Metal-assisted chemical etching of silicon in HF–H2O2
Introduction
Since the 1990s, porous silicon is under intense investigations due to its potential interest in several applications such as emitting materials in optoelectronics [1], bone growth media in biology, gas and humidity sensors in chemistry [2], surface texturization in photovoltaics [3] or sacrificial layers in micromachining [4]. The anodic electrochemical dissolution in HF solutions is well known [5] and is the most widely applied technique for porous Si formation. Porous Si can also be obtained by stain etching in mixtures of HF and HNO3 [6], [7].
Recently, a new method to form porous Si based on metal-assisted etching was proposed for photonic [8], [9], [10], photovoltaic [11], [12] or diffusion membrane applications [13]. This technique was also proposed to produce large-area Si nanowire arrays [14] and for the synthesis of porous gallium nitride [15] or Si carbide [16]. Metal particles or films are deposited at the Si surface prior to chemical etching to enhance the Si dissolution. This metallization is performed by various techniques such as sputtering [8], thermal evaporation [9], electrochemical deposition [11] or electroless deposition in HF solutions [17]. After metallization, the Si sample is etched in a solution containing HF and an oxidizing agent. H2O2 is commonly used as oxidizing agent but other chemicals are reported in the literature, such as potassium dichromate K2Cr2O7 [9], sodium persulfate Na2S2O8, potassium permanganate KMnO4 [18], a metal ion (Ag+ [19], Au3+ [20], and Fe3+ [14]) or dissolved oxygen gas [11].
Although substantial work has been devoted to metal-assisted Si chemical etching, the influence of the etching solution composition in the etching process has not been studied yet. Here we present the formation of porous Si by chemical etching in HF–H2O2 solutions with Ag nanoparticles as catalyst agents. The relative composition of the etching bath was varied in order to reveal the different dissolution regimes taking place. We found that the etching bath composition is a major parameter that influences the formation and morphology of porous Si and show for the first time that Si polishing can also be obtained by this method. The goal of this work is to highlight the mechanisms involved in metal-assisted Si etching to ensure a better control of the pore formation for various potential applications.
Section snippets
Experimental
Polished single crystalline (1 0 0) p-type boron-doped Si wafers with a resistivity of 1–2 Ω cm (∼1016 cm−3) were cleaved into 1 × 1 cm2 samples and weighed.
Ag nanoparticles were deposited on Si samples by immersion in 0.14 M HF and 5 × 10−4 M AgNO3 metallization aqueous solution. After the electroless metallization, the wafers were etched in aqueous solutions containing HF (40%), H2O2 (35%) and ultra-pure H2O (18.2 MΩ cm) at different concentration ratios and for different etching times. In the text, the
Ag nanoparticle deposition
After a few minutes of immersion in the metallization bath, the Si surface is loaded by isolated and nearly spherical Ag nanoparticles, as shown in Fig. 1. SEM observations of the nanoparticles at higher magnifications reveal a granular morphology indicative of their polycrystalline nature (insert of Fig. 1).
In order to optimize the nanoparticle deposition, a systematic study has been carried out by varying the metallization time and the bath temperature. The results are displayed in Fig. 2. Ag
Electroless metal deposition
Electroless metal deposition on Si in presence of HF has been well described by Morigana et al. regarding the Cu particle initiation deposition and growth on Si [27]. A similar sketch can be drawn for Ag nanoparticle deposition on Si. This is well discussed in Ref. [28] and only a summary of the process will be presented. Electroless silver deposition on a Si surface in HF solution containing Ag+ ions is based on the galvanic displacement reaction, in which two simultaneous processes occur at
Conclusion
The influence of the etching solution composition has been found to be an important parameter in metal-assisted Si chemical etching. We have shown in the case of Ag-assisted etching of Si in HF–H2O2 that a large variety of surface morphologies can be produced by changing the etching solution composition. As the composition varies from high to low HF/H2O2 ratio, mesopores, cone-shaped macropores, craters and eventually smooth surfaces are obtained. The different surface morphologies originate
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