Thin Solid Films, 92 (1982) 1-17 PREPARATION AND CHARACTERIZATION

R O L E OF IONS IN ION-BASED F I L M F O R M A T I O N *

T. TAKAGI Ion Beam Engineering Experimental Laboratory, Kyoto University, Kyoto (Japan)

(Received December 24, 1981 ; accepted January 12, 1982)

In ion-based film formation, the properties of a film are controlled by changing the deposition conditions such as the acceleration voltage and the content of ions. The role of ions in film formation is discussed under three headings: the effect of inert gas ion bombardment after or during film formation, the effect of the kinetic energy of source material ions, and the effect of the charges of the ions. A comparative description of various experiments according to the above categories is given. In the case of inert gas ion bombardment after or during film formation, ions of a few hundred electronvolts to a few kiloelectronvolts were used. In depositions where the source material is ionized, the effective incident energy of the ions is found to be of the order of a few electronvolts to several hundred electronvolts which is much lower than that in inert gas bombardment.

Various kinds of film formation technique are reviewed from the viewpoints of kinetic energy and charge content. Although these parameters may vary according to the film formation technique, it was found that their role in influencing film properties does not change.

1. INTRODUCTION

The object of ionization in the ion-based techniques for film formation is to utilize the inherent properties of materials by giving some kinetic energy and/or enhancing chemical activity through the ionization process. Chemical activities include the film formation activity and the chemical reaction activity. The fundamental effects of the kinetic energy of ions are sputtering, implantation, nucleus formation, heating, migration etc. The presence of the ionized particles in the evaporated materials, even without acceleration of the ionized particles or even when only a few per cent of ionized particles are included, greatly influences the critical parameters of the condensation processes for film formation and enhances the chemical reaction. In the ion-based technique, the kinetic energy and the ion

* Paper presented at the Fifth Symposium on Ion Sources and Ion-assisted Technology and International Workshop on Ion-based Techniques for Film Formation, Tokyo and Kyoto, Japan, June 1-5, 1981.

0040-6090/82/0000-0000/$02.75 © Elsevier Sequoia/Printed in The Netherlands

2 T. TAKAGI

content of the deposited materials are easily controlled by changing the operating conditions. Therefore, film properties such as the mechanical, optical and crystallo- graphic properties can be controlled three-dimensionally.

In this paper, the role of ions for film formation will be discussed. Further, several kinds of ion-based technique including the ionized cluster beam (ICB) techniques will be investigated and summarized, on the basis of the processes of ionization and the acceleration of the source materials.

2. ROLE OF IONS FOR FILM FORMATION

Ions transfer energy, momentum and charge with all the inherent properties of the materials to a substrate or to a depositing film surface 1-s. The kinetic energy due to an accelerating voltage and/or the chemical activity enhanced by the existence of charge can be utilized effectively for deposition and epitaxy. Figure 1 shows the energy ranges typical of the preparation methods for film formation in terms of the kinetic energy of the source materials.

Conventional Vacuum Sputter Ion Plating and Evaporation Deposition Ion Beam Deposition

F . . . . . . . . . . . . .

10 -2 lO "I 1 IO i IO 2 lO 3

KINETIC ENERGY ( eV )

i)i)iiii Impl antatl on ~!~::~)

1O 4

Fig. 1. Energy ranges typical of the preparation methods for film formation.

TABLE I INFLUENCE OF KINETIC ENERGY ON FILM FORMATION

Fundamental effects of kinetic energy Influence on film formation

Surface cleaning by sputtering

Deep etching Blending of sputtered material with incident

evaporant particles Creation of suitable amount of activated centres

such as defects and displacements of surface atoms which act as centres of nucleus formation

Suitable ion bombardment and sputtering during deposition

Ion implantation (including recoil implantation) Heating by the thermal energy converted from

kinetic energy Migration of depositing particles on the

substrate

Improvement of adhesion Removal of the surface oxide or contaminated

layers just before the deposition Mechanical improvement of adhesion Formation of interfacial layer

Enhancement of the growth of nuclei at the initial stage of film formation

Formation of an inteffacial layer and increase in bonding energy between substrate and de- posited atoms

Change in the morphology Stimulation of nucleation, growth of nuclei and

coalescence Enhancement of interfacial layer formation Change in the morphology Increase in the chemical reactivity Enhancement of surface diffusion energy keep-

ing a relatively low substrate temperature, resulting in change in morphology or growth of epitaxial film

ION-BASED FILM FORMATION 3

The kinetic energy of the ions affects the fundamental processes in film formation, i.e. sputtering, implantation, nucleation, heating, migration etc., as shown in Table I. These effects influence the crystallographic and physical properties of the films. For example, the adhesion strength, pac.king density, surface roughness, crystalline state and structure of the deposited films, or the efficiency of synthesis of deposited compound materials etc., are improved remarkably by the acceleration of the ionized particles.

Furthermore, the presence of ions produces a remarkable change in the chemical activity, which includes the film formation activity and the enhancement of chemical reactions. In particular, it greatly influences the critical parameters of the condensation process for film formation, even without the acceleration of the ionized particles or even when only a few per cent of the ionized particles are included in the source materials. Therefore, the physical and crystallographic characteristics of the deposited films can be remarkably improved by adjusting the ion content.

