1. Laboratory for Micro‐ and Nanotechnology, Paul Scherrer Institut, CH‐5232 Villigen PSI, Switzerland
2. Present address: Mechanical Engineering Department, Louisiana State University Baton Rouge, LA 70803, USA.
3. Institute for Microtechnology, Rue A.‐L. Breguet 2, CH‐2000 Neuchâtel, Switzerland