Fundamental Processes in SiO2 under Ion Bombardment

Doctoral thesis, 1993

Investigations of sputtering, electron emission and oxide growth under 40-300 keV H+, He+, N+, Ne+, Ar+, Kr+ and Xe+ ion bombardment of SiO2 are presented in this thesis. To accurately determine SiO2 sputtering yields, a new measurement method was developed where the electron emission was monitored during sputtering through thin SiO2 films on Si substrates. Sputtering yields were thereby determined both in ultrahigh vacuum (UHV) and during exposure to O2, N2 and CO. The UHV sputtering yield was found not to be proportional to the energy deposited in elastic collisions at an SiO2 surface and therefore it is suggested that SiO2 sputtering is controlled by a mixed collisional-electronic mechanism. The sputtering of SiO2 films was found to decrease by up to 50% with increasing O2 partial pressure but no dependence on N2 or CO partial pressure was observed. The reduction in sputtering at O2 exposure is larger the smaller the ion mass and it is argued that this may be due to mass dependent preferential sputtering of oxygen and/or ion stimulated incorporation of oxygen in the surface layers. The ion induced electron yield of SiO2 films on Si was measured as a function of oxide thickness, ion mass, ion energy and angle of ion incidence. For oxide films thicker than 200 ], the electron yield was found to be approximately independent of oxide thickness, but close to the SiO2/Si interface unexpected variations in the electron yield with oxide thickness were found. Furthermore, it was found that the electron yield of a thick oxide increased with the energy deposited in electronic excitations approximately to the power of 0.65 and not to the power of 1 as expected from theory. A satisfactory interpretation of the data could be obtained with a suggested model where the positive holes, left behind in the oxide by the liberated electrons within electron cascades of individual ions, cause the probability of escape of electrons to decrease with increasing electron yield. For the first time, it was observed that a 40 keV N+ ion beam could induce oxidation of Si at room temperature if the bombardment is performed in oxygen at low pressure. It was noted that the attainable oxide thickness increases with increasing availability of oxygen on the target surface. The maximum oxide thickness produced is estimated to be in the range 50-100 ] and, from XPS measurements, it is also concluded that the oxide formed is stoichiometric SiO2. A number of possible growth mechanisms for this have been pointed out.