Cluster-solid interaction experiments

Nuclear Instruments and Methods in Physics Research B 102 (1995) 305-311

B Beam Interactions with Materials & Atoms

ELSEVIER

Cluster-solid interaction experiments Walter L. Brown ~' *, Marek Sosnowski

b

a A T& T Bell Laboratories, Murray Hill, NJ 07974, USA b Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA

Abstract Experiments to investigate the interaction of clusters with solids include a number of different types of observations, but with very little systematic study in most of them. In rough order of increasing cluster energy, experiments with non-metallic clusters involve surface cleaning, smoothing of surface topography, sputter erosion, crystallographic damage and impurity doping. The results of these experiments are different than if individual atoms of equivalent velocity had been incident on the solid. Studies of the impingement of metallic clusters on solids have been motivated primarily by interests in the deposition of thin films. The suggested advantages of clusters in this role are only beginning to be established experimentally.

1. Introduction The use of atomic and molecular clusters as tools for modifying the properties of solids is still in an early stage of evolution. The interest in it is driven by the expectation that cluster impingement on solids will exhibit new effects not present in collisions of individual atoms with those solids [1]. Clusters are different in several ways. With a total energy of 10s or even 100s of keV but energies per cluster atom of < 1 keV/atom, the entire energy of collision with a solid goes directly into energy of atomic motion. There is no picosecond delay for conversion of that portion of the energy which goes into electronic excitations into energy of atomic motion [2], in contrast to the case of atomic ions. The depth of penetration of clusters into a solid is very shallow. Molecular dynamics calculations indicate it is even shallower than for individual atoms at the same velocity because of the collective " p l u n g e r " effect of a number of cluster atoms interacting with the same target atoms at the same time [3-5]. The combination of direct energy input to atomic motion and shallow depth of penetration results in very high transient energy densities in duster impacts. Local energy densities above the cohesive energy of a solid can be produced [5]. The fragmentation properties of clusters are another uniqueness that seems to depend upon the relative mass of the cluster atoms compared with atoms of the solid. For light element clusters impacting a heavy target, strong backscattering of the first atoms of the cluster to arrive at the target cause them to collide with the last atoms of the

* Corresponding author. Elsevier Science B.V. SSDI 0 1 6 8 - 5 8 3 X ( 9 4 ) 0 0 7 7 7 - 2

cluster, still arriving, to enhance fragmentation and the lateral velocity of fragments [5]. Fig. 1 schematically illustrates some of the expectations for clusters: surface cleaning, surface smoothing, sputtering and implantation and film deposition. We will discuss the experimental status of these effects in paragraphs to follow. A number of different methods of producing clusters have been explored with a variety of different emphases [6-16]. Control of cluster type (rare gas, inorganic molecules, metals, etc.), cluster size and homogeneity in size, and cluster energy are major objectives. In the case of metal clusters in particular, obtaining an adequate cluster fluence with clusters of well defined properties is a major challenge [10-15]. Two examples of cluster sources are illustrated in Fig. 2. Expansions through nozzles are effective sources of rare gas [16,17] and molecular gas [15] clusters. Sources in which metal atoms aggregate to form clusters in non-reactive carrier gases are effective sources of metallic clusters [11,18]. Broad categories of cluster effects are characterized in Fig. 3 with two parameters, the number of atoms or molecules in the cluster and the cluster velocity or energy per atom or molecule in it. Results in these categories will be discussed in Section 2 below. The classification of Fig. 3 does not contain all of the significant features. It overlooks, for example, the nature of the duster - whether it is a tightly bonded refractory metal or a loosely bonded Van der Waals molecular solid - and the nature of the solid on which the cluster will impinge - the strength of its binding and the atomic mass of its atoms compared with those of the cluster. The ordinate in Fig. 3 covers a huge range and the scale has been broken to include more compactly both very

V. CLUSTER-SOLID INTERACTIONS

306

W.L. Brown, M. Sosnowski /Nucl. Instr. and Meth. in Phys. Res. B 102 (1995) 305-311 RARE GAS & MOLECULAR CLUSTERS ~, Arn

(002) n

METAL CLUSTERS

Arn

-,0Oo s

Cun

O ,o°om,s

,O%m,,

CLEANING

SMOOTHING

IMPLANTATION & SPU'FI-ERING

DEPOSITION

(a)

(b)

(c)

(d)

Fig. 1. Effects of cluster impingement on solids: (a), (b) and (c) for non-metallic clusters; (d) for metallic clusters. (a) cleaning of particulates; (b) smoothing a surface; (c) sputtering and implantation; (d) film deposition.

