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What do you characterize with thermal imaging?
Keywords: thermal imaging , power map, device characterization
Thermoreflectance thermal imaging is a technique that leads to a better understanding of the transient temperature distribution (submicron spatial resolution and 1-100 ns time resolution). The thermoreflectance method detects top surface temperatures with visible light and bottom surface temperatures through a transparent (silicon) substrate with a near IR (NIR ~1000 nm) light source. We will present our investigation with respect to what the temporal thermal images disclose about the device and how you can relate this information to what is occurring within the device. Knowledge of the technical details of the device, such as dimensions, material properties, interfaces, and the temporal and inhomogeneous heat generation in the devices, lead to an understanding of the temperature propagation in the solids structures. Temporal surface temperature is the result of thermal diffusion occurring in the device and a convolution of multiple events of temporal hotspots in the enclosed space (solids). Such thermal diffusion will be explained. The first step is to recognize the raw data obtained from a surface temperature time-series map. Observing a location on the surface where an especially high temperature occurs is indicative of at least one hotspot somewhere nearby in depth but not necessarily on the surface. If the device is under an investigation due to a failure, the temperature map can provide important information that may lead to the cause of the failure. Also, a time delay of temperature rise after a biasing pulse at a particular circuit provides information about the thermal diffusion through the layer(s). Sometimes, the delay is explained with respect to a network of thermal time constants. If the hotspot information in the circuit is missing, there is a way to determine the temporal and local heating map (power-map) by using Greens function but this is limited to an effective power-map on the characterized surface (or circuit if characterized through a silicon substrate). The generated power-map can be compared with the designed value in a time-series. The time delay or advance with respect to the design provides critical information for the block level time sequence. Further numerical thermal analysis with FEA software can regenerate a thermal map with designed dimensions and properties. Comparison to the original thermal map will provide the differences caused by the design parameters. This approach helps to identify the property value(s) by manipulating the parameters such that both temporal temperatures match. In summary, time domain analysis with thermal mapping is important. The thermal map on the surface not only identifies temperature information for the hotspot(s) but also contains a foot print for localizing electrical hotspots in space and time. The temperature map also contains dimensional information and properties of solid layers. The challenge remains, to fully consider the three dimensional thermal diffusion including interface contact(s) in the device structure. We will add discussion on an approach for this challenge as well.
Kazuaki Yazawa, Research Scientist
Microsanj LLC.
Santa Clara, CA

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