ELSEVIER 1 rhin Solid Films 270 ( 199.5) 346-350

Segregation of Cu in Cu( Ti) alloys during nitridation in NH,

Daniel Adams *, T.L. Alford, N.D. Theodore, S.W. Russell

Abstract

Cu(Ti 17 at.%) alloys on SiO, were reacted in NH, for 30 min over the temperature range 400-700 “C. Rutherford backscattering

spectrometry in conjunction with high resolution transmission electron microscopy were utilized to investigate reaction products At 400- 450 “C, Ti is observed to segregate to the free surface to react with NH,, forming an Ti oxynitride layer. Above 500 “C, Ti segregates to both

the free surface and to the alloy/SiO, interface, leaving relatively-pure Cu layer. Reaction between Ti and SiOz results in a TiO/Ti,Si, bilayer

structure. By use of high spatial resolution energy dispersive X-ray spectroscopy. the presence of a Cu-containing layer at the TiO/Ti,Si3 interface is observed. This layer may also contain Ti, Si and/or 0. We propose a mechanism for Cu segregation to this interface which requires

Cu diffusion across TiO and subsequent dissociation of Ti,Si,. Thermodynamic calculations support this mechanism.

Ke,~~‘or&: Copper; Titanium; Nitrides

1. Introduction

The miniaturization of interconnect and contact feature

sizes have become a prerequisite to obtain high packing dcvicc densities in ultra-large-scale (ULSI) technology.

However, decreasing feature sizes penalize the overall per- formance of interconnect and contact materials by increasing both interconnect resistance and current density, leading to reliability concerns due to electromigration. Therefore, matc- rials with lower resistivity, higher resistance to electromigra- tion and to stress migration are being considered to replace the currently used Al and its alloys. Copper is especially attractive, but several obstacles must be overcome to take advantage of its potential. Among these problems is the high diffusivity of Cu in many materials such as Si, silicides and SiO, under bias thermal stress [ I]. Pure copper does not

adhere well to SiO,-based dielectrics [ 21. For shallow junc- tion devices, even a very limited penetration of Cu through the silicide in the contact area can influence device perform- ance, since Cu in Si can generate deep-level states and provide excess generation-recombination centers, that lead to a reduc- tion in minority carrier lifetime [ 31. Cu segregation near the Si-SiO, interface results in a high density of interface states. Therefore, it is important to understand the thermal stability of Cu-based metallization for successful application in IC technology.

* Corresponding author.

0040.6090/95/$09.50 Q 1995 Elsevier Science S A All rights reserved

SSD10040-6090(95)06712-4

Rapid diffusion of Cu can be impeded by effective diffu-

sion barriers. It has been shown that Cu can be encapsulated

by nitridation of a Cu-refractory metal alloy on silicon diox-

ide in an ammonia (NH,) ambient [4]. Reaction with SiO, vastly improves Cu adhesion [ 51, while nitridation encap-

sulates the copper with a Ti or Cr-nitride diffusion barrier

[J 1. Rutherford backscattering spectrometry ( RBS), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) analyses used in these investigations were not sensitive enough to detect the distribution of Cu in the interfacial reaction zone. In this paper, high-resolution energy dispersive X-ray spectroscopy (EDS) was used to profile Cu in this region.

2. Experimental details

A 175 nm thickCu(Ti 27 at.%) alloy film wascodeposited by electron-beam evaporation onto 110 nm thermally grown SiO, on Si( 100) substrate. The original targets were elemen- tal Cu and Ti of 99.999% purity. Base and operating pressures of the deposition system were - lo-‘and lo-’ Torr, respec- tively. Samples were annealed for 30 min at temperatures ranging from 400-700 “C in aquartz tube furnace in a flowing electronic grade ammonia (NH,: 99.99%, with HZ0 < 33 ppm and O7 + Ar < 10 ppm) ambient. The cham- ber was purged overnight with NH, before each anneal. Flow rates of about 100 cc min- ’ were maintained during anneals.

D. Adams Ed ul. / Thin Solid Film 270 (I 995) 346-350 341

Samples were allowed to cool in flowing ammonia before

removal.

