An SU-8-silicon(100)-SU-8 flexible composite sandwich structure is studied. Besides preventing corrosion to the thin silicon membrane, SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS finite element simulation. Using a plasma enhanced chemical vapor deposited SiO2/Si3N4 composite film as an etch mask on the polished side, a 4″ (100) silicon wafer was thinned to 26 µm without rupture in a 30 wt% KOH solution. The wafer was coated on both sides with 20 µm of SU-8 photoresist and then bent over cylinders with various diameters, to flex the sandwich in a controlled manner. The determined minimal bending radius of the fabricated thin silicon-based sandwich structure is approximately 3.5 mm. The fabrication of this sandwich structure is compatible with conventional microelectronic fabrication processing. Thus, it allows one to thin fully fabricated devices in a post-fabrication process to make high-performance flexible electronics.