 60 s, the IR band has increased and reaches a magnitude similar to that recorded on crystalline silicon. It is now centered at 2210 cm-1, with a low energy tail. It suggests that species like (N N N)SiH or (N N)SiH2 now prevail. The large value of the SiH IR absorption is likely due to a strong roughening of the surface, highlighting the etching action of the H2 plasma. After 300 s of plasma, the signal is quite broad,



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and a distinct contribution appears in the 2000-2050 cm-1 range. Such a contribution requires the appearance of species like (Si Si Si)SiH or (Si Si)SiH2, which can be generated by the etching of the silicon substrate only. The subsequent stabilization of the signal consistently reveals that part of the layer have then been completely etched out. This is confirmed by examination of the XPS narrow scans of the Si2p region (cf Figure 4B). The shape of the peak characteristic of the signal from Si3.4N4 remains unchanged until 120 s of plasma. Upon prolonged plasma exposure, the peak is significantly broadened, with the appearance of a new peak at 99.5 eV, again characteristic of the signal from the silicon substrate, confirming the degradation or the partial dissolution of the silicon nitride layer. As shown from the F1s XPS spectra displayed in Figure 4C, SiH formation is correlated with a progressive fluorine loss as a function of plasma time. After 30 s of plasma, there is still a large amount of fluorine, suggesting that the initial surface Si-F bonds are not broken by the plasma, but that the silicon nitride film is progressively etched during the treatment. After 12 min of plasma treatment, no more fluorine is detected.

Figure 4. (A) ATR-IR spectra (p polarization), (B) XPS spectra (Si2p peak) and (C) XPS spectra (F1s peak) of the etched Si3.4N4 surface (30 s 0.2% HF) (grey), then treated with a room temperature H2 plasma of 10 s (blue), 30 s (green), 60 s (red), 120 s (pink), 300 s (orange) or 720 s (black). For the IR spectra, the reference is the same surface before etching.


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To summarize, a post-etching H2 plasma treatment actually induces the formation of surface SiH species by slowly etching the silicon nitride layer. In this way, SiH species may become surface majority species, at the expense of a significant surface roughening.

4. DISCUSSION 4.1 Surface concentrations A major outcome of the previous results is to give a quantitative determination of the surface chemical composition of silicon nitride layers after various etching treatments. Such a determination is not completely available in the literature. The qualitative picture of an HFetched Si3.4N4 is that of a surface mostly covered by fluorine atoms bound to silicon (8.1 F atoms.nm-2), with some surface SiOH species (1.2 Si-OH.nm-2) and a few NHx species (0.5 NHx.nm-2). Strikingly, no SiH species are found in those conditions. This picture is consistent with that given by Liu et al.34 but the comparison in terms of coverages is not immediate since coverages were determined as percentages of a reference surface which is different for all the considered species. The quantitative estimation of surface concentrations appears at first sight somewhat problematic, since the total amount of surface species is on the order of 10 nm-2 for a Si3.4N4 surface etched in 0.2% HF. This amount appears too large, since a dense plane of silicon atoms corresponds to a Si surface concentration of ~7.8 nm-2.54 This high surface concentration might be ascribed to a significant roughening of the surface. We rather assume that it mostly comes from the existence of SiF2 surface species. No direct spectroscopic proof is available, though, for the existence of SiF2 species on the HF-etched Si3.4N4 surface. The Si-F vibrations are outside the spectral range accessible in our ATR geometry and the F1s peak



