Direct Measurements of Thermal Stress Distributions in Large Die Bonds for Power Electronics
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I. Introduction In a typical power module, multi-device bare semiconductor dies, such as insulated gate bipolar transistors (IGBT), are soldered to direct bond copper (DBC) substrates, which are subsequently soldered to a metal base plate. In turn, the DBC substrate is comprised of three layers: two thin outer layers of copper and a central, electrically insulating layer of a ceramic, e.g. aluminum oxide or aluminum nitride. Manufacture requires heating the components above the melting point of solders. Because the coefficient of thermal expansion (CTE) of most metals is several times that of silicon and other common semiconductors, significant residual stress is developed during cooling back to ambient temperature. In some instances, this stress causes mechanical failure or changes the operational characteristics of the device [1]. In all cases, it governs the reliability of the power module during use. The nature of this residual stress depends on the elastic and thermal properties of the devices, solder and substrates. Furthermore, as will be described in detail, for die shorter in lateral dimension than a critical value, the peak magnitude of the residual stress is also proportional to the size of the die. The critical dimension depends on the properties of the solder material. However, for die larger than the critical dimension, the stress reaches a constant level that is independent of die size. The dependence of this residual stress on the physical and architectural properties of the system is well known [2,3]. However, detailed experimental data for the spatial distribution and time/temperature dependence of residual stress are lacking. Therefore, the present investigation was motivated by the need to better understand the fundamental physical properties that govern the development of such stress. Specifically, for the first time, we measure directly the absolute magnitudes and spatial distributions of time-dependent thermal residual stress in silicon die attached to substrates by a new, piezospectroscopic technique with high stress (±15 MPa) and 99 Mat. Res. Soc. Symp. Proc. Vol. 515 ©1998 Materials Research Society
spatial (1 ptm) resolution. Representative stress profiles have been measured in several systems pertinent to power electronics. In conjunction with simple mechanical models, the effects of size and physical properties on the thermal residual stress are analyzed and discussed.
11. Experimental Methodology 2.1. Silicon Raman Piezospectroscopy Piezospectroscopy is concerned with the effect of strain on the spectroscopic properties of solids. In a typical measurement, a laser beam in an optical microscope is used to excite a spectral signal (fluorescence or Raman) from a region selected in the microscope. From the frequency shift and broadening of intensity peaks in the resulting signal, the local stress and stress gradients in the material can be determined. The techniques are non-destructive and can be used to study the stress from regions just a few microns in dimension. This technique has b
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