ELECTRON-BEAM-INDUCED DAMAGE IN AMORPHOUS SIO2 AND THE DIRECT FABRICATION OF SILICON NANOSTRUCTURES

We have investigated the behaviour of self-supporting amorphous SiO2 ( a-SiO2) thin films under 100 keV electron-beam irradiation in a high-d ose regime (10(7)-10(9) C m(-2)). Electron-energy-loss and energy-disp ersive X-ray measurements show that oxygen is preferentially lost duri ng the damage process which leaves the irradiated a-SiO2 oxygen defici ent. The results are discussed in terms of previously reported models, which suggest that the mass loss from a-SiO2 is attributed to a combi nation of high-energy sputtering, surface desorption and volume-dissoc iated mechanisms. The oxygen can be totally removed from a-SiO2 layer 15 nm thick after a dosage of approximately 3 x 10(9) C m(-2) electron s. On the basis of the discovery of this effect we develop a controlle d way of making nanostructures of silicon directly from a-SiO2. In par ticular, if a-SiO2 is irradiated with a highly intense electron beam o f nanometre scale, then a column of silicon is formed, which can be as small as 2 nm in diameter. If the beam is moved in a straight line, t hen a thin plate of silicon is formed. These silicon nanostructures ar e formed directly under electron irradiation after a dose of 10(9) C m (-2) of 100 keV electrons and no resists or chemical development are r equired.