UDK 539.2:533.5:620.179.11 ISSN 1318-0010 Izvirni znanstveni članek KZLTET 33(6)493(1999) U. VALBUSA ET AL.: NANOSTRUCTURING SURFACES BY USING ION SPUTTERING NANOSTRUCTURING SUR FACES BY USING ION SPUT TERING NANOSTRUKTURIRANJE POVRŠIN S POMOČJO IONSKEGA JEDKANJA Ugo Valbusa, Corrado Boragno, Francesco Buatier, Giovanni Costantini INFM and CFSBT of CNR, Physics Department University of Genova, Via Dodecaneso 33 16136 Genova Ital y Prejem rokopisa - received: 1999-10-15; sprejem za objavo - accepted for publication: 1999-11-19 Sur face etch ing by ion sput ter ing, be sides pro duc ing equi lib rium-oriented pat terns sim i lar to those ob tained by Mo lec u lar Beam Ep i taxy, can also be used to pat tern the sur face along non-equilibrium ori en ta tions, thus ex tend ing the pos si bil i ties of Mo lec u lar Beam Ep i taxy. By tun ing the com pe ti tion be tween ero sion at graz ing an gle and dif fu sion in duced sur face re-organisation, it is for ex am ple pos si ble to pat tern a sub strate char ac ter ised by a square sym me try with a we ll or dered rip ple struc ture run ning along any de sired di rec tion. Po ten tial ap pli ca tions of such nanofabricated sub strates can be found among oth ers in the field of sur face coat ings be cause a con trol of the lo cal rough ness on the nanometric scale is ex pecte d to im prove the ad he sion as well the tribological prop er ties of the films. The pa per will dis cuss the use of sput ter etch ing i n ma te rial sci ence as a pow er ful method which per mits the in situ pro duc tion of sub strates with well de fined ver ti cal rough n ess, lat eral pe ri od ic ity and with a con trolled step size and ori en ta tion. Key words: ion sput ter ing, sur face nanostructuring, equi lib rium pat terns, ero sion, sur face mor phol ogy, con trolled nano rough ness Ionsko jedkanje površine, ki ustvarja ravnotežne oblike podobno kot pri molekularni epitaksiji (MBE ), se lahko uporabi tudi za neravnotežno preoblikovanje površine, kar poveča možnosti MBE. Z uskalditvijo tekmovanja med erozij o pri vpadnem kotu ter reorganizacijo površine zaradi difuzije, je na primer mogoče oblikovati substrat kvadratne oblike z dobro urejeno valovito površino vzdolž poljudno želene smeri. Potencialna uporaba za tako nano-izdelane substrate je med d rugim na področju površinskih prevlek, kjer pričakujemo izboljšanje adhezije kot tudi izboljšanje triboloških lastnos ti trdih prevlek. V članku je obravnavana uporaba ionskega jedkanja kot močnega orodja na področju znanosti o materialih, ki omog oča in situ izdelavo substratov z dobro definirano vertikalno hrapavostjo, lateralno periodičnostjo in s kontrolirano ve likostjo in orientacijo stopnic. Ključne besede: ionsko jedkanje, nanostrukturiranje površine, ravnotežne oblike, erozija, morfologi ja površine, kontrolirana nanohrapavost It is well known that when an off-normal incidence ion beam etches an amorphous solid a ripple topography can be observed. One of the first observations was done in 1956 by Navez et al. 1 by bombarding glass with an ion beam. They found that for incident angles close to the normal the ripple morphology is oriented in a direction perpendicular to the ion beam and that the ripple orientation is rotated 90° when the beam is close to grazing incidence. The authors did not find an accurate explanation at that time, but simply tried to find analogies with macroscopic phenomena like the ripple structures when air or water flows over a sand bed or the ripple formed during the sandblasting of solids 2. An explanation of the mechanism was first proposed by Bradley and Harper 3. They proved that, since the sputtering depends on the surface curvature, under certain conditions this dependence gives rise to a surface instability where the erosion is faster for the bottom of a trough than for the crest of a peak. This instability can be described by the continuum equation 3 reported in figure 1 that describes the time evolution of the local surface height h (t). The derivative of h is proportional to the Laplacian of the surface height and the proportionality factor ? is the surface tension. Whenever the surface tension is negative it tends to maximize the surface in contrast to the surface tension that minimizes the surface. KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 It is the competition between this surface instability and the smoothing effect of diffusion that forms the ripple morphology. The periodic modulation occurs along the direction for which ? is negative and, in absolute value the largest one. Experimentally those structures were observed on amorphous materials 4 and on semiconductors 5. In the case of metals additional consideration should be done. The diffusion effects are not negligible and they introduce in equation 3 an additional term, which is again proportional to the Laplacian of the surface height. What happens if the atom diffuses on the island and approaches its edge from above? There is an additional potential barrier (Schwoebel barrier see figure 2 ) at the edge of the island that atom must overcome in order to descend, so the probability to be reflected is higher than the probability to jump off 6. This contribution adds to equation 3 a term which is proportional to the Laplacian of h. In this case the coefficient in front of the Laplacian S is now depending from the crystallographic directions and from the surface temperature and is not related to the initial conditions of the ion beam (scattering angle, energy, penetration depth). The combination of the two effects induces the formation