FIB implantation and anisotropic etching for 3D device fabrication

A modern technique to fabricate three dimensional micromechanical structures is the doping of high concentrations of p+-type ions with the focused ion beam. In this case, p+-type ions, e.g. gallium, act as an etch stop. A subsequent anisotropic and selective wet chemical etching makes it possible to fabricate 3D structures. Micromechanical test structures were implanted into (100) oriented silicon wafers at room temperature with a 35 keV Ga+ beam, using the Rossendorf focused ion beam equipment IMSA-100. The ion beam current was 1.5 nA, and the implantation dose was varied between 1*1014 cm-2 and 3*1016 cm-2. In order to obtain different structure shapes, which depend on the orientation of the crystal planes at the edges and corners, two orientations for the designed structures were chosen. One was orientated along the (110) direction in the wafer surface, a second one at an angle of 45o. The subsequent anisotropic wet chemical etching was carried out in an etchant of KOH / H2O etch solution of 30 % concentration in the temperature range from 40oC to 80oC for a time of some tens to about a few minutes. The Ga+ FIB patterned and wet chemical etched test structures were characterized by scanning electron microscopy. A possible application of these structures are in the field of special sensors in micro system technique. An important aspect is the complete compatibility to microelectronic fabrication technology.
The left picture is an SEM image of quadratic line structures demonstrating the undercut at convex corners. The right picture shows an SEM image of a free standing grid pattern. The grid is oriented under 45o to the main flat of the (100) Si wafer.

B. Schmidt, L. Bischoff and J. Teichert,
Writing FIB Implantation and Subsequent Anisotropic Wet Chemical Etching for Fabrication of 3D Structures in Silicon
Sensors and Actuators A61 (1997)  369-373
Proc. of the 10th European Conference on Solid-State Transducers, September 8 - 12, 1996, Leuven, Belgium 

 
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