Three-Dimensional Sacrificial Etching

In MEMS fabrication micro-mechanical components have to be partially released from a substrate. Selectively etching away sacrificial layers, such that a free standing structure remains, is a widely used technique for this purpose. Free standing structures allow MEMS devices to induce or to sense mechanical movements or vibrations. During sacrificial etching lower etch rates than the blanket ones are observed. This reduction can be explained by additional factors like the transport of the etch medium and its etch reactants via the relatively narrow (in relation to the etch depth) already etched channel under the free standing structure. Sacrificial etching is mainly controlled by process parameters like the etch agent concentration, chamber temperature, and pressure. Furthermore, local geometrical features and the nature of chemical reactions are responsible for different etch speeds at material boundaries and, therefore, they influence the propagation of the etch front. In order to analyze these effects we have developed a three-dimensional topography simulation tool and the required models for the etch rates. 1 Modeling During etching the surface of the etched material forms an interface to the chemical solution, where the reactions take place [1]. Depending on involved materials and acid concentrations, locally varying etch rates determine the evolution of the etch front. These etch rates are interpreted as the speed function F of a three-dimensional level-set calculation [2]: F |∇Φ(~x,t)| = − ∂Φ(~x,t) ∂ t , (1) where Φ(~x,t) represents the level-set function. This differential equation delivers the evolving boundary, which is the etch front, for all points ~x that satisfy Φ(~x,t) = 0 at time t [3]. 2 Material Transport In the most simple case, the transport of the etch agent can be described by a diffusion process governed by the etchant concentration in the reactor. With the assumption that the system reacts in a quasi-static way, the transient processes ∂/∂ t can be neglected, which results in the Laplace equation: △c(~x) = 0 for~x inside the etchant domain, (2) SIMULATION OF SEMICONDUCTOR PROCESSES AND DEVICES Vol. 12 Edited by T. Grasser and S. Selberherr September 2007 433