Japan. Keywords: wet etching, nanoscale region, dhf (dilute hydrofluoric acid solution), electric double layer, solid-liquid interface

Transcription

1 Solid State Phenomena Online: ISSN: , Vol. 29, pp 58 doi:.428/ 25 Trans Tech Publications, Switzerland Impact of electrostatic effects on wet etching phenomenon in nanoscale region Atsushi Okuyama,a, Suguru Saito,b, Yoshiya Hagimoto 2,c, Kenji Nishi 3,d, Ayuta Suzuki 4,e, Takayuki Toshima 3,f and Hayato Iwamoto 2,g Sony Semiconductor Corporation, 4 Haramizu, Kikuyomachi, Kikuchigun, Kumamoto, 8692 Japan 2 Sony Corporation, 44 Asahicho, Atsugishi, Kanagawa, 2434 Japan 3 Tokyo Electron Kyushu Limited, Fukuhara, Koshishi, Kumamoto, 8696 Japan 4 Tokyo Electron Limited, 53 Akasaka Minatoku, Tokyo, Japan a Atsushi.Okuyama@jp.sony.com, b Suguru.Saito@jp.sony.com, c Yoshiya.Hagimoto@jp.sony.com, d kenji.nishi@tel.com, e ayuta.suzuki@tel.com, f takayuki.toshima@tel.com, g Hayato.Iwamoto@jp.sony.co.jp Keywords: wet etching, nanoscale region, dhf (dilute hydrofluoric acid solution), electric double layer, solidliquid interface Introduction The microminiaturization of semiconductor devices has made it necessary to control the wet etching process on the nanometer order. It is therefore extremely important to understand wet etching reactions in the nanoscale region of solidliquid interfaces, in order to assist in optimizing process conditions to satisfy the severe demand for semiconductor devices. Simulations performed to analyze the behavior of liquid molecules in the nanoscale region have been reported [], but there have been few reports of detailed experimental results. We here report detailed experimental results on the wet etching behavior of SiO 2 film in the nanoscale region between Si materials. Wet etching in nanoscale region Experimental procedure. A thermal silicon dioxide (thickness: 5,, 2 nm) film and a polycrystalline silicon film were formed on a Si substrate. The mask pattern of the polycrystalline silicon film was made by photolithography and dry etching on each sample. These samples were etched with dilute hydrofluoric acid solution (dhf,.5 wt%) at room temperature using a single wafer etching system. Then, the etching amount in the nanoscale region was measured using a scanning electron microscope (SEM) (Figure ). Figure : Measurement point of etching amount on SEM image. Results and discussion. Figure 2 (a) shows the etching rate dependence on SiO 2 thickness in the nanoscale region. Figure 2 (b) shows the relationship between the etching rate of bulk SiO 2 and that of SiO 2 in the nanoscale region for each SiO 2 film thickness (5,, 2 nm); the latter rate decreases as the film thickness decreases, and at 5 nm thickness becomes less than half the bulk SiO 2 rate. This is one possible reason that the replacement performance of the etching solution in the nanoscale region becomes worse. However, it is incompatible with the fact that the etching rate does not decrease with an increased etching amount. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA5/3/6,4:34:29)

2 Etching amount in nanoscale region (nm) Etching rate in nanoscale region (nm/min) 6 Ultra Clean Processing of Semiconductor Surfaces XII 2 8 5nm nm 2nm Etching rate of bulk SiO Etching amount of bulk SiO 2 (nm) Film thickness of SiO 2 (nm) (a) (b) Figure 2: Etching rate in nanoscale region. Another possible reason is that the electric potential of the wall near the SiO 2 film affects the etching rate. It is known that an electric double layer that does not satisfy electroneutrality is formed in the interfacial layer between a solid and a liquid. This is because the layer has insufficient ions with the same polarity as the solid surface zeta potential. The thickness of the electric double layer is expressed as λ D = (εε k B T/2C bulk N A z 2 e 2 ), () whereεis the dielectric constant of the liquid,ε is the dielectric constant in a vacuum, k B is Boltzmann s constant, T is the absolute temperature, C bulk is the bulk ion concentration, N A is Avogadro s constant, z is the ion valence, and e is elementary electric charge [2]. It is known that Si in a dhf solution, which is acidic, has negative electric potential [3], and that it has the same electric potential as HF 2, which is the etchant for the SiO 2. Therefore, the etchant is insufficient for use in an electric double layer (Figure 3). Figure 3: Schematic of electric double layer. Numerical simulation Simulation model. The finite element method was used to perform twodimensional numerical analysis. Figure 4 shows the simulation model. The dhf density ranges of.5 wt% and 2.5 wt% and the SiO 2 film thickness ranges from.56 nm. Considering the chemical species HF, HF 2, H, and F, we use the following formulas to calculate chemical equilibrium, where K and K 2 are equilibrium constants at 25 degrees Celsius [4]. HF H F, K = [H ][F ]/[HF] =.3 3 [mol/l] (2) HF 2 HF F, K 2 = [HF][F ]/[HF 2 ] =.4 [mol/l] (3) The chemical species transportation in the standstill solution, which is considered an electric field, is expressed by the diffusion equation for steady as