The role of ions for film formation can be discussed through investigations of the following effects: (a) the effect of inert gas ion bombardment after or during film formation, (b) the effect of the kinetic energy of the source material ions and (c) the effect of the presence of ions on the film quality.

2.1. Effect of inert gas ion bombardment after or duringfilm formation The study of the effects of inert gas ion bombardment on film properties has

been in progress since the beginning of the 1970s. The irradiation effects of inert gas ions after or during film formation reported so far are listed in Table II. For example, the effects of ion bombardment (10 keV argon ions with an ion current of 8 laA) on the crystallographic orientation in vacuum-condensed silver and gold films were investigated by Dobrev and Marinov in 19739. It was also shown by the same group that vacuum-condensed cadmium and cobalt thin films on glass substrates change their crystallographic structure after bombardment with 10keV energy ions (2-3 IxA mm-2)1 o. Marinov has also discussed the effect of ion bombardment on the initial stages of thin film growth by using silver deposition onto an amorphous substrate with accelerated argon ions of 1-10 keV energy 11. A study of the effects of ion bombardment (Ar +, Ne +, 100eV to 3 keV, 0-10 ~tA c m -2 ) o n the process of metallic (zinc, antimony) film formation on semiconductor (Cu20) and ionic crystal (NaC1, KC1) substrates has been made by Babaev et al. 12 In the experiment, the effects of inert gas ion bombardment on the critical pressure for the condensation process were investigated.

Studies on germanium films deposited under a simultaneous argon ion bombardment were made by Hirsch and coworkers in 1978 to investigate the effect of ion bombardment on the adherence of germanium films 13. In the experiment, the ions constituted only 4% of the total particle flux reaching the substrate, but nevertheless ion bombardment had a profound effect on the properties of the germanium films. From the results obtained by these workers, it follows that ion bombardment causes stronger bonding between the deposited film and the substrate, or that it leads to a decrease in the intrinsic film stress, or that possibly it produces a combination of these two effects.

A dual-ion-beam deposition technique has been proposed by Weissmantel and

4 T. TAKAGI

TABLE II I R R A D I A T I O N EFFECTS OF INERT GAS ION B O M B A R D M E N T AFTER OR D U R I N G FILM FORMATION

Bombarding ion Deposited Substrate Results materials a materials

Ar +, 100 eV-3 keV Zn, Sb Semiconductor He + (0-10 ~tA) (Cu20),

ionic crystal (NaC1, KC1)

Ar + 10 keV (8 laA) Ag, Au Glass 10 keV Cd, Co Glass (2-3 IxA mm- 2) 1-10 keV Ag Amorphous b

1.65 keV Ge Glass (2 ~A)

0.5-3 keV (~ 102 laA cm-2 to 102 mA cm- 2)

C c Glass

Change in the critical pressures for the condensation process

Change in the capture cross section of an atom of deposit material on the sub- strate surface

Increase in adatom mobility and nu- cleation rate

Stimulation of coalescence Crystallographic orientation (1 l~0)-oriented film formation

Enhancement of the mobility of deposit- ing atoms

Acceleration of the nucleation, growth of nuclei and coalescence

Preferential crystal orientation Improvement of adhesion Uniform film formation without peeling

off Formation of diamond-like film

a By vacuum evaporation. b The amount of irradiated Ar + is only 4~ of the total incident number of germanium atoms. c Deposited by sputtering; dual-ion-beam deposition.

his g roup 4. In the exper iments , no rma l ion beam sput te r ing of a g raphi te target yielded only blackish a m o r p h o u s layers. However , when the growing film was b o m b a r d e d with a rgon ions of a b o u t 1 keV at a cur rent densi ty of the o rder of 100 ~tA c m - 2 , the layers became t r anspa ren t and very hard. D ia mond- l i ke ca rbon films were obta ined . I t should be no ted tha t b o m b a r d m e n t by the inert gas ions dur ing the film growth by sput ter depos i t ion also causes dis t inct changes in the film propert ies .

Th rough the results of these exper iments , it m a y be conc luded that the effects of inert gas ion b o m b a r d m e n t are as fol lows: (1) enhancement of the surface mobi l i ty of ada toms , (2) s t imula t ion or accelera t ion of the nucleat ion, the g rowth of the nuclei and the coalescence at the init ial s tage of film format ion , (3) c rea t ion of ac t iva ted sites that s t imula te the nuclea t ion process, (4) deve lopmen t of nucleus o r ien ta t ion on a s ingle-crystal subs t ra te by the increasing concen t ra t ion of small condensa t ion centres, (5) a recrys ta l l iza t ion effect in the depos i ted film p roduced by the incident energy of the ions, (6) increase in the bond ing energy between the depos i ted film layer and the subs t ra te and (7) decrease in stress in the film.

