Sputtering rates are normally measured by recording the depth of a crater after a several minutes of sputtering. The sputtering rate per unit area (g/s/m2) is then equal to the depth of the crater (um) divided by the time of sputtering (s) and multiplied by the density of the material (g/cm3).
Several types of instruments are used to measure crater depths. Amongst these are profilometers, also called surface roughness instruments, and interferometers. The most common profilometers use either a contact diamond stylus, optical focus, or laser. Some profilometers are capable of recording the whole crater but many are capable of only a single trace across the crater.
Line Scan vs Area Scan
A sputter crater formed on an iron sample, recorded with a scanning laser profilometer. The peaks on the edge of the crater are an artifact caused by flaring of the laser at the sharp edges of the crater.
To determine the average depth of the crater it is first necessary to level the slope of the sample and then to identify the areas inside (blue) and outside (yellow) the crater. The difference in height of these two regions is then given here as 16.8 mm.
A single laser scan across the crater is shown below, the rough bottom is caused by the differential sputtering of different grains in the iron sample:
Again it is a matter of identifying the regions inside and outside the crater:
The difference in height of these two regions from the single trace is 16.9 mm.
Here the crater depths estimated by scanning the whole crater or by a single scan across the crater give almost the same answer. This is usually the case when the crater bottom is nearly flat, as here. But will not generally be the case if the crater bottom is not flat.