Lattice-strain Enhanced Chemical Reactivity of Silicon for Efficient

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Etching Shapes the Topography of Silicon Wafers: Lattice-strain Enhanced Chemical Reactivity of Silicon for Efficient Solar Cells Thomas Langner, Tim Sieber, and Jorg Acker ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00906 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Applied Nano Materials

Etching Shapes the Topography of Silicon Wafers: Lattice-strain Enhanced Chemical Reactivity of Silicon for Efficient Solar Cells Thomas Langner, Tim Sieber, Jörg Acker* Brandenburg University of Technology Cottbus-Senftenberg, Faculty 2 – Environment and Natural Sciences, Department of Physical Chemistry, Universitätsplatz 1, D-01968 Senftenberg, Germany

*corresponding author: [email protected], phone: +49 35 73 85 839, fax: +49 35 73 85 809

ACS Paragon Plus Environment

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Abstract

Multi-wire sawing of silicon (Si) bricks is the state-of-the-art technology to produce multicrystalline Si solar wafers. The massive indentation of the abrasive Si carbide or diamond particles used leads to a heavily mechanically damaged layer on the wafer surface. Etching the surface layer using typical HF-HNO3-H2SiF6 acid mixtures reveals an unevenly distributed etch attack with etch rates several times higher than known for bulk Si etching. The present study follows the hypothesis that lattice strain, introduced by the sawing process, leads to an increase of the etch rate and determines the topography of the etched wafer, the so-called texture. Scratches were introduced into single crystalline Si surfaces in model experiments and the magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements. The essential parameters used to describe the local reactivity of Si is the local etch rate, which was derived by confocal microscopy from the local height before and after etching. It was found that the reactivity of Si increases linearly with the magnitude of lattice strain. An increase in tensile strain raises the reactivity of Si significantly higher than an increase of compressive strain. The second decisive parameter is the reactivity of the etch mixture that correlates with the total concentration of the acid mixtures. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and, therefore, more reactive acid mixtures can attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed. The presence of unevenly distributed lattice strain of different magnitude and the resulting unevenly distributed reactivity of Si explains the features of a heterogeneous etch attack observed and the resulting topography of the etched wafer surface.


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Keywords: silicon, lattice strain, Raman microscopy, confocal microscopy, etching, reactivity


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Introduction

Solar wafers are produced from monocrystalline silicon (Si) ingots or from multi-crystalline Si bricks by multi-wire sawing using an abrasive Si carbide (SiC) slurry or diamond wires.1 The massive mechanical load applied during the sawing process creates a surface layer of latticedamaged Si, referred to as saw-damaged Si. This damage is characterized by lattice defects, amorphous Si and some high-pressure Si modifications, as well as fractures, rifts and half-penny shaped cracks, each of which can be several micrometers long in the bulk material.1-4 Material removal during the sawing process is caused by multiple indentations and the release of diamond or SiC particles into Si crystals by the moving wire:2,5,6 Local indentations form a plastic deformation zone around the indentation center. When the critical value of the indentation force is exceeded, cracks are generated beneath the plastic deformation zone and are perpendicular to the load axis of the material; these are the so-called lateral cracks. So-called shallow radial cracks are formed at the edges of the plastic zone; they are initially directed parallel to the surface. If both cracks coalesce, half-penny shaped cracks occur. Residual stress while unloading the indentation force results in radial cracks beneath the plastic deformation zone. If they reach the surface, material is chipped away in the shape of a semicircle. Apart from the generation of fractures, mechanical loading and unloading lead to the formation of several high-pressure modifications in Si, including the generation of amorphous Si close to the surface.7-11 Furthermore, indentation leads to plastic deformation and the generation of dislocations, which can migrate, dissociate, accumulate or decay at certain positions within the lattice.11-14 Similar to impurities and grain boundaries in multi-crystalline Si, dislocations affect the electronic properties of Si as they act as recombination sites for light-induced electron-hole


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pairs, which reduce the lifetime of the minority carriers and the associated diffusion length dramatically. Therefore, the saw-damaged layer at the surface of the sliced Si wafer needs to be removed in order to manufacture powerful Si solar cells. In the case of multi-crystalline Si, the saw-damaged surface layer is removed by wet-chemical etching using mixtures of nitric acid (HNO3), hydrofluoric acid (HF) and hexafluorosilicic acid (H2SiF6), which are known to be isotropic etchants.15 This etching process, known as texturization, shapes the surface morphology of bulk Si; the “texture” consists of shallow valleys (similar to partly hemispherical structures) in the micrometer range. Additionally, the mechanical stability of the wafer is significantly increased by the removal of fractures and cracks. In the case of multi-crystalline Si, a typical texture consists of etch pits with lengths of 5–10 µm and widths of 2–5 µm. A semicircular cross-section with a depth of several micrometers is formed.3,4 This texture reduces the overall reflection of the incidental sunlight owing to multiple reflections of the reflected light on the wafer surface and increases light harvesting. This, in turn, enhances the solar cell’s efficiency.16 Previous studies have revealed that the etching of multi- and monocrystalline Si subjected to saw damage is considerably different from the etching of multi- and monocrystalline bulk Si.3,4,17,18 Regardless of the slicing technique, microstructure and crystal orientation, saw damage is removed at a considerably higher etch rate than for multi- and monocrystalline bulk Si at identical etch mixture compositions and temperatures.3,4,17 The etch rate particularly of the topmost layer (about 0.3 µm) of the saw-damaged material is higher by a factor of 10–15 compared to bulk Si.17 This is not a geometric effect caused by the corrugated wafer surface, which is about 1.2–1.5 times greater than the projected geometric surface area.3,17 Amorphous Si or high-pressure modifications, which might explain the higher reactivity, were not found.


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Finally, the distribution of hydrogen-terminated surface species (SiHx, x = 1, 2 and 3), measured from the topmost saw-damaged surface of multi-crystalline bulk Si after each etching step, was almost identical across the whole etch depth.17 However, the oxidation of the Si-Si back bonds of the SiHx saw-damaged surface species to Si-O-SiHx occurs more than twice faster when compared to the SiHx-terminated surface on undisturbed bulk Si.17 Amorphous Si and the high-pressure phases identified have no significant impact on the etch rate within the saw-damaged layer, as their appearance (the relative surface coverage) and thickness in the areas where they are present is so low that they could be removed in less than 2 s of etching.21 These phases play no role in the industrial solar cell etch process since they are removed completely during posttreatment following wafer sawing. None of these phases were found on post-treated as-cut solar wafers.3,4,17,18 Finally, the question remains whether the lattice strain introduced has any significant impact on the etching behavior of the saw damage and on the final topography of the etched wafer. This question is addressed in the present study.