2.2. Effect o f kinetic energy o f source material ions The kinet ic energy of the source part icles (deposi t mate r ia l vapour) great ly

ION-BASED FILM FORMATION 5

influences the formation of films with the inherent properties of the materials. The efficiency of material collection on a substrate is controlled by the sticking probability and the self-sputtering ratio. Film formation cannot be obtained at large incident energies of the deposit material vapour on the substrate, which corresponds to the sputtering rate being equal to unity. However, if the incident energy is too low, the film" cannot be formed effectively either, because the sticking probability decreases to too low a value. According to the results obtained by Fontell and Arminen 14 the sticking probability of some kinds of material decreases gradually below energies as low as 100-200 eV, whereas in other experiments that of other materials does not show remarkable decreases even below several electron- volts15-1 s. Of course, the energy dependence of the sputtering rate and the sticking probability depend on the kinds of deposit material and on the combinations of deposit and substrate materials used. Following these considerations about material collection, the optimum energy should be less than that for which the sputtering rate S(E) = 1 and also larger than that at which the sticking probability is extremely low. In addition, the adsorption energy of physically adsorbed atoms on the substrate surface is 0.1-0.5 eV, and that of chemically adsorbed atoms is 1-10 eV 19. Therefore it is desirable to adjust the kinetic energy of the source ions to a value larger than the adsorbed energies of surface impurities, which results in cleaning of the substrate surface.

For good quality film formation, the migration effect is one of the important factors. The effect causes a change in the momentum of depositing materials from the direction of bombardment to directi6ns along the substrate surface, resulting in an increase in surface diffusion energy and a change in the quality of the deposited films. Furthermore, some recent studies suggest that suitable ion bombardment enhances the growth of crystal nuclei, and a proper amount of defects and displacements ofsubstrate surface atoms is a good influence for the film formation in the initial stage 2°. From this viewpoint, the kinetic energy of the source ions should be in the range of energy corresponding to the formation of films with good quality.

A summary of the above is listed in Table III. Taking account of these considerations, an optimum value of the kinetic energy of ionized and accelerated particles is estimated to be in the range of a few electronvolts to a few hundred electronvolts for film formation. The optimum energy should be selected according to the required characteristics, e.g. mechanical property, optical property, mor- phology and the combination of deposit and substrate materials.

2.3. Effect of the presence of ions It is significant to summarize the influence of the ionized particles on the film

quality from the viewpoint of enhancing the film formation activity and the chemical reactions through the presence of ionized atoms as distinct from the effects of kinetic energy. A profound effect caused by the presence of the ions, even though the content of ions is only a few per cent in the total flux or even without an acceleration voltage, is the production of a marked change in the critical condensation process of the deposited materials. This process is closely related to the mechanisms of nucleation, growth of nuclei and coalescence 21.

Some of the remarkable effects due to changing of the ions during deposition

6 T. TAKAGI

TABLE III O P T I M U M C O N D I T I O N S FOR THE KINETIC ENERGY OF IONS I N C I D E N T ON THE SUBSTRATE FOR FILM

FORMATION

Condition Required incident ion energy Result

Deposition

Surface cleaning

Good quality film formation

Less than the energy corresponding to the sputter- ing rate (S(E) = 1)

Larger than the energy at which the sticking probability becomes too low

Larger than the energy of adsorption on the substrate surface, i.e. 0.1-0.5eV for

physically adsorbed gases and 1-10eV for chemically adsorbed gases

In a range where enhanced adatom migration influences proper-

ties of the deposited film suitable ion bombardment affects the growth of

nuclei a suitable amount of defects or atomic displace-

ment near the substrate surface contributes to film formation during the initial stage

--, Optimum value of kinetic energy: a few to a few hundred electronvolts

can be understood through the experimental results as shown in Table II, where the presence of ions influences very much the critical pressures for the condensation process and the capture cross section of an atom of the deposit material, resulting in increases in the surface mobility of adatoms and the nucleation rate and in stimulation of coalescence.

Evidence of the influence of ions can be seen in the case of ZnO films prepared by a reactive ionized cluster beam (RICB) technique to be described later. In the experiments carried out by Takagi e t al . , a good quality film with a preferential orientation to the c axis could be obtained by increasing the ion content in the cluster beam. Figure 2 shows the reflection high energy electron diffraction (RHEED) patterns of ZnO films deposited onto a glass substrate with varying electron current I c for ionization under no acceleration voltage for the ionized clusters. The proportion of the clusters which were ionized was approximately 0~, 7.5~o and 30~ for Ie = 0 mA, 150 mA and 300 mA respectively in the experiment 22 and, moreover, each ionized cluster had only a single charge in spite of consisting of 500-2000 atoms. It should be noted that remarkable improvements in crystallinity can be seen in spite of the small change in ion content in the total flux. The films deposited and grown in the presence of ions not only have good crystallinity or a highly lustrous surface compared with the case where no ions are involved 23' 24 but also have high adhesion to the substrate surface 25. The following discussion refers to the nucleation and film growth processes for MnBi films prepared by the ICB technique. Figure 3 shows the change in the Faraday rotation as a function of temperature rising at a rate of I0 °C min ~, where the results of Chen e t al. 26 for a double-layer film on a mica substrate formed by using a conventional method is compared with the present result. As shown in this figure, there are distinct differences with regard to the magnetic domain growth process between the conventional and the ICB technique, for which the Faraday rotation suggests that the growth of ferromagnetic MnBi can be observed at temperatures below 80 °C,

ION-BASED FILM FORMATION 7

o

O

(a) (b) (c)

Fig. 2. Influence of the electron current for ionization on the crystallinity of the ZnO films: (a) I= = 0 mA; (b) I= = 150 mA; (c) I e = 300 mA.