(a)

large clusters (aerosols) of 109 to 10 l° atoms or molecules and clusters in the 10-1000 atom size range where extensive efforts are being made to select different bands of sizes for experimental tests of their effects. Fig. 3 does not distinguish between neutral and ionized clusters. It is very difficult to obtain clusters with even 1 e V / a t o m (or molecule) by acceleration in a gas expansion since this requires gas velocities in excess of 105 c m / s . Ionized clusters provide the opportunity for acceleration to arbitrarily high energies. Several experiments have been carried out using clusters having 100s or even 1000s of e V / d u s ter atom. As far as their impact effects are concerned, there seem to be no reasons to expect that ionized clusters are different from neutral clusters. In all cases the clusters are moving much more slowly than electrons bound to them so that the ionic state of a cluster at the time of collision will be determined by electron exchange as the cluster "slowly"oapproaches the solid, no matter what its charge was 100 A away.

2. Experimental results of cluster impacts on solids

(b)

2.1. Rare gas and molecular clusters

Fig. 2. Effective sources of (a) non-metallic and (b) metallic clusters: (a) supersonic expansion through a nozzle from a high pressure chamber results in cluster formation; (b) metal atoms evaporated from a crucible aggregate in a rare gas and are then carried out of the coallescence chamber entrained in the gas flow.

10l° b

2~

s.Cleaning : , ~ ~

l

F :~*:*:!! :ii! ~? :

;

f

~

Cratering

\"

Sputtering, Imolantation

] 04 10 3

o

<

102 10 10-4

10"2 1 102 eV/atom (or Molecule) I

I

I

5x10 3

5x10 4

5x10 5

104

5x10 6

Velocity cm/sec (Mass 40) Fig. 3. Broad characterization of effects of cluster impingement on solids. The ordinate is broken to include large aerosol clusters as well as much smaller clusters. The diagonal dashed lines indicate total cluster energy. The boundaries between different regions are much less distinct than shown.

2.1.1. Cleaning The presence of particulates on the surface of silicon wafers is of major concern at many different steps in VLSI processing. At present they are controlled almost entirely by wet d e a n s in aqueous baths. The possibility of removing particles in a dry process, for example by bombardment with clusters as illustrated in Fig. la, is an intriguing alternative. Since typical particles that affect chip yields are a few tenths of a micrometer in diameter or larger, quite large clusters are needed for a process of momentum transfer to be effective in removing them. The clusters that are being tested for this purpose contain 10 9 to 1010 atoms or molecules as shown in Fig. 3. They are aerosols formed by expansion of high pressure CO 2 or Ar through small nozzles into vacuum. The clusters are accelerated by the gas stream in the expansion to velocities between 3 X 103 and 8 X 104 c m / s . As the clusters and accompanying gas meet the wafer surface to be cleaned, the clusters have enough momentum even at these relatively low velocities to penetrate the boundary layer of gas that flows across the surface and to impact particles adhering to it. Particles that are dislodged from the surface are carried off the wafer in the fast moving gas stream. CO 2 is an effective and convenient aerosol material. However, it is extremely difficult to clean of residual hydrocarbons to the level demanded in silicon processing. Ar is another possibility. Because of its low boiling point it can be distilled to very high purity. Work on Ar aerosols is currently being carried out at Krytek [19], following up the published work of McDermott et al. [20]. Kryteck has used ~ 0.5 Ixm Ar " c l u s t e r s " whose size was determined by

W.L. Brown, M. Sosnowski / Nucl. Instr. and Meth. in Phys. Res. B 102 (1995) 305-311 PRINCIPLE OF OPERATION Ar

.

~--POINT OF USE FILTER

N2 ,

F[~

~

LN2 PRESSURE (TLN2 )

HEAT EXCHANGER 3 )

( (

D

(

Ar AEROSOL

i

--L~

PURGE GAS

..@...'J

~

I

VACUUM CHAMBER

1 PUMP

Fig. 4. Schematic of the wafer cleaning setup due to Krytek [19].

