FABRICATION OF A NEW HIGHLY-SYMMETRICAL, INPLANE ACCELEROMETER STRUCTURE BY ANISOTROPIC ETCHING OF (100)SILICON

In this paper, we present a silicon bulk-microfabrication method which helps to overcome simultaneously several limitations of multi-axis mi cro-accelerometers. The method demonstrates an orginal solution to the building of a symmetrical structure by using double-side wet etching. This is a low-cost alternative to existing techniques for the fabrica tion of highly-symmetrical, single crystal silicon structures. The pro posed approach provides low mechanical cross-sensitivities as well as the possibility of a batch fabrication process of the whole three-dime nsional device without loss of accuracy due to assembly operation. For the fabrication of thin suspended beams with vertical sidewalls, a no n-conventional alignment of +/-45 degrees from the [110] wafer flat wa s used. This alignment allows one to fabricate two perpendicular devic es on one wafer in the same etching step. The etching was performed wi th a simple standard wet etching process in a KOH solution. A number o f structures were fabricated to demonstrate the feasibility of this me thod. Aspect ratios (beam height over beam thickness) of over 35 were easily achieved. Undercut directions were determined and design rules for the mask layout were established. To describe the mechanical behav iour of the fabricated structure, an analytical model was implemented and a finite-element simulation was performed. First measurements of t he seismic mass displacement were performed with an optical comparator , and they agree with theoretically obtained results. The new design o ffers the possibility of a two-axis accelerometer system on one wafer, consisting of two sensor elements rotated by 90 degrees. A three-axis monolithic accelerometer system with intrinsic perpendicular alignmen t due to the rectangular symmetry of the (100) planes can be realized, by including a third sensor element sensitive to vertical acceleratio ns.