Z

Chen's data double layer deposited

( by conventional method )

le=lOOmA

le=/oO mA o ~ " ~f~, ~ x~ ~

- i 7 ~ ~x'" - ' " " / ~",.=o

o ~ t l ' / o/ , J

_ _ , . . ~ . ~ . . . ~ . - ~ . , , , I J , , , , , ,

0 1 O0 200 300 ANNEALING TEMPERATURE ( * C )

Fig. 3. MnBi growth process during annealing measured by the magneto-optical Faraday effect. Y, = 0 (ejection speed only).

which is about one-third of the corresponding temperature (220°C) in the conventional method. The annealing of films at elevated temperatures from 80 to 300 °C is found to increase the Faraday rotation, and in this region a pronounced increase in the rotation angle can be seen at higher amounts of ionized clusters. This result leads us to believe that the presence of ions and the kinetic energy of clusters bombarding the substrate produce a marked change in the critical condensation pressure for the source materials, which is closely related to the nucleation and growth mechanism of the films.

Moreover, with an ionization of a few per cent, it was observed by Itoh et al. 27 in 1977 that the substrate temperature for epitaxial growth can be decreased and so the amount of strain in the growing film can be reduced (c f Table IV). The effects of the presence of ions which influence the deposited film quality were also reported by Namba and M6ri in 1976 (c f Table IV) 2 s ' 2 9 Heteroepitaxial growth of silver or ZnTe on NaCI substrates was notably enhanced by the presence of ions in the deposit particle flux, and a distinct dependence on ion energy could be found.

Enhancement of chemical reactions through the presence of ionized atoms is effective for film formation using reactive methods such as reactive evaporation, reactive sputtering, reactive ion beam deposition etc. These ideas explain not only

8 T. TAKAG1

TABLE IV CHARACTERISTICS OF THE PARTIALLY IONIZED VAPOUR DEPOSITION METHOD

Deposited material Substrate Operating conditions Results material

Partially ionized Si Si(111) 10 keV (100 ~tA cm- 2) with neutral Si (25~o-30~ of the total vapour incident Si atoms are

ionized and accelerated)

Partially ionized Ag, NaC1 1.5 keV (Ag about ZnTe with neutral 10~) vapour

Ga +-As +, Ga ÷ -P +, GaAs(100), No additional I n + -P + Si (111) acceleration voltage

(two-beam flux from different sources)

Reduction in the substrate temperature for epitaxial growth (Si epitaxial growth on p-doped Si(111) substrate at room temperature)

Good effects for improving the crystalline states due to the presence of ions; sometimes the influence of the ion content in the total flux on the morphology is greater than the effects of acceleration

Synthesis of III-V compounds

the good quality film formation of oxides, nitrides and carbides with the reactive methods, but also the synthesis of compound materials. For example, the synthesis of III-V compound material films with two beam fluxes from different sources was reported by Murayama in 1975 (cf. Table IV) 3°.

In the reactive method, radical or metastable state atoms which are produced through the ionization process have an important role in the chemical reactions. The diagnostics of the plasma, i.e. measurement of the amount of these activated atoms and/or the properties of the plasma such as electron or ion temperature etc., becomes therefore an important factor in making compound material films with high reliability and reproducibility.

3. VARIOUS KINDS OF FILM FORMATION METHOD IN RELATION TO ION-BASED

TECHNIQUES

3.1. Vacuum evaporation methods A source material is evaporated from a heated metal wire or boat with thermal

energy corresponding to about 10-2-1 eV as shown in Fig. 1. Even in this method, physical and crystallographic properties of the film deposited onto the substrate can be expected to be altered by applying an acceleration voltage parallel or perpendicular to the substrate 21'31, because a very small number of ions are produced in evaporant materials by thermal dissociation or electron bombardment, especially in the case of electron beam evaporation.

On the basis of the enhancement of chemical reactions due to the presence of ions, the activated reactive evaporation process has been developed by Bunshah and Raghuram to prepare compound films such as carbides, oxides and nitrides 32'33. The metal or alloy vapours are produced in the presence of a partial pressure of reactive gas to form a compound either in the gas phase or on the substrate. In this

ION-BASED FILM FORMATION 9

process, the low energy secondary electrons from the plasma sheath on top of the melt are pulled upwards into the reaction zone by a positive potential of 20-100 eV applied at an electrode placed above the pool where the metal is melted and vaporized. These low energy electrons ionize or activate the metal and gas atoms, which results in increasing the reaction probability on collision of these atoms.

