Ion beam studies

NUCLEAR

INSTRUMENTS

AND

METHODS

143 ( 1 9 7 7 )

99-115

;

(~)

NORTH-HOLLAND

PUBLISHING

CO.

ION BEAM STUDIES Part IV: The Use of Multiply-Charged and Polyatomic Ions in an Implantation Accelerator J. H. F R E E M A N , D. J. C H I V E R S and G. A. G A R D Received 3 January 1977 Polyatomic and multiply-charged ions provide a c o n v e n i e n t m e a n s of e x t e n d i n g the energy range of an implantation accelerator. T h e molecular species are also of interest in certain special b o m b a r d m e n t studies. This report considers s o m e of the factors which affect the production and utilisation of s u c h beams. It introduces the concepts of hetero- and autoc o n t a m i n a t i o n , and particular attention is given to the modification of t h e charge or m a s s of the ions resulting from inelastic collisions in the various beam transport regions of the accelerator.

1. Introduction 1.1. GENERAL Ion implantation has been described as the process of modifying the physical or chemical properties of a solid by embedding into it appropriate atoms in the form of a beam of ionised particles. Such dopant particles are normally monatomic and have a single positive charge (X+), but the bombardment can also be effected with a variety of other ionic species. For example, negative ions (X-), compound ions (XY +) and even energetic neutral particles (X°), may have advantages in particular circumstances. In this report, we are concerned with the production of multiply-charged (Xn+) and polyatomic (Xm +) particles and with their application to extend the energy range and the experimental flexibility of an implantation accelerator. As we shall see below the use of even this restricted range of ionic species is often subject to some quite severe constraints. In order, therefore, to understand the need to resort to such particles for ion doping, we begin by reviewing some limitations of accelerator performance and the bounds which these may impose upon the scope of implantation experiments. 1.2. THE ACCELERATORCAPABILITY The growth, in recent years, of ion bombardment and implantation studies has resulted in a corresponding resurgence of interest in heavy ion accelerators and sources. This is now evidenced in the much increased availability of ion-beam facilities, often custom-built, and of much improved performance. Nevertheless, it is apparent that the accelerator still commonly provides a major constraint on doping capability.

This situation is a measure of the very broad scope of ion implantation and of the resultant demanding range of ion-beam requirements. For example, even a commonplace experimental programme of silicon semiconductor doping could require the use of dopant ions ranging from boron with a mass of 11 amu to antimony at 123 amu. It might involve fluences from 1011 to 1016 ions/cm 2 and ion energies from 5 to 500 keV. Inevitably there would be additional requirements in respect of the quality of the ion beams, and of the uniformity and precision of doping. Taken together these correspond to an exacting accelerator specification; yet they are modest when set alongside the ion-beam demands which are now associated with the implantation of metals, compoundsemiconductors and insulators. There is no single design of heavy ion accelerator which can satisfy all of these needs and since most laboratories are unable to instal a comprehensive range of facilities the choice of machine is frequently in the nature of a compromise which subsequently dictates the scope of the ion-beam programme. If we neglect the increasing, but rather specialised, use of nuclear physics accelerators for bombardment studies at high energy the problem is somewhat simplified. It is apparent that much of the remaining reported work is carried out at energies below 600 keV on conventional accelerators, which incorporate some degree of mass analysis. Even these boundary conditions still encompass a variety of accelerator and ion-source concepts, but the essential problem of machine selection generally reduces to a choice between energy and intensity. Thus heavy ion accelerators which operate at energies of a few hundred kilo electron volts are

100

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FREEMAN

usually restricted to beam currents on target of, at most, some tens of microamperes. In contrast, it is relatively simple to extract and manipulate ion beams of milliampere intensity at primary acceleration voltages below one hundred kilovolts. Such low voltage accelerators, which are the subject of this series of ion-beam studies, are also capable of combining high resolution with an extended mass spectrum1,2), but the upper limit of beam energy is a serious constraint for certain implantation applications. 1.3.

THE ENERGY RANGEOF THE HIGH INTENSITY ACCELERATOR

A restricted, but relatively simple technique which extends the upper energy range of the high intensity, low voltage accelerator is to employ a negatively biased target to give a-further stage of acceleration after mass analysis*. This procedure has been extensively used by Freeman et al. 5'6) to implant a wide range of dopants at energies up to 200 keV and this report is primarily concerned with the use of doubly and triply charged ions to extend the capability of such facilities to cover the important energy range 200-600 keV. The use of multiply charged ions is not, of course, confined to such machines and they can also be exploited on low current implantation accelerators. Insofar as an increase in charge state reduces the magnetic field requirements for ionbeam analysis, they may alternatively provide a convenient means of extending the effective mass range of high energy accelerators. It is, however, important to recognise that such multiply-charged particles are normally obtained in relatively low yield from the ion sources which are currently used for ion implantation. Typically, the doubly charged ions may have an intensity which is only a few percent that of the singly charged primary beam, whilst the corresponding triply charged ratio will generally be well below one percent. Since a wide range of doping applications can be satisfied using ion beams of microampere intensity, this factor is not generally too important in the case of an accelerator with a primary beam * An alternative means of achieving the same increase in energy is to m o u n t a high current accelerator onto a high voltage stage. This involves a more radical machine redesign, but it has the advantage of leaving the target at earth potential. Implanter designs which are based on this concept and which combine intensity with energy are beginning to emerge 3,4).

et al.

