Improved thermal stability of cobalt silicide formed by ion beam assisted deposition on polysilicon

~:~:::..~... . . . . . . . .

..~.:.~::~,

applied surface science ELSEVIER

Applied Surface Science91 (1995) 19-23

Improved thermal stability of cobalt silicide formed by ion beam assisted deposition on polysilicon S. Ravesi a, F. La Via b,,, V. Raineri b, C. Spinella b a Physics Department, Corso Italia 57, 95129 Catania, Italy b CNR-IMETEM, Stradale Primosole 50, 95100 Catania, Italy Received 19 March 1995; accepted for publication 17 May 1995

Abstract

The thermal stability of thin cobalt silicide films obtained by ion beam assisted deposition of Co on polycrystalline Si has been studied. A large improvement has been obtained depositing Co at 470°C with an Ar + beam energy of 1000 eV: no increase of the sheet resistance was observed until 1000°C. The improvement has been connected to the stability of the CoSi2/polysilicon interface.

1. I n t r o d u c t i o n

Cobalt silicide has attracted a large interest in submicron metal-oxide-semiconductor technology, because of its superior characteristics compared to other silicides such as TiSi 2. In addition to the low resistivity (10-18 /xO- cm) and the excellent chemical stability, the dopant drive-out technique using CoSi 2 for p + / n ultra-shallow junction and for degenerately doped polycrystalline silicon gate formation has also been demonstrated [1-3]. However, one potential problem with CoSi 2 is its inferior thermal stability compared to TiSi 2 and WSi 2 in polycide structures. Thermal degradation, which is due to agglomeration of CoSi 2 films, leads to an increase in the sheet resistance of CoSi 2 and in the leakage current of silicide p / n junction diodes. It is well known that degradation of silicide is strongly af* Corresponding author.

fected by the silicide thickness and, for the same thickness, the silicide/polycrystalline Si [4] structure is more thermally unstable than the silicide/monocrystalline Si [5] structure, especially for undoped polycrystalline substrates, The degradation of CoSi 2 has been explained by thermal grooving and epitaxial growth of Si grains into CoSi2. Analysis of the grooving process at the top and bottom surface of a thin film has shown that it can lead to morphologies which may be predicted using equilibrium calculations [6]. This local equilibrium is defined by a balance of the surface (or the interface) and grain boundary energies. The resultant equilibrium morphology of a grain would be spherical caps on top and bottom with the perimeter held at the equilibrium grooving angles. These calculations have been used to determine whether a given morphology can result in agglomeration. It has been shown that agglomeration depends on the ratio of the grain size to the as-deposited film thickness and that

0169-4332//95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 5 ) 0 0 0 8 8 - 7

20

s. Ravesi et al. / A p p l i e d Surface Science 91 (1995) 19-23

a critical grain size L c for which smaller grains will prevent agglomeration, may be determined. For TiSi 2, agglomeration was not observed when the grain size was about four times the as-deposited film thickness, but was observed for a ratio of seven. For cobalt silicide no direct measurement of this critical grain size was performed. In a previous work [7] it was observed that the grain size of the - F e S i 2 can be controlled with the ion beam assisted deposition (IBAD). In particular it was shown that under an ion beam bombardment of the sample surface with an energy of 500 eV the grain dimension was reduced from 230 to 30 nm. In all these experiments the temperature of the substrate during Fe deposition was maintained at 600°C. This technique is extremely interesting to control the grain size and then to improve, according to the theories previously described, the thermal stability of the silicide. In this work a CoSi 2 thin layer has been formed on polysilicon by the ion beam assisted deposition. These layers show a large improvement in the thermal stability and several analyses were done t o explain this behavior.

The thickness of the CoSi 2 layer was 41 nm as measured by Rutherford backscattering spectroscopy (RBS). The sample was heated during the deposition by forcing a current through the Si itself, and the temperature was detected by both a thermocouple and an infrared optical pyrometer with an accuracy of 5°C. The substrate temperature was varied in the range between 470 and 600°C, The beam current density was fixed at 7 6 / z A / c m 2 and the energy was varied in the range between 0 and 1000 eV. During the IBAD process one third of the substrate was shadowed from the ion beam by a small shutter and used as a reference of the not ion beam assisted deposition (NO IBAD). The samples were then isochronally annealed for 30 s in a Heatpulse 610 Rapid Thermal Furnace at

'

'

'

'

I

'

'

'

'

I

. . . .

I

. . . .