3.2. Sputter deposition methods Sputter deposition methods utilize the process by which surface atoms of a

target are ejected, i.e. sputtered by bombardment with the energetic ions and atoms created region in a plasma (10-3-10-1 Torr) or by bombardment with accelerated ions moving in a high vacuum region (10-7-10-4 Torr), and the sputtered surface atoms deposit onto a substrate to form a thin film 3+' 3 s. The experimental boundary condition used to distinguish between the plasma and ion beam methods of sputter deposition is the vacuum pressure in the deposition chamber, in practice 10- 4-10 - a Torr, where the mean free path of the gas atoms is comparable with the working distance in the chamber. As shown in Figs. 1 and 4, the energy range of the depositing material, most of which comprises neutral atoms and molecules, lies between.a few and a hundred electronvolts, although the incident ions on the target may have an energy from a few hundred electronvolts to a few tens of kiloelectronvolts.

I Incident energy of sputtered material J If tom the target onto the substrate 2

[ ,^-2 ._-1- ~ I 10 eV / / [ I0 - - 1 ~ (Several eV- 100 sV )

Target L ~ / Hollow Anode Type X ~ . ~ ~ I " ~ r - - ~ ' ~ I • Ion source !1 ~ 10 .7 - lO'+Torr II I ~ ' / I • 'JI \ " II Target

- + Holder

- - T a r g e t + , ~ -~- ~ + - - ~:==~_--_:=_:=-~_ : "

I I I " IIII--III Su,.rat, - - ' " ' ' 'l I i [ - ~ n- J " - ' ~ - - t Multiple Hole ' '

Ar _,. J..,.~JJ ~ "~ ~ Ar Extractor

i+i To pump To pump

(a) (b)

Fig. 4. Schematic diagram of the sputter deposition process: (a) plasma method; (b) ion beam method.

D.c. sputtering, as shown on the left-hand side of Fig. 4, is a method by which material is sputtered mostly in the neutral state from the metal target (cathode) bombarded by argon ions into the space between the target and the substrate (anode), where the d.c. discharge Voltage, e.g. a few kilovolts, is applied. In this method, sputtered particles contribute to film formation with higher energies than particles generated by thermal evaporation, and the mean energy is as high as 10 eV.

A magnetron-type sputter deposition system was developed in order to increase the sputter rate, where the magnetic field is used to trap electrons near the

10 T. TAKAGI

target (cathode) surface, resulting in an increased ionization efficiency of argon and an increased amount of ionized argon bombarding the target. Under these conditions, the amount of ionized atoms in the total flux of sputtered particles from the target surface may be expected to be increased by passing through the high density trapped plasma towards the substrate. The growing film is subjected to bombardment by energetic neutral atoms and ionized particles, which have an important influence on structural quality and possibly on physical properties.

The ionized and/or radical particles of reactive gas included in the plasma promote chemical reactions and are used to produce compound films of high quality. When a reactive gas such as oxygen or nitrogen mixed with an argon gas is introduced in a glow discharge region, it reacts with the depositing material and films of completely reacted materials, i.e. oxide or nitride films, can be obtained. This method is known as reactive sputtering and occurs not only with the magnetron sputtering method but also with any of the sputtering methods referred to above.

In the ion beam method of sputter deposition as shown on the right-hand side of Fig. 4, an inert gas such as argon is extra: ~ed at a few kilovolts from an ion source into a high vacuum region, where a target material is sputtered by the energetic ion beam, and then the material which is sputtered mostly in a neutral state deposits onto a substrate to form a thin film. Most of the work on ion beam sputtering has been done so far with the Duoplasmatron type of ion source, which is capable of producing 5-25 mA of ion current with energies of 1-25 keV. In the ion beam method, the sputtered particles have a mean energy as high as up to 100eV, depending on the energy of the bombarding ions 36 and on the crystallographic direction of the target 37. Studies on the energies of gold sputtered from the target bombarded along two different crystal directions indicate that the energies of species sputtered along the more close-packed (110) directions are significantly higher, having a mean value of 93.5 eV compared with 22.7 eV for the (100) directions.

3.3. Ion plating and ion beam deposition methods The ion plating method, which has been developed by Mattox 3 s, is essentially a

combination of evaporation and the use of a glow discharge as shown on the left- hand side of Fig. 5. The ionization of vaporized metal takes place in an inert gas (argon) glow discharge and the ions are accelerated across the cathode dark space to a substrate (cathode). Each ion will experience many collisions before arriving at the cathode and will lose its energy in symmetric charge transfer collisions, i.e. it transfers its energy to neutral atoms and a lot of energetic neutral atoms are produced. According to the results obtained by Teer 39, if the mean free path 2 and the length Z c of the cathode dark space are assumed to be 0.5cm and 10cm respectively under typical conditions, e.g. applied voltage V¢ = 3 kV and gas pressure of 10- 2 Torr, the ions lose almost 90~o of the energy by transferring this energy to neutral atoms in traversing the cathode dark space, and a total number (Zd2)N o = 20N o of energetic neutrals are produced, where No is the number of ions leaving the edge of the cathode dark space towards the cathode in unit time. The average energy of the ions arriving at the cathode is approximately (Vc/Z¢)22 = 300 eV and that of the neutral particles is Vc(Z~-22)(2/Zc)/Z~ = 135 eV for the same conditions, assuming they suffer no collisions after gaining their energy. From these considerations, it can be said that energetic neutral atoms (less than 135 eV) are

ION-BASED FILM FORMATION 1 1

in the majority and ions with moderately high energy (about 300 eV) are in the minority for film formation, even in the method where the substrate is set in the plasma region and the applied voltage is 3 kV. As mentioned above, even such a small amount of ions in the total flux is enough to have an effective influence on film formation.

