Native oxides on Si surfaces of deep-submicron contact-hole bottoms

The effects of cleaning and treatment on the characteristics of contact-hole-bottom Si surfaces are investigated in order to reveal the origin of the increased contact resistance and to find treatment processes that can be used to obtain low contact resistance. Contact-hole-bottom Si surfaces were analyzed by using thermal desorption spectroscopy, transmission electron microscope, and energy-dispersive x-ray spectroscopy. Nonpatterned Si surfaces, which roughly simulate the properties of the contact-hole-bottom Si surfaces, were also analyzed by using x-ray photoelectron spectroscopy. It is revealed that suboxide-rich native oxide layers are formed on dry-etch-damaged Si surfaces. The oxide layer persists after the samples are cleaned with a mixture of NH4OH, H2O2, and H2O, and with a mixture of HCl, H2O2, and H2O, and even after dipping in diluted HF. The roughly 1.3-nm-thick oxide layer remains at the plugging-poly-Si/Si–substrate interface, increasing the contact resistance. The carbon contamination in the dry-etch-damaged layer contributes less to the increase in contact resistance. The dry-etch-damaged Si layer is removed by chemical dry etching. On the resultant damage-free surfaces, native oxides with low suboxide density appear after NH4OH/H2O2/H2O and HCl/H2O2/H2O cleaning. Such oxides are easily removed by treatment with diluted HF, resulting in low contact resistance. An integrated contact-hole treatment sequence is thus achieved to control the Si surface condition. The resultant low-contact-resistance deep-submicron contact holes, plugged with P-doped poly-Si, can be applied for deep-submicron contacts of 256 Mbit and larger dynamic random access memories.