I

'

'

'

O No ZB~D A 200 eV + 500 eV × 650 eV 0 I000 eV

6

"~4 C~

~

a) i

RDE 600 °C

2. Experimental On Si(001) wafers, 110 nm S i t 2 was grown by dry oxidation and subsequently a polysilicon layer of 130 nm was deposited by chemical vapor deposition (CVD) at 620°C. Cobalt silicide layers were prepared in a high vacuum (10 -8 Torr) evaporation system equipped with a 3 cm Kaufman-type ion source oriented at 45 ° with respect to the substrate surface; the ion beam was extracted from N50 pure Ar gas and the ion current density was measured by a retractable Faraday-cup placed near the surface of the sample. During the I B A D process the Ar partial pressure was 3 × 10 - 4 Torr. The samples were organically cleaned and dipped in aqueous 10% HF solution just before introduction into the vacuum chamber; moreover, they were cleaned in situ by 200 eV Ar + sputtering and the crystalline order was recovered by thermal annealing at 800°C. Cobalt evaporations were made by a 2 kW e - - g u n and film thicknesses were estimated by a crystal quartz micro-balance. CoSi 2 was formed by depositing Co with a rate o f 0.1 n m / s on a hot substrate.

0

*

'

'

'

i

I

r

'

'

'

I

'

'

'

'

I

'

'

700 800 900 Temperature

600

' ~ ' ' I ' ' ' ' I ....

'

'

I

I

r

I

(°C)

I ' ' ' ' I ' ' ' '

O 600 °C a 520 + 470 °C

4

I

1000 1100

b)

IBAD 1000 eV

%

0

i

600

t

i

}

700

i

r

i

i

1

1

1

,

800

Temperature

,

l

900

....

I .... 1000 1100

(°C)

Fig. 1. Normalized sheet resistance as a function of 30 s RTA annealing temperature in N2: (a) deposition at 600°C; (b) IBAD 1000 eV.

S. Ravesi et al. /Applied Surface Science 91 (1995) 19-23

(a)

No IBAD 600 °C

21

temperatures ranging from 800 to 1050°C in N 2 ambient. The sheet resistance was measured with a four point probe while the surface roughness and the film morphology were analyzed with a Nanoscope III atomic force microscope (AFM) in tapping mode and a JEOL JEM 2010 transmission electron microscopy (TEM), respectively.

5

3

K 2

3. Results and discussions

1

0

,

/zm

3

4

5

IBAD 600 °C 1000 eV

(b) I

2

,

43

K 2

0 0

(c)

1

2

/zm

IBAD 4 7 0

,3

4

°C 1 0 0 0

5 eV

3

K

Fig. la shows the normalized sheet resistance versus the annealing temperature for several samples deposited at the same temperature (600°C) but with different beam energies. In all the cases the sheet resistance remains constant up to 800°C and then, increases more for the samples deposited with a beam energy up to 500 eV. If a constant beam energy is used (1000 eV) and the deposition temperature is decreased, a larger thermal stability improvement can be observed (see Fig. lb). For the sample deposited at 470°C no increase of sheet resistance was observed until 1000°C. This result is similar to that one reported for CoSi 2 monocrystalline silicon [5], where agglomeration of grains started at 1050°C. In a previous paper [6], the main factor that influences the silicide stability has been indicated in the ratio between the average grain size and the film thickness. Following these arguments the silicide film will become discontinuous when this ratio exceeds a critical value. Then, at a given film thickness, a silicide with a smaller grain size should be more stable. This is not true in our experiment. From the AFM images shown in Fig. 2 it is possible to observe that the film with the smaller grain size is not the more stable. The silicide layer (Fig. 2a) with the lower average grain size (90 nm) has been deposited at 600°C without beam. This sample shows a poor stability, as it is possible to observe from Fig.

2

0

1

2

/~m

3

4

5

Fig. 2. AFM images of: (a) sample deposited at 600°C without the assistance of the ion beam (NO IBAD); (b) sample deposited at the same temperature of (a) and with an ion beam energy of 1000 eV; (c) sample deposited at 470°C with an ion beam energy of 1000 eV.

