# Assessing the Stress Induced by Novel Packaging in GaN HEMT Devices via Raman Spectroscopy
## 1 Introduction
Furthermore, the overall effect on the device lifetime and performances induced by the packaging is not predictable since there are many elements that constitute the packaging (e.g., wires, solders, die, etc. [13]). In this work, we studied a commercial electronic device produced by STMicroelectronics equipped with a wire-bonding-free packaging designed to be integrated around the chip to save space and optimize thermal dissipation. The investigation proposed in the manuscript emphasizes the importance of choosing and designing the appropriate packaging to ensure performance and lifetime suitable for the device's mission profile. The proposed package comprises a lead frame onto which the die is soldered using a silver sinter paste. Fiberglass is used to plane the intermediate space. Holes are then dug into the glass fiber, onto which copper is deposited, and subsequently etched down using a dedicated mask, leaving the final connection. Despite its utility, the packaging encapsulation process may induce unwanted additional mechanical residual stresses across various layers of the device and at the die/packaging interface. The additional mechanical stress can be even more burdensome under normal operating conditions due to differences in thermal expansion coefficients between the various materials composing the device [14]. The above results in a continuous stream of reliability challenges in the field of advanced packaging technology. This work focuses on the effect of packaging on the stress among the semiconductor layers constituting the device. In particular, the thermomechanical stress contributes to premature device failure through a variety of mechanisms, including packaging, die, wire, and solder bump crack mechanisms, as well as delamination. The study of the aging effect and failure mechanism caused by thermal and thermomechanical phenomena on electronic devices is currently an active research field [11; 15]; therefore, the evaluation of the built-in stress and strain is mandatory to correctly estimate the device lifetime. To explore this issue, many non-destructive approaches have been employed to measure mechanical strain and stress (e.g., X-Ray Diffraction, Photoluminescence, and Terahertz Time-Domain Spectroscopy [9]). In this context, micro-Raman spectroscopy emerges as a potent non-contact, local, and non-destructive molecular characterization technique [16] capable of assessing localized residual strain/stress across the entire thickness of the AlGaN/GaN HEMT [17] with a sub-micron lateral resolution. Significantly, when crystals are subjected to mechanical stress, the frequency of their characteristic Raman modes shifts due to stress-induced crystal deformations [9]. The GaN crystal Raman spectrum can be populated by eight different optical modes, four of which are Raman active in our geometry, namely A1(TO), E1(TO), E2(low), and E2(high) [18]. Among the detectable phonon modes, the E2(high), which arises from atomic oscillations along the c-plane, is more susceptible to lattice strain elongations, constituting a good marker for the quantification of residual stresses in AlGaN/GaN HEMT devices [19]. According to the previous consideration, the frequency center (\(\omega\)) of the E2(high) mode was monitored along a vertical direction spanning from the bottom to the top of the HEMT devices, crossing the entire AlGaN/GaN heterostructures. This comprehensive analysis was conducted for both AlGaN/GaN HEMT-PD and AlGaN/GaN HEMT-B samples. This approach, together with using a specific strain/stress relation, enables correlation of the optical data, as achieved by micro-Raman spectroscopy, and the residual stress existing within the layers of the device under test [1, 7, 20, 21]. Knowledge of the aforementioned aspects not only helps evaluation of the performance of GaN-based electronics in various applications, but also provides valuable information for the understanding of the unknown failure mechanisms which compromise their lifetime.