SIMS analysis of isotopic impurities in ion implants

Vacuum/volume36/numbers 11/12/pages 1001 to 1003/1986

0042-207X/86S3.00 + .00 Pergamon Journals Ltd

Printed in Great Britain

SIMS analysis of isotopic impurities in ion implants D E Sykes, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TF, UK and R T B l u n t , Plessey Research (Caswell) Ltd, Allen Clark Research Centre, Caswell, Towcester, Northamptonshire, NN 12 8EQ, UK

The n.type dopant species Si and Se used for ion implantation in GaAs are multi-isotopic with the most abundant isotope not being chosen because of potential interferences with residual gases. SIMS analysis of a range of 29Si implants produced by several designs of ion implanter all showed significant 2aSi impurity with a different depth distribution from that of the deliberately implanted 29Si isotope. This effect was observed to varying degrees with all fifteen implanters examined and in every 29Si implant analysed to date 29Si + , 29Si + + and 3oSi implants all show the same effect. In the case of Se implantation, poor mass resolution results in the implantation of all isotopes with the same implant distribution (i.e. energy), whilst implants carried out with good mass resolution show the implantation of all isotopes with the characteristic lower depth distribution of the impurity isotopes as found in the Si implants. This effect has also been observed in p-type implants into GaAs (Mg) and for Ga implanted in Si. A tentative explanation of the effect is proposed.

1. Introduction

2. Experimental

Apart from the review by Freeman 1 there has been very little work published on the isotopic purity of ion implants. Most modern implanters have sufficient mass resolution to separate adjacent isotopes up to mass 125 (M/AM~ 100). In the most commonly used implantation species for Si (B, P and As), both P and As are mono-isotopic whilst B has only two isotopes which, since the mass numbers are low (10 and 11), are easily separated. However, most of the species used for implantation into III-V compounds are multi-isotopic and, for reasons of elemental beam purity, the most abundant isotope is unsuitable for implantation. Isotopic impurity will not cause any obvious problems with electrical activity provided that the impurities have the same energy as the main implant. However, such isotopic impurities are unwelcome because they often represent an implanter fault and can also result in discrepancy between SIMS and electrical profiles unless they are allowed for. Furthermore, the use of ion implants as SIMS reference samples may lead to inaccurate quantification of SIMS depth profiles. In contrast, energetic impurities (i.e. the implant species having an energy different from that chosen) will cause deviations from the required profile, and some work has been published on the effects of charge exchange reactions in implants of doubly charged species, particularly for the case of P + + implantation 2. However, to our knowledge, no cases of simultaneous energetic and isotopic impurities have been reported. The purpose of this paper is to show that such phenomena do occur and that SIMS can be used to identify them.

The SIMS analyses were performed in a Cameca IMS 3F SIMS system equipped with mass filtered duoplasmatron and surface ionization (caesium) sources. The depth profiles for electronegative species (Si, Se) were performed using Cs + ion bombardment at 10 keV and detecting negative secondary ions whilst for the electropositive species 02 + bombardment at 15keV was employed and positive secondary ions detected. In both cases a 60 #m diameter analysed area was used with a 250 #m raster for the primary beam.

3. Results Silicon is the most commonly used donor implant species in GaAs technology. In general the most abundant isotope (2sSi) is avoided due to the possible contamination with 14N 2 or t2C160, both of which will compensate the donor activity of Si in GaAs (refs 3, 4). Consequently the minor isotope, 29Si, is most frequently used. Figure 1 shows the SIMS profile recorded from a singly charged 29Si implant (10 TM atoms cm -2, 200 keV). There is also a distinct 28Si profile (dose 4 x 1 0 1 2 a t o m s c m -2) with a distribution corresponding to a lower energy of implantation (120 keV). The same effect is seen in a doubly charged implant of the same dose and energy from the same implanter, Figure 2 (i.e 29Si++ implanted at 100 kV). In this case the 2sSi level is even greater but the implant peak occurs at the same depth (120 nm) as in Figure 1. Similarly in a 3°Si implant at 240 keV (3°Si+ + at 120 kV), a 2sSi impurity is found with the same peak range as for the 29Si 1001