Sputter Type A HoSUbstratel de r Ion Source ~ Substrate

R " ~ ~ Ar Filament ~ ~ Holder Substrate (Cathode> 1 Anode ~ ~ - -

Cathode Dark II ~ L ' l I~tlr,ctor Plasma II ~ _JI (-4oov~-4ov) I Plasma t~__~.(A,:,o"-,~' ~ ' 112_U. 11,

i , r e'''l ~...~ hOOeV-aOeV all Material to l ' ~ / / / ~ I . . . . . . be depost ted . l . - ¢W- - /~, I - ~ / - L - - . ~ . L . . J I ~~ . - - ~ l ~ :~:::

sputtering-IT, ~ / ' " I I " - - I i ' ~111 Crucible I Y r ~ - - - - ) ~ li li Su~st.te (Anode> I I Electrode ~ t ' / - - - - ~ :t II , "

H e a t e r ~ lon Source Chamber ~ / ~ mL~l. -6 -h , I1"---'11 ~i0 --io Torr) Material to ~ a

be deposited I T I I I Ar ~ rfor example:~ ' I ~ I

- ~Carbon '

To pump (a) (b) TO pump

Fig. 5. Schematic diagram of (a) ion plating and (b) ion beam deposition methods. (a) Typical parameter values: argon gas pressure, 10 -2 Torr; applied voltage, 3 kV; energy of ionized evaporant material, 300 eV; energy of neutral evaporant material, 135 eV.

Ion beam deposition is the generic term for the method of film deposition in a high vacuum region (10-7-10 -4 Torr) using ions of the deposit material with or without a neutral atom component. Sometimes, inert gas ions or reactive gas ions are used separately or together.

The ion beam deposition methods including their modified type are classified as follows: (i) hybrid methods involving vacuum evaporation and ions or an ion beam and (ii) the ion beam deposition method using an ion source as the main constituent. The methods (i) will be classified into (A) the method of simultaneous vacuum evaporation and ion implantation or ion irradiation and (B) the partially ionized vapour deposition method. In the former an auxiliary ion source for producing metal ions or reactive gas ions is combined with a conventional evaporation method, and in the latter the source material which is partially ionized is used for film formation together with the neutral state source materials. Some of the results reported so far according to this classification are listed in Tables I V 27-3° and V 4° 43.

An attempt to combine conventional vacuum evaporation and low energy ion implantation has been carried out by Takagi et al. in which low impedance d.c. electroluminescent ZnS:Mn cells without any coactivator have been developed 4°. A similar study has been performed by Dodonis and Pranevi~ius where 02 ion implantation was used during film growth of evaporated aluminium and SiO 41. In this technique, the ion acceleration voltage is about a few kilovolts which is not enough to cause deep penetration into the depositing material. However, even for these low energy ions, the effect of the ions is enough to change the electrical characteristics of the film, if irradiation with the ions is continued during the film formation simultaneously or alternately. A similar method employing molecular

12 T. TAKAGI

TABLE V C H A R A C T E R I S T I C S OF THE M E T H O D OF S I M U L T A N E O U S V A C U U M EVAPORATION A N D ION I M P L A N T A T I O N OR

ION I R R A D I A T I O N

Deposited material Operating Results conditions

Mn + during ZnS evaporation 5 keV

0 2 + during film growth of evaporated AI and SiO

0 2 + sputtering of Pb atoms Zn ÷ beam doping in molecular

beam epitaxy of GaAs and AlxGal _xAs

1-10 keV (3 mA maximum)

50 eV 200 eV-1.5 keV

(0.1-10 ~tA)

Low impedance d.c. electroluminescence ZnS:Mn cells without coactivator Investigation of thin SiO and Si films

doped by 02 ion implantation during film growth

Oxide formation Remarkable improvement of the effective

sticking coefficient by ionization of and irradiation with dopant Zn

TABLE VI ION BEAM DEPOSITION U S I N G THE ION SO U RCE AS A MAIN C O N S T I T U E N T

Deposited Substrate material Operating conditions Results material

Without mass analyser Cr + Glass

Si +, C ÷ Single-crystal stainless steel and glass

230 eV, 2 I~A (8.7 ~tA mm- 2)

40eV for C

Low energy mass-analysed deposition Pb +, Cu ÷ Si single-crystal 60 eV, 15 ~tA for Pb ÷

(ion extraction voltage, 20 kV)

Pb ÷ C and NaCI

Mg ÷ C

Ag ÷ Polycrystalline Pd, single-crystal Si

Zn+,Ag +

Ag ÷

24-256 eV, 9-12 ~tA (ion'~ extraction voltage, [ 5 kV) /

24-500 eV (100 eV | optimum) J

20-100 eV, 5-25 ~tA (ion extraction voltage, l0 kV)

Single-crystal A1 and Cu, 30-300 eV stainless steel

Single-crystal Si 5-300 eV (ion extraction voltage, 30 kV)

