Study of the diffusion path during the lateral growth in the salicide process

Applied Surface North-Holland

Science

53 (1991) 391-305

applied

surface

science

Study of the diffusion path during the lateral growth in the Salicide process M. Bakli ‘, G. Giiltz, S. Caranhac

and G. Bomchil

CNE7; BP 98, 38243 Meylan Cedex, France Received

24 March

1991; accepted

for publication

25 April

1991

Self-aligned formation of silicides allows selective formation of MSI, on poly- or mono-crystalline Si and is typically used for gate, source and drain metallization. Under certain conditions lateral growth of a parasitic layer is observed during silicide formation. For example, TiSi, formation under Ar atmosphere or in a vacuum results in a thin conductive layer on SiO,, which can expand over several microns in the neighbourhood of Si. will be presented. Different conditions have been In this paper results of experiments with WSi, and TiSiz formation investigated with respect to lateral growth: anneal under vacuum, Ar, Nz or different dilutions of NH,, or deposition of an encapsulation layer on top of the metal prior to the anneal. Results of both silicides will be compared. Based on these experiments a model will be proposed for the fast diffusion which causes the lateral growth.

1. Introduction

Self-aligned formation of silicides (“Salicide”) is a well-known process [l-3]. It allows selective formation of MSi, on poly-Si or monocrystalline Si and is typically used for gate, source and drain metallization of MOS transistors. For such a technology a critical issue remains the control of lateral growth during self-aligned formation of silicides, which is required for the electrical insulation The “Salicide” process requires SiO, spacers and the typical distance in submicron MOS structures beJween gate and source and drain is 1000-3000 A. A large number of publications is available on the formation of silicides, mostly on TiSi, and CoSi,. Formation of TiSi, under vacuum or in an inert gas atmosphere results in a lateral growth of a thin conductive layer over distances up to several microns; this parasitic growth disappears completely if this anneal is performed under nitrogen and this feature

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Potential Diffusion Path

Si Fig. 1. Possible

diffusion

path of Si during

lateral

growth

is used in production. Surprisingly, to our knowledge there is no specific experiment or model which explains consistently this lateral growth. For WSi, and TiSi, the main diffusing species during silicide formation is Si and it will also play the major role in lateral growth. But there remain different possibilities as shown in fig. 1. During anneal under N, one part of Ti is transformed into a TIN layer (1.50-250 A thick) on top of TiSi,. TiN is well known for its barrier properties, but the role it plays in the control of lateral

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growth has not been elucidated up to now. A model should also take into account other approaches: it is also reported in the literature that ion-beam mixing of the metal/% interface prior to silicide formation eliminates lateral growth [4-

61. In this paper results of experiments with WSi, and TiSi, formation will be presented. The formation of WSi, is more sensitive to the presence of impurities than TiSi, and a specific process has been developed for this application [7]. Also the formation of WN is more difficult to obtain, the presence of N2 during WSi, formation is not sufficient and a more reactive agent such as NH, is required. Different conditions have been investigated for both silicides with respect to lateral growth: anneal under vacuum, Ar, Nz or different dilutions of NH,, or deposition of a capping layer on top of the metal prior to the anneal (fig. 2). Except for the vacuum anneal, which has been performed in a classical vacuum furnace, the wafers have been processed in an RTP machine. Also, the first steps of the reaction (fig. 3) have been investigated and results of both silicides will be compared.

2. Results

Fig. 2. MOS

transistor

with self-aligned

WSI,.

using a ‘LN

capping layer formed thermally.

2.1. Study of the role of TiN TiN has been deposited on top of Ti on patterned wafers (Si versus SiO,). The subsequent anneal ( I 700 o C, - 20 s) for the formation of TiSiz has been performed under Ar atmosphere in order to avoid additional growth of TIN. The lateral growth has been studied as a function of the thickness of TiN. The minimum thickness of TiN investigated was - 50 A. The lateral growth disappeared for thicknesses 2 1.50 A. Independent of the thickness of TiN the Ti layer is almost completely transformed into TiSi,. The composition of TiSi.,N,O, near the surface has been investigated in more detail by Rutherford backscattering (RBS). The results show a strong correlation between the Si concentration nearby the surface and the thickness of the TIN layer deposited. Similar experiments have been per-

formed with TiN on top of W and the results arc consistent with those obtained with Ti as shown in fig. 4. 2.2. Study of the self-aligned fortmtior~ of WSi, influence of NH., In analogy to the role of TIN in the formation of TiSi?, WN is supposed to play a role in the control of lateral growth during the formation of WSi?. As already mentioned. the formation ot WN requires a strong agent for nitridation as NH,. In addition, a good control of the NH, concentration is required: too large conccntrations will prohibit any formation of silicidcs, concentrations < 1000 ppm are required for WSi, formation. Nearby this upper limit the lateral growth is well-controlled, but the process window

M. Bukli rr ul. / The diffusion path during the lu&mI growth in the Salicidr procm

Fig. 3. First steps of an annealed selective withdrawal

of W on a patterned

of the unreacted

(Si/SiOz)

P g s:(u

0,4

I

I Towards better Control of lateral growth

-

x go,3G’t

\

i

o,o !

