Sensor objective with tuning fork module
Another possibility of integrating the deflection detection system into the measuring probe consists in using a cantilever arm made of quartz. In operation, this quartz is - just like the tuning fork in a quartz clock - excited to oscillate after an electrical voltage has been applied. Measurement of the distance between probe and surface and thus imaging the surface is performed by recording the current flowing through the quartz. This signal is proportional to the lever arm vibration and reacts very sensitive to changes of the damping when the distance between tip and surface varies. Diamond tips designed at PTB are fastened on these tuning forks to allow high lateral resolution of the measurement (tip radii < 100 nm). Figure 4b shows a quartz probe with tip. The selection of diamond as tip material is based on both, the mechanical properties (stability and resistance to abrasion) and the optical properties which are important for the future use of the probes in optical near-field microscopy. To test the efficiency of the tuning fork measuring head, topographic measurements were performed on structures with dimensions in the nanometer range. The samples used here are made of self-organized, punctual InAs quantum dots on a GaAs substrate. These quantum dots have pyramidal geometries (width: approx. 20-30 nm, height: approx. 4-6 nm). The mechanical stability of the whole microscope is sufficiently high to image such nanostructures. Investigations of the noise resulted in values of less than 0.6 nm (root mean square value) on a profile 2 µm in length. Due to their extremely slim construction and their adjustment-free deflection detection, the tuning fork sensors can be tilted relative to the surface without any problem. This also allows measurements to be performed on object areas difficult to access such as structure edges or inclined areas. These properties allow these as well as the piezolever sensors to be used as sensitive probes in a coordinate measuring machine. Relevant developments have already been initiated at PTB.
Figure 4a: "Tuning fork" module with positioning mechanics and adapter ring for the microscope objective Figure 4b: a micrograph of the tuning fork lever arm with the diamond tip
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