High Resolution Thermal Imaging of Integrated Circuits
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1022-II06-02
High Resolution Thermal Imaging of Integrated Circuits Gilles Tessier, Mathieu Bardoux, CÈline Filloy, and DaniËle Fournier Laboratoire d'optique, ESPCI / UPRA005 CNRS, 10 Rue Vauquelin, Paris, 75005, France ABSTRACT Thermoreflectance is an non contact optical method using the local reflectivity variations induced by heating to infer temperature mappings, and can be conducted at virtually any wavelength. In the visible, the technique is now well established. It can probe temperatures through several micrometers of transparent encapsulation layers, with sub-micron spatial resolution and 100 mK thermal resolution. In the ultraviolet range, dielectric encapsulation layers are opaque and thermoreflectance gives access to the surface temperature. In the near infrared, thermoreflectance is an interesting solution to examine chips turned upside down, since these wavelengths can penetrate through silicon substrates and give access to the temperature of the active layers themselves. Here, we explore the possibilities of each wavelength range and detail the CCD-based thermal imaging tools dedicated to the high resolution inspection of integrated circuits. INTRODUCTION Heating is a major cause of failure or performance limitation in integrated circuits (ICs). High frequencies and integration densities make temperature and thermal management a crucial aspect of integrated circuit design. To improve the performance and reliability of microelectronic devices, and also to validate thermal models, an accurate knowledge of local temperatures and thermal properties is required. This demand has indeed led to the development of a diverse array of tools for high resolution temperature measurements. Infrared emission imaging is a popular technique, either in the 3-5 µm 1, 8-12 µm, or in the near infrared ranges2. However, it is limited to long wavelengths, or high temperatures for which the thermal emission is measurable, forbidding sub-micrometer resolution. The most promising techniques regarding spatial resolution are those involving contact probes 3. One major drawback is that, like liquid crystal thermography and fluorescence microthermography, this technique is mostly sensitive to the surface temperature. Modern ICs are highly tri-dimensional devices, with several layers of metal interconnects and dielectrics covering the active semiconductor regions. In some cases, thermal management concerns have even led to the adoption of flip-chip configurations, in which the active side of the integrated circuit is attached to a heat sink, and only the back side of the substrate is visible. In both cases, most of the heat is produced in deeply buried regions, inaccessible to most thermal measurement techniques. Non contact optical techniques, like Raman spectroscopy4 or thermoreflectance, can give access to the temperature of buried features, provided that materials are transparent at the wavelength of investigation. Thermoreflectance measures the local reflectivity variations induced by heating, and can be conducted at virtuall
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