Si nanowires by a single-step metal-assisted chemical etching process on lithographically defined areas: formation kinetics
© Nassiopoulou et al; licensee Springer. 2011
Received: 18 May 2011
Accepted: 16 November 2011
Published: 16 November 2011
In this paper, we investigate the formation kinetics of Si nanowires [SiNWs] on lithographically defined areas using a single-step metal-assisted chemical etching process in an aqueous HF/AgNO3 solution. We show that the etch rate of Si, and consequently, the SiNW length, is much higher on the lithographically defined areas compared with that on the non-patterned areas. A comparative study of the etch rate in the two cases under the same experimental conditions showed that this effect is much more pronounced at the beginning of the etching process. Moreover, it was found that in both cases, the nanowire formation rate is linear with temperature in the range from 20°C to 50°C, with almost the same activation energy, as obtained from an Arrhenius plot (0.37 eV in the case of non-patterned areas, while 0.38 eV in the case of lithographically patterned areas). The higher etch rate on lithographically defined areas is mainly attributed to Si surface modification during the photolithographic process.
PACS: 68; 68.65-k.
KeywordsSi nanowires metal-assisted chemical etching lithographically defined areas formation kinetics
Si nanostructures such as quantum dots, nanocrystals, porous Si, and Si nanowires [SiNWs] exhibit interesting properties [1–4] that are very different from their bulk counterparts and make them interesting for several applications. These properties include a diameter-dependent bandgap, very-high-density electronic states, an increased surface-to-volume ratio, an enhanced exciton binding energy, enhanced thermoelectric properties, and increased surface scattering for electrons and phonons. These properties make SiNWs interesting for application in electronic and photonic devices [4–16], sensors , energy harvesting devices, and solar cells [18–20].
Different methods have been developed for the fabrication of SiNWs either by Si etching [12, 21, 22] or by Si nanowire synthesis [1, 23]. Among them, the technique of metal-assisted chemical etching [MACE] [24–29] has gained an increasing interest in the last years due to its simplicity and the high crystalline quality of the obtained SiNWs, resulting from etching of the single crystalline Si material. SiNWs with lengths ranging from a few micrometers to several tens of micrometers can be obtained using either a two-step process involving metal (Ag, Pt) nanoparticle deposition on Si followed by etching or a single-step chemical dissolution process in an aqueous HF solution containing the metal salt. The SiNWs can be fabricated on large areas, which can cover the whole Si wafer. However, for the different applications (Si devices), it is interesting to form the SiNWs on specific confined areas of the Si wafer. It is thus important to develop a technology for their local formation on Si on preselected areas.
In this work, we report on the formation kinetics of SiNWs on lithographically defined areas on the Si wafer using a single-step MACE process based on an aqueous HF/AgNO3 solution. We investigated the etch rate of Si, and the corresponding SiNW length, on lithographically defined Si areas compared to that on large non-patterned areas. Field emission scanning electron microscopy [FE-SEM] was used to characterize the samples.
The substrates used in this work were p-type (100) Si wafers with resistivity ranging from 1 to 2 Ω cm. SiNWs were formed with the single-step MACE technique that consists of immersing the sample in an HF/AgNO3 aqueous solution for a process time that determines the SiNW length. With this technique, the mechanism of SiNW formation involves two different processes that occur simultaneously: (a) deposition of Ag nanoparticles on the Si surface and (b) catalytic etching of Si at the sites where the Ag nanoparticles have been deposited. The composition of the AgNO3/HF/H2O solution used was 0.67 g:35 ml:182 ml. Experiments were carried out in a temperature ranging from 20°C to 55°C. Confined areas on the Si wafer were defined by photolithography using the AZ5214 photoresist. This photoresist was used as the masking material for SiNW formation on confined areas, and it was found to constitute an excellent masking material since it withstands the MACE solution for a long time and it is easily removed with acetone after the end of the process. Square-shaped windows in the photoresist having a surface area ranging from 2 × 2 μm2 to 400 × 400 μm2 were used in our experiments.
SiNWs were characterized by FE-SEM using a JSM-7401F microscope (JEOL, Tokyo, Japan). The Ag dendrite-shaped structures that usually grow on the SiNW surface during the single-step MACE process were removed in an HNO3/water solution with a volume ratio of 1:1.