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Experimental

Sample treatment: The experiments were conducted on 15 × 15 mm2 wafer pieces laser cut

from pure single crystalline p-type Si wafers (diameter 4 inch, double-side polished, thickness 675 µm, resistivity 24–36 Ω cm; Silchem GmbH Freiberg, Germany) with (100) and (110) orientation. Scratching experiments were conducted with an indentation system equipped with a Vickers indenter type diamond tip. They were performed under identical conditions with a force of 15–25 N and a speed of 1 mm·s-1. The force applied in preliminary experiments was tuned in such a way that neither high-pressure modifications nor large amounts of amorphous Si were formed. The scratched samples were inspected to select those with no visible radial cracks inside the scratches formed. Thereafter, the wafer pieces were ultrasonicated in deionized water (18 MΩ cm–1, Milli-Q) and, finally, rinsed and dried. Etching experiments: HF/HNO3/H2SiF6 etching mixtures were prepared by weighing aliquots from HNO3 (69 % (w/w)) and HF (40 % (w/w)), both of which were purchased from Merck (Darmstadt, Germany), and H2SiF6 (35 % (w/w)) obtained from Alfa Aesar (Karlsruhe, Germany), according to Table 1. Etching was performed at 20 °C. In each etching step, the wafer piece was vertically immersed in an aliquot of a freshly prepared etch solution for a certain time period and then rinsed with deionized water. The surface of the wafer piece was analyzed by confocal microscopy and Raman microscopy after scratching and then after each etch step. Subsequently, the etch procedure was repeated with an aliquot of freshly prepared acid mixture.


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Table 1. Composition of the etch mixtures used Etch mixture

HF (w/w) (%)

HNO3 (w/w) (%)

H2SiF6 (w/w) (%)

#1

4.0

28.0

17.0

#2

3.6

25.2

15.3

#3

3.3

22.8

13.9

#4

2.8

19.3

11.7

Confocal microscopy measurements: The surface topography of Si wafers was analyzed by confocal microscopy using a 3D profiler, Sensofar PLµ neox (Sensofar), equipped with a 50× objective having a numerical aperture of 0.95. This allows us to measure a surface spot area of 257 × 190 µm2 with a spatial sampling resolution of 0.33 µm and lateral resolution of less than 3 nm. Several surface spot areas were stitched together to obtain a surface area of about 2 × 2 mm2. Confocal microscopy mappings before and after etching were adjusted and overlaid by means of marker positions located on the sample surface away from the scratches to calculate the local removal of Si due to etching from the height difference between the overlaid mappings. The local etch removal divided by the etch time applied yields the etch rate, r. Raman confocal microscopy: The wafer surfaces were mapped using a confocal Raman microscope (DXR SmartRaman, Thermo Fisher Scientific) in the back-scattering configuration; the microscope was equipped with a 532 nm excitation laser (penetration depth ≈ 0.7 µm22) and a 900 grooves/mm grating, which enabled the acquisition of Raman spectra in the wavenumber range of 150–1250 cm–1. The incident laser light was focused onto the sample surface through a 50× microscope objective, resulting in a focused spot with a diameter of ~ 0.8 µm. The Si surface was probed with a spot distance of 10 µm and laser power of 2 mW in the experiments


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described in the section on “correlation between local lattice strain and local etch rate.” The experiments described in the Results section on “Raman analysis and quantification of local lattice strain” were performed with a spot distance of 2 µm and laser power of 0.5 mW to minimize localized annealing or heating effects. Additionally, a Mercury emission source with 484.4560 cm–1 was used as the reference for wavelength correction of the spectra recorded. The Raman spectra were deconvoluted in the 513–528 cm–1 range by three Lorentzian functions representing unstrained Si in the cubic diamond structure (space group Fd3m); these are designated as Si-I (~ 520 cm–1), Si under compressive strain (> 520 cm–1) and Si under tensile strain (< 520 cm–1), according to the biaxial stress model.21,33 In case a signal around 300 cm–1 appears, the range of 500–515 cm–1 was fitted as the Lorentzian function in order to cover nanocrystalline Si.10 Amorphous Si is indicated by a signal in the range of 475–485 cm–1; this range was fitted by a Gaussian function. Combination of confocal microscopy and Raman microscopy: The confocal microscopy and Raman microscopy mappings were adjusted and overlaid by means of marker positions located at the wafer surfaces away from the scratches to correlate the local removal with the local state of Si (please see the section on “Raman analysis and quantification of local lattice strain”). The area of the wafer was virtually overlaid with a quadratic grid of 2-µm wide rows and columns; the excitation laser spot used had a diameter of 0.8 µm. The high resolution of confocal measurements (0.33 µm lateral resolution) corresponds to 7 × 7 individual confocal spots within the 2 × 2 µm2 Raman spot. The individual surface heights of these 49 individual spots were averaged and attributed to the corresponding 2 × 2 µm2 Raman spot. The Raman spectra measured were deconvoluted to extract individual Raman signals corresponding to Si-I, amorphous Si and nanocrystalline Si, as well as for tensile and compressive strained Si. The


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difference between the Raman shifts of the respective lattice strain components, ωstrain , and the Raman shift of unstrained Si-I, ω0 , (Eq. 1) leads to a peak shift, ∆ω , with ∆ω > 0 for compressive strain and ∆ω < 0 for tensile strain. ∆ω = ωstrain – ω0

(1)

∆ω is considered to be linearly related to the degree of lattice strain, as it exhibits a linear relationship with the residual stress in plastically deformed Si after mechanical loading.7,22-25 Spots consisting of amorphous or nanocrystalline Si were excluded from the etch rate and strain analysis owing to their high reactivity.21 Finally, this results in a total of 600 – 800 individual points which are used to correlate the lattice strain components with the local etch rate of one scratch etched with one mixture.


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Results

Visualization of wafer etching The heterogeneous effects of an etch attack across the surface of a solar wafer have been demonstrated recently.17 Such heterogeneity also exists on the microscopic level, as visualized in the movie S1. The topography of a 127.92 × 95.45 µm2 spot of an as-cut slurry-sawn multicrystalline Si wafer was measured before etching and after each step (10 s etching) using etch mixture #1. The micrographs obtained were assembled in the form of a movie which is found in S1 of the Supporting Information. Fig. 1 shows exemplarily four confocal images from the movie. The images were recorded after a mean etch removal of 0.57 µm (Fig. 1a), of 3.43µm (Fig. 1b), of 6.29 µm (Fig. 1c) and of a final removal of 16.6 µm (Fig. 1d). Three distinct features can be recognized: (i) There are spots which are constantly etched from the top with a moderate etch rate (cf. Fig. 1a and 1b). (ii) After a short induction period without significant etching, certain spots experience a rapid burst of etching (cf. Fig. 1b and 1c). At later time periods, these pits are found to maintain a constant depth, but their widths increased due to lateral etching of the side walls (cf. Fig. 1c and 1d). (iii) Some spots are practically not attacked from the surface. These spots are removed at longer etch times through lateral side wall etching of the surrounding pits (Fig. 1d). The movie demonstrates the aspect of unexpected heterogeneity of the etch attack at different spots, which must be attributed to the differences in the reactivity of the Si atoms involved.


