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Measuring material softening with nanoscale spatial resolution using heated silicon probes
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) Principle of local thermal analysis. A heated probe is in contact with the sample to be characterized. Heat is generated at the bridge of the probe, marked by the hot temperature , and flows down the legs of the probe, across the air gap, into the ambient environment, and through the tip to the ambient temperature . Phase transitions in the sample cause the tip to sink into the softened sample and change the thermal resistance between the probe and sample. The onset of phase transition can be detected electronically due to the coupling between the thermal and electrical properties of the probe or optically by using a reflected laser to monitor the vertical position of the probe.

Image of FIG. 2.
FIG. 2.

(Color online) SEM image of microfabricated heated silicon probes. Current to heat the probe flows through the highly doped electrically conductive legs and the low-doped resistive heater at the end. Joule heating in the resistive region results in a temperature rise at the end, which includes the AFM tip. The inset shows a tip with radius of curvature near . The scale bar in the inset corresponds to .

Image of FIG. 3.
FIG. 3.

(Color online) Thermal circuit for heat flow through the tip of a heated AFM probe. The relative sizes of the thermal resistances of the tip interface and substrate determine the temperature at the interface between tip and substrate. The dominant mode of heat transfer from probe to substrate is through the air gap, which has a thermal resistance . The calibrated temperature of the probe corresponds to the temperature of the heater region above the tip.

Image of FIG. 4.
FIG. 4.

(Color online) Inverted topography (top) and measured depth (bottom) of indentations made with a heated probe held at various temperatures for each at . Above the indent depth increases rapidly with temperature. The scan size of the topographical image was .

Image of FIG. 5.
FIG. 5.

(Color online) Amplitude of lateral tip motion as a function of probe temperature with the probe tip in contact with a PS substrate that is subjected to of lateral oscillation. Above the glass transition, the tip sinks into the PS, which increases the torque on the cantilever and increases the lateral tip motion of the probe.

Image of FIG. 6.
FIG. 6.

(Color online) Experimental setup for performing thermal analysis with method 3. A small ac temperature dither is connected in series with a slow temperature ramp. A lock-in amplifier measures the ac phase and amplitude response of the probe, and the reference power, amplitude, and phase are subtracted out numerically.

Image of FIG. 7.
FIG. 7.

(Color online) Total and differential dissipated power (top) and differential ac phase and amplitude (bottom) of the probe during a temperature ramp on polystyrene. The differential power exhibits a clear change in slope as the substrate softens, while no transition is visible in the total dissipated power. The differential phase gives a weak indication and the differential amplitude gives no indication of softening. The reference signal for the differential data is taken from a temperature ramp with the probe in contact with a glass substrate.

Image of FIG. 8.
FIG. 8.

(Color online) and deflection as a function of probe temperature while in contact with a PS film using method 4. The optical deflection signal shows significant convolution from vertical deflection mechanisms while exhibits a clear drop as the tip begins penetrating into the film.

Image of FIG. 9.
FIG. 9.

as a function of indentation temperature for indentations formed using method 1. The data shown were taken during the same experiment as that shown in Fig. 4. Measuring indentation depth from gives a clearer indication of substrate softening than topographical measurement and also eliminates the need for data postprocessing.

Image of FIG. 10.
FIG. 10.

(Color online) Typical tip crater left behind in the PS substrate by a heated silicon probe after performing a temperature ramp. An ideal crater size for a Wollaston probe is shown for reference and is two orders of magnitude larger in linear dimension.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Measuring material softening with nanoscale spatial resolution using heated silicon probes