Vacuum/volume 36/numbers 1-3lpages 27 to 31 I1986 Printed in Great Britain

0042-207X/86$3.00 + .OO Pergamon Press Ltd

Ionized cluster beam technique T Takagi. Ion Beam Engineering Experimental Laboratory, Kyoto University, Sakyo, Kyoto 606, Japan

Ionized cluster beams (ICB) are widely used to deposit metal, semiconductor and insulating films. This paper describes the current state of this technology in both fundamental and applications areas. ICB provides tight control of the kinetic energy and ion content of the beam, which gives the technique a unique advantage compared to MBE, VPE, LPE, MOCVD, sputtering or other thin film deposition methods. The development of this technology should allow the deposition of high quality material at low temperature onto a wide variety of substrate surfaces and even permit the formation of thin film materials not previously possible.

1. Introduction

More than 10 yr have passed since ICB was introduced’. The major problem to be initially addressed was the production of high quality thin films for functional devices at high deposition rates by utilizing the effects ofion bombardment. Since then, study of cluster formation has been carried out theoretically and experimentally. Preparations of various kinds of thin film materials have also been made. These include semiconductors, metals. dielectrics for electronic devices, optical coatings, opto- electronic devices, magnetic materials, thermoelectric materials and organic materials’.

In the last 2 yr, ICB studies have made much progress in both cluster formation and thin film formation. Fundamental study of cluster formation by using condensation theory has brought much understanding of ICB operation conditions3. Through these studies, one important ambiguity in the classical condensa- tion theory of metal cluster formation could be addressed. Recent emphasis in cluster formation study has been placed on the measurements oftranslational velocity, translational temperature

and mass spectrum. Several types of ICB equipment are now commercially available in Japan and in the United States4.‘.

Film formation study has also brought much progress in semiconductor device and opto-electronic device applications. Since the experimental results have clearly shown that the kinetic energy of the individual atoms in the accelerated clusters is much lower than those by single atom@, further development of structurally sensitive materials could be addressed. In the low energy range which ICB can offer, we have already demonstrated explicitly the effects of acceleration voltage upon adatom migration, nucleation density, sticking coefficient and enhance- ment of chemical reaction’. These studies contributed to under- standing the film formation mechanism by ICB. Al epitaxy on Si and dielectric materials, high quality and large area GaAs film epitaxy, CdTe/PbTe super-lattice formation, high quality SiO,

and CaF, depositions on low temperature substrates, and controlled crystalline organic film deposition were highlights in the last 2 yr.

The paper describes only novel results. Other papers presented

in IPAT conferences complement the present paper by presenting information on many other thin film applications3.8~‘0.

2. Equipment and operation conditions

The basic deposition system construction is shown in Figure 1. Vaporized source material is ejected through the nozzle of a crucible at high temperature into a high vacuum chamber. Operating conditions are selected so that vapour emerging through the crucible nozzle undergoes adiabatic expansion and subsequent cooling to a supersaturated state. This leads to the formation of atomic aggregate clusters. Energetic electrons injected into the transport stream cause impact ionization ofsome of the clusters; these positively charged clusters can then be accelerated toward the deposition surface of the substrate by an applied high potential.

Theoretical strategies of studying cluster formation have made much progress in the last 2 yr. The classical theory told us that the

SUBSTRATE

!:

0-lOkV

Figure 1. Schematic diagram of a laboratory ICB system

27

T Takagf. lonlred cluster beam technique

b;~rricr height for cluster formation is strongly influenced h\

surface tension (T. so that the value of the nucleation rate is ccr! sensitive to cnriation in 0 ’ ’ This information had for many years

been responsible for the following misunderstanding: ‘metal clusters are hard to form because of the high surface tension‘. The

situation is. however. more subtle. The author and co-wol-hers

have pointed out that the harrier height of cluster formation and

nucleation rate for metals arc. in fact. quite similar to those 01

alk;tli metals. gases and welter. cvcn though metals :jre char;lcter-

itcd by high surface tension’. The vaporization temperatures 01

metals and semiconductors arc normally higher than those oft he

gases for the pressureunder consideration. and the nucleation I-ate

IS ii function of (f~ Tj’. not just f13. ConsequentI!, the nucleation barrier heights and critical cluster G/es become comparable to

those of gases when proper operating conditions were taken fog-

the theoretical calculation. We have made several calculations on

these problems for metals and gases and shown clearly the aho\c

mentioned formation conditions. Lu ~‘f ctl have also calculated the

Gibbs free energy AG for cluster formation as ;I function of number of atoms”. The results show that energ) barriers fol-

nucleation of metals are indeed comparable to. or c\ CII lowel- than

thoi;e of gas and water.

