Metal-assisted chemical etching of Ge(100) surfaces in water toward nanoscale patterning
© Kawase et al.; licensee Springer. 2013
Received: 27 December 2012
Accepted: 11 March 2013
Published: 2 April 2013
We propose the metal-assisted chemical etching of Ge surfaces in water mediated by dissolved oxygen molecules (O2). First, we demonstrate that Ge surfaces around deposited metallic particles (Ag and Pt) are preferentially etched in water. When a Ge(100) surface is used, most etch pits are in the shape of inverted pyramids. The mechanism of this anisotropic etching is proposed to be the enhanced formation of soluble oxide (GeO2) around metals by the catalytic activity of metallic particles, reducing dissolved O2 in water to H2O molecules. Secondly, we apply this metal-assisted chemical etching to the nanoscale patterning of Ge in water using a cantilever probe in an atomic force microscopy setup. We investigate the dependences of probe material, dissolved oxygen concentration, and pressing force in water on the etched depth of Ge(100) surfaces. We find that the enhanced etching of Ge surfaces occurs only when both a metal-coated probe and saturated-dissolved-oxygen water are used. In this study, we present the possibility of a novel lithography method for Ge in which neither chemical solutions nor resist resins are needed.
KeywordsDissolved oxygen Machining Catalyst Lithography Oxygen reduction Atomic force microscopy
Germanium (Ge) is considered to be a substitute for Si for future complementary metal-insulator-semiconductor devices because of its higher carrier mobility than silicon (Si) . Although wet-chemical treatments are essential for the fabrication of Ge-based devices, they have not been well established yet. The primary reason for this is the chemical reactivity of Ge and its oxide (GeO2) with various solutions. For example, Ge oxide (GeO2) is permeable and soluble in water, unlike the more familiar silicon oxide (SiO2). Ge surfaces are also not resistant to various chemical solutions. For example, a piranha solution (a mixture of H2SO4 and H2O2) is commonly used in removing metallic and organic contaminants on the Si surface. However, we cannot use it for Ge because it damages Ge surfaces very easily. Although in several earlier works, the etching property of Ge surfaces has been investigated [2, 3], the unique chemical nature of Ge prevents researchers from developing surface treatment procedures for Ge using solutions.
One of the surface preparation steps needed is wet cleaning. For Si, sophisticated cleaning procedures have been developed since the 1970s [4, 5]. For Ge, however, researchers have just started developing wet cleaning processes together with some pioneering works [6–9]. Furthermore, a variety of solutions have been used in lithography processes (e.g., development, etching, and stripping) to fabricate Si-based devices. However, patterning techniques are not well optimized in the case of Ge. To realize these surface preparation methods, the impact of various aqueous solutions on the morphology of Ge surfaces should be understood on the atomic scale.
In this study, we pay attention to the interaction of water with Ge surfaces in the presence of metals on the Ge surface. In the case of Si, a metal/Si interface in HF solution with oxidants added has been extensively studied [10–18]. Metallic particles on Si serve as a catalyst for the formation of porous surfaces, which can be applied in solar cells. A similar metal/Si interaction is also used to form either oxide patterns or trenches . Recently, we have found that similar reactions occur on Ge surfaces even in water [20, 21]. On the basis of these preceding works, we show the formation of inverted pyramids in water on Ge(100) loaded with metallic particles in this study. We also discuss the mechanism of such formation on the basis of the relationship of redox potential as well as the catalytic role of metals. Then, we apply this metal-assisted chemical etching to the nanoscale patterning of Ge in water.
We used both p-type and n-type Ge(100) wafers with resistivities of 0.1 to 12 Ω cm and 0.1 to 0.5 Ω cm, respectively. The wafers were first rinsed with water for 1 min followed by treatment with an ultraviolet ozone generator for 15 min to remove organic contaminants. They were then immersed in a dilute HF solution (approximately 0.5%) for 1 min.
We conducted two experiments. One is the etch-pit formation by metallic particles in water. Here, we used both Ag and Pt nanoparticles. Ag nanoparticles with a diameter (φ) of approximately 20 nm were mainly used. To deposit these nanoparticles, Ge surfaces were dipped in HCl solution (10-3 M, 100 ml) with AgClO4 (10-4 M, 100 ml) for 5 min. After dipping, they were dried under N2 flow. We also used Pt nanoparticles of approximately 7 nm φ, which were synthesized in accordance with the literature . They were coated with a ligand (tetradecyltrimethylammonium) to avoid aggregation and were dispersed in water. This enabled us to obtain near monodispersed particles. The Ge samples were immersed in the resulting solution and dried under N2 flow. Then, the Ge surfaces loaded with the Pt particles were treated with the ultraviolet ozone generator for 6 h to remove the ligand bound to the Pt surfaces. In this experiment, we used two types of water with different dissolved oxygen concentrations, both of which were prepared from semiconductor-grade ultrapure water. The first type was water poured and stored in a perfluoroalkoxy (PFA) beaker. This water has a saturated dissolved-oxygen concentration of approximately 9 ppm. The second type contained a very low oxygen concentration of approximately 3 ppb. We, hereafter, call these two types of water ‘saturated dissolved-oxygen water’ (SOW) and ‘low dissolved-oxygen water’ (LOW), respectively. By putting a Ge sample in a PFA container connected directly to an ultrapure water line faucet, we were able to treat samples in LOW. The change in the structure of Ge surfaces loaded with metallic particles by immersion in water in the dark was analyzed by scanning electron microscopy (SEM, HITACHI S-4800, Hitachi Ltd., Tokyo, Japan).
