Sequence of rectangular trenches in Si, milled with equal total ion doses but using an increasing number of sequential scans. The left structure gives an example of the -profile, which is observed when milling is done in single scan mode. A shallower trench with flat bottom surface is obtained when the same dose is distributed sequentially over 50 or 100 scan cycles (right structures). A further increase in the number of scans does not change the shape any more.
(a) Example of a milled structure. It was sputtered as well as imaged at the sputtering angle (normal incidence). The structure consists of a square hole, that is milled first, and two lines. The first line crosses the square hole in the middle, while the second line runs at a fixed distance of . Both lines are parallel to the tilt axis. As all structures are written with the same beam steering commands, structures milled at angles are in reality elongated by along the direction perpendicular to the lines. (b) The same structure imaged at the inspection angle (at 1.5 times higher magnification). In order to determine the depth of the sputtered structure, the parallel offset , as it is visible in the first line, is measured from the image. The known distance between the two lines compared to their distance, as measured from this image, allows for self-calibration of the length of , which is proportional to the depth of the hole.
Cross-sectional sketch of a structure milled at a sputtering angle after tilting into the inspection angle . The distance is the shift of the first line. From this shift, the depth of the hole can be calculated. For and the triangle ABC is isosceles, so that depth and shift are equal.
Comparison of the sputter yields of single crystalline Fe (squares) and W (circles; lines serve to guide the eyes). Both crystals were tilted from at normal incidence into crystallographic directions. Low-Miller-index orientations are marked on the top axis to indicate the directions of the strongest ion channeling. Each point of this plot is an average value of the sputter yields calculated from four squares milled with different ion doses.
Sketch of the alignment of atoms in a bcc lattice model. The atomic lattice is tilted to the left from to crystallographic directions. The atoms of a single elementary cell are rendered in black. The view from the top is shown for tilt angles of 0°, 7°, 26°, and 45°. At , there is plenty of space between the atoms for the incident ions. Already at 7°, the ions cannot avoid hitting upon an iron atom near to the surface. At 26°, narrow channels open up and at 45°, corresponding to the direction, broad channels open up, causing channeling and therefore significant reduction in the sputter yield.
Comparison of sputter yield (squares) with secondary electron yield (circles) of a single crystal of Fe. The crystal is tilted from at normal incidence into crystallographic direction. While the sputter yield is given in absolute units, the electron yield data have been scaled in magnitude. The solid line reproduces qualitative electron yield data from Yahiro et al. (Ref, 17) on this system, which has been adjusted, both in offset and magnitude, for a close match.
Atom alignment as viewed for 0°, 26°, 40°, and 54° tilt angle when tilting a bcc lattice to the left from to crystallographic direction. The atoms of one elementary cell are rendered in black. Narrow channels remain open at all sputtering angles. Channeling always takes place and sputter yield values are smaller compared to those obtained for crystals tilted into crystallographic direction.
Comparison of measured sputter yields with values simulated by TRIM (Ref. 8). For the two examined single crystals of Fe and W, the smallest and the largest values of the measured sputter yields for tilt angles ranging from 0° to 60° are shown. The sputter yield values for Si and GaAs were found with the ion beam being perpendicular to the target’s (100) surface. For polycrystalline Co, the maximum and minimum values of the sputter yield found for different crystallites are given. For polycrystalline Permalloy, the average sputter yield is given.
Article metrics loading...
Full text loading...