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A new method to improve the thickness uniformity of a silicon-on-insulator layer in a defined area by photoetching with N -fluoropyridinium salts using a projector was proposed. The method involved initial calculation of the red–green–blue tone distribution from the top silicon layer thickness distribution to be etched using the relationship between the etching depth and red–green–blue tone in a projector. Photoetching was then conducted by projection of an image with the required tone distribution onto a silicon layer surface coated with N -fluoropyridinium salts. The uniformity of silicon layers with a thickness of less than 10 nm was improved in an area of approximately 20 × 20 mm by photoetching after prior thinning by thermal oxidation and wet etching. The developed method enables the thickness uniformity in a defined area to be improved while avoiding exfoliation of the silicon layer or the formation of silicon islands in subsequent thermal treatment. In addition, this method has the potential to be cost-effective because it uses a projector.

Control of the microroughness of Ge surfaces is required to realize field-effect transistors with high performances. We propose a novel surface-flattening process for Ge that involves the preferential transformation of surface protrusions on Ge into soluble GeO2 with the help of a catalyst in water. To carry out this process, we developed a setup comprising a catalyst plate covered with a Pt film in contact with a Ge surface in saturated O2-dissolved water. The role of the metallic film is to enhance the oxygen reduction reaction in water that accompanies the oxidation of protrusions or microbumps on a Ge surface. After presenting the fundamental etching properties of a Ge surface treated with this metal-assisted chemical etching method, we demonstrate that our water-based process creates a flattened Ge surface that has few protrusions with a lateral size on the order of 10 nm.

Metal-assisted chemical etching is a simple and low-cost etching process for semiconductor surfaces. There have been many reports on Si surfaces loaded with noble metals in HF solutions with oxidants such as H2O2.[1,2] Noble metals catalyze the reduction of the oxidant (H2O2), resulting in enhanced oxidation of the Si surface around the loaded metals. Because the oxide is immediately dissolved in the HF solution, selective etching of the Si surface occurs. This etching mode is used to form a variety of three-dimensional nanostructures not only on Si but also on other semiconductor surfaces. We have applied this etching mode to the machining of a Ge surface in O2-containing water. So far, we revealed fundamental etching properties such as pore formation and patterning in this system with noble metals (Pt and Ag),[3] and recently, we demonstrated Pt-assisted chemical flattening.[4,5] In this scheme, a catalyst plate comprising a soft elastomer coated with a sputtered Pt film, and a Ge wafer were brought into contact and rotated independently in the same plane in water. The processed Ge surface includes few protrusions with a lateral size on the order of 10 nm, which is probably caused by the selective removal of protrusions from the Ge surface by the catalytic activity of Pt. However, a problem in this system is the use of noble metals as catalysts. After metal-assisted chemical etching, residual metals on a semiconductor surface have to be removed. For example, aqua regia is effective for dissolving Pt. However, such a strong oxidative solution causes severe damage to a Ge surface. To solve this issue, graphene can be used as a substitute for noble metals to achieve catalyst-assisted chemical etching.[6] In this talk, we discuss the etching properties of a p-type Ge(100) surface loaded with reduced graphene oxide (RGO) in O2-containing water. In order to obtain the RGO, a graphene oxide (GO) ink, used as a starting material, was either heated at 900°C in Ar ambient for 10 min or immersed in a solution comprising the GO ink, hydrazine and N,N-dimethylformamide.[7] Then we deposited the obtained RGO on a Ge surface in the form of aggregated particles or dispersed flakes. After immersing the samples into water exposed to air, we found that the Ge surface was preferentially etched around the loaded RGO. The etching rate as well as the etched morphology was revealed by atomic force microscopy observations, and the etching mechanism is proposed. These findings show the possibility of using RGO as a catalyst to enhance the chemical etching of a Ge surface in water. [1] Z. Huang, N. Geyer, P. Werner, J. de Boor and J. Gösele, Adv. Mat., vol. 23, no. 2, pp. 285-308 (2011). [2] X. Li, Current Opinion in Solid State and Materials Science, vol. 16, pp. 71-81 (2012). [3] T. Kawase, A. Mura, K. Nishitani, Y. Kawai, K. Kawai, J. Uchikoshi, M. Morita and K. Arima, J. Appl. Phys., vol. 111, no. 12, pp. 126102 1-3 (2012). [4] T. Kawase, Y. Saito, A. Mura, T. Okamoto, K. Kawai, Y. Sano, M. Morita, K. Yamauchi, and K. Arima, ChemElectroChem, vol. 2, no. 11, pp. 1656-1659 (2015). [5] T. Kawase, A. Mura, Y. Saito, T. Okamoto, K. Kawai, Y. Sano, K. Yamauchi, M. Morita, and K. Arima, ECS Transactions, vol. 75, no. 1, pp. 107-112 (2016). [6] J. Kim, D.H. Lee, J.H. Kim and S.-H. Choi, ACS Appl. Mat. and Inter., vol. 7, no. 43, pp. 24242-24246 (2015). [7] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni and R.S. Ruoff, Nano Lett., vol. 9, no. 4, pp. 1593-1597 (2009).

