High-resolution scanning thermal probe with servocontrolled interface circuit for microcalorimetry and other applications

This article presents a scanning thermal microscopy sensing system equipped with a customized micromachined thermal imaging probe and closed loop interface circuit. The micromachined thermal probe has a thin film metal bolometer sandwiched between two layers of polyimide for high thermal isolation and mechanical flexibility, and a tip with a diameter of approximately 50 nm which provides fine spatial resolution. The circuit includes a proportional-integral (PI) controller which couples to a Wheatstone bridge circuit in which the bolometer forms one leg. The PI controller adjusts power supplied to thermal probe, compensating change in heat loss from probe tip to sample and keeping the resistance bridge balanced. It permits precise control of probe temperature to within 2.3 mK, and widens its applications to microcalorimetry. The probe is used in thermal mapping and microcalorimetry applications. A calibration method based on microcalorimetric measurements of melting temperature is presented for the probe. Scanning thermal images show a high signal-to-noise ratio of 15.7 for 300 nm thick photoresist in which the minimum detectable thermal conductance change is <23 pW/K (which corresponds to a topographic change of 7.2 nm). Subsurface scans show a signal-to-noise ratio of 15.5 for variation of 1.0% in thermal resistance for a topographically smooth surface.