Effect of Hydrogen for Preservation of Reconstructed Surfaces

Abstract

It is well known that a smooth surface of Si wafers can be obtained by Si surface reconstruction via high-temperature annealing. However, there remains a possibility of smooth Si surfaces deteriorating by accidental oxidation (called reflow oxidation) during the unloading process, i.e., taking out Si wafers from a vertical furnace after high-temperature annealing. Therefore, we considered it important to investigate the atomic-scale effects of oxidation on surface steps and terraces on Si wafers during the unloading process. We examined the effect of unloading temperature on oxide formation on Si (100) and Si (110) surfaces. The change in surface roughness was also measured. Our results indicated a significant improvement in the root mean square values of the surface roughness of terraces on the reconstructed surface. Moreover, this improvement was dependent on the decrease in the oxidation layer thickness in the case of low-temperature unloading. Furthermore, for suppressing reflow oxidation, we replaced the injected Ar gas with H2 in the cooling process during high-temperature Ar annealing and evaluated the thickness of the reflow oxidation layer and surface structure of Si (100) and Si (110). H2 annealing during the cooling process resulted in the formation of H-terminated Si surfaces, and this formation effectively suppressed reflow oxidation. However, the H2 atmosphere also caused etching of the reconstructed Si surfaces. Atomic force microscopy measurements revealed that in spite of the etching, Si (100) and Si (110) surface roughness drastically decreased because of subsequent roughness variation, regarded as being caused by oxidation. In the case of Si (110), characteristic line oxidation was effectively suppressed, resulting in a smooth terrace-and-step structure. In summary, the obtained results suggested that our method is effective for restraining the increase in atomic-scale surface roughness due to oxidation.