VPE - Technology

Technology: vapor phase etching

First experiments on vapor phase etching were carried out by Holmes & Snell in 1966 [1]. They observed that silicon dioxide on a wafer is etched with a comparable etch rate even when the wafer is not in the etch bath but close to. Helms & Deal established that the role of water is to provide a condensed solvent medium for the HF on the surface. Offenberg et al. [2] proposed a two step reaction where first the oxide surface is opened by formation of silanol groups by adsorbed water (H2O ) . Subsequently silanol groups are attacked by the HF:

SiO2 + 2H2O -> Si(OH)4

Si(OH)4 + 4HF -> SiF4 + 4H2O

The above formula shows that water acts as initiator of the etching process as well as reactant. This fact suggests that the etching process can be temperature controlled in order to maintain in equilibrium the amount of water needed to initiate the process and the amount of reactant water . In idonus' Vapor Phase Etcher this equilibrium is achieved by heating the wafer. The water film on the wafer is evaporated at moderate temperatures. The etch rate decreases with increasing temperature and stops completely at temperatures above 50°C.
Sticking free MEMS release is achieved at etch rates around 5 µm/h.

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[1] P. J. Holmes and J. E. Snell, Microelectronics and Reliability (Pergamon, New York 1966), Vol. 5, p. 337.

[2] M. Offenberg, B. Elsner, and F. Lärmer, "Vapour HF etching for sacrificial oxide removal in surface micromachining", Extended Abstracts: Electrochem. Soc. Fall Meeting (Miami Beach) vol 94-2, pp 1056-7, 1994.