FIB Incidence Angles Impact Lift-Out Sample Success
by Cheryl Hartfield
The interaction of an ion beam with a material, at a given kV, varies as a function of the material type and the angle of incidence (AOI). The AOI is the angle of a beam relative to a vertical unit vector drawn normal to the sample surface. It defines the approach angle of an ion beam to a surface that will be imaged or milled. The AOI influences milling rate, amount of redeposition, and sample geometries. Since the ion beam axis is fixed on the majority of ion beam microscopes, in practice the AOI is varied by adjusting the stage tilt.
Small changes to the AOI, depending on application, can have a large impact to the end result. One such example is the case of final TEM thinning, where tilt offsets of plus or minus 1 to 2 degrees from normal can make the difference between sloped or vertical lamella faces. Likewise, the AOI plays a large role in the success of in situ lift-out. Large lift-out samples destined for TEM/STEM, EDS, or 3D imaging can be created rapidly by performing “wedge preparation”.
Wedge preparation is a common approach used by Omniprobe’s Total Release™ in situ lift-out method and atom probe preparation. The wedge is created by milling at 2 angles of incidence. If not understood and correctly configured, the final geometry of the lift-out sample may not contain the region of interest. High AOIs create shallow samples, while low AOIs create deep samples.
One way to rapidly calculate different AOIs is by use of any of the free online triangle calculators, such as those available at HandyMath.com, ostermiller.org, or csgnetwork.com. These are useful to identify the AOI required for a given depth.
Once the required AOI is identified, it is simple to calculate the required stage tilt to achieve the AOI by the formula:
AOI = ION BEAM AXIS ANGLE - STAGE TILT
The ion beam axis angle is defined by the position of the ion column relative to the vertical line (electron column axis). Most instruments on the market use a primary ion beam axis of zero degrees in the case of single beam FIBs, or an ion beam axis between 52 to 54 degrees for FIB-SEM instruments. For the creation of wedge samples at high incidence angles >40 degrees, a 2 degree AOI variation impacts the final depth by ~1µm. Thus in practice, the milling recipe to achieve the geometry should readily translate between the different microscopes without requiring adjustment for the different ion beam axes, if a lift-out wedge is designed 1µm longer than the requirement. Note however, beam tailing may vary between columns, so fine tuning may still be required to adjust for beam tailing.
The example below can be used to estimate depth or AOI for a wedge with an 8um side.
The online triangle calculators are useful to create similar graphs for alternate side lengths. Understanding incidence angle and planning milling geometries beforehand make in situ lift-out preparation easier and faster.
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