Focused ion beam sputter yield change as a function of scan speed

In many of the applications of focused ion beams, such as integrated circuit sectioning and TEM sample preparation, considerable volume of materials may need to be removed. Thus optimizing the sputter yield is important. For very rapid scan speeds at normal incidence, each pass of the beam removes a thickness of material which is much smaller than the beam diameter. In this case, this milling yield corresponds to the yield at normal incidence. However, if the scan speed is slowed down so that the thickness removed per pass is comparable to the beam diameter, then locally under the beam the ions are not normally incident even though the beam is normal to the surface. The milling yield of Si and SiO2, for example, increases by a factor of seven to eight in going from normal incidence at 0° to 75°–85°. Thus the material removal rate can be significantly increased by reducing the scan speed. We have measured the milling yield of Si and SiO2 as a function of scan speed in one axis by milling boxes, typically 7 μm×9 μm using 30 keV Ga+ ions. In the other axis, the scan speed is many orders of magnitude faster so that the beam can be thought of as a sheet or “blade.” To measure the dependence of yield on scan speed, we milled the boxes with a single scan in the slow direction, and then measured the depth of the boxes with an atomic force microscope. The beam currents used were 1 and 2.9 nA. At the slowest single-axis speeds, the sputter yield increased by a factor of two. The observed dependence on scan speed agrees with existing models, which assume a single local angle of incidence under the beam. We also measured the redeposition on the bottom of the pit and found that it increased as the scan speed was decreased. In many applications, a small amount of redeposition would be unimportant.