Plasma oxidation of TaSix thin films

Applied

Surface Science 53 (IO41

1X4-250

applied surface science

North-Holland

Plasma oxidation

of TaSi,

thin films

and

We

have

3lK-800°C composition different

investigated temperature of samples

the

RF

plasma

oxidation

hackscattering

and to determine

the oxide

regimes in the oxide growth depending

about 550 o C. llowever.

at floating

range. Ruthertord

both temperature

potential

spectrometry

growth

kinetics.

on the temperature.

and silicide Qoichiomrtry

nature

film on the surface in the 300-X00

of the oxide depends

500-700 enriched

provided

that the temperature

almost pure SiO,

is high enough

UAM.

address:

the

play an important composition

cilicidc

temperature

composition

hetwecn

for 7~

thew

there

arc taco

two regime\

role in the nature of the grown oxide\. of a disilicide (i.e.

550°C‘

I\ The

Y i 7) Icads to the growth cnrichcd

and an oxide mixture

(i.e. .v 2 7) the

(Ta,Oi~SiC),)

in the

film on the surface cvcn if the \ilicTdc i\ Ta

2. Experimental

Departamento

Cantohlanco.

0169-4332/91/$03.50

(I.6 5-t K 3.6). in

TaSi,

( = X00” C).

The possibility to use silicides as interconnect material in integrated circuits has aroused a new interest in the oxide growth on these silicides. Until now. the more widely used technique has been pure thermal oxidation [ 1.21. Lower temperature techniques have been developed to produce insulating oxides on silicides; plasma oxidation is one of them. Preceding studies on plasma oxidation of titanium disilicide [3] have evidenced quite unexpected phenomena related to the nature of the grown oxides and growth kinetics.

XII.

the initial

that it is possible to grow ;L pure SiO,

1. Introduction

’ Permanent

ailicides.

o C range. On the other hand. if the Glicide is tantalum

on the temperature:

o C range. It has alho been evidenced

Whatever

The transition

fact that the silicide is silicon enriched with respect to the stoichiometrlc of a pure SiO,

of tantalum

and nuclear reaction analysis have been used to study the

0

de Fisica Aplicada

E-2XO4Y Madrid,

1991 - Elsevier

Spain.

Science Publishers

C‘-

TaSi, thin films (X ranging between 1.6 and 3.6) deposited by co-sputtering from different targets have been used in these experiments. They have been grown on SiO, substrates (SiO, thickness 180 nm) in which SiO, film is supposed to act as a good diffusion barrier to avoid either silicon diffusion from substrate to the silicide or metal diffusion from the silicide to the silicon substrate, but also on (100) n-type silicon substrate for purposes of comparison. The oxidations of the as-deposited films were carried out in a RF oxygen plasma [4]. The plasmas were obtained at 2.5 Pa oxygen pressure with a capacitively coupled RF (13.6 MHz) electrode. The average RF power density was about 0.1 W/cm-‘. All the

B.V. All rights reserved

QQUG 4ooT &&a!& 600°C ~ODDO 800°C o,,'

I

c ;

/’

%,~ /Ii y

,” ,/’ AN

Ii-

,/A’

3

L

I

o---

I

100

0

200

Time Fig. 1. Variation for

plasma

300

400

oxidation

0

4

t

of TaSi, (, /SiO,

at various

tempera-

12

l/2

16

kinetics at several temperature\

SiO,

(note: IO” atoms/cm’

oxidation

20

24

( min1”2)

Fig. 7. Growth

tures.

for TaSi, ,, /

of oxygen is equivalent

to a 0.72 nm thickness of SiO,).

oxidations were performed without any external bias on the sample (i.e. at floating potential). Nuclear microanalysis by direct observation of nuclear reactions [S] ( ‘“0 (d. p) “0 * 1 was used to determine the oxide growth kinetics. Rutherford backscattering spectrometry (RBS) was used to analyze the depth distribution of Ta and Si in the silicides and in their oxides by using the RUMP simulation program [h].

3. Results The kinetics and the temperature dependence of the plasma oxidation of TaSi, have been investigated. As a typical example, fig. 1 represents the amounts of incorporated oxygen atoms versus time, t, at various temperatures for plasma oxidation of TaSi,,,/SiO,. The shape of all the curves allows an analysis by means of the following equation: X” = kt.

