In order to obtain a sensor with force resolution better than 100 pN for biomolecular detection, U-shaped piezoresistive cantilevers made of crystalline silicon have been fabricated. The resistors have been defined by ionic implantation of As+, yielding a shallow and thin resistor and they have been patterned parallel to the (100) crystallographic direction, attaining a maximum for the piezoresistive factor. The sensitivity of the devices is characterized together as the force resolution, being both of them 60 mu V/pN and 65 pN, respectively, for a cantilever with a length of 250 mu m, a width of 8 mu m and a thickness of 340 nm. (C) 2008 Elsevier B.V. All rights reserved.