Results and discussion
SiNW formation and morphology on large surface areas
In order to understand the experimentally observed differences of the etch rate between large areas and confined areas, it is necessary to recall here the mechanism involved in the one-step MACE process. In this process, the SiNWs are produced by simply immersing the wafer into an HF/AgNO3 aqueous solution of a given concentration for an appropriate time. The reaction that takes place is a galvanic displacement reaction. A galvanic cell is established when the Si wafer is immersed into the solution because the reduction potential of the Ag+/Ag couple is more positive than the flat band potential of Si. Two simultaneous processes occur in the galvanic displacement reaction at the Si surface: (a) reduction of Ag+ ions by hole injection into Si, which produces metallic Ag deposits (cathodic reaction, electron-consuming type) and (b) oxidation of Si by the injected holes (anodic reaction, electron-releasing or hole-consuming type). In this process, the bonding electrons of surface Si atoms are transferred to Ag+ ions in the aqueous HF solution as described in detail by Peng et al. . The oxidized Si is dissolved by the HF, leading to Si etching and resulting in pore or NW formation. Ag+ reduction and Si oxidation result in the deposition of silver atoms on the cathodic sites of the Si surface, forming nanoscale Ag nuclei at the beginning of the process, with higher electronegativity than Si and thus, strongly attracting electrons from Si to become negatively charged, providing a catalytic surface for further Ag+ reduction. A quasi-Schottky Ag/Si interface is formed, with a relatively low-energy barrier for holes. Charge transfer from the Ag nuclei to Ag+ ions in the solution occurs by hole injection through the quasi-Schottky Ag/Si interface.
Formation of SiNWs on confined areas on the Si substrate
The confined surface area ranged from 1 to 4 × 104 μm2. The cross-sectional images are from SiNWs formed in confined areas of 100 × 100 μm2, and they are shown at two different magnifications. A first observation is that the NWs formed on confined areas are regular and well defined. They are vertical to the (100) surface, as in the case of NWs formed on non-patterned areas. Their formation at the lithographic edges is relatively anisotropic (see Figure 3f).
In order to understand the mechanism responsible for the increase of the formation rate of Si NWs on lithographically patterned areas compared to the non-patterned ones, we performed a series of experiments and examined different factors that could be at the origin of the different reaction kinetics in the two cases, as follows:
Effect of surface area
In order to elucidate the effect of surface area localization on the formation rate of SiNWs, we performed a series of experiments on a single wafer with different lithographically defined surface areas. The wafer was immersed into the solution for 30 min at a temperature of 30°C for a reaction time of 1 h. A comparative study of cross-sectional SEM images of the SiNWs from the different surface areas of the sample showed that there was no significant difference in the SiNW length in small and large confined surface areas in the examined range from 50 × 50 μm2 to 400 × 400 μm2.
Effect of silver dendrite density
Effect of surface modification through the lithographic steps
After excluding all the factors above, we examined the possible effect of the lithographic steps on the reaction kinetics by MACE in the case of lithographically defined areas. The different process steps include the use of a resist adhesion promoter, resist spinning, UV exposure, and a developer for resist stripping. The first experiment was to perform all the lithographic steps above on a blank wafer without any mask exposure, remove the resist normally, and then, etch the sample by MACE. In this way, we exclude the surface area effect from the origin of the etch rate difference. Indeed, after applying all lithographic steps without mask and stripping the resist, the etch rate of the sample was much higher than the etch rate of a virgin Si wafer. This leads to the conclusion that surface modification during lithography is at the origin of the etch rate effect described above. The step responsible for surface modification is the adhesion promoter. The one used was hexamethyldisilzane [HMDS], known to remove -OH groups from the Si surface and form a hydrophobic surface with the methyl groups of the HMDS fragment. The so-formed hydrophobic surface improves resist wetting and adhesion. We demonstrated that it also increases the etch rate by MACE.
After the above result, we investigated a number of other surface chemical treatments on the MACE kinetics. We compared the SiNW length after etching by MACE at 28°C for 1 h on a non-treated Si wafer, a Si die sample after piranha cleaning for 30 min, and a Si die after HF (10%) dip for 15 min and after a dip into the above-mentioned developer for approximately 1 min. The result was that the non-treated wafer resulted in 20-μm-long NWs; the same was obtained after piranha cleaning, while there was approximately 15% increase in length after the developer dip and 7.5% decrease in length after the HF dip.
From the above experiments, it is clear that chemical surface modification plays an important role in the etching kinetics by MACE. It mainly affects these kinetics at the beginning of the process.
A comparative study on the formation kinetics of SiNW formation by MACE of non-patterned and lithographically patterned Si surfaces showed that the etch rate of Si for SiNW formation is higher in the case of lithographically patterned surfaces compared to that of the non-patterned ones. The origin of this effect is at the surface modification by the promoter used for resist adhesion. Resist promoter is known to form a hydrophobic surface covered by methyl groups. Moreover, it was shown that surface treatment by an HF dip, which leads to surface passivation by hydrogen, results in retardation of the etching process, thus leading to shorter SiNWs.
Funding for this work was received from the European Union's 7th Framework Programme (FP7/2007-2013) through the ICT NoE project 'Nanofunction' (grant agreement number 257375).
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