Behaviors of small molten metal islands on several substrates† T. Ichinokawa,a H. Itoha and Y. Sakaib aDepartment of Applied Physics, Waseda University, 3–4–1, Ohkubo, Shinjuku, Tokyo 169, Japan bJEOL Ltd., 3–1–2 Musashino, Akishima, Tokyo 196, Japan Received 28th August 1998, Accepted 8th December 1998 Electro- and thermomigration of metallic islands of mm size produced by vapor deposition on the Si(100)2×1 surface were investigated by ultra-high-vacuum scanning electron microscopy at substrate temperatures higher than the melting-points of islands.The direction of electromigration due to electric current passing through the Si substrate depends on the type of metal, whereas the direction of thermomigration caused by a temperature gradient on the substrate surface is always from low to high temperature, independent of the type of metal. The speeds are 0.1–10 mm s-1 and are approximately proportional to the island radius and increase exponentially with temperature in both cases.The driving forces of the island migrations are explained by the diVusion theory of metals in Si due to the electric field or the thermal gradient. Furthermore, it was found that the melting-points of metal islands on the carbon substrate are lower than those of the bulk and, moreover, the wettability (the contact angle) of the molten Cu or Ag islands changes in an oscillatory manner on the SiO2 substrate with periods of 100–0.1 s depending on the island diameter and substrate temperature. The origin of the contact angle oscillation is explained by the periodic change of the interface profile between island and substrate and by the failure of Young’s equation due to the change of orientations of surface and interface tensions.sequently annealed at 600 °C in UHV. The substrate tempera- 1. Introduction ture was measured by the resistance of the substrate, which The interface properties of metal films deposited on Si or SiO2 had been calibrated using an infrared pyrometer.Various substrates with increasing temperature are important not only metal films of Au, Ni, Pd, Ag, In, Cu and Al were deposited for the fabrication of electronic devices in the semiconductor on the substrates at room temperature from a heated tungsten industry, but also for the investigation of metal–ceramic wire basket and the thickness was measured with a quartz interfaces. Moreover, the epitaxial growth of metal films on thickness monitor.The deposited specimens were transferred Si or SiO2 crystals is very complicated, depending on the type from a UHV specimen preparation chamber to the UHVof metal. A number of studies have been carried out for SEM system through a transfer tube. SEM observations were systems of metals on Si and SiO2. carried out at a primary electron energy of 10 keV. Dynamic During our investigation of metallic films on Si or SiO2 motions of islands on the substrates were observed by TV crystals by ultra-high-vacuum scanning electron microscopy scanning at temperatures around the melting-points of islands (UHV-SEM) as a function of temperature, we found several with resistive heating and were stored on a video tape.interesting phenomena for liquid and quasi-liquid metal islands. In the present experiments, several phenomena, e.g., 3. Results (1) electro- and thermomigration of liquid metal islands on Si substrates,1,2 (2) rotation of facets of Au quasi-liquid islands 3.1.Electro- and thermomigration1,2 on graphite at temperatures lower than the bulk melting-point Fig. 1(a) and (b) are UHV-SEM images showing the electro- and (3) contact angle oscillation of the small molten metal and thermomigration of Au islands observed by passing direct islands on SiO2,3 have been observed in real time. The results current through the Si substrate. The larger the island, the and preliminary discussions on these phenomena are presented higher is the speed.For Au islands, the direction of the in this paper. electromigration is opposite to that of the electric field. The velocity increases exponentially with temperature and approxi- 2. Experimental mately proportionally to the island radius. Palladium islands also migrate in the same direction as the Au islands, but Al The experiments were carried out using a JAMP-30 UHVislands migrate in the opposite direction to the Au islands. SEM system. A p-type silicon wafer with a resistivity of Ag, Cu and Ni islands do not migrate.On heating with 4–6 V cm and a thermally oxidized Si (100) wafer of size alternating current, no electromigration was observed. The 20×5×0.4 mm were used as substrates. The substrate crystals eVect of the gravity is negligible and the level of the trace left were held between tantalum electrodes and heated by passing behind after migration is lower than that of the surrounding direct current through the substrate.The surface of the Si substrate surface. The direction of the electromigration is crystals was