Experimental evaluations of cluster size have been made so far by Theetcn’“. Kn,iucr’4 ‘ and Harrison”. Structural analysis of

metal clusters was made at Northwestern University collabora-

ing with Kkoto Universit> ” Elctensivestudiesofthefund;m~ental

characteristics of the cluster beam. specifically cluster size. cncrg).

velocity, structure and bonding configurations have been carried

out at Kyoto University. Recently. translational velocit! and

translational temperature ofthe cluster beam have been measured

b! u time-of-flight (TOF) method. An apparent contrast w;15

observed by comparing the translational tempcrltture of beams

from ;I cluster source with those of simply evaporated atoms from

an open cruciblei. The translational temperature of the cluster.

source beam is much lower than the crucible temperature and

decreases with incrcasinil_ crucible temperature. whereas the

temperuture for the simply evaporated atoms is nearly equal to

the crucible temperature and increases with increasing crucihlc

temperature. The results demonstrate that the efficient adirrbatic

expansion enables the onset of the condensation of the vapour. that is. cluster formation is taking place.

Figure 2 shows the size distributions of Ag clusters measured b!

retarding field method bl Yamada and Harrison’ _. Thr si/cs

showed broad distribution in the range from several hundred to the thousand atoms. cluster. The beam intensity and the cluster

I SILVER CLUSTERS

NOZZLE

imm x Imm

a 7 .-

I 500 I 000 I500 2000

CLUSTER SIZE (atoms/cluster )

Figure 2. (~‘lustel- \~zt‘ distrlbuclon mxsured hq rt~,mi~n_c eicctr~c ticlJ rnethnd

s17c incrc:tsc h! increasing the crucible temperature. Figure 3

shows the mass spectra of Te clusters measured hl the TOF

spectrometer. The Te cluster sizes also distribute from 500 1500

atoms. and the intensity of the high mass component increases h>

increasing the crucible temperature. Since the ahove LMO different

mcasuremcnts showed similar results. the measuring errors which

might inherently bc caused in the equipment cmploycd. for

r~ample. effects h> multiple charge. charge transfer collislclnh. spice charge in the ionizci-. etc.. I% cre sho\vn to hc ncglipihlj sm;ill.

Te CLUSTERS

100 200 TIME OF FLIGHT hsec)

1, , 1 , , - HO loo 500 lam 2000

CLUSTER SIZE (ATOMS 1

3. Bombarding effect by IC‘H

Kinctlc energy of ions appropreatc for tilm gro\\ th 15 in ;I I-ange 01

a few cV to a few hundred eV’“.“‘. Crystallographic and other

physic:ll properties of depositing tilm can bc controlled h!

changing the kinetic energy c,f ICB. WC 1x1~ ;tlready dc-

monstrated cxplicitl! the cffccts of acceleration voltage upon

nucleation dcnaitk. sticking coefticient und the enhanccmcnt of

adatom migration”“.“,

Recenti!. tbs’o important e4pcrinicnts related to the film

formation mechanisms h> ICB hate been done. The one is ii1 ,\ifr,

ohscrk ation of initi:ll stage of film growth. The ohscr\ Lition during

41 deposition on the clenn Si surface Showed that Incre;lse of the

accctcration voltage on I<‘B resulted in nearI! Ia!er-by-layel-

growth and flat film surfa&‘. The other is mcasurcment of the

surface disorder caused h> ICB bombardment. M hich C;III also hc

;i nie;isure of the cluster size’.“. After the Al wiis chemically ctchcd

from the Si substrate. the surface peak of the substr-ate w;~\

measured by 0.925 MeV He F ch:mnelling. I-~gure 4 shows the

comparison of the surface peaks for 5 kcV and 0.2 keV ICB Al

bombarded surfaces, and 0.5 keV atomic Ar ion bombarded

surfLIce to a dose of 3 x IO’ ’ ions cm ‘. The results show that the

displacement ofsurfacc atoms produced b! Al ICB bombardment

is much smaller than that cuuscd by the atomic ion beam. Notice

that the disordered layer thickness I-I! Al ICB bombardment is

comparable to that of native oxide on Si substrate. These inherent

properties of ICB enable the technique to perform low temper;l-

ture epitax?. ultra-thin film depositlon. and crSstallopraphicall?

and compositionall> designed film FormatIon.

