Focused Ion Beam
Passes/Dwell time
The dwell time (or scan speed) sets the time that the ion beam remains in one spot. The dwell time is linked to the passes: the shorter the dwell time the more passes are needed to cut to a specific depth. Passes determine how often the ion beam is scanned over the entire area. A single pass only scans over an area once. Multiple passes allow the beam to have a shorter dwell time per spot but the area is scanned multiple times instead of just once. Shorter dwell times (~1µs) are selected when the sample shows heat damage or when redeposition needs to be reduced (ion beam mills over the redeposited areas and reduces redeposition). Longer dwell times are used when fast milling is desired or redeposition is not an issue. The long dwell time and single pass option means that each point is scanned until the desired milling depth is achieved. The next point is then milled to the desired depth and so forth. This means that the beam is slowly progressed along an edge and the resulting incidence angle (non-zero) leads to an increased sputtering yield (faster patterning). Redeposition into the already milled areas will occur and is more dominant for this setting. The multiple passes and short dwell times option means that the ion beam only mills each spot to a specifc depth and then moves on the next spot. The sample is milled down layer by layer.