BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON

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BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON

F. Cappellani, M. Henuset, G. Restelli

To cite this version:

F. Cappellani, M. Henuset, G. Restelli. BEHAVIOUR OF LOW DOSE ARSENIC IM- PLANTS IN SILICON. Journal de Physique Colloques, 1973, 34 (C5), pp.C5-145-C5-149.

�10.1051/jphyscol:1973529�. �jpa-00215316�

JOURNAL DE PHYSIQUE

Colloque C5, suppldment au no 11-12, Tome 34, Novembre-Ddcembre 1973,page

BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON

F. CAPPELLANI, M. HENUSET AND G . RESTELLI Electronics

Joint Research Centre, Ispra, Italy

RBsumb. -

Le comportement d'ions d'arsenic de 80 keV implantks

temperature ambiante dans des cristaux de silicium orientks ou non, en dessous de la dose d'amorphisation, a kt8 Btudik en mesurant les profils physique et tlectrique suivant des recuits isochrones.

A

partir de l'analyse du profil physique, les paramhtres de pknktration, Rp, AR, et Rmax pour des ions canalisks le long de l'axe < 11 1 >, ont kte mesurks.

Aprhs un recuit isochrone d'kchantillons implantks hors canalisation, un coefficient de diffusion effectif k 900

pour l'arsenic a ktt calculk Cgal k 5

10-16 cm2.s-1.

11 est montrk que pour des ions d'arsenic implant& dans le silicium hors canalisation, la fraction klectriquement active, qui est d'environ 0,4 aprhs un recuit de 30 min a 600

augmente continuel- lement jusqu'k ce qu'une activation Bectrique des ions arsenic de 100

soit atteinte

envi- ron 900

Une tendance similaire est observke pour les implantations en canalisation < 11

> .

Le procCdt de recuit semble suivre une reaction cinktique du premier ordre avec une Cnergie d'activation kgale

0,4-0,5 eV.

I1 est donne un essai d'interprktation du mCcanisme impliquk.

Abstract. -

The behaviour of 80 keV arsenic ions implanted at room temperature in oriented and disoriented silicon crystals below the amorphization dose, has been studied by measuring the physical profile and the electrical profile following isochronal anneals.

From the analysis of the physical profile, the penetration parameters, ED,

and

for ions channeled along < 1 11 > axis, have been measured.

After isochronal annealing of off-channeling implants, an effective diffusion coefficient at 900

for arsenic equal to 5 x 10-16 cm2.s-1 has been calculated.

It is shown that for arsenic ions implanted in disoriented silicon, the electrically active fraction, which is about 0.4 after 30 min annealing at 600

increases continuously until 100 % electrical activation of the arsenic ions is reached at about 900

Similar trend is observed for < 11 1 >

channeled implants. The annealing process appears to follow a first order reaction kinetics with an activation energy equal to 0.4-0.5 eV. A tentative interpretation of the mechanism involved is given.

1. Introduction.

- Extensive investigations on the behaviour of boron and phosphorous implants in silicon have been reported

much less attention has been devoted to arsenic.

Previous studies on atom location and electrical properties of low energy (20-40 keV) arsenic implants have been published before 1970

the results can be found in the book by Mayer, Eriksson and Davies [I].

Detailed studies on the electrical behaviour of 280 keV arsenic implants in silicon have been published by the IBM group [2]-[6] and data on the penetration parameters of arsenic implants by Kleinfelder et al,

and by E. Davies [8]-[9].

The annealing behaviour of arsenic implants in silicon for doses below the amorphization limit has been the subject of the present research. Since the implantation energy around 100 keV has not been investigated, the value of 80 keV was chosen. At this energy the range of arsenic ions allows the determi- nation, with sufficient precision, of the penetration profile, using anodic sectioning techniques.

The determination of the total concentration dis- tribution using radiotracer techniques has allowed the evaluation of the penetration parameters

mean projected range %, standard deviation of the pro- jected range

R,,, for As'

ions channeled along < 111 > axis.

An effective diffusion coefficient for arsenic at 9000 in off-channeling implants has been calculated equal to 5 x 10-l6 cm2.s-l.

The number of electrical carriers has been calcu- lated by integration of the carrier concentration pro- file obtained from differential sheet resistivity and Hall effect measurement, together with anodic sec- tioning, after each isochronal annealing step. Attri- buting the carriers to substitutional uncompensated arsenic atoms, the electrically active fraction of implanted atoms which is about 0.4 after 30 min annealing at 6000C increases continuously until 100 % electrical activation is reached at about 900 OC.