ImA

O oA C, IO#A

AIf o WA

IO

20

30

40

KeV MAXIMISED BEAM CURRENTSAS A FUNCTION OF ACCELERATING POTENTIAL

Fig. 1. Beam current as a function of accelerating potential.

capability of around a milliampere. It can, however, severely restrict the value of multiply charged ions in low current facilities. Additionally, because of the complex spectrum of ions which is a commonplace feature of implanter operation, such low yield beams can only be unambiguously identified and usefully applied if the instrument has an adequately high resolving power. We conclude this general introduction to the application of multiply charged ions to implantation by noting that high charge states are also used routinely for certain plasma studies and for injection into high energy acceleratorsT,S). The ion sources which have been specially developed for these requirements are often capable of producing useful intensities of quite high charge state and they might thus have a future role in extending the performance of conventional implantation facilities. However, for any rigorous doping requirement, it would be necessary to make a more detailed assessment of the quality of the ion beams and some modification of the high voltage terminal and the power supplies might be required in order to match such ion sources to conventional implantation accelerators. In contrast, we are concerned in this report with the extended application of an unmodified implantation facility.

ION BEAM S T U D I E S

1.4. IMPLANTATIONAT tOW ENERaIES Although the requirement for an increase in ion energy has provided the principal stimulus for this study, the problems associated with ion doping at energies below the normal operating regime of the accelerator are also important. In the case of certain conventional high energy machines, this can prove to be a particularly restrictive factor because of the rapid fall-off in the quality of the beam as the accelerating voltage is reduced. The higher current machines considered here have the advantage for surface studies that they will provide useful (>lEA) intensities of most dopants even down to quite low energies. This feature is illustrated in fig. 1 and in an earlier paper in this series 9) we have also described the retardation of intense ( - m A ) beams to energies below 1 keV. Nevertheless, there are situations where the use of polyatomic ions provides a useful alternative means of satisfying low energy requirements and since such ions are also of particular interest in certain radiation damage and atomic collision studies 1°-~3) we include below a consideration of their behaviour.

Fig. 2. Low energy antimony beam.

101

2. Experimental The implantation facility used for this series of ion beam studies is based on a modified electromagnetic isotope separatorm). It has a mass range of 1 to _+400 ainu at the normal operating voltage of 40 kV. The sharp focussing capability of the accelerator is relatively insensitive to the extracted beam current and milliampere intensities of most elements can be resolved at the target stage. Fig. 1 shows the relationship between the intensity and the energy for some typical heavy ion beams. The data, which were obtained by simply reducing the extraction voltage of an unmodified ion source, demonstrate that usable beam currents (>/zA) can still be obtained down to kilovolt energies. Fig. 2 shows that an acceptable quality of beam resolution is maintained at such low energies. Implantation energies above 40 keV are obtained by post-acceleration of the ions at the target stage. In this way, a maximum final energy of 200 keV can be achieved using singly charged ions. In spite of the apparent inconvenience of working with a target at high voltage, such an implantation facility 6) incorporating automatic process control has been successfully operated over a period of several years for a very wide range of implantation requirements. Since the primary objective in using multiply charged and polyatomic ions was to extend the day-to-day operational versatility of the implantation facility, the results described below were obtained without any modification to the ion source or the accelerator.

3. Results and discussion The two most important factors which govern the successful use of multiply charged and polyatomic particles for ion implantation are the intensity and the purity of the required dopant beam. As we shall see below, the large variations in the ionisation behaviour of the required range of ion-source feed materials preclude the formulation of a simple and general description of ion-beam quality. We can, however, discern some systematic trends which at least provide a useful operational guide. The examples of ion-beam behaviour described below cover only a fraction of the broad spectrum of implantation requirements, but they have been chosen to illustrate the general pattern of doping behaviour.

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3.1. ION-BEAMINTENSITY The performance of the ion source used in these experiments has been described elsewhere14,1s). The detailed behaviour is complex and varies from element to element, but we are simply concerned here to make some general observations which are relevant to the production of suitable beams of multiply charged or polyatomic ions: 1) The most intense beams in the ion spectrum are usually - although not inevitably - the singly charged elemental beams. 2) Milliampere intensities are achievable for most elements. 3) The intensity of multiply charged beams depends upon a number of factors (ionisation potential, source operating conditions etc.), but as a simple guide the yield of doubly charged ions tends to increase from several microamperes with light ions such as boron (B2+) to some tens or even hundreds of microamperes at higher masses (e.g. Sb ++, Bi ++, Pb++). Such beams are of adequate intensity for a variety of ion doping requirements. Although the yield of triply charged ions is much more restricted - typically only microamperes even at high mass - such beams can nevertheless be usefully applied to certain high energy applications. In favourable cases, the intensity can be increased to at most some tens of microamperes by careful selection of the ion-source operating conditions. For example, although phosphorus trichloride is the most convenient ion-source feed material for t h e routine production of phosphorus ions, the use of the element results in a substantial increase in the yield of P+, W + and W + ions. This gain has been extensively employed for high-energy (>500 keV) implantations using the triply charged phosphorus beam. 4) Polyatomic ions tend to be produced in significant yields only with the more covalent, or electronegative, elements. The best results are usually obtained when the element itself is used in the ion-source discharge (e.g. C12, 02, P, As, Sb), but some association may occur even when compounds are used. Thus the spectrum of phosphorus trichloride shown in fig. 10 shows evidence of a significant beam of PJ-. In certain particularly favourable cases (e.g. arsenic) the diatomic beam XJ- may correspond to a dopant flux which

