# Assessing the Stress Induced by Novel Packaging in GaN HEMT Devices via Raman Spectroscopy
## 1 Introduction  
Among Wide-Bandgap Semiconductors (WBSs), Gallium Nitride (GaN) is emerging as the latest breakthrough in developing novel energy-efficient power electronic devices and optoelectronic components [1]. It is characterized by a crystallographic arrangement typical of wurtzite, where Gallium (Ga) and Nitrogen (N) atoms are spatially arranged to form a superimposed hexagonal packed lattice. Power electronics has recently leveraged the characteristics of wide-bandgap materials to overcome the limitations imposed by silicon. In particular, the increasing demand for the next generation of high-efficiency power converters has pushed the development of devices capable of operating at higher frequencies and temperatures, achieving higher conversion efficiency. Among these devices, GaN-based technology has been employed to develop AlGaN/GaN high-electron-mobility transistors (HEMTs) [2, 3, 4]. These devices find applications in electric vehicles, phone chargers, renewable energies, and more as they meet the above specifications, are able to operate at high temperatures, frequencies, voltages, and currents, and allow the design of systems that occupy less space and generate less heat [5]. However, due to the limited commercial availability of bulk GaN substrates [6], GaN-based electronics are typicallydeveloped using alternative substrates such as sapphire (Al2O3), Silicon Carbide (SiC), and silicon (Si) [7]. Nevertheless, owing to its cost effectiveness and availability in large wafer sizes (e.g., 12-inch wafers), Si substrate is widely used and predominant compared to SiC and Al2O3 substrates, which are available in smaller-sized wafers (e.g., 6 or 8 inches) [8]. Moreover, silicon technology and fabrication processes are more mature and advanced. Additionally, the presence of an AlGaN (Aluminum Sallium Nitride) modulation layer on silicon substrate plays a crucial role in mitigating defects in GaN semiconductor materials. These factors, along with the higher costs associated with SiC, have positioned silicon as the preferred substrate for electronic industrial production. Although silicon is a dominant choice and more commonly used for electronic applications, it is worth noting that there are specific applications where SiC or sapphire substrates might be preferred, depending on the performance requirements and specific characteristics of the devices being developed. The packaging of electronic devices plays a critical role in ensuring their functionality, reliability, and protection from external factors (chemicals, light exposure, and mechanical impact) [9] while simultaneously guaranteeing a good connection between the chip and the board. For these reasons, many studies have been conducted to identify the best kind of packaging for the desired applications, testing different designs and materials. More specifically, the electronic device packaging research field has become increasingly active as the demand for power devices has increased [10]. Among all the analyzed factors that can affect both performance and lifetime of a device, the effect of the mechanical stress on the chip surface is rarely taken into account [11]. Due to the lattice mismatch between the AlGaN/GaN HEMT and the chosen substrate, along with the difference in thermal expansion coefficients [12], there is a significant built-in strain/stress capable of modifying the overall performance of GaN-based devices. Moreover, regardless of the type of substrate used, AlGaN/GaN HEMTs must be encapsulated within specific packaging. Minimizing the stress and strain experienced by the devices has been one of the main goals of the research on new packaging materials and designs. Typically, stress and strain can be induced mechanically and thermally; therefore, for different kinds of device, different materials have to be employed. For instance, polymer-based packaging materials provide flexibility and shock absorption, while ceramics offer high stiffness and thermal stability.