A similar trend is observed for ions implanted along

< 11 1 > axis in channeling conditions. In both

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973529

C5-146 F. CAPPELLANI, M. HENUSET AND G. RESTELLI

cases from an Arrhenius plot an activation energy equal to 0.4-0.5 eV has been calculated.

2.

- 1-100 a. cm P-type silicon slices, cut normally to the < 111 > axis with a precision of

0.50, were implanted at room tem- perature at an energy of 80 keV using a 100 keV ion accelerator [lo]. The dose was fixed at

below the critical value required to form an amor- phous layer [Ill. The dose rate was in the range 0.1-0.3 ~ A . c m - ~ to avoid heating effects

secondary electron suppression was provided.

For off-channeling implants, the specimens were mounted with the beam direction 80 to the < ^{11 1} >

axis.

Scanning of the specimen over a diameter of 20 mm was provided using electrostatic deflection giving a maximum beam divergence of + ^1.50

the critical angle for 80 keV As ion channeling along < 111 >

axis is about 3.50.

The silicon slices were chem-mechanically polished

a final cleaning procedure (HF dip, wash in methanol and blow-dry with nitrogen) was provided immedia- tely before mounting the slices in the target chamber.

Annealing in 30 min steps was carried out in a flow of dry nitrogen.

The total arsenic concentration distribution vs penetration depth was obtained by radioactivation in conjunction with layer removal by anodic oxi- dation and H F stripping using an automated machine 1121, 1131.

Carrier concentration profiles were obtained by differential sheet resistivity and Hall effect measure- ments. A Van der Pauw pattern was defined using photomasking and mesa-etching

this procedure was found preferable to the direct implantation of the pattern using mechanical collimators.

3. Results. -

3.1 PENETRATION PARAMETERS.

From the physical profile, the mean projected range &

for 80 keV arsenic ions in disoriented silicon has been found equal to 450 + 50 A with a standard deviation

25 A.

The mean projected range agrees with the value calculated according to the LSS theory [14], while the standard deviation shows a large discrepancy with respect to the calculated value of 169 A [14]. This situation is reversed if a comparison is made with the experimental values of reference [6]

i?,

600 + 60 A and AX,

250 + ^{25 A .}

The range distribution profile fits a gaussian curve up to Rp and then exhibits a nearly exponential penetrating tail most probably due to channeling effects favoured by the large critical angle, the beam divergence and the low implantation dose. The chan-

neled fraction calculated by subtracting an area cor- responding to a gaussian curve centered at % and having a standard deviation equal to AX,, accounts for 12-15 % of the implanted dose (Fig. 1).

I I 1 I I

500 1000 1500 2000

DEPTH ( A )

FIG. 1. - Total concentration distribution curve of 80 keV arsenic ions implanted a t room temperature in disoriented silicon crystals, after 30 min isochronal annealing at 500 and 900 OC respectively. Implanted dose 7

x

The analysis of the total concentration distribution of Asf ion implanted along < 111 > in channeling conditions, shows a nearly exponential tail extending up to 4 000 f 200 A (Fig. 2). This value of R,, is in fairly good agreement with that obtained extra- polating the data of Kleinfelder

[7].

500 1500 2500 3500 4500

DEPTH

(A)

FIG. 2. - Total concentration distribution curve of 80 keV arsenic ions implanted at room temperature along

<

>

axis in silicon (full line).

Circles represent the carrier concentration profile after 30 min at 550 OC anneal.

Implanted dose : 7 x 1013 ions .cm-2.

Beam divergence : f 1.5".

3.2 ENHANCED

Figure 1 shows the

physical profiles of 80 keV arsenic implants in diso-

riented silicon after isochronal annealing for 30' at

5000 and 900 0C respectively. The 5000 annealing

was required by the necessity to anneal out fast

neutron irradiation defects occurring during the

reactor activation of arsenic atoms. The agreement

BEHAVIOUR O F LOW DOSE ARSENIC IMPLANTS I N SILICON C5-147

in the projected range standard deviation with the experimental value reported by Crowder [5], can be used to exclude diffusion phenomena after a 500

anneal. On the other hand, the broadening of the distribution after 900 0C annealing is evident and must be attributed to arsenic diffusion.