D E Sykes and R T B~nt.'SIMS analysis ofisotopicimpuritiesinionimplants 1020

1020

28

i 0 i9

28

,tOi9

29

29

/ / / / ~ \ \\\ I o tn 0 I-

I0 la

\

-J /

/ \

, \\

\ \ \

10t7 ~

Q,

t0 x8

\\

o° \ \\ \ \\\

-

[E IZ ILl (J Z 0 o t0i8

u

X\ \

\

rr Z ,,,

\\'i,

\

(JoZ o t01B

\\\ ,,,/,\\

~

\

t~,t5¢~ 0

0.2

0.4

0.6

0.8

t,. , t.0

, 1.2

, 1.4

1o t s

,

0

DEPTH (MICRONS)

0.2

,

0.,4

,

0.6

V ~v, I K IAJAIVV~II O.S

t.0

t.2

I

t.4

DEPTH (MICRONS)

Figure 1. SIMS profile of 2sSi and 29Siin a 1 x 101. atoms cm -2 200 keY

Figure 2. SIMS profile of 2ssi and 295iin a l x l0 ~4 atoms cm -2 200 keV

29Si ÷ implant (i.e. 29Si + at 200 kV accelerating potential). Source voltage

29Si÷ ÷ implant (i.e. 29Si÷÷ at 100 kV acclerating potential). Source voltage 32.5 kV, post acceleration voltage 67.5 kV.

32.5 kV, post acceleration voltage 167.5 kV

implants. This effect is not confined to the major isotope and can indeed be seen in the minor isotopes: in the 3°Si implant the 29Si isotope is also seen with the same distribution as the 2sSi isotope whilst for the 29Si implants the 3°Si level is partly masked by the 29Sill interference but is apparent in the sub-surface region. This effect is not confined to one design of implanter, indeed it is seen with non-commercial systems and commercially produced systems and has been observed in all 298i implants analysed to date, (from a total of 15 different machines). The level of the 2aSi impurity varies from 2 to 14% of the implanted dose. Selenium is also used as a donor implant species in GaAs and here also the major isotope a°Se is not often used because of interference from 4°Ar2. In one 1 x 10 is atoms cm -2 7SSe+ implant analysed, all six isotopes were found with the same Gaussian shaped implant profile. (Figure 3). This quite clearly results from poor mass resolution in the ion implanter. However, in a 101. atoms cm -2 7 7 5 e + + implant carried out in a different implanter with much higher nominal mass resolution (Figure 4) all the isotopes are found again but in this case with a depth distribution similar to the 2ssi impurity in the 29Si implants, the implant energy being approximately 160 keV but note that the profiles are far from Gaussian. Table I shows the relative amounts of the various isotopes in the two implants together with their natural abundance. SIMS analysis of a 25Mg doubly charged implant into GaAs also shows the major isotope, 24Mg, appearing with a shallower depth distribution as seen in the Si and Se implants.

1002

4. Discussion Isotopic and simultaneous isotopic and energetic impurities have been noted in ion implants into GaAs. Although the study has not systematically covered all multi-isotopic elements, the effect has been observed in implants of Si and Se, as shown in this paper, as well as in Mg in GaAs and B and Ga in Si (ref 5). Thus it is probable that this effect is universal in ion implantation. It does not appear to be related to mass resolution as poor resolution leads to implantation of all isotopes with the same depth distribution as in Figure 3. Moreover, in implants of 298i and 3°Si the 28Si impurity level is of similar proportions which would not be the case if the effect was related simply to mass resolution. It is known that Se does undergo significant charge exchange reactions: for instance, Jamba et al 6 have reported significant differences between the profiles of 600 keV Se implants made on a pre-analysis acceleration system and a post-acceleration system which appear to be due to charge exchange. The magnitude of the effects they observed was much greater than found in the present work but unfortunately they did not look for !sotopic effects. However, it is difficult to envisage a charge exchange reaction which would account for the effects observed and in the case of the Si implants the use of a beam filter did not significantly alter the level of 2aSi impurity. Recoil implantation of species absorbed on the surfaces of the samples can be dismissed as a possible mechanism because (a) the profile shape is inconsistent with recoil profiles 7 and (b) the impurity isotopes are not found in their

D E Sykes and R T Blunt: S I M S analysis of isotopic impurities in ion implants iO20

l o 2o

i o i9

~o le

a-"

..............

