New “self-sensing” cantilever with integrated electronic and actuation mechanisms

Integrating electronic and actuation mechanisms into micromechanical devices such as MEMS cantilevers are widely used to transduce an applied force to an electrically measurable signal. Employing such “self-sensing” cantilever in an atomic force microscope (AFM) allows getting ride of the optical read-out by a laser reflected at the backside of the cantilever. This opens new applications where there is not enough space for the optical read-out module above the sample to be investigated or where the sample has to be immersed into – even – nontransparent liquids like for biological samples.

In most of the traditional AFMs, the deflection of the cantilever is detected using the optical beam deflection method where a laser beam reflected from the back of a cantilever is centered on a quadrant photo diode. An alternative is measuring the cantilever bending by integrated piezo-resistive sensors. A high sensitivity is achieved by measuring the resistance of the sensors by means of a Wheatstone bridge.

The deflection sensitivity of the self-sensing cantilever increases with its thickness due to the increased distance of the sensors from the neutral axis of the cantilever. However, the spring constant of the cantilever increases strongly with its thickness.

The design of these new cantilevers incorporates an elastically soft polymer core enveloped by 2 hard thin film layers. This patented new cantilever concept provides additional degrees of freedom to optimize the performance of the MEMS cantilever. The

most striking advantages compared to standard Si or SiN self-sensing cantilever are:

• For a given resonance frequency f0, the force sensitivity of the trilayer is up to 10 times higher than that for silicon cantilevers.

• The use of a polymer as core of the AFM cantilever increases its bandwidth f0/Q, – Q is the mechanical quality factor. This is particularly advantageous when imaging in vacuum when fluid of air damping is absent. Excellent resolution can be achieved even at imaging speeds up to 10 times higher than with Si or SiN self-sensing cantilever.

• All sensing elements and electrical connections are hermetically sealed inside the MEMS device, which makes the trilayer devices inherently suited for measurements in fluids – even in opaque, electrically conductive or harsh chemical environments.

The trilayer technology is not limited to self-sensing cantilevers. Examples are fluid-compatible membrane-type surface-stress sensors or force-sensors with resolution in the nN-range.

For a detailed description see “A hybrid polymer / ceramic / semiconductor fabrication Platform for high-sensitivity fluid-compatible MEMS devices with sealed integrated electronics” by Nahid Hosseini et al. (arxiv_2307.05221)