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Figure 1. Confocal microscopy images from a SiC slurry-sawn multi-crystalline Si wafer surface at an etch removal of 0.57 µm (a), of 3.43µm (b), of 6.29 µm (c) and etching with a final removal of 16.6 µm (d). The size of the images is 110 µm times 87.5 µm. The images were taken from the movie S1 of the Supporting Information.


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Correlation between local lattice strain and local etch rate Figure 2 illustrates the experimental approach for a scratch on a polished single crystalline Si (100) wafer. Figures 2a–2d show the overlays of confocal microscopic images with mappings of the normalized Raman intensities for selected Raman shifts at 517 and 525 cm–1 before and after etching. Figure 2 can be discussed only qualitatively, as the Raman spectra recorded were not deconvoluted and only the net intensities at the selected Raman shifts were used to calculate the mappings. Furthermore, the Raman spectra were recorded at a low lateral resolution of 10 µm. Figure 2a shows the normalized Raman intensities at 517 cm–1; this wavenumber represents tensile strained Si.22 The highest normalized intensities, indicated in red, are found as isolated spots at the deepest positions in the center of the scratch close to the right-hand side wall of the asymmetric scratch. A Raman shift of 525 cm–1 was chosen to represent compressive strained Si.22 The highest normalized intensities of this signal (red color) are widely distributed in the middle part and on the left side of the scratch. The light blue color surrounding the scratch indicates unstrained Si-I. Etching with mixture #1 for 40 s removes almost the entire amount of strained Si, as indicated by the evenly distributed light blue color (Figs. 2c and 2d). The mapping in Fig. 2e represents the etch removal of Si, which was calculated from the height information of the confocal images before and after etching. The highest local removal was found at the center and the right-hand side wall of the scratch, where tensile strained Si was predominant (Fig. 2a). Removal was significantly less in the middle and on the left-hand side wall, where high normalized intensities of compressive strained Si were found. Although the polished wafer surface had already been attacked (Figs. 2c and 2d), material removal at the wafer surface was negligible compared to the depth of the scratch.


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Figure 2. Correlation between lattice strain and etch removal at the site of scratch on a polished Si (100) wafer; the wafer was etched with mixture #1. Confocal microscopic images are overlaid with mappings of the normalized Raman intensities at selected Raman shifts. (a) Before etching


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at 517 cm–1 (tensile), (b) before etching at 525 cm–1 (compressive), (c) after 40s of etching at 517 cm–1 (tensile) and (d) after 40s of etching at 525 cm–1 (compressive). The color scale of the relative Raman intensities is valid for (a)–(d). (e) shows the local etch removal of Si calculated from the confocal microscopic images before and after etching. The numbers on the color scale indicate the depth of local removal in µm regarding the polished wafer surface. Length of the scratch: ~ 1.2 mm; width: ~ 80 µm.

The volume of the scratch was calculated before and after each etching step from the confocal microscopic measurements to quantify the etch progress. The volume of the pileup zones at the scratch edges was neglected and the height of the polished surface was linearly extrapolated through the piles towards the scratch edge for simplification. The corresponding scratch volumes, vt, were normalized to the initial scratch volume before etching, v0, to yield the relative scratch volume, vrel, according to Eq. 2, to compare the measurement values at different etch times (t). vrel = (vt – v0)/v0

(2)

The most reactive etch mixture #1 generates a graph (Fig. 3a) with a plateau of vrel = 4.5 after etching for a total of 200 s. Further immersion of the sample in mixture #1 prolongs etching inside the scratch and at the polished wafer surface. The less concentrated mixture #3 requires 950 s to achieve its maximum vrel value of 0.5 (Fig. 23). Further immersion in mixture #3 did not increase vrel. Therefore, this scratch was treated subsequently with etch mixture #1. Two consecutive etch steps lasting 30 s and 65 s increased vrel to a value of 4.0, as shown in Figure 3b.


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Raman microscopic measurements revealed that strained Si was removed completely after etching with mixture #1 within 40 s (Fig. 2c and 2d), which corresponds to a vrel of about 2.6 (indicated by arrow A in Fig. 3a). A further increase in vrel from 2.6 to 4.5 has to be attributed to the etching of less strained Si, which is no longer detectable by Raman microscopy, and the etching of unstrained Si-I. By contrast, lattice strain is still detectable after etching for 180 s using mixture #3, which corresponds to a vrel of about 0.35 (indicated by arrow B in Fig. 3b). The less reactive etch mixture #3 can only remove significantly strained Si with a degree of strain detectable by Raman microscopy. Etching stops once all such strained Si is removed.

rel. scratch volume vrel=(vt-v0)/v0

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5

5

4

4

3

3

A

2

etched with mixture #1 etched with mixture #3 subsequently etched with mixture #1

2

B

1

1

a)

0 0

50 100 150 200

etch time / s

b)

0 0

200 400 600 800 10001200

etch time / s

Figure 3. Relative scratch volume, vrel, vs. cumulative etch time for scratches generated on a polished Si (100) surface by etching with (a) mixture #1 and (b) a combination of mixture #3 and mixture #1. The arrows indicate the etch time at which strained Si is completely removed (A – mixture #1 and B – mixture #3).


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Raman analysis and quantification of the local lattice strain Peak assignment: The Raman spectrum of single crystalline Si in the cubic diamond structure, designated as Si-I, originated from the scattering of incident light by the optical phonons of Si.10 In the absence of stress, the three optical phonon modes, one longitudinal (LO) and two transversal (TO), are degenerate in the center of the Brillouin zone.22 This leads to a single peak in the Raman spectrum at around 520 cm–1 with a full width at half maximum (FWHM) of about 3 cm–1.7,22,26 Mechanical stress affects the frequencies of the Raman modes by changing the unit cell dimensions and lowering the symmetry of the crystal, leading to individual shifts in the three phonon modes.22,27-32 A biaxial compressive stress results in an increase in the Raman shift wavenumber, while a biaxial tensile stress reduces the Raman wavenumber.22 The peak shifts depend linearly on the magnitude of the corresponding tensile and compressive stress components, leading to a simple equation to calculate stress as internal pressure directly from the peak shift.23,24 Stress leads to a change in the Raman shift but does not contribute to line broadening (except through the Fano effec33).34-36 In this study, Raman spectra deconvolution is performed according to the model of biaxial stress proposed by DeWolf,22,32 in which each component and the signal of remaining Si-I is fitted as a Lorentzian function (Fig. 4). Apart from lattice strain, scratching can result in several high-pressure modifications in Si and in amorphous Si;20 their chemical reactivities are different compared to those of crystalline and lattice-strained Si. High-pressure modifications in this study were avoided by choosing proper conditions for scratching. Amorphous Si was proven to have a significantly high reactivity against acid etchants.21 Amorphous Si is characterized by broad bands at around 160, 300, 390 and 480 cm–1.10,37,38 Their wavenumber positions and linewidths depend on the degree of