28

T Takagi: Ionized cluster beam technique

ENERGY ( MeV)

0.40 30 II

0.45 0.50 0.55 0.60 1 1 I 1 I

Sl(llll IRRADIATE0 BY

25 - Ar+ 2x IO” /cm2

Al ICB -l&/cmz, IZnmlmin

0

O.ZkeV Al ICB

iCI8XlO” Sl/cm’l

PROBE : 0.9MeV He+

IIIIIALIGNEO

NO IRffADlATlON

/I6.9~10’~ Sl/cmzl

_. . _ 80 90 100 I IO I20 130

CHANNEL NUMBER

Figure4.0.925 MeV He+ channelling spectra for Si bombarded by Ar’ and Al cluster (Al has been etched off).

4. Applications

Here we describe only a few areas of recent important progress in

ICB applications. Recent review articles and symposium reports have described in detail areas of applications and specific film

result+‘.

4.1 Semiconductor devices (a) Metallization and interconnects. Metallization and interconnect technologies are important key factors enabling further advancement in VLSI technology. Aluminium is widely used for contact electrodes and intercon-

nects in silicon semiconductor devices. It is, however, important to establish the quantitative limits of the AI/Si system imposed by such problems as electromigration, hillock growth, alloy penetra- tion and interface stability. We have shown these limits and further extension of device performance by using epitaxial Al films. The films could be grown by ICB at room temperature on Si(ll1) and Si(100) substrates. Crystalline orientation of the Al film on the Si( 111) substrate is determined as Al( 11 l)//Si( 11 l), Al[ilO]//Si[ilO]. Two orthogonal orientations, Al(l lO)//Si(lOO), AI[OOl]//Si[Ol l] denoted as Al( 110) and Al(l lO)//Si(lOO), Al[ilO]//Si[Oll] denoted as Al(llO)R were observed in the film on the Si(lO0) substratez3. Ho reported that MBE deposited Al film on Si(ll1) substrate can be epitaxial only after the heat treatment, but not on the Si(100) substratez4. The thermal stability of the epitaxial Al/Si system has been studied collaborat-

ing with Mayer’. ICB deposited Al films on Si substrate were annealed for 30 min in the 450-55O’C temperature range. No hillocks and valleys, as normally observed in the Al films prepared by conventional vacuum deposition, could be seen. Also, no degradation of the film crystallinity was observed by Rutherford backscattering spectroscopy (RBS). Auger electron spectroscopy (AES) measurement showed that the interface remained abrupt even after annealing at 550°C. Electrical stability was evaluated by the Schottky barrier diodes. The barrier height and the n value remain stable after the annealing. This behaviour is in stark contrast to that of the films made by sputtering or evaporation which show changes of more than 10%. An electro-migration test was made for 10 pm wide, 1000 pm long strips of 400 nm thick films. After flowing current at a density of lo6 A cm-’ at 250°C there was no change in resistance after 400 h of operation as

opposed to sputtered Al films that normally fail after 20 h. The results show that many difficulties encountered with Al metalliz- ation for VLSI application are thus apparently not intrinsic to the

AI/Si system itself, but are consequences of particular deposition

techniques. Our previous experiments on Al/Si showed that ICB appeared

to be capable of making a significant contribution to the

interconnects. Epitaxial deposition of metals on single crystalline

dielectric materials thus becomes important for three-dimensional VLSI devices. A trial of Al epitaxial growth on sapphire and CaF, substrates has been made by the ICB method. Epitaxial Al films on these substrates could be obtained. Evaluation of electrical characteristics has just been started.