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in XPS spectra is practically identical for SiF and SiF2 species. The Si2p XPS peak is expected to be somewhat different, but in silicon nitride, this difference will be less obvious than in silicon, since the energy difference between (N N N)SiF and (N N)SiF2 will be reduced; moreover, these relatively small contributions will be superimposed on the dominant (N N N)SiN signal from the bulk silicon nitride, making any deconvolution of the Si2p peak in three components (actually three doublets) mathematically non unique. The analysis of the Si2p line can only unambiguously show that there is a high-frequency contribution on the high energy side of the Si2p line, whose amplitude is too high for being accounted for by SiOH surface species only. We indistinctly ascribe this contribution to SiF and SiF2 species. It therefore appears out of reach to determine an experimental value for the SiF/SiF2 ratio. As a mere support to the SiF2 presence at the HF-etched Si3.4N4 surface, one might only observe that a SiF/SiF2 ratio of 1 would be large enough for keeping the total (SiFx+SiOH+NHx) surface concentration within the bounds of a dense Si plane. The question of the high value of the total concentration in surface species appears more severe for the HF-etched Si5N4 surface, since the total (SiFx+SiOH+NHx) surface concentration remains practically identical to that measured for the HF-etched Si3.4N4 surface and that a significant amount of SiH species is also present. Here again, the major cause of the apparent paradox can be solved by keeping in mind that many (if not most) of the SiH species present at the surface might be bonded to a fluorine atom (as mentioned above, species like (Si Si F)SiH, (Si N F)SiH or (N N F)SiH exhibit spectral characteristic consistent with the experimental IR spectra). Roughening of the silicon nitride layer might also contribute, a possibility supported by the slightly increased roughness in the AFM images of the HF-etched Si5N4 surface as compared to those of the HF-etched Si3.4N4 surface (see Supporting Information, Fig. S7).


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4.2 Etching mechanisms The absence of SiH species on the HF-etched Si3.4N4 surface is a striking result, in marked contrast with the well-known case of silicon which is known to be fully covered with SiHx species after etching in HF.24 The absence of SiH species is a direct consequence of the polar nature of silicon nitride. Silicon nitride is an insulator, which implies that only chemical mechanisms are taking place during its etching (no electrochemical mechanism is achievable, in contrast with silicon). In quasi-stoichiometric material, the only bonds to be broken are Si-N bonds. In view of the marked polarity of the Si-N bond, the chemical etching in an HF-based solution, by insertion of a HF molecule in the Si-N bond, can only result in the formation of Si-F and H-N bonds. No Si-H nor N-F bonds could be formed in such a process, in agreement with the experimental observations. Liu et al. have proposed an etching mechanism of quasi-stoichiometric silicon nitride on the previous grounds.34 This mechanism is depicted on Figure 5: the dissolution of one surface Si atom (from stage A to stage E) takes place by successive breaking of four Si-N bonds. As underlined by Liu et al., the experimental observation of both Si-F and Si-NHx bonds implies that the first two steps are slower than the last steps. Our observation of a significant amount of surface SiF2 species (deduced from the value of the surface F concentration) implies that the first three steps are the slowest. This conclusion can be rationalized by observing that the accessibility of the Si-N back bonds of surface Si atoms remains generally limited as long as two or three of these bonds are still present. To put it another way, a surface Si atom bonded to the material through a single Si-N bond is short-lived, whereas Si atoms of higher coordination to the surface are much less reactive. Such a picture of a reaction limited by the accessibility of the surface-



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species back bonds to solution etching agents has already been put forward by Liu et al.34 and in older works.32

Figure 5. Schematics of the etching mechanism proposed by Liu et al.34 The etching proceeds by successive breaking of Si-N bonds. Such a breaking takes place by insertion of a HF molecule in the initial Si-N bonds, yielding the formation of Si-F and N-H bonds. After the first three steps (stage D), a surface SiF3 species has been formed. The next step yields the dissolution of the Si atom (formally under the SiF4 form, likely under the equivalent SiF62- form in solution).

After NH4F etching of Si3.4N4, a lower amount of silicon-bound fluorine atoms is measured. We propose that it is due to a reduced efficiency in the transformation of SiF species into SiF2 species, thereby lowering the steady-state concentration in surface SiF2 groups. Such reduced kinetics can be expected in a process where SiF groups are transformed into SiF2 groups by breaking of one Si-N bond. According to Knotter et al.32 and as schematized in Fig. 6 (a


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decomposition in two elementary steps of the transition from B to C in Figure 5), such a process is favored by first protonating the N atom. In view of the reported pKa value of amino-terminated surfaces,55 the first step is easier in low-pH HF solutions where the surface is positively charged and hampered in concentrated NH4F solutions where the surface is neutral. According to this process, the transition from A to B in the mechanism depicted in Figure 5 should also be affected. As a consequence, the stability of surface SiNHx groups should also be increased in NH4F; it should lead to an increased steady-state concentration of surface SiNHx groups in NH4F. As above reported, such an increase in the surface concentration of NHx groups is consistently measured after NH4F etching as compared to HF etching.

