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Overview of KOH etching in microfabrication of MEMS structures E-mail
Written by Mitsuhiro Shikida   
Tuesday, 17 August 2010 00:00

guest article

Wet etching technologies can produce complicated Micro-Electro-Mechanical Systems (MEMS) structures onto a Si wafer with a batch process by combing a photolithography. Generally, MEMS structures are produced by the three steps, a thin film deposition, a patterning of the film defining the etching region by the photolithography, and the etching to create the 3D structure into Si wafer. The etching process, especially anisotropic wet one, becomes a key technology and requires know-how in the fabrication of MEMS. This is because the final MEMS structures are defined by the etching process.

 

1. Basic characteristics

 

1-1. Etching solution

Two alkaline solutions, potassium hydroxide (KOH) and tetramethl ammonium hydroxide (TMAH), are normally used as the etchants for the anisotropic wet process. The former has an excellent uniformity and reproducibility, but not-compatible with an electrical circuits. To overcome this drawback, TMAH used in the development process in photolithography became to be applied in the anisotropic wet etching. Generally, the usage of KOH becomes the best choice in the case of that the engineers simply produce the micro-structures onto the Si wafer. Therefore, the etching characteristics by KOH are focused in this article. The detail information of the differences between the two solutions is described in [1].

 

1-2. Etching mechanism and orientation dependency

The etching rate by KOH strongly depends on the crystallographic orientations of the Si material. That is the why we are able to produce the 3D micro-structure by the KOH anisotropic wet etching. The overall chemical etching reaction by alkaline solution is given by [2],

chemical reaction KOH

Silicon reacts with water and an OH- ion and produces hydroxide ion and hydrogen gas bubbles. The dependency of the etching rate on the crystallographic orientation said to be the differences of the number of dangling bond at the surfaces and of the atomic step structures [3-4]. However, the mechanism of the orientation dependency remains inconclusive, and still mystery in the present.

Up to now, a wagon-wheel masking pattern formed on the flat Si wafer [3] and a hemispherical Si specimen [5] were proposed to measure the etching rate for many crystallographic orientations to produce the sophisticated microstructures. Especially, the latter approach enables us drawing the etching rate diagram for all crystallographic orientations. Under the high concentrated KOH conditions normally used in MEMS fabrications, a {100} and a {111} planes are located at the local minimum and the minimum in the diagram, respectively. A {110} one placed on the near to the maximum. An etching simulator, which can predicts the etched shape by a certain masking pattern, was developed by applying the etching rates database [6, 7] and commercialized. By applying the simulator, MEMS engineers also can produces the rounded shape at the bottom of a cantilever by KOH etching [8].

 

1-3. Concentration and temperature dependencies

The rate-limiting factor in KOH etching said to be changed with the change of the concentration. That is, the chemical reaction happened on the Si surface limits the overall systems in the case of the high concentration. On the other hands, the system is dominated by the diffusion of reacting species and reaction products at low concentration. The border KOH concentration between the reaction- and diffusion-limiting processes is said to be around 25 wt.% generally.

The etching rate by KOH becomes the maximum around the border of 25 wt.%, and the etched surfaces become smooth with the increase of the KOH concentration. The uniformity and the reproducibility also increase with the increase of the concentration. High KOH concentrations more than 25 wt.% are normally applied in the fabrication of the MEMS structures. However, the throughput in the etching process will be down because of the decrease of the etching rate in the case of higher KOH concentration. Therefore, MEMS engineers have to decide the KOH concentration by considering the quality of the etched Si surface and the productivity of the etching process. The etching-rate- and the surface-roughness-dependency on the orientation by KOH etching are described in [5] and [9], respectively.

The anisotropic wet etching is obeyed by an Arrhenius equation [3]. It means the etching rate drastically increases with the maximally increase of the etching temperature. When the temperature is 130℃, the etching rate of both of {100} and {110} planes becomes 8.0 µm/min and 16.0 /min, respectively [10].

 

2. Application consideration

The etch pits sometimes are appeared on the etched Si surface when the high concentrated KOH solution is used in the fabrications. The morphologies of the pits happened on {100} and {111} planes become the shallow circular- and the triangular-shapes, respectively. The crystal defects induced by the stress in Si wafer are related with these etch pits formations [11, 12]. The etch pits normally deteriorate the quality of the etched surface and the final shapes, therefore, the mechanical stress, for example in a mechanical handling and a thermal process, acting the Si wafer have to be considered.

