AFM cantilever beam mechanical behavior optimization

Atomic Force Microscopes make use of consumable probes, characterized by a body (permitting easy handling and mounting) onto which is attached a cantilever with a tip at the end. The cantilever beam primarily acts as spring or resonant structure, permitting the forces between the tip and sample to be measured. The vast majority of AFM probes are based around one of two cantilever designs, rectangular or 'dual triangular' beams. The reasons for the dominance of these designs are as follows:

1. These designs are well established and proven to be highly effective, since their introduction at the very start of AFM development
2. The manufacture of these cantilevers can be easily achieved with commonly available microfabrication tools, such as optical lithography, chemical vapor deposition and wet/dry etching. This permits the scale production of nominally identical probes at relatively low cost.
3.The mechanical behavior of simple cantilevers can be calculated or modeled using simple mathematical approaches. This makes their calibration and interpretation of experimental results straightforward.

These points offer a strong disincentive to the development and use of more complex cantilever geometries, unless absolutely necessitated by the measurements being made. However, these ubiquitous geometries do impose limitations on the behavior and properties of most probes, including:

1. Probes with differing mechanical properties (stiffness, resonance frequency) have markedly different geometries, limiting their use in AFMs that employ automated or kinematic probe loading and setup.
2. The modification of one mechanical property (e.g. spring constant) has an inevitable impact on the other properties of the cantilever (resonance frequency, thermal conductivity).
3. Probe mechanical properties are highly dependent upon cantilever thickness, t (spring constant and resonance frequency). Variations in this thickness >10% are not uncommon in conventionally available probes, necessitating individual calibration if accurate knowledge of probe properties is required.
4. Bilayer cantilevers (e.g. providing additional coatings for reflectivity or electrical conduction) exhibit undesirable bending caused by thermal (ref) or surface chemical(ref) interactions.

As a result of these limitations, AFM is limited to relatively simple measurements, that often suffer from poor accuracy and reproducibility. The aim of this project is to design, model, produce and evaluate AFM probes with advanced mechanical characteristics, permitting them to make measurements previously inaccessible to the AFM technique. The primary target will be the production of probes with variable spring constants, allowing much improved measurement of soft materials, such as biological cells and tissues. However, the steps required to reach this goal will also result in other, entirely new, AFM probes made from novel materials and offering enhanced mechanical performance e.g. lateral stability.

Initially, this work falls into the 'Productivity' prosperity outcome outlined by EPSRC, in particular the 'Sensors and Instrumentation' research area. However, it is also anticipated that it will assist in measurements that fall into other into other, more diverse areas such as the 'Biomaterials and Tissue Engineering' research area in the 'Health' prosperity outcome.

Ultimately, this project will result in improved and entirely new AFM probes that can be easily employed by a large number of researchers in the charaterisation of materials. This is desperately required as the study of mechanical properties on the nanoscale (e.g. nano materials such as graphene or advanced composites in material science, all the way through to individual biological cells in single-cell diagnostics).