The effect of Cr, Mn, Fe, Ni, Cu, Zn, Pd, Ag, Pt, or Au metal particles on the rate of Si etching using N-fluoropyridinium salts was examined. The average etching depth increased when N-fluoropyridinium salts were mixed with Cu particles. The activation energy determined from the temperature dependence of the average etching rate for the Cu particles is lower than that without a metal. The Cu particles greatly improve the etching of Si with the salts compared with the other metals. The magnitude of the effect of the Cu particles on the rate of Si etching with the salts is larger than that of the Mn, Fe, Ni, or Zn particles. This can be explained by the order of the relative stabilities of the complexes formed by bivalent ions of transition metals. The etching method involves the application of easy-to-handle salts containing Cu particles as etchants on the Si surface and has the potential to be a simple and less expensive form of Si etching.

Mori D., Oka H., Hosoi T., et al. Comparative study of GeO2/Ge and SiO2/Si structures on anomalous charging of oxide films upon water adsorption revealed by ambient-pressure X-ray photoelectron spectroscopy. Journal of Applied Physics, 120, 9, 095306. https://doi.org/10.1063/1.4962202., The energy difference between the oxide and bulk peaks in X-ray photoelectron spectroscopy (XPS) spectra was investigated for both GeO2/Ge and SiO2/Si structures with thickness-controlled water films. This was achieved by obtaining XPS spectra at various values of relative humidity (RH) of up to ∼15%. The increase in the energy shift is more significant for thermal GeO2 on Ge than for thermal SiO2 on Si above ∼10-4% RH, which is due to the larger amount of water molecules that infiltrate into the GeO2 film to form hydroxyls. Analyzing the origins of this energy shift, we propose that the positive charging of a partially hydroxylated GeO2 film, which is unrelated to X-ray irradiation, causes the larger energy shift for GeO2/Ge than for SiO2/Si. A possible microscopic mechanism of this intrinsic positive charging is the emission of electrons from adsorbed water species in the suboxide layer of the GeO2 film to the Ge bulk, leaving immobile cations or positively charged states in the oxide. This may be related to the reported negative shift of flat band voltages in metal-oxide-semiconductor diodes with an air-exposed GeO2 layer.

Si sublimation from a silicon carbide (SiC) surface at elevated temperatures is a convenient and simple method of forming graphene [1]. The increase in carbon concentration at the monolayer scale on SiC prior to graphene growth is the key to obtaining graphene with reduced pit density. There have been some attempts to achieve this, one example of which is the use of vacuum evaporation from a solid carbon source [2, 3]. We have proposed a novel chemical route to enrich the carbon concentration on SiC. Namely, we have found that aggregated carbon compounds with a few monolayers are formed when an initial hexagonal SiC surface is treated by plasma oxidation at near room temperature and then the oxide layer (SiO2) with a thickness of ~10 nm is stripped off by HF etching [4, 5]. No additional carbon clusters were formed in the conventional thermal oxidation of SiC at 1000°C. Neither the composition of the aggregated compounds nor the mechanism of the generation of such carbon species by the plasma-assisted procedure is clear, however. In this study, we reveal that, after the plasma oxidation followed by wet etching, the SiC surface is more hydrophobic than the initial SiC surface terminated by hydroxyls. Taking the electronegativities of Si, C and O atoms into account, this result indicates the formation of C-O and C-C, which is confirmed by the chemical-shift analyses of O1s and C1s spectra taken by X-ray photoelectron spectroscopy. In thermal oxidation, it is widely accepted that the reaction (2SiC+3O2 -> 2SiO2+2CO) forms a SiO2 layer whereas CO molecules diffuse into the oxide and are finally emitted in the gas phase. However, it has also been reported that CO molecules can diffuse a high-quality SiO2 film at temperatures higher than 900°C [6]. It is reasonable to suppose that, in the case of plasma oxidation at near room temperature, CO molecules reside at the SiO2/SiC interface and form carbon compounds. In addition, a consideration based on thermodynamics implies the direct formation of solid carbon clusters during oxidation at low temperatures by the reaction SiC+O2 -> SiO2+C(s) [7]. These growth kinetics of SiO2 on SiC well explain the obtained results described in the previous paragraph. Such a carbon residue at the SiO2/SiC interface can degrade the performance of metal-oxide-semiconductor (SiC) devices. But it may serve as an additional carbon source for epitaxial graphene growth on SiC. [1] C. Berger et al., J. Phys. Chem. B, 108, 19912 (2004). [2] A. Al-Temimy et al., Appl. Phys. Lett., 95, 231907 (2009). [3] J. Park et al., Adv. Mater., 22, 4140 (2010). [4] N. Saito, K. Arima et al., Carbon, 80, 440-445 (2014). [5] N. Saito, K. Arima et al., ECS Transactions, 64, 17, 23 (2014). [6] O.H. Krafcsik et al., Jpn. J. Appl. Phys. 40, 2197 (2001). [7] K. Kita et al., ECS Transactions, 64, 8, 23 (2014).