8

(minutes)

with time t of the oxygen atom incorporation

I I

0

500

I

(1)

The fitting of the curves in fig. 1 leads to values of n close to 2 for each temperature. This value implies that the rate-controlling process is

diffusion. The amount of incorporated oxygen with time has been analyzed by a parabolic law: A”-X,‘=Bt,

(3)

were X, accounts for the initial transient time of oxidation. Fig. 2 shows the amount of incorporated oxygen versus t ‘I7 at several temperatures for TaSi,,/SiO, oxidation at floating potential. Whatever the temperature and initial composition, the kinetics of oxide growth can be expressed by a pure parabolic law. Table 1 shows the respective values of B for different TaSi, compositions at 400, 600 and 800 o C.

Table

I

Parabolic of TaSi,

rate constant,

at several

B. for different

R(lO’i

Silicide film

Substrate

Si

Si( 100)

51.5

100 a c

1,,

initial

atoms/cm’? 600 o C 95.6

SiO,

73.3

TaSi 2.2

Si( 100)

32.6

92.5

TaSi,,

50,

29

72

TaSi;,

SiO 2

44.5

TaSi

compositions

temperatures

I35

454

min-’ XOO”C 232.2 ‘W.3 301.5 _

FL

0’:

t

L

G’l

EO

PI

91

0:

B-0

21

01

00

L

I

straightforwardly followed for silicon-rich silitides. Similar results have been obtained for silitides deposited onto silicon substrates in these temperature ranges 131. In order to understand this behaviour we have studied the nature of the grown oxide by means of Rutherford backscattering spectrometry (RBS). 3. I. Silicon-rich

SiO,). Fig. 4 shows the RBS spectra of two samples (TaSi,.,/Si and TaSi,,,/SiOz) before and after being oxidized at 800 “C during 4 h. The spectra of the oxidized samples show that the Ta peak is shifted towards lower energies while the Si signal is always at its theoretical surface position. This evidences the growth of a pure SiO, layer on the surface. If the silicide is on a silicon substrate, one observes a little change in the stoichiometry of the underlying silicide to a new value TaSi II tx’
silicide

The RBS analysis of the silicides enriched in silicon with respect to the disilicide stoichiometry (X > 2) demonstrates the formation of pure SiO, films on the surface whatever the nature of the substrate on which the silicide is deposited (SiO, or Si) in the 300-700 o C temperature range. The oxide growth rate is much slower in the low-temperature regime (T < 550 “C> than in the hightemperature one (T > 550 o C). However, if the oxidation is carried out on silicides that are slightly silicon enriched ts = 2.2) at high enough temperatures (T = X00 o C) differences can occur according to the nature of the substrate on which silicide is deposited (Si or

3.2. Tantalum-rich

The growth kinetics also show two temperature regimes with a transition temperature of about 550 “C. The analysis of RBS spectra demonstrates the formation of an almost pure SiOZ layer on the surface in the low-temperature regime. In fact. it can be noticed that the activation energy in this regime is very close to that

Energy 0.6 I

I 130

t

1.0

silicide

1.2 I

(MeV) 1.1 I

1.6 /

1.6 I

Ref

Ta

- - 600 ‘C-4h

Channel Fig. 5. RBS spectra

of TnSi,

JS~ oxidized at 600 o C. 4 h and at X00 ’ C. 2 h

obtained for the silicon-rich silicide in the hightemperature regime, where SiO, is always grown. But on the other hand, the spectra of samples oxidized at high temperature show the presence of plateaux in the Ta and Si contributions. This is characteristic of the formation of a surface layer in which Ta and Si cations are both present. i.e. a mixture of Ta and Si oxides. The depth profiles obtained from the RBS measurements indicate ;I mixture of stoichiometric oxides with a composition directly deduced from that of the silicidc. For instance, the composition of an oxide grown on TaSi ,.(, is 0.S(Ta105)1.MSiO, ). All these rcsuits are independent of the substrate nature. One more time. at 800 a C, rcmarkablc differcnces appear with substrate nature. Fig. 5 shows the RBS spectra of TaSi, ,,/Si before oxidation. oxidized at hO0 o C during 4 h and at X00 “C during 2 h. The spectrum of the sample oxidized at 600 0 C shows the growth of a mixture of oxides (Ta,O,-SiO. ). However, at 800 ’ C. the surface oxide is prac&lly pure SiO,. On the other hand. the oxidation is carried out in a catastrophic way if there is a diffusion barrier (SiO,) between the silicide and the substrate.

