Si multilayers

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+,,

+

ELSEVIER

Thin Solid Films 289 (1996) 180-183

Surfaces,Interfacesand ColloidalBehaviour Reinvestigation of the first nucleated phase in Nb/Si multilayers Ming Zhang a, Wen Yu ", W.H. Wang a, W.K. Wang b,c i Institute of Physics, Academia $inica, Beijing 100080, People's Republic of China b International Centerfor Materials Physics, Academia $inica, Shenyang 110015, People's Republic of China c Institute of Physics, Academia Sinica, Beijing 100080, People's Republic of China Received i September 1995; accepted 17 April 1996

Abstract The first nucleated phase in compositionallymodulatedNb/Si multilayerswas identifiedusing transmissionelectron microscopy (TEM) and X-ray diffraction (XRD). A cubic NbjSi phase was found to be formed when the Nb/Si multilayers were deposited at 200 °C and 300 °C. The formationof the crystallinesilicide at such low temperaturesis explainedon the basis of theoriesof the effective heat of formation and classical nucleation. Keyworda: Mulfilayers;Depositionprocess;X-raydiffraction;Transmissionelectronmicroscopy(TEM)

L Introduction Silicide has attracted much attention in recent years for practical applications in microelectronics devices as well as for fundamental research [ 1]. Nucleation of the first phase is one of the prominent problems in the interracial reactions of metal-silicon thin films. Although there exist other compounds in the equilibrium phase diagram, only the NbSi2 phase is reported so far to be formed by the reaction of thin film Nb/Si systems [2]. Recently, it was found that Nb3Si as the first-nucleated phase forms in the Ni/Nb/Si [3], Nb/ Cu/Si, Cu/Nb/Si [4] and crystalline Nb/Si thin film systems [5] during annealing at 540, 400, 500 and 500 °C respectively. However, the exact mechanism of thin film reaction is not clear yet. In order to predict ~heprimary phase of crystallinesilicides, different criteria have been proposed. Some, which do not use the thermodynamic data directly, rely heavily on phase diagram information such as the lowest eutectic temperature, phase stability and diffusion species [6-8]. Other criteriause the concept of the effective heat of formation, so that thermodynamic data can be used directly to predict the first phase formation [9,10]. At present, almost all the criteria predict that NbSi2 is the first nucleated phase, in Nb-Si systems. In this study, the phase formation in Nb/Si multilayers deposited at different temperatures was investigated. Both the

kineticfactorand the thermodynamic drivingforce (heatof formation) are integrated to interpret the formation of the 0040~901%1515.00 © 1996 El~vier Scie~e S A All rtgl~s mmexvcd PH $ 0 0 4 0 - 6 0 9 0 ( 9 6 } 0 8 8 9 7 - 9

first crystalline phase in Nb/Si multilayers deposited onto the substrates at different temperatures.

2. Experimental details The multilayer specimens were prepared by ion beam sputtering deposition with alternating elemental targets in a high vacuum chamber, The base pressure of the chamber was about 1 × 104 Pa. 99,999% Si and 99.99% Nb were used as sputtered elemental targets. Samples for X-ray diffraction (XRD) were deposited onto single-crystal Si wafers; transmission electron microscopy (TEM) samples were deposited onto freshly cleaved NaCi slices, The overall thickness of the .mltilayers for XRD measurement was about 0,8 p,m. The thickness of the TEM specimens was less than 50 nm so that the electrons could penetrate them. The selected weight ratio of Nb to Si was 3:1, All TEM observations were carried out in a Hg000NAR transmission electron microscope operating at 300 kV. The XRD was performed on a Rigakku PSPC/ MDG diffractometer with Cu Kot radiation.

3. Results and discussion Fig. I (a) shows a TEM diffraction pattern of an Nb/Si multilayer deposited at room temperature with the modulation period L ~ 4 nm. As shown in Fig. l (a), only a typical amorphous halo is observed which indicates no crystalline

M. Zlmng et al. /Thin Solid Films 289 (1996) 180-183

|8|

Fig. 1. TEl~imagesof Nb/Simuldlayers withL=4 m : (a) d c t ~ o n at 25 +C, (b) depo~doa at 200 "(~o(c) ~ 300°C for2 h.