Recently, metal-assisted chemical etching has attracted a great deal of attention because it is a simple and low-cost process for the fabrication of nanostructures on a semiconductor surface, mainly Si [1, 2]. This electroless etching is made possible by using noble metals, such as Pt and Ag, to catalyze the reduction of a chemical oxidant, such as H2O2, in the presence of HF. Namely, the reduction is accompanied by the selective oxidation around the loaded metals followed by the dissolution of oxide (SiO2) in HF solution. By controlling the etching conditions, a vast variety of nanostructures from porous structures to nanowire arrays can be fabricated in a chemical lab without the need for expensive equipment. Ge is regarded as a promising channel material in future electronics. It is necessary to develop and/or optimize wet processes, such as lithography, planarization and cleaning, for Ge. Ge oxide (GeO2) is soluble in water, unlike SiO2. Also, O2 molecules dissolved in water are oxidants. These chemical properties enable us to develop a novel process for a Ge surface using metal-assisted chemical etching in water. In this study, we immersed a Ge(100) surface loaded with Pt or Ag particles with the size of 10 nm order into O2-containing water and observed the formation of inverted pyramidal etch pits around the particles. This is caused by the catalytic effect of metals whereby O2 molecules in water are reduced, resulting in an enhanced oxidation of a Ge surface to form soluble GeO2. We confirmed that the O2 concentration in water is a key in etch rate [3, 4]. Then, we tested nanoscale patterning of a Ge surface by metal-assisted etching in O2-containing water. This was achieved by scanning a metal-coated cantilever on a Ge surface in the contact mode in an atomic force microscopy setup. We found that the Ge surface scanned by the cantilever was selectively removed, and the etched depths depended on experimental conditions such as the O2 concentration in water, the pressing force of the cantilever to the Ge surface, and the conduction type (n-type or p-type) of Ge. These findings indicate that the mechanism of inducing enhanced etching along the trajectory of the cantilever probe is a catalytic chemical effect rather than a mechanical one [4]. Finally, we used this metal-assisted chemical etching to improve the microroughness of a Ge surface. In this scheme, a thin film of noble metal (Pt) was deposited on a soft elastomer pad. This Pt surface was brought into contact with a Ge wafer surface by applying 0.01 MPa in O2-containing water. Both the pad coated with the metal and the Ge wafer were rotated independently in the same plane. Consequently, the Ge surface was flattened. The flattening mechanism is as follows. First, protrusions on the Ge surface have a higher probability of coming into contact with the metal surface than do grooves. These protrusions were selectively oxidized by the catalytic effect of the metallic film in water. After the oxidized protrusions were dissolved in water promptly, a flattened surface was created [5]. [1] X. Li et al., Appl. Phys. Lett. 77, 2572 (2000). [2] Z.P. Huang et al., Adv. Mater. 23, 285 (2011). [3] T. Kawase et al., J. Appl. Phys., 111, 126102 (2012). [4] T. Kawase et al., Nanoscale Res. Lett., 8, 151 (2013). [5] T. Kawase et al., ChemElectroChem, 2, 1656 (2015).

This paper proposes an efficient control method for the large-scale integration of microvalves in microfluidic systems. The proposed method can control 2n individual microvalves with 2n + 2 control lines (where n is an integer). The on-chip valves are closed by applying pressure to a control line, similar to conventional pneumatic microvalves. Another control line closes gate valves between the control line to the on-chip valves and the on-chip valves themselves, to preserve the state of the on-chip valves. The remaining control lines select an activated gate valve. While the addressed gate valve is selected by the other control lines, the corresponding on-chip valve is actuated by applying input pressure to the control line to the on-chip valves. Using this method would substantially reduce the number of world-to-chip connectors and off-chip valve controllers. Experiments conducted using a fabricated 28 microvalve array device, comprising 256 individual on-chip valves controlled with 18 (2 × 8 + 2) control lines, yielded switching speeds for the selected on-chip valve under 90 ms.