Mie scattering. The clusters typically are formed in a series of jets from a linear array of small orifices and are arranged to impinge on a wafer surface at ~ 45 ° as shown in Fig. 4. A wafer is translated under the array of jets to expose the entire surface. In one experiment [19] polystyrene particles ~ 0.3 ixm in diameter were randomly distributed on a wafer surface. Their positions are indicated in Fig. 5a based on a Tencor laser scan. In this case they number 184. Fig. 5b shows the result after a pass under the Ar duster jets in which the clusters were estimated to have a velocity of ~ 3 x 103 c m / s . Approximately 95% of the polystyrene particles were removed. There are a few larger particles that were present on the

(a)

.... ::,.-.":%~.

(b)

..:..: - ~

E""-

\~..:--

-.

.,.-:.

,

....

:;-/

. i.;" .... -

.,

~'~

,."/

......:

"

:/

Fig. 5. Tencor laser scan measurements of contaminant particles on the surface of a Si wafer: (a) before and (b) after cleaning with 0.5 ~m Ar aerosol particles. The wafer was intentionally contaminated with (184) 0.3 p~m polystyrene spheres shown as + 's. Particles larger than those intentionally added are shown as: small dots 0.4-1.0 p.m; big dots > 1 p,m (courtesy of Krytek [19]).

307

wafer before the smaller particles were added. Between 50 and 75 percent of these were also removed by the Ar cluster cleaning treatment. Other experiments have shown that fingerprints are not removed at this cluster velocity and neither is residual photoresist. Recent experiments [21] have been carried out with Ar clusters at velocities of 5 - 8 × 104 c m / s ( ~ 0.05-0.13 eV/atom). Over this range the cleaning process makes a transition from cleaning to cratering and surface erosion, an undesirable limit for wafer cleaning. It seems clear that control of cluster velocity to optimize cleaning without damaging surface topographir patterns, which may be submicron aluminum interconnect lines for example, will be critical. Some types of particles may be bonded too well to the surface to permit removal by this process short of the surface erosion regime. Small areas of contaminant surface film (as contrasted with individual contaminant particles) seem likely to be particularly difficult to remove. 2.1.2. Sputtering, implantation and damage A relatively small number of experiments have been reported on bombardment of solids with much smaller and much higher velocity clusters than those discussed in Section 2.1.1 above. H 2 0 , CO 2 and Ar have been the principal species studied and the clusters have been produced as shown in Fig. 2a by gas expansion through small nozzles, sometimes with a carrier gas (He with At, for example) [16] to assist the clustering process. Pressure and temperature of the stagnation gas(es) allow control of the cluster size. To achieve high velocities the clusters are ionized, usually by electron impact after formation, and are subsequently accelerated. Quadrupole and time of flight spectrometry provide information on cluster size and size distribution. 2.1.2.1. Sputtering. We will include in the generic term "sputtering" all of those phenomena which result in ejection of material from a solid as a result of cluster impact. Sputtering may provide a way of smoothing a rough surface or it may be used to do either broad or localized machining of a solid. A graphic example of these effects is shown in Fig. 6b [22]. The setup of the experiment is shown in Fig. 6a. A pair of 100 txm crossed Ta wires was bombarded by a relatively broad beam of 150 keV CO 2 clusters which contained about 1000 molecules/cluster. An SEM of the lower wire (which was masked by the upper wire) is shown in the figure. The step in the surface is a clear indication of removal of tens of micrometers of material. In addition it is evident that the bombarded surface is much smoother than the surface in the masked region. Two interpretations have been suggested to explain this smoothing. In one, atoms of the cluster are envisioned as fragmenting from the cluster and being given high lateral velocities across the surface which enhance the removal of asperities on the surface and hence produce smoothing. An alternative interpretation is that the individV. CLUSTER-SOLID INTERACTIONS

308

W.L. Brown, M. Sosnowski / Nucl. Instr. and Meth. in Phys. Res. B 102 (1995) 305-311

(a) 150 keV (CO2)(lO00)

E~

1111111

~

&

£ 001.tm T a Wires 0

"(

I 1

i 2

INCIDENT CLUSTERS

(b)

