ISSN 10637850, Technical Physics Letters, 2010, Vol. 36, No. 11, pp. 991–993. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.N. Gerasimenko, A.A. Chamov, N.A. Medetov, V.A. Khanin, 2010, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2010, Vol. 36, No. 21, pp. 38–45.
Specific Features of Relief Formation on Silicon Etched by a Focused Ion Beam N. N. Gerasimenko*, A. A. Chamov, N. A. Medetov, and V. A. Khanin Moscow State Institute of Electronic Engineering (Technical University), Zelenograd, Moscow, Russia Molecular Electronics Research Institute and “Micron” Plant, Zelenograd, Moscow, Russia *email:
[email protected] Received June 7, 2010
Abstract—The effect of a focused ion beam on the state of a silicon crystal surface has been studied. Periodic ringshaped ribs have been observed on the walls of an etched cylindrical hole. The formation of periodic structures depends on the conditions of ion beam etching. The observed phenomenon is explained based on the notion of the radiationinduced plasticity. DOI: 10.1134/S1063785010110064
Focused ion beams (FIBs) are widely used in the technology of solid state microelectronics, in particu lar, for the formation of various profiles (grooves, cylindrical holes, etc.) on the surface of wafers and the preparation of sample structures to transmission elec tron microscopy examinations. It is commonly accepted that the walls of the ion beam etched profiles are absolutely smooth, that is, an etched hole repre sents the ideal cylinder. However, we have established that, depending on the experimental conditions, the cylinder surface can differ from ideally smooth and exhibits periodic ringshaped ribs spaced by a certain distance. The present investigation was devoted to elu cidating specific features of the formation of surface profiles on the surface of FIBetched silicon. Results of investigations of the effect of lowenergy (10–30 keV) FIBs on the surface structure of irradi ated crystals (including single crystal silicon wafers) have been repeatedly discussed in the literature. In particular, researchers observed and studied a phe nomenon, whereby periodic structures were formed on the surface of ionirradiated materials. This effect was conventionally attributed to anisotropic sputtering of the sample surface under the FIB action. The results of our observations suggest that the FIBinduced peri odic structures are of the same general nature as those observed previously. However, based on the obtained results, we propose an alternative explanation of the observed phenomena.
50 µs) at each point and (ii) fixed ion beam position on the sample surface. We used single crystal silicon wafers with {111} and {100} orientations, but the observed phenomena proved to be independent of the sample orientation. During the exposure, the sample wafers were arranged perpendicularly to the incident ion beam. It was established that the process of periodic struc ture formation by FIBs depended on the ion beam current density. The probing beam diameter was fixed at ~2.75 µm. At an ion beam current density of J = 2.16 µA/cm2 (Fig. 1), the surface of etched cylinder walls exhibited clearly pronounced ribs, which were absent when the current density was reduced to J = 1.07 µA/cm2. This result indicates that an increase in
We have studied the surface profiles formed on the walls of cylindrical holes etched by FIBs under various experimental conditions. The experiments were per formed on a FEI Quanta 200 3D setup using the ion energy E varied within 5–30 keV and the ion beam current I varied within 1–20 nA. The samples were etched in two regimes, with (i) ion beam scanning over a given region at a preset time of exposure (within 1– 991
15 kV 17 nA
5 µm
Fig. 1. Scanning electron microscopy (SEM) image of a cylindrical hole formed in a silicon sample etched at an ion beam current density of J = 2.16 µA/cm2.
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(а)
d = 10, I = 10, t = 1 µs 30 kV, 20 nA
(а)
spot, t = 20 s 30 kV, 20 nA
5 µm
(b)
d = 10, I = 10, t = 50 µs 30 kV, 20 nA
2 µm
(b)
30 kV, 20 nA spot 100 s
5 µm
Fig. 2. SEM images of cylindrical holes formed by a scan ning ion beam with E = 30 kV and I = 20 nA at different scan frequencies such that the time of exposure was t = 1 µs (a) and 50 µs (b) at each point (with a spot size corre sponding to the beam diameter).
4 µm
Fig. 3. SEM images of a cylindrical hole formed at a fixed point under the constant action of an ion beam with E = 30 kV and I = 20 nA for an exposure time of t = 20 s (a) and 100 s (b).
