Sample 1 is a silicon lamella shown after FIB lift-out and thinning in the SEM image in . shows the TEM high angle annular dark field (HAADF) image of the lamella after HIM modification. The three dark vertical grooves indicate the areas modified in the HIM. In one effect of the helium ion modification process is clear; material is selectively removed from the sample sidewalls. The thickness map of the modified area is shown in . From this map we can calculate quantitative thickness values based on the mean free path of a 300 keV electron in silicon (180 nm) [22
]. The arrows on the image indicate the regions from which the integrated intensity profiles were plotted in . The dashed red line is from a region prepared by gallium ions only. The solid blue line is from the helium ion modified area. The profiles both show an increase in thickness with increasing distance from the top of the lamella, indicating that the sample has a wedge shape. This increase is more gradual and noticeably smoother after helium ion modification. The modified area was observed to be consistently thinner than the unmodified region when compared at the same distance from the top of the lamella. The suitable area for TEM extends further from the top of the sample in the modified region, this results in a larger area useable for TEM in samples modified by the HIM. shows the EFTEM gallium map of the region. The areas of higher intensity in this map have a greater concentration of gallium contamination. The solid red arrow indicates the region from which the integrated intensity profile in was plotted. The solid white line is below the FIB prepared areas, while the dashed green line is beneath the areas further modified by helium ions. In the intensity profile from is plotted with the corresponding ion beam used to modify the area indicated below. This graph clearly shows a significant reduction in gallium contamination implanted by the FIB lift-out process. Typically around a 32% reduction in gallium concentration is achieved by HIM modification.
Figure 1 (a) SEM image of the silicon lamella (sample 1) after FIB preparation. (b) HAADF TEM image of the sample after three separate areas (observed as three vertical dark streaks) were modified by helium ions. (c) Thickness map of the modified area. The dashed (more ...)
High resolution TEM (HRTEM) was performed on the HIM modified grooves and the unmodified sidewalls to afford further insight into the material modification. is the HRTEM image of the unmodified region of silicon; is the corresponding selected area diffraction (SAED) pattern from the region. is the HRTEM image of the HIM modified region of the sample, is the corresponding SAED pattern. displays a noisy HRTEM image when compared with that of from the HIM modified region of the sample. The inset FFT of the images also show the increase in high frequency information attained from the modified region. The uniform background contrast of the modified area indicates that it has a more uniform thickness. These images indicate that the amorphous layer of material on the sample, which contributes to background noise only, is reduced by HIM modification. Similarly, the SAED pattern in shows less information than that in . The extended high frequency information in the diffraction pattern recorded from the HIM modified region in indicates that this area of the sample is thinner, while still retaining its high quality crystalline structure. The diffraction patterns show that the sample was measured along the  direction.
HRTEM (a) and diffraction (b) information from gallium finished region. HRTEM (c) and diffraction information (d) from helium modified region.
This analysis of sample 1 clearly shows us that HIM modification of a FIB prepared TEM lamella can be used to further reduce sample thickness while removing contamination and also retaining the crystallinity of the material. In order to investigate this polishing effect further we used the HIM to modify a TiO2 TEM lamella prepared by FIB lift-out. We called this sample 2. is a HAADF image of the sample after FIB lift-out, gallium contamination is observed as the small white spots on the image, the background noise is also large. The same sample was analyzed again after modification with 35 keV helium ions. The HAADF image of the modified region in shows a striking improvement as the surface agglomerations were removed and the contrast in the image was greatly improved.
(a) HAADF image of the TiO2 sample (sample 2) after FIB lift-out. (b) HAADF image of the sample after HIM modification.