22

s. Ravesi et aL /Applied Surface Science 91 (1995) 19-23

la, while the ~ample (Fig. 2c) with the largest grain size (140 nm) is instead the most stable. Furthermore from the data reported in Fig. 2 it is possible to observe that the behavior of CoSi 2 is completely different with respect to that of -FeSi 2 [7] deposited by the IBAD technique. In that case the grain dimensions were reduced by increasing the ion beam energy, while, in the present case of cobalt silicide, the average grain dimension increases (Figs. 2a and 2b), In general the modifications of the films grown by IBAD with respect to the standard reactive deposition technique should be interpreted as a result of two different effects: the increase of the superficial mobility of the atoms impinging on the sample surface and the increase of the nucleation sites at the silicides/silicon interface. During the early stages of the film growth, i.e. until the film thickness is greater than the projected ion range, the silicide/silicon interface is directly hit by the ion beam. These ions produce a great amount of defects that are good sites for the nucleation of the silicide film with an increase in the rate and a decrease of the grain size. At the same time the Ar ions produce an increase of the mobility of the Co atoms impinging on the surface. These atoms can then easily reach the main nucleation sites and a larger grain size is obtained. Probably in the case of -FeSi 2 the increase in the nucleation sites during the IBAD process is predominant and the grain size is reduced. Instead the enhancement of the superficial mobility is the main process for the deposition of the CoSi 2 and an increase of the grain size has been observed. After a high temperature process the average grain size does not increase, while the surface roughness increases a small amount for both the samples shown in Fig. 2a and Fig. 2c. To measure the roughness of the silicide/polysilicon interface, the silicide layer was etched in a dilute (1 : 10) HF solution. This etch completely removes the CoSi 2 layer, as tested by RBS, and then the silicide/polysilicon interface can be measured by AFM. An example of the surface and interface roughness is shown in Fig. 3 for the NO IBAD sample after a thermal process at 950°C for 30 s. The AFM images show that the surface (Fig. 3a) roughness is much lower than the CoSi2/Si interface (Fig. 3b). On the surface the roughness is typically of about 49 nm and is larger for the IBAD samples; while, at this annealing temperature, the

interface roughness goes from 14.6 nm, for the sample shown in Fig. 3, to 12.9 nm for the sample deposited at 470°C with an Ar + beam energy of 1000 eV. At a higher annealing temperature, the surface roughness does not increase, while the CoSi2/Si interface becomes more and more rough. The total roughness of the silicide layer, i.e. the sum of the surface and interface roughness, can be also obtained from the fit of the low energy Co signal of the RBS spectra. In Fig. 4a and Fig. 4b the total roughness of the CoSi 2 is plotted versus the anneal temperature. These plots are similar to those ones of Fig. 1 and indicate that the sheet resistance increases considerably when the total roughness is larger than 50 nm. Then the ion beam assisted

Fig. 3. AFM images of the sample deposited at 600°C without the ion beam (NO IBAD) after a thermal process at 950°C for 30 s: (a) surface;(b) after HF etch of the CoSi2 layer.

S. Ravesi et al. /Applied Surface Science 91 (1995) 19-23 140 120

A + x 0

100 ~

80

200 eV

librium groove angle [6] decreases. From the Nolan model we know that if this angle decreases, the interfacial energies should become larger and then the critical grain size L c should increase [6].

/

500 eV

750 e V i 0 0 0 eV

/

RDE 600 °C

23

/

4. C o n c l u s i o n s

O

z0

_

0

. . . .

600

I

,

,

,

700

,

I

,

,

,

800"

i

I

. . . .

000

. . . .

. . . .

I

. . . .

.... i

i

0 600 °C oC A 520 °C

120

loo 80

/+

IBAD 1000 eV

6o

~ o

. . . .

b) A

+

L--.d

~o

I

. . . .

(°C)

Temperature 140

I

1000 1100

o

/

References

40

zo ~ 0

We have studied the thermal stability of thin cobalt silicide film deposited on polysilicon by ion beam assisted deposition. The use of the Ar beam produces a large increase of the thermal stability of the film and results similar to those ones reported for CoSi 2 on monocrystalline silicon have been obtained. The ion beam produces clearly an increase of the average grain size, that should induce a greater instability, but at the same time the stability of the silicide/polysilicon interface increases for annealing at high temperatures.

I

. . . .

600

I

....

700

I,,:,l

800"

Temperature

. . . .

900

I,,,

1000 I100

(°C)

Fig. 4. Roughness as a function of 30 s RTA annealing in N2: (a) deposition at 600°C; (b) IBAD 1000 eV.

deposition gives a large improvement of cobalt silicide thermal stability because it increases the stability of the CoSi2/polysilicon interface, i.e. the equi-

[1] Q. Wang; C.M. Osburn and C.A. Canovai, IEEE Trans. Electron Devices ED-39 (1992) 2 4 8 6 . [2] F. La Via, C. Spineila and E. Rimini, Semicond. Sci. Technol., to be published. [3] W.M. Cben, J. Liu, S.K. Banerjee and J.C. Lee, J. Appl. Phys. 73 (1993) 4712. [4] W.M. Cben, S.K. Banerjee and J.C. Lee, Appl. Phys. Lett. 64 (1994) 1505. [5] B.S. Chen and M.C. Cben, J. Appi. Phys. 74 (1993) 1035. [6] T.P. Nolan, R. Sinclair and R. Beyers, L Appl: Phys. 71 (1992) 720. [7] A. Terrasi, S. Ravesi, M.G. Grimaldi and C. Spinella, J. Vac. Sci. Technol. A 12 (1994) 289.