Nb silicide has formed. It is conceivable that amorphous Nb silicide has formed at the interfaces between amorphous Nb and amorphous Si during the deposition process because of the large negative heat of formation in the Nb/Si system. These results can also be obtained from the cross-sectioual TEM image. Fig. 1(b) and (c) shows TEM patterns of Nb/Si mnltilayers with L = 4 nm deposited at 200 °C and 300 °(2 respectively. When the deposition temperature increases up to 200 °C, instead of an amorphous Nb silicide, crystalline Nb silicide is formed. "lhe l~b siticide for,t~d is identifiedas the Cubic ]qb3Si phase with CusAu structure. All of the diffraction rings match w,:!l with the diffraction patterns of the cubic Nb3Siphase. When the deposition temperaturereaches 300 °C, the crystalline. Nb silicide crystallizes more easily than at 200 °C. The compound silicide formed is also identiffed as cubic Nb3Siphase. Fig. 1(d) shows the TEM pattern of annealed Nb/Si multilayer deposited at room temperature; the annealing temperature and annealing tin~ were 300 °C and 2 h respectively. The selectionelectron diffractionpattern does not differ obviously from that of the multilayers deposited at room temperature. No diffraction rings of crystalline Nb silicide phase can be detected. Fig. 2 shows the XRD

~ 3(X}°C, (d) ~

of (a)

pattern of the l ~ / S i multilayer deposited at 200 °(2withL = 4 nm. It also i ~ i ~ t e s that the crys~llim silici~ I ~ $ i has formed during the deposition Ia'ocess at 200 ~ . In order to understand our results, a memstab~froe-enersy diagram of the Nb/Si system at the reaction ~mperatta'e TR= 500 K was obtained using the schemes proposed by Schwarz and Johnson [ 11] and Miedema and cow~rkers

i

~.

F~. 2. X-ray~ wi~h L =4 emm~

~

,

~

~

,

~

~

,0

of Nb/Sim~Jayer ~Vositcdat 200 ~C

mt%~,ls due to the Nb;Si p ~

~ shown.

182

M. Zhang et al. /Thin Solid Films 289 f 1996) 180-183

10

mSi + c-No

o ............ ~ -~o

40 -50

l~Si; I 20

I

40

T~500k

NbSi2

i

60

810

100

We at. % Si Si Fig. 3. Gibbs flee-energydiagramfor the Nb/Si binary system. AGe and AG~ agethe drivingforceforthe formationof an amorphousand a crystallinesilicidephase respectively. [ 12]. Fig. 3 shows the curve of free energy vs. composition in the Nb/Si system. Phase selection in the interfacial reactions is controlled by kinetic constraints because the difference in the driving force between the formation of amorphous and crystalline silicide (AGad and AGxd) is not substantially large. According to classical nucleation theory, the activation threshold AG* for the formation of a critical nucleus is given by [ 1 3 ] . 16¢ro"3 AG = ' 3 ( - - ~ " 2

( 1)

where o" is the interfacial energy, and AG is the driving force for the formation of an amorphous or crystalline Nb silicide phase. A lower interfacial free energy means a smaller nucleation barrier for silicide formation. Although the exact values of interfacial energy in the Nb/Si system are not known, the interfacial energy for various interfaces can be estimated to be a fraction of the high-angle grain-boundary energy Ee (a typical high-angle grain-boundary energy En is about 0.5 eV atom- !). The energy of a solid-liquid interface is about 0.5En, an incoherent solid-solid interfacial energy is about 0.9E,. The interfacial energy of a liquid-liquid interface can be assumed to approach zero [ 14]. Here any amorphous phase is treated as an undercooled liquid phase and the configurational freezing that occurs as the glass transition temperature is approached is ignored. When an N'b monolayer is deposited onto a-Si, or an Si monolayer is deposited onto alqb, the activation energy barriers for the formation of amorphous and crystalline silicide compound phases are zero and 0.5En respectively. Although amorphous silicide forms normally in Nb/Si multilayers deposited at room temperature, the crystalline silicide phase may be formed at higher deposition temperature because of the small kinetic barrier. However, because the crystallization temperature of amorphous silicide at the interface between a-Nb and a-Si is very high (above 500 °C), the forming crystalline silicide must over-

