Use of the Rise Distance Method to Measure Beam Size of a FIB
J. Orloff 1 * and L. Roussel2 1FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, OR 97124, USA 2FEI Company, Building AAE, PO Box 80066, 5600 KA Eindhoven, Te Netherlands
*
Jon.Orloff@fei.com
Introduction The performance metric of greatest interest to the user
of a focused ion beam (FIB) system is generally its resolution. Because of the difficulty in defining and measuring the resolution of a FIB system directly, its performance is often assessed using a method related to the beam quality instead [1, 2]. This consists of the measurement of the rise distance of the beam current as the beam passes across an edge, which, for low currents where spherical aberration can be neglected, is closely related to the full width at half maximum (FWHM) of the current density of the ion beam [3]. The edge, also known as the “knife edge,” corresponds to a sharp discontinuity in a specimen, as can be practically found on the surface of a graphite specimen. Because the rise distance can be used to obtain an idea of the dimension of the waist of a beam, it is, perhaps, an indication of the quality of an instrument. Because the rise distance depends on the quality of an edge, it is sometimes called edge sharpness. This concept bears similarities with the image sharpness method developed to assess the performance of SEMs, usually on gold nanoparticles on carbon specimen [4]. Rise distance is actually a convolution of the current density distribution with the properties of
the knife
edge and depends strongly on the spatial distribution of the secondary electron yield of the edge. By using the rise distance, different systems can be compared in a quantitative way [5]. To compare instruments, the identical specimen must be used and the measurements must be done in an identical way. This article discusses the method and some pitfalls in its application.
Rise Distance Method Te rise distance of a focused beam system is oſten resolution.
called It does not relate straightforwardly
to resolution because the definition oſten proposed for resolution, following Rayleigh, refers to the ability to distinguish two objects in an image. Te edge sharpness is a convolution of the beam current density distribution with the spatial distribution of the edge. Te concept of the rise distance measurement is shown in Figure 1 for a beam described by a current distribution J(r) (A·cm-2). As a beam crosses a knife edge (either a physical discontinuity or an actual edge), the current striking the knife edge increases, and a plot of the intercepted beam current against position looks like what is depicted in Figure 2. In this case, the current density distribution J(r) of a FIB was calculated and then integrated to give the current I(x) as a function of position. Experimentally, the intercepted beam current is measured via secondary electrons (Figure 3).
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Some Experimental Problems Although the rise distance method appears to be an
appropriate way to estimate the beam size of a FIB, some source of errors or misinterpretation must be kept in mind. First, what is detected is not the beam current itself but rather the secondary electrons produced by the beam; if the secondary electron yield is not constant, artifacts can be produced. Second, instrumental problems such specimen driſt and damage may play a role, but provided the right selection of experimental parameters and specimen—so far graphite has proven favorable in this respect—they turn out to be fairly minor. Tird, the noise issue is important because in a single beam sweep any individual pixel may contain only a few ions. Te variability of the secondary electron yield may be important. Figure 3 shows an actual rise distance measurement made
by sweeping a 1 pA Ga+ ion beam rapidly over a discontinuity in a graphite specimen. Te effect of noise is significant as each pixel in the image sees only a few ions. Although the actual FWHM of the beam is ~4 nm, in the single sweep shown in Figure 3 the beam has a 20-percent to 80-percent rise distance of 1.8 nm. Clearly a single measurement can lead to significant error. In Figure 4 we show the distribution of 10,000 20-percent rise distances
to 80-percent resulting from a theoretical
calculation of the “noisy” current for a FIB, from Figure 2b. For simplicity, the data are presented as a series of bins of width 0.5 nm located at xi = 2.75 nm, 3.25 nm, 3.75 nm, 4.25 nm, 4.75 nm, and 5.25 nm, containing 2, 110, 2192, 6717, 964,
Figure 1: The concept of a rise distance measurement. As the beam sweeps from left to right, the current intercepted by the knife edge rises from 0 to 100 percent of its full value. The distance over which this takes place is a measure of the beam size (see Figure 4).
doi:10.1017/S1551929511000253
www.microscopy-today.com • 2011 May
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