How the Oxidation Stability of Metal Catalysts Defines the Metal

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How the Oxidation Stability of Metal Catalysts defines the Metal Assisted Chemical Etching of Silicon Max Owen Williams, Daniel Hiller, Thomas Bergfeldt, and Margit Zacharias J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12362 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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The Journal of Physical Chemistry

How the Oxidation Stability of Metal Catalysts defines the Metal Assisted Chemical Etching of Silicon Max O. Williams†, Daniel Hiller†, Thomas Bergfeldt‡, Margit Zacharias*,†. †

Laboratory for Nanotechnology, Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-Koehler-Allee 103, D-79110 Freiburg im Breisgau, Germany.

‡

Karlsruhe Institute of Technology, Institute of Applied Materials, Hermann von Helmholtz Platz 1, D-76344 Eggenstein Leopoldshafen, Germany.

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Abstract

Metal-assisted chemical etching (MACE) is done with different metal species. The resulting silicon nanostructures appear strongly dependent on the choice of metal, but a deeper understanding of the MACE process is still missing. We report here direct evidence that the etching solution composition plays a major role in the chemical stability of the metal catalyst used. We show from an elemental analysis of post-MACE etch baths that dissolved silver is found in the bath with concentrations up to 3 orders of magnitude larger than when gold is used. Furthermore, the dissolved silver content also correlates with the amount of H2O2, either in different initial conditions, or as would be expected from its decomposition over time. We also show that silver dissolution leads to unintended etching elsewhere on the substrate. This species-dependent behavior of the metal catalyst is responsible for the different kinds of control possible over the nanostructures produced with silver- and gold-based MACE.


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Introduction Metal-assisted chemical etching (MACE) is a promising technique capable of producing a variety of microstructures in a simple wet chemical process, with an anisotropic and high aspect ratio character1,2. In this way, it can be a cheap alternative to other anisotropic processes like deep reactive-ion etching (the Bosch process), which have a relatively limited throughput, higher cost, and limitations on the maximum aspect ratio3. However, reproducibility and control are important for the success of any nanostructuring technique, if using it for creative design of structures. Hence, a deeper understanding of the important factors controlling the MACE process is needed. Up to now, etchant bath composition4–7 , substrate orientation4 or doping8, reaction temperature5,6,8, etching time1,5 and the species or amount of metal catalyst9–12 are shown to influence the etching of a variety of nanowires5,6,8,13,14, pores10, micromachined trenches1,2,4,9 and other complex structures15,16. A comprehensive view of the interrelation between all these parameters and possible resultant structures is still missing. The morphological stability of the metal catalyst during MACE is one such influential parameter requiring consideration. Changes in the size and shape of silver catalysts due to chemical attack are often observed17–19, with recent work proposing that dissolution of silver into Ag+ ions significantly affects the etching mechanism13. In comparison, gold catalysts may change shape under the influence of mechanical forces, but their chemical resistance during etching appears much higher2,9,16,19. A greater tendency for silver to dissolve can thus be inferred from the literature, yet to date no direct and quantitative measurements have been reported showing how metal dissolution is influenced by other etching parameters, such as etch bath composition. Towards this end, we retained etch baths from several MACE experiments and performed trace metal analyses to determine the silver and gold concentrations remaining in solution. In these experiments, etching was performed on pieces taken from silicon wafers deposited with either gold or silver films, and metal levels were quantified as a function of both etch time and initial bath composition. The lower levels of gold detected in all samples indicates agreement of our results with the trends seen in literature, where gold catalysts appear morphologically stable in post-etch imaging. New evidence showing a dependence of silver dissolution on etch bath composition and etch time is also reported here for the first time ACS Paragon Plus Environment