3 Y Position [nm] Y position[nm] Solid State Phenomena Vol. 29 7, (4) where c i is the concentration [mol/m 3 ], D i is the diffusion coefficient [m 2 /s], z i is the valence of ions, μ i is the mobility [s mol/kg], F is Faraday s constant [C/mol], and V is the electric potential [V]. The electric potential is expressed by Poisson s equation as, (5) whereρis the charge density [C/m 3 ],ε is the dielectric constant in a vacuum [F/m], andε r is the dielectric constant. A 2 mv electric potential was set on the Si and polysi surfaces as a boundary condition, and a mv electric potential and the bulk chemical density were set in the border of the upper domain. Figure 4: Simulation model. Electric potential distribution. Figure 5 and 6 show the calculation results of the electric potential distribution and the schematics of the electric double layer in the nanoscale region (Figure 5: SiO 2 = nm, Figure 6: SiO 2 =5 nm). Electric potential decreases under the influence of an electric double layer near the Si surface. The portion of the nanoscale region the electric double layer occupies increases so that SiO 2 becomes thin. 5 nm Electric potential[mv] nm Figure 5: Electric potential distribution (SiO 2 : nm). PolySi Si nm Electric potential[mv] 5nm PolySi Si Figure 6: Distribution of electric potential (SiO 2 : 5 nm).

4 Concentration of HF 2 in nanoscale region /Concentration of HF 2 in bulk 8 Ultra Clean Processing of Semiconductor Surfaces XII Comparison of simulation results for etchant concentration and experimental results for etching rate The electric double layer thickness changes with ion concentration as shown in equation (). We simulated the dependency of the mean etchant concentration in the nanoscale region on dhf concentration and SiO 2 film thickness. Figure 7 shows the results; the concentration decreases as the SiO 2 film becomes thinner. This tendency is weak when the dhf density is high because a high dhf density causes the electric double layer to become thinner. We also conducted an experiment on the dependency of the etching rate in the nanoscale region on dhf concentration and SiO 2 film thickness. Figure 8 shows the results; the same tendency as that for the simulated mean etchant concentration is observed. These results lead us to consider that the decrease in the etching rate in the nanoscale region is caused by the decrease in the etchant density due to the electrical influence from the wall % dhf 2.5% dhf Film thickness of SiO2 (nm) Etching rate in nanoscale region /Etching rate of bulk SiO % dhf 2.5% dhf Film thickness of SiO2 (nm) Figure 7: Etchant concentration (Simulation). Figure 8: Etching rate (Experiment). Summary We performed experiments on wet etching behavior in the nanoscale region between Si materials. It was found that under certain conditions the etching rate decreases so that the etched region becomes small, and it was assumed that this was due to the influence of an electric double layer formed in the vicinity of the solidliquid interface. We simulated the electric potential distribution and confirmed that the electric potential near the Si changed. Furthermore, calculated results for the mean etchant density and experimental results for the etching rate in the nanoscale region showed similar tendencies in terms of dependence on SiO 2 film thickness and dhf density. These results strongly support the hypothesis that electrical influence from the wall causes a decline in the etching rate in the nanoscale region. References [] S. Kosaka, G. Kikugawa, T. Nakano, and T. Ohara: International Forum on Heat Transfer, [2] P. Pungetmongkol, R. Hatsuki, and T.Yamamoto: The 7th International Conference on SolidState Sensors, Actuators and Microsystems & Eurosensors XXVII, 2326, 23. [3] M. Itano, T. Kezuka, M. Ishii, T. Unemoto, M. Kubo, and T. Ohmi: J Electrochem Soc 42, 97978, 995. [4] R. E. Mesmer, et al., Fluoride Complexes of Beryllium(2) in Aqueous Media, Inorg. Chem., Vol.8, No.3, 969.

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