In samples 1 and 2 we have used lamellae finished only with 30 keV gallium ions in the FIB. A lower energy gallium ion beam can be used to produce lamellas with significantly less FIB induced damage [23
]. FIBs with low energy capability have become more widely available over the past few years. We prepared the silicon lamella in sample 3 with a 5 keV gallium ion beam final polish in order to reduce FIB induced artifacts which would obscure our analysis of the patterning and subsurface modification effects of the HIM modification. Sample 3 is shown after FIB lift-out in the SEM image in . is an illustration of the beam–sample geometry used to modify the sample in the HIM. The sample sidewall was inclined 15° to the beam. This geometry was used in order to mill a wedge shape within the lamella in order to observe the minimum thickness dimensions which can be fabricated by HIM. This geometry also allows us to observe the extended effects of the modification process. is a bright field TEM image of the area of the sample after controlled sidewall modification by helium ion irradiation. is a HAADF image of the same area. A rectangular hole is observed at the center of the image, this was fabricated due to the high dose used. Below this hole is a circular region with rapid variations in contrast. This circular area has been heavily modified by the HIM. is a thickness map of the area. Below this map is the integrated intensity profile of the area indicated by the solid red arrow on the thickness map. This profile shows the sloping thickness of the wedge fabricated by helium ion irradiation, followed by the hole where the beam penetrated the lamella. Finally, the circular feature is observed to have rapidly varying thickness. The hole has a non-zero thickness due to the limitations of the thickness mapping process, such as its tendency to overestimate the thickness of very thin areas [24
]. It is well known that helium ion irradiation, with an appropriate beam energy, can produce helium bubbles beneath the surface of a sample [25
]. In this case the center of the circular feature is approximately at the implantation depth of 35 keV helium ions in silicon, about 318 nm (SRIM) [26
]. This is made clear by the SRIM simulation image inset in showing the distribution of 35 keV helium ions in silicon; this simulation has the same scale as the image and correlates well with our experimental data. What we observe in this region is the implantation of helium ions, where the incident helium ions have been scattered by the silicon and have come to rest within the sample. These implanted ions then lead to the formation of bubbles beneath the surface which stretch and distort the silicon. The contrast observed corresponds to regions where helium bubbles have formed and silicon has been displaced.
Figure 4 (a) SEM image of the silicon lamella (sample 3) after FIB lift-out and a 5 keV gallium ion polish. (b) Illustration of the geometry of the helium ion beam irradiation. The red arrow represents the helium ions which are incident on the face of the sample (more ...)
Thickness map of the modified region. The arrow indicates the area along which the integrated intensity profile below the image was plotted.
At this point we have described the morphological changes induced by a high dose of HIM irradiation on sample 3. We then investigated the effect of the HIM modification on the structure of the silicon. is a graph of the EELS spectra recorded from three different locations on the sample. The black solid line at the top was recorded at a region which was not modified by helium ion irradiation. The red dashed line in the middle was recorded from the wedge shape region fabricated by helium ion irradiation (marked “I” in ). And finally the blue dotted line at the bottom was recorded at the circular feature (marked “II” in ). When we analyzed our three EELS spectra in (spectra are offset for clarity) we found that the intensity of the first peak in the spectra at ≈99 eV was observed to degrade from the top spectrum to the bottom. We compared our data to the spectra for crystalline and amorphous silicon from an online database [27
]. The intensity of the peak at ≈99 eV is used as an indication of the crystallinity of the silicon. A higher intensity indicates more crystallinity in that area, a lower intensity corresponds to an area which is more amorphous [28
]. The top spectrum in our data corresponds to an area of high crystallinity, as indicated by the presence of a peak in this region of the spectrum. This result was to be expected as the spectrum was recorded from an unmodified region of the sample. However, the spectrum from the wedge shape area fabricated by the HIM (marked “I” in ) shows a high degree of amorphization as the intensity of the first peak at ≈99 eV is greatly reduced. The spectrum from the area of the sample containing the bubbles (marked “II” in ) shows an area which is even more amorphous again.
Figure 6 (a) EELS spectra of an unmodified area of silicon (solid black line), the HIM fabricated wedge marked I in (d) (dashed red line) and the circular featured marked II in (d) (dotted blue line). The spectra are offset for clarity. (b) HRTEM image (more ...)
is a HRTEM image from the wedge area (marked “I” in ) with an inset FFT of the image. This image shows only amorphous material is present at this location. is a HRTEM image of the area with the circular feature (marked “II” in ). This image shows the presence of circles created by the growth of helium bubbles. No crystal structure was observed in this region either. We have observed that a high dose of HIM irradiation on sample 3 was used to fabricate a smooth wedge of material on a TEM lamella with minimum thickness dimensions of just a few tens of nanometers. This thickness may even be less than our thickness map indicates as significant errors may be present in the mapping process for very thin samples due to surface excitations, which can lead to overestimation of the thickness in this region [24
]. The lateral dimensions of the pattern can also be tailored with a high degree of precision; the HIM can fabricate 4 nm nanopores quickly and reliably [29
]. The EELS spectra and HRTEM from the HIM modified areas show that the wedge fabrication process caused significant amorphization of the sample in that region. The beam particles were also observed to implant in the sample below the wedge causing bubbles to form in the material, again resulting in significant amorphization of the silicon, as observed by EELS and HRTEM.