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Experimental Methods Four identical lowly-doped (1-100 Ω cm) p-type Si wafers from the same batch were cleaned individually with piranha etch (3 parts 18 M H2SO4 to 1 part 10 M H2O2) for 10 minutes, rinsed with DI-H2O, and then UVO3 cleaned for a further 10 minutes. Metal was immediately evaporated on each wafer after cleaning, so that two wafers were deposited with ~300 nm thin films of Au, and the other two with ~150 nm of Ag. Nominal thicknesses were initially intended to be equal, but subsequent SEM investigation revealed the gold films to be thicker, as reported above. Wafers were then cleaved into 2cm-by-2cm squares, such that a consistent surface area between samples was achieved. 23 M HF and 10 M H2O2 were used in preparing the 5 mixtures for etching shown in Table 1, and ~15 mL aliquots of each were retained without further processing to act as controls. Etching was performed on individual wafer squares with ~15 mL volumes of each solution in a Teflon dish, covered with a Teflon watch glass to reduce evaporative losses, and left for 5, 60, 360 or 1080 min before removing the square. The etch bath solutions, including the controls, were retained in individual vials for subsequent batch analysis, and the wafer squares were imaged with an FEI Nova NanoSEM 430 scanning electron microscope. Table 1. Etchant Compositions solution volume of volume of total HF (mL) H2O2 (mL) volume (mL)

[HF] (M)

[H2O2] (M)

ρa

A

0

15

15

0

10

0

B

1.9

13.1

15

2.9

8.9

0.25

C

4.6

10.4

15

7.0

7.0

0.50

D

8.6

6.4

15

13

4.3

0.75

E

15

0

15

23

0

1

a

Molar ratio, ρ, is defined as [HF]/([HF] + [H2O2])

Elemental analysis of etch baths by way of inductively coupled plasma optical emission and mass spectroscopies (ICP-OES and ICP-MS) was performed to quantify the metal concentrations for each species. Both techniques convert any aqueous species into their atomic components by way of a plasma flame, and subsequently analyze the elemental composition from either characteristic optical emission, or by mass ACS Paragon Plus Environment

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spectroscopy. Silver concentrations were measured with ICP-OES, and gold with the more sensitive ICP-MS technique. This was due to the much higher silver levels, which allowed the simpler ICP-OES setup to be used. Different preparation methods were also employed to match the requirements of the two techniques, and the two metals. The Au samples were diluted gravimetrically 3:1 in aqua regia, to dilute samples with high HF concentrations and keep Au in solution. The samples were heated overnight to 364 K in order to dissolve any precipitated Au. For the quantitative mass spectrometry measurements (ICP-MS, Agilent 7500ce), samples were diluted by 3 to 13 gravimetrically in HCl (5%), with two different dilution values. The analysis was accomplished with four different calibration solutions, an internal standard (In) and a matrix adapted for Si and HF. The range of the calibration solutions did not exceed two orders of magnitude. Masses of 197 for Au and 115 for In were used in calculations. The Ag samples were diluted gravimetrically 3:4 in HNO3 (5%), to dilute samples with high HF concentrations. For the quantitative optical emission spectrometry measurements (ICP-OES, Thermo Fisher Scientific iCAP 7600), samples were diluted by 5 to 200 gravimetrically in HNO3 (2%), with two different dilution values. The analysis was accomplished with four different calibration solutions, an internal standard (Sc) and a matrix adapted for Si. The range of calibration solutions did not exceed an order of magnitude. Two major Ag wavelengths were used in calculations. Results and Discussion All etch mixtures containing H2O2 contained large amounts of silver (Figure 1). Gold concentrations were often below the 0.2 µg/g quantification limit for ICP-MS, with the ρ = 0 results for gold representative of the maximum concentrations found. Silver concentrations are therefore up to three orders of magnitude higher than gold. Notably, concentrations were negligible for pure HF samples (ρ = 1.0) in both metals, signifying that almost no dissolution occurs without H2O2.


