NF3/NH3 Dry Cleaning Mechanism Inspired by Chemical and Physical Surface Modification of Si, SiO2, and Si3N4

As Si-based semiconductor technology has entered the 10 nm gate length regime, every semiconductor process technology has been developed to meet the severe process specification. Recently, chemical dry cleaning process technique using NF₃ and hydrogen-containing gas, such as H₂ and NH₃, has been introduced into Si-based IC fabrication industry to remove the surface native oxide selectively in the name of COR or NOR (1, 2). Fig.1 schematically depicts the main mechanism of the dry cleaning process with NF₃ and NH₃ gases using indirect plasma activation like down-flow plasma (3). Although the etchant-surface reaction mechanism of the NF₃/NH₃ dry cleaning is different from the common RCA cleaning or dry etching mechanism, the NF₃/NH₃dry cleaning technique has been mainly studied in the aspect of its application in the device fabrication.

                  In this paper, we have investigated the NF₃/NH₃ dry cleaning process with different Si-based materials, Si, SiO₂, and Si₃N₄ in terms of surface modification because Si-based process technology should provide techniques to control devices with high surface-to-volume-ratio in near future. The surface study of the NF₃/NH₃ dry cleaning has been explored using atomic force microscopy (AFM), spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) with changes in process parameters of gas ratio, pressure, and plasma power. Fig. 2 shows the surface morphology and removed film thickness results after the dry cleaning for 30 s at various process conditions and materials. We also found that SiO₂ surface roughness increased as the pressure of the plasma reaction chamber increased from 1Torr to 3Torr but the etched amount decreased for both NF₃:NH₃ratios of 1:1 and 1:2. SEM observation before the sublimation by heating step shown in Fig.3 indicates the rough surface was formed during the reaction in plasma chamber. Surface chemical compositions measured using XPS are shown in Fig. 4 in comparison with the wet chemical cleaning using 1% HF. Traces of residual F atoms on the surface were detected for all the Si-based materials after the dry cleaning and its chemical shift was depended on the process condition.

                  In conclusion, we have found that the NF₃/NH₃ dry cleaning process using down-flow plasma technique can cause significant surface modification on SiO₂ depending on the NF₃:NH₃ ratio, plasma power, and pressure of the plasma reaction before (NH₄)₂SiF₆ by-product sublimation step. These results might provide empirical evidence that there are competing reaction pathways producing different intermediate species in the condensed phase on the SiO₂ surface to form (NH₄)₂SiF₆ by-product depending on the process condition in contrast to the understanding on the NF₃/NH₃dry cleaning mechanism (3).

1. W.S. Kim, W.G. Hwang, I.K. Kim. K.Y. Yun, K.M. Lee, and S.K. Chae, Solid State Phenom, 103, 63 (2005).

2. Y. Hagimoto, H. Ugajin, D. Miyakoshi, H. Iwamoto, Y. Muraki, and T. Orii, Solid State Phenom, 134, 7 (2008).

3. H. Nishino, N. Hayasaka, and H. Okano, J. Appl. Phys., 74, 1345 (1993).