(b) Compound semiconductors. GaAs thin films with high crystalline quality were recently prepared. Compared with MBE, ICB could be more suitable for industrial production because of much higher allowable deposition rates for crystal growth, the possibility of using much larger substrate area, the possibility of using lower substrate temperature and more versatile doping methods. The films were deposited on (100) LEC GaAs substrates using dual Ga and As cluster beams, both partially ionized and accelerated to the substrate held at 5obdoo”C under a back- ground pressure of only 10m6 torr. It is expected that the sticking coefficient of ICB deposited Ga is enhanced and less arsenic flux is required compared to that used in MBE. The RED pattern of the film deposited at acceleration voltage of 1 kV showed a streaked

pattern and the Kikuchi lines, which indicated surface smoothness in addition to the excellent crystallinity. RBS channelling spectra shown in Figure 5 demonstrate that there is no measurable difference in the spectrum of the film compared to that of the substrate5,2s.

The study of thin multilayered films in the l-10 nm range has become very important in both fundamental physics and ad- vanced device application. CdTe-PbTe multilayered structures have been constructed on InSb substrates at 250°C by means of dual ICB source using CdTe and PbTe ingots as source materials. The CdTe-PbTe multilayer was grown on a 150 nm CdTe buffer layer which was epitaxially grown on an InSb substrate. The existence of an n = 1 miniband in the potential well structure of 60 period films with layer thickness of 6.5 and 8 nm respectively was confirmed by optical absorption spectrum26.

ENERGY (MeV)

50. 1.6 I 1.8 2.0 2.2 2.4 2.6 I

He++’ ICE GaAs/GaAs 11001

A:RANDOM, El:Va=l kV. Ts=500°C

40 - c:va=05kv, TS=~O~~~(NEUTRAL ~01

D:Va= 1 kV, Ts=GOO~. E:WBSTRATE

CHANNEL

Figure 5. 3.0 MeV He+ channelling spectra for GaAs films.

29

T Takagi: lonved cluster beam technique

(c) Insulating films. Insulators play very important roles in integrated circuits as gate oxides for MO% isolation regions

between active devices, passivating coating, and interlevcl dielectric for multilayer interconnects. Development of process technology for formation of a very thin dielectric film at low substrate temperature would bring about significant progress in device fabrication. The conventional highest quality SiOZ has been, so far, high temperature (800- 1200 C) thermally grown oxide. The necessity of high temperature restricts the device processing that can be done before the thermal oxidation.

Encouraging ICB SiO, deposition at substrate temperatures of 200 300 C has been achieved by Minowa VI (I/“. Wong et trl deposited SiOz films which displayed high dielectric strength

(ho”,, yield exceeding 8 x IO” V cm ’ PH. RecentI!, stoichiometric, dense SiOZ has been deposited by ICB at :I substrate temperature of 100 C. Presently. we are studying deposition of various kinds of single crystalline dielectric films on which epitaxial metal can be deposited. We have alreadq deposited single crystalline CaF, on Si substrate and then grown epitaxial Al film on it by ICB. Figure 6 shows a preliminary result of RBS channelling analysis of CaFz film on Si substrate. The x,,,,,, is 2”,, for (1 IO) incidence, 4”,, for (I 1 I), and 6”,, for (41 I). The film showed smooth surface. good adhesion and no cracks.

1.0

0

2OMeV He+

aligned to

CaF2<l IO>

iSi<411>1

Va IkV

Ts 7OO’C

I I loo 200 300 400 5ot

CHNNEL NUMBER

4.2. Application to magneto-optical devices. Currently. bulk material is used for optical isolator, circulator and modulator as well as for the magneto-optical recorder. This is presently the major factor limiting the device performance in high speed and high density operations. Cd, _.Mn,Te film. which shows a large Faraday rotation, could be one of the candidates for fabricating a

high sensitive thin film type magneto-optical device. Epitaxial

Cd, ,Mn,Te thin film with large Faraday rotation wasdeposited on 300 C sapphire (0001) substrate by simultaneous deposition of ionized CdTe and neutral MnTeclusters. The Faraday rotation of the epitaxial film is greater than that by bulk material. Through ICB deposition of Cd, _,Mn,Te. we found that the film composi- tion. that is .x, can be changed by controlling the ion acceleration voltage“‘. Since the optical bandgap of this material varies linearly with the composition .x. the photon energy where the Faraday rotation changes from a negative. direction to positive can also be varied by depositing the film at different acceleration