78

i o ta

SO Z 0

",

!

77

77

t 0 LB -/ o

/

ioi7

SO

\

\,,

o

I /'

78

'\\

', \

i:

"\

,/

\'

H I'-" < rr IZ LLI

\

Z W

t 0 t7 .

~

\"\ ',

O

Z 0 r_,l

z 0

0 lOle

t0 i5

;\

O '. l

0 '. 2 0 i 3 0 .,4 ',3 0 .x'.5 DEPTH

0 .,6

0 .,7

O.' 8

0 .'9

"",,,.,,%

t 0 i e " ,,

lo t5

= 0

0.t

Xl O.E

~= 0

= 3

0

4

DEPTH

(MICRONS)

Figure 3. SIMS profile of isotopic impurities in a 785e 1 x l0 ts atoms cm -2 380 keV implant (TSSe+ at 380 kV accelerating potential) showing poor mass resolution in the implant. Only isotopes 77 and 80 are shown for clarity, see Table 1 for implanted doses of all isotopes.

Table 1. Relative abundance of Se isotopes found in a 7sSe+ and a 77Se+ + implant carried out in poor and high mass resolution conditions in different implanters.

Mass

78Se implant (%)

77Se implant (%)

Natural abundance (%)

74 76 77 78 80 82

0.02 0.85 0.40 97.8 0.90 0.02

0.02 4.46 94.1 1.31 0.11 0.02

0.9 9.0 7.6 23.5 49.8 9.2

015

OlB

0 .t7

t8 O.

0 I

.9

(MICRONS)

Figm-e 4. SIMS profile of isotopic impurities in a 77Se 1 × 1014 a t o m s cm -z 380 keV implant ( 7 7 8 e + + at 190 kV acclerating potential). Only isotopes 78 and 80 are shown for clarity, see Table 1 for implanted doses of all isotopes.

5. C o n c l u s i o n

Energetic and isotopic impurities in ion implants are a common occurrence in routine implants from multi-isotopic elements. Although not presenting an immediate problem in terms of electrical activity they may lead to a difference in profile shape depending on the method of measurement. Operators of ion implanters need to be aware of the effect as it may signify an implanter fault whilst SIMS analysts need to be alert to the possibility of some of the implanted dose being present in the nonselected isotopes.

Acknowledgement

natural abundances, Table 1. It appears more likely that during mass analysis the non-selected isotopes are scattered by and/or sputtered from the walls of the magnet housing into the selected isotope beam to be implanted into the sample with a lower energy. This would account for the generally lower energy of the isotopic impurities and its dose variation from implanter to implanter and from run to run, since minor changes in magnet layout or vacuum would affect the a m o u n t of contaminant scattered back into the beam. In the profiles shown in Figures 1 and 2 the effective energy of the 2sSi impurity is the same, but not consistent with either the source potentials or post acceleration voltages. However the isotopic impurities must be charged as they are not filtered out by the neutral traps in the ion implanters.

We would like to thank all those who have supplied the implants used in this investigation.

References

1 j H Freeman, Nucl Instrum Meth, 38, 49 (1965). 2 j H Freeman, D J Chivers and G A Gard, Nucl Instrum Meth, 143, 99 (1977). 3 S Matteson, J Vac Sci Technol, 132, 145 (1984). 4 W M Duncan and S Matteson, J appl Phys, 56, 1059 (1984). 5 R T Blunt and D E Sykes, unpublished work. 6 D M Jamba, Nucl Iastrum Meth, 189, 253, (1981). 7 R T Blunt, R Sweda and I R Sanders, Vacuum, 34, 281, (1984). 1003