Variable Frequency Microwave and Convection Furnace Curing of Polybenzoxazole Buffer Layer for GaAs HBT Technology

Abstract

Photosensitive polybenzoxazole (PBO) film has been used in GaAs heterojunction bipolar transistor (HBT) technology for stress buffer and mechanical protection layer applications. However, this film needs to be cured at high temperatures for a long period of time in order to obtain its desired excellent material characteristics. High-temperature curing can result in degradation to the electrical characteristics and performance of the underlying GaAs devices due to limited thermal budget. In this paper, we have characterized the effects of curing the PBO film on GaAs HBT wafers using a conventional convection furnace and using a variable frequency microwave (VFM) furnace. The results show that a VFM cure can achieve similar excellent physical, mechanical, thermal, and chemical material characteristics at a lower curing temperature and in a much shorter time, as compared to convection furnace curing, therefore resulting in minimal GaAs device degradation. Based on these results, an optimum curing condition using the VFM method can be obtained that satisfies both stress buffer layer material and device requirements for GaAs HBT technology.

Development of New Photosensitive Low CTE Materials

“…It is known that low CTE strongly depends on rigidity and orientation of the molecular backbone. On the other hand, the PSPIs require high curing temperature, typically more than 300 o C. Because of the high temperature, there are concerns about damage to low heat resistant substrates and other materials [7]. Therefore, we started development of low temperature-curing type photosensitive low CTE materials.…”

Low Temperature Curable Photo-sensitive Insulator

“…After dipping, the films were treated with water and dried. The film thickness change ratio was calculated with the film thickness before and after dipping, and we also checked the appearance for cracking, swelling, wrinkling and discoloring [13]. As a result, Sample 4 showed good chemical resistance with the various process chemicals.…”

“…In general, these commercial insulation materials need to be applied by high temperature curing (> 300 o C). This can cause a significant film shrinkage leading to wafer bow and possible damage to the device [13]. In contrast, there is a strong demand for low temperature curable (around 200 o C) photo-sensitive insulators for remarkable packaging structures such as WL-CSP, FO-WLP and Through-Silicon Via (TSV).…”

Integrated process characterization and fabrication challenges for 2.5D IC packaging utilizing silicon interposer with backside via reveal process

“…Besides the first buffer layer, a second buffer layer was also added to strengthen the adhesion between UBM and isolation films. After material property analysis and experimental trial runs, Polybenzoxazole (PBO), one kind of passivation material which has been extensively investigated was selected because of its excellence and merits in dielectric, electrical, thermal, and mechanical properties, such as low dielectric constant, low stress, high modulus, chemical resistance, high electrical resistivity, and thermal stability and low coefficient of thermal expansion [6]. In many cases, PBO was preferred over polyimide (PI), as it absorbed less water, and can be cured at lower temperature and still retained its excellent mechanical characteristics.…”