Hard Mask Fabrication for IBC-SHJ Solar Cells & Material Properties Investigation

Abstract

This thesis studies how certain fabrication methods influence the properties of nanocrystalline silicon (nc-Si:H) layer for interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells using MoOx. The study focused on two main objectives: first, characterizing nc-Si:H and amorphous silicon (a-Si:H) layer depositions with hard masks already fabricated in the PVMD group and improving hard mask design, and second, optimizing the hard mask production process and evaluating the resulting layer quality. In the initial phase, Raman spectroscopy and profilometry were used to assess layer properties. The influence of PECVD deposition of nc-Si:H layer with CO2 plasma treatment was investigated. Raman analysis indicated that a CO2 plasma treatment step promotes better crystalline growth in nc-Si:H layers. Profilometry revealed that depositions using old hard masks resulted in finger profiles that deviated from the target rectangular shape, with thicknesses below the desired 50nm and arched walls. To address these limitations, a new hard mask design incorporating 40nm supports on all fingers was developed, preventing structural deformation during deposition.

The second phase addressed production challenges, specifically overheating during dry etching caused by the use of carrier wafers. Introducing an AlSi landing layer enabled etching without a carrier wafer, reducing the number of etching cycles from 625 to 425 and improving hard mask integrity. SEM analysis confirmed smoother edges and more uniform structures with the new process. PECVD depositions of nc-Si:H layer using the improved masks showed significantly enhanced finger profiles, achieving target thickness, steeper walls, and nearly right-angled edges, particularly when combined with CO2 plasma treatment. Photoconductance lifetime measurements further demonstrated that layers produced with the optimized process exhibited longer minority carrier lifetimes, and patterns created with hard masks outperformed those patterned via conventional lithography.

Overall, this work demonstrates that improving the hard mask production process, alongside its design, plays a critical role in enhancing nc-Si:H layer properties, thereby contributing to higher-performance IBC-SHJ solar cells.