Figure 6. Decomposition of the transition from B to C in Figure 5 in two elementary steps, the first one being pH dependent. Si-N bond breaking is favored by first protonating the N atom in order to form the leaving group at the first step of the reaction.

In summary, a model of polarity-driven successive breaking of Si-N bonds with stericallyhindered kinetics gives a fair account of the existence of surface SiFx and NHx species after fluorinated wet etching of quasi-stoichiometric silicon nitride. The major experimental trends are accounted for, including the absence of surface SiH species in such conditions. The reaction schemes are drawn in Fig. 7, with elementary steps yielding some of the species identified as plausibly present after HF etching. From the above considerations, it is clear that the



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presence of Si-Si bonds in silicon nitride is needed for the formation of SiH bonds during HF etching (cf. Fig. 7A). Conversely, the breaking of a Si-Si bond by an HF molecule will result in the creation of Si-H and Si-F bonds. This is the primary reason for the presence of SiH bonds after etching of Si-rich silicon nitride. However, not only Si dimers but small clusters of Si atoms are required for the formation of some of the species identified at the surface of HF-etched Si5N4. For instance, (Si N N)SiH requires the presence of 3-Si-atoms cluster in the etched material to show up : two Si atoms for creating the SiH bond, a third one back bonded to the generated SiH group (cf. Fig 7B). Along the same lines, it is easy to see that 4-Si-atoms clusters are needed for generating (Si Si F)SiH or (Si N)SiH2 species (cf. Fig. 7C). Many other environments of the SiH species generated according to similar process may be similarly obtained, which straightforwardly accounts for the broad spectroscopic signature obtained from infrared spectra. The representative examples selected in Fig. 7 have been chosen among those yielding the most kinetically stable products, present in sizeable amount at steady state and therefore detectable in infrared spectra.


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Figure 7. Possible mechanisms to form the hydrogenated species present on Si5N4 HF etched surface. In each case, the dissolution proceeds through the sequential breaking of Si-N or Si-Si bonds by insertion of a HF molecule. Breaking of a Si-N bond always yields one N-H and one Si-F bond (see, e.g., the first step of A, B, or C). Breaking of a Si-Si bond yields one Si-H and one Si-F bond, the fluorine atom attaching to the Si atom linked to the most electronegative neighbors (see, e.g., the second step of A or B). One Si-Si bond is needed for generating Si-H species (A). Clusters of three (B) or four (C) Si atoms are required for the formation of siliconlinked SiHF species.

The reaction schemes depicted on Fig. 7 describe elementary mechanisms starting from the etching of surface SiNHx species. However, the kinetic stability of such species, and therefore their presence in detectable amounts at steady state, could appear questionable. When Si-N bonds are directed towards the etching solution, they are indeed potential targets for efficient reaction with HF. This vulnerability should make the surface concentration in NHx groups very low, which is not experimentally verified since a measurable concentration of surface NHx species is measured. As a matter of fact, N-H bonds are continuously re-generated in the etching process. When they are created in hollow sites (when the surface roughens upon etching), they are transiently sterically protected until the etching progresses in their vicinity. This is a plausible cause for the weak but non negligible concentration in NHx species present at the surface. In summary, a consistent picture of the presence of SiHx, SiFx and NHx at the HF-etched siliconrich silicon nitride surface is obtained on the basis of polarity-driven reactions and stericallyhindered kinetics.