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structural disorder.10,38 In the present study, the broad band centered at around 480 cm–1 was fitted as an envelope using a Gaussian function. Furthermore, some of the spectra recorded in this study exhibited signals at around 300 cm–1 and between 500 and 515 cm–1, as reported earlier by several authors.7,9,10,39 Unambiguous attribution to a single Si phase is not possible using Raman spectroscopy. These bands can be attributed to nanocrystalline Si, which is assumed to be formed when the critical value of ductile deformation is exceeded, leading to the formation of micro-cracks under the action of a local strain relaxation.10 Silicon nanocrystals, less than 8 nm in size, show a significant shift in their Raman signals to lower wave numbers when compared to bulk Si. Therefore, the broad and asymmetric Raman signal between 500 and 515 cm–1 is explained by a mixture of Si nanocrystals with individual particle grain sizes below 5 nm.40-42 In contrast to bulk Si showing an overtone peak at around 300 cm–1, nanocrystals exhibit an overtone signal with a higher intensity and a slight maximum shift to values around 280 cm– 1 26

.

A certain number of Raman spectra recorded in this study, mainly after indention, exhibit

signals attributed to amorphous and nanocrystalline Si. This agrees with the findings of Pizani et al., who identified Si nanocrystallites dispersed in the amorphous media in single-point indentions on Si (100).43 Furthermore, the Raman signals observed can be attributed to the formation of the high-pressure modification Si-IV (hexagonal diamond structure, space group P63/mmc).10 Because the formation of Si-IV requires stress and elevated temperature,44-46 we attributed the signals observed in this state to nanocrystalline Si fitted as a Gaussian function (Fig. 4).


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relative Raman intensity / a.u.

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measured compressive strained Si-I tensile strained nanocrystalline

1.0 0.8 0.6 0.4 0.2 0.0 505

510

515

520

525

530

-1

Raman shift / cm Figure 4. Deconvolution of a typical Raman spectrum

Reactivity of amorphous and nanocrystalline Si: Amorphous and nanocrystalline Si were detected after scratching polished Si (110) and subsequent surface cleaning only at certain isolated spots inside the scratch. These phases are detected especially for 200 to 550 Raman spots out of a total of 3900 spots of one scratch mapping in each etch step. The absolute intensities of the corresponding signals differ significantly from spot to spot, which might be explained by the variation in the sample volumes probed, i.e. layer thicknesses. The count of spots containing one of these phases were analyzed regarding the variation in etch time to estimate the reactivity of amorphous and nanocrystalline Si.


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The most reactive etch mixture #1 removes any spots along with amorphous and nanocrystalline Si within 5 s. Amorphous Si could be etched away in 10 s and nanocrystalline Si vanished after 20 s using mixture #2. Both modifications can be distinguished by their reactivity values when etching is carried out with more diluted etch mixtures (#3 and #4). Amorphous Si is removed completely after 15 s of etching with mixture #3 (Fig. 5a) and after 500 s using the least reactive mixture #4 (Fig. 5b). Nanocrystalline Si is significantly less reactive than amorphous Si; etching times of 500 s (Fig. 5a) and 2400 s (Fig. 5b) are required for its complete removal with mixture #3 and #4, respectively.

350

a)

300

count of spots

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amorphous Si nanocrystalline Si

250

350 300 250

200

200

150

150

100

100

50

50

0

0 0

100 200 300 400 500

b)

0

500 1000 1500 2000 2500

etch time / s

etch time / s

Figure 5. Count of Raman spots showing shifts for amorphous Si (485 cm–1) and nanocrystalline Si (515 cm–1) as a function of etch time using (a) mixture #3 and (b) mixture #4 Quantification of the relationship between lattice strain and etch rate: Figure 6 shows the relationship between the degree and direction of lattice strain, ∆ω, and the corresponding


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chemical reactivity, i.e. etch rate, using the example of mixture #1. The etch rate (Figs. 6a and 6b) increases linearly with the absolute of the increasing lattice strain. The absolute value of the slope corresponding to tensile strained Si (∆ω ranges up to –5.5 cm–1) is higher than that for compressive strained Si (∆ω ranges up to +7.5 cm–1). A high etch rate correlates with a high normalized Raman intensity of the tensile strain component, as shown in Figure 6c. By contrast, the plots of etch rate vs. normalized Raman intensity of the compressive strained component (Fig. 6e) and etch rate vs. normalized Raman intensity of the Si-I component (Fig. 6d) show an inverse relationship. The etch rates increase with a decrease in the relative intensity. This is explained by the ratios of the normalized intensities of the strain components and Si-I (Fig. 4). The probed spots with medium or high relative intensities in the tensile strain component always exhibited low to medium intensities for unstrained Si-I, while very low intensities were observed in the compressive stained component. The etch rate is dominated by the most reactive modification, in this case, the tensile strained Si. Other components, namely SiI and compressive strained Si, are of less importance because of their lower etch rates and small amounts in the sample volumes probed. Etching of unstrained bulk Si-I in etchant mixture #1 resulted in an etch rate of 40–50 nm·s–1.3,4,17 Figure 6e shows that a mean etch rate of about 57 nm·s–1 is obtained in the Si-I component at the highest relative intensity of about 0.7. This slight increase in comparison to the bulk etch rate is attributed to the small fraction of the highly reactive tensile strained Si in the spot probed. A decrease in the relative intensity of the Si-I component (Fig. 6e) corresponds to an increase in the amount and relative intensity of the tensile strain component. Therefore, etch rate increases with increasing tensile strength and relative intensity of tensile strained Si (Figs. 6a and 6c).


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There are only a few probed spots at which the relative intensity of the compressive strained component exhibits high values of around 0.55. The resultant components are always unstrained Si-I and tensile strained Si. Therefore, the local etch rate attributed to compressive strained Si is affected by the amount of tensile strained Si present and its corresponding etch rate, however, this contribution cannot be quantified. The impact of Si-I on the etch rate is considered to be negligible. Table 2 summarizes the etch rate range at the maximum strain for the etch mixtures studied. The tensile strained component is the most reactive, followed by the compressive strained component. Unstrained Si-I has the lowest reactivity. As expected, the etch rates decrease with a reduction in the acid concentration in the etchants. Etch rates of the most diluted etchant, mixture #4, indicate only a weak attack, probably on the thin layers of the Si spots most strained and most disturbed in the first etching step (the spots consisting of amorphous and nanocrystalline Si are already excluded). In spite of the low etch rates, these values are considered reliable, as they were obtained at etch times 10–20 times higher than those observed in other mixtures. The local etch rates obtained (Table 2, Fig. 6) show a significant spread. The major reason behind such a spread is the presence of more than one strain component in the spot probed. Thus, the etch rate calculated is actually a net etch rate determined from all of the existing components. Another source of data scatter arises from the unknown thickness of the strained Si. The etch rate calculation relies on the assumption that the immersion time of a wafer in the etch mixture is identical to the time of etching. Assume that a thin layer of strained Si is removed entirely within a fraction of the immersion time and etching stops thereafter with the exposure of Si-I. Under such conditions, only a very low etch is observed and does not reflect the actual etch rate of the etched strained Si. This result underlines the importance of the careful removal of debris from


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the wafer surface after etching. Tiny particles, observed in Raman microscopic images as isolated spots of highly strained Si, are etched away; however, the surface spots where these particles were situated show practically no etch removal.