cleaned by flashing above 1000 °C for a few independent of the type of Si substrate (p- or n-type). minutes at a vacuum pressure of <5×10-10 Torr. The oxide The islands also migrate owing to the temperature gradient surface was cleaned by Ar+ ion bombardment and subon the substrate surface from low to high temperature, independent of the type of metal. The velocity of the thermomigration for Au islands is 0.1–1.2 mm s-1 depending on the †Presented at the Fifteenth International Congress on X-ray Optics and Microanalysis (ICXOM), Antwerp, Belgium, August 24–27, 1998.island radius and substrate temperature. For thermomigration, J. Anal. At. Spectrom., 1999, 14, 405–408 405migrations of metallic islands of mm size, we consider that electro- and thermomigrations are caused by diVusion of metal atoms into the Si substrate across a solid–liquid interface caused by an electric field or temperature gradient.The diVusion velocity v of the migrating atoms depends on the concentration gradient, the temperature gradient and the electric field, as shown by the following equation using Fick’s diVusion equation: v=-D d ln c dx - DQ*dT kT dx +BeZ*E (1) where D is the diVusion coeYcient, c is the concentration of metal atoms, Q* is an ‘apparent heat of transport’ for a Fig. 1 (a) Electromigration and (b) thermomigration of Au islands migrating atom as given by Davies,4 k is Boltzmann’s constant, on Si(100).B is mobility, Z* is the ‘eVective charge’ of the migrating atom as given by Huntington5 and E is the electric field. From our experiments, it can be seen that Q* is negative for every type of metal used because all islands migrate from the cold to the hot side. The interpretation of thermomigration was described by JaVe and Shewmon6 for several impurity atoms in Cu, Au and Ag metals, and it was reported that almost all impurity atoms migrate from the cold to the hot side at lower speeds than in Si.For electromigration, there are two sources as the driving force: the first arises from the direct Fig. 2 Low temperature melting of a Cu film deposited on graphite (at 1000 °C). it was also found that the larger the island, the faster is the speed. Furthermore, we observed island migration due to an electron beam. If an electron beam of 0.1 mm diameter with a current density of 106 A cm-2 is scanned slowly near an island, the island moves, following the electron beam.Hence, we can write a letter of mm size with an electron beam. Such an eVect is regarded as an island migration due to the temperature gradient, because all islands migrate towards the electron beam Fig. 3 Rotation of the facets of an Au island on a carbon substrate independently of the type of metal. (side view) at temperatures lower than the melting-point of the bulk Au.Although the phenomena observed in this experiment are Fig. 4 Contact angle change in a period of the oscillation for a Cu island of 20 mm diameter on an SiO2 surface. The maximum contact angle is 120° and the minimum is 30°. 406 J. Anal. At. Spectrom., 1999, 14, 405–408action of the external field on the charge of the migrating ion 20 mm diameter gradually spreads and then suddenly contracts in an oscillatory manner in a period of 200 s at 1100 °C. The (‘direct force’)7 and the second from the momentum transfer due to scattering of conduction electrons by migrating atoms oscillation period changes from 200 to 0.1 s depending on the island diameter and substrate temperature.The higher the (‘wind force’).5 For impurity atoms in metals, the theoretical estimation of the eVective force for electromigration is diYcult. temperature, the shorter is the oscillation period, and the larger the island, the longer is the oscillation period. The In semiconductors, however, the ‘direct force’ is of great importance, because the electric field in a semiconductor is oscillation phenomena were investigated experimentally as a function of the type of metal and substrate material in order greater than that in a metal.Therefore, the charge transfer from Si to migrating atoms, which is probably deduced from to reveal the mechanism of the contact angle oscillation. Similar phenomena were observed in Al–Al2O3 and Ag–SiO2 the relative value of the electronegativity between metal and Si atoms, is an important factor for estimating the direction systems.During the contact angle oscillation, concentric depression of electromigration. In fact, the opposite directions of electromigration of Au and Al are interpreted in terms of electronega- rings of 50 nm depth around an island were formed on the substrate surface, because the island diameter just before the tivity, because the electronegativity of Au is larger than that of Si and that of Al is smaller than that of Si.Examples of contraction in each period decreases by thermal evaporation and the interface between the molten island and SiO2 substrate the electronegativities of relevant