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4.3 Surface chemical reactivity In addition to SiHx, SiFx and NHx, the experimental results also point the presence of surface SiOH species on HF-etched surfaces. We do not think that these species are generated during the etching itself. Such a possibility might be considered if water would be an active etching agent, an expected situation for highly diluted HF solutions. It does not seem to be the case, since the same amount of SiOH species is found on the Si3.4N4 surface after etching in 0.2% HF and 2% HF solutions. Following previous works,21,

34, 56, 57

we ascribe the formation of surface SiOH

species to the hydrolysis of surface Si-NHx bonds during the water rinse after the etching. As explained above, these bonds are not very stable and are readily hydrolyzed when they are not protected from water access. On the opposite, Si-F bonds are very stable. This is primarily why during exposure to H2 plasma, the amount of surface fluorine species slowly decreases: Si-F bonds are not broken by the plasma treatment, but fluorinated silicon atoms are progressively removed from the surface during the silicon nitride etching, when Si-N back bonds are transformed into Si-H bonds, in a kind of liftoff process. Since SiFx are the majority species at the etched surface, and that in view of their stability these species are poorly reactive, few anchoring points are available at the etched surface for further modification through covalent bonding. SiOH and SiNHx species are left only, which are not ideal candidates for stable surface modification. At the HF-etched Si-rich silicon nitride surface, the situation is much different and the mildly reactive surface SiH bonds covering the surface are good anchoring points for further covalent surface functionalization. This is why using Si-rich silicon nitride may be an attracting possibility in view of covalent surface functionalization. When using stoichiometric silicon nitride is preferred, exposure of the


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HF-etched silicon nitride surface to H2 plasma is a useful alternative for generating such SiH bonds, through a dry etching process.

5. CONCLUSION The composition and the surface chemistry of various SixN4 coatings were quantitatively determined for the first time by the combination of XPS and IR-ATR spectroscopies and colorimetric assays. We demonstrated that an optimized etching in diluted HF leads to the formation of mainly Si-F bonds with some SiOH and few SiNHx bonds on the quasi-stoichiometric Si3.4N4 coating. The presence of Si-F and SiNHx bonds at the surface is consistent with a reactive mechanism in which Si-N bonds are broken by polarity-driven insertion of HF molecules with sterically-hindered kinetics. Si-OH surface species are thought to arise from SiNHx hydrolysis during the rinsing step. Si-H bonds are attractive surface species for further surface functionalization, as well documented on Si surfaces. Such Si-H species can be generated from the etching of Si-rich silicon nitride. In that case, surface composition is more complex, since the generation of SiNHx, Si-H, Si-F and combinations thereof is possible; but a sizeable amount of Si-H species is readily obtained. Inducing Si-H species at the surface of quasi-stoichiometric silicon nitride is also possible, using a post-etching H2 plasma treatment. In that case, the silicon nitride layer in slowly etched; it results in an increase of the specific area and the generation of a large amount of Si-H species. Such a comprehensive view of the surface etching mechanism and chemical composition of the etched silicon nitride surface should provide a firm basis for a well-controlled functionalization of silicon nitride surfaces.

ASSOCIATED CONTENT



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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following Figures are displayed: XPS Si2p and F1s spectra of Si3.4N4 before and after etching in HF and NH4F (Fig. S1); XPS F1s spectra of Si3.4N4 after etching in 0.2% HF for 30 s and quantification of Si-F bonds from XPS (Fig. S2); Coomassie Brilliant Blue (CBB) dosing (Fig. S3); Si-OH determination by IR-ATR (Fig. S4); Dissolution rate of Si3.4N4 surface in 40% NH4F solution (Fig. S5) ; SiHx determination by IR-ATR (Fig. S6); AFM images of the HF-etched Si3.4N4 and Si5N4 surfaces (Fig. S7)

AUTHOR INFORMATION Corresponding authors *

E-mail:

[email protected].

Phone:

+33

[email protected]. Phone: +33 1 69 33 47 04. Author Contributions All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


1

69

33

46

80;

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The authors thank Anne Lelarge and Pascal Naël for their help with the silicon nitride deposition.

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Etching and Chemical Control of the Silicon ... - ACS Publications

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