Table 2. Range of etch rates in lattice-strained Si at maximum ∆ω and the highest relative Raman intensity obtained with different etch mixtures for Si-I. The range covers the spread between 25 and 75 % of all the data points measured. Maximum etch rate range (nm·s–1) obtained with etch mixture Component Compressive strained Si Unstrained Si-I Tensile strained Si

#1

#2

#3

#4

187–348

45–72

-

0.4–1.5

38–67

1–15

0.7–2.5

< 0.45

115–561

34–439

3–60

0.4–5.4


23


0.6

0.4

0.2

0.0

-1

0.8 25% - 75% median average

etch rate / µms

etch rate / µms-1

0.8

-6

-5.5

-5

-4.5

peak shift ∆ω / cm

0.4

0.2

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

-1

peak shift ∆ω / cm-1

b) 0.8

0.8 25% - 75% median average

0.6

etch rate / µms-1

etch rate / µms-1

25% - 75% median average

0.6

0.0

-4

a)

0.4

0.2

0.0

0.35

0.4

0.45

0.5

0.55

0.6


0.6

0.4

0.2

0.0

0.3

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

relative Raman peak intensity / a.u.


d)

c) 0.8

etch rate / µms-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 51

0.6


0.4

0.2

0.0

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7


e) Figure 6. Ranges of etch rate as a function of peak shift, ∆ω, for (a) tensile and (b) compressive strained Si. Etch rate as a function of the relative intensities of deconvoluted Raman spectra for


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(c) tensile strained Si, (d) compressive strained Si and e) unstrained Si-I. To reduce the number of points, ∆ω values are grouped into classes with a width of ∆ω = 0.5 cm–1 and relative peak intensities are grouped in classes with a width of 0.05 arbitrary units (a.u.). The horizontal box dimensions cover 25–75 % of all the data points measured. The line within the box indicates the median and the open circle symbol indicates the arithmetic average.

Scratch profiles The time-dependent development of scratch profiles provides additional insight into the etch process. The confocal microscopy image in Figure 7a shows a scratch that was etched with mixture #3 over a total time of 1040 s. In spite of this long etch time, the polished wafer surface was not attacked. However, selective etch attack occurs inside the scratch, as shown by the scratch profiles at increasing etch time (Fig. 7b). The asymmetric shape of the scratch can be observed until 60 s, along with a moderate attack on the side walls. Etching for longer than 60 s increases the scratch width and depth; the increase in width, measured at half depth, is roughly twice as high as the increase in depth. The width at the wafer surface remains almost unchanged until 440 s of etching, after which it increases with increasing etching time. Keeping the asymmetric peak shape in mind, the aspect ratio of the etched scratch was calculated as the ratio of the width to depth; it is ~ 0.5, which corresponds to an even removal at the bottom and side walls of the scratch.


25


0 -2

depth / µm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 51

-4

etch time 0s 30 s 60 s 180 s 300 s 440 s 620 s

-6 -8

-10 -12 75

100

125

150

175

length / µm a) b) Figure 7. Etching a scratch with etch mixture #3. (a) Confocal microscopy image after etching for 620 s. The color scale indicates the depth regarding the surface. The arrow indicates the direction of gravity. (b) Profile of the etched scratch. Profile data was obtained from the region indicated by the black box in (a). Note the different scales for the x- and y-axes. Etching with the more reactive mixture #1 leads to a significant etch attack at the polished wafer surface in the close neighborhood of the scratch (Fig. 8a), especially in the areas above the scratch, which was immersed vertically into the etchant. The surface is not attacked only at the wafer edge below the scratch. The scratch profile in Figure 8b is significantly different from that in Figure 7b. After only a short etch time, the width of the upper part close to the surface increases significantly and finally reaches ~ 200 µm. Removal at the polished wafer surface increases linearly with etch time at different distances from the scratch edge, considering the edge position before etching as the reference, as shown in Fig. 9a. This corresponds to a constant etch rate at each position and to their linear decrease with increasing distance from the initial scratch edge (Fig. 9b). This behavior is remarkable, as the etched Si consists of Si-I without any lattice strain. Freshly prepared mixture #1 cannot etch a polished wafer surface as rapidly within


26

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the scratch. Therefore, this phenomenon is attributed to the highly reactive and intermediary species generated and stabilized in concentrated etchants during the etching reaction.58,59 In contrast to the preferential attack on the upper side walls, the narrow shape in the lower part can be observed throughout the experiment, which indicates a moderate etching on the lower side walls. The narrow part of the scratch has an aspect ratio of about 0.3. 0 -5

depth / µm

Page 27 of 51

etch time 10 s 20 s 30 s 40 s 50 s 60 s 90 s 120 s 150 s 300 s

-10 -15 -20 150

200

250

300

350

length / µm a) b) Figure 8. Etching a scratch with etch mixture #1. (a) Confocal microscopy image after etching for 540 s. The color scale indicates the depth regarding the surface. The arrow indicates the direction of gravity. (b) Profile of the etched scratch. Profile data was obtained from the region indicated by the black box in (a). Note the different scales for the x- and y-axes.


27

0.030

-2

0.025

Page 28 of 51

-1

0

etch rate / µms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

etch depth / µm


-4 distance

to trench 0 µm 25 µm 50 µm 75 µm 100 µm 125 µm 150 µm 175 µm 200 µm 225 µm

-6 -8

-10 -12

0

0.020 0.015 0.010 0.005

a)

50 100 150 200 250 300

b) 0.000

etch time / s

0

50 100 150 200 250

distance to trench / µm

Figure 9. Etching a scratch with etch mixture #1. (a) Time-dependent etch depth measured at varying distances from the initial scratch edge and corresponding linear fits. (b) Etch rate as a function of distance

Discussion

The present study reveals that there are at least three decisive parameters that affect the etching behavior of Si. The results in Table 2 show that lattice strain in Si leads to a significant increase in the chemical reactivity of Si when compared to that of unstrained Si-I. The increase in reactivity correlates linearly with the degree of lattice strain, ∆ω, i.e. the extent of lattice strain. The major difference is attributed to the nature of the strain; tensile strain leads to a higher