Ge +, Si ÷ Single-crystal Si, sin#e- crystal Ge

Cu ÷ in 02, H2S gas

Several tens to several hundreds of electronvolts (ion extraction voltage, 30 kV) 20-500 eV for 02, H2S ambients

Cr film formation (deposition rate 100/~ min- 1)

Diamond-like carbon film formation (deposition rate 300/~ min- 1)

Pb film formation (deposition rate 100/~ h - 1)

Si/Cu/Pb multiple-layer film formation

Strongly preferred (11 I) orientation

Epitaxial- growth of Ag(111)/Si(111) at room temperature

Fundamental research on deposition mechanism

Maximum density in polycrystalline state is obtained at about 20 eV

Epitaxial growth of Ge film on Si(l 11) or Ge(111) substrate at a substrate temperature of 300 °C

Formation of CuO or CuS films

ION-BASED FILM FORMATION 13

beam epitaxy instead of conventional vacuum evaporation has been studied by Matsunaga et al. 43 to investigate the impurity doping in GaAs and AlxGal _xAs films.

Some of the results reported so far of the partially ionized vapour deposition method are listed in Table IV 27 30. These studies were described in Section 2.1 from the standpoint of the influence of the presence of ions.

The ion beam deposition method using an ion source as the main constituent can be classified into (A) the ion beam deposition method without mass analysis and (B) the low energy and mass-analysed ion beam deposition method. The former method was proposed by Flynt in 1961 to fabricate microminiature electronic circuits 44. Since then, many experimental results have been reported and some of them are listed in Table VI 45,46. As shown on the right-hand side of Fig. 5, ions of the deposit material are generated in the ion source and extracted into a high vacuum region by a suitable acceleration voltage. In the experiments on diamond- like carbon film coating by Aisenberg and Chabot 46, the substrate was cleaned by sputtering due to carbon and to argon ions extracted at - 400 V before deposition and the film deposition occurred at about - 4 0 V after the extraction voltage had been reduced.

In addition, facilities for film deposition using the latter method, i.e. a mass- and energy-selected ion beam in the high vacuum region, have been developed since the beginning of the 1970s. In this method, a pure ion beam without neutral particles is used after deceleration to a suitable incident energy (a few tens to a few hundreds of electronvolts) for the film formation. Table VI lists some of the results reported so far.

Colligon et al. 47 have investigated the deposition of thin films by retardation of an isotope separator beam using lead and copper ions which were deposited onto silicon single-crystal substrates. The 20 keV ion beam of a selected isotope is deflected by a potential on the deflector plates and retarded to 60 eV, while the substrate is kept at a high potential.

Studies of thin film deposition have been carried out by Amano and coworkers17,18,4.8,49 using a modified ion implantation machine which consists of an ion source, an extractor, an einzel lens, an g × B mass analyser, a decelerator, neutral trap plates etc. The deposition of lead ions which are accelerated up to 5 kV and then decelerated to 24-256 V was carried out on carbon and NaC1 substrates with 9-12 laA ion beams under 10 -a Torr vacuum. An ion energy of 50eV is an optimum energy of deposition when parameters such as space charge expansion, self-sputtering, film thickness and surface coverage are taken into account. As the deposition energy is increased above 50 eV, the crystalline structure of deposits grown on single-crystal NaC1 substrates becomes more ordered with a strongly preferred (111) orientation. The deposition of magnesium ions onto carbon substrates was studied in a range of energies between 24 and 500 eV. An incident magnesium ion beam energy of about 100 eV produced the optimum film. The adhesion of the film was improved at high energies.

A similar system with an g x B mass analyser has been used for the deposition of silver ions onto a polycrystalline palladium and single-crystal silicon substrate by Thomas and de Kluizenaar 5°. Silver ions were deposited onto the substrate which had a special surface treatment such as sputter cleaning, under 2 x 10 - 9 Torr

14 T. T A K A G I

vacuum. In the energy range 25-100 eV, silver layers deposited onto an Si(ll 1) surface at room temperature grew epitaxially with Ag(111)//Si(111).

An apparatus controlled by a sector-type mass analyser has been developed by Tsukizoe et al. to study the fundamental deposition mechanism 51. Zinc and silver ions with energies from 30 to 300 eV were plated onto single-crystal aluminium and copper substrates and stainless steel substrates.

Silver deposition at room temperature onto a silicon substrate surface, which has no special surface treatment such as sputter cleaning, has been carried out using a mass-analysed ion beam at an energy range of 5-300eV by Cheng and coworkers15.16. The maximum density of polycrystalline film with the maximum intensity X-ray diffraction peak could be obtained at around 20eV. The results suggest that extremely low energy deposition at 5-50 eV may be interesting for fundamental research into film formation mechanisms and special applications.

The design and characteristics of a low energy mass-analysed ion beam deposition system which improved the ion implantation facilities by using a sector- type mass analyser have been investigated by Tokuyama and coworkers 52-55. In the target chamber where the ion beam was decelerated, a weak magnetic field of 300 G was applied perpendicular to the ion beam axis and electron bombardment of the substrate was prevented by the linear magnetron motion. Single-crystal germanium films were epitaxially grown on Si( l l l ) and Si(100) or Ge(111) substrates with 100 eV ions and at a substrate temperature of 300 °C. Single-crystal silicon films were epitaxially grown on Si(100) substrates with 200 eV ions and at 740 °C.