come a large activation energy barrier (the crystallization of a-Nb is neglected). So even if the multilayer is annealed at the temperature of 300 °C, no crystalline silicide phase is formed. A lower interfacial free energy means a smaller nucleation barrier for the formation of crystalline silicide. The lower the temperature of the eutectic is, the lower is the interfacial free energy [6]. However, one can see from the phase diagram [ 15] that the difference between the temperatures of the eutectic points near 19 at.% Si (1880 °C) and 58 at.% Si (1850 °C) is so small that it can be neglected. Because the concentration at the interface (50:50 at.% of the elements) needs to change to the concentration of the eutectic that is closest to the center of the phase diagram given in atomic per cent [6], the lowest eutectic point near 96 at.% Si will not affect the first phase formation; in other words, the Si cannot supply so many Si atoms to react with Nb at low temperature. Thus the effect of the interfacial free energy for predicting the first crystalline phase formation need not be considered under this condition. The mobility of atoms during the growth of crystalline silicide interlayers should be considered to play an important role. Si was determined to be the dominant diffusing species during silicide formation at low temperature [ 16]. For solid state reaction in metal-metal systems, one species with much smaller atomic size often diffuses fast in the other species. However, in the present study, the difference in the atomic size between Nb (0.146 nm) and Si (0.132 nm) is very small. The mobility of Si diffusion is likely to be rather low although it is high enough to sustain reactions [ 2]. Despite an average composition of 50 at.% Si at the given reaction interfaces, some Si atoms do not contribute to form the crystalline silicide because of the low mobility of Si atoms. Hence, the true thermodynamic driving force for such reactions is not the heat of formation AH calculated by Miedema's method; the heat of formation should be modified, and the effective heat of formation AH* can be defined as [171 (2)

AH*=AHk

where k is the ratio of the effective concentration of Si atoms to the compound concentration of Si in the silicide. In order to explain this more clearly, here the effective concentration of Si atoms is assumed to be 20 at.%. The calculated effective heat of formation of Nb/Si multilayers is shown in Table 1. From Table 1, one can see that Nb3Si has a lower heat of formation than NbsSi 3 or NbSi2. So Nb3Si is the preferred phase to form during deposition at 200 °C and 300 °C. The Table l Effectiveheatof formationAH* forthe Nb-Sisystemat an effectivesilicon concentrationof 20 at.% Si (at.%)

AH (kJ tool-j)

AH* (kJ mol-j)

66.67 37.5 25

- 37.8 - 38.6 - 30.4

I 1.34 20.58 24.32

M. Zhang e! al. /Thin Solid Films 289 (1996) 180-183

reason why the NbSi2 phase is considered as the first crystalline silicide phase in a previous study is that the amorphous interlayer formed during room temperature deposition crystallizes at a very high annealing temperature at which the mobility of 3i is quite high. 4, Conclusion The crystalline cubic Nb3Si is observed to be the first phase formed in Nb/Si multilayers during deposition only at 200 °C and 300 °C. It is mainly because of the kinetic factor that the crystalline silicide phase forms at such low temperatures. Using the concept of the effective heat of formation, the initial phase formation can be explained for Nb/Si multilayers deposited at 200 °C and 300 °C. The Nb3Si phase, with the largest negative effective beat of formation, is the preferred phase formed at the interfaces ~.f Nb/Si multilayers. At low temperature the atomic mobility of Si plays an important role in determining the formation of the first crystalline silicide phase in Nb/Si multilayers.

183

References

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Acknowledgements

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The work was supported by the National Natural Science Foundation of China. We thank Mr J. Zhang for his help with the experiments.

[16] J.E.E. Baglin, F.M. d'Hma~, W.N. ~ a n d S, P~crs.zon,NucL Instrum.Methods, 168 (1980) 491. [ 17] R. Pretofius,R. de Reus, A.M. Vn:danbe~ and F.W. Sxia, J. AppL Phys.. 70(1991) 3636.