Because of the presence of a channeled fraction, the calculation of the effective diffusion coefficient Dx was performed using the gaussian portion of the curve. A

x 10-l6 cm2.s-I at 900 OC was calculated [15]. This value is only about an order of magnitude higher with respect to the diffusion coefficient obtained from extrapolation of the values measured at higher temperatures in radia- tion damage free silicon [16]. From our results there is no evidence of a temperature independent enhanced diffusion coefficient as it has been observed for anti- mony implants [17]-[19]. Both these results seem to support an hypothesis of absence of enhanced diffusion in low dose arsenic implants.

3 . 3

ELECTRICALLY

Figure 3 shows the sheet resistivity us anneal temperature of RT 80 keV off-channeling arsenic implanted silicon Iayers for different implantation doses of loL3, loi4 and lo1' i ~ n s . c m - ~ . The duration of the annealing at each temperature was always 30 min. The measure- ments were performed using four point probe tech- nique.

ANNEALING TEMPERATURE ( "C )

FIG. 3. - Sheet resistivity vs anneal temperature for room temperature 80 keV arsenic implanted layers with doses of 1013, 1014 and 10'5 ions.cm-2 respectively. The duration of

each annealing was 30 min.

It is evident that the 10'' i o n ~ . c m - ~ implant shows a predominant annealing step before 550 OC in cor- respondence with the reordering of the amorphous silicon layer formed at these doses. On the contrary, the

loi3 and 1014 ions.cm-2 implants which are below the amorphization dose, exhibit non saturating charac- teristics with a continuous increase of the layer conductivity. This trend is in agreement with the results obtained for 280 keV arsenic implants by Crowder and Morehead [2].

In this experiment the attention was concentrated on the annealing behaviour of layers implanted with a dose (7 x 1013 Asf .cm-') below the amorphiza- tion limit.

The determination of the number of carriers from sheet conductivity and sheet Hall effect measurement can be imprecise because of the effect of non-uniform distribution of carrier density and mobility. There- fore the total number of carriers has been calculated by integration of the carrier concentration proaes, obtained from sheet resistivity and sheet Hall coeffi- cient measurements in conjunction with sectioning by anodic oxidation and stripping.

The mobility values obtained after 550 OC annealing agree within + ^{20 %} with the data reported in reference [20] for diffused silicon samples.

Assuming that the carriers are due to substitutional uncompensated arsenic atoms, by comparing the carrier concentration with the total arsenic concen- tration obtained from the physical profile, an elec- trically active fraction after isochronal annealing steps has been calculated (Table I).

Figure 4 shows a set of carrier concentration pro- files determined after 30 min annealing at 650, 700, 800 and 900 OC. After the 900 OC annealing the elec- trical and the physical profiles coincide.

Assuming a first order reaction mechanism for the electrical activation of the arsenic atoms, an Arrhenius plot has been drawn for the temperature

m

DEPTH

(A)

FIG. 4. - Set of carrier concentration vs penetration depth profiles after 30 min isochronal annealing at 650, 700, 800 and

900 OC.

Electrically active fraction of im-

planted Asf .ions 0.38

10 % 0.46 + ^{10 % 0.56} + ^{10 % 0.78}

^{10 % 0.92}

^{10 %}

C5-14 8 F. CAPPELLANI, M. HENUSET AND G . RESTELLI

range 600-900 OC. The points lie close to a straight

line and from the gradient an activation energy AE equal to 0.4 eV has been calculated [21], [22].

The same analysis of the data obtained from the comparison of the physical profile and the electrical profile after isochronal annealing steps, in the same temperature range, has been performed according to Ishino

channeling conditions along < 111 >. Also in this case the data fit a first order reaction mechanism with an activation energy calculated from the Arrhenius plot equal to 0.5

4. Discussion. - As an implanted ion slows down and comes to rest in a crystal, it suffers collisions with lattice atoms which result in the production of highly disordered regions around the path of each ion. At sufficiently high doses a layer is formed which is characterized by absence of long range order (amorphous layer). In silicon this amorphous layer anneals by epitaxial reordering on the underlying crystalline material at temperatures between 550 and 6500C. The presence of the amorphous phase has a pronounced effect on the anneal characteristics of ion implanted layers as observed by electrical measu- rements.

When the implantation dose is such that an amor- phous layer is not formed, the correlation between disorder anneal and electrical characteristics is not observed and the achievement of electrical activity of the implanted ions requires temperatures as high as 1 000 OC as well known for many dopants and demonstrated by these results in the case of arsenic.