The other experiment is the nanoscale machining of Ge surfaces by means of the catalytic activity of the metallic probes, using a commercial atomic force microscopy (AFM) system (SPA-400, Hitachi High-Tech Science Corporation, Tokyo, Japan) equipped with a liquid cell. It was carried out in the contact mode using two types of silicon cantilever probe from NANOWORLD (Neuchâtel, Switzerland): a bare Si cantilever and a cantilever coated with a 25-nm thick Pt/Ir layer (Pt 95%, Ir 5%). The resonant frequency and spring constant of both probes were 13 kHz and 0.2 N/m, respectively. An AFM head was covered with a box capable of shutting out external light. A conventional optical lever technique was used to detect the position of the cantilever. Ultrapure water exposed to air ambient and poured in the liquid cell contained approximately 9 ppm dissolved oxygen (SOW). We added ammonium sulfite monohydrate (JIS First Grade, NACALAI TESQUE Inc., Kyoto, Japan) to the water in the liquid cell. Performed according to the literature [23–25], this method enabled us to obtain ultralow dissolved-oxygen water with approximately 1 ppb oxygen (LOW).
Results and discussion
One may wonder why p-type Ge releases electrons to be oxidized as shown in Equation (2), because electrons are minority carriers for p-type samples. In the pore formation on Si by metal-assisted chemical etching in the dark, researchers mentioned that the conductivity type of the Si substrate (p-type or n-type) does not directly influence the morphology of pits formed [11, 12]. This is in agreement with our result in which a Ge surface with either conductivity type was preferentially etched around metallic particles in saturated dissolved-oxygen water in the dark. As described previously, we confirmed that similar etch pits to those on p-type wafers were formed on n-type ones. We presume that n-type Ge samples emit electrons in the conduction band (majority carriers), whereas p-type samples release them in the valence band.
In our experiments, most etch pits were pyramidal, one of which is shown in Figure 1c. The outermost Ge atoms on the (111) and (100) faces have three and two backbonds, respectively. This probably induces a (100) facet to dissolve faster in water than a (111) facet, forming a pyramidal etch pit on the Ge(100) surface, as schematically shown in Figure 2b. This anisotropic etching is very unique, because it has not been observed on Si(100) surfaces with metallic particles immersed in HF solution with oxidants. It should be noted that Figure 1e exhibits some ‘rhomboid’ and ‘rectangular’ pits together with ‘square’ pits. We believe that the square pits in Figure 1e represent pyramidal etch pits similar to those with Ag particles in Figure 1c. On the other hand, the reason for the formation of the rhomboid or rectangular pits in Figure 1e is not very clear at present. It is possible that the shape of a pit depends on that of a metallic particle. Although Ag particles (φ is approximately 20 nm) appear spherical in Figure 1a, the shape of the Pt particles (φ about 7 nm) is hard to determine from the SEM image in Figure 1d. To answer this question, etch pits should be formed with Ag and Pt particles of similar diameters and shapes, which remain to be tested.
As mentioned in the ‘Background’ section, Ge is not resistant to a variety of chemical solutions. Hence, wet-chemical treatments such as wet cleaning and lithography for Ge have not been well optimized compared with those for Si. The results in this study present several important messages for future semiconductor processes for Ge. First, residual metallic particles on Ge can increase surface microroughness even in water. For Ge surfaces, LOW should be used for rinsing to prevent unwanted pit formation. However, the metal-assisted chemical etching presented here can be a novel patterning technique for Ge surfaces in water, one example of which is demonstrated in Figures 3 and 5. This method is unique and promising because it requires no chemical solution that degrades Ge surfaces but is used in conventional wet-chemical treatments in Si processes.
We studied the metal-induced chemical etching of Ge(100) surfaces in water. We showed that noble metal particles such as Ag and Pt induce anisotropic etching. The mechanism of this formation is the catalytic activity of noble metals to reduce O2 molecules in water, which promotes preferential oxidation around metallic particles. Etch pits are formed to roughen the surface due to the soluble nature of GeO2. A key parameter for controlling the reaction is the dissolved oxygen concentration of water. We proposed that enhanced etching can be used positively toward the nanoscale patterning of Ge surfaces in water. This idea was confirmed by a set of AFM experiments in which a cantilever probe on Ge(100) was scanned in either water or air. We investigated the dependences of probe material, pressing force, and dissolved oxygen concentration on etched depth. We demonstrated the metal-assisted patterning of Ge surfaces in water, the mechanism of which is similar to that of the metal-induced pit formation mentioned above.
Atomic force microscopy
Low dissolved-oxygen water
Normal hydrogen electrode
Scanning electron microscopy
Saturated dissolved-oxygen water
The authors would like to thank Dr. Yusuke Yamada for the preparation of the Pt particles. The work was supported in part by a Grant-in-Aid for Young Scientists (A) (grant no.: 24686020) from Japan Society for the Promotion of Science. It was also supported in part by grants from Amano Institute of Technology and Ichijyu Industrial Science and Technology Promotion Foundation.
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