Measurements of the rate of mass collection on a substrate exposed to a monoenergetic beam of copper ions in the presence of 02, H2S or other gaseous ambients have been made by Shepherd56. No gas absorption was observed when the deposition energy was below approximately 20eV. Above this value, a general increase in absorption rate up to a saturation value sufficient to form CuO or CuS was observed.

From the considerations mentioned above, for conventional ion plating in the plasma, the kinetic energy of the depositing particles is about 100-300 eV and the neutral particles also play an important role for film formation. For ion beam deposition with a decelerating field, highly energetic ions extracted into a high vacuum region are utilized for film formation after deceleration with energy losses of the order of several to several hundred electronvolts.

3.4. Ionized cluster beam technique The ICB technique, which has been developed by Takagi and other

workersSV 73, is one of the ion beam deposition methods where macroaggregates of atoms (clusters) formed from deposit material vapour are utilized instead of atomic or molecular state particles. The vaporized-metal clusters are formed in supercondensation phenomena following adiabatic expansion through the nozzle leading to the deposition chamber. Each cluster contains 500-2000 atoms loosely coupled together, which is different from the droplet (liquid particle) in that the droplet contains from 5 × 108 to 5 × 1 0 9 atoms per droplet closely coupled to each other. The clusters are ionized to be singly charged by electron bombardment in the ionization electrode assembly located above the crucible. The cluster ions are accelerated towards the substrate by a high negative potential applied to the

ION-BASED FILM FORMATION 15

accelerating electrode. Besides the ionized clusters, there are neutral clusters which remain unionized during flow in the ionization region. Although the neutral clusters are not accelerated by the voltage, they move towards the substrate at ejection velocity. Compared with other atomic- or molecular-ion-based techniques such as sputter deposition, ion plating etc. which were described previously, the attractive features of the ICB technique are as follows. When the clusters are broken up into the atomic state again on their arrival at the substrate, the incident momentum of the clusters is transformed into the surface diffusion energy of each atom effectively. The migration effect of adatoms due to this enhanced surface diffusion energy contributes to forming good quality films or to adjusting the morphology of deposited films. Each atom of the clusters has an average energy E = eVJN, where e is the electronic charge, V a is the acceleration voltage and N is the cluster size (number of atoms per cluster ). By controlling V~, it is possible to provide each atom with energies high enough for surface diffusion (E ,~ 1 eV) and surface cleaning due to sputtering of surface contamination (E ~ 0.1-10 eV) and to adjust the energies keeping the substrate temperature constant. Furthermore, sufficient energy for inducing a suitable amount of defects and atomic displacements in the surface layer (E >~ 10eV) can be provided, if necessary. From the viewpoint of ion beam transport, it is possible to transfer high current and low energy (1-20 eV) ion beams without space charge repulsion. In addition, because the ionized clusters possess a small charge-to-mass ratio, deposition onto an insulating or semiconductor substrate is possible without charging up due to accumulation of positive charge.

Concerning the effect of the presence of charge on film formation, it can be shown that the charge effect of a cluster is sufficient to influence film formation, although the actual electric charge content in the total cluster is very low, as described in Section 3.3 and in many experiments. These features open the low energy range suitable for film formation with a high deposition rate without space charge trouble, which requires the energy range to be as high as possible in the case of charged atomic or molecular ion transportation with high intensity.

For the RICB technique, a reactive gas, e.g. oxygen, nitrogen and other reactive gases, is supplied from a gas nozzle near the metal vapour ejection area through a controlled leak valve to maintain the desired pressure in the chamber during the deposition. The reactive gas pressure is always maintained below the pressure (approximately 10-5-10-4 Torr) at which the mean free path corresponding to the pressure is longer than the distance between the nozzle and the substrate, to prevent the plasma region from occurring in the chamber. If the plasma region occurs in the chamber, clusters are destroyed by plasma heating and the advantages of the ICB technique are lost. Some reactive gas atoms or molecules are ionized together with vaporized-metal clusters and are accelerated towards the substrate, although the ionization efficiency of the reactive gas or molecule is generally smaller than that of the cluster. The vaporized-metal cluster and the reactive gas atoms react to form compound films with the aid of the electric charge and/or the converted energy, e.g. surface heating energy and migration energy, from the accelerated cluster ions. Therefore film formation of oxides, nitrides or other compound materials is carried out effectively by the impingement of ionized and neutral particles of vaporized- metal clusters and of ionized and neutral gas molecules.

16 T. TAKAGI

4. CONCLUSION

In c o n c l u s i o n , t he i o n - b a s e d f i lm f o r m a t i o n t e c h n i q u e h a s h i g h p o t e n t i a l i t y for

f o r m i n g v a r i o u s k i n d s o f f i lms b y c o n t r o l l i n g t h e d e p o s i t i o n c o n d i t i o n s , so t h a t

a p p l i c a t i o n s c a n b e e x t e n d e d .

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ION-BASED FILM FORMATION 17

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