Helium And Hydrogen Ion Co-implantation Induced Surface Exfoliation In Si-based Materials And The Involved Mechanisms

The invention of the'smart-cut'process by Bruel has spurred increasing attention on high dose light gas ions implantation into Si-based materials, owing to the ability to fabricate high quality silicon-on insulator (SOI) wafers. In this work, two kinds of Si-based materials were implanted by He and H ions singly or successively. The formation and thermal evolution of the surface damage as well as micro-defects have been studied in detail by using various techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transimission electron microscopy (XTEM). The possible mechanism on the damage production has been also discussed and provided. The main research contents and results are as follows:(i) Cz n-type Si (100) wafers with a 220 nm top oxide layer were sequentially implanted with 40 keV He ions at a dose of 5×10¹⁶/cm² and 35 keV H ions at a dose of 1×10¹⁶/cm2, and were then subjected to furnace annealing in the temperature range from 400℃to 1000℃. Techiques of scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transimission electron microscopy (XTEM) have been used to characterize the surface morphology as well as defect microstructures. Our results clearly show that subsequent annealing at temperatures of 500oC and above leads to appearance of surface features as blisters and the localized exfoliation regions. The exfoliation occurs at a depth of about 420 nm from the sample surface, which is slightly larger than the projected range of He or H ions. XTEM observations reveal that He and H co-implantation together with high temperature annealing produces a variety of defect microstructures, including cavities, platelets, nanometer or micrometer sized cracks and dislocations. The platelets and cracks mainly distribute at the depth of about 420 nm from the sample surface, which are responsible for occurrence of the observed surface features.(ii) Cz n-type Si (100) wafers with a thermal grown Si3N4 layer of thickness about 170 nm were sequentially implanted with 40 keV He ions at a dose of 5×10¹⁶/cm2 and 35 keV H ion at different doses of 1, 5 and 10×1015/cm2, and were then subjected to furnace annealing in the temperature range from 400℃to 800℃. Techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM), have been used to characterize surface morphologies, defect microstructures as well as their thermal evolutions. Our results indicate that the creation of surface damage depends strongly on the H ion dose. Blistering or localized exfoliation of the top Si3N4 layer has been observed for the additional H ion implantation at the high dose of 1×1016/cm2. Two layer surface exfoliations have been well observed upon annealing, which correspond to the top Si3N4 layer at the intermediate annealing temperatures and the implanted Si layer at high temperatures, respectively. However, only the top Si3N4 layer exfoliations have been observed for the additional H implantation at the intermediate dose of 5×1015/cm2. However, no surface exfoliation has been found for the sample under additional H implantation at the low dose of 1×1015/cm2. The exfoliation efficiency has been found to increase with increasing annealing temperature, and seems to saturate at temperatures of 600℃and above. XTEM observations show formation of micro-cracks in both Si bulk and the original interface, which is responsible for the observed surface features. Based on the results from cross-sectional transmission electron microscopy (XTEM), the formation mechanism of micro-cracks has been discussed and presented. Moreover, the dependence of surface damage on the H dose has also been discussed.