the beam current density leads to the formation of a periodic relief on the walls of a FIBetched cylindrical hole. It was also established that the process of periodic structure formation by a scanning FIB depended on the scan frequency under otherwise equal conditions (area, depth, and total time of etching). Figure 2a shows a clearly manifested relief on the walls of a cylindrical hole, which was formed when the time of exposure at each point was reduced to 1/50 of that for the hole imaged in Fig. 2b. This result indicates that (i) a decrease in the scan frequency (i.e., increase in the exposure time at each point) leads to vanishing of the relief on the etched cylinder walls and (ii) this change is more pronounced in regions occurring at greater depth from the irradiated sample surface. It is most interesting to consider the results obtained for the surface profile of holes etched without
scanning, that is, under the constant ion beam action at a fixed point. In this case, we observed a nonmono tonic character of the formation of ribs on the walls of an etched hole. For an initial etching time of t = 5 s, the surface relief was absent. An increase in the expo sure time led to the gradual manifestation of periodic structures (see Figs. 3a and 3b), which first appeared from one side (t = 20 s) and then spread over the entire cylindrical surface (t = 100 s). As the exposure time was increased further (t = 360 s), the relief disap peared, which was similar to the vanishing of profile in the region etched at a low scan frequency (Fig. 2b). We believe that phenomenon of ribbed relief for mation and disappearance on the FIBetched cylin drical wall surface can be described based on the notion of the radiationinduced plasticity, which is developed in the nearsurface layers of an irradiated crystalline material under the action of highdensity
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ion beans. In the case under consideration, the process can be divided into two stages, involving the formation and smoothing of the relief. At the first stage, the target material exhibits radiationinduced flow under the FIB action, which is related to the generation of defects in the nearsurface layers of walls of the etched cylindrical hole. This results in the formation of peri odic ringshaped ribs on the cylinder walls. At the sec ond state, the surface relief is subject to smoothing under the action of the same ion beam. This cycle is sequentially repeated until the profile front will go beyond the boundary of the FIB action. Let us consider the observed phenomena and the proposed explanation to the analogous results and their interpretations in the available literature. There is a wellknown phenomenon whereby f a periodic relief is formed on the surface of solids (in particular, single crystalline semiconductors including silicon) irradi ated by low and mediumenergy ion beams at small incidence angles (see, e.g., [1]). All researchers previ ously attributed the observed effects to the anisotropic sputtering of material from the irradiated surface. However, Mayr et al. [2, 3] showed both experimen tally and theoretically that the irradiated materials can acquire plasticity due the accumulation of radiation defects. Then, a periodic relief can be formed on the target surface under the action of a grazing ion beam. On the other hand, there are rather many published results showing that the roughness on solid surfaces can be eliminated (smoothed) under the action of ion beams [4–7]. It was emphasized that this smoothing effect was related both to the production of radiation defects and to the radiationinduced plastic flow [2, 3]. We must recognize that, although the aforemen tioned published data do not refer immediately to the use of FIBs, there are nevertheless some important characteristic features of the FIB action on a solid state structure that renders these data interrelated with our results: (i) In all cases, the process involves the generation of radiation defects at a high rate. This is especially characteristic of the bombardment with cluster ions [5]. The etching with a FIB also involves a highrate production of radiation defects (ion beam density at the spot of action amounted to 2.16 µA/cm2). (ii) In many cases, researchers pointed out that the transition to plastic flow followed by smoothing pro
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ceeded via the stage of amorphization. Apparently, in the case of FIB action–especially in the case of expo sure to a fixed beam (i.e., without scanning)—one can expect that the stage of evaporation (sputtering) of the target material is preceded by the single crystal–amor phous phase transition. From this standpoint, the entire set of specific fea tures observed in our experiments, including the relief formation and its subsequent disappearance under the FIB action, can be explained as follows. In the case of etching by a scanning FIB, a periodic structure is formed on the etched cylinder walls as a result of the plastic flow manifestation. It should be taken into account that, during the cylindrical hole formation, the beam is incident on its walls at a grazing angle. A decrease in the beam scan frequency leads vanishing (smoothing) of the relief, which is only retained near the sample surface. This behavior is related to an increase in the time of continuous ion beam action (and, hence, radiation effect generation) at each point of the etched profile. On the other hand, the etching of a cylindrical hole under the action of a fixed beam involves the sequen tial passage through all stages of relief formation on the cylinder walls and the its subsequent vanishing as a result of the smoothing as a result of the radiation induced plastic flow. REFERENCES 1. E. Chason and M. J. Aziz, Scripta Mater. 49, 953 (2003). 2. S. J. Mayr and R. S. Averback, Phys. Rev. Lett. 87, 6106 (2001). 3. S. J. Mayr, Y. Ashkenazy, K. Able, et al., Phys. Rev. Lett. 90, 5505 (2003). 4. C. A. Volkert, J. Appl. Phys. 70, 3521 (1991). 5. A. Nakai, T. Aoki, T. Seki, et al., Nucl. Instrum. Meth. Phys. Res. B 206, 842 (2003). 6. D. K. Goswami and B. N. Dev, Phys. Rev. B 68, 3401 (2003). 7. Z. Insepov, A. Hassanein, J. Norem, et al., Advanced Surface Polishing Using Gas Cluster Ion Beams, Pre print ANL/MCSP14090407 (2007).
Translated by P. Pozdeev
2010