exceeds that of the singly charged ions. Such beams are particularly convenient for highflux applications such as semiconductor predeposition, where it may be desirable to carry out the implantation at low energy in order to minimise the ion beam power, or irradiance~6). The intensities quoted above relate to the total transmitted and resolved beam of the required element. It should be stressed that in practice, as a result of isotopic dispersion and the common need to restrict the beam dimensions or angular divergence, the usable beam of dopant ions on target may be significantly reduced. 3.2. ION-BEAMPURITY The quality of the ion beam is of primary importance in most implantation applications. It is determined by a variety of experimental factors the nature of the ion source, the performance of the beam analysing system, the quality of the vacuum, etc. Even in accelerators which have a very high mass resolution, as well as the capability of measuring the detailed ion spectrum, the optimisation of beam quality is not necessarily straightforward. The problem merits particular attention in the case of applications involving the use of multiply charged and polyatomic ions. This is because of their relatively low abundance in the overall mass spectrum and because they are particularly prone to contamination by charge-exchange. Ideally the resolved particle flux at the target stage of an implantation accelerator should be chemically pure and should be homogeneous in energy, mass, and charge-state. In practice, this situation is never completely achieved and in unfavourable cases, it is quite possible to have impurity levels of some tens of per cent. Even in elementary doping applications involving the use of ,single charged ions, considerable care may have to be taken to reduce contamination of the beam to below five percent. Although isotopic contamination is generally unimportant in ion implantation we see below that a high degree of mass, and therefore isotopic, resolution significantly assists the overall determination of the beam purity. This factor is of particular significance in the two causes of loss of beam quality - h e t e r o c o n t a m i n a t i o n and a u t o c o n t a m i n a t i o n - which we consider below. These do not usually result in a significant confusion of the resolved image since the impurity

ION

BEAM

particles are focussed along with the dopant ions at the required mass position*. In consequence, a careful examination of the spectrum, possibly with variation of the accelerator operating conditions, may be required to identify their presence and to assess their relative importance. 3.3. HETEROCONTAMINATION Adventitious chemical degradation of the ion beam can lead to spurious doping effects as well as to errors in dose measurement. Such impurity effects - which we can designate collectively as heterocontamination - may result from a variety of causes. For example, simple isobaric contamination can be particularly serious in implantations involving the use of rare earths or transition elements where there is commonly overlap of the isotope spectra between adjacent elements (e.g. 58Ni-S8Fe, 1445mJaaNd). Because of fractionation (arising from differences in volatility) of the ionsource feed material, these effects may be quite significant even when high purity materials are used. In such circumstances, they are often characterised by an abnormal isotopic spectrum which gradually improves as the source operation continues. They are generally most easily identified in that part of the spectrum corresponding to the singly charged species, but they also affect the purity of the multiply charged and polyatomic ions. Whilst there is no general guide to such chemical contamination of the ion beam, the impurities can generally be recognised by careful examination of the mass spectrum. In most instances they can be reduced to an acceptable level by varying the mode of operation of the ion source or by exploiting the capability of isotope separation. For example, the contamination of a platinum beam (Pt ÷) by tungsten oxide (WO ÷) can be reduced by taking care to minimise the amount of oxygen in the ion source. It can be eliminated by the use of a different cathode material such as niobium. In contrast the contamination of doubly charged silicon ions (28Si++) by nitrogen ( l a N + ) c a n often be most readily avoided by using the lower abundance 295i++ ion. This has the advantage of being analysed at a . f i ' a c t i o n a l m a s s position (14.5). As we shall see below this particular feature of multiply charged ions can frequently be exploited to sig* T h e resolution of m o s t implantation accelerators is inadequate to d i s t i n g u i s h s u c h small isobaric m a s s differences as m a y exist.

STUDIES

103

nificantly reduce the degree of heterocontamination. 3.4.

AUTOCONTAMINATION

In addition to the chemical contamination effects described above the ion-beam quality may also be degraded by the presence of particles .of the required element, but of incorrect charge-state, energy or mass. These parasitic effects, which can be more difficult to detect, and which we can designate collectively as a u t o c o n t a m i n a t i o n , result from inelastic collisions of the ion-beam particles with residual gas molecules. Such interactions tend to be of particular importance in implantation applications involving the use of multiply charged or polyatomic ions. They are of the general form: X~+

--* xqb+ ,

(1)

and they result in a change in the atomicity (p), or in the charge-state (a) of the ions, X, in the primary beam. They reduce the precision of beam monitoring and they may also modify the implantation profile. They can also cause doping nonuniformity in accelerators which employ electrostatic scanning to obtain uniform target coverage. An important feature of such autocontamination is that the impurity particles can almost always be filtered out of the ion beam by the use of an electrostatic analyser. This solution which has been extensively adopted to eliminate neutral particles from the ion beam in a variety of low current accelerators is unfortunately less appropriate to the higher intensity beams considered here. The consequences of such beam contamination processes depend critically upon the region of the accelerator in which the inelastic collisions occur. In a typical implantation facility, the problem is complex and a complete analysis would involve: 1) the ion-source accelerating region; 2) the field-free region before analysis; 3) the analysis section; 4) the field-free region after analysis; 5) the region of secondary acceleration (if present); and 6) the region between the electrostatic neutral beam filter and the electrostatic scanning assembly (if present). We shall restrict this consideration to reactions occurring in regions (2) and (4) since these constitute the major source of autocontamination. Because of the proximity of the ion source, the

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FREEMAN

pressure in region (2) may be relatively high ( - 1 0 -5 torr). In most accelerators region (4) is characterized by lower pressures ( ~ 10 -6 torr), but the path length may be longer. 3.4.1. Collisions before analysis An ion of mass M which undergoes a change of mass, or charge, as a result of a collision in region (2) is subsequently analysed at a new position in the ion spectrum corresponding to a mass M ' where: M,=q__ 2 a

P ~ M,

(2)

and p and q represent the atomicity, and a and b the charge state as defined above. The spurious beam, M', is known as an Aston BandiT). Although such reactions can lead to heterocontamination (e.g. 4°Ar--l°B, 64Zn--, 32S) they occur only rarely and we are concerned here with the more commonplace and systematic contamination arising from inelastic collisions which involve the various ion species of the dopant element. The most important reactions of this nature, which result in contamination by ions of incorrect charge-state, mass and energy are shown schematically in fig. 3. A general pattern of behaviour can be discerned: contamination of the higher chargestates arises from the dissociation of polyatomic ions, while, in turn, the polyatomic spectrum is degraded by the charge exchange of multiply charged ions. Both types of reaction have a reasonable probability of occurring in the energy range of interest. Thus, Moline Is) has reported electron stripping cross-sections, al.2, in the range (2-9) x 10 -16 cm 2 for boron, nitrogen and phospho-

et al.

