The dual role of silver during silicon etching in HF solution
© Abouda-Lachiheb et al.; licensee Springer. 2012
Received: 25 April 2012
Accepted: 23 July 2012
Published: 13 August 2012
It was reported that during silicon etching, silver was subjected to have a controversial role. Some researchers debate that silver protects silicon, and, at the same time, other ones confirm that silver catalyzes silicon underneath. In this paper, we give experimental results arguing the dual role that silver has during the formation of silicon nanostructures. We give a proof that the role of silver depends on the experimental details and the intrinsic properties of silver during its deposition on the silicon wafer. Through our investigations, we tracked the silver particles that indicated which mechanism is involved. Characterizations of the prepared samples were made using a scanning electron microscope.
By etching silicon in a HF aqueous solution, porous silicon can be formed. When silicon etching in the HF solution is assisted by silver, well-organized nanostructures like silicon nanowires [1–3] are formed. The latter are very interesting in electronic and photovoltaic application. In order to enable significant improvements in the devices’ technologies, understanding of the fabrication mechanism must be attained to produce materials with precise control. Herein, we are interested in the silver-assisted chemical etching of silicon in HF solution. This technique can be realized either by a one-step process or a two-step process. The one-step method includes, at the same time, metal deposition and electroless etching. In this process, the silicon substrate is immersed in an AgNO3/HF aqueous solution. The two-step method consists first of metal deposition and second, the chemical etching. In the latter method, the silver particles are evaporated or chemically formed on the surface of the silicon sample then introduced in a HF aqueous solution. With these two processes, we obtain two different behaviors of the silver particles; in one case, the silver catalyzes silicon, and in the other one, it protects the silicon underneath. Therefore, using a scanning electron microscopic (SEM) characterization, we tracked silver particles during silicon etching to determine which mechanism is privileged. To interpret the obtained results, we analyze charge displacement at the metal/Si interface. The developed model clarifies the role that the silver has during silicon metal-assisted etching experiences. Hence, the controversy regarding this role is solved.
For cleaning, samples were boiled in acetone for 10 min, followed by their immersion in ethanol for 5 min to remove organic greases. Then, we rinsed them three times with deionized water. Finally, samples were dipped in a 5% aqueous HF solution for 1 min to eliminate native silicon dioxide. This study was realized at two ambient conditions: room temperature and atmospheric pressure. The etching duration was 1 h. We used high purity single-crystalline silicon. The wafers are p-type, boron-doped and (100)-oriented, with a thickness of 525 μm and a resistivity of 1 to 10 Ωcm. The samples were then separated in sample A and sample B to be treated differently. Sample A was immersed in an etching solution composed of HF/AgNO3[1–3]. The concentrations in the latter were 40% and 0.02 M, respectively. On sample B, we first evaporated a 30-nm silver film, and then it was immersed in a 40% HF aqueous solution .
Our SEM observations show two different structures; in the case of the one-step process (sample A), we obtained silicon nanowires (SiNWs) covered with a dendritic silver film. We did not observe any silver nanoparticle at the bottom of SiNWs. In the two-step process (sample B), we obtained silicon pores. However, Fang et al.  obtained SiNWs by etching a silicon wafer on which they evaporated a 20-nm silver film.
Results and discussion
From the obtained results for samples A and B, we estimate the etching rate for sample A at about 0.5 μm/min and the penetration rate of silver particles for sample B at about 2.7 μm/min.
The electroless metal deposition mechanism was presented by Morinaga et al. with copper particles . Then Peng et al. started with the protecting mechanism that silver has during the silicon etching in the one-step method . After that, they provided in  their new position regarding the etching processes of silicon wafers in aqueous HF/AgNO3 solution where the catalyzing effect was described. Qiu et al.  gave experimental evidences that silver protects the silicon underneath. Consequently, the role that silver has was still not elucidated.
Values of electronegativity of Ag, Si, O, and B
During metal-assisted etching experiences, some researchers reported that silver protects silicon. However, others confirmed that silver catalyzes the silicon underneath. In this paper, we give experimental results arguing the dual role that silver has during silicon etching. We propose a model based on the effect of electron transfer at the silver/silicon interface when silver is deposited during the HF etching (one-step method) and when silver is deposited prior to the etching in a HF solution (two-step method). By the proposed model, it becomes clear how silver nanoparticles can protect the silicon underneath in some experimental conditions and how they initiate pore formation by etching silicon underneath in others.
Authors are grateful to the Mitsubishi Corporation for the financial support in the form of a scholarship of 10 months, via a memorandum with the Ecopark of Science and Technology of Borj-Cedria, Tunisia.
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