Nanometer Thermal Conductivity Mapping Using Laser-based Scanning Thermal Microscopy
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Nanometer Thermal Conductivity Mapping Using Laser-based Scanning Thermal Microscopy Jeremy Goeckeritz1, Gary Aden1, and Ami Chand1 Applied Nanostructures, 415 Clyde Ave. Suite 102, Mountain View, CA 94043, U.S.A.
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ABSTRACT A new measurement technique using a cantilever probe with an integrated thermal sensor is investigated for measuring thermal conductivity at the nanometer scale. The probe is used in a configuration wherein the laser from an atomic force microscope (AFM) heats the tip of the probe above ambient temperature. Heat is transferred from the probe to a sample based on the thermal conductivity of the sample. The heat flow creates a temperature change, as small as 0.01 °C, which is detected by the thermal sensor. The measurement technique presented offers a simple and effective method for mapping the thermal conductivity of a number of materials. We explore the ability of the technique to map silicon oxide on silicon, carbon fibers and gold nanoparticles. Analysis shows that the technique can be used to produce images with a thermal resolution surpassing 25 nm. INTRODUCTION A large number of physical, biological and engineered materials and phenomena require thermal imaging at high spatial resolution. In recent years, scanning thermal microscopy (SThM) has attracted considerable attention as a promising nanoscale thermal imaging technology. In SThM, a thermal probe consisting of a cantilever with a sharp tip is scanned over a sample while a temperature-sensing element near the tip measures thermal properties at the sample surface. The temperature sensor may be, for example, a thermistor, thermocouple, or Schottky diode [13]. Typically, an atomic force microscope (AFM) is used to scan the probe enabling simultaneous topography and thermal imaging. The thermal probe may be operated in passive or active mode [4], [5]. In passive mode, the sample heats the tip while a temperature-dependent tip property is monitored. In active mode, the tip is heated above ambient temperature. The tip is then brought in contact with a sample and heat is transferred from the probe to the sample based on the thermal conductivity of the sample. For example, a resistive heater may be used to elevate the tip temperature while the temperaturedependent resistance of the heater is monitored during the scan. Such active mode SThM requires precise current control to heat the probe and electronics to monitor the temperaturedependent tip property. Here, we present an alternative method for mapping thermal properties without a highprecision current controller or custom readout electronics. The setup requires only the feedback laser of an AFM to heat a thermal probe. The probe uses a thermocouple junction at the tip which produces a temperature-dependent voltage response when the tip is brought in contact with a sample. The voltage is amplified and sent to the AFM controller input for display. The setup does not require active current heating thus allowing the same probe to be used in passive or active modes by simply moving the AFM las
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