Patterning Basics

Table of Contents

Overview #

Patterning is the process of milling, depositing, or etching a pattern into a sample surface with the ion beam. During patterning, the ion beam is rastered across the sample surface. The beam stays in one scan point for a designated time (known as the dwell-time or scan-time) before it moves on to the next spot (similar to electron beam imaging processes). The scan points are usually overlapped so that a smooth cutting edge is achieved. The principles covered here are valid for all ion species and FIB systems.

Figure 1. Schematic of ion beam patterning parameters.

Pattern Parameters #

Beam Current #

The ion beam current describes the flux of ions to the sample surface. The ion beam as generated by the ion source is a column of ions that is the maximum amount of ion current that the source can produce. To reduce the beam current, a series of holes of different diameters are used to reduce the amount of the ion beam column that reach the sample (Figure 2). Lowering the beam current also reduces the ion beam spot size, enabling milling of finer features. Changing the beam current will also effect the beam dose the sample receives.

Figure 2. Schematic of the ion beam aperture strips function to reduce the ion beam size and the resulting beam spot size on the sample surface.

Beam Accelerating Voltage #

The acceleration voltage has a dramatic effect on the sputtering yield. The highest acceleration voltage (usually 30 kV) is usually used for sputtering as it provides fast sputtering and the best resolution, which is an important factor for nanostructuring. Lower voltages are conventionally only used to control the size of damage layers / ion dose or to reduce the ion beam induced heating of the sample.

Stage Tilt #

The stage tilt determines the incident ion angle, and therefore the location of the interaction volume in the sample. Patterning is usually performed with the ion beam perpendicular to the sample surface. Polishing the sample for EBSD analysis or final TEM lamella preparation polishing is usually performed at larger glancing angles, enhancing sputtering rates and confining the interaction volume and sample damage to the sample surface. Tilting is also often used in TEM lamella preparation to compensate for the beam tails or enhance patterning speed.

Scan Direction #

The scan direction is used to control the direction of redeposition. Redeposition of sputtered sample atoms occurs at the backside of the scan direction (where the scan started) and is indicated by the purple lines in Figure 3. Various different scan strategies can be used to reduce the effects of redeposition on patterning speed (top-to-bottom, bottom-to-top, serpentive), or spread the redeposition over the scan area (rotating edge) and therefore reduce the impact of it in any single area.

Figure 3. Schematic of different scan direction schemes.

Repeats or Passes #

Repeats or passes describe the number of times the ion beam is rastered over a pattern. In a single pass setup, the ion beam stays at each point of the raster until the desired depth is reached, then moves onto the next raster point. In multi-pass setup, the milling is done to a portion of the final depth at each spot before returning to the starting point of the pattern and repeating the pattern.

Sample sputtering yield depends partially on the incident angle of the ion beam, therefore if a milled edge is created and the ion beam is advanced slowly enough to keep the beam on the edge, faster sputtering occurs than if milled into the flat area of the surface (Figure 4 left). The edge naturally occurs due to the ion beam tails; walls are always sloped to some degree. A single repeat of the milling pattern advances the beam slowly enough to keep the edge and maintain fast sputtering.

Figure 4. Sketch of the effect of a single repeat vs. a multiple repeats of a milling pattern on the depth of a trench cut in a sample.

Using multiple pattern repeats lessens the milled edge depth and decreases the sputtering yield (Figure 4 right). The advantage of the multi-repeat, however, is that it removes redeposited material on each pass. The redeposited material removal also slows down the patterning. As shown in Figure 5, the same pattern milled in a single pass method results in a box that is deeper at the bottom of the pattern where the milling pattern ended, while the multi-pass pattern is shallower. The single pass pattern has more redeposition at the the top side of the pattern where the milling began, while the multi repeat pattern has uniform milling depth across the entire pattern.

Figure 5. Ion beam image of a the results of milling the same pattern with a single or multi-pass option.

The patterning pass option that is chosen depends on the application. Single repeat/pass is usually used for fast sputtering and material removal where redeposition is not critical, e.g. bulk cross-sectioning and TEM/APT sample preparation. Multiple repeats are predominantly used if even surfaces and high structure quality are of importance.

Pitch and Overlap #

The pattern pitch or overlap set the distance between the ion beam dwell points during patterning. While the two terms describe the scan points, each has a different definition that are illustrated in Figure 6:

  • Pitch: Distance between the centers of the scan points
  • Overlap: Percentage of beam overlap between two adjacent scan points
Figure 6. Diagram of the differences between overlap and pitch, and the affect of ion beam spot size (blue circles) on the overlap of scan points with identical pitches (red arrows).

Overlap consistently describes how much of the beam interacts with the adjacent scan points and is independent of the beam spot size, but the distance between points will vary. Pitch has a consistent distance between the scan points, but the amount of interaction between adjacent scan points varies. Larger ion beam currents typically have larger beam diameters.

Figure 7. Illustration of the effect of beam overlap. In the top image, the larger overlap creates a smooth finish on the cross-section, while in the lower image with less overlap leaves an uneven polish on the section.

The ion beam conventionally overlaps 50% between the scan points to ensure a smooth milling edge (Figure 7 top).  Reducing beam overlap is done to either reduce the ion beam induced heating of a sample or speed up milling, but results in a milled edge that is less smooth and rippled (Figure 7 bottom).

Serial or Parallel Milling #

Multiple patterns can be created in either serial or parallel mode. During serial milling, the patterns are  created one after another: pattern 1 is created completely first, pattern 2 is created completely next, then finally pattern 3 is created.

Parallel patterning mills all patterns at the ‘same’ time: pattern 1 is created to a specific depth, then pattern 2 is created to the same specific depth, pattern 3 is created to the same specific depth afterwards. The ion beam is then returned again to pattern 1 to mill to the set depth, and the process is repeated. All of the parallel milled patterns are completed at the same time.

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