elements decrease in the following order:8 Au (2.4)>Pd (2.2)>Ag (1.9)>Cu moves downwards during spreading, as shown in Fig. 6. The geometric change of the interface leads to a change in the (1.9)>Ni (1.8)>Si (1.8)>Al(1.5)>Mg (1.2).This order agrees well with the results of the present experiments. The orientations of the liquid and interface tensions and the failure of Young’s equation for the force components between out- driving force acting to an island is a sum of ‘direct forces’ acting on an individual ion and is proportional to r2, because ward and inward tensions induces the spread and contraction the eVective volume relating to the island migration is probably underneath the substrate surface, whereas a reaction force acting on the island is a surface tension proportional to r.Thus, the size eVect on the island migration has been explained. The work function is one of the factors that can explain the electromigration of islands. However, the values of work functions are irregular and cannot explain the directions of electromigration reasonably. 3.2. Rotation of facets of metallic islands at temperature lower than the bulk melting-point Fig. 2 shows a UHV-SEM image of island formation for a Cu deposited film several hundred nanometers thick on the carbon substrate at 1000 °C (the bulk melting-point of Cu is 1083 °C).The Cu film becomes quasi-liquid at temperatures lower than 1000 °C and forms islands caused by surface energy minimization through the liquid-like flow. For Cu on a graphite substrate, the island shape is almost hemispherical and the contact angle is larger than 120°. The Au islands have facets of low crystallographic indices, as shown in Fig. 3, and the facets move at temperatures several degrees lower than the bulk melting-point (1064 °C). The facets disappear at the bulk melting-point. It should be noted that the quasi-melting-points of deposited metal films are several tens of degrees lower than that of the bulk and the crystallographic orientation of liquidlike islands moves at temperatures lower than the bulk melting-point. 3.3. Oscillation of wettability of liquid Cu islands on SiO2 3 The oscillation of the wettability of molten Cu islands of several mm diameter on amorphous or single-crystal SiO2 was observed by UHV-SEM. Fig. 4 shows the change in the island Fig. 5 Scanning electron microscope images of a Cu island on an SiO2 crystal, (a) before and (b) after milling by a focused ion beam. shape with the period of oscillation. A Cu molten island of rSV = rLS + rLV cos q rSV = rLS cos a¢¢ + rLV cos q¢¢ rSV > rLS cos a¢ + rLV cos q¢ rSV = rSL + rLV cos q (a) (b) (c) (d) a¢ Fig. 6 Interpretation of the contact angle oscillation due to a failure of Young’s equation for horizontal components of outward and inward tensions. J. Anal. At. Spectrom., 1999, 14, 405–408 407motions of the liquid metal island. The cross-section of the substrates, were observed by UHV-SEM. To analyze the interface properties of a micro-area as a function of tempera- interface profile between island and substrate was obtained ture, it is suggested that three-dimensional analysis by using a with a focused Ga+ ion beam as shown in Fig. 5 and it was combination system of a focused ion beam and UHV-SEM is proved that the interface level is lower than that of the promising. substrate surface. Fig. 6 provides an explanation of the contact angle oscillation for a molten island taking into account the surface and interface tensions according to Young’s equation. References The shift of the interface level is probably caused by diVusion. 1 T. Ichinokawa, H. Izumi, C. Haginoya and H. Itoh, Phys. Rev. B, From the experimental fact that the contact angle returns to 1993, 47, 9654. the initial value after the contraction, we can see that the 2 T. Ichinokawa, C. Haginoya, D. Inoue and J. Kirschner, Jpn. substrate surface is not contaminated by Cu. Further analysis J. Appl. Phys., 1993, 32, 1379. of the interface property, however, should be performed to 3 M. Ohya, D. Inoue, H. Itoh and T. Ichinokawa, Surf. Sci., 1996, 369, 169. clarify the mechanism on the contact angle oscillation. 4 R. O. Davies, Rep. Prog. Phys., 1956, 19, 327. 5 H. B. Huntington, in DiVusion in Solids—Recent Development, ed. A. S. Nowick and J. J. Burton, Academic Press, New York, 1975, Conclusion ch. 6. 6 D.JaVe and P. G. Shewmon, Acta Metall., 1964, 12, 515. Several interesting phenomena of liquid metal islands on Si or 7 A. H. Verbruggen, IBM Res. Dev., 1966, 32, 93. SiO2 substrates, e.g., (1) electro- and thermomigration of 8 L. Pauling The Nature of the Chemical Bond, Cornell University liquid metal islands on an Si substrate, (2) rotation of facets Press, Ithaca, NY, 3rd edn., 1960. of quasi-liquid metal islands at temperatures lower the bulk, and (3) contact angle oscillation of liquid Cu islands on SiO2 Paper 8/06746E 408 J. Anal. At. Spectrom., 1999, 14, 405–408