CHARPAN
Charged Particle Nanotech
Project
Achievements in Nano-Research
In the CHARPAN project the influence of ion beam bombardment on various materials and structures is investigated to identify opportunities and limits to industrial application of the technology.
Emerging applications subsume research into NEMS, sensors, polymer substrates, bio-nano-devices and alternative ion sources.
Nano research into materials and structures
An example for nano-research accompanying tool development in the CHARPAN integrated project is work performed by Fraunhofer IISB on the "Electrical Characterization of FIB Induced Damage in Silicon"[8]. This topic is important for CHARPAN PMLP process development, just as it is today for FIB users: "...sputtering with focused ion beams (FIB) is an important technique for patterning in the semiconductor technology down to nanoscale size (...). However, the highly energetic ions induce severe damage in the target material. For silicon, this usually even leads to an amorphization of the processed area and its surrounding." In the SSR image (left) the colour of the structures corresponds to Ga+ doses from 4.1013 cm-2 to 1.1015 cm-2. In the topography image the height scale is 10 nm.
W. Li and colleagues of Cardiff University studied the parameters influencing the applicability of FIB technology for manufacturing UV-NIL templates as compared to e-beam widely in use today([9]and[10]). To this end a widely used system (fused silica substrate with 15nm Cr coating) was analysed. The milled depth, surface roughness and the evolution of feature profiles as a function of ion fluence were investigated. It was found that the milled depth in fused silica substrates was linearly dependent on the ion fluence within the studied range. Features topography measurement revealed that after removing completely the Cr layer, an increase of the ion fluence resulted in a slight, almost negligible rise of surface roughness (root mean square values were still less than 2.5 nm). More importantly, compared with the roughness of an unprocessed area on the substrate, an improvement of the surface finish of about 300% was obtained.
At Vienna Technical University, A. Lugstein and colleagues investigated how GaSb reacts to focused ion beam bombardment with Ga+ ions, an ion source interesting for implantation. A honeycomb-like structure consisting of many cells evolved under the GaSb surface implanted with 50 keV Ga+ ions. The cell diameter and the thickness of the walls partitioning the cells were about 60 and 20 nm respectively. Upon further FIB implantation the subsurface cavities expanded in the surface direction and formed a micro texture of filaments about 25 nm in diameter. Above a Ga+ fluence of 6.25 × 1016 ions/cm2 the onset of nanofibers growth was observed. The images show similar experiments for SB and Ge surfaces.
Meanwhile at Cardiff University it was analysed if masters for injection moulding and hot embossing can be produced using FIB[11] (and probably later PMLP). These masters often are made from amorphous or polycrystalline Ni-based alloys like Ni78B14Si8. When milling amorphous Ni78B14Si8, it was found that the sputtering yield first decreased when increasing the beam scan speed, then kept constant. It was also found that the milled depth was almost proportional to the ion beam fluence. The patterning of polycrystalline Ni78B14Si8 resulted in anisotropic milling-rates due to the varying orientation of the grains in the material. The analysis of the profile evolution in both materials indicated that the surface finish of trenches depended on scan speed, ion beam fluence and scan strategy.
CrC coatings are often used to produce the specific properties required in micro tools. For applications in micro injection moulding and thermal imprinting these CrC coatings need to be structured. To investigate the response of the CrC coatings to FIB milling a team of researchers at Cardiff University produced a series of rectangular trenches. Especially, the effects of the ion beam current, exposure time and ion fluence on the sputtering yield and roughness of the produced micro structures were studied. Some essential parameter' windows for performing FIB milling with relatively high sputtering rates, higher than 1 µm/min, and at the same time achieving the best possible surface integrity were determined during the experiments. The images show FIB milled CrC with Iion =200mA, ts =120s, NL=1 (a) and NL=50 (b). (NL is the number of layers)
Not only surfaces but also structures like transistors made fromcarbon nanotubes (CNT-FET) will be subject to irradiation with particle beams. G.Rius and colleagues at Instituto di Microelectronica de Barcelona studied their response to electron beam exposure. Results showed a temporarily degradation, which depends on the beam energy and irradiated area, and is produced by the charging of the underlying silicon ([12]and[13]).
[8] Beuer, S. et al.: "SSRM Characterisation of FIB Induced Damage in Silicon", Poster presentation: ICN+T 2007 (32nd International Conference on Nano Science and Technology), Stockholm, Sweden, 2007 July 2-6
[9] Li, W. et al.: "Focused-ion-beam direct structuring of fused silica for fabrication of nano-imprinting templates", Microelectronic Engineering, Vol. 84, 829-832, (2007)
[10] Li, W. et al.: "A study of fused silica micro/nano patterning by focused-ion-beam", Applied Surface Science, Vol. 253, 7, 3608-361430, (2007)
[11] Li, W.: "Patterning of amorphous and polycrystalline Ni78B14Si8 with a focused-ion-beam", Applied Surface Science, Vol. 253, 12, 5404-5410, (2007)
[12] Rius, G.: "Response of carbon nanotube transistors to electron beam exposure", Microelectronic Engineering 84, 1596-1600, 2007 [13] Rius, G.: "Characterization at the nanometer scale
