edfas.org 23 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 commonly found in integrated circuits (Si, SiO2, Cu, Al, W) for 30 kV Ga+ ions using SRIM 2008.04. The defocus and overtilt allow each ion to interact with the milling surface at an angle that increases the sputter yield. It also creates a condition where curtaining will be minimized because each ion has a line-of-sight impact along the milling surface. For example, tungsten has a much lower milling rate than copper or aluminum; when FIB-milling a crosssection with a traditional recipe and a well-focused beam, the material below a tungsten via will be shadow-masked and will create an uneven surface that leads to a curtain, however the defocused method is almost like a vertical delayering instead of milling a vertical cross-section. Figure 4 shows a FIB-milled cross-section of an unpolished, mechanically thinned Si substrate using this beam defocus concept. The thinning was conducted on an X-Prep (Allied High Tech) leaving an extremely rough top surface. The FIB cross-section was conducted on a Thermo Fisher Scientific Nova NanoLab using its maximum voltage and current (30 kV, 21 nA) producing a surface with almost zero curtaining. A modified “regular cross-section” milling recipe was used for this cut, where “passes” was set to 1 and the “scan ratio” was set to 0.99. The scan ratio is defined as the beam dose delivered at the first line divided by the dose delivered at the last line where the milling dose of all intermediary lines are linearly interpolated, e.g., a scan ratio of 0.25 means the beam will deliver four times the dose at the final line as it does at the initial line, i.e., one mills four times as long at the final line than the initial line. The cross-section in Fig. 4 has nominal dimensions of 250 µm wide, by 18 µm deep milled at a stage tilt of 55° with a total beam spot diameter of approximately 5 µm giving an instantaneous current density of 1.0 nA/µm2, a dwell time of 0.2 µs, an x-overlap of 50% and a y-overlap of 99%. A couple points to note concerning the minimal curtaining of the cross-section seen in Fig. 4. First, the dwell time is kept relatively low so that the beam essentially “sweeps” across the milling surface and does not sit in a specific location for any length of time. Second, the y-overlap is very high so that a particular location on the milling surface sees the ion beam for many consecutive milling lines. In this case the y-pitch is 50 nm, meaning that the surface will be milled by ions for 100 consecutive lines. These two factors together allow for the creation of a smooth milling surface even though the top surface is what some might consider to be a “hot mess.” THERMAL DAMAGE MITIGATION The final, and possibly most important advantage that ion beam defocus provides is the ability to keep the thermal spikes from individual ion/sample impact events separate even within the defocused ion beam. The ion/ sample interaction is a very complex process, for a thorough explanation see this book on ion bombardment[4] and the references therein. It has been long known that continuous ion/sample events bear little resemblance to individual events.[5] Thermal damage from FIB-milling is recognized as a major artifact for polymers[6] and biological materials,[7] and is generally mitigated for these classes of materials by using lower energy ions and by cooling the sample with a cryogenic stage. For example, when FIB-milling using a standard recipe, thin films of polymer corrosion barrier coatings have been shown to develop voids as far as 5 µm away from the milling surface due to vaporization of the ceramic pigment nanoparticles in the film.[8] Similar behavior can be seen in Fig. 5a which shows a FIB-milled cross-section of AZ10XT photoresist lines on a silicon substrate using a medium ion beam (30 kV, 9.1 nA) with a standard cleaning cross-section recipe (dwell of 1 µs, x-overlap of 85%, y-overlap of 50%, beam diameter of 267 nm with zero defocus giving an instantaneous current density of 162 nA/ µm2). Figure 5b shows the same photoresist lines milled with a defocused ion beam (30 kV, 9.1 nA) and a modified cleaning cross-section recipe (dwell of 200 nm, x-overlap of 0%, y-overlap of 90%, beam blur of 500 nm for a total beam diameter of 567 nm giving an instantaneous current density of 36 nA/µm2). There is a small amount of photoresist volatilization along the lines’ top surfaces in Fig. 5b, however it is not nearly as damaged as the lines in Fig. 5a. How hot does the milling surface get in Fig. 5a? Thermal gravimetric analysis (TGA) in an inert atmosphere shows that AZ10XT volatilizes at temperatures between 350- 450 °C,[9] very hot indeed. Fig. 5 AZ10XT photoresist FIB-milled using a 30 kV, 9.1 nA ion beam with (a) standard cleaning cross-section recipe exhibiting large amounts of volatilization compared to (b) defocused ion beam recipe showing almost no volatilization. (b) (a) (continued on page 26)
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