4. Discussion

and conclusions

TaSi, plasma oxidation has been studied in the 300-800 o C temperature range. Whatever the temperature treatment. the oxidation kinetics seem to follow a purely parabolic law. However. two different regimes of growth have been observed depending on temperature with a transition temperature of about 550 0 C. Differences in nature of the oxides grown in each temperature regime have also been observed depending on the initial composition of the silicides. For silicon-rich silicide. the behaviour is simlar to silicon plasma oxidation [3], but with a faster reaction rate at high temperature (at 800 0 C the R value is two or three times greater than the silicon). In this temperature regime. the RBS spectra show a change in the stoichiometry fo the silicide. Ta yields increases due to the loss of Si atoms in excess which diffuse through the silicide towards the external surface to form SiO,. If all

silicon in cxccss is dcplcted and the oxidation temperature is high enough (= 800” C), atoms coming from the substrate can diffuse through the silicide and produce SiO,. This mechanism would be similar to thermal oxidation of TaSi, [7]. In the other case, i.e. the silicide is deposited on SiO, substrate where no free silicon is prcsent. the silicon in cxccss is deplcted to form ;I SiO, layer on the surface until the TaSi? is rcachcd. As the trcatmcnt time incrcascs. the incorporated oxygen forms an underlying TaSi ,TaTSi ,-SiO, mixture according to Si-()-‘?a ternary equilibrium diagrams [Xl. For tantalum-rich silicide, in the low-tcmpernturc rcgimc the nuclear reaction “‘O(d. p) “OZiLISCC~ to mcasurc the absolute “‘0 contents rcvealed a very low oxygen incorporation. So. for oxidations at 300 and 400 0 C during 2 h the oxide thickness is below the limit of accuracy of RBS ( = IO nm). At 500 o C for 2 h the oxide grown was about 20 nm. l‘hc RBS analysis showed the formation of almost pu~‘c SiO, on the surface. This result agrees with thermodinamic predictions. If the oxygen incorporated is very low. it is most probable to form :I SiO, thin layer. Anyway. it is important to remcmhcr that the oxygen content is so low that WC arc very close to the limit ol accuracy of RBS. In the high tcmpcraturc range (550 I 7’s 700 0 c’), the RBS spectra evidenced the formation of a mixed oxide. In this cast, the amount of incorporated oxygen was much higher than in the Iaxt USC. Starting from composition obtained from depth profiles, WC can verify that it corresponds to a point in the stable lint bctwccn Ta,O, and SiOl in the ternary phase diagrams [Xl. Howcvcr, at higher oxidation tcmpcraturc (7. = 800 o c’) and if the silicidc is on a silicon substrate, the silicidc stoichiometry changes to a new value TaSi ,, (x’ - 2). This is attributed to formation of the disilicidc. At this tcmpcraturc the rapid diffusion of silicon atoms from the substrate within the silicidc layer insures a supply of “fret” silicon atoms at the silicide/oxidc intcrface for the formation of SiO, [I]. The decrcasc of the oxidation rate at X00” C with respect to the at lower tempcraturc (550 5 7‘ 5 oxidation 700 0 C) would agree with different diffusion rates of oxygen through SiO, and through l‘alOi-SiO,.

25’)

In summary, the plasma oxidation of tantalum silicides shows two different regimes as a function of temperature. The transition temperature is about 550 o C. These two regimes are characterized by different growth kinetics and the oxide nature is different in each of them. In fact, they have been observed in the RF plasma oxidation of other silicides [3]. Both thermodynamics and atomic transport are needed to understand the oxide growth on silicidcs in RF plasma environments.

References [I]

F.M.

D‘fIeurlc.

[2] L.N. Lie. W.A. (IYXJ)

This work has been supported by the CNRS (GDR 861, by The French-Spanish Integrated Actions Program and by the CSIC-MEC.

Frampton

nnd EA.

Irene,

Tiller

[3] R. Ptrez-Casero. Films lY3/lY4 [4] J. Pcrriire.

J. PerrGre

and J.P. Enard.

J. Siejka. N. Remili. J. Appl.

G. Amscl, J.P. Nadai. and J. Moulin,

[6] L.R.

Doolittle,

[7] S.P.

Murarka.

Thin

Solid

(IYYO) 627.

and B. Vuillermoz. [j]

and K.C. Saraswat. .I. Appl. Phys. 56

2127.

Sinha. J. Appl.

Acknowledgments

A. Cros. R.D.

Phil. Msg. B 55 (IYX7) 201.

A. Laurent.

E. D’Artemare.

Nucl. In\tr.

Meth.

Nucl. Instr. Meth. D.B.

Fraser.

D. David. E. Girard

Y2 (lY71)

W.S.

Phys. 56 (10X1)

4X1.

B Y (10X.5) 334. Linclenhergcr

Phy\. 51 (IYXO) 2241.

[X] R. Beycrs, J. Appl.

A. Strahoni

Phys. SY (1YKh) 2752.

147.

and K.