Fig. 6. (a) The experimental setup of Henkes and Klingelhofer [22] in which a pair of Ta wires are bombarded with a CO 2 cluster beam. The first wire serves as a mask for the second. (b) The result of the bombardment on the second wire.

ual energetic cluster impacts produce small, transiently molten regions that tend to smooth the surface through surface tension. It is difficult to see how a series of very small random melting events can produce smoothing on a scale large compared with the size of the individual melted regions as Fig. 6 would seem to require. However, it will be interesting to see how computer modeling of these effects and further experiments designed to investigate smoothing as a function of the mass and binding energy of the clusters (compared with the solid), the kinetic energy of the cluster and the angle of impact resolve the mechanism question. The clusters masked by the crossed wires of Fig. 6 bombarded a Cu sample, leaving a cross of unaltered material and producing deep sputter erosion of the unprotected Cu [22]. The sputtering coefficient of the Cu from this result was estimated to be > 600 Cu atoms/150 keV CO 2 cluster. More precise measurements of cluster induced sputtering were made for the case of 29 keV At,, clusters bombarding Au films on Si substrates [16]. The clusters had n = 300 + 75 atoms as determined by time of flight.

(xl 013)

Fig. 7. The number of sputtered Au atoms as a function of the number of incident 29 keV At300 clusters [16].

The few hundred picoampere beam of clusters contained no small clusters and no atomic ions. It was focused to a spot about 1 mm in diameter on the A u / S i films and produced a loss of Au atoms whose spatial profile and total volume were determined by Rutherford backscattering. The result is shown in Fig. 7 as a function of cluster fluence. The slope of this line indicates a sputter yield of 105 Au atoms/29 keV Ar30o cluster. Another set of erosion ("sputtering"?) experiments was carried out with (H20)" clusters with n between 25 and 150 and with energies up to 300 keV [23,24]. In this case the bombarded targets were very thin C foils coated with Au and Ta at sufficiently low coverage as to be present as ~ 100 ,~ islands. TEM observations were made of the loss of islands and a sputtering coefficient deduced from them. A summary of the results of the three sputtering experiments described above is shown in Fig. 8. Taken at face value they indicate an extremely steep dependence of sputter yield on total cluster energy at high energy. However, the three highest points, derived from the last experi-

106

~

I

~

I

BAu

lO 5

[] w

lO 4

LU

AC

T- lO 3

A Cu

lO 2

10

I

O Au

0

i 5O

I

I

I

I

I

100

150

200

250

300

350

TOTAL ENERGY, keV

Fig. 8. Summary of sputtering by non-metallic clusters: ©, mr300 [16]; zx, (COa)10oo [22]; D, (H20)s0_a00 [23,24].

309

W.L. Brown, M. Sosnowski / Nucl. Instr. and Meth. in Phys. Res. B 102 (1995) 305-311

ment, need to be considered carefully. In this case the energy deposited by a cluster impact in an island of the film may be enough to thermally evaporate the whole island in the absence of lateral thermal conductance that would be present in a bulk material or even in a continuous thin film. Extraordinary sputter yields were noted for energetic heavy ions in similar discontinuous metal films [25]. More systematic experiments are needed to determine the critical parameters in the sputtering process. It is interesting to compare the experimental sputtering yield for AS clusters on Au with MD calculations reported by Insepov et al. [5]. Fig. 9 shows the results of the calculations for 100 e V / a t o m clusters of increasing size. The linear extrapolation of the three calculated points in this log-log plot to clusters of 300 argon atoms predicts a yield about 60% of the yield deduced in Fig. 7. Considering the trend of the three calculated points and the accuracy of the experiment, the agreement between extrapolation and data seems quite satisfactory. 2.1.2.2. Damage and implantation. Accelerated Ar, cluster ions with n between 150 and 300 were used to bombard Si(100) samples to examine the damage produced and also the implantation of As [16]. Fig. 10a shows 2 Mev He Rutherford backscattering spectra for a sample bombarded with a focused beam of 30 keV Ar15o clusters. The fluence in this case was estimated to be ( 0 . 7 - 1 . 5 ) X 1015 clusters/cm 2, the uncertainty due primarily to uncertainty in the area of the beam spot. Comparing the pre- and post bombardment channeled spectra it is evident that the bombardment resulted in a large increase in the Si near-surface peak. This increase corresponds to the formation of an amorphous Si layer approximately 50 ,~ thick. Such a thickness is comparable with the depth of an individual molten region that is estimated from molecular dynamics simulations to be formed in the very fast ( < 10 -13 s)

I

I

O - CALCULATIONS •

- EXPERIMENT

100 o

E o I.U >-

10

j

1 I I 80 100

.,1 4 /

I 200 Ar CLUSTER SIZE

l 300

400

Fig. 9. Molecular dynamics calculations [5] of Ar sputtering by 30 keV At, clusters for different n. Also shown is the experimental value for At300 [16].

