Fabrication of self-actuated piezoresistive thermal probes

• We fabricated novel thermally sensitive cantilevers.
• A thermally sensitive tip with 11.2 μV/K could be achieved.
• A full Wheatstone bridge is implemented to the cantilever.
• Electronics were developed to measure resistance changes upon temperature changes.
• A calibration procedure of the thermally sensitive tip was performed.

The fabrication and characterization of the novel thermal sensitive probes presented here are established on a well-known physical effect of the electrical resistivity changes of a nanometric metal (Au or Pt) filament. The filament is fabricated from the high aspect ratio (1:4) silicon AFM cantilever tip (approximately 3–5 μm high) covered by a metal layer (approximately 50 nm thick), but keeping its tip diameter of about 20 nm. Therefore, a focused ion beam (FIB) milling process is used to remove the silicon underneath the metal coating of the tip in order to increase the thermal and electrical resistance. Silicon is milled away very precisely with ion beam settings of only 69 pA beam current and 30 kV acceleration voltage. As a result, the electrical resistance of the metal filament increases in the order of 5–6 times in comparison to its original value (≈25 Ω). The new probe tip is undergoing a vast resistance change with the slightest temperature fluctuations measured at the tip apex. Typical characteristic thermal sensitivity (SA) value of the sensor is measured to be SA = 40 ± 4 nm/mW. In addition, AFM thermal probe has a full Wheatstone bridge integrated, which is used for an ultra-sensitive and non-optical deflection read out. Deflection sensitivity (SV) of the thermal probes is measured to be SV = 9 ± 0.5 μV/nm. This self-sensing method allows recording simultaneously data about the topography, as well as the thermal properties of the sample’s surface. Therefore the probe is operating in a contact mode. Thermally-calibrated sensors are implemented in a measurement setup, which includes self-developed electronics connected to a lock-in amplifier for a fast data acquisition. The novel approach is the combination of CMOS technology, bulk and surface micromachining with advanced FIB milling processes to fabricate unique nanoprobes with a thermal resolution of 10−3 K.

The following two pictures show the fabricated thermally sensitive sensor (left) having a FIB modified sharp metallized cantilever tip. The second picture (right) shows the principle which is used to measure the temperature above a sample (after the sensor is calibrated) due to a change of the resistance of the metal tip.Figure optionsDownload as PowerPoint slide