Reactions were analyzed by RBS. A4.3 MeV He+ + beam

energy was used for all analyses; the samples were tilted at

7”. Spectra were taken at a backscattering angle of - 170”. We utilized the program RUMP for simulation and interpre-

tation of the RBS spectra [ 61. High-spatial resolution chem- ical analysis was done using a Vacuum Generators-HB.501 STEM field-emission gun microscope coupled to a window-

less energy-dispersive X-ray spectrometer operating at

100 kV; the probe diameter is 1 nm and the spatial resolution

- (l-2) nm. Windowless detectors have the advantage that quantum energies as low as a few hundred eV. such as those

produced by Km quanta from light elements (C, N and 0)

can be detected. However, the signals overlap since their full- width half maximum (FWHM) is too large [7]. In this exper-

iment no attempt has been made to separate the 0 and N overlapping signals in the surface layers. The cross-section

TEM specimens used for the energy-dispersive X-ray anal-

ysis (EDS) were made by dimpling and polishing, followed by ion-milling to obtain electron-transparent regions of inter-

est.

3. Results

Fig. 1 shows the RBS spectra of the as-deposited alloy and

that after thermal annealing at 450 “C in an NH3 ambient.

Only the Ti and Cu backscattered signals are shown. Simu- lation of the RBS data of the as-deposited sample indicates a composition of Cu(Ti 27 at.%) and an alloy thickness of

175 nm, shown in Fig. 1 (a). The 4.50 “C anneal results in segregation of part of Ti in the Cu(Ti) alloy to the free

surface, as indicated by the presence of the peak labeled as ‘surface Ti’ in Fig. 1 (b). Titanium remaining in the alloy is labeled ‘unreacted Ti’. Simulation of the RBS data obtained

from the 450 “C anneal, indicates a composition of - Cu( Ti 15 at.%) in the near surface region. No accumulation of Ti

at the Cu(Ti) /SiO, interface is observed at this temperature. In Fig. 2 the RBS spectrum obtained from the sample

annealed at 550 “C, is compared with that at 650 “C. Fig. 2(a)

shows that the sample annealed at 550 “C has two separate Ti signals, resulting from the diffusion of Ti to both the free surface and to the alloy/SiO, interface. At the surface the Ti reacts with the NH3 ambient to form a Ti oxynitride (labeled TiN( 0) ); while at the interface (labeled ‘interfacial Ti’), it reacts with SiO,. Fig. 2(b) indicates that features of the spec- trum of the sample annealed at 650 “C are similar to the

spectrum at 550 “C. Although the higher surface Ti signal indicates more Ti at the free surface at 650 “C than at 550 “C, it is still less than the corresponding amount of Ti at the interface. The thickness of the TiN( 0) layer is - 52 nm at 650 “C.

High resolution depth profiling by EDS was used to sup- plement RBS analysis in establishing the segregation of Cu during nitridation. Fig. 3 shows EDS line scans obtained

As deposited

I I I I (0) 4.3 t&V ti2+

Cl.

Ti

J \ I I I I I I

45O”C, 30 min in NH,

1 I I I

(b)

surface Ti

Unreacted Ti I

/ \ I I I I I I Channel

Fig. 1. RBS spectra of (a) as-deposited Cu( Ti 27 at.%) alloy, and (b) alloy

annealed at 450 “C for 30 min in NH>. Only the Ti and Cu backscattered

signals are shown. A 4.3 MeV He+’ beam energy was used.

550°C. 30 min in NH,

(a)

Interfacial cu Ti

J \

650°C, 30 min in NH,

[Ill)

Channel

Fig. 2. RBS spectraof Cu(Ti) alloy nitrided for 30 min in NH, at (a) 550 “C.

and (b) 650°C. Only fhe Ti and Cu backscattered signals are shown. A

4.3 MeV He+? beam energy was used.

TiN(0) Cu(Ti) SiO ? Si

* ,Titonium 450°c

2lio 360 460

Distance (nm)

Fig. 3. EDS line scans obtained across the Cu(Ti) structure annealed at

450 “C for 30 min in NH{ ambient. Signals due to Cu. Ti, Si, and 0 are shown.

34x

CU(TI,SI,O) I ’ 0

TiN(0) CU TIO 5 50, Si +

0 160 200 300 400 560

Distance (nm)

EDS line scans obtnmed across the Cu(Ti) structure annealed ;II

for XI rnin In NH, ambient Signals due 10 Cu, Ti, Si and 0 are

across the cross-section sample annealed at 450 “C. Cu, Ti,

Si and 0 signals are shown. The Ti and Cu lint scans clearly

show the dealloying of the titanium from the original alloy.

A Ti-depleted region within the alloy just below the surface

is exhibited, wherein the Ti signal decreases and the Cu signal

increases. The Si scan shows Si in the substrate and SiO?. A

surface 0 peak confirms the presence of oxygen in the surface

layer. Fig. 4 presents EDS line scans obtained across the

reacted structure annealed at 650 “C. The Cu. Ti, Si and 0 signals differ significantly from those presented in Fig. 3. The

Ti lint scan shows a surface peak and two interfacial peaks.

