Three-dimensional force sensor by novel alkaline etching technique

Silicon based three-dimensional (3D) force sensor was designed and fabricated for detection of normal and shear forces. The sensor contains a rectangular rod emerging out of the centre of the membrane. The technology involves ion-implanted piezoresistors formation on the backside and a novel anisotropic etching on the front side of an n-type wafer. The two-component, two-step anisotropic etching process forms the rectangular side-wall rod and the membrane simultaneously, using a double layer silicon oxide and nitride mask. A mask-less alkaline etching in the second step results in the central rod protruding over the chip surface while leaving a frame of reduced thickness around the membrane providing mechanical stability. The cavity underneath the membrane was formed by bonding, facilitating also the signal read-out from the ion-implanted strain gauge resistors at the bottom (backside) of the membrane. The results of the mechanical test of the realised 3D Si force sensors are in good agreement with the theoretical predictions.