The design and fabrication of a thermal microprobe integrated on an atomic force microscope probe tip

A thermal microprobe has been designed and built for high resolution temperature sensing. The thermal microprobe consists of a very-thin-film thermocouple junction confined to the very end of a low mass Atomic Force Microscope (AFM) probe tip. Essential to high resolution temperature sensing is the confinement of the thermocouple junction to a short distance at the AFM tip. This confinement is achieved by controlled photoresist coating. Experimental prototypes have been made with the junction confined to within 0.3 µm of the tip. The couple is made of Au/Pd, and the two metals are electrically separated elsewhere by a thin insulating layer. The device is designed for insertion in an AFM instrument so that topographical and thermal images can be made with the same tip. Large contact pads permit mechanical and ohmic contacting with spring clamps. Processing begins with double-polished, n-type, 4-inch-diameter, and 300 µm thick silicon wafers. Probe tips are formed by a combination of RIE, wet chemical etching, and oxidation sharpening, which makes the tips atomically sharp. The hot thermocouple junction is formed by controlled photoresist coating. The metal layers are sputtering deposited and the cantilevers are released by KOH etching and RIE. The thermal microprobe gives a high temperature resolution and a high spatial resolution. The thermal mass is kept low in order to cause minimal disturbance of the component under measurement. The thermal output of the microprobe is 5.6 µV/°C and is linear over the temperature range 25 - 110°C