Functional microscopy using quartz tuning fork position sensors.

Tuning fork resonance curves

Scientists in the Quantum Detection group have developed a simple and versatile scanning probe platform which equally suites the needs of industry and challenging research projects.

The new system is based on a quartz tuning fork force sensor – a small, accurate and stable electro-mechanical oscillator, which one can find in all and every electronic quartz clock, costs 10p apiece, and requires only 2 wires to operate. The tuning forks used in our system have a very sharp resonance close to 2¹⁵ = 32768 Hz, when the prongs oscillate in opposition and the centre of mass of the fork remains at rest.

A tuning fork approaches
a sample surface

The resonance conditions are strongly modified when one of the prongs is brought very near a surface. This change can be easily detected due to the sharpness of the resonance and used to measure the surface topography.

AFM cantilever placed on the prong
and the tip cut from the cantilever

The tuning fork sensors are very robust and can have various functionalised probes mounted onto them. The probes can measure for example charge or magnetic field distributions with high spatial resolution. However, to begin with a probe has to be attached to one of the prongs in order to perform functional microscopy using a tuning fork as a position sensor. This operation readily destroys the balance of the two prongs and introduces substantial dissipation. The balance can in theory be re-established by attaching a counterweight (e.g., nominally the same probe) on the opposite prong. In practice the attachment is carried out using some sort of glue and it is the weight of this substance that makes re-balancing extremely difficult, if at all possible. It is therefore desirable to find a glue-less way to attach functional probes.

AFM tip attached to the prong
at its very edge using FIB-CVD

Focussed ion beam (FIB) systems equipped with various add-on tools have emerged as the tool of choice for an increasingly wider range of microfabrication applications. We have used an FIB system equipped with a micromanipulator and gas-injection channels for local chemical vapour deposition (FIB-CVD) to position, shape and attach a standard AFM tip to a standard tuning fork in one fabrication cycle.

AFM scan example

The NPL system has a scanning range 30 x 30 x 15 μm with up to 6 mm offset and sub-nm resolution. It works in air or vacuum, in cryogenic environment or at room temperature and ideally suites the needs of industry (e.g. for inspection of ICs) as well as challenging research projects.