Resolution Limits of Frequency Domain Thermoreflectance for Interconnect Damage in Heterogeneously Integrated Microsystems

In both industrial and consumer electronics, there is demand for increased computational power with a smaller physical footprint. Heterogeneously Integrated (HI) microsystems, using combinations of application-specific chiplets, have emerged as a way to deliver increased performance in smaller areas. HI architectures are typically constructed using metal microbumps to connect the set of chiplets. Damage to the microbumps can be detrimental to overall microsystem performance. These interconnects can be easily harmed during manufacturing processes or use, but damage identification in the metal interconnects is challenging for existing measurement techniques because the interconnects lie beneath the surface of the chiplets. Electron microscopy techniques offer detailed pictures of damage, but require cross sectioning of the HI device to interrogate the relevant interconnects. Other techniques such as acoustic microscopy or ultrasound are less destructive, but require extremely high frequencies (GHz) to image small features, and offer more limited data on interconnect failure.<br/><br/>To offer more detailed assessment of HI interconnect state of health, this talk demonstrates use of Frequency Domain Thermoreflectance (FDTR) to assess these subsurface geometries. The typical implementation of FDTR is a two-laser pump-probe setup which interrogates the sample of interest in the frequency domain. A metal transducer layer is patterned onto a sample to convert the incident pump laser into heat, which diffuses into the sample. Measured changes in reflected probe signal are proportional to the temperature rise on the sample surface in response to the heating from the pump beam. Typically, an analytical heat transfer model assuming radial symmetry is used to fit for thermal properties of the sample based on the phase difference between the reflected signal and pump beam assuming radial symmetry of the sample.<br/><br/>To demonstrate FDTR capability to assess HI microsystem interconnect state of health a thermal finite element analysis (FEA) model is validated and used in Sandia’s structural dynamics FEA code (Sierra/SD). The FEA model frees the analysis from relying on radially symmetric geometries, and shows that FDTR can be leveraged to sense buried rectilinear features. This model is used to calculate expected phase deviation due to a damaged interconnect, and is additionally used to examine the relationship between depth and resolvable feature size. FDTR is shown to be viable for sensing HI features through silicon layer thicknesses of over 100 microns. These model predictions are compared to FDTR imaging data collected on mock HI microsystems, and effects of experimental noise floor are examined.<br/><br/><i>Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.</i>