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increase in reactivity compared to compressive strain. At a given etchant composition, the etch rate, r, exhibits the following trend, rtensile strained Si > rcompressive strained Si > runstrained Si. The second decisive parameter is the reactivity of the etchant that is determined by the concentration of the acids used. If a sufficient amount of HF is present, as in the case of the etch mixtures used in this study, the reactivity, i.e. the oxidation strength, of the etchant is correlated with the concentration of HNO3.14,15 Nitric acid of a specific concentration can remove Si with a specific degree of lattice strain (Fig. 3b). This is especially true in the case of mixture #4, which has an extraordinarily low reactivity against highly reactive amorphous and nanocrystalline Si, owing to which several etch steps are necessary to remove these modifications (Fig. 5). Thus, the etch rates for mixture #4, given in Table 2, are attributed to the removal of very thin layers of Si with the highest strain. The more concentrated mixture #3 is already able to remove strained Si, as shown in Figure 3b. Etching stops, although weakly strained Si is still detectable, indicating the insufficient oxidation strength of mixture #3. The higher concentrated and, thus, more reactive mixture #1 can continue etching even weakly strained Si, leading to a maximum volume removal which is five times higher than that obtained with mixture #3 (Fig. 3a). It should be noted that even the polished wafer surface is attacked by mixture #4, as shown in Figures 2c and 2d, however, without a significant removal at the same time. The immediate start of etching with mixture #1 inside the already etched scratch (Fig. 3b) and the corresponding increase in vrel underlines the importance of lattice strain. The remaining weakly strained Si inside the scratch has a higher reactivity than the Si atoms on a polished single crystalline Si surface leading to a steep increase in vrel (Fig. 3b). As a first estimation, the material volume that is affected by lattice strain due to


29


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scratching and measurable by Raman spectroscopy is about 2.6 times higher than the initial volume of the scratch (point A in Fig. 3a). Finally, reactive intermediary species have to be considered as the third parameter affecting the etching of Si. Acid mixtures with high concentrations of HNO3 can stabilize and accumulate highly reactive intermediary species generated by HNO3 reduction during the etching process.5860

These intermediates attack Si with a considerably higher etch rate than freshly prepared acid

mixtures.58 Mixture #1, a typical composition used in the photovoltaic industry with a medium initial etch rate, has the ability to induce the accumulation and stabilization of reactive intermediates and dissolved nitrous oxides, whereas mixture #3 does not have the ability to do so. During the etching of the more reactive strained Si, a sufficiently high concentration of intermediary species is generated only when the initial etch attack by HNO3 is rapid (faster than the decay of the intermediates58-60). These species seem to be preferentially formed in the lower part of the scratch, as schematically depicted in Scheme 1. Once formed, they are forced upwards against gravity, presumably due to the temperature and density gradients of the highly exothermic reaction61 and evolution of gaseous reaction products (H2, N2O, NO and NO2).62-64 The intermediates attack the upper side walls of the scratch and even the polished wafer surface after emerging from the scratch (Figs. 8a and 8b), regardless of lattice strain. At the surface, these species are consumed by the etching process and by an oxidative decay,58-60 as indicated by the decrease in removal and etch rate with increasing distance from the scratch edge (Fig. 9). This nonselective attack produces an inverted pyramid-shaped cross-section of the upper scratch profile, as shown in Figure 8b. The width of the upper edge in Figure 8b is ~ 150 µm, while the width of the scratch is roughly 40 µm, as shown in Figure 7b. However, the lower parts of the scratches studied are comparable


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in width, but not in depth. Assuming the scratches in this study were generated in comparable conditions, etching with mixture #1 creates a scratch profile of almost 20 µm depth (Fig. 8b), roughly twice as deep as the scratch etched with mixture #3 (Fig. 7b). The similar scratch widths indicate a minor or no impact of the reactive intermediates on the etching of lower side walls. It cannot be ruled out that the mechanical load along the direction of the normal force creates massive microscopic defects in addition to the plastic deformation and lattice strain; these cannot be detected by Raman microscopy, but they affect the chemical reactivity of the material considerably.65

Scheme 1. Model of silicon etching inside a scratch using a concentrated HF-HNO3 mixture The results presented provide an insight into the major features involved in the etching of multi-crystalline solar wafers. Etching proceeds at spots with a high lattice strain at a considerably higher etch rate; the etch rate at such spots is much higher than that at spots with a lower strain level or even unstrained Si. This feature is shown in the movie S1; however, it has


31


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been previously identified in slurry-sawn as-cut multi-crystalline Si wafers.29 Sharp and large SiC grains at the wire entrance create the deepest indentation pits on the total surface and, therefore, the deepest strain fields. The highly-strained Si around the indentation pits is etched at a higher rate compared to the pit’s side walls, leading to the deepest etch pits in the texturized wafer. Due to abrasion and breakage, a higher number of smaller and rounder SiC particles indent the surface at the wire exit side and create shallower indentation pits compared to the wire entrance side. The resulting strain field is not as deep and etching results in etch pits roughly half as deep as those at the wire entrance side. Such a heterogeneous etch attack is also depicted in the movie. When spots with a higher degree of strain are next to a low or unstrained spot, this results in preferential etch removal at the higher strained spots. This might be considered as isotropic etch behavior, unlike the well-known anisotropic etching that takes place with HFHNO3 mixtures. Finally, spots that are not etched from the surface are attacked from their side walls. This is explained by the formation of highly reactive intermediates in certain etch pits. These species accumulate over time, owing to which the etch attack is retarded. The intermediates attack the side walls, which consist of less-strained or unstrained Si, and lead to a relatively fast widening of the lateral pit size. Concurrently, the spots not attacked from the surface are constantly removed by the lateral growth of the surrounding pits.


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Conclusion

Studies on scratch etching on the surfaces of single crystal Si wafers show that mechanically created lattice strain alters the chemical reactivity of the Si atoms affected. This is the first time that the reactivity of lattice-strained Si has been determined in terms of the magnitude and direction of lattice-strain. Tensile strain resulted in the highest increase in etch rate; comparatively, compressive strain resulted in a moderate increase in the etch rate. The second parameter affecting the reactivity of strained Si is the reactivity of the etchant, which is determined primarily by HNO3 concentration. Low acid contents lead to etching mixtures with low reactivity, which can remove only highly strained Si, including nanocrystalline and amorphous Si. Moderate acid concentrations lead to moderately reactive etchants, which can easily attack strained Si, whose range of lattice strain can be simply detected by Raman spectroscopy. Concentrated acid mixtures exhibit the highest reactivity and cause a change in the etching mechanism, which is the third parameter. The generation and stabilization of highly reactive intermediary species leads to the etching of unstrained and strained Si, regardless of the magnitude and direction of the lattice strain. The results show that lattice strain and its heterogeneity in magnitude and lateral distribution across a solar wafer surface is the determining parameter that controls the local etch rate and, therefore, the final topography and reflectivity of the etched solar wafer. Consequently, the wafer sawing process contributes to the performance of a multi-crystalline Si solar cell to a significant extent.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. (J.A.) ORCID T. Langner: 0000-0002-5979-6977 T. Sieber: 0000-0003-0172-357X J. Acker: 0000-0002-1325-1111 Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Supporting Information. movie of time-dependent etching a multicrystalline Si solar wafer surface (mpg)