Om the other hands, the etched {100} surface was covered by micro-pyramids, sometimes called as hillocks, in the case of low KOH concentration. This phenomenon is related to the change of the etching rates ratio between the {100} and the {110} planes with the change of the KOH concentration. The ratio becomes to be less than 1.0 at low KOH concentration from 2.0 at high one. This means the reduction of the etching rate at the edge line in the micro-pyramids. Therefore, the micro-pyramid becomes to be appeared in this condition when the micro-masking is present on the Si surface.

The undercut mechanism for an island pattern on {100} wafer, and the effects of impurities and additives on the etching are also explained in [13-16].

 

References

[1] M. Shikida, et al., "Differences in anisotropic etching properties of KOH and TMAH", Sensors & Actuators: A80, 2, 179-188, (2000).

[2] H. Seidel, et al., "Anisotropic etching of crystalline silicon in alkaline solutions, 1. Orientation dependence and behavior of passivation layers, J. Electrochem. Soc., 137, 3612-3626, (1990).

[3] M. Elwenspoek, et al., "Silicon micromachining", Cambridge studies in semiconductor physics and microelectronic engineering, Cambridge University Press, (1999).

[4] Y. Gianchandani, et al., "Comprehensive microsystems", 1, 183-215, (2007).

[5] K. Sato, et al., "Characterization of orientation dependent etching properties of single-crystal silicon: Effect of KOH concentration", Sensors & Actuators: A64, 87-93, (1998).

[6] A. Koide, et al., "Simulation of two-dimensional etch profile of silicon during orientation-dependent anisotropic etching, Proc. IEEE MEMS Workshop, 216-220, (1991).

[7] K. Sato, et al., "Development of an orientation-dependent anisotropic etching simulation system MICROCAD", Electronics and Communications in Japan, Part 2, 83, 4, 13-22 (2000).

[8] A. Koide, et al., "A multi step anisotropic etching process for producing 3-D Si accelerometers", Tech. digest 11th Sensor symposium, 23-26, (1992).

[9] K. Sato, et al., "Roughening of single-crystal silicon surface etched by KOH water solution", Sensors & Actuators: A 73, 1-2, 122-130, (1999).

[10] H. Tanaka, et al., "Fast etching of silicon with a smooth surface in high temperature ranges near the boiling point of KOH solution", Sensors & Actuators: A114, 516-520, (2004).

[11] M. Shikida, at al, "Nano-mechanical method for seeding circular-shaped etch pits on (100) silicon", Sensors & Materials, 15, 1, 21-35, (2003)

[12] K. Sato, et al., "Difference in activated atomic steps on (111) silicon surface during KOH and TMAH etching", Sensors & Materials, 15, 2, 93-99, (2003)

[13] M. Shikida, et al., "A model explaining mask-corner undercut phenomena in anisotropic silicon etching: A saddle point in the etching-rate diagram", Sensors & Actuators: A97-98, 758-763, (2002).

[14] H. Tanaka, et al., "Characterization of anisotropic wet etching properties of single crystal silicon: Effects of ppb-level of Cu and Pb in KOH solution", Sensors & Actuators: A128, 125-131, (2006).

[15] H. Tanaka, et al., "Effect of magnesium in KOH solution on the anisotropic wet etching of silicon", Sensors & Actuators: A134, 465-470, (2007).

[16] P. Pal, et al., "Study of corner compensating structures and fabrication of various shapes of MEMS structures in pure and surfactant added TMAH", Sensors and Actuators: A, 154, 192-203, (2009).

 

Mitsuhiro Shikida has been an associate professor since 2004, and he is now working at the center for micro-nano mechatronics from 2009, at Nagoya University. His research interests include integration of micro-sensors and actuators for intelligent systems, micro-fabrication of 3D microstructures for medical applications, and micro-total analysis systems for biotechnologies. His personal website is http://www.mech.nagoya-u.ac.jp/mems/staff/shikida/shikida_index.html.

 

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