2D Micro-Raman mapping of thermomechanical local stress in assembled GaN LEDs and Si chips

Integrated circuits and optoelectronic devices are complex systems consisting of various materials with different characteristics that can develop mechanical stress when assembled. Since the presence of local stress influences the electronic performance of the device, it is useful to investigate the presence of stress and its distribution in different samples. Micro-Raman spectroscopy allows for determination, mapping, and quantification of local stress and is a powerful tool in understanding how various assembly methods of different materials influence the stress distribution in each layer of complex electronic systems such as LEDs.

This work focusses on the Raman investigation of both metal and semiconductor material properties in integrated circuits: silicon chips that simulate part of the structure of an optoelectronic device, and GaN LEDs. Silicon chips (120 μm thick) are soldered to copper substrates. Blue GaN-based LEDs, bonded to a silicon carrier using gold silicon, are soldered with an AuSn alloy on copper substrates, with different thicknesses. Stress is determined by 2D Raman mapping of the surface, at 514.15 nm, in a wide temperature range, from -50 to 180°C: from the determination of the Raman peak position of Silicon centered around 520 cm^-1, and GaN, centered around 568 cm^-1, the presence of tensile and compressive stresses on the samples are evaluated.