105

I

(a) 1

0

Si 4

~

10a ;- .... -.......

--~-- Arr~ (b)

I Si

--

, iA' 100

}

200 300 200 30(] BACKSCATTERED ENERGY (CHANNEL)

i'"'' 40(]

Fig. 10. RBS spectra of Si(100) sample. (a) channeled spectrum before (dotted line) and after (dashed-dotted line) bombardment with 30 keV Arls 0 clusters and random spectrum (solid line) (b) bombarded with 30 keV Ar monomers: random on the bombarded spot (dotted line) and random off the bombardment spot (solid line) [16].

transient "thermal spike" produced by a single cluster ion " h i t " . The rapid quenching of a sub-picosecond thermal transient produced by a 100 fs laser pulse is experimentally known to produce amorphous Si [2]. The MD simulations are consistent with this, indicating retention of regions of highly disordered Si after individual cluster-induced " m e l t s " [26]. The amorphous Si spot of Fig. 10a had a diameter of about 3.5 mm, much larger than the approximately 1 mm FWHM diameter of the beam. This suggests that the 50 amorphous thickness is a steady state thickness reached even in the wings of the beam. If each cluster impact amorphizes a region 50 A in diameter, the cluster fluence required for amorphization of the whole surface would be only 5 x 1012 cm -2, more than two orders of magnitude less than the fluence delivered to the center of the spot in the experiment. There is a small but statistically significant peak in the Fig. 10a spectrum corresponding to RBS from Ar implanted into the Si surface. It is due to approximately 1 X 10 I4 A s / c m 2. This is only about 0.1% of the Ar atoms that arrived at the center of the beam spot. This very small fraction arises from two factors. First, sputtering will reduce the retention of Ar if the material removed by sputtering is comparable to the depth of As implantation. The sputtering yield of Si was not determined in the experiment of Fig. 10a, but even if it were as large as 100 Si a t o m s / 3 0 keV Arl50 cluster (the value determined for Au sputtering by 29 keV Ar3oo clusters discussed above) sputtering would account for removal of at most 200 ~k of Si. Comparing this maximum with a 50 /k molten zone depth, sputtering will account for at most a factor of 4 reduction in retained AS. A factor of more than 100 seems to be due to the insolubility of AS in Si and its rejection from the shallow, transiently " m o l t e n " zones. V. CLUSTER-SOLID INTERACTIONS

310

W.L. Brown, M. Sosnowski /NucL Instr. and Meth. in Phys. Res. B 102 (1995) 305-311

The Ar~ cluster case is very different than the situation in which mr+ atomic ions are present. Fig. 10b shows the RBS result for the latter case. The beam provided 1.3 × 1014 30 keV mr+ ions in beam spot approximately 0.6 mm FWHM in diameter, together with 4 × 10 a2 30 keV Ar20o clusters. The Ar RBS peak in Fig. 10b corresponds to 1.8 X 10 16 mr a t o m s / c m 2. It was concluded that a large fraction of the monomer ions were implanted as expected from conventional ion implantation in Si. The order-ofmagnitude larger number of mr atoms arriving as constituents of mr20o clusters were not retained. The monomer ions in this comparison have much higher velocity than the cluster, penetrate more deeply and do not produce molten zones from which they are efficiently rejected. The technologically interesting possibility that soluble dopant atoms (B or P for example) might be introduced in a very shallow layer by using clusters has not been tested experimentally. MD simulation suggests that the point defect damage produced by cluster bombardment may be low because of the formation of molten zones [26]. Hence, annealing of the damaged (amorphous) layer might not result in undesirable defect enhanced diffusion of the dopant atoms which would add a more deeply penetrating tail to the depth distribution of the shallow implant. 2.2. Metallic clusters

