Hf bilayers on Si(100)

Hf bilayers on Si(100)

Microelectronic Engineering 37/38 (1997) 483-490 ELSEVIER Epitaxial CoSi 2 formation by Co/Hf bilayers on Si(100) B. G e b h a r d t * , M. F a l k ...

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Microelectronic Engineering 37/38 (1997) 483-490

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Epitaxial CoSi 2 formation by Co/Hf bilayers on Si(100) B. G e b h a r d t * , M. F a l k e , H. G i e s l e r , S. T e i c h e r t , G. B e d d i e s , H.-J. H i n n e b e r g Institute of Physics, Technical University of Chemnitz, D-09107 Chemnitz, Germany

Abstract The reaction of Co/Hf bilayers with Si(100) was investigated in the temperature range between 700°C and 950°C. After the thermal treatment at 700°C we found a multilayer system, which consists of an epitaxial CoSi 2 layer on Si(100), a CoHfvSiz layer, and a CoSix layer on the top. An increase of the annealing temperature leads to a decrease of the thickness of the CoSix top layer. After annealing at 950°C we found a two layer system consisting of an epitaxial CoSi 2 layer and a ternary agglomerated top layer. After the removal of the top layer a second annealing process (1050°C, 51 s) was carried out to improve the crystalline quality of the epitaxial CoSi 2. The minimum yield of 10% and the low resistivity of 15 1~12cm at room temperature and of 2.9 IX1]cm at 4,2 K as well as a Hall coefficient of 1.85 × 1 0 - 4 c m 3 A-t S-I revealed the positive effect of this second thermal treatment.

I. I n t r o d u c t i o n Polycrystalline metal silicide films are c o m m o n l y used in silicon very large scale integrated (VLSI) device technology as gates, ohmic contacts, Schottky barriers and interconnects. The production of submicron-sized electronic devices requires a high quality interface to the Si substrate and good thermal stability o f the films used. Epitaxial silicides have the advantage of a smoother interface to Si and a better thermal stability compared to polycrystalline films. CoSi 2 is suitable for ohmic contacts and interconnects in VLSI technology due to its low resistivity and its high thermal stability. The similar cubic crystalline structure and the small lattice mismatch to Si (only - 1 . 2 % at room temperature [1]) make it possible to grow epitaxial CoSi z on silicon. An epitaxial CoSi 2 film on Si(100) can be formed by the Ti-interlayer mediated epitaxy ( T I M E ) [2-8]. Co and Si are separated by a thin Ti layer. It was reported [2] that Ti is able to reduce the native oxide during the early stages o f thermal treatment. During the annealing in a N 2 ambient Co diffuses through Ti and reacts with the Si. Different structures of the annealed samples are known from the literature. Dass and co-workers [2] obtained a TiN top layer on the epitaxial CoSi z layer. But in contrast Hsia et al. [3] found a T i - C o - S i alloy layer above the epitaxial CoSi 2. Several other metals, as Cr, V, Ta and Zr [9] have also been used instead o f Ti. In our study we replaced Ti by H f due to similar chemical properties o f H f and Ti. *Corresponding author. Fax: + 49 371 5313077; e-mail: [email protected] 0167-9317/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0167-9317(97)00150-0

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2. Experimental The substrates used were n-type 3-inch Si(100) wafers of 3 . . . 4 fl cm resistivity. Before they were loaded into a vacuum chamber with a base pressure of 7 × 10 -5 Pa the substrates were dipped in a diluted HF solution ( H F : H 2 0 = 1:100, 3 min) to remove the native oxide. Hf and Co were sequentially deposited by magnetron sputtering with a deposition rate of 6 nm m i n - ' for Hf and 1.8 nm min -1 for Co. The Ar pressure was kept at 2.5 × 10 -1 Pa during the deposition. The bilayer was prepared with 3 nm Hf thickness and 10 nm Co thickness and in a second series with Hf thicknesses of 5 nm or 10 nm and a Co thickness of 20 nm. The samples were isochronally (60 s) rapid thermal annealed [AST SHS 10] in the temperature range between 700°C and 950°C in a N 2 ambient (heating rates 5 K s -~ and 125 K s-~). A wet chemical removal of the top layers was not successful, therefore we used ion beam sputtering to remove the top layers above the epitaxial CoSi 2. The composition and the thickness of the layers were analyzed by 1.5 MeV He + Rutherford backscattering spectrometry (RBS). The scattering angle of the detected particles was 170 °. The simulations of the RBS spectra were made by means of RUMP [10]. Channelling spectrometry was used to determine the crystalline quality of the films. The morphology, the interfaces and the crystalline structure were characterized by cross-section transmission electron microscopy (XTEM). The electrical properties were determined by measurements of the resistivity and the Hall coefficient using the van der Pauw method.