Comparison with the 0 scan indicates an oxygen peak located

at the peak position of the first interfacial Ti-containing layer.

farther away from the SiO? substrate, suggesting that this

layer is a Ti oxide. The peak corresponding lo the second

interfacial Ti layer (closer to the Si02 substrate) has a cor-

responding Si signal increase, suggesting that this layer is a

Ti-silicidc which contains some oxygen. Apart from the

broad Cu signal of the dealloyed layer, an additional small

Cu peak is observed at the Ti-silicide/Ti-oxide interface. This

Cu peak corresponds to a minimum in the Ti and 0 signals. This indicates that an intermixed Cu layer which may contain

Ti. Si and/or 0, is sandwiched between Ti-oxide and Ti-

silicide. EDS data further shows a small shoulder on the

surface oxygen peak. indicating the presence of nitrogen. However, the poor resolution of EDS for low Z elements makes it difficult to quantify nitrogen. The presence of nitro- gen in this layer was verified using ion resonance in a previous

paper [gl. Only at and above 700 “C could evidence of the interfacial

Cu be seen by RBS. Fig. 5 is an RBS spectrum of Cu(Ti) / SiO? annealed at 700 “C. A small step to the left of the major Cu peak (labeled ‘Dealloycd Cu’) is due to Cu at the Ti- oxide/Ti-silicide interface. The signal contribution from interfacial Cu is shaded on the figure. At 700 “C, 1.8 X 10” Cu cm-’ is present in the interfacial layer.

4. Discussion

Annealing of Cu(Ti)/SiO, in NH, in the temperature

range 400-700 “C results in segregation of Ti out of the Cu

to both the free surface and to the alloy/SiOl interface. At the surface, Ti reacts with nitrogen dissociated from NH3 and ambient 0, to form a Ti oxynitridc layer. The large negative

heat of formation of TiO, (242.5 kcal g-mol- ’ at 900 K) compared to that of TIN (9 I .4 kcal g-molt ’ ) is compensated

by the high partial pressure of NH, relative to that of Oz [ 91,

leading to nitride rather than oxide formation. RBS and EDS data indicate significant out-diffusion of Ti to the alloy/SiO, interface above 500 “C. The interfacial reaction starts with

the dissociation of SiO, in the presence of Ti according to:

1lTi + 3Si0, --) Ti,Si, + 6TiO (1)

Released 0 and Si react with Ti to form a bilayer structure at

the alloy/SiOz interface. Electron diffraction analysis (not included) revealed that the reaction results in formation of a

TiO/Ti,Si,/SiO, stack at temperatures in the range of 500- 700 “C. This is consistent with those previous reported for

the reaction of pure Ti on SiOl ] 10,l I].

EDS results further revealed the presence of an intermixed Cu layer located atthc TiO/Ti,Si, interface. The well-defined nonzero minimum in the Ti and 0 signals of the EDS data as

well as the broad Si tail suggest the presence of Ti, Si and/ or 0 in this intermixed layer. Signal overlap from adjacent

regions may also be a contributing factor. Samples exhibit the configurations TiN( 0) /Cu(Ti) /SiO?/Si between 400-

450°C and TiN(O)/Cu/TiO/Cu(Ti.Si,O)/Ti,Si,/SiO,/Si above 500 “C, shown schematically in Fig. 6.

The presence of a Cu-containing layer at the TiO/Ti,Si,

interface may be explained by diffusion of Cu across the TiO layer to react with Ti,Si,. Reactions between Cu and Ti,Si,

have not been previously studied. A recent study by Hong and Mayer [ 121 revealed diffusion of Cu through TiSi, to

Energy (MeV)

2.8 3.0 32 3.4

16, , 1 I I I

550 600

Channel

Fig. 5. RBS spectrum of Cu(Ti) alloy after annealing at 700 “C for 30 min

in NH, ambient Only the Cu and Ti bsckscattered signals are shown. A

4.3 MeV He+’ beam energy was used.

T = 400-45O’C

1 TIN(O)

m

Si SiO, Cu(Ti) :I

Ti

TG?

