Short Notes K13 1

phys. stat. sol. (a)% K131 (1985) Subject classification: 11; 21 Electrical Engineering, Hosei University, Tokyo 1)

Arsenic Implantation in CVD Tungsten Silicide

BY T. HARA, H. TAKAHASHI, and S.-C. CHEN

Introduction Molybdenum and tungsten silicides have been o r will be exten- sively used in gate electrodes and interconnections in high speed and high packing density LSI’s because of their low resistivity. Interfacial reaction of silicide has been studied by several authors /1 to 3/. When these materials a r e employed to gate and ohmic electrodes, impurity concentration profiles in these layers must be measured for process design. But few papers have reported these data /4/.

AES meausrement of As implantation in sputtered silicide through the sur- face coated CVD oxide has been reported /5/. Significant redistribution of im- planted P with annealing has been observed in sputtered tungsten silicide /6/. However, detailed features of impurity concentration profiles have not been observed.

4

When ion implantation is done in sputtered refractory metals, such as W, much deeper impurity profiles than theoretical ones have been observed /7/.

In this material, channeling of implanted species is dominant and the masking effect is insufficient for the use in self-sligned implantation process.

Projected ranges for ion implantation in M&iZ and TaSiZ have been studied by Chow et al. / 8 / . However, these for tungsten and tungsten silicides with different compositions employed in gate electrodes have not yet been reported. Recently, high quality non-stoichiometry tungsten silicide layers have been de-

posited by low pressure chemical vapor deposition (LFCVD) technique /9/. Layer properties and interfacial reaction in a silicide-polycide M06 gate structure have been studied /1 O/.

This note describes impurity concentration profiles of As ion implanted in CVD tungsten silicides. Detailed features of the channeling effect in this profile have been reported elsewhere /ll/.

1) Kajinocho Koganei, Tokyo 184, Japan.

physica status solidi (a) 88

Projected ranges of P, As, and B implantatiqns in MoSi2, TiSi2, and TaSi2 have been calculated by Chow et al. / 8 / . However, these values for tungsten and tungsten silicides with different compositions have not yet been studied. In CVD tungsten silicide MOS gates, non-stoichiometric

silicide, for instance WSi2. layer, has been employed. Since a self-aligned implantation process has been used in this process, ion implantation data for these silicides are needed for process design.

K132

Theoretical background

In this study, projected range, R and projected strzyggling, A R for As

ion implantation in W, WSi2. o, and WSi2.6 were calculated by the interpolation program based on Brice /12/, where densities of 19.3, 9.8, and 6.5 were used for W, WSiz. o, and WSi2. 6, respectively. The calculated projected

range, R and projected straggling, AR are drawn by solid and broken curves P P

in Fig. 1 as function of acceleration energy. It can be seen that these values are strongly dependent on composition, x, of WSix and-increase with increasing

energy. It has been proved by RBS measurements /13/that As concentration profiles

P’ P’

implanted in the silicide cannot be observed in samples with conventional M a silicide gate structure, because the observed As and W spectra overlap. Fig. 2

shows simulation spectrum for As-doped (5 at%) WSiZs6 layers obtained by a recently developed simulation program /13/. Spectra for W of WSi2.6 and for As implanted in WSi2*6 a re shown schematically. The values of higher and lower channel side edges of As and W spectra, CAs and Cw, defined in this figure a re shown in Fig. 2 as a function of layer thickness of WSiZm6. In con- ventional gate structure, for instance, with 250 nm thick silicide layer, Cw is much lower in energy than that of CAs. In this case, overlapping of these two dpectra occurs as also examined by RBS measurements. Therefore, detailed impurity concentration profiles cannot be observed from the overlapped RBS spectrum. As seen in this figure, however, Cw shifts to higher channel side with decreasing layer thickness and separation of these spectra becomes pos- sible at layer thickness below 90 nm, as shown by the broken line figure. This suggests that impurity concentration profile measurements can be attained precisely i f only these samples a re prepared.

in this

Experimental procedures Optimized electrode structures for As-profile

measurements mentioned above were manufactured. Thin tungsten silicide

Short Notes K133

501

450

c a 9 F 2 COL

s -5

- cn E

35L

fig. 7 Fig.2

Fig. 1. Calculated projected range, R (solid line), and projected straggling, P

A R (broken line) for arsenic ion implantation in tungsten, WSi2. o, and

WSi2 .6,and silicon layers as function of acceleration energy. Closed and open circles are observed values of R and AR for WSi2.6, where the measure- ment was made by 1.5 MeV He RBS

P

+ p P

Fig. 2. RBS spectrum simulation for As-doped (5 at%) WSi2 layers. Spectra for W of WSi2,6 and As-doped uniformly in WSiZa6 a re shown in the upper area

of the figure, where C and Y express channel and yield, respectively. The lower channel side edge for the W spectrum of WSi2. layer, Cw, and the higher edge of the As spectrum, CAs, defined in this figure, are shown as function of silicide layer thickness. 'Overlapping of these spectra disappears at layer thicknesses below 90 nm

layers (80 nm thick) were deposited directly on p-type (100) Si substrate by a cold-wall type LPCVD reactor at 370 OC with a pressure of 0.2 Torr, where

SiH and W F Source gases were fed with flow ratesoft000 and 14 cm /min,

together with He car r ie r gas. High purity silicide layers can be deposited by this t'echnique /9/. The composition, x, of WSix determined from the RBS yield ratio of W-to-Si spectra was 2.6. This composition for as-deposited layers has been extensively used a s gate electrode and interconnection /9, 10/ because of i ts promising performance after annealing. However, this value decreases ra-

3 4 6

K134 physica status solidi (a) 88

pidly to near stoichiometry of 2.1 with annealing at above 900 OC /lo/. X-ray and electron diffraction studies have shown that the as-deposited layer is amorphous o r has microcrystalline structure /9, lo / . In this experiment, As ions were implanted directly in silicide layers with a dose of 1 1 ~ 1 0 ~ ~ cm-2. Impurity concentration profiles in silicide layers were obtained by means of 1 .5 MeV He' RBS measurements.