z 2.0 - Y

Cd,_xMnx Te Film

Lz LCdTeI+-(MnTeI

z l.O- Ie= lOOmA

c5 -

s

k 0

-2.0’ ’ 1 ’ ’ 1 ’ 1.8 2.0 2.2 2.4 2.6 2.8

ENERGY (eV)

voltages. Figure 7 shows one esamplt: of the shift of the dispersion characteristics. This versatility in the ICB method could extend further development of functional devices with special optical

characteristics in mind.

4.3. Application to organic materials. Just ;I!, the l’ormation 01’

highly oriented inorganic films has been made possible by ICH. the fundamental effects associated with ICB could be exploited to produce high quality organic film. Recent researches on anthra- cene. polyethylene and Cu phthalocyanine film depositions have shown the above possibilities in organic film formation. An evaluation of the crystalline characteristics of the polyethylene fihn has been made. Figure 8 shows the RED patterns of the tilms deposited at different acceleration voltages. The film deposited at the higher acceleration voltage indicated high crystal orientation. The molecular chains in the crystallites stand perpendicularly to the substrate. In addition. adhesion strength was increased bq increasing the acceleration voltage and ion current densit>. Molecular weight distribution of the film deposited by neutral

cluster beam was measured by the gel permeation chromatograph (GPC). Figure 9 shows the comparison of the results for the source material and the deposited film. The average mol wt did not show much difference. but the film had smaller mol wt distribution than that of the source material. Thcsc results show that highly crystalline and adhesive polyethylene can be deposited by ICB. Recently. we have carried out investigation of the ICB effects on the electrical characteristics of an organic semi-

iio, l llO

Too0 l ooo l 100

ii0* .iio

le:ZOmA Tsub. 70°C

30

T Takagi: Ionized cluster beam technique

DEPOSITED FILM

SOURCE MATERIAL REMNANT IN CRUCIBLE

100 I 000 I 0000

MOLECULAR WEIGHT

Figure 9. Molecular weight distributions of polyethylene for source material. deposited film and remnant in crucible.

conductor”‘. In the case of Cumphthalocyanine films, the activa- tion energy of the conductivity can be increased by applying the

appropriate acceleration voltage during the deposition. Effective doping method for oxygen or Mg during Cu-phthalocyanine deposition has been investigated. Depositions in oxygen gas in a range of 10-‘~-10~4 torr and evaporation of Mg during the

deposition have been effective and practical methods. The electrical property of oxygen-doped film showed high photo- sensitivity with small dark conduction. For ME-doped films, it was shown that Mg atoms in Cumphthalocyanine-substituted Cu atoms which were coordinated in the centre of the molecule and consequently Mg-phthalocyanine was formed. Detailed investi-

gation is now in progress.

5. Conclusions

Thus far most of the ICB applications have been directed toward the area of electronic device fabrication. Other applications exist where there is a crucial need for dense, smooth, adherent films with well-controlled microstructure. One example is the optical coatings industry where multilayer laser mirrors and antireflec- tion coatings frequently display variations in scattering, sen- sitivity to moisture, and (for laser mirrors) uncontrolled damage threshold. The development of ICB techniques for the deposition of TiO,, SiO,, AI,O, and MgF_, for these applications is in progress. Yet another application where no work has been done is surface modification by thin film coatings which are hard and corrosion resistant and have a low coefficient of friction. Again, ICB offers control precisely over those deposition parameters which may make these coatings useful.

Acknowledgements

The author wishes to thank his many associates in Japan and in other countries for their help and cooperation. He would also like to thank them for valuable data which is not explicitly credited.