I

5oooc

TIN(O)

N

m

Si $ CU

Ti,Si,

CU(T’ S’ o;io 1, 1,

Fig. 6. Schematic showmg the evolution of the multilayer structure after

annealing the Cu(Ti I /SIO,/Si system for 30 min in NH, ambient.

react with the underlying Si substrate, forming Cu,Si. No

evidence of TiSi, decomposition nor of Cu-Ti intermetallic

formation were observed. These results were explained by

thermodynamic calculations for binary intermetallic alloys

using Miedema’s model [ 131, which predict stability between Cu and TiSi, but reaction between Cu and Si. If we

employ similar calculations, we generate the Cu-Ti-Si ter- nary phase diagram, as shown in Fig. 7. We find that while

stable tic lines exist between Cu and TiSi, and between Cu

and TiSi, none exist between Cu and more Ti-rich silicides. Therefore. reaction between Cu and Ti,Si, to form both a

Cu-Ti intermetallic and either TiSi or Ti,Si, is expected,

whereas the formation of TiSi, or any other Cu-silicides is not. We predict the following reaction between Cu and Ti,Si,:

2OCu + 12Ti,Si, + 9Ti$i, + SCu,Ti, (2)

The enthalpy change for (2) is - 19.9 kJ mole-G_ [ 131, implying that this reaction is thermodynamically favorable. Unlike TiSi?, TisSi,3 is not stable in contact with Cu, and at high enough temperatures a reaction should occur. Other reactions exist between Cu and Ti,Si,, but the products of reaction (2) are in turn stable in contact with the remaining

unreacted Ti,S&, as is evidenced by the Cu,Ti,-Ti,Si,-Ti,Si, three phase region which exists on the phase diagram of

Fig. 7. As a result, we propose (2) as a mechanism for Cu segregation to the TiO/Ti,Si, interface. Reactions of Cu with TiO arc unfavorable, which explains the lack of reaction at the Cu/TiO interface.

Since Cu is unable to reduce TiO or SiOz, one may omit oxygen and consider this reaction in terms of a quasi-ternary phase diagram. Solid lines indicate tie lines between phases which are stable in mutual contact. Cu adjacent to Ti,Si3 is unstable, so contact between them is represented by a dashed line. Dissociation of Ti,Si, by a small amount of Cu by (2) results in three phase equilibrium between Ti,Si,, Ti,S& and Cu.,Ti,. If additional Cu diffused across TiO, Ti,Si, could be completely consumed. and other reaction products would

Ti Ti,Si, Ti,Si,TiSi liSi, Si

Fig. 7. Calculated Cu-Ti-SI ternary phase diagram relevant to the segrega-

tion of Cu to the TiO/Ti,Si, interface via reaction to form Ti,Si, nnd Cu,Ti 1 as per reaction ( ?),

appear. The reaction is apparently self-limiting, in that both

Cu and unreacted T&S& remain within the film, separated by

the TiO layer. We propose that the initial stage of SiO,

decomposition via reaction ( 1) results in formation of a TiO

layer which is too thin to prevent diffusion of Cu to the

underlying Ti,Si,, resulting in reaction (2). As additional Ti

reacts with SiO,, the TiO layer thickens, and Cu diffusion slows accordingly. Thus, a sufficient flux of Cu through TiO

to supply reaction (2) is only available during the earliest

stages of reaction ( 1). The total amount of Cu at the TiO/

Ti,Si, interface increases slightly with increasing anneal tem-

perature, but clearly TiO is a viable diffusion barrier to Cu in

the temperature range studied.

No Cu diffusion through the silicide was detected at these

temperatures. The absence of Cu diffusion through the Ti,Si,

may be due to two factors. First, diffusion may be inhibited

by dissolved oxygen in the silicide layer. A high solubility of

oxygen of - 9 at.% in Ti,Si, has been reported [ II]. Sec-

ondly, SiO, provides no driving force for Cu diffusion across

the silicide. compared to Si in the Cu/TiSi,/Si structure.

5. Conclusions

Nitridation of CutTi 27 at.%) /SiO,/Si at temperatures of 400-700 “C in NH, for 30 min resulted in diffusion of Ti to

both the free surface and alloy/SiO, interface, leaving behind

an almost pure Cu layer. Ti segregated to the surface reacted with NH, to form a Ti oxynitride. Above 500 “C, Ti reacted with Si02 at the alloy/SiO, interface to form a TiO/Ti,Si, structure. RBS and EDS data revealed a thin intermixed Cu- containing layer at the TiO/Ti,Si, interface. Thermodynamic calculations were used to propose a reaction between Cu and

Ti,Si, as the mechanism for Cu segregation to the TiO/Ti,Si, interface.

350

Acknowledgements

The authors would like to thank Dr. Moon Kim (Center

for Solid State Science, ASU, AZ) and Dr. Stella Hong

(Motorola, Mesa, AZ) for their fruitful discussions and con-

tributions. A special thank you to Dr. J. Liu (Center for Solid

State Science, ASU, AZ) for assisting with the EDS analysis.

We also acknowledge the financial support provided by the

National Science Foundation (L. Hess, DMR-9307662 and L. Salmon, ECS-9410399).

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