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References (1) Wu, H. Wire Sawing Technology: A state-of-the-art review. Precis. Eng. 2016, 43, 1–9. (2) Möller H.J. Basic Mechanisms and Models of Multi-Wire Sawing. Adv. Eng. Mater. 2004, 6, 501-513. (3) Meinel, B.; Koschwitz, T.; Blocks, C.; Acker J. Comparison of Diamond Wire Cut and Silicon Carbide Slurry Processed Silicon Wafer Surfaces after Acidic Texturization. Mater. Sci. Semicond. Process. 2014, 26, 93-100. (4) Meinel, B.; Koschwitz, T.; Heinemann, R.; Acker J. The Texturisation Process during Horizontal Acidic Etching of Multi-Crystalline Silicon Wafer. Mater. Sci. Semicond. Process. 2014, 26, 695-703. (5) Möller, H.J. Wafering of Silicon Crystals. Phys. Status Solidi A 2006, 203, 659-669. (6) Weber, B. Untersuchung der Material begrenzenden Einflüsse beim Multidrahtsägen von Silicium unter Verwendung gerader und strukturierter Drähte. Ph.D. Thesis, Freiberg University of Mining and Technology, Freiberg, Germany, 2015. (7) Gogotsi, Y.; Baek, C.; Kirscht, F. Raman Microspectroscopy Study of Processing-Induced Phase Transformations and Residual Stress in Silicon. Semicond. Sci. Technol. 1999, 14, 936– 944. (8) Domnich, V.; Gogotsi, Y. High-Pressure Surface Science. In Handbook of Surfaces and Interfaces of Materials, Volume 2: Surface and Interface Analysis and Properties; Nalwa, H.S., Ed.; Academic Press: San Diego, 2001; Chapter 5, pp 195-237.


35


Page 36 of 51

(9) Gogotsi, Y.; Zhou, G.; Ku, S.S.; Cetinkunt, S. Raman Microspectroscopy Analysis of Pressure-Induced Metallization in Scratching of Silicon. Semicond. Sci. Technol. 2001, 16, 345352. (10) Domnich, V.; Gogotsi, Y. Phase Transformations in Silicon under Contact Loading. Rev. Adv. Mater. Sci. 2002, 3, 1-36. (11) Bidiville, A. Wafer Sawing Process: From Microscopic Phenomena to Macroscopic Properties. PhD Thesis, University Neuchatel, Neuchatel, Switzerland, 2010. (12) Sumino, K. Deformation Behavior of Silicon. Metall. Mater. Trans. A 1999, 30, 1465– 1479. (13) Rabier, J.; Cordier, P.; Demenet, J.L.; Garem, H. Plastic Deformation of Si at Low Temperature under High Confining Pressure. Mat. Sci. Eng., A 2001, 309–310, 74–77. (14) LeBourhis, E.; Patriarche, G. TEM-Nanoindentation Studies of Smiconducting Structures. Micron 2007, 38, 377–389. (15) DeWolf, S.; Choulat, P.; Vazsonyi, E.; Einhaus, R.; van Kerschaver, E.; DeClercq, K.; Szlufcik, J. Towards Industrial Application of Isotropic Texturing for Multi-Crystalline Silicon Solar Cells. In Proceedings of the 16th EUPVSEC Glasgow, 2000, pp. 1521-1523. (16) Khan, F.; Baek, S.H.; Kaur, J.; Fareed, I.; Mobin, A.; Kim, J.H. Paraboloid Structured Silicon Surface for Enhanced Light Absorption: Experimental and Simulative Investigations. Nanoscale Research Letters 2015, 10, 376. (17) Acker, J.; Koschwitz, T.; Meinel, B.; Heinemann, R.; Blocks C. HF/HNO3 Etching of the Saw Damage. Energy Procedia 2013, 38, 223-233.


36

Page 37 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Acker, J.; Langner, T.; Meinel, B.; Sieber, T. Saw Damage as an Etch Mask for the Acidic Texturization of Multicrystalline Silicon Wafers. Mat. Sci. Semicond. Proc. 2018, 74, 238-248. (19) Gassilloud, R.; Ballif, C.; Gasser, P.; Buerki, G.; Michler, Deformation Mechanisms of Silicon during Nanoscratching. Phys. Status Solidi A 2005, 202, 2858–2869. (20) Bidiville A.; Wasmer K.; Kraft R.; Ballif C. Diamond Wire-Sawn Silicon Wafer from the Lab to the Cell Production. Proceedings of the 26th European Photovoltaic Solar Energy Conference and Exhibition; 2009, 1400-1405. (21) Herold, S. Raman Study of Silicon Surfaces. Internship Report, Brandenburg Technical University Cottbus-Senftenberg, Senftenberg, Germany, 2014. (22) De Wolf, I. Micro-Raman Spectroscopy to Study Local Mechanical Stress in Silicon Integrated Circuits. Semicond. Sci. Technol. 1996, 11, 139-154. (23) Teixeira, R.C.; Doi I.; Zakia, M.B.P.; Diniz, J.A.; Swart, J.W. Micro-Raman Stress Characterization of Polycrystalline Silicon Films Grown at High Temperature. Mat. Sci. Eng. B 2004, 112, 160–164. (24) Kulshreshtha, P.K.; Youssef, K.M.; Rozgonyi, G. Nano-Indentation: A tool to Investigate Crack Propagation related Phase Transitions in PV Silicon. Sol. Energy Mater. Sol. Cells 2012, 96, 166–172. (25) Jana, M.; Singh, R.N. A Study of Evolution of Residual Stress in Single Crystal Silicon Electrode using Raman Spectroscopy. Appl. Phys. Lett. 2017, 111, 063901 (26) Zhao, X.S.; Ge, Y R.; Schroeder, J.; Persans, P.D. Carrier-Induced Strain Effect in Si and GaAs Nanocrystals. Appl. Phys. Lett. 1994, 65, 2033-2035.


