Emission-Based Temperature Mapping with STEM EBIC

In situ TEM heating and biasing capabilities enable high-resolution imaging of temperature-induced changes in materials and devices. Heaters used in the microscope, which are now typically MEMS-based rather than older furnace-style systems, produce a large area of uniform temperature, with temperature calibrated by external optical thermometry and/or 4-point resistance measurements. While these systems are well-calibrated for uniform heating, spatially-resolved in situ thermometry techniques are necessary for measuring temperature gradients, such as those produced by operating micro- and nanoelectronic devices. Recent advances in TEM hardware have enabled thermal measurements in the microscope via signals that change very subtly with temperature, including EELS-based measurements of plasmons [1] and phonons [2,3] and precise diffraction measurements [4]. These techniques have narrow restrictions on applicable materials, and the long acquisition times and complicated computation involved makes temperature mapping difficult. Here we discuss measurement and mapping of temperature via precise measurement of secondary electron (SE) emission using STEM electron beam-induced current (EBIC) imaging. A recently demonstrated mode of EBIC, called SE emission EBIC (SEEBIC), measures small hole currents produced in samples by SE emission [5] with high spatial resolution [6]. The SEEBIC signal changes obviously with temperature, and gradients are visible in a single image without any post-processing or computation. Unlike other TEM-based thermometry techniques, SEEBIC temperature mapping appears compatible with most materials. The Fig. 1 data shows the obvious change in SEEBIC contrast with temperature. In Fig. 1, a heater is patterned just outside, and to the left of, the field of view of the Pt electrode. When the heater is off the SE signal on the electrode is mostly uniform, with slight variations due to surface topography. When the heater is energized in 50 µW steps throughout the image, SEEBIC-measured SE emission increases obviously with temperature for a given power value and changes can be seen in the raw image without the need for filtering or processing. Within each horizontal strip (power value), a horizontal gradient is also visible, with contrast brightest on the left near the heater. Using plasma energy expansion thermometry PEET [1], an EELS-based temperature mapping technique, we calibrate the SEEBIC temperature response for different probe electrode materials. Our SEEBIC results match the temperature dependence expected from emission of electrons from a potential barrier as described by the free electron model [7]. Calibrated SEEBIC thermometry was successfully demonstrated on several different materials, including Pt, a crystalline metal, and amorphous carbon. Each material tested exhibited a similar temperature response, with a material-dependent correction factor to compensate for difference in SE yield [8]. The left pair of images show annular dark-field (ADF) and STEM EBIC images of a Pt electrode patterned on a thin SiN membrane while a heater, which sits to the left of the electrode, is turned off. In the images to the right, the heater is turned on and power is increased in 50 µW steps during the image, producing an obvious change in the SEEBIC signal for each horizontal strip (power value). For each heater power, a horizontal gradient is also visible, with brighter signal on the left side of the electrode, which is nearest the heater. STEM EBIC current is measured by connecting an amplifier to the Pt electrode and the contrast is produced by SEEBIC in both images.