Paper NS-TuM6
Atomic Resolution AFM with a Purely Electrical QPlus Sensor

Tuesday, October 16, 2007, 9:40 am, Room 616

In contrast to conventional optical detection AFM, self-sensing or purely electrical detection schemes have not yet been established as reliable and routine techniques for atomic resolution under UHV conditions. The QPlus sensor however, has successfully been used for LT STM with 5 K operation and is now intruduced for variable temperature operation with the VT STM. The QPlus sensor employs a quartz tuning fork for force detection in non-contact AFM operation mode. One prong of the tuning fork is fixed while the SPM probe tip is mounted to the second prong. It thus acts as a quartz lever transforming it's oscillation into an electrical signal as a result of the piezo-electric effect. The feedback signal is based on frequency shift originating from tip-sample force interaction. A dedicated pre-amplification technique ensures distance control based on the vibrational signal. The main motivation for the QPlus sensor is to improve AFM resolution for short range forces by the high spring constant of the sensor (approx. 1800 N/m, cantilever typ. a few ten N/m) and small oscillation amplitudes in the range of 1nm or below (cantilever typ. 10 nm), which more precisely match the range of the involved (chemical) forces. Optimal image performance was achieved using conventionally wet-chemically etched tungsten tips, glued onto the tuning fork. This allows for highest performance in simultaneous or alternative STM/STS operation. Measurements on Si(111) 7x7 show that tunnelling current and vibrational signal are clearly separated. In addition, benchmark measurements on NaCl with a typical corrugation of approx. 10pm prove that resolution on insulation samples is competitive to best cantilever based AFM results.