37


Page 38 of 51

(27) Anastassakis, E.; Pinczuk, A.; Burstein, E.; Pollak, F.H.; Cardona, M. Effect of Static Uniaxial Stress on the Raman Spectrum of Silicon. Solid State Comm. 1970, 8, 133-138. (28) Ganesan, S.; Maradudin, A.A.; Oitmaa, J. A Lattice Theory of Morphic Effects in Crystals of the Diamond Structure. Ann. Phys. 1970, 56, 556-594. (29) Creira, F.; Buchenauer, C.J.; Pollak, F.H.; Cardona, M. Stress-Induced Shifts of FirstOrder Raman Frequencies of Diamond- and Zinc-Blende-Type Semiconductors. Phys. Rev. B 1972, 5, 580-593. (30) Anastassakis, E.; Liarokapis, E. Polycrystalline Si under Strain: Elastic and LatticeDynamical Considerations. J. Appl. Phys. 1987, 62, 3346-3352. (31) Anastassakis, E. Strain Characterization of Polycrystalline Diamond and Silicon Systems. J. Appl. Phys. 1999, 86, 249-258. (32) DeWolf, I. Relation between Raman Frequency and Triaxial Stress in Si for Surface and Crosssectional Experiments in Microelectronics Components. J. Appl. Phys. 2015, 118, 053101. (33) Gupta, R.; Xiong, Q.; Adu, C.K.; Kim, U.J.; Eklund, P.C. Laser-Induced Fano Resonance Scattering in Silicon Nanowires, Nano Lett. 2003, 5, 627-631. (34) Lucazeau, G. Effect of Pressure and Temperature on Raman Spectra of Solids: Anharmonicity. J. Raman Spectrosc. 2003, 34, 478–496. (35) Weinstein, B.A.; Piermarini, G.J. Raman-Scattering and Phonon Dispersion in Si and Gap at very High Pressure. Phys. Rev. B 1975, 12, 1172-1186.


38

Page 39 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Beechem, T.; Graham, S.; Kearney, S.P.; Phinney, L.M.; Serrano, J.R. Simultaneous Mapping of Temperature and Stress in Microdevices using Micro-Raman Spectroscopy. Rev. Sci. Instrum. 2007, 78, 061301. (37) Smith, J.E.; Brodsky, M.H.; Crowder, B.L.; Nathan, M.I.; Pinczuk, A. Raman Spectra of Amorphous Si and Related Tetrahedrally Bonded Semiconductors, Phys. Rev. Lett. 1971, 26, 642-646. (38) Maley, N.; Beeman, D.; Lannin, J.S. Dynamics of Tetrahedral Networks: Amorphous Si and Ge. Phys. Rev. B 1988, 38, 10611-10622. (39) Kailer, A.; Nickel, K.G.; Gogotsi, Y.G. Raman Microspectroscopy of Nanocrystalline and Amorphous Phases in Hardness Indentations. J. Raman Spectrosc. 1999, 30, 939–946. (40) Doğan, I.; van de Sanden, M.C.M. Direct Characterization of Nanocrystal Size Distribution using Raman Spectroscopy. J. Appl. Phys. 2013, 114, 134310. (41) Ke, W.; Feng, X.; Huang, Y. The Effect of Si-Nanocrystal Size Distribution on Raman Spectrum. J. Appl. Phys. 2011, 109, 083526. (42) Faraci, G.; Gibilisco, S.; Pennisi, A.R.; Faraci, C. Quantum Eize Effects in Raman Rpectra of Si Nanocrystals. J. Appl. Phys. 2011, 109, 074311. (43) Pizani, P.S.; Jasinevicius, R.G.; Duduch, J.G.; Porto, A.J.V. Ductile and Brittle Modes in Single-Point-Diamond-Turning of Silicon probed by Raman Scattering. J. Mater. Sci. Lett. 1999, 18, 1185-1187. (44) Kobliska, R.J.; Solin, S.A. Raman Spectrum of Wurtzite Silicon. Phys. Rev. B 1973, 8, 3799-3802.


39


Page 40 of 51

(45) Besson, J.M.; Mokhtari, E.H.; Gonzalez, J.; Weill, G. Electrical Properties of Semimetallic Silicon III and Semiconductive Silicon IV at Ambient Pressure. Phys. Rev. Lett. 1987, 59, 473-476. (46) Pirouz, P.; Chaim, R.; Dahmen, U.; Westmacott, K.H. The Martensitic Transformation in Silicon I. Experimental Observations. Acta Metall. Mater. 1990, 38, 313-322. (47) Steinert, M.; Acker, J.; Krause, M.; Oswald, S.; Wetzig, K. Reactive Species Generated during Wet Chemical Etching of Silicon in HF/HNO3 Mixtures. J. Phys. Chem. B 2006, 110, 11377-11382. (48) Steinert, M.; Acker, J.; Oswald, S.; Wetzig, K. Study on the Mechanism of Silicon Etching in HNO3-rich HF/HNO3 Mixtures. J. Phys. Chem. C 2007, 111, 2133−2140. (49) Steinert, M.; Acker, J.; Wetzig, K. New Aspects on the Reduction of Nitric Acid during Wet Chemical Etching of Silicon in Concentrated HF/HNO3 Mixtures. J. Phys. Chem. C 2008, 112, 14139−14144. (50) Roever, I.; Roewer, G.; Bohmhammel, K.; Wambach, K. In Freiberger Siliciumtage 2003- Halbleitermaterialien, Prozesstechnologie und Diagnostik: Freiberger Forschungshefte B327; Möller, H.-J., Roewer, G., Eds.; Technische Universität Bergakademie Freiberg: Germany, 2004; pp 179−193. (51) Kooij, E.S.; Butter, K.; Kelly, J.J. Silicon Etching in HNO3/HF Solution: Charge Balance for the Oxidations Reaction. Electrochem. Solid-State Lett. 1999, 2, 178−180.


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(52) Hoffmann, V.; Steinert, M.; Acker, J. Analysis of Gaseous Reaction Products of Wet Chemical Silicon Etching by Conventional Direct Current Glow Discharge Optical Emission Spectrometry (DC-GD-OES). J. Anal. At. Spectrom. 2011, 26, 1990-1996. (53) Acker, J.; Rietig, A.; Steinert, M.; Hoffmann, V. Mass and Electron Balance for the Oxidation of Silicon during the Wet Chemical Etching in HF/HNO3 Mixtures. J. Phys. Chem. C 2012, 116, 20380–20388. (54) Kissinger, G.; Pizzini, S. Silicon, Germanium, and Their Alloys: Growth, Defects, Impurities, and Nanocrystals; CRC Press: Boca Raton, 2014.


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graphical abstract

The production of a Si solar wafer by multi-wire sawing produces an uneven distribution of tensile and compressive lattice strain across the wafer surface. Depending on the strain component and its magnitude, a preferential etching is stimulated which determined the topography of the etched wafer surface.


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Fig. 1a 148x118mm (96 x 96 DPI)



Fig. 1b 147x118mm (96 x 96 DPI)


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Fig. 1c 147x118mm (96 x 96 DPI)



Fig. 1d 146x119mm (96 x 96 DPI)


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Fig. 2 149x171mm (150 x 150 DPI)



Fig. 7a 210x176mm (150 x 150 DPI)


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Fig. 8a 230x176mm (150 x 150 DPI)



Scheme 1 246x163mm (150 x 150 DPI)


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graphical abstract 44x24mm (300 x 300 DPI)


Lattice-strain Enhanced Chemical Reactivity of Silicon for Efficient

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