The temperature range 600-1 000

is well above any identified primary defect anneal range [23]-[25].

Above 600 OC in fact, only secondary defects such as dislocation loops, stacking faults and dislocation networks have been observed.

Annealing in the temperature range of 600-1 000 OC has been investigated in low dose room temperature arsenic implants by EPR studies indicating an increase in substitutionality of arsenic atoms [26].

Measurements by channeling technique, previously performed by Eriksson

revised by Haskell

ment of a beam induced off lattice motion of arsenic atoms. However, no conclusive results can be derived from atom location measurements since substitutional atoms can be electrically inactive due to the presence of compensating defects.

An interpretation of the physical process governing the annealing can be inferred from the activation energy AE obtained from the Arrhenius plot. Such an analysis has been performed e. g. for boron and phosphorous implants [29]-[31].

Ishino

energy AE and order of reaction may be determined from isochronal studies. Using their treatment the

experimental data fit a first order reaction of the type Dl1

Ci-

In -

exp

ci k%

where

Ci-, and Ci are the defect concentrations before and after the

th anneal of duration

T i is the temperature of the

th anneal,

is the rate constant for the reaction, AE is the activation energy of the process

which comes out equal to 0.4-0.5 eV (as reported in the preceeding paragraph).

A second calculation of the activation energy of the process can be performed following the treatment outlined in the paper by Gusev and Titov [29].

In the approximation given by these authors the following equation holds

Ace - ^C,(T)

^exp

AH

where

Ce(T)

concentration of electrically active defects at temperature T ,

AC,

change in the concentration of electrically active defects in a time t,

B =

quantity which is constant for each annealing stage (for constant short annealing duration), AH

annealing activation energy.

From the Arrhenius plot AH comes out to be equal to 0.25 eV. However in this second case the inter- pretation of AH follows from the mechanism assumed for the physical process.

Infact, the creation of an electrically active arsenic atom (assuming that the carriers are due to substi- tutional uncompensated arsenic) can result from two possible processes

a) the dissociation of a complex defect ;

b) the combination of two defects during an encoun- ter.

In the case that the process is the dissociation of a complex defect which leaves a substitutional uncom- pensated arsenic atom the annealing activation energy AH is equal to the dissociation activation energy AEd of the complex defect.

The second annealing process, i. e. the combina-

tion of two defects during an encounter, has been

discussed by Waite [32] and by Jurkov [33]. The

migration of defects is similar to diffusion and can be

represented by a diffusion coefficient with an activa-

tion energy for defect migration AE,. Assuming that

the electrically active atom is created by the encounter

of an interstitial type and a lattice type defect and that

the diffusion of the interstitial defect is much faster,

it can be shown that the annealing activation energy

AH is equal to AEm/2. Therefore the activation energy

BEHAVIOUR OF LOW DOSE ARSENIC IMPLANTS IN SILICON C5-149

of the process would be case and 0.5 eV in the two defects.

equal to 0.25 eV in the first case of the combination of Experimental data are available only for the E-like center, substitutional arsenic atom vacancy pair

the dissociation energy has been measured equal to

1.27 eV [34] and the activation energy for motion equal to 1.07 eV [35]. No data are available for defects like a closely spaced vacancy interstitial pair or interstitial associated with an impurity atom or complexes of two arsenic atoms [28]. The agreement between the value obtained according to Ishino

the electrical activation results from a combination of two defects (0.5 eV) can be used as a criterion of choice.

In this case the migrating defect would be an inter- stitial arsenic atom which drifts during the annealing to a lattice type defect and the activation energy of the process would be the migration energy of inter- stitial arsenic.

As pointed out by Gusev and Titov 1291 the lattice

type defects are not necessarily single vacancies but they may be more complex defects. When they combine with an interstitial impurity atom, an unstable complex is formed which then dissociates rapidly, producing an electrically active center.

The activation energy for interstitial arsenic migra- tion (0.4-0.5 eV) is in rather good agreement with the values theoretically estimated for insterstitialcy diffusion mechanism [16], [36].

Further studies are in progress to elucidate the reliability of this mechanism, however it appears that current concepts and inventory of identified defects are not sufficient for a clear interpretation either of the annealing mechanism either of the defects involved.

Acknowledgments. -

The authors gratefully acknowledge the continuous encouragement of Pro- fessor G. Bertolini and the help of F. Mousty, R. Adam and G. Melandrone for the experimental techniques.

Preliminary results of this work were obtained by E. Baldo in the course of his thesis.

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