edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 2 6 As an example, Fig. 4 shows the results obtained using the SThM method in active mode on a fully flat, thermally active test device (manufactured at IEMN in Lille, France). The device is based on the implantation of boron ions into the top layer of a silicon-on-insulator substrate to create three heat-generating resistive channels ranging in width from 20 µm to 100 nm. The two sides of each channel are connected by two platinum (Pt) wires for four-point electrical measurements. Experiments were carried out on unheated channels and on lines heated with an electrical current of a few mA, while they had a separate ground and a common ground to create a current leak in the device. As Si and ion-implanted Si have virtually the same thermal conductivity, implanted lines are not detected when they are not heated (Fig. 4, right top). Platinum wires, on the other hand, are detected because their height is around 40 nm, and the thermal conductivity of Pt is lower than that of Si. When one of the three lines is heated, it is clearly detected in both measurement configurations (different grounds and common ground in Fig. 4, right, middle, and bottom, respectively). The other two lines, however, appear heated when all three lines have a common ground, indicating that in this case, a current leak occurs in the upper Si layer of the device. Using a higher current led to a high voltage short circuit. SThM can then be used for failure analysis of electronic devices, helping to explain why a device fails and, more importantly, to define the safety margin of device operation. All of these results were obtained in air. Under such conditions, the spatial resolution of the technique can be degraded due to thermal conduction in the air between the probe and the sample. Thermal conduction through the water meniscus at the tip-sample contact can contribute to the measurement. Because modeling these heat transfers Fig. 3 SiO2 step sample: top left, cross-sectional diagram of a sample; bottom left, sample topography; right, thermal signal as a function of SiO2 thickness. The inset shows a thermal image of the sample.[3] Fig. 4 Left, top view and cross section of the sample’s active zone. Middle, topography, and right, thermal images of the active zone with three lines of ion implanted silicon.
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