Self-sensing smart AFM cantilevers for ultra-low noise applications.

Atomic Force Microscopy (AFM) is aubiquitous technique found in the laboratories of many scientists and engineers. It makes use of consumable probes, characterised by a body (permitting easy handling and mounting) onto which is attached a cantilever with a sharp tip (<50 nm apex radius). The cantilever beam primarily acts as spring or resonant structure, permitting the forces between the tip and sample to be measured. By bringing the tip into contact with a surface, the mechanical loading between tip and sample can be calculated from the mechanical behaviour of the cantilever. This is typically achieved using an approach called an 'optical lever', whereby a laser spot is reflected off the cantilever and onto a photosensitive detector. Deflections in the cantilever then result in movement ofthe reflected light on the photodetector, which allow the cantilever movement to be calculated.[1]This is a robust and simple approach for many experiments. However, illumination of the AFM probe and sample can have a significant impact on the behaviour of the probe or sample being studied. Examples include materials that have light sensitive behaviour (e.g. semiconductors), or samples where temperature stability is essential. This second point is particularly important whenyou consider many AFM probes have a thin mirror (assisting laser reflection) coated on one side of their silicon (or silicon nitride) cantilever. This bi-metallic structure deflects when exposed to temperature changes due to the differing thermal expansion coefficient of the materials it is made from. Inturn, this can easily be misinterpreted as a tip-sample interaction, producing erroneous AFM data. Another type of AFM probe is designed to simultaneously measure sample temperature alongside the normal tip-sample forces. This technique is called ScanningThermal Microscopy (SThM), and the data it acquires can be seriously impacted by the undesirable probe/sample heating caused by the AFM laser.[2][3]The only realistic method to eliminate laser heating in AFM experiments is removal of the laser itself. However, this leaves the problem of how to detect cantilever deflection without an operating optical lever. This is a problem that has been addressed in commercial MEMS sensors, such as the microphones, accelerometers and gyroscopes already employed in mobiles and computers. These sensors typically employ strain gauges or capacitive approaches to measure deflection of their mechanical sensing elements.[4][5]This project proposes to integrate such a sensor into AFM and SThMprobes, before using the new probes to make previously unobtainable measurements. This will require the following steps to be undertaken:1) Design and modelling of a range of sensors integrated into AFM probes: This will allow sensor sensitivity to be predicted and the best candidate designs to be selected.2) Fabrication of test sensors, followed by full AFM probes that include the sensors.3) Development and test of instrumentation that will be used to measure the sensor and deliver its signal in a form suitable to replace the laser signal in commercial AFMs: This will employ the test devices fabricated in 2) above to minimise delay whilst the complex AFM probes are fabricated.4) Evaluation and use of the completed system to make previously unachievable measurements.These measurements will include using SThM to accurately measure the operating temperature of semiconductor devices (essential to validate thermal models that inform future designs). Kelvin probe force microscopy (measuring surface work function) will also be undertaken on semiconductors both with and without illumination.
[1] D. Rugar and P. Hansma, "Atomic Force Microscopy," Physics Today, pp. 23-30, October 1990. [2] A. Majumdar, Lai, M. Chandrachood, O. Nakabeppu, Y. Wu and Z. Shi, "Thermal imaging by atomic force microscopy using thermocouple cantilever probes," Review of Scientific Instruments, 1995. [3] L. Ramiandrisoa, A. Allard, Y. Joum