Roads to Ultrathin Silicon Oxides

Ultrathin gate dielectrics for complementary metal–oxide–semiconductor (CMOS) devices, with suitable structural and electrical properties, are crucial for the further development of silicon based microelectronics. The effective (SiO2-equivalent) thickness of 10 Å or below needed in the next generations of CMOS devices has been found too low to prevent tunneling, and leakage, with current processes for SiO2 based gate insulators.

Before abandoning SiO2 completely, however, there are good reasons to look for improved procedures or alternative processes to grow or form ultrathin SiO2 films on silicon, and possible improvements through the controlled addition of nitrogen. The present article initially describes an attempt to grow ultrathin oxides in a furnace, but this was limited to 50-Å-thick layers or above.

It then unveils some particularly simple, easily controlled, low-thermal budget, low-pressure based processes for thinner oxide layers, which have not been met earlier. These later processes are all done in an ultrahigh vacuum (UHV) based environment, starting from a clean and perfectly ordered Si surface.

Thus we formed the thinnest possible (≈4Å) uniformly covering oxide layers on the Si(111) and Si(001) surfaces. They are made very simply from cycles of oxygen adsorption at room temperature and short anneals, and are self-saturating at this thickness. Following these processes we explored isothermal methods in UHV at low temperatures and pressures.

Such processes, at low pressures, were found to lead to a universal, self-limiting growth of an approximately 7-Å-thick oxide at a range of temperatures between 300 and 700 °C. Further, up to about 10 Å oxides are grown in a series of steps, in each of which a layer of freshly deposited Cs on top of already grown oxide is retaining oxygen on this otherwise passivated surface.

The Cs layer also catalyzes oxidation during a subsequent rapid annealing step. Higher thicknesses (up to 50 Å) are obtained by using a precursor layer of Cs–oxide formed in alternating Cs and oxygen dosing processes, which is converted into SiO2 by heating. The present investigations are focused on structural properties of the systems studied with the use of electron spectroscopy, mainly photoemission with synchrotron radiation, in UHV.