Results and discussion The observed RBS spectra a r e shown in Fig. 3, where As was implanted in CVD WSi2 .6 at energiesof 00 and 160 keV, re- spectively. In this figure, the RBS yield is plotted in logarithmic scale. This figure depicts that the spectra due to W and Si of a thin WSi2.s layer (80 nm thick) appear in the channels 41 0 to 470 and around channel 31 0, respectively. The spectrum of ion implanted As locates in the channels 350 to 410. Clearly, the As spectrum does not overlap with that of W (channel 4lO to 470). There- fore, it has become possible from these spectra to study detailed features of As profiles in the silicide.

Fig. 4 shows a typical As profile at 40 keV in WS$.6 (80 nm thick) ob- tained from RBS measurement, where observed points of projected range and projected straggling are shown by open and closed circles in Fig. 1, re- spectively, and the curves show theoretical profiles calculated by using a projected range of 0: 018 p m and a projected straggling of 0.012 p m at 40 keV given in Fig. 1. As is evident from this figure, the observed As profile in silicide agrees well with LSS profiles although much broader profiles were observed for channeling in ion implantatioh in sputtered tungsten and tungsten silicide /7/. These observed profiles give a projected range of 0.019 p m and

I

300 coo 500 channel number -

Fig. 3. 1.5 MeV He+ RBS spectra from CVD WSi2.6 layer (80 nm thick) de-

posited on silicon, where As was implanted in WSi2a6 at 100 (- -4 and

160 keV (-) with a dose of 1x10l6 cm-2

Short Notes K13 5

+SI substrate

observed 6y RBS

Fig. 4. Ckserved As concentration profiles in CVD WSi2. layers (80 nm thick) deposited on Si

substrate obtained by RBS measurement, where As implantation was performed at 40 keV with a dose of l x l 0 cm . Theoretical profiles cal- culated by using R = 0.018 p m and AR =

= 0.012 p m obtained from Fig. 1 a re shown by the solid curve

16 -2

P P

a projected straggling of 0.014 pm. Projected

ranges and projected stragglings for 40, 100, and

160 keV obtained from observed As profiles a r e

depth @m)- plotted in Fig. 1 by closed and open circles, re- spectively. It is evident from this figure that the

observed values agree well with calculated ones in case of CVD WSi2.6. There- fore, the observed profiles can be described by theoretical LSS profiles. De-

tailed features of the channeling effect have been studied and reported elsewhere /ll/. This indicates that channeling is not dominant in CVD WSi2*6.

tation in W, WSi2. o, and WSi2 .6 layers were calculated as function of accele-

rationvoltage. Theoretical impurity concentration profiles in CVD tungsten sili- cide at various implant conditions can be obtained from these data.

Conclusions 1) Projected range and projected straggling for As implan-

2) As concentration profiles in CVD WSi, .6 with different implant con- ditions were observed. These observed profiles without annealing show the

Gaussian distribution and agree well with theoretical profiles obtained by using

projected range and projected straggling given in Fig. 1. These data a r e re- quired and useful for design of the self-aligned implantation process in tungsten

silicide gates.

References /1/ S. INOUE, N. TOYOKURA, T . NAJSAMURA, and H. ISHIKAWA, J. Electro-

/2/ M.Y. TSAI, F.M. D’ HEURLE, C.S. PETERSON, and R.W. JOHNSON,

/3/ S. P. MURAKA, M.H. READ, and C.C. CHANG, J. appl. Phys. -’ 52- 7450

chem. SOC. 128, 2402 (1981).

J. appl. Phys. 1 52 5350(1981).

(1 981).

K136 physica status solidi (a) 88

/4/ T. HARA, H. SUZUKI, H. TAKAHASHI, and S.C. CHEN, Electrochem.

/5/ P. PAN, N. HSIEH, H.J. GEIPET, Jr., and G.J . SLUSSER, J. appl.

/6/ T. P. CHOW, C.S. GRANT, W. KATZ, G . GILDENBLAT, and F. REIHL,

/7/ T. HARA, S.C. CHEN, H. TAKAHASHI, and N. OHTSUKA, Japan. J.

/8/ T . P. CHOW, C.S. GRANT, W. KATZ, G. GILDENBLAT, and R.F.

/9/ K.S. SARASWATT, D. L. BR06, J .A. FAIR, K.A. MONNIG, and R. BEY-

SOC. Cincinnati Meeting, May 1984.

Phys. - 53, 3059 (1982).

J. Electrochem. SOC. 130, 952 (1983).

appl. Phys., to b e published.

REIHL, J. Electrochem. Soc. - 130, 933 (1982).

E m , IEEE Trans. Electron Devices =, 1497 (1983).

/ lo / T. HARA, S. ENOMOTO, and T . JINBO, Japan. J. appl. Phys. - 23, 479

/11/ N. YAMAMOTO, S. IWATA, N. KOBAYASHI, K. YAGI, and Y. WATA, (1984).

VLSI Science and Technology, 1984. The Electrochemical Soc. Inc.,

1984 (p. 361). /12/ D.K. BRICE, Ion Implantation Range and Energy Distributions, Vol. Ib,

JFI/Plenum Data Company, New York 1975. /13/ S. ENOMOTO, T. JINBO, and T. HARA, Proc. 2nd Symp. Ion Beam Tech.,

Hosei Univ., Tokyo 1983 (p. 25).

(Received February 13, 1985)