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Figure 1. Silver and gold concentration in MACE etch baths after increasing etching times, using four etchant compositions for silver, and ρ = 0 alone for gold. Error bars are smaller than the symbols. Lines are used as guides to the eye only. Dissolved silver concentrations peak at short etch times and, clearly at least in the case for the lowest two ρvalues, decrease after longer etches (Figure 1). This is suggestive of a time-dependent shift in the relative rates of dissolution and deposition, favoring the latter process. However, the higher ρ-value samples show variances away from this trend which have not been fully accounted for. In particular, silver levels after 1080 min of etching in solution D (ρ = 0.75) are about three times higher than after only 360 min. Although it is possible that there are small, uncontrolled differences between samples, it is unknown how these difference would only lead to a strong effect for high-ρ cases. Even more precise control over parameters such as metal thickness or bath temperature may be helpful, but a connection to ρ-value itself cannot yet be dismissed. A decrease with respect to ρ is also noticeable, and is better highlighted in Figure 2. By definition, higher ρvalues correspond with a lower initial amount of H2O2, so our results can also be interpreted as showing a correlation between silver dissolution and H2O2 concentration. High-ρ values are also less diluted, but with only an ~11% difference in the amount of water between the HF and H2O2 stock solutions, we assume any effect to be minor.


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Figure 2. Silver and gold concentration in MACE etch baths after 5 min for different ρ-values. In light of recent studies involving silver nanoparticles exposed to H2O2, both trends make sense within a model of cyclical silver catalyst dissolution and reformation20–22. A direct effect of raising the initial H2O2 concentration would be an increase in silver dissolution, and thus the peak Ag+ concentration, as seen in the trend of Figure 2. On the other hand, because H2O2 is consumed by way of decomposition into other oxygen species, and this process is catalyzed by metallic silver, the H2O2 concentration continues to drop over time. As the tendency for any redeposited silver metal to redissolve therefore decreases, there is a consequent drop in aqueous silver levels, as seen in Figure 1. The formation of reactive oxygen species upon H2O2 decomposition also introduces the possibility that the dynamics of competing reaction mechanisms leads to a more complex system, which could help explain deviations in the trend for lower initial H2O2 concentrations. It is important to understand the influence of aqueous species, such as Ag+ ions, in MACE. These species move around the etch bath, whereas solid particles remain localized. This is particularly clear in one-step MACE23–28, where AgNO3 solutions introduce known quantities of oxidizing Ag+ and NO3- ions. In such studies, highly porous nanowires are etched all over the substrate, and the degree of porosity correlates with the initial AgNO3 concentration23,28. Analogously, Ag+ ions coming from unintentional silver dissolution by H2O2 in our experiments must play a similar role. The etching mechanism proposed by Geyer et al.13 indeed suggests that hole injection into silicon from H2O2 reduction is mediated by the Ag/Ag+ redox couple. In this case, the actual concentration of Ag+ now becomes an important variable when distinguishing its role in etch structuring from the role of other species, such as NO3- or H2O2 decomposition products. ACS Paragon Plus Environment

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Figure 3a,b illustrates how different metal stabilities of silver and gold, respectively, affect MACE-produced structures. Silver morphology is fundamentally unstable due to metal dissolution, and the front side surface in Figure 3c consequently appears rough. The influence of dissolved silver ions is highlighted in Figure 3d, which shows etch structures on the back side of the wafer piece, where no silver was initially deposited. These observations are in accordance with Chiappini et al.14, who also saw porous structures in separate blank wafers that were exposed to solutions previously used for MACE. As discussed above, aqueous species moving within the solution can take part in etching in an increasingly delocalized fashion over the entire substrate. Given the results of our elemental analyses, we know that dissolved silver ions are present in large amounts in solution, and thus are likely to have played a role in forming these structures.

