Smooth Etching of Si

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Ultra-Fast Anisotropic Silicon Etching with Resulting Mirror Surfaces in Ammonia Solutions

Transducers'01, Munich, Germany, June 10-14, 608-611

SUMMARY

The effect of an arsenic(III) salt addition on the anisotropic etching performance of aqueous ammonia solutions is presented. Here we demonstrate that the intake of small amounts of this salt considerably alters the etching behavior of <100>oriented silicon. During the etching process, an elementary As-film precipitates at the dissolving Si-surface and floats on it as etching proceeds. The real surfactant like behavior of the arsenic compound results in etch rates as high as 95 um/h at 70 oC. In addition, the etched surface appears mirror like to the naked eye and shows a rms-roughness below 5 nm as confirmed by AFM measurements. 

Keywords: Ammonia Etching, Surfactants, Surface Roughness 

INTRODUCTION 

Anisotropic etching remains a widely used tool for the manufacture of micromechanical devices. Etching in basic solutions such as KOH, TMAH, Hydrazine or EDP solutions all show similar anisotropic etching characteristics, but have specific drawbacks as well as advantages when compared to each other. KOH and other alkaline solutions cause metal ion contamination and show low passivation layer selectivities. The organic etchants overcome these problems but are usually toxic, unstable, show smaller etch rates, and are more difficult to handle. Considering these aspects, ammonium hydroxide water (AHW) solutions show distinct advantages. They combine high passivation layer selectivities, allow the use of metal masks including aluminum [1, 2] and are completely IC compatible, since they can easily be prepared in ultra clean quality. Compared to EDP or hydrazine etchants, AHW solutions are also less toxic. Thermal oxide etch rates are of the order of EDP solutions. 
Two significant drawbacks of AHW solutions are the low etch rates, and very rough resulting etching fronts. Here we present data demonstrating that the addition of arsenic (III) oxide (As2O3) to the etching solution has a significant impact on the silicon dissolution characteristics and results in unparalleled etching rates, as high as 95 um/h at 70 oC. Furthermore, the surface quality is considerably improved, outperforming all other conventional anisotropic etching mixtures in this regard. Previously we have shown [3] that arsenic and antimony compounds also influence the surface roughness in KOH solutions to a similar extent. Although the etch rates are, in contrast to AHW, not affected. When dissolved in water, the strong base KOH becomes almost completely dissociated whereas the weak base NH3 only produces minor quantities of OH-. Having demonstrated that the surfactants work in both of these extremes, it is reasonable to assume a similar result in the other commonly used etching media. The very high etch rates combined with the excellent surface finish and the effortless processing make the modified AHW solutions extremely attractive for micromachining purposes. 

Fig. 1: Light microscopy images and profilometer scans of patterned silicon samples etched in concentrated AHW solution at 70 oC. a) Without additional As2O3 . b) With the addition of 10 x 10-3 mol/L As2O3.

Fig. 2: AFM topography images of a <100>-silicon surface etched at 70 oC in a fresh 6 M AHWsolution (a)) and a used 6 M AHW solution (b)). 

Fig. 3: Si-<100>-etch rate in concentrated AHW at 70 oC as a function of As(III) concentration. 

Fig. 4: a) AFM topography image of a <100>-silicon surface etched at 70 oC in a 6 M AHW solution with the addition of 10 x 10-3 mol/L As2O3 . b) Surface profile obtained in a 40% wt. KOH solution at the same temperature. 

REFERENCES

1. U. Schnakenberg, W. Benecke, and B. Lochel, NH4OH-Based Etchants for Silicon Micromachining. Sensors and Actuators, A21-A23, 1031, 1990. 
2. U. Schnakenberg, W. Benecke, B. Lochel, S. Ullerich, and P. Lange, NH4OH-Based Etchants for Silicon Micromachining: Influence of Additives and Stability of Passivation Layers. Sensors and Actuators A, 25-27, 1, 1991. 
3. C. Mihalcea, A. Holz, M. Kuwahara, J. Tominaga, E. Oesterschulze, and N. Atoda, Improved Anisotropic Deep Etching in KOH-Solutions to Fabricate Highly Specular Surfaces. To appear in: Microelectronics Engineering. 
4. H. Nojiri and M. Uchiyama, Bias-Dependent Etching of Silicon in Aqueous Ammonia. Sensors and Actuators, A34, 167, 1992. 
5. E. D. Palik and O. J. Glembocki, Etching Roughness for (100) Silicon Surfaces in Aqueous KOH. J. Appl. Phys., 70, 6, 3291, 1991. 
6. M. Elwenspoek and H. Jansen, Silicon Micromachining. Cambridge University Press, 1998.