Piezoresistive cantilevers with “on board” thermomechanical actuator (so called “Active Cantilever”)
Components of piezo-resistive self-actuated cantilever
The first pioneer work toward autonomic cantilevers has already been accomplished in 1998 at the group around Prof. Rangelow at University of Ilmenau, Germany, using thermo-mechanically driven cantilevers with an integrated resistive heater. The group achieved a significant improvement in the performance of the piezoresistive cantilever with respect to deflection sensitivity using a Wheatstone bridge circuit, with temperature drift compensation. These compact, self-actuating and self-sensing cantilevers (called “active cantilever”) are also the basis for parallel probes with arrays already realized.
The challenge in the realization of next generation atomic force microscopes is to maintain atomic resolution while scanning at speeds needed for high-throughput imaging and metrology. This requires precise and faster detection of the interaction between the sample and the tip. As such, cantilevers must be small and soft with high signal-to-noise ratios (SNR).
A common approach for probe measurement is based on optical read-out. However, the shortcomings of optical detection, associated with diffraction and reduced sensitivity, necessitate the development of novel measurement methods to quantify probe–sample interactions. The optical read-out components tend to make the whole system bulky and expensive, and they limit the dimensional downscaling of conventional scanning probes. In particular, a minimum reflective area is required on the cantilever beam backside, in the range of 3µm in high-end AFM systems facilitating a small laser spot. Hence, the direct integration of readout and actuation onto the cantilever is essential for further miniaturization.
The active cantilever can be used in contact on or non-contact (dynamic) modes of atomic-force microscopy, offering new possibilities for imaging, because of the availability of the various vibrational modes of the probes. In this case, the cantilever is oscillating at its resonant frequency and its amplitude damping or frequency shift are used for tip-sample distance control and topography tracking.
The self-actuated piezoresistive cantilevers allow for easier system integration and significant reduction in the systems weight. Microscopes incorporating them offer a better controllability and significantly better imaging throughput. Furthermore, in contrast to conventional optical read-out systems, these systems offer the potential for full lithographic and metrological automation. For Field Emission-Scanning Probe Lithography, the implemented active probes enable an in situ inspection capability, a quantitative mapping at unprecedented resolution, as well as an integrated overlay alignment system.