Figure 3. Schematic representation of the MACE process, using (a) silver and (b) gold metal catalysts. SEM images of (c) front side and (d) back side of a silicon piece after 5 min of Ag-MACE in ρ = 0.75 and ρ = 0.50 baths, respectively. Part (e) shows the front side after 18 h of Au-MACE in a ρ = 0.25 bath. Comparatively, gold maintains its shape, appears to sink into the silicon, and has been described as a travelling catalyst16. Because very little dissolution takes place, the challenge of tightly controlling etching ACS Paragon Plus Environment

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shifts partly into a problem of understanding the movement of the gold. For example, Figure 3e shows an etch hole with a very consistent shape, suggesting the gold structure causing this feature did not change its shape over time. We can also attribute the pits seen on the top to the gold film moving relative to the silicon surface during etching, without any chemical instability being involved. Conclusions Our results prove unambiguously that Ag-MACE catalysts dissolve at significant levels, particularly in low-ρ etch baths, while Au-catalysts are stable. We further show a dependence of the dissolved Ag+ concentration on etchant bath composition, with enhanced silver dissolution in H2O2-rich mixtures. These factors related to metal catalyst stability should therefore be included in models seeking to explain MACE. The combination of silver and low-ρ etch baths is thus clearly to be avoided wherever direct etching with a stable travelling catalyst is required. This accounts for the preference for high-ρ mixtures in the Ag-MACE literature, and for the tendency of reports to show a particular subset of structures, often requiring less direct control over the catalyst morphology. In contrast, gold or other catalysts resistant to a wider range of etch bath compositions are desirable when precise transfer of the catalyst structure into silicon is needed. AUTHOR INFORMATION Corresponding Author *Address correspondence by e-mail to [email protected]. Author Contributions All authors designed the experimental plan. M.W. performed the MACE sample preparation and SEM imaging. T.B. performed the ICP-OES and ICP-MS chemical analysis, and wrote the related parts of the manuscript. M.W. wrote the manuscript, with comments and approval of the final version from all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT ACS Paragon Plus Environment

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We acknowledge financial support from the German Research Foundation (DFG) under grant ZA 191/34-1, and the Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/knmf) of the KIT for provision of access to instruments at their laboratories. REFERENCES (1) Booker, K.; Brauers, M.; Crisp, E.; Rahman, S.; Weber, K.; Stocks, M.; Blakers, A. Metal-Assisted Chemical Etching for Very High Aspect Ratio Grooves in n-Type Silicon Wafers. J. Micromechanics Microengineering 2014, 24, 125026. (2) Li, L.; Liu, Y.; Zhao, X.; Lin, Z.; Wong, C.-P. Uniform Vertical Trench Etching on Silicon with High Aspect Ratio by Metal-Assisted Chemical Etching Using Nanoporous Catalysts. ACS Appl. Mater. Interfaces 2014, 6, 575–584. (3) Wu, B.; Kumar, A.; Pamarthy, S. High Aspect Ratio Silicon Etch: A Review. J. Appl. Phys. 2010, 108, 051101. (4) Lai, R. A.; Hymel, T. M.; Narasimhan, V. K.; Cui, Y. Schottky Barrier Catalysis Mechanism in MetalAssisted Chemical Etching of Silicon. ACS Appl. Mater. Interfaces 2016, 8, 8875–8879. (5) Balasundaram, K.; Sadhu, J. S.; Shin, J. C.; Azeredo, B.; Chanda, D.; Malik, M.; Hsu, K.; Rogers, J. A.; Ferreira, P.; Sinha, S.; et al. Porosity Control in Metal-Assisted Chemical Etching of Degenerately Doped Silicon Nanowires. Nanotechnology 2012, 23, 305304. (6) Kim, J.-C. J.; Han, H.; Kim, Y. H.; Choi, S.-H.; Lee, W. Au/Ag Bilayered Metal Mesh as a Si Etching Catalyst for Controlled Fabrication of Si Nanowires. ACS Nano 2011, 5, 3222–3229. (7) Kolasinski, K. W.; Barclay, W. B.; Sun, Y.; Aindow, M. The Stoichiometry of Metal Assisted Etching (MAE) of Si in V2O5+HF and HOOH+HF Solutions. Electrochim. Acta 2015, 158, 219–228


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How the Oxidation Stability of Metal Catalysts Defines the Metal

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