rus between 100 and 300 keV. Although there is a paucity of other experimental measurements we can anticipate, as an approximation, that the crosssections for such reactions will generally be around 10-15-10 -16 cm 2 but may fall, in particularly favourable cases, to 10-~Tcm 2. On the basis of this quite simple assumption, we can reach some useful conclusions both about the general pattern of charge-exchange behaviour and about the probable limits of contamination in particular implantation situations. We begin by considering the general case of an element, X, in which a required dopant beam X~ of charge-state, a, and atomicity, p, is contaminated by a flux of particles X~ arising from inelastic collisions before analysis of a related beam X~q(X~q--,X~). Let the intensities of the three beams X~, X~qand X~ be 11,12 and 13 respectively. In a consideration of contamination in ion implantation, we are ultimately concerned not with measurements of ion charge, but instead with the ratio, R, of the flux of contaminant atoms to the associated flux of dopant atoms. The respective beam currents which measure the fluxes of charged particles must, therefore, be corrected for charge-state and atomicity. Thus R , the fractional contamination of X~ by X~ is given by the expression:

However, in a consideration of the collision-induced reaction X~q--,X~ which acts as the source of contaminant particles we are concerned with the respective fluxes of ions, regardless o f their atomicity, and: I3 c

-

b

R =

x""

~

x""

x"

x"

x~

I2

3.3 x 1018 apl,

(4)

where tr is the cross-section, p the pressure in torr, and 1 the path length in meters. Eqs. (3) and (4) give:

X~-~X +

01

(3)

R = 13 (r/c)/Ii(P/a).

x~

J

x;.

×**..t~X° X**.~_~ X +

Fig. 3. Schematic representation of ion beam contamination arising from charge-exchange processes (Aston Bands).

--12 __ra3.3 X 1018 trpl.

(5)

I~ b-p

For the most important reactions illustrated in fig. 3 the term ra/bp varies between 1/9 and 4. Thus for the contamination o f 1) 2) 3) 4) 5)

X~- by the reaction X 3+ X + by the reactionX 2+ X z+ by the reactionX + X 3+ by the reactionX~ X 4+ by the reaction X +

+, ra/bp = 1/9 ; +,ra/bp=l/4; +, r a / b p = 2 ; +, r a / b p = 3; and ~ X 2+, r a / b p = 4 .

~ ~ ~ ~

X X X X

ION BEAM S T U D I E S

105

For a known, or assumed, cross-section, ix, the fractional contamination, R, of a multiply charged, or polyatomic, dopant beam can therefore be estimated from measurements of the accelerator operating conditions. Even without such experimental data, the expression (5) above can be used to obtain an indication of the likely limits of applicability of such beams for particular implantation requirements. For example, in the case of the high current accelerator used for these experiments the product of the pressure and the path length before analysis, pl, would typically lie in the range 10-s-10-6torr.m. For cross-sections of 10 -Is and 10 - t 6 c m 2, it follows that the fractional contamination will lie between

tions (3) and (4) above] could be as high as 10% under the worst conditions assumed above. Caution should therefore be observed when the ratio of polyatomic to multiply charged ions approaches unity. It is fortunate that this situation only occurs with a limited number of electronegative elements.

R = 3.3x10 -4 12 ra I 1 bp' for the best conditions, pl = 10 - 6 torr.m, and R = 3.3×10 - z I 2 ra 11 bp'

f = 3.3 x 1018 trpl. ix--

1 0 -16 c m 2

and

for the worst conditions, ¢r=10 -~Scm 2 and pl = 10 -s torr. m. If we now consider the contamination of the quadruply charged beams by charge-exchange of the singly charged primary ions [reaction (5) above] we can readily see that the loss of quality will be unacceptably large for most applications. This is because the ratio 1+/14+ will normally be at least 103 and it follows that even under favourable conditions, (or= 10 -16, p l = 10-6), the contaminant beam will be comparable with, or greater than,, the required flux of quadruply charged particles (R >~l). Fortunately, this is an extreme case and with the same assumptions about cr and pl the expression can be used more constructively to indicate the large number of instances where such autocontamination will not seriously degrade the energy and charge homogeneity of the dopant beam. Thus, for the polyatomic beams X~- and X~[reactions (1) and (2) above] it can be seen that the contamination will be less than 1% ( R < 1 0 -2) if 12/11 < 1. This is usually the case for the covalent elements which are of particular interest for such applications, but care may be needed when the lower abundance polyatomic beams of more electropositive materials are required. On the same basis, however the contamination of the multiply charged ions X 2+ and X~+ [reac-

3.4.2. Collisions after analysis In comparison with the previous section, the situation in respect of inelastic charge-exchange reactions after analysis is much simpler. The only important effect of such inelastic collisions is to modify the charge of a fraction, f, of the resolved flux of dopant particles where: (6)

Thus, if we assume a fairly adverse pressure-path length product of 10 -s torr.m the probable contamination will vary between about 0.3% and 3% for cross-sections in the range 10-16-10 -is cm 2. It is thus apparent that for elementary doping situations, the degree of contamination will normally be low unless the pressure-path length product is particularly high or the charge-exchange cross-section unusually large. Such conditions may occur, for example, when a long beam flight tube is used with ions of a gas such as krypton or argon. We also note below that in certain special circumstances even minor levels of such autocontamination may result in signifieant effects. 3.4.3. The consequences o f autocontarnination We have seen above that whereas autcontamination does not lead to a loss of chemical purity, it can result in a significant degradation of the charge, mass and energy homogeneity of the dopant beam. This loss of beam quality can have important consequences for implantation applications. Firstly, and most commonly, it may cause errors in beam monitoring as a result of the incorrect charge-state or atomicity of the contaminant particles. Secondly, it may modify the implantation profile as a result of the contaminant particles having an incorrect energy at the target stage of the accelerator. It is apparent that this effect will evitably occur when the contamination arises from charge-exchange before mass analysis. Such impurity ions must have both an incorrect energy and

106

J.n.

j

tO0

200

300

FREEMAN

Pro file

t,O0 500 600 Penetrofion(nm)

700

800

900

Fig. 4. Computed depth profile of doubly-charged phosphorus in which each of the reactions: p + + ~ p 0 , p + + ~ p + and P+ +--,P+ + + results from charge-exchange of 5% of the primary beam.