Interest in metallic clusters has arisen from the idea that the quality of metal films can be improved if the film material is brought to the deposition surface as energetic cluster ions with appropriate energy. Fragmentations of the clusters, as described in Section 2.1.2.1 above, will assist in cleaning the surface of impurities. It will also enhance surface mobility to enable atoms to find their lowest energy surface sites and thus produce dense films free of voids and possibly to fill topographic features on a deposition surface. The idea is related to ion beam assisted deposition in which rare gas ions bombard a surface simultaneously with the arrival of the deposition species from an independent source. In the cluster deposition no foreign species (even rare gases) are needed at the growth surface. Ion cluster deposition is also related to ion beam deposition in which atomic ions provide all of the deposition flux. Because the mass to charge ratio of clusters is large the transport of cluster ion beams has less demanding space charge requirements than an atomic ion beam. Furthermore, the choice of cluster size provides another adjustable parameter for optimizing the deposition process. The idea of cluster deposition was strongly propounded by Takagi et al. starting in the early 1970s and labeled ICB, ionized cluster beams [27]. However, effective sources of metallic cluster ions have only recently been developed. Neat metal expansion sources of high vapor pressure metals, akin to the sources of rare and molecular gas dusters of Fig. 2a, have been demonstrated [10], but the most versatile source of metallic cluster ions is due to Haberland

(a) ~

Glass

---Io.8.ml--(b)

Fig. 11. Cross section of thin film deposition on a 0.8 txm diameter window in 1.2 I~m thick glass on a Si wafer: (a) 30 keV Mo3000 at room temperature; (b) 30 keV CUls00 at 500 K [28]. [18]. It involves rare gas sputtering of the metal species of interest to put atoms into the gas phase and in the presence of a strong source of ionization, aggregation of the atoms to form ionized clusters in the rare gas, transport and minor acceleration of the ionized (both + and - ) clusters in a flow of the rare gas through an aperture into higher vacuum and subsequent acceleration of the cluster ions to the substrate for deposition. This type of source is shown generically in Fig. 2b (where the atoms are schematically indicated to be evaporated from a crucible to aggregate in a rare gas) but without the presence of a source of ionization to provide a high yield of ionized clusters. Haberland has called his deposition process ECI, energetic cluster impact. He has reported results for the deposition of several different materials two of which are compared in Fig. 11 [28]. The deposition surface in both cases was topographically patterned as a series of 0.8 p.m diameter windows in a 1.2 p~m thick glass layer on Si, typical of the topography of current VLSI technology. Fig. 11a shows results for Mo n clusters with n = 3000 deposited at 30 keV on a substrate at room temperature. Dense Mo pyramids form in the window and the windows are completely covered with a Mo film. The pyramidal geometry in the window is very similar to a low energy sticky ball simulation in which clusters stick wherever they first touch on landing. The low energy sticky ball model results in a porous film but it seems plausible that the 10 e V / M o atom deposition energy could be responsible for the difference. Window filling as in Fig. 11a is totally unacceptable in VLSI technology. Fig. l l b is a striking contrast. This case is for Cu,, with n = 1500, cluster energy 15 keV and a substrate temperature > 500 K. The windows are now completely filled. The authors interpret the result (in comparison with the Mo case) as due to the diffusivity of Cu at > 500 K and the hammering action of the impinging energetic clusters.

W.L. Brown, M. Sosnowski /Nucl. Instr. and Meth. in Phys. Res. B 102 (1995) 305-311

Any cornice that starts to develop at the edge of the window is not allowed to build up. The observed behavior is different than would be expected from simply increasing the diffusivity of Cu atoms as clusters are fragmented at the deposition surface. An increased diffusivity would not result in such nearly straight-walled indentations in the Cu film. A more quantitative model of this process is clearly needed.

