Thermomechanical stress in GaN‐LEDs soldered onto Cu substrates studied using finite element method and Raman spectroscopy

Abstract

Local thermomechanical stress can cause failures in semiconductor packages during long-time operation under harsh environmental conditions. This study helps to explain the packaging-induced stress in blue GaN-LEDs soldered onto copper substrates using AuSn alloy as lead-free interconnect material. Based on the finite element method, a virtual prototype is developed to simulate the thermomechanical behavior and stress in the LED and in the complete LED/AuSn/Cu assembly considering plastic and viscoplastic strain. The investigations were performed by varying the temperature between −50 C and 180 C. To validate the model, the simulation results are compared to experimental data collected with Raman spectroscopy. Studies of the E H 2 phonon mode of GaN semiconductor are elaborated to understand the induced thermomechanical stress. The model enables evaluation of the stress in the interfaces of the assembly, which otherwise cannot be accessed by measurements. It serves to predict how assemblies would perform, before committing resources to build a physical prototype.

Coherent-interface-induced strain in large lattice-mismatched materials: A new approach for modeling Raman shift

“…at 658 cm −1 is nearly identical with that of bulk AlN, (AlN bulk ) at 656.7 ± 2.4 cm −1 [45][46][47][48][49][50][51][52]. The shift to the higher frequency in both cases indicates compressive residual stress in the template layers [33]. In contrast, a significant shift of the is observed for both the GaN and AlN SL layers, which indicates that the SL structure is not pseudomorphic to the underlying…”

“…where is a constant with, , the Poisson ratio and C 13 and C 33 elements of the elastic constant tensor [33,39,40,44]. Unfortunately, there are wide ranges of the values of , , C 13 and C 33 found in the literature, with averages and standard deviations of common values summarized in Table 1 (also see Table S1 in the Electronic Supplementary Material (ESM)).…”

“…Raman spectroscopy and X-ray diffraction (XRD) are powerful methods for reliable and non-destructive analysis of strain in heterostructured materials [28][29][30][31][32][33][34][35][36][37][38][39]. One big advantage of Raman spectroscopy over the laboratory-based XRD technique is the ability to spatially resolved assessment of inhomogeneities in strain distribution at the nanoscale.…”

“…Nanofocused scanning XRD with high spatial resolution can be measured only by using a synchrotron source [38]. Therefore, Raman spectroscopy has been effectively used to study strain-induced band gap engineering in GaN/Al(Ga)N core-shell nanowire heterostructures [31], energy transport in Ga 0.84 In 0.16 N/GaN heterostructures [32], the strain state of ultrathin GaN QWs embedded in an AlN matrix [33], packaging -induced stress in blue GaN -LEDs soldered onto copper substrates [34], spatial non-uniformity in heavily Si-doped GaN layers [35], and phonon modes and in-plane strain (ε xx ) in GaN/AlN multi-QW structures [37,39]. However, a wide range of values of the deformation potential constants and elastic constants of Al(Ga)N used for calculation of strain via Raman spectroscopy lead to an uncertainty in the strain values.…”

“…Moreover, the previous model in the literature for calculating strain from Raman data was based on experimental data limited to a small range of biaxial strains (0% < ε xx < 0.2%) [34,40]. Consequently, a disagreement between strain values calculated via Raman spectroscopy and from other methods is often found in the literature, especially for high biaxial strains (ε xx > 0.2%) [33,39].…”