•J-

-~-

et al.

charge-state. The effect is exacerbated when postacceleration is employed after analysis in order to increase the implantation energy. In such a case, charge-exchange reactions which occur after analysis, but before the final stage of acceleration will also result in a modification of the dopant profile. This is illustrated in fig. 4 which shows a computed profile for a hypothetical implantation of doubly-charged phosphorus. The following conditions are assumed: (a) initial acceleration 40 kV, (b) post-acceleration of 160 kV to give a total energy of 400 keV for the doubly charged ions (p2+), (c) charge-exchange of 5% of the p2+ ions to each of the charge states p0, p+ and p3+. The p0 would thus be implanted at 80keV (80+0), the P+ at 240keV (80+160), the W + at 400 keV (80+320) and the p3+ at 560 keV (80+480). The broken lines in the illustration show the computed profiles for the various charge states and the full line shows the composite profile. It can be seen that the effect of such charge-exchange would be to modify both the shape and the peak height of the main dopant profile and to introduce a satellite profile closer to the surface.

J _ [ x 50 Kr_B6 +**

,ll.. .]....

~

I

N~

Kr ~

Fig. 5. 4 mA krypton spectrum.

Kr* 4mA total

x 20

ION BEAM

Finally, such changes in charge-state and energy may also have consequences which relate more to the general use of dopant beams than to the application of multiply charged and polyatomic ions. Such effects, which are considered elsewhere in this series of articles, may be important when even a small fraction (say 1%) of the beam is degraded. They include (a) the loss of uniformity when electrostatic scanning is used, (b) undesirable radiation damage when beam deceleration is employed and (c) electrical breakdown problems and X-ray generation in electrostatic mirrors and lenses. 3.5. EXAMPLES OF ION-BEAMBEHAVIOUR We have described above the several factors which can influence the successful use of multiply charged and polyatomic ions. We have also noted the large variations in the ionisation behaviour of different elements which preclude the formulation of a systematic description of ion-beam quality. Nevertheless, we can obtain a clearer understanding of the scope of the technique and of the associated problems by an examination of the detailed mass spectra of a selection of elements representative of those used for implantation and related ion beam studies.

3.5.1. Krypton The spectrum of an intense krypton beam ( - 4 mA) in fig. 5 illustrates several important features of ion-beam behaviour. The first point of note is that the spectrum contains relatively few impurity ions. This is normal for source operation with an inert gas. The doubly charged beam, Kr 2÷, of several hundred microamperes is well defined and has an adequate intensity for a wide range of requirements. The triply charged beam which has a much lower, but still useful, intensity of about 10 ~zA, is adjacent to a significant impurity beam of N~ or CO + at mass 28. Nevertheless, the 86Kr3+ peak at the mass position 28.66 is clearly resolved J

I

I 18M÷

I

~_

! l

i,:J

,~c

i OxtO-5

2 0 x 10-5

39110-5

72x10-5

MASS

SPECTRA ot

Jj' i 4xlO-4

STUDIES

107

and usable. It illustrates the value of isotopic resolution. As would be anticipated for an inert gas spectrum there is no evidence of polyatomic krypton ions. It follows, therefore, that the doubly and triply charged beams are free of contamination due to charge-exchange before analysis. The beam which can be seen at the diatomic (Kr~) position around mass 172 is in fact an Aston Band (see section 3.4.1) arising from the charge-exchange reaction Kr 2÷-> Kr ÷. This can readily be deduced from its detailed mass spectrum which corresponds to Kr and not to Kr2, and from the increase in its relative intensity which is observed when t h e pressure is raised in the vacuum region before mass analysis*. It has an intensity about 5% that of the parent beam and it exhibits some loss of resolution due both to scattering and to the fact that a fraction of the charge-exchange collisions occur during the mass analysis. There is no evidence of a Kr 4÷ beam in the figure, but the similar higher gain trace of the appropriate region of the mass spectrum in fig. 6 shows a characteristic krypton pattern of isotopes. This is adjacent to a small impurity beam at mass 18 (probably H20+). The figure shows the effect of varying the pressure in the flight tube between 10 -5 and 2.5 x 10 4 torr. As expected the normal beam at mass 18 decreases steadily because of scattering and neutralisation. In contrast the beam at the Kr 4+ position increases rapidly in intensity indicating that it is mainly due to the charge-exchange reaction Kr + --,Kr 2÷. The low resolution of the beam is further evidence of its origin. This behaviour corresponds closely to the analysis in section 3.4.1 above and confirms the particular danger of attempting to use quadruply charged ions to achieve higher energies. 3.5.2. Dysprosium, gallium and boron The spectra of these three electropositive elements in fig. 7, 8 and 9 respectively, provide further illustrations where the use of multiply charged ions is favoured by the virtual absence of covalent polyatomic ions. The dysprosium spectrum (fig. 7) obtained using dysprosium trichloride in the ion source shows how effective the discharge is in breaking down a molecular vapour to give a significant flux of the

25 x(O-4

Kr4+POS[TION

Fig. 6. Variation of krypton mass spectrum at the quadruply charged position as a function of pressure.