References

’ T Takagi. I Yamada, M Kunori and S Kobiyama, Proc 2nd I~lt Conf’lo~ Sources, Vienna, ijsterreiche Studiengesellschaft fiir Atomenenergie, p 790 (1972). ‘T Takagi, in lo11 Implantation und ion Beam Processing of Materials. (Edited by G K Hubler, 0 W Holland, C R Clayton and C W White). p 501, North-Holland. New York (1984). 3l Yamada, Proc Int Ion Engng Congr- ISlATN3 & IPATX3. Kyoto. (Edited by T Takegi) p 1177, IEEJ, Tokyo, (1983). ’ TTakagi, in Emergency Process Methodsjiw High-Technology Ceramics. (Edited by R F Davis, H Palmour III and R L Porter), p 425, Plenum, New York (1984). ’ I Yamada, T Takagi, P R Younger and J Blake, SPIE’s 1985 Los Angeles Tech Symp Advanced Applications of Ion Implantation, Tokyo, 1982, Vol 530, p 75, Int Sot Optical Engrs, Washington (1985).

’ I Yamada, C J Palmstrnm, E Kennedy and J W Mayer. Material Res Sot Symp Proc, Vol 37, p 401, Material Res Sot (1985). ’ T Takagi, J Vat Sci Technol. 2, 382 (1984). ’ T Takagi, I Yamada and A Sasaki, Proc Coqf Ion Plating & Mied Techniques, Edinburgh, p 50, CEP Consultants Ltd (1977). 9 T Takagi, K Matsubara, H Takaoka and I Yamada, Proc Int Co$Ion Plating & Allied Techniques, London, p 174, CEP Consultants Ltd (1979). ” I Yamada and T Takagi, Thin Solid Films, 80, 105 (1981). I’ G D Stein. Proc Int Ion Engng Congr ISIA7”83 & IPAT83, Kyoto, (Edited by T Takagi) p 1165, IEEJ, Tokyo (1983). I* T M Lu, J appl Phys. to be published. I3 J B Theeten, R Madar. A Mircea-Roussel. A Rocher and G Laurence. J Crystal Growth, 37, 3 17 (1977). ” W Knauer, ICB Workshop, Dayton OH (1984). ” S Harrison and P Younger, ICB Workshop. Dayton OH (1984). I6 I Yamada, G D Stein, H Usui and T Takagi, Proc, 6th S,mp Ion Sourws & Ion-Assisted Technol, Tokyo, 1982, (Edited by T Takagl), p 47, The Res Group of Ion Engng, Kyoto Univ, Kyoto (1982).

” I Yamada, private communication. ‘” T Takagi, Thin Solid Films, 92, 1 (1982). ” T Takagi, Proc Int Ion Engng Congr ISIATX3 & lPATX3. Kyoto, (Edited bv T Takagi). v 785. IEEJ. Tokvo 119831. ” I Yamada H Takaoka, H Inokawa. fi Usui. S’C Cheng and T Takagi, Thin Solid F;lmb, 92, 137 (1982). ” T Takagi. I Yamada and A Sasaki, Thin Solid films. 39, 207 (1976). “1 Yamada, H Inokawa and T Takagi, Thin Solid Films, (to be published). 23 I Yamada, H lnokawa and T Takagi. J uppl Phys. 56, 2746 (1984). 24 F K LeGoues P S Ho and M Henzler, Material Res Sot Symp Proc, Vol 37, p 359, M’aterial Res Sot (1985). 25 P Younger, J Vat Sci Technol, A3, 588 (1985). 26 T Takagi K Matsubara and H Takaoka, Thin Solid Films, (to be published). ’ ” Y Minowa and K Yamagishi, J Vat, .%i Trchnol, Bl, I I48 (1983). ‘” J Wong, T M Lu and S Metha. SPIE’s 19X.5 Los Angeles Tech Sj,mp Advanced Applications ofIon Implantation, Vol 53, p 84, Int Sot Optical Engrs, Washington. 29 T Koyanagi, Y Obata, K Matsubara, H Takaoka and T Takagi, Proc 8th Symp Ion Sources & Ion Assisted Technol, Tokyo, 1984, (Edited by T Takagi), p 85, The Res Group of Ion Engng, Kyoto Univ, Kyoto (1984). 3o H Usui, M Naemura, H Nakanishi, I Yamada and T Takagi, Proc 8th Symp Ion Sources & Ion Assisted Technol, Tokyo, 1984. (Edited by T Takagi), p 271. The Res Group of Ion Engng, Kyoto Univ. Kyoto (1984).

31