Batch-fabrication of novel nanoprobes for SPM
A micromachining method has been developed for fabricating 20µm tall silicon atomic force tips with flat tops less than 2µm wide suitable for defining nanosensors upon, and with low aspect ratio sides suitable for defining electrical connections to the sensor. Methods have been developed to allow flat substrate processing techniques to be applied to such non-planar micromachined substrates. This has necessitated the development of a novel resist-coating technique and the use of defocused electron-beam lithography. Methods for through-wafer alignment by electron-beam lithography and accurate alignment to the tips using micromachined alignment markers have also had to be developed. The fabrication process has been designed to enable a wide variety of sub-micron sensors to be defined on the atomic force probes, with little additional development beyond that of : sensors themselves. This flexibility has enabled very different sensors meant for very different scanning probe microscopy techniques to be designed without significant redevelopment of the underlying fabrication process. The main restrictions on the type of sensor that can be used are the physical dimensions of the sensor, the number of alignment levels necessary, the degree of alignment accuracy required and the choice of sensor materials. However, within these constraints it has been found that probes optimised for scanning near-field optical microscopy (SNOM), scanning thermal microscopy, modulation differential scanning calorimetry (MDSC) and scanning Hall-probe microscopy can be fabricated. For the SNOM probes three methods for fabricating sub-l00nm diameter apertures have been developed, analysed and compared with each other to evaluate both the process latitude. and, the size and reproducibility of apertures that can be fabricated, as a function of electron beam dose, pattern shape and size, and metallisation material and thickness. Two methods, both utilising multilayer 'resist' schemes have been found suitable for this purpose, one based on conventional electron-beam lithography with PMMA and a new dry etching process for titanium, and the other based on a novel electron-beam lithography technique utilising cross-linked PMMA for lifting off nichrome. A simple analytical model has also been developed for these probes allowing the effects of changes in the sensor design parameters on the light throughput to be compared qualitatively, if not quantitatively. For the scanning thermal probes a method for lifting-off sub-l00nm, thin-film thermocouple sensors on silicon tips without the loss of electrical continuity has been developed. For the MDSC probes, a similar method has been developed for defining thermal resistors. A method has also been presented for fabricating sensors for scanning Hall-probe microscopy based on an evaporated germanium sensing layer. This has been found to require annealing and optimisation of sensor design and geometry to reduce sensor resistance to acceptable levels.