* This characteristic increase in intensity, as a function of pressure, provides a very simple and convenient m e a n s of detecting Aston Bands.

108

J.H.

FREEMAN

et al.

2OOpA

IOOpA

,1,,.. L,.

U

II

ct +

Dy2 ~r

' '~-~ O y + Er + DYSPROSIUM

I

To +

' OyC t ÷'

~

TaCe+

I

I

o,c,,"

T R I C H L O R I DE

Fig. 7. Dysprosium trichloride spectrum.

elemental ions C1 ÷ and Dy ÷. The spectrum is more complex than that obtained with krypton and it shows evidence of reaction of the corrosive halide with the ion-source constructional materials (Ta ÷, TaCI+). The Dy 2+ beam is well defined and relatively intense.

In contrast, the gallium spectrum (fig. 8) which was obtained by vaporising the pure metal in the ion source illustrates the importance of adventitious impurities and the need to examine the mass spectrum with care when using beams of low abundance. In this case, the 80 # A beam of dou-

5OOpA

I

, ! B÷

C t 2.1-

GAIN APPROX xS

Fig. 8. Gallium spectrum.

AjlL

GaS+ 3pA

L~

~801JA Go~ J 3SCC+ 37CE ÷

GALLIUM GAIN x I

Go +

~lmA

totol

ION BEAM S T U D I E S

8+

_

c~+

109

~,+

1_1 j,j, j J!lll Lt_ "'

BORON

TRICHLORIDE

Fig. 9. Boron trichloride spectrum.

bly charged gallium is dominated by the neighbouring beam of 35C1+. The chlorine contamination reflects the earlier use of the ion source for a different doping requirement. In spite of the high relative intensity of the impurity beam the accelerator resolution is adequate in this instance to allow the Ga 2+ to be used for implantation. Since both of the gallium isotopes are focussed at a fractional mass position (34.5 and 35.5 respectively) they are also likely to be free of isobaric contamination. Similarly, the even lower intensity 71Ga3+ beam at mass position 23.66 is likely to be of an adequate purity. On the other hand, the triply charged beam of the other gallium isotope (69Ga3+) arrives at the mass position 23 and would therefore require careful assessment before use. In contrast to the previous examples of ionisation behaviour the boron spectrum (fig. 9) illustrates the much reduced yield of multiply charged ions with elements of low atomic weight. In this case for a primary beam of - 1 mA of B ÷ and a total extracted current of several milliamperes, the intensity of the doubly charged beam, B2+, is only 3 ttA. Because of the charge-state this corresponds to an equivalent flux of only 1.5/~A of singly charged ions. The high gain section of the spectrum illustrates the effect of the reactive halide vapour in producing small, but significant, impurity peaks at almost every position in the mass spectrum. A final feature of note is the absence of any detectable level of B£ compared with the

quite significant yield of the more electronegative

Cl~-. 3.5.3. Phosphorus, arsenic and antimony Figs. 10-14 illustrate the ionisation behaviour of the more covalent elements (P, As and Sb) commonly used for semiconductor implantation. Figs. 10 and 11 show the difference between the spectra obtained using phosphorus trichloride and elemental phosphorus as source-feed materials. Although phosphorus trichloride is more convenient and is extensively used for the routine implantation of singly charged phosphorus ions, it is apparent that the element is preferable when other charged species of phosphorus are required. It has the additional advantage that a much higher overall intensity can be obtained. Thus, the P÷ beam of fig. 11 corresponds to an intensity of about 5 mA, whilst the lower current peak of P~- ions corresponds to an almost identical dopant flux, but at half the equivalent energy. As in the case of the krypton spectrum discussed above the beam at the p4+ position is mainly spurious and arises from the reaction P+ --, P+ ÷ before analysis. The inset in the illustration shows the effect on the triply charged beam of raising the pressure in the beam transport region before analysis. At the higher pressure, the p3+ intensity is reduced by about a factor of two because of charge exchange and a significant level of contamination due to the reaction Pj~--, P+ has appeared at the base of the peak. A more quart-

110

J. H. F R E E M A N

et al.

A,

A|

Gqin x l

"~

~

.c,+

ct~+

1602-I-

~-,~+

c,c-q~-i~+

~-~c~+

POCt PHOSPHORUS TRICHLORIDE

Fig. 10. Phosphorus trichloride spectrum.

4-6x10 - 6 torr

1.10 - 4 torr

Quahty of P+++ as a f u n c t i o n of p r e s s u r e

p3+ Gain xlO0

Gain x 5 10;+ 11B~" ,

Gain x l

'

p2 +

p+

J 1

p +

Fig. 11. Phosphorus spectrum showing multiply charged and polyatomic beams.

L

f W 2+

1 p3 +

111

ION BEAM S T U D I E S

Gain x 100 d

i1..... A~÷÷

Gain xl - - ~ A~ '~

AS

As~

AS

As~

Fig. 12. Arsenic spectrum showing multiply charged and polyatomic beams.

titative interpretation of such pressure effects is considered later, but even a cursory inspection indicates that the quality of the triply charged beam is high at the normal operating pressure. The arsenic spectrum, shown in fig. 12, is similar in most respects to that for phosphorus described above. The main feature of note is the even higher abundance of polyatomic ions. Thus the As~ beam corresponds to a dopant flux which is almost double the intensity of the monatomic beam As ÷. Even the tetratomic beam As~- has an intensity equivalent to approximately 50% of the monatomic beam and thus offers a very convenient intense source of low energy ions. In the second illustration (fig. 13) showing the detailed low mass region associated with a milliampere As ÷ beam, the charge-exchange contamination of the As 2÷ peak can be seen as a triangular extension of the base of the peak, even under normal operating conditions. The higher level of contamination in this instance arises from the unusually large intensity of the As~ beam which provides the source of the impurity flux. Even in this case the beam is still adequately pure ( > 9 8 % ) for most requirements, but it is evident that at higher operating pressures the level of contamination would be unacceptable. As in the previous examples the small beam at the As 4+ position (mass 18.75) is mainly spurious and it is interesting to note the broad distribution of low level impurity beams (10-2-1.0 - 1 % ) which can be detected at high amplification.