3. Summary and perspectives With the possible exception of wafer cleaning with aerosols, the technological application of cluster bombardment of solids is still years in the future. While the non-linear effects of duster size and energy in sputtering (and surface smoothing?) are technically and scientifically interesting, the development of cluster sources and systems to utilize these effects will require extensive development. The possibility of shallow implantation of dopant atoms with minimal creation of point defects in silicon is intriguing and clearly deserving of experimental verification. The fluence of clusters that would be required should be quite small in this application. However, until these potential advantages are demonstrated, it remains an intriguing idea and prediction of molecular dynamic calculations. The use of metallic clusters to deposit high density metal films with good " s t e p coverage" on topographically patterned surfaces is a long-envisioned goal. The recent demonstration of complete hole filling in the case of Cu films is a fascinating result, although it is not yet clearly understood. The evolution of both understanding and the technical capability to exploit it will be a challenge in the next several years. Taken as a whole, clusters provide a new set of parameters for material processing, but the technological significance of this added flexibility has still to be developed.

References [1] R.J. Beuhler and L. Friedman, Chem. Rev. 86 (1976) 521. [2] R. Yen, C.V. Shank and C. Hirliman, Mat. Res. Soc. Proc. 13 (1982) 13.

311

[3] G.Q. Xu, S. Bernasek and J.C. Tully, J. Chem. Phys. 88 (1988) 3376. [4] J. Petterson and N. Marcovitc, Chem. Phys. Lett. 201 (1993) 241. [5] Z. Insepov, M. Sosnowski and I. Yamada, Proc. 3rd Int. Conf. on Advanced Materials, Symp. U, Tokyo 1993 (Elsevier) in press. [6] A. Kantorovitz and J. Gray, Rev. Sci. Instr. 22 (1951) 328. [7] E.W. Becker, K. Bier and W. Henkes, Z. Phys. 146 (1956) 333. [8] O.F. Hagena, Phys. Fluids 17 (1976) 894. [9] O.F. Hagena, Z. Phys. D 4 (1987) 291. [10] J. Gspann, Nucl. Instr. and Meth. B 37/38 (1989) 775. [11] O.F. Hagena, in: Physics and Chemistry of Finite Systems: From Clusters to Crystals, eds. P. Jena, S.N. Khana and B.K. Rao (Kluwer, 1992) p. 1233. [12] K. Sattler, J. Muhlbach and E. Recknagel, Phys. Rev. Lett. 45 (1980) 821. [13] D.E. Powers, S.G. Hansen, M.E. Geusic, A.C. Pulu, J.B. Hopkins, T.G. Dietz, M.A. Duncan, P.R.R. Langridge-Smith and R.E. Smalley, J. Phys. 86 (1982) 2556. [14] G. Gantefor, H.R. Siekmann, H.O. Lutz and K.H. MiewesBroer, Chem. Phys. Lett. 165 (1990) 293; H.R. Siekmann, C. Luder, J. Faehrmann, H.O. Lutz and K.H. Meiwes - Broer, Z. Phys. D 20 (1991) 417. [15] R.J. Beuhler and L. Friedman, J. Chem. Phys. 75 (1982) 2549. [16] J.A. Northby, T. Jiang, G.H. Takaoka, I. Yamada, W.L. Bro6cn and M. Sosnowski, Nucl. Instr. and Meth. B 74 (1993) 336. [17] R. Sherman and W. Whitlock, J. Vac. Sci. Technol. 89 (1991) 1970. [18] H. Haberland, M. Karris and M. Mall, Mat. Res. Soc. Symp. Proc. 206 (1991) 291. [19] Krytek Corp., Danvers, MA, USA. [20] W.T. McDermott, R.C. Ockovic, J.J. Wu and R.J. Miller, Microcontamination, Oct. (1991) 33. [21] P.H. Rose, Private communication. [22] P.R. Henkes and R. Klingelhofer, Vacuum 39 (1989) 541. [23] M.W. Matthew, RJ. Beuhler, M. Ledbetter and L. Friedman, Nucl. Instr. and Meth. B 14 (1986) 448. [24] R.J. Beuhler and L. Friedman, J. Phys. 50 (1989) C2-127. [25] H.H. Andersen, private communication. [26] G.H. Gilmer, Private communication. [27] T. Takagi, I. Yamada, M. Kunori and S. Kobiyama, Proc. 2nd Int. Conf. on Ion Sources, Vienna (1972) p. 790; I. Yamada, H. Inokawa and T. Takagi, J. Appl. Phys. 56 (1984) 2746. [28] H. Haberland, M. Mall, M. Moseler, Y. Qiang, Y. Thurner and T. Reinere, J. Vac. Sci. Technol. A, to be published.

V. CLUSTER-SOLID INTERACTIONS