Christopher Ahles1 Jong Youn Choi1 Andrew Kummel1

1, University of California, San Diego, La Jolla, California, United States

As devices are scaled to sub 5nm, it is critical to prepare clean and atomically flat surfaces. The traditional aqueous HF clean for removal of native Si oxide suffers from an inevitable air exposure resulting in re-oxidation of the Si surface as well as carbon contamination. The Siconi™ process is a dry clean which utilizes a low temperature (<30C) NF3/NH3 based plasma to selectively etch the native oxide layer on Si without significantly etching the underlying Si layer. The selectivity is based on the plasma chemistry NF3 + NH3 --> 3F + 3H + 2N --> 3HF + N2 where HF is formed in the gas phase and then proceeds to etch the native oxide in a similar manner to aqueous HF. However, unlike aqueous HF the Siconi™ process leaves behind a hexafluoroammonium silicate salt, (NH4)2SiF6, which must be removed in a subsequent anneal. In this work we present a modification of the Siconi™ process which eliminates (NH4)2SiF6 salt formation and provides a surface which is just as flat as with the traditional aqueous HF clean.
The silicon surface following removal of the native oxide with an NF3/NH3 plasma was studied using X-Ray Photoelectron Spectroscopy (XPS) for elemental analysis as well as Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) for surface roughness. It was found that when the silicon substrate is held at 65-70C during the etching no (NH4)2SiF6 salt formation is observed. The NF3:NH3 ratio was found to be a critical parameter for controlling the surface roughness, with the surface roughness decreasing with decreasing NF3:NH3 ratios. A NF3:NH3 ratio of 1:5 gave a Si surface with an RMS roughness of 3.4nm while a NF3:NH3 ratio of 1:10 gave a Si surface with an RMS roughness of 1.9nm. Further decreasing the NF3:NH3 ratio is expected to provide an even smoother surface, and experiments towards this end are ongoing. The plasma time was also optimized, and it was found that exposure to the NF3/NH3 plasma for 10 seconds and 1 minute resulted in essentially no removal of the native oxide, while exposure for 2 minutes resulted in complete removal of the native oxide. For comparison, Si surfaces cleaned with aqueous HF in our laboratory typically have ~10% oxygen and ~10% carbon contamination, while after cleaning with a NF3/NH3 plasma with NF3:NH3 = 1:10 for 2 minutes at 70C there was 3% O, 0% C, 4% F and 4% N impurities. Additionally, all of the silicon was in an oxidation state of 0 which means that there was no (NH4)2SiF6 salt formation since this salt has silicon in an oxidation state of +4. To validate the applicability of this process, a molybdenum silicide film was deposited via atomic layer deposition (ALD) on dry-cleaned Si as well as on HF cleaned Si. The RMS roughness of the ALD MoSix film on the dry-cleaned Si was 2.26nm while on the HF cleaned Si the RMS roughness was 2.78nm. This shows that the dry clean developed in this study is capable of producing cleaner and smoother Si surfaces than the traditional aqueous HF clean.