Fig. 1. The XTEM micrograph of an as deposited sample shows a 10 nm thick Co layer above the Hf layer with an amorphous interlayer at the interface to Si and to Co.

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3. Results The XTEM micrograph (Fig. 1) of an as-deposited sample shows the layer sequence of polycrystalline Co above the Hf intermediate layer• At the interface of the Hf layer to Co as well as to Si we found an 1.5 nm thick amorphous interlayer• The thicknesses of the amorphous interlayers remained constant while the thickness of the Hf layer was increased. In the following section we describe the results of the reaction of Co(10 nm)/Hf(3 nm) bilayers with the Si substrate after annealing at various temperatures. Silicon diffusion to the sample surface and Co diffusion into the Si substrate have been observed by RBS after annealing at 700°C for 60 s (Fig. 2). Simulating the RBS-spectrum of a sample annealed at 700°C we found a multilayer system which consists of 3 different layers. A CoSi 2 layer has formed at the interface to the Si substrate. It is followed by a CoHfySiz layer and a CoSi x layer on the top of the system. The ternary layer consists of about 17 at.% Hf, 25 at.% Co and 58 at.% Si and the top layer is composed of about 34 at.% Co, 63 at.% Si and a small content of Hf (3 at.%). To determine the epitaxial quality of the formed CoSi 2 films channelling investigations were camed out using an incident beam direction parallel to the [100] direction of the Si substrate. Only a slight decrease of the backscattering yield was detected in the CoSi 2 layer nearest to the substrate. This

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Fig. 3. The XTEM micrograph of a Co/Hf/Si(100) sample after rapid thermal annealing at 700°C for 60 s in N 2 ambient.

indicates that only a small amount of the crystallites has the same orientation as the Si substrate. This result is confirmed by the XTEM micrograph (Fig. 3) of this sample, which shows the multilayer structure and a polycrstalline growth of the CoSi 2 layer. Detailed investigations showed that some of the crystallites are aligned to Si. The interface to the substrate is rough. With the increase of the annealing temperature a thicker CoSi 2 layer was formed and the thickness of the top CoSi x layer decreased. The amount of epitaxial CoSi 2 increased considerably. The minimum yield Xmin, the ratio of the backscattering yield at an aligned orientation to that of a random orientation, decreased to 25% in the Co peak after annealing the sample at 950°C for 60 s. The RBS-spectra are shown in Fig. 4. After this first thermal treatment the XTEM micrograph (Fig. 5) shows only an epitaxial CoSi 2 and an agglomerated top layer. The interface of the epitaxial CoSi 2 layer to Si is quite smooth with steps of up to 10 nm in height. In a second series of experiments we investigated the influence of the Hf thickness on the reaction of C o / H f bilayers with Si. For this purpose we used samples with 5 nm and 10 nm Hf thickness and 20 nm Co thickness. All samples were annealed at 750°C for 60 s. The formation of the CoSi x layer on the top of the samples could not be prevented by a thicker Hf layer. However, its composition changed from CoSi 2 to CoSi. The increase of the Hf thickness led to an increase of the thickness of the ternary CoHfySi layer, but the layer composition of about 17 at.% Hf, 31 at.% Co, and 52 at.% Si stayed unchanged. To measure the electrical properties of the CoSi 2 film the ternary top layer was removed by ion beam sputtering. A second thermal treatment (1050°C, 51 s) was carried out to improve the crystalline quality of the epitaxial CoSi 2 causing a minimum yield of 10% (Fig. 6). The resistivity of the film

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was determined to be 15 ixfI cm at room temperature and 2.9 txl~ cm at 4.2 K. The Hall coefficient of 1.85 × 10 - 4 c m 3 A l s- J was found to be independent of the temperature in the range from 4.2 K to 300 K. High resolution electron microscopy (HRTEM) was carried out to examine the CoSi2/Si interface. The (110) cross-sectional HRTEM image (Fig. 7) shows the sharp and smooth (100) interface between the formed CoSi 2 layer and the Si substrate.