0

I

200

Thickness Fig. 4. Self-aligned structure

formation

1

I

100

consists of a patterned

300

of TiN (8

of WSi,; ware,

of lateral

before

anneal

(Si and SiO,)

subsequent layers of W ( b 1000 A) and TiN on top.

the with

growth) bt edge.

growth, annealed under Ar. Whereas in the last case a homogeneous composition with depth of WSi,., is observed, the first layer presents a strong gradient ranging from WSi,, x = 3.3 nearby the interface to x = 0.9 near the surface. The concentration of nitrogen in the WSi, is still very low; the total amount of 5 X 10” cmp2. It seems that the nitrogen is distributed in the grain boundaries and plays the role of a barrier. 2.3. Analysis

&Z = al 0,2Z’ I-& c o,t .C

x

under vacuum (typical situation

metal. Note the presence of a cloudy region near the silicide/SiOz

is narrow and at lower concentrations the control is lost. Also, NH, must be present from the beginning of the thermal anneal, adding large concentrations of NH, afterwards will not improve the control. RBS analysis has been performed for a layer without lateral growth and for comparison - also for a layer with lateral

0.5

wafer

3Y3

of the

laterally grown zone

Deliberately conditions have been chosen where lateral growth is very strong: WSi2 is formed by vacuum anneal. Exceptionally, this anneal has been performed in a classical vacuum furnace (740 “C, 10 min) with a vacuum in the lo-’ mbar range. After the anneal a cloudy region is observed (similar to fig. 3) several microns large surrounding the areas with Si underneath W prior to rhe anneal; this result is similar to those obtained with Ar and also with TiSi,. Analysis has been performed by SIMS just after the

Fig. 5. Ion-beam

images (two-dimensional

consists of II patterned

wafer (Si and SiO,)

SIMS) with

showing the presence of Si’ vacuum in the IO-’

anneal, the unreacted metal layer on top of SiOz has not yet been withdrawn. Fig. 5 shows the two-dimensional SIMS signal of Si’, SiO; and OP. the bright areas indicate the presence of the spe:ies. All three are present on top of Si/WSi, and in the cloudy area. The presence of Si in the cloudy area is consistent with the well-established fact that Si is the dominant species for WSiz and - more important - this proves that the diffusion path of Si is at the surface of the metal. In contact with air most of the Si layer has been transformed into SiOz. After an erosion of - 100 A the cloudy region disappeared and the SIMS signal of Si+ is confined to the areas of Si/WSi,. This indicates that the top layer is still rich in Si; more Si than required for silicide formation has diffused to the surface.

3. Discussion

(left), 0

(middle)

and SiO;

- 1000 A of W on top. the anneal has been performed

and conclusion

Conditions of lateral growth during self-aligned formation of silicides have been studied for both Ti and W. With respect to lateral growth both metal show a similar behaviour: the presence of nitrogen from the beginning of the reaction re-

(right). The structure

for IO min at 740 o c‘ under

mbar range.

duces or suppresses lateral growth. Choosing voluntarily conditions where lateral growth occurs we observe a Si layer on top of the metal on areas several microns away from the next underlying Si supply. In contact with air this layer tra!sforms into a type of SiO, layer often > 100 A thick. The Ti lateral growth during TiSi, formation disappears for anneals under N, or with a TIN capping layer, for W the presence of NH, in the annealing atmosphere is required. The study 01 the TIN capping layer reveals an interesting feature: with increasing thickness of this layer control of lateral growth is improved and at the same time the Si concentration nearby the TiN/TiSi: interface decreases. This is a strong indication for a fast Si diffusion path along the sulfate of the silicide. In a similar way the control of lateral growth during WSi, formation is improved with increasing NH, concentration and again it is observed that the Si concentration at the surface decreases. This is also in agreement with results reported in the literature [S-IO] comparing formation under Ar and N,: they observe a lower Si concentration in the latter case. Also they report that at the very beginning of the silicide formation Si can reach the surface of the metal. The

authors of the paper on ion-beam-mixing as a technique to control lateral growth [I I] observe the same phenomenon: comparing samples with and without mixing they report a lower concentration of Si for the latter. Consequently, our model for the explanation of the lateral growth assumes first a rapid diffusion of Si towards the surface of the metal from the very beginning of the silicide formation. Secondly, a rapid diffusion along the surface of the metal over several microns can take place during the formation of less than N 1000 A of silicides. Within our model two way exist to control the lateral growth: either to limit the Si diffusion towards the surface, this is probably the case for WSi2 formation under NH,, but an interaction with the silicide formation is possible; or to “stuff” the fast horizontal diffusion path of Si; we assume that this is the case for the TiN capping layer; in addition a modification of the surface energy may reduce the attraction force of Si towards the surface of the metal. This model has been verified by a two-dimensional SIMS analysis of W layers on patterned wafers annealed under vacuum. Effectively, a thin Si layer (< 100 A) is found on top of W in the area where after the selective withdrawal of the unreacted metal lateral growth of silicides will be observed. This SIMS analysis together with the effect of the capping layer confirms the fast diffusion path of Si along the surface of the metal.

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