of a ripple for normal incidence 7 as shown in figure 3 , in the case of a surface with a 110 symmetry, but produces for surfaces with a 493 U. VALBUSA ET AL.: NANOSTRUCTURING SURFACES BY USING ION SPUTTERING Fig ure 1: The fig ure de scribes the ef fect of the ion ero sion, which is faster for the bot tom of a trough than for the crest of the peak be cause of the cur va ture de pend ence of the sput ter ing. In the con tin uum equa tion that de scribes the sur face evo lu tion dur ing sput ter ing, h and ? de pend on ion flux and in ci dent an gle ?, D is the dif fu sion co ef fi cient and takes into ac count of the sto chas tic ar riv als of at oms. ? is the sur face ten sion. The pe ri odic mod u la tion set in along the di rec tion for which ? is neg a tive and in ab so lute value, the larg est one Slika 1: Prikazan je vpliv ionske erozije, ki je hitrejše za dno brazde kot za greben vrha, zaradi odvisnos ti ionskega jedkanja od ukrivljenosti. V zvezni enačbi, ki opisuje nastanek površine med ionskim jedkanjem sta h in ? odvisna od gostote ionskega toka in od vpadnega kota ?, D je difuzijski koeficient, ki upošteva stohastično prihajanje atomov in n je površinska napetost. Perio dična modulacijska smer je vzdolž strani, za katero je n negativna in absolutno največja Fig ure 2: Sche matic view of the Schwoebel bar rier at the edge of an is land Slika 2: Shematičen pogled na Schwoebel oviro na robu otoka 100 symmetry nice and regular checkerboard patterns. In the case of an amorphous material one is expecting a rough surface without any periodic structure. But if one sputters at grazing angles the erosion mechanism becomes again dominant and both (100) and (110) (see figure 3) surfaces present a well defined ripple morphology, which depends from the beam orientation and not from the crystallographic directions. 494 The peculiar role played by the Schwoebel barrier is also illustrated clearly in figure 4 where a (110) surface is reported after ion bombardment of Ar+ for different temperatures. At low temperature (180 K) the surface is rough and no ripple structures are present since the adatoms mobility is low. At T= 250 K, diffusion sets in along <1-10> but the temperature is too low to overcome the Schwoebel barrier in this direction and thus a ripple is formed along the <100>. By increasing the temperature, atoms start first to diffuse also along <100> (T=320 K) and then start to descend the edge of the islands along the <100> stops thus forming a clear ripple structure along the <1-10> . For higher temperatures the adatoms descend the edges of the islands along both direction <100> and <1-10> thus forming a flat surface. The effect is known as ripple rotation and was first reported in ref. 8. The application of ion sputtering techniques in producing novel structures on the nanometer scale may be of great interest for future applications. Quite recently Facsko et al. 9 produced with this method crystalline dots 35 nanometers in diameter, in a regular hexagonal lattice on gallium antimonide surfaces demontrating that this method is useful in device fabrication. The capability of controlling the defect on a surface can be an other example of future application of this method for modifying the catalytic properties of a surface. Quite recently 10 it has been in fact demonstrated that by tuning the orientation of the ripple on a substrate Ag (001) it is possible to change the density of defects onto the surface (in particular kinks) allowing for KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 U. VALBUSA ET AL.: NANOSTRUCTURING SURFACES BY USING ION SPUTTERING Fig ure 3: A g (001) and Ag(110) af ter sput ter ing with Ne +; a) is Ag (001) sput tered in con di tions where the dif fu sion mech a nisms are dom i nant 570 nm 2, b) is Ag (110) sput tered in con di tions where the dif fu sion mech a nisms are dom i nant 400nm 2, c) is Ag ( 001) sput tered in con di tions where the ero sion is dom i nant 200 nm 2, d) is Ag (110) sput tered in con di tions where the ero sion is dom i nant 200 nm 2 (From ref. 7) Slika 3: Ag(001) in Ag(110) po jedkanju z Ne+; a) Ag (001) ionsko jedkan v pogojih, ko prevladuje difuzijsk i mehanizem, 570 nm 2; b) Ag (110) ionsko jedkan v pogojih, ko prevladuje difuzijski mehanizem, 400 nm 2; c) Ag (001) ionsko jedkan v pogojih, ko prevladuje difuzijski mehanizem, 200 nm 2; d)Ag (110) ionsko jedkan v pogojih, ko prevladuje difuzijski mehanizem, 200 nm 2 (po referenci 7) Fig ure 4: Rip ple for ma tion on Cu (110) as func tion of tem per a ture. The in set re ports the unit cell. Rip ple ro tates 90° by in creas ing tem per a ture (From ref. 8) Slika 4: Oblika valov na Cu (110) v odvisnosti od površine. Vstavki so merilo mrežne celice. Valovi se obrn ejo za 90° pri povečanju tem per a ture. (Po referenci 8) KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 495 U. VALBUSA ET AL.: NANOSTRUCTURING SURFACES BY USING ION SPUTTERING Oxygen dissociation at low temperature while this mechanism is not allowed on a clean and flat surface. REFERENCES 1 M. Navez,C. Sella and D. Chaperot C. R. Acad. Sci. 254 (1962) 2 4 0 2 Carter et al. Rad.Eff. 33 (1977) 6 5 3 R. M.Bradley and J. M. E. Harper J.Vac.Sci.Technol. A6 (1988) 2390 4 E. A. Eklund, R. Bruinsma, J. Rudnick and R. S. Williams Phys. Rev. Lett. 67 (1991) 1759 5 E. Chason, T. M. Mayer, B. K. Kellerman, D. T. Mellroy and A. J. Howard Phys. Rev. Lett . 72 (1994) 3041 S. Rusponi, C. Boragno and U. Valbusa Phys. Rev. Lett. 78 (1997) 2795 F. Buatier, G. Costantini, C. Borgano and U. Valbusa Appl. 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