Finally in this description of ion-beam spectra, we consider an illustrative example of the contamination of a polyatomic antimony beam. In contrast with the increase in intensity of the polyatomic ions which we noted in passing from phosphorus to arsenic, antimony manifests its more metallic nature through a significant reduction in the yield of such ions. Typically the ratio of Sb~/Sb ÷ is only about 0.1 compared with about 0.5 for phosphorus and 1.0 for arsenic. The intensity is adequate for most requirements, and in particular, such beams have been successfully used for radiation damage studies13). However, because of their lower abundance they are clearly more subject to autocontamination from charge-exchange of the multiply charged ions. In the experimental condition illustrated in fig. 14 the requirement was for the precisely controlled implantation of Sb~- at an exceptionally low fluence (N 109 atoms/cm3). In order to reduce the beam intensity, the ion source was operated with argon plus a very small partial pressure of antimony. The unusually high background pressure contributed to the degradation of the relatively low abundance of Sb~ by the reaction Sb2+-Sb ÷. This can clearly be seen in the badly resolved peaks at the mass positions 242 and 246 corresponding to the diatomic ions 121Sb~- and 1235b~- respectively. However, the central beam which corresponds to the ion 121Sb123Sb+ at mass 244 is much more sharply resolved and was successfully used for the implantation. The marked improvement in quality arises

112

J H. FREEMAN et al.

t

t, - -

75~JA

c ca

g

2--

oE oo

L I

10

I

12

I

14

I

15

I

18

.__.__A

.

A

As3"

AS~o

I

20

[

22

I

24

I

I

26

Atornrc. H o s s

28

I

30

I

32

I

34

I

36

I

38

I

40

Units

Fig. 13. Detail of multiply charged arsenic spectrum.

because there is no equivalent charge-exchange product at the compound isotopic mass. This is a general feature of the behaviour of polyisotopic elements and it provides a convenient solution to the problem of autocontamination arising from charge-exchange before analysis. As in the somewhat analogous gain observed in the case of multiply charged ions which are focussed at fractional mass positions it also illustrates the value of a high resolving power. 3.6. CHARGE-EXCHANGE CROSS-SECTIONS We have remarked earlier on the general paucity of charge-exchange data for the reactions of interest to this application of ion implantation. We estimated, however, on quite general principles, that, over the energy range with which we are

concerned, they were unlikely to be significantly in excess of 10 -]5 cm 2 and that in favourable cases they might be as low a s 10 -17 c m 2. The results of Moline, quoted earlier, fall into this range, as does some very recent and as yet unpublished data of Leyland and Armour on the charge-exchange neutralisation of a wide variety of singly charged ions in various gases at energies from 5 to 40 keV. As an example a selection of 40 keV neutralisation cross sections in argon is listed in table 1. We have also noted that an approximate estimate of dopant beam quality can be obtained by inspection of the detailed mass spectrum. This simple test is adequate for many requirements but it is also possible to use the variation of such spectra with pressure to make a more quantitative estimate of purity.

113

ION BEAM STUDIES TABLE 1. Neutralisation cross section in argon at 40 keV. [Unpublished data from K. Leyland and D. Armour, Univ. of Salford, U.K. (1976)] Ion

liB+

12C+ 2°Ne+ 27A1+ 35C1+ 4°Ar+ 52Cr+ 6°Ni+ 63Cu+ 84Kr+ ll2Cd+ 13°Te+ 131Xe+ 184W+

a ( x 10 16 cm 2)

11.5

10.3

6.8

0.9

0.8

If, for example, the pressure is raised in the beam transport region before analysis the consequent fractional reduction in intensity of a beam is given by eq. (6): f = 3.3x10 is crpl, where a is the total cross-section for all possible charge-exchange and dissociation reactions. The expression is only rigorously applicable at low pressures but for the purpose of beam purity estimation it is normally quite adequate to plot the variation of transmitted beam with pressure and then to extrapolate the curve back to zero pressure. Since this provides a measure of the fraction of the beam which has undergone an inelastic collision at the normal operating pressure it is relatively simple to then set an upper limit on the possible resultant contamination of the required flux of dopant particles. Alternatively the slope of the curve extrapolated to low pressure can be used to provide an approximate value of the total cross-section or. In spite of the inherently large uncertainty in the measurement of the pressure-path length product, pl, such

Fig. 14. Variation of diatomic antimony (Sb~-) spectrum as a function of pressure.