4. Summary films have been grown by the reaction of C o / H f bilayers with Si(100). The formation of a multilayer system consisting of an epitaxial CoSi 2 layer, a ternary CoHfySi z layer and a CoSi x layer on the top of the sample has been observed at 700°C. With the increase of the annealing temperature the thickness of the CoS L top layer decreased and after annealing at 950°C a two layer system consisting of an epitaxial CoSi 2 layer and a ternary agglomerated top layer was formed. For an increased thickness (up to 10 nm) of the Hf layer the formation of the multilayer system takes place in the same way. A good crystalline quality (Xmin = 10%) can be obtained by a second thermal treatment after the removal of the top layers. Our investigation shows that Ti can be replaced by Hf. Epitaxial

CoSi 2

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Fig. 7. The < 110 > cross-sectional HRTEM image of a Co/Hf/Si(100) sample after the removal of the top layers and a second thermal treatment at 1050°C for 51 s in N 2 ambient.

Acknowledgements This research was financially supported by the " B u n d e s m i n i s t e r i u m ffir B i l d u n g und F o r s c h u n g " u n d e r grant n u m b e r 0 3 H E 3 C H E 4 . T h e authors are grateful to the R e s e a r c h C e n t e r R o s s e n d o r f for the o p p o r t u n i t y to carry out the R B S m e a s u r e m e n t s .

References [1 ] R.T. Tung, Epitaxial silicides, in: Mahajan (Ed.), Handbook on Semiconductors, Vol. 3b, Elsevier, Amsterdam, 1994, p. 1917. [2] M.L.A. Dass, D.M. Fraser, C.-S. Wei, Growth of epitaxial CoSi 2 on (100) Si, Appl. Phys. Lett. 58 (1991) 1308-1310. [3] S.L. Hsia, T.Y. Tan, P. Smith, G.E. McGuire, Resistance and structural stabilities of epitaxial CoSi 2 films on (001) Si substrates, J. Appl. Phys. 72 (1992) 1864-1872. [4] A. Lauwers, R.J. Schreutelkamp, B. Brijs, H. Bender, K. Maex, Technological aspects of epitaxial CoSi 2 layers for CMOS, Appl. Surf. Sci. 73 (1993) 19-24. [5] G.B. Kim, H.K. Baik, S.M. Lee, Control of Co flux through ternary compound for the formation of epitaxial CoSi 2 using Co/Ti/Si system, Appl. Phys. Lett. 69 (1996) 3498-3500.

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[6] A. Vantomme, M.A. Nicolet, N.D. Theodore, Epitaxial CoSi z films on Si(100) by solid-phase reaction, J. Appl. Phys. 75 (1994) 3882-3891. [7] F. Hong, G.A. Rozgonyi, B.K. Patnaik, Mechanism of epitaxial CoSi 2 formation in the multilayer Co/Ti-Si(100) system, Appl. Phys. Lett. 64 (1994) 2241-2243. [8] S. Ogawa, J.A. Fair, M.L.A. Dass, E.C. Jones, T. Konzaki, N.W. Cheng, D.M. Fraser, Epitaxial CoSi 2 layer formation technology on (100) Si and its application for reduced leakage, ultra shallow p+/n junction, Extended Abstracts of 1993 Int. Conf. on Solid State Devices and Materials, Makuhari, 1993, pp. 195-197. [9] J.S. Byun, W.S. Kim, M.S. Choi, H.J. Cho, H.J. Kim, Silicidation mechanism of Co/(refractory-metal) bilayer and epitaxial growth of CoSi 2 on Si(100) substrate, Mat. Res. Soc. Symp. Proc. 320 (1994) 379-384. [10] L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering spectra, Nucl. Inst. Meth. B 9 (1985) 344-351.