6.8

2.2

0.5

0.3

20.3

0.3

22.1

0.3

0.8

practical cross-section estimates related to the accelerator behaviour are particularly relevant to the autocontamination calculations outlined in section 3.4.1 above. As an alternative to the inferential measure of beam quality based on the attenuation of intensity with pressure, a more direct estimate can sometimes be made by measuring the appearance of the somewhat defocussed contaminant peak at the required mass position. Such peak broadening occurs in all inelastic collisions but it is particularly noticeable in reactions involving the dissociation of a polyatomic ion. Both the attenuation and the appearance effects can be seen clearly in figs. 15 and 16 which illustrate the behaviour of multiply charged and polyatomic phosphorus as a function of pressure. Measurement of the detailed spectra gives the following approximate cross-sections: 1) cr~r,~~p+) < 0.9x 10 -16 c m 2, 2) O'(p3+~p+)~ 1 . 5 x 1 0 - 1 5 c m 2 , 3) a(pI_~p+) < 0.6x10-15cm 2. Although it should be stressed that the main purpose of such observations of beam behaviour is to provide an assurance of implantation quality, they furnish the only present source of charge-exchange data for a number of important reactions. For example, in certain beam scanning systems it is important in estimating doping uniformity to have at least an approximate measure of the crosssection for the neutralisation of the singly charged dopant ion. Some typical so,0) results obtained in this way for some typical implantation species at 40 keV in air are: B+-3.4 × 10 -16 c m 2 ; P+-4.0 × ×10 16cm2;As+-3.1×10 16cm2;Ar+-7×10 16cm2; Sb+-2.5 × 10 16 c m 2. In spite of the possible errors in such measurements it is possible to conclude with reasonable confidence that the neutral fraction will, in all cases, be below 1% for a typical pressurepath length product, after analysis, of 2 × 10 -6 torr. m.

114

J

H. F R E E M A N

et al.

p÷÷

p;

I

i

P2*

I

p+*

p; ÷+

p; ,÷+

p;

L~, 3'6x 10-6 t o r r

1 xlO -5 t o r r

_JL_

2"5.10 .5 t o r r

VARIATION OF p++ AND

5,,10 -s t o r t

p~" WITH

l x 1 0 "~ torr

PRESSURE

Fig. 15. Variation of P + + and P~ with pressure.

3÷

p;

p; p3+

p; 3+ p3+

pC

_ 2"4 xlO -5 forr

-J-. l x 1 0 -5

J

J torr

5=10 s torr

J l x 1 0 -~ t o f r

VARIATION OF p3+ AND p~ WITH PRESSURE(SCALE p3+ P3" = 1 I00)

Fig. 16. Variation of p3+ and Pj~ with pressure.

2xlO-4tofr

ION BEAM STUDIES

4. Conclusions The production of multiply charged and polyatomic ions in an accelerator with a milliampere primary beam has been described. It has been shown that the doubly and triply charged ions provide a convenient means of extending the high energy capability of such a facility. The beam intensities which can readily be obtained for a wide range of dopants are adequate for a variety of implantation requirements. Polyatomic ions are useful for certain doping experiments and in favourable cases they provide a simple means of obtaining a high flux at low energy. They are, however, only obtained in significant yield for a restricted range of covalent dopants. Such special ion beams usually have a relatively low abundance in the overall mass spectrum. In consequence, they are sensitive to a variety of adventitious chemical doping effects which we have designated heterocontamination. They are also particularly subject to a systematic loss of beam quality arising from the inelastic collisions of a fraction of the ion beam with residual gas in the beam transport system of the accelerator. 'These effects, which we have designated autocontamination, degrade the energy, mass and charge homogeneity of the required flux of dopant particles. The isotopic resolution of the accelerator has been exploited to assess the extent of such contamination for a selection of typical dopants. It has been demonstrated that, with the exception of the singular degradation of the quadruply charged beams, it is usually possible, by careful operation, to obtain a sufficiently low level of contamination (<2%) to satisfy a wide range of ion implantation requirements.

115

References l) j. H. Freeman, Proc. Int. Mass Spectrometry Conf. (Kyoto, Japan, 1969) p. 369; also AERE, Harwell, Report R-6354. 2) j. H. Freeman, Proc. Int. Conf. on Electromagnetic isotope separators (eds. H. Wagner and W. Welcher; Marburg, Germany, 1970), p. 373; also AERE, Harwell, Report R-6497. 3) D. Aitken, Proc. Syrup. on Electron and ion beam science and technology (ed. R. Bakish; The Electrochemical Society Inc., New Jersey, U.S.A., 1970) p. 504. 4) G. Ryding, A. B. Wittkower and P. H. Rose, ibid., p. 518. 5) j. H. Freeman and G. A. Gard, AERE, Harwell, Report R6330 (1970). 6) j. H. Freeman, L. R. Caldecourt, K. C. W. Done and R. J. Francis, Proc. European Conf. on lon implantation, Reading, England (Peregrinus Ltd., Stevenage, England, 1970) also AERE, Harwell, Report R-6496. 7) Int. Conf. on Multiply charged heavy ion sources and accelerating systems, IEEE Trans. Nucl. Sci. NS-19 (1972) no. 2. 8) Proc. Int. Conf. on Heavy ion sources, IEEE Trans. Nucl. Sci., NS-23 (1976) no. 2. 9) j. H. Freeman, W. Temple. D. G. Beanland and G. A. Gard, Nucl. Instr. and Meth. 135 (1976) 1. 10) j. B. Mitchell, J. A. Davies, L. M. Howe, R. S. Walker and K. B. Winterbon, lon implantation in semiconductors (ed. Namba; Plenum Press, New York, 1975) p. 493. 11) j. A. Moore, G. Carter and A. W. Tinsley, Rad. Effects 25 (1975) 49. 12) D. G. Beanland, J. H. Freeman and C. A. English, Inst. Phys. Conf. Ser. no. 28, Ion Implantation (1976) p. 262. 13) H. H. Anderssen and L. H. Bay, J. Appl. Phys. 46 (1975) 2416. 14) j. H. Freeman, Nucl. Instr. and Meth. 22 (1963) 306. 15) j. H. Freeman, Proc. Int. Ion Source Conf. (INSTN-Saclay, France, 1969) p. 369; also AERE, Harwell, Report R-6138. 16) j. H. Freeman, D. J. Chivers, G. A. Gard, G. W. Hinder, B. J. Smith and J. H. Stephen, [on implantation in semiconductors (ed. Namba; Plenum Press, New York, 1975) p. 555. 17) B. Cobic, D. Tosic and B. Perovic, Nucl. Instr. and Meth. 25 (1963